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VARIABLE FREQUENCY DRIVES

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Until the advent of the AC drive, industry could not take advantage of the superior features of AC motors for many applications. Until recent advances in solid-state electronics decreased the cost of AC drives, industry did not take advantage of AC drives. Industry is now paying more attention to AC drives. AC drives convert the constant frequency and voltage of incoming AC line power to variable frequency, variable voltage, and sometimes variable current AC power. By converting the power to variable frequency and voltage, it is possible to control the speed of an AC motor. The result is that industry can control the speed of a process for a given application (for example, the speed of a mixer) and take advantage of the excellent qualities of AC motors. This article introduces you to the hardware components that make up AC drives. This article also describes some of the major circuits in the power and logic sections of a drive, and it explains how these circuits operate. Different parts of the interface and how it works with the logic section to control drive performance are also covered in this article.

Menu
1.2.1 Processor
2.1.1.1 VVI Drive
2.3.1.1 Advantages
3.2.1 Torque
3.2.2 Speed
9 SCR
12 Diac
15.1.1 Escape Key
15.1.2 Select Key
15.1.4 Enter Key
15.2.1 Start
15.2.2 Stop
15.2.3 Jog
15.4.1 Display
15.4.2 Program
15.4.3 Process
15.4.4 EEPROM
15.4.5 Search
15.4.7 Password
16.2.1.1 Metering
16.2.8 MOP Hz
16.2.9 Drive Temp
16.2.10 Last Fault
16.3 Setup
16.3.1 Input Mode
16.3.5.1 Base Voltage
16.3.5.3 Minimum Freq
16.3.5.4 Maximum Freq
16.3.5.5 Stop Select
16.3.5.6 Current Limit
16.3.5.7 Overload Mode
16.3.6.2 Minimum Freq
16.3.6.3 Maximum Freq
16.3.12 Start Boost
16.3.13 Run Boost
16.3.17 Stop Select
16.3.18 DC Hold Time
16.3.20 DB Enable
16.3.21 Motor Type
16.3.22 Compensation

AC Drive Systems

An AC drive hardware system has some basic similarities to many of todays motor vehicles. For example, some cars contain operator controls, an engine, and a microprocessor. The operator controls allow a driver to start and stop the vehicle, change its speed, and change its direction. The engine converts chemical energy, fuel mixed with air, to mechanical energy, or drive shaft rotation, to produce motion and speed. The microprocessor monitors the status of the operator controls and controls when and how fast the engine should change chemical energy to mechanical energy to drive the vehicle.

Similarly, AC drives also have power conversion and control units. The power conversion unit is called the power section. The control unit is called the logic section. In addition, various controls interface to the AC drive to control the drives output frequency and voltage. Many of the operator controls are outside of the AC drive. Some, however, are located on the drive, such as start and stop pushbuttons. All controls that connect to the drive are referred to as the interface.

An AC drive system generally consists of the drive, the motor, the load, and associated hardware like gearboxes and limit switches. Figure 1 shows a simple AC drive system.


Figure 1: AC Drive System

The drive itself consists of two major sections: power and logic. Figure 2 shows the two major sections and the interface in an AC drive.


Figure 2: AC Drive Sections and Interface
The drive receives the incoming AC line power from the plants electrical power source. The AC line power has the following two important characteristics:

  • Constant voltage (for example, 460 V or 230 V)
  • Constant frequency (usually 60 Hz in North America)

Although the actual values of the incoming lines power voltage and frequency may vary a few volts and hertz, the power company guarantees these values within a certain range. Thus, they are constant values for all practical purposes. The drive converts the constant voltage and frequency from the incoming AC line power to variable voltage and frequency AC power. By being able to change frequency and voltage, the drive is able to change a motors speed and control its torque.

Power Section

The power section is the main part of an AC drive. Most modern AC drive power sections consist of solid-state electronic circuits. The solid-state electronics change the incoming AC line power to control the speed and torque of an AC motor. Figure 3 shows the three major circuits (stages) in the power section. The power section of an AC drive consists of three stages:

  • Input stage
  • Intermediate stage
  • Output stage


Figure 3: Power Section Stages

Input Stage

The first stage is the input stage. The input stage is basically a rectifier circuit. The primary function of the input stage is to change the fixed magnitude and frequency of the incoming AC line power to DC power. The output of the input stage is a DC voltage. There are two types of input stages used in most AC drives:

  • Diode rectifier
  • Silicon-controlled rectifier (SCR)

Diode Rectifier Input Stages

Diode rectifier input stages use a pair of diodes for each phase. They convert the incoming AC line voltage to a constant DC voltage. They then apply this constant DC voltage to the intermediate stage.

SCR Input Stages

SCR input stages use a pair of SCRs for each phase. They also convert the incoming AC line voltage to DC, but unlike the voltage produced by diode rectifier input stages, SCR input stages produce variable DC voltage.

Intermediate Stage

The second stage, the intermediate stage, is a filter circuit. The primary function of the intermediate stage is to smooth the DC ripple voltage that the intermediate stage receives from the input stage. There are two types of intermediate stages used in AC drives: filter and chopper.

Filter Intermediate Stage

Filter intermediate stages use capacitors and inductors. Capacitor filters smooth out the voltage waveforms applied to them; inductor filters smooth out the current waveforms applied to them. The filtered DC voltage that may have either a fixed or variable magnitude is then applied to the output stage.

Chopper Intermediate Stage

Chopper intermediate stages use transistors to chop the DC voltage applied to them. This results in a DC voltage pulse train. By varying the duty cycle of pulses in the pulse train, the average DC voltage level is varied. This changing pulse train is applied to the output stage.

Output Stage

The third stage is the output stage. The output stage is an inverter circuit. The primary function of the output stage is to change the smooth DC voltage back to an AC type of power. The AC power output is variable frequency. Some output stages also vary the apparent voltage amplitude. In other drives, the voltage amplitude is varied in another stage. However, remember the amplitude may be varied in the input, intermediate, or output stages; but frequency is varied only in the output stage. There are three types of output stages used in most AC drives. These types of output stages are as follows:

  • Transistor
  • SCR
  • Gear turn-off thyristor (GTO)

Transistor Output Stage

Transistor output stages consist of a pair of transistors for each output phase. The logic section applies and removes a pulse to the base of each transistor to turn it on and off. Turning the transistors on and off at the proper time generates an AC type of voltage. This voltage is applied to an AC motor to control its speed.

SCR Output Stage

SCR output stages also consist of a pair of SCRs for each output phase. The logic section applies a pulse to the gate of each SCR to turn it on. However, removing the pulse from the gate of the SCR does not turn it off because the DC bus never crosses zero. The SCRs in the output stage must be turned off by a method called forced commutation. In forced commutation, capacitors are charged up and discharged across the SCRs at the proper time to force them off. The resultant AC voltage is applied to an AC motor to control its speed.

GTO Output Stage

Like transistor and SCR output stages, GTO output stages consist of a pair of GTOs for each output phase. GTOs are like SCRs except that GTOs have two gates: one for turning the device on and another for turning it off. The logic section applies pulses to one gate to turn the GTO on and another pulse to the other gate to turn the device off. Turning the GTOs on and off at the proper time generates an AC type of voltage that is applied to an AC motor to control its speed.

Logic Section

The logic section is another important part of AC drives. Most AC drive logic sections use microprocessors and other large-scale integrated circuits. The microprocessor and LSI chips can be divided into several functional circuits as follows:

  • Processor
  • Acceleration/deceleration circuit
  • Volts circuit
  • Frequency circuit
  • DC boost circuit
  • Driver circuits
  • Protection sensing circuits
  • Low-voltage power supply circuits

Figure 4 shows a simple diagram of the major circuits in the logic section of the drive.


Figure 4: Logic Section Circuits

Processor

The processor monitors signals from the acceleration, deceleration, volts, frequency, DC boost, start/stop, and protection circuits in the logic section. The processor also monitors signals in the input, intermediate, and output stages of the output section.

Once the processor determines the frequency and voltage that the drive should output, the processor sends signals to the driver circuits. The driver circuits output the proper pulses to turn on and turn off the semiconductor devices in the power section. The power section produces the required output frequency and voltage to run the motor and its load.

To properly run a motor and its load, the drive must be designed and set to match the specific operating requirements of the motor and the load. Parameters are used to set the requirements. These requirements generally include the following:

  • Maximum and minimum speed limits
  • Acceleration and deceleration limits
  • Volts-per-hertz ratio
  • DC boost level

These parameters are set by controls that are usually mounted in the logic section. However, some may be found as part of the interface. These controls may be digital or analog.

Maximum and Minimum Speed

AC motors have a maximum safe operating speed, and many have minimum safe operating speeds. Should the motor exceed these limits, it could be damaged. The logic section ensures that the drive does not allow a motor to rise above its maximum safe operating speed, nor fall below its minimum safe operating speed. The maximum and minimum speed limit controls send reference signals to the frequency circuit, which then conditions these signals so the processor can use them.

Acceleration and Deceleration Limits

AC motors also have certain acceleration and deceleration limits. Should the motor exceed these limits, it or the load may be damaged. An AC drives logic section must be set so that it safely accelerates and decelerates the motor within these limits. The acceleration control establishes the time that the drive allows the motor to accelerate from zero speed to full speed. Conversely, the deceleration control establishes the time that the drive allows the motor to decelerate from full speed to zero speed. Both controls send reference signals to an acceleration/deceleration circuit in the logic section that, in turn, conditions these signals so the processor may use them.

Volts per Hertz Ratio

AC motors have a certain volts-per-hertz ratio requirement. As frequency increases or decreases, voltage must increase or decrease proportionally to maintain the required torque. The logic section establishes the proper ratio between volts and hertz as motor speed is increased or decreased. The volts-per-hertz control sends a reference signal to the volts circuit, and the volts circuit provides a properly conditioned signal to the processor.

DC Boost Level

Many AC motors have a special requirement called DC boost. At zero or near zero speed, they require an additional boost of voltage for starting torque. The logic section establishes the additional DC boost level for motors that require this DC boost. DC boost control establishes a reference signal to a DC boost circuit that, in turn, conditions the signal so the processor can use it.

Drive Circuits

The driver circuits receive signals from the processor. They modify the signals from the processor to turn on the power devices (SCRs, transistors, and GTOs) in the power section. The drive circuits also control the desired motor operating speed and motor startup and shutdown. A speed control in the interface sends a signal to the frequency circuit in the logic section to establish the desired operating speed. Similarly, a start/stop control, also in the interface, sends signals to a start/stop circuit in the logic section to start or stop the drive.

Protection and Sensing Circuits

The protection-sensing circuits monitor the status of certain circuit and environmental parameters, such as overvoltage conditions or higher than normal temperatures in the power section. When a problem emerges, the protection-sensing circuits send a signal to the processor to protect the drive. They light an indicator, typically a light-emitting diode (LED), to warn a maintenance person or operator about the problem. They also isolate the power devices from the processor to protect the processor from excessive voltages and currents. The protection-sensing circuit provides protection to both the motor and the drive. Generally, this includes protection against the following kinds of conditions:

  • Overcurrent
  • Under current
  • Overvoltage
  • Under voltage
  • Overtemperature
  • Short circuit
  • Instantaneous overcurrent

Low-Voltage Power Supply

The low-voltage power supply actually may be a part of the power section, the logic section, or part of both. The low-voltage power supply generates the low voltages and currents that the circuits in the logic section need to operate properly. This supply takes a part of the incoming AC power, usually in the range of 5 to 30 VDC.

Interface

A variable frequency AC drive is divided into two main sections: the power section and logic section. The interface consists of a group of components that interacts with the logic section to control the drives performance.

To some people, the interface includes only those components that link to the logic section, because these components send control information to, or receive control information from, the logic section. However, to other people, the interface includes any component that is mounted externally to the drive, including breakers, contactors, line fuses, and transformers. For this course, the interface includes all the devices that are mounted externally to the drive.

Interface components vary in complexity. They range from simple remotely mounted stop/start switches to more complex binary coded decimal (BCD) modules in a programmable controller. This section presents an overview of some of the many kinds of interface components. They include the following:

  • Control Interface components that link directly to the logic section; they are divided into two groups:
    • Person-to-Machine Interface components must be adjusted or read by the operator.
    • Machine-to-Machine Interface components link an external control device to the logic section; they control some aspect of drive operation without operator influence.
  • Power Interface components that connect to input or output power and do not link directly to the logic section.

Person-to-Machine Interface

The most varied group of interface components includes those that link an operator directly to the logic section. This is a person-to-machine interface, where the operator interacts with the drive at either the drive control panel or a remote panel or station. Some of these components alter set points in the logic section and change drive performance. Other interface components link directly with the logic section, but have no direct control over the drive. These components provide information to control the drives performance.

Interface Components

Interface components that allow the operator to communicate with the drive vary depending on the application. Inputs and outputs to the logic section can be remotely or directly mounted on the logic boards. Almost any logic section adjustment or indicator can be remotely mounted and labeled an interface component. For example, an application may require the operator to change the acceleration/deceleration rate of the drive many times per day. The acceleration/deceleration potentiometer can be mounted with the stop/start switch and frequency potentiometer on the outside of the drive. Examples of remotely mounted interface components are as follows:

  • Run/Jog switches
  • Stop/Start switches
  • Forward/Reverse switches
  • Frequency controls

Diagnostic and Performance Indicators

Diagnostic and performance indicators and meters are also interface components in the person-to-machine link to the logic section. An interface control panel could include voltage meters and current meters, overload indicators, fault indicators, and frequency adjustments. Figure 5 shows an interface containing these components.


Figure 5: Example Interface Panel

Application Requirements

Depending on the manufacturer, environment, and option packages available, interface components follow the application requirements. Any information the logic section handles can be displayed on an interface. Interfacing configurations can become complex, especially in situations where multiple motors run under the direction of multiple drives.

The more complex the interfacing, the more important it becomes to follow all of the manufacturers specifications regarding length of cables, configuration of cables, and shielding requirements. Separating interface cables from high-power input or output cables and other wiring is especially important. This prevents cross-talk. Cross-talk is interference or noise that is induced in one set of cables by another adjacent set. With a person-to-machine interface, the operator usually sets up, monitors, or changes the drives performance as the application requires.

Machine-to-Machine Interface

Many applications involve machine-to-machine interfacing. These are usually highly automated systems. Although an operator may set up these systems and monitor their performance, the process controls the logic section.

An example of machine-to-machine interfacing is an application where an AC drives speed is varied by a pressure transducer to maintain a constant airflow. Figure 6 shows a representation of this type of interface.


Figure 6: Machine-to-Machine Interface Example
The output frequency of an AC drive must match the required production and process requirements. For example, it must match the speed, flow rate, and pressure requirements of an application. Many methods are available for this sort of speed match. Usually, a sensor sends a voltage or current signal proportional to frequency to the logic section.

Proper interface operation also often requires impedance matching. This may involve the use of specially shielded cables or cables cut to a certain length.

Power Interface

Power interface components do not directly control the logic section. Therefore, they do not control drive operation. However, they must function properly for the drive to operate properly. Power interface components may include the following:

  • Fuses external to the drive
  • Other loads connected to the drive input
  • Circuit breakers or contactors at the input or output
  • Input isolation transformers
  • Output transformers

Some of these components are shown in Figure 7. Although these components do not control the drive, they do affect the performance of the drive application. Therefore, they must be examined carefully in installation, maintenance, and troubleshooting. For example, fuses, input cables, and output cables must be properly sized.


Figure 7: Power Interface Components

Existing installations may have used power factor correction capacitors with a motor starter. These capacitors or other power conditioning equipment are seldom used in a drive application, so they usually must be removed.

Thermal Overload Relay

One type of device found in the interface is the thermal overload relay. Figure 8 shows a thermal overload relay symbol. Thermal overload relays are used with AC drives to protect the motor. They are the same types as those used with motor starters. However, when used with AC drives, the thermal overload relays are sized differently because of the unique characteristics of the waveforms in the drive outputs.


Figure 8: Thermal Overload Relay

Transformer

Another type of device found in the interface is the transformer. Figure 9 shows a symbol of an industrial transformer. Transformers may be used at both the input and the output of AC drives. At the input, power transformers decrease or increase the incoming line voltage, provide short-circuit protection for the drive, and provide noise rejection when digital equipment is present. At the output, power transformers increase or decrease the voltage delivered to the motor. Use of power transformers generally requires factory consultation.


Figure 9: Transformer

Pushbuttons, Limit Switches, and Temperature Switches

To start and stop an AC drive, many kinds of devices are used in the interface. These include pushbuttons, limit switches, temperature switches, proximity switches, and others. Figure 10 shows symbols for these kinds of devices.


Figure 10: Input Devices
Programmable controller output signals also can provide start, stop, and speed control signals to an AC drive. There are many other devices used to change the speed of the drive. Speed control devices include speed potentiometers, digital thumbwheel switches, motor-operated potentiometers, and programmable controller binary coded decimal (BCD) output signals.

Types of AC Drives

Various items are classified in different ways; for example, consider cars. Cars are grouped in many ways. Sometimes they are grouped as sub-compacts, compacts, midsize, and full size. Other times, they are grouped as sports cars, sedans, station wagons, and recreation vehicles. How items are grouped often depends upon their design, use, or what seems to be convenient at the time.

AC drives also are classified in different ways. They are typically classified according to two factors:

1. The design of their power sections
2. The types of waveforms they generate to run an AC motor

Recall that an AC drives power section consists of an input stage, an intermediate stage, and an output stage. These stages may consist of a number of different types of circuits. The type of circuits used and the output waveforms they generate determine the type of drive.

This section presents an overview of several common types of AC drives. The main focus is on several kinds of voltage source drives, but it also briefly discusses current source drives.

Voltage Source Drives

Voltage source drives provide a voltage source to the output stage in the power section of an AC drive. Voltage source drives are divided into two major categories, as follows:

  • Variable-voltage source
  • Constant-voltage source

Variable-Voltage Source Drives

Variable-voltage source drives provide a variable-magnitude DC voltage to the power sections output stage. There are two major kinds of variable-voltage source AC drives:

  • Variable-voltage input (VVI) drive
  • Chopper drive

VVI Drive

Variable-voltage input drives are one of the most common types of variable-voltage source drives found in industry. They have a power section that consists of an SCR input stage, a filter intermediate stage, and an SCR output stage. Figure 11 shows the power section of a VVI drive.


Figure 11: VVI Power Section
The output stage converts the variable-magnitude DC voltage that it receives from the intermediate stage to a variable-frequency and variable-magnitude AC signal. The VVI drive produces a six-step voltage level waveform and a sinusoidal current waveform. The attached AC motor runs as if the voltage waveform is a sinusoidal waveform. Figure 12 shows VVI output waveforms.


Figure 12: VVI Output Waveforms
VVI drives use SCRs to vary the DC bus voltage in the input stage. By determining when to turn on the SCRs, the logic section sets the voltage amplitude. The intermediate stage then smoothes the DC and transfers it to the output stage. Then the output stage, by gating SCRs on and using commutation circuitry to turn them off, creates a six-step waveform at the appropriate frequency. To the AC motor, the output waveform appears like a sinusoidal AC voltage.

Chopper Drive

Chopper drives are the second type of variable-voltage source drive. The chopper drive has a diode rectifier input stage and a chopper type of intermediate stage. The output stage is made up of power transistors. The power section and output waveform for a chopper drive are displayed in Figure 13.


Figure 13: Chopper Power Section
The chopper drive also produces a six-step, variable-voltage waveform and a sinusoidal current waveform. The AC motor reacts to the voltage waveform in the same way as it does for the VVI drives voltage waveform.

Chopper drives use diodes in the input stage. These rectify the current but do not vary the voltage. A transistor chopper circuit in the intermediate stage varies the DC bus voltage. It turns on and off to create pulses. By varying the duty cycle of the pulses, it creates an average DC voltage of the desired amplitude. A filter smoothes this voltage. The output stage uses transistors to create a six-step AC waveform.

Constant Voltage Drive Summary

The output stage in a variable voltage source drive controls only the frequency of the AC voltage output. Varying the amplitude of the DC bus voltage varies the amplitude of the voltage. This is done in either the input or intermediate stages.

Advantages and Disadvantages

Each type of drive has advantages and disadvantages. The type of semiconductors used in each determines these advantages and disadvantages.

Because SCRs can handle high voltage and current, VVI drives are designed for high horsepower ranges (500 HP ranges). However, the switching of the SCRs in the input stage can cause a power factor shift and send noise back to the plant power. Noise and power factor shifts may be controlled with isolation transformers. The switching rate of SCRs limits the output of VVI drives to 2.5 times the line frequency.

The diodes used in the input stage of chopper drives do not affect plant power. However, transistors cannot handle as much current as SCRs, so chopper drives cannot produce as much horsepower as VVI drives. Transistors do have faster switching rates than SCRs, so they can produce higher output frequencies than VVI drives.

Constant-Voltage Source Drives

Constant-voltage source drives provide a constant-magnitude DC voltage to the power sections output stage. The three major kinds of constant-voltage source AC drives are:

  • Transistor pulse-width-modulated (PWM) drive
  • Gate turn on/off (GTO) PWM drive
  • Silicon-controlled rectifier (SCR) PWM drive

Transistor PWM Drive

The transistor PWM drive has a power section that consists of a diode rectifier input stage, a filter intermediate stage, and a transistor output stage. Figure 14 shows a Transistor PWM Drive.


Figure 14: Transistor PWM Drive
The transistor power circuit generates a variable-duty cycle waveform to drive an AC motor. The output voltage waveform consists of pulses of varied widths. Figure 15 shows the waveform for a transistor PWM drive. Although the PWM voltage waveform is not a sinusoidal waveform, the AC motor reacts to it as if it is one. However, note that its current waveform is sinusoidal. The major advantage of the transistor PWM drive is the fast switching time of the transistors. It is capable of speed changes faster than the other PWM drives. The disadvantage of the transistor PWM drive is that the power limits are less than SCR or GTO power limits.


Figure 15: Transistor PWM Drive Waveform

GTO PWM Drive

GTO PWM constant-voltage source AC drives also have a power section input stage and intermediate stage like that of the transistor PWM drive. The input stage consists of a diode rectifier circuit, and the intermediate stage consists of a filter circuit.

The output stage of the GTO PWM drive is different from that of transistor PWM drives. The output stage of the GTO PWM drive consists of GTOs. Figure 16 shows the power section for GTO PWM drives.


Figure 16: GTO PWM Power Section

Like the transistor PWM drive, the GTO PWM drive generates a PWM output voltage waveform and a sinusoidal current waveform. Figure 17 shows the waveforms for the GTO PWM drive. The AC motor reacts to these waveforms as if they were sine waves, in the same manner as it does to the transistor PWM drive.


Figure 17: Constant-Voltage Output Waveform (PWM, GTO, SCR)

SCR PWM Drive

SCR PWM constant-voltage source AC drives also have a power section input stage and an intermediate stage like those of the transistor PWM and GTO PWM drives. The input stage consists of a diode rectifier circuit, and the intermediate stage consists of a filter circuit. However, the output stage consists of an SCR inverter circuit. Figure 18 shows the power section for a SCR PWM drive.


Figure 18: SCR PWM Power Section

Unlike the transistor PWM drive or the GTO PWM, the SCR PWM drive generates a constant duty cycle output voltage waveform. Figure 19 shows the waveforms for an SCR PWM drive. It is still a PWM waveform, but all pulses have an equal duty cycle. The SCR PWM drive also produces a sinusoidal output current waveform. The AC motor reacts to the voltage waveform as it does to the other PWM drives. However, because the output voltage waveform has equal duty pulses, the motor does not accelerate and decelerate as smoothly as it does with the other PWM drives.


Figure 19: SCR PWM Drive Waveform

Constant-Voltage Drive Summary

All constant-voltage source drives have similar input and intermediate stages. Neither stage varies the voltage. The input stage simply rectifies the AC line voltage with diodes. As a result, constant-voltage source drives do not cause plant power factor shifts. The filter intermediate stage smoothes the DC voltage.

The output stage of constant-voltage source drives sets the frequency and varies the apparent voltage by pulse-width modulation. One of three types of semiconductors is used:

  • Transistors
  • GTOs
  • SCR

Advantages and Disadvantages

Each type of constant-voltage source drive has certain advantages and disadvantages. These are based primarily on the type of semiconductor devices in the output stage. Because transistors and GTOs are capable of high switching rates, they can vary the voltage pulse widths to produce smooth variations in the average voltage. To the motor, the average voltage variations appear as a sine wave. Therefore, GTO and transistor PWM drives are sometimes called sine PWM drives. Both types are able to produce an output frequency that is three times the input frequency.

SCR are incapable of switching rates as high as those of transistors or GTOs. They must be turned off by forced commutation. Forced commutation requires more support electronics. They are capable of somewhat less output frequency than sine PWM drives, about 2.5 times the input frequency. In addition, SCR PWM drives normally produce a constant pulse width, so the average voltage does not change as smoothly as with sine PWM drives. Some newer SCR PWM drives are designed to produce smoother sinusoidal voltages at low frequencies and six-step voltages at higher frequencies.

Since SCRs can handle high current, SCR PWM drives currently go up to the 500 HP range. Constant-voltage source drives using lower-power GTOs and transistors currently can go to only the 50 HP range.

Current Source Drives

Current source drives are commonly referred to as current source inverters (CSI). CSI drives provide a current source to the output stage in the power section of an AC drive. They have many similarities with voltage source drives, but they also have some unique differences. Figure 20 shows a block diagram of a common CSI drive.


Figure 20: Current Source Drive
One of the major differences between CSI drives and voltage source drives is in the intermediate stage. In CSI drives, the intermediate stage has a filter circuit that consists mainly of a large series inductor that opposes current fluctuations. In voltage source drives, the filter or intermediate stage consists of large series inductors as well as large capacitors or banks of capacitors placed across the bus.

The input stage in CSI drives may be identical to the input stage of voltage source drives. Like voltage source drives, it may be made up of diodes or SCRs.

The output stage in CSI drives also is quite similar to that in voltage source drives. Because CSI drives normally handle larger horsepower ratings, the output stage usually contains SCRs. The output waveforms for CSI drives are the opposite of those produced by voltage source drives. Figure 21 shows the output waveform for a CSI drive. In voltage source drives, the output voltage waveform is a six-step waveform. In CSI drives, the output voltage waveform is a sinusoidal waveform. However, in CSI drives, the output current waveform is a six- step waveform, whereas, in voltage source drives, the output current waveform is a sinusoidal waveform.


Figure 21: CSI Drive Output Waveform
Finally, CSI drives usually require a feedback loop, which provides stability and control. As shown in Figure 22, the feedback loop consists of a tachometer coupled to the AC motor.


Figure 22: Feedback Loop
In the example shown in Figure 22, a tachometer monitors motor speed. The signal generated by the tachometer is fed back to the logic section. The logic section compares the signal with a speed reference signal to regulate drive performance and motor speed. The result is that the control circuits in the logic section are more complex than those for voltage source drives.

A CSI drive also requires some load for commutation to occur in the output section. Finally, it cannot operate at frequencies as high as in a voltage source drive.

Current Source Drive Summary

Current source drives supply regulated current to the output stage. The output stage uses the regulated DC current to generate a six-step current waveform.

Current source drives, like voltage source drives, have a logic section and a power section. Both types of drives include input, intermediate, and output stages, though there are important differences in each type of drive. The main difference is in the type of waveform produced by the output stage; current source drives supply regulated current, while voltage source drives supply regulated voltage.

Either voltage source drives or current source drives could be used successfully depending on the application. The features of the drive determine its suitability for a given application.

Advantages

Some advantages of current source drives include the following:

  • They operate in higher horsepower ranges than voltage source drives.
  • They are very rugged, and they are not harmed by short circuits.
  • They have simple commutation circuitry in the output stage.

Disadvantages

Some disadvantages of current source drives include the following:

  • They do not operate well with multiple motor applications.
  • They do not handle opens well.
  • They need an additional circuit to monitor motor operation.
  • They are unable to match the high frequencies at which voltage source drives operate.

AC Induction Motors

Motors are the workhorses of modern industry. Motors are found virtually in every manufacturing application in modern industry. They move conveyors, they rotate mixers and cement kilns, and they run pumps and fans. Whenever materials, fluids, or gases need to be moved, motors are at work running equipment to complete the job.

AC motors are one common type of motor found in many phases of manufacturing. They are divided into two major classes:

  • AC induction motors
  • AC synchronous motors

Induction motors are the simplest and the most rugged. They are the most frequently encountered AC motors in drive applications.

Synchronous motors are generally more complex and less rugged. They are less frequently encountered in manufacturing equipment.

This article focuses on three-phase (3) AC induction motors ( = phase, phi). Three-phase induction motors are the kind most often used with AC drives. This section primarily discusses AC induction motor operation and characteristics. Three-phase induction motors are the most common type of AC motor built by manufacturers. They have a fixed stator and a movable rotor. The rotor turns because of magnetic interaction (magnetic coupling) between electromagnets in the stator and rotor.

Motor Construction

The stator consists of a set of electromagnets built into the motors frame. These electromagnets are stationary. They are referred to as the stator windings or stator. The rotor consists of another set of electromagnetic windings built into the motors rotating shaft. These electromagnets rotate with the shaft (inside the stator electromagnets). They are referred to as the rotor or rotor windings. Figure 23 shows an exploded view of an induction motors physical components. The stator electromagnets are physically mounted to the cylindrical motor frame. The rotor electromagnets are physically mounted to the motor shaft. Two end bells support the rotor shaft, one at each end of the motor. They also serve as the end caps to the motors frame.


Figure 23: Induction Motor

Stator Construction

The stator electromagnets consist of insulated wire wound around iron cores. The cores consist of a collection of disks with slots in them for the wire. The cores are wound in pairs called pole-pairs. Each pole in a pole-pair is positioned on opposite sides of the inside of the cylindrical motor frame. An induction motors stator may be constructed of any even number of pole-pairs, but the most common are the 2, 4, 6, and 8 pole-pairs.

When line power is applied to a pole-pair, the magnetic field produced in each is of the opposite polarity. The magnetic field of one pole repels (pushes) the rotor, while the magnetic field of the opposite pole attracts (pulls) the rotor. When the polarity of the line power changes, the two poles change magnetic polarity - the opposite one now pushes while the other now pulls. The end effect of the constant changing is a revolving magnetic field around the circumference of the stator windings.

Rotor Construction

Unlike the stator, the rotor of an AC induction motor usually consists of rows of conductive bars rather than wire windings. Rings at each end of the rotors shaft connect the conductive bars together to form a closed circuit. Its physical construction gives the appearance of a squirrel cage.

The rotors of induction motors are very rugged and safe. They are rugged because of their heavy conductive bar construction. They are also safe because electrical connections or brushes are not used to supply voltage to the rotor. Instead, the stators magnetic field induces voltage in the rotor. The rotors voltage then generates a magnetic field that interacts with the stators magnetic field. The result is that the rotors shaft turns as the stators magnetic field attracts the rotors magnetic field.

Motor Torque and Speed

The number of pole-pairs in an induction motors stator affects motor torque and motor speed. Torque is directly proportional to the number of pole-pairs, whereas motor speed is inversely proportional to the number of pole-pairs.

Torque

Torque is directly proportional to the number of pole-pairs because a larger number of electromagnets increase the magnetic coupling between the stator and the rotor. An increased magnetic coupling results in increased rotational force on the motor shaft. For example, four pole-pair stators have twice the number of places where their magnetic fields touch the rotor when compared to two pole-pair stators. The result is twice the amount of turning force or torque on the motors shaft.

Motor torque is also directly proportional to the amount of current that flows through stator windings. An increased amount of current produces a larger magnetic field in the stator and more magnetic coupling between the stator and the rotor. As stated previously, a greater magnetic coupling results in more rotational force on the motors shaft. Therefore, increasing the current two times produces two times the amount of rotational force or torque.

Speed

Unlike torque, speed is inversely proportional to the number of pole-pairs in a motors stator. As the number of electromagnets increases around the stator, the rotor has less distance to rotate before the pushing and pulling of the next electromagnet influences it. For example, a motor with one pole-pair is controlled for 180 degrees of rotation for each pole. A motor with two pole pairs, however, is controlled for only 90 degrees of rotation for each pole. Thus, motor speed decreases as the number of pole-pairs increases.

Motor speed is also affected by the rate at which each pole-pair pushes and pulls the rotor. The frequency of the incoming line power determines the rate that the pole-pairs push and pull the rotor. Therefore, the incoming line-powers frequency has a major effect on motor speed.

Motor Slip

When power is applied to an induction motor, the induction motors rotor tries to catch up to the speed at which the stators magnetic field changes (rotates or revolves around the stator). The rotor never catches up to the stator for the reasons listed below:

  • A load connected to the rotor slows it down.
  • The amount of torque on the rotor decreases as the rotor accelerates and starts to catch up to the stators revolving magnetic field.

The speed at which the stators magnetic field rotates is called the motors synchronous speed:

Synchronous speed = speed of stators rotating magnetic field

The speed at which the rotor rotates is called the motors running speed:

Running speed = speed of rotor

The difference in speed between the stators rotating magnetic field and the rotor is known as slip. Motor slip is often expressed as a percentage. The equation below shows how to calculate motor slip in a percentage.

AC motors have a rated amount of slip. It is equal to the synchronous speed of the motor minus its rated speed at full load. Motor rated slip is often expressed as a percentage. The equation below shows how to calculate motor rated slip.

However, the greater the load connected to the motor, the greater the amount of slip.

Motor Control

As previously discussed, two major factors affect an induction motors operation: speed and torque. Speed (S) is determined primarily by the number of poles (P) in a motor and the line powers frequency (F):

Because the manufacturer fixes the number of stator poles in an induction motor, this factor cannot be changed easily to adjust the speed of a motor on the manufacturing floor. Although two-speed motors are available, rewiring is required to change from one speed to another, and the motor is limited to only two speeds. To allow continuous speed variation, the frequency of the line power applied to the motor would have to be changed.

As it turns out, manufacturers have found convenient ways to use solid-state electronic equipment to change the frequency of the line power applied to AC motors. However, changing only the incoming lines power frequency is not enough.

Frequency changes the motors impedance (effective resistance). Motor impedance (Z) increases if you increase the frequency of the applied line power because inductive reactance (XL) increases with frequency.

Conversely, motor impedance decreases if you decrease the frequency of the applied line power because of inductive reactance. Remember from basic electricity that impedance increases when frequency increases due to the inductive reactance of the motors stator windings:

X L = 6.28 x F x L

Z 2 = R 2 + X L 2

Remember that, according to Ohms Law (E = I x Z or I = E/Z), motor current decreases as impedance increases if the voltage is constant. Therefore, current decreases as the frequency of the applied line voltage increases because frequency determines impedance (voltage amplitude is still constant). Motor current increases as the frequency of the applied line voltage decreases because impedance decreases. Therefore, the amount of current is proportional to voltage divided by frequency:

I ~ E/F

The relationship between frequency, speed, current, and torque shows that increasing frequency can increase motor speed. When motor speed is increased by increasing frequency without increasing voltage, the motor torque is also decreased, which creates a problem in driving a machines load. Similarly, decreasing motor speed by only decreasing frequency causes an increase in motor current. This increase in current does not result in an increase in torque. Rather, it results in overheating the motor, which reduces torque.

A solution to the problem is to change motor speed by changing the voltage of the applied line power as the frequency changes. Changes in motor current are proportional to changes in the frequency of the applied line power so that if frequency increases, current decreases.

The negative effects (overcurrenting and under-torqueing the motor) of changing the lines power frequency can be overcome by making corresponding changes in the lines power voltage. By increasing voltage (E) as frequency (F) increases and vice versa, the proper relationship between the two can be maintained to produce constant current and constant torque. The relationship between voltage and frequency to produce constant torque is called the constant volts-per-hertz (V/Hz) ratio. It is a fundamental principle on which AC drives are designed.

Three-Phase Induction Motors

Three-phase AC induction motors are more frequently encountered in industrial machinery applications than other kinds of induction motors. These motors have three-phase line power applied to their stator windings. As shown in Figure 24, three-phase line power may be applied to the stator in different ways: directly across the line or from an AC drive.


Figure 24: Three-Phase Induction Motor Connected across the Line
Figure 24 shows a motor connected directly to the line. Incoming line power is first wired to the motor starter. When the starter is closed, three-phase line power is applied directly to the motors stator windings.

Figure 25 shows a motor connected to an AC drive. As with the motor starter, incoming line power is first wired to the AC drive. When started up, the AC drive applies three-phase power to the motor. The AC drive, however, converts the line power to a variable frequency and variable amplitude signal so the motors speed can be changed easily.


Figure 25: Three-Phase Induction Motor Connected to AC Drive

Single-Phase Induction Motors

Single-phase induction motors are less frequently encountered in industrial machinery than three-phase induction motors. Single-phase line power is applied to the stator windings of single-phase induction motors.

The magnetic field produced by the stator windings of single-phase induction motors tends to oscillate when line power is first applied to the stator windings rather than rotate as in three-phase induction motors. The result is that there is no starting torque to move the rotor.

Several methods may be used to start single-phase induction motors. Two common start methods are the split-phase and the capacitor. They tend to draw more current than is required, which can damage solid-state electronic devices (such as drives) and cause nuisance trips. Therefore, single-phase AC induction motors generally are not used with AC drives.

AC Motor Nameplate Data

Most modern equipment has a nameplate attached to it. The nameplate identifies the devices model number and important data or information about the equipments operation. For example, the automobile you drive has a nameplate in its engine compartment. Its nameplate may specify the spark plug gap, engine idle speed, and other important data. You use the nameplate data for numerous reasons. With regard to your automobile, you use the nameplate data when servicing the vehicle.

Like automobiles, AC motors have a nameplate. The nameplate identifies important data about the motors operation.

Nameplate Data

Figure 26 shows a typical AC motor nameplate. A typical AC motor nameplate has the following kinds of data:

  • Voltage (V)
  • Current (A)
  • Horsepower (HP)
  • Speed (RPM)
  • Service factor (SF)
  • Insulation class (B, F, or H)
  • Enclosure type (DPFG, TENV, TEFC, or X-PLO)


Figure 26: Typical Motor Nameplate
The values associated with each of the above kinds of data are known as the rated motor values. Normally, they are specified at standard American voltages of 208, 230, and 460 VAC and standard line power frequency of 60 Hz. These values are used to determine the operational limits of the motor when attached to a machine load, the types of power that can be wired to the motor, and the type of environment in which the motor can operate.

Rated Voltage

The volts listed on the motor nameplate are known as rated voltage. Rated voltage should be the voltage of the line power to which the motor stator windings are connected. Motor voltage normally is either 230 VAC or 460 VAC. Most AC motors are designed to be connected to either voltage. In the case of three-phase induction motors, for instance, a set of windings for each phase may be connected together in series for 460 VAC operation or they may be connected together in parallel for 230 VAC operation.

Rated Current

The current (amps) listed on the motors nameplate data is known as rated current. Rated current is the maximum amperes that the motors stator wires can conduct over a long period of time. If this value is exceeded, the motors stator windings will overheat and the motor will be damaged. Thus, rated current depends on the size of the wire used in the stators windings.

Rated current also is used to determine other important information, such as kilovolt-amperes (KVA). Kilovolt-amperes are calculated from a motors rated voltage and rated current to properly match a motor to an AC drive and line fuses or circuit breakers.

Rated Horsepower

The horsepower (HP) listed on the motors nameplate data is known as the motors rated horsepower. Rated horsepower is the maximum guaranteed horsepower that the motor can output continuously without overheating. This rated horsepower depends on motor current and speed. Since torque is proportional to current, rated horsepower depends on the maximum continuous torque the motor can produce at 60 Hz line power frequency.

Rated horsepower is used to calculate other values, such as rated torque. The rated torque is calculated from the motors rated horsepower and rated speed.

Rated Speed

The speed listed on the motors nameplate data is known as the motors rated speed. Rated speed is the maximum speed the motors shaft will turn in revolutions per minute (RPM) at the line powers frequency (60 Hz). Recall that speed is equal to a constant (120) times frequency divided by the number of stator poles.

For example, if the number of stator poles is six and the line frequency is 60 Hz, the speed is 1200 RPM. This speed is called synchronous speed. Synchronous speed is slightly different then the rated speed found on the nameplate. Rated speed is equal to the synchronous speed minus the motors slip when it is operating at full torque.

Sometimes the value of rated speed is given two values, such as 1750/3500. The first value is the rated speed, just discussed. The second value is a type of mechanical limit. If the motors speed ever reached this limit, it could be damaged. This value should never be exceeded.

As previously mentioned, rated speed also is used to determine other values, such as rated torque.

Service Factor

Service factor is a value that the rated current (which is given on the motors nameplate) is multiplied by to determine the maximum current that the motor could handle if a load caused an unforeseen amount of motor torque. Excessive torque demands from the load would cause excessive current in the motors stator windings that would damage the motor. Therefore, service factor really is a multiplication factor that is used to determine if a safety factor has been built into the motor.

Motors typically have a service factor of 1.0 or 1.15. A service factor of 1.0 (unity) means that an additional safety factor is not built into the motor because 1.0 times any value equals that value. A service factor of 1.15 means that a safety factor of 15% is built into the motor. For example, a service factor of 1.15 times a rated current of 100 amps is equal to 115 amps. Therefore, the motor could handle 15 amps above rated current for a short period of time.

Insulation Class

The insulation class rating identified on a motors nameplate is its temperature rating. Temperature ratings are of three types:

  • Class B
  • Class F
  • Class H

Class B insulation ratings indicate that the insulation on a motors stator windings can withstand temperatures up to 105 degrees Celsius. Class F insulation ratings indicate that the insulation on a motors stator windings can withstand temperatures up to 135 degrees Celsius. Class H insulation ratings indicate that the insulation on a motors stator windings can withstand temperatures up to 155 degrees Celsius.

These temperatures are the maximum motor temperatures that the stator winding insulation can withstand for the motor to function properly over its normal lifetime. If the temperatures exceed these values, the insulation will break down in time and the motor will be damaged.

There is a rule of thumb that is useful when analyzing motor insulation class ratings. The motors lifetime is cut in half for each 10 degrees Celsius that a motor is operated above its insulation class rating. For example, a motor with a 10-year insulation lifetime that is operated 10 degrees Celsius above its insulation class rating would now have an expected lifetime of 5 years.

Enclosure Type

The motor enclosure type that is stated on the motors nameplate identifies the type of environment that the motor can operate in with its enclosure. Motor enclosures are of three general categories:

  • Drip-proof-fully-guarded (DPFG)
  • Totally enclosed (TENV or TEFC)
  • Explosion-proof (X-PLO)

DPFG enclosures are for indoor use only. They prevent dripping liquids from entering the motor. These types of enclosures are for use in clean and dry air environments.

TENV and TEFC enclosures are for use in dirty or outdoor environments. They protect the motor from rain, snow, dirt, and other liquids. TENV enclosures are non-vented. They are available for applications that use up to 10 HP.

TEFC enclosures are fan-cooled. They use a fan to vent warm air from the motor to cool it. The fan is usually mounted on the motors shaft. TEFC enclosures are available for applications that use 10 HP and up.

Explosion-proof enclosures are for hazardous duty environments where an explosion is possible. These types of enclosures (as are the others) are described fully in the National Electrical Code (NEC). It always is advisable to carefully consult with the motor manufacturer for explosion-proof applications.

Semiconductor Fundamentals

Variable-speed drives are used in many applications. The major components associated with the drives are transistors, silicon-controlled rectifiers (SCR), and operational amplifiers (op-amps).

Transistors and SCRs are described in this section. This section also provides an overview of operational amplifiers. However, once transistor circuits are understood, you will have a basic working knowledge of the op-amp.

Atomic Review

Each group of elements that make up the current periodic chart has different electrical properties. An understanding of the electrical properties and the basic atomic structure of elements is necessary in order to understand how semiconductors operate.

The smallest portion of an element is the atom, which is composed of a combination of electrons, protons and neutrons. The structure of the atom itself is similar to a miniature solar system. The neutrons and protons form the center of the atom or nucleus, and the electrons surround the nucleus in groups of orbits called shells.

The electrons, which orbit the nucleus of an atom, are confined to specific shells based on their energy level. Energy must be gained or lost for an electron to move from one shell to another. The electrons contained in the outermost or valence shell of an atom are known as the valence electron.The number of electrons that are contained in the valence shell will determine the electrical and chemical properties of the atom. The electrons associated with an atom exist in one of three major energy bands:

  • Valence Band
  • Conduction Band
  • Forbidden Band

The valence band is created by the individual energy levels of each valence electron within it. The electrons in this band are firmly attached to the parent atom. The valence band is the one that is closest to the nucleus of the atom.

The upper-most band, or the band furthest from the nucleus is known as the conduction band. Electrons that make up the individual levels of this band are essentially free from the influence of the parent atom. These electrons can easily be moved throughout a material by applying an external force such as voltage or heat.

The forbidden band, which exists between the valence and conduction bands, is known as an energy gap. No electrons can exist in the forbidden band. This energy gap represents the amount of energy that is required to move an electron from the valence band into the conduction band. The size of the forbidden band determines whether a substance is an insulator, a conductor, or a semiconductor.

As previously stated, elements can be classified electrically into one of the three following categories:

  • Insulators
  • Conductors
  • Semiconductors

An insulator is an element in which electrons cannot easily flow. They have a large forbidden band, which means that a large amount of voltage potential must be placed across them in order to move an electron from the valence band into the conduction band. Elements in this category have five or more valence electrons. An example of a good insulator is an inert gas that contains eight valence electrons.

A conductor is an element in which electrons can flow very easily. Elements classified as conductors have a very small or, in some cases, no forbidden band at all. Very little potential is required to move electrons from the valence band into the conduction band. Conductors will have three or less valence electrons.

As the name implies, a semiconductor is an element that has electrical properties that are somewhere between an insulator and a conductor. Semiconductors have a smaller forbidden band than an insulator, so for a given amount of applied potential, more current will flow in a semiconductor than in an insulator. Elements that fall into this category have four valence electrons, and the most common examples are germanium and silicon.

The number of electrons in its valence shell, as previously stated, determines the chemical activity of an atom. When the valence shell is complete (eight valence electrons), the atom is stable and shows little tendency to give up electrons or combine with other atoms. These atoms are referred to as inert or inactive atoms, and make very good insulators due to their large forbidden band. However, if the valence shell of an atom has less than the required number of electrons to complete the shell, the stability of the atom decreases. These atoms tend to combine with other atoms and give up valence electrons more easily, making them better conductors (smaller forbidden band).

Semiconductor Materials

Silicon and germanium are the most frequently used semiconductors. Both are quite similar in their atomic structure and chemical behavior. Each has four electrons in the valence shell. Since each has fewer than the required number of eight electrons needed in the outer shell, their atoms will unite with other atoms until eight electrons are shared. This gives each atom a total of eight electrons in its valence shell: four of its own and four that it shares with the surrounding atoms. Each individual silicon or germanium atom is now more stable than it was by itself. This sharing of valence electrons between two or more atoms produces a covalent bond between the atoms. It is this bond that holds the atoms together in an orderly structure called a crystal. A crystal is just another name for a solid whose atoms or molecules are arranged in a three-dimensional, geometrical pattern commonly referred to as a lattice.

The silicon atom in a lattice is bonded to four other atoms. As a result, the electrons are not free to move about the crystal. The net effect of this bonding makes pure silicon or germanium a poor conductor of electricity. The reason that they are not insulators is because, with the proper application of heat or electrical potential, electrons can break free from their bonds and move into the conduction band.

Semiconductor Conduction

Applying a voltage to a semiconductor crystal that contains conduction band electrons causes the electrons to move through the crystal towards the applied voltage (negative to positive). This movement of electrons is referred to as electron current flow.

This movement of electrons causes another type of current flow in a pure semiconductor known as hole flow. This current flow occurs when a covalent bond is broken and a vacancy is left in the atom by the missing valence electron. The vacancy is commonly referred to as a hole and is considered to have a positive charge because its atom is missing one electron. As a result of this hole, a chain reaction begins when an electron from a neighboring atom breaks its own covalent bond to fill the hole, and, in turn, creates another hole in the process. Each time an electron fills a hole with this process, it enters into a covalent bond. The important thing to remember is that even though an electron is moving from one covalent bond to another, the hole is also moving in the opposite direction. The major difference in the two current flows aside from energy level is that holes flow from the positive potential to the negative potential.

In the theory just described, the breaking of covalent bonds created two current carriers:

  • Negative Electron Flow
  • Positive Hole Flow

These carriers are referred to as electron-hole pairs. Since the semiconductor we have been discussing contains no impurities, the number of holes is always equal to the number of electrons. The term intrinsic is used to describe a pure semiconductor material that has no impurities.

Extrinsic Semiconductors

A pure semiconductor contains no free electrons in its conduction band and is very stable. Even with the application of thermal energy, only a few covalent bonds are broken, yielding a relatively small current flow. A much more efficient method of increasing current flow in semiconductors is by adding very small amounts of selected impurities, generally no more than a few parts per million. The process of adding these impurities to semiconductor crystals is referred to as doping.

The purpose of semiconductor doping is to increase the number of free charges that can be moved by an externally applied voltage. When an impurity increases the number of free electrons, the doped semiconductor material is called negative or N-type. An impurity that reduces the number of free electrons, causing more holes, creates a positive or P-type semiconductor. Semiconductors that are doped either with N-or P-type impurities are referred to as extrinsic semiconductors.

Although the N-type material has an excess of free electrons, it is still electrically neutral. This is because the donor pentavalent atoms in the N material have five valence electrons and a corresponding number of protons in the nucleus, maintaining the electrical neutrality of the atom. The end result is that the N material has an overall charge of zero. By the same reasoning, the P-type material is also electrically neutral because the number of electrons exactly balances the excess of holes in this material. Recall that the holes and electrons are still free to move in the material because they are only loosely bound to their parent atoms.

N-Type Semiconductors

The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and, in doing so, increases the conductivity of the material by contributing a free electron. This type of impurity has five valence electrons and is called a pentavalent impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called donor impurities.

When a pentavalent impurity, such as arsenic, is added to germanium, it will form covalent bonds with the germanium atoms. The arsenic atom is in the center of the lattice. It has five valence electrons in its outer shell but uses only four of them to form covalent bonds with the germanium atoms, leaving one electron relatively free in the crystal structure. Pure germanium may be converted into an N-type semiconductor by doping it with any donor impurity having five valence electrons in its outer shell. Since this type of semiconductor has a surplus of electrons, the electrons are considered majority carriers, while the holes, being few in number, are the minority carriers.

Current Flow in the N-Type Material

With voltage applied across the N-type material, electrons move through the semiconductor. The positive potential of the battery will attract the free electrons in the crystal. These electrons will leave the crystal and flow into the positive terminal of the battery. As an electron leaves the crystal, an electron from the negative terminal of the battery will enter the crystal, thus completing the current path. Therefore, the majority current carriers in the N-type material (electrons) are repelled by the negative side of the battery and move through the crystal toward the positive side of the battery.

P-Type Semiconductors

Doping a pure semiconductor with an impurity having only three valence electrons makes P-type semiconductor material. These are known as trivalent impurities, and they compensate for their deficiency of one valence electron by acquiring an electron from a neighbor. Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept one electron from the doped material, they are also called acceptor impurities.

A trivalent impurity element is used to dope germanium. The impurity (Indium) is one electron short of the required number of electrons needed to establish covalent bonds with four neighboring germanium atoms. Thus, one covalent bond will have only one electron instead of two. This arrangement leaves a hole in that covalent bond and in the crystal lattice structure. Gallium and boron, which are also trivalent impurities, exhibit these same characteristics when added to germanium. The holes can only be present in this type of semiconductor when a trivalent impurity is used. Note that a hole carrier is not created by the removal of an electron from a neutral atom, but is created when a trivalent impurity enters into covalent bonds within a tetravalent (four valence electrons) crystal structure. The holes in the P-type semiconductor are considered to be the majority carriers since they are present in the material in the greatest quantity. The electrons, on the other hand, are the minority carriers.

Current Flow in the P-Type Material

Current flow through the P-type material is by positive holes, instead of negative electrons. The hole moves from the positive terminal of the P material to the negative terminal. Electrons from the external circuit enter the negative terminal of the material and fill holes in the vicinity of this terminal. At the positive terminal, electrons are removed from the covalent bonds, thus creating new holes. This process continues as the steady stream of holes (hole current) moves toward the negative terminal. Notice in both N-type and P-type materials, current flow in the external circuit consists of electrons moving out of the negative terminal of the battery and into the positive terminal of the battery. Hole flow only exists within the semiconductor material itself.

Diodes

Diodes are widely used in the industry, primarily to rectify AC current into DC current. The diode operates as an electrical check-valve. Current is easily passed in one direction, but is almost totally blocked in the reverse direction. This block-or-pass operation is determined at the PN junction. Whether or not the diode will pass current depends on the polarity of the voltage difference applied to the junction itself. This polarity is called bias. Figure 27 shows the diode schematic symbols.


Figure 27: Diode Schematic Symbols
If the diode is placed in a DC circuit, with the positive potential of the battery connected to the P material, and the negative potential of the battery connected to the N material. The voltage polarities applied to the leads are passed within the diode to the PN junction. In this example, the positive polarity is applied to the P side of the junction, and the negative polarity is applied to the N side of the junction. One can say that the polarity difference across the junction bias agrees with the type of material on each side of the junction. Due to the molecular make-up of the diode, this bias arrangement causes the junction to allow current to pass through, producing free conduction of current through the circuit. A favorable polarity difference across a PN junction that allows free conduction is called forward bias. The following happens when battery polarity is reversed. Now the bias across the PN junction is contrary to the material type on each side of the junction. In this case, the junction will block current flow, causing the entire diode to appear open. This condition of contrary polarity difference across a PN junction that blocks current flow is called reverse bias.

Diode Applications

Two important types of circuits that use diodes are diode rectifiers and diode-regulated power supplies. One of the most common uses for diodes is the converting alternating current to direct current. Since it passes current in only one direction, a pulsating DC voltage can be obtained.

Half-Wave Rectifier

Figure 28 shows a half-wave rectifier; as the input signal goes positive, the diode D1 is forward biased and offers low resistance. Thus, the input voltage appears across the load resistor R1. When the input voltage goes negative, the diode is reverse biased and offers a high resistance. Therefore, essentially none of the input voltage is passed on to the load resistor. Figure 29 shows the resulting output wave shape from the half-wave rectifier. A capacitor placed across the load resistor R1 will filter the ripple in the direct current output.


Figure 28: Half-Wave Rectifier

Figure 29: Half-Wave Rectifier Output Waveform

Full-Wave Rectifiers

To further improve the efficiency of the direct current output, a full-wave rectifier using two diodes, a transformer, and a filter can be assembled (Figure 30).


Figure 30: Full-Wave Rectifier
When the input voltage is positive, diode D1 is forward biased, and positive half-cycle voltage appears across the load resistor. When the input goes negative, diode D2 is forward biased, causing the negative half-cycle voltage to appear across the load. The circuit is designed so that both diodes conduct in a way that produces the same polarity of voltage across the load resistor R1. Another method of constructing a full-wave rectifier is shown in Figure 31. This is called a diode bridge rectifier.


Figure 31: Bridge Rectifier
When the applied alternating current input voltage is positive, diodes D1 and D2 are forward biased: current flows through diode D1, the output circuit, diode D2, and back to the source. When the input is negative, diodes D3 and D4 are forward biased. Current flows through diode D3, the output circuit, and diode D4 and back to the source. The current flow through the output circuit is in the same direction for both positive and negative input cycles. Figure 32 shows the waveform for the bridge rectifier.


Figure 32: Bridge Rectifier Output Waveform
The diode bridge rectifier has some definite advantages over the other full-wave rectifier. The diode bridge rectifier does not require a transformer to accomplish full wave rectification. In addition, the diode can have a lower voltage rating since there are two diodes in each conducting path. Under conditions of reverse bias, the two diodes will divide the reverse voltage that they must block. If the voltage to the diodes is 200 volts peak-to-peak, each diode need only be capable of blocking 100 volts. In the full-wave rectifier, the diodes would have to block 200 volts.

Diodes in UPS applications are used in a variety of ways. Diodes will appear in a circuit as blocking diodes, bleed down diodes, as part of a SCR three-phase bridge, and clamping diodes.

Blocking diodes prevent reverse power flow. Bleed down diodes dissipate relay magnetic fields on unit shutdown. Bridge circuits supply a return path for power continuity. Clamping diodes maintain SCR gates at a low value when reversed biased.

Transistors and Transistor Applications

Transistors are solid-state circuit devices that have three or more terminals, each connected to a P-type or an N-type region of semiconducting material. There are many different kinds of transistors. The most common are PNP transistors and NPN transistors. Other types of transistors are field-effect transistors (FETs) and metal oxide semiconductor field effect transistors (MOSFETs). The transistor is a semiconductor device that can be used for amplification or switching operations.

A transistor can be thought of as a combination of two diodes joined back-to-back with their like material mating regions made very thin, forming a three-region semiconductor device. A transistor can be arranged as either a PNP type or an NPN type semiconductor device. The three elements of the transistor are called the emitter, base, and collector. The circuit symbols for the PNP and NPN transistors are shown in Figure 33.


Figure 33: Transistor Symbols
The emitter-base PN junction is forward biased, while the collector-base PN junction is reverse biased. This is always the case in transistor amplifier circuits.

A transistor has very high resistance as long as there is no current flow across the emitter. When current flows across the emitter, the transistor has very low resistance. Applying a voltage to the base can control the current flow across the emitter. Thus, a transistor acts like a potentiometer: off when the base-emitter junction is reverse biased, and on when the base-emitter junction is forward biased.

SCR

Silicon controlled rectifiers, or SCRs, also called thyristors, are four-layer PNPN semiconductors with three electrodes: cathode, anode, and gate. Figure 34 shows the schematic symbol for the SCR.


Figure 34: SCR Schematic Symbol
There are many solid-state devices that are not used for signal amplification. However, they have wide application in power supply circuits, control circuits, and oscillator circuits. These devices have one or more PN junctions and are generally operated in either an ON or OFF state. The silicon controlled rectifier (SCR) and the diac and triac are examples of these bistable-switching devices.

The silicon controlled rectifier, usually referred to as an SCR or thyristor, is one of the family of semiconductors that includes transistors and diodes. Not all SCRs use the casing shown, but this is typical of most of the high power units. Although it is not the same as either a diode or a transistor, the SCR combines features of both. Circuits using transistors or rectifier diodes may be greatly improved through the use of SCRs.

The basic purpose of an SCR is to function as a switch that can turn on or off large amounts of power. It performs this function with no moving parts that wear out and no points that require replacing. There can be a tremendous power gain in the SCR; in some units, a very small triggering current is able to switch several hundred amperes without exceeding its rated abilities. The SCR can often replace much slower and larger mechanical switches.

An SCR is an extremely fast switch. It is difficult to cycle a mechanical switch several hundred times a minute, yet SCRs can be switched 25,000 times a second. It takes just microseconds (millionths of a second) to turn SCRs on or off. Varying the time that a switch is on as compared to the time that it is off regulates the amount of power flowing through the switch. Since most devices can operate on pulses of power (alternating current is a special form of alternating positive and negative pulses), the SCR can be used readily in control applications. Motor speed controllers, inverters, remote switching units, controlled rectifiers, circuit overload protectors, latching relays, and computer logic circuits all use the SCR.

SCR Construction

The SCR is a four-layer device with three electrodes. The terminal connected to the outer P-type layer is called the anode. The terminal connected to the outer N-type layer is called the cathode. The gate terminal is connected to the inside P-type layer. The gate is physically closer to the cathode and is designated as such in the schematic diagram.

SCR Operation

An SCR has much in common with a diode. First, the anode of the circuit will be made positive with respect to the cathode, but the gate will be left open. Under these conditions, the NPN transistor will not conduct because its emitter junction will not be subjected to a forward bias voltage that can produce a base current. The equivalent SCR circuit will not allow current to flow from its cathode to its anode under these conditions.

This first state is called the forward blocking region, or the off state. The anode-to-cathode resistance of the SCR is very high and will only allow a small flow of leakage current between the anode and the cathode. The leakage current will only slightly increase when the output of the supply battery is increased. However, the increase in the supply voltage will cause the current carriers to increase in energy and speed.

In its second operating state, the SCR is switched to its high conduction region. This will occur if the output of the supply battery is further increased. At some value of positive voltage at the anode, the SCR begins full conduction. This point is called the forward breakover voltage (VBO).

At the forward breakover voltage, the current carriers require sufficient energy and speed to produce an avalanche effect, hence increasing the amount of current flow and decreasing the anode-to-cathode opposition, and cathode-to-anode current flow through the SCR will increase. In short, regeneration completes the switching process and the anode-to-cathode opposition decreases to a minimum value.

Note that very little current will flow from anode-to-cathode until the anode-to-cathode voltage is equal to the forward breakover voltage (Vbo). Above Vbo, the SCR is switched from an off state of high anode-to-cathode resistance to an on state of low anode-to-cathode resistance with high forward current. It also should be noted that the anode-to-cathode voltage would decrease to a value that is much lower than Vbo. This is important because it indicates that the SCR will continue to conduct a flow of forward current after it has been switched to its on state even if the supply voltage is decreased below its forward breakover voltage.

Forward current will continue until the supply voltage is reduced to a point that produces a value of forward current that is less than the SCRs holding current. Holding current is the minimum value of anode current that will allow the SCR to remain in its on state. The SCR cannot be switched off until its anode current is decreased below its holding current rating. The anode-to-cathode voltage drop is extremely small when the SCR is in its on state. Voltage drops between 1V to 3V are not unusual. This includes SCRs that conduct very large currents, such as 2000 amps, after they are fired into conduction.

It should be noted that the SCR functions like a conventional diode when a reverse voltage is applied to it (negative at the anode with reference to the cathode). Very little reverse current flows until a critical value of reverse voltage is applied. When the reverse voltage is increased beyond this point, the SCR breaks down and a reverse avalanche current will flow. This point is called the reverse breakdown voltage.

Now let us consider the effects of a positive potential at the gate with reference to the cathode. A positive potential at the gate will forward bias the gate-cathode circuit. This will produce a flow of current between the cathode and the gate that will increase the number of available current carriers and lower the anode-to-cathode opposition of the SCR. As a result, the value of anode-to-cathode voltage that is necessary to produce breakover is reduced. The SCR will be switched to a high state of conduction at a lower anode-to-cathode voltage. If the forward bias current between the cathode and the gate is further increased, the forward breakover voltage will be further decreased.

The breakover voltage is high when there is not forward bias current (IgtO) between the gate and the cathode. When a small forward bias current (Igtl) is flowing, the SCR is switched to a high conduction state at a lower breakover voltage. At (Igt2), the forward bias current between the cathode and the gate is sufficient to produce a high state of conduction when the anode is only a few volts positive.

From the preceding discussion, it can be seen that a voltage at the gate can control the firing of the SCR. After the SCR is fired, or turned on, the gate loses control over the SCR. This means that the gate can maintain control until the SCR is switched on, but after the SCR is in a high state of conduction, the gate voltage has no effect on the SCRs operation. In fact, the gate voltage can be removed after the SCR is conducting. Also, it is only necessary for the gate voltage to be a pulse of short duration to drive the SCR into conduction.

After the SCR is in its on state, it is switched back to its off state by decreasing the anode current below its holding current level. If a given SCR has a holding current of 3mA, then the SCR will not switch to its off state until the anode current is less than 3mA. This can be accomplished by removal of the supply voltage or other circuit arrangements that would prevent the SCR from conducting a value of current that is equal to or greater than holding current.

The SCR is normally operated with a positive potential at the anode with reference to the cathode. The SCR will not conduct a significant flow of current unless the forward breakover voltage is exceeded. Normally, SCRs are chosen to have a forward breakover voltage in the absence of a gate signal that is much higher than any of the circuit voltages.

Both Vgt and Igt must be available to trigger the SCR. The Vgt rating may be 0.8V, which means the gate must be 0.8V positive with reference to the cathode. The Igt rating for the same SCR may be 200 A. If a 0.8V source is available but it cannot deliver 200 A of gate-cathode current, then the SCR will not fire.

After the SCR is triggered, the gate voltage has no effect on the operation of the SCR. It is only necessary for Vgt and Igt to be present for a period of time that will allow the SCR to switch from its off state to its on state. Although the SCR may begin a transition from off to on, it will return to its off state if the gate signal is removed too soon. There must be sufficient cathode-to-anode current to complete the regenerative process after the gate signal is removed. This current value is called latching current. After the cathode-to-anode current is at its latching current value, the SCR will complete the transition to its state of high conduction without the gate signal. The latching current will be about three times the holding current. The gate pulse must always have a duration that will allow the SCR to reach latching current.

Forced Commutation

An SCR ceases to conduct and the gate regains control only after the anode current falls to zero. The current may cease flowing naturally, or it can be forced to zero artificially. Such forced commutation is required on some circuits in which the anode current has to be interrupted at a specific instant. Figure 35 provides a circuit in which an SCR and a resistor are connected in series across a DC source (E). If a single positive pulse is applied to the gate, the resulting DC load current (I1) will flow indefinitely. However, conduction in the SCR can be stopped in one of three ways:

  • Momentarily reduce the supply voltage (E) to zero.
  • Open the load circuit by means of a switch.
  • Force the anode current to zero for a brief period.


Figure 35: Forced Commutation

This third method is of particular importance. In Figure 35, a current source (CS) delivering a current (I2) is connected in parallel with SCR Q1. As I2 is increased, the net current (I1 - I2) flowing in the SCR decreases. However, the SCR continues to conduct, with the result that the current flowing in the resistor is unchanged. But if I2 is increased until it is equal to I1, the SCR ceases to conduct, and the gate regains control. In practice, I2 is a brief current pulse, usually supplied by triggering a second SCR. For example, in Figure 35C, a capacitor, initially charged as shown, is suddenly discharged by triggering the gate of SCR Q2. Discharge current I2 immediately cancels the current in Q1; consequently, it stops conducting. Current now flows through resistor R, capacitor C, and thyristor Q2. The capacitor quickly charges up and, when its voltage is close to E, the charging current becomes so small that Q2 also stops conducting. Thus, current ceases to flow in the load shortly after Q2 is triggered. This type of forced commutation, using a commutating capacitor, is common in many industrial components.

Theory of Inverter Operation

Many methods can be used to convert DC to AC. One of the most common methods is to use a device called an inverter. An inverter takes a DC voltage and power switches it using solid-state switching devices, thereby converting it into an AC waveform. Figure 36 is a typical block diagram of an inverter. The functions of the blocks are as follows:

  • Rectifier The purpose of this block is to provide a means to convert an AC input to a DC output.
  • Power Switching This block is where the inverter function is performed. This block contains the power switching devices (SCR) used to convert the DC into AC.
  • Controls This block is the control panel to allow the user to operate the inverter. This block also contains the circuits that generate the gating pulses. The gating pulses are used to switch the power switching devices on and off. Also this block contains circuits used to synchronize the inverter output with an alternate power source.
  • Isolation, Output, and Harmonic Filter This block provides for transformer isolation of the output for safety and reliability. The transformer used is a constant voltage transformer (CVT) that provides for output voltage regulation. Also, since the inverter produces square waves and they contain a large number of odd ordered harmonic frequencies, a harmonic filter is used to remove these undesired signals from the output.


Figure 36: Typical Inverter Block Diagram
Virtually all inverter systems will contain these basic blocks. The circuitry and electronic components used will vary from manufacturer to manufacturer. A more detailed discussion of the purpose of each of these blocks will follow, starting with the heart of the inverter, the power switching circuit.

Power Switching Circuit

The power switching circuit converts the filtered DC input into a square wave output. To be able to deliver substantial power to the loads, the switching devices used must be able to interrupt very large current flows. In the early days of inverter technology, mechanical switches called relays were used to chop the DC into a square wave AC. These relays were bulky and required constant replacements of their contacts due to shock and pitting. Because of this, early inverters were unreliable and costly. Also, because of the switch bounce associated with mechanical switches, the output waveforms of these inverters were electrically noisy. The development of reliable solid-state high power switching devices came shortly after the invention of the transistor. These static electronic switches eventually replaced the old-fashioned mechanical relays as the power switching devices in inverters.

During the years since the invention of the transistor, a wide variety of solid-state switching devices have been produced. Thyristors are a family of switching devices whose members include the silicon controlled rectifier (SCR), triac, and diac.

Other, more exotic, power switching devices include gate turn-on devices (GTOs), metal oxide semiconductor field effect transistors (MOSFETs), and insulated gate bipolar transistors (IGBJTs). These devices are also very useful as switching devices. The theory behind their operation is beyond the scope of this article.

No matter what power-switching device is used, it must be able to take the DC input and reshape it into a square wave AC output. For the purpose of explanation, we will examine the operation of a simple power switching circuit using mechanical switches. This circuit is for the purpose of understanding only, and would not be practical to use for a real world inverter. This circuit is shown in Figure 37 below.


Figure 37: Simulated Static Inverter Using Mechanical Switches
Switches may simulate the operation of an inverter. Switches 1 and 1 are operated together. Switches 2 and 2 are also operated in unison. When 1 and 1 are closed, and 2 and 2 are open, load current flows in the direction shown by the arrow above the load. With 2 and 2 closed and 1 and 1 open, load current flows in the reverse direction. The important thing to note is that while the current from the DC battery input always flows in the same direction, the current through the load reverses direction with each different switch combination. This means that the voltage polarity being supplied to the load by the load current also reverses its polarity. An inversion of the polarity has been performed by this circuit, hence the name inverter. The mechanical switches used in this circuit would not be used in an actual inverter. A power-switching device such as the SCR would be used instead in the place of each switch. This will cause the simple mechanical switching circuit to evolve into the circuit shown in Figure 38.


Figure 38: Bridge SCR Static Inverter
The circuit shown in Figure 38 contains SCRs and various other components to support the SCRs operation. These include the addition of the inductors above and below the SCR bridge and a capacitor connected across the bridge. The inductors and capacitor are commutating components. These components are necessary to turn one pair of SCRs on as the other pair is being switched off. These reactive components ensure that the switching is accomplished smoothly.

The SCR gate circuits are the triggers that actually turn the SCRs on at the proper times. The gate circuit applying a positive voltage to the gate terminal of the SCR accomplishes SCR turn on. The gate circuitry is complex, but is fundamentally an oscillator circuit.

This circuit also includes the rectifier diodes, D1, D1, D2, and D2. These diodes are not part of the inverter process, but are used to clamp the amplitude of the load voltage to a value approximately equal to the magnitude of the source voltage.


Figure 39: Center-Tap SCR Inverter
Figure 38 shows the diagram of a four-SCR parallel-commutated inverter bridge. Figure 39 is a two-SCR bridge. The operation of the two-SCR bridge is similar to that of the four-SCR bridge. It differs in that half the number of controlled rectifiers is used, and each must hold off a voltage approximately equal to twice the supply voltage. The center-tap of the transformer provides a return path for the DC.

The circuits used to power inverters will now be discussed. These circuits are the battery and battery charger (rectifier).

Rectifier

The inverter is powered via a rectifier. The output voltage depends on the output voltage that the inverter is designed to produce. The rectifier must be capable of providing a filtered DC source for the power switching circuit. This ensures that the output of the power switching circuit is transient free.

Since the AC power may vary in voltage at different times of day and under different load conditions, the rectifier must be capable of producing a constant output voltage. This voltage regulation is accomplished by using a rectifier whose output waveform is controllable. Figure 40 shows a basic block diagram of the rectifier. The circuit is supplied by an AC input. The functions of the blocks are described below.


Figure 40: Block Diagram of Battery Charger / Rectifier

  • Input Transformer The input transformer is designed to step the incoming AC supply voltage up or down to the proper value for the rectifier. The transformer also provides isolation between the supply and rectifier circuits for safety and ground reliability.
  • Rectifier This block is where AC is converted into a pulsating DC output. Typically, the rectifier is made up of three diodes and three SCR (or 6 thyristors) connected in a full wave array. The conduction interval of the SCRs is adjusted by firing at the proper time during each half-cycle. This change in the conduction interval allows the rectifier to maintain a constant output with varying input conditions. A longer conduction period increases the DC voltage output, and, conversely, a shorter conduction period lowers the DC output voltage. This method of regulation is called phase angle control.
  • Output Filter This block contains the circuits that smooth the pulsating DC output from the rectifier. The filter is made up of a choke (inductor) and a capacitor bank. These are connected together in an "L" configuration. The capacitors oppose changes in voltage, and the inductor opposes changes in current. After filtering, the DC output voltage will be relatively smooth and ripple free. The capacitor part of the filter may be the capacitors located in the inverter.
  • Control Board The control board supplies the pulses that gate the SCRs at the proper time. The exact point of pulse generation is a function of the output voltage of the rectifier. The control board compares the output of the rectifier to an internal reference and then generates an error signal, which, in turn, adjusts the firing angle of the SCRs. If the output of the rectifier begins to drop, a signal is generated that increases the conduction interval of the SCRs and thus returns the output voltage to its normal value. In addition, the control board has built in over-voltage protection. If the DC output should go abnormally high, the gate pulses to the SCRs are immediately reduced. This feature protects the load against the high DC voltages that may occur if a drastic increase in AC supply voltage occurs. To better understand the operation of the rectifier, a simple half-wave rectifier using an SCR for phase control will be examined. See Figure 41.


Figure 41: Half-Wave Rectifier
Figure 41 shows a half-wave rectifier using an SCR for phase control. The SCR provides the rectification. The gate circuit controls the firing of the SCR. As an example, suppose that at steady state, the conduction angle of the SCR is 90 degrees. The input and output waveforms are shown below in Figure 42.


Figure 42: Rectifier Input and 90 degree Output Waveforms
The output is a pulsating DC half-wave output with half of the waveform eliminated. Although the filtering of the output filter is not shown, the final output would be a constant DC voltage of a certain level. The level would represent the average DC value of the output waveform shown above. Suppose, for example, that the AC input voltage increased. The gate circuit would sense this and adjust the conduction angle so that it would be less than the 90-degree angle shown in Figure 42 above. The conduction angle may decrease to 45 degrees; this output waveform is shown below in Figure 43.


Figure 43: Rectifier Input and 45 degree Output Waveforms
After filtering, the average DC output voltage of this waveform would be less than the waveform shown in Figure 42. This would compensate for the increase from the AC input.

If, for example, the AC input voltage decreased, the gate circuit would detect this and generate a new firing angle that would cause the SCR conduction angle to increase from the 90-degree steady state value to a value of 135 degrees. This is shown in Figure 44 below. Now the output is receiving power for a larger percentage of each sine-wave input; the average output voltage of this waveform after filtering would be higher than the steady-state value shown when the conduction angle was 90 degrees. This increase in average output voltage would compensate for the decrease in the AC input voltage.


Figure 44: Rectifier Input and 135 degree Output Waveforms
The half-wave, single-phase rectifier circuit shown above has some serious disadvantages. First, since this is a half-wave rectifier, one half of the AC input waveform is wasted since it is blocked by the SCR. The negative half-cycle is never used. This makes the half-wave rectifier very inefficient. Second, since there are large time gaps between the DC output peaks, filtering this output is difficult and would require a large capacitor and inductors to produce an acceptable ripple-free output.

Using a full-wave rectifier instead of a half-wave rectifier increases the efficiency of the circuit. This allows the entire sine-wave input to be used. Also, full-wave rectification causes the peaks to be moved closer together in time, which again makes the output waveform easier to filter.

The control board controls the firing of the SCRs in the proper sequence. This is important so that the average DC output of the rectifier can be controlled.

Three-Phase SCR Controlled Converter Bridge-Semi-Converter

To understand the operation of the three-phase SCR controlled converter bridge, look at the circuit as being made up of three single-phase, full-wave bridges.

A to B phase is being represented by the top circuit of Figure 45. SCRs 1 and 2 are turned fully on and act as diodes. The voltage starts to rise at 0 degrees to begin the cycle. Its rectified waveform is shown. As can be seen by this waveform, the resulting DC voltage has a very large 120 Hz ripple.

The middle and bottom figures show similar waveforms for B to C and A to C phases. As can be seen in these figures, each waveform is shifted 120 degrees.

Since the size of the output filter choke (L2) is inversely proportional to the ripple frequency, a smaller choke is required in the three-phase circuit then one in a single-phase circuit to accomplish the same amount of filtering action.

With the SCRs fully on, as shown in the figures, no output regulating is taking place. The sine wave will be distorted when the SCRs are regulating the output using phase control.

L2 is sized to reduce the output ripple to less than 2% of the output voltage when connected to a properly sized battery bank. If less ripple is desired, additional chokes or capacitors may be added to the output.

The semi-converter (3 SCR) and full converter (6 SCR) have similar input/output characteristics. The full converter, which also controls the firing angle of the SCRs in both the (+) and (-) legs of the bridge reflects less harmonic distortion to the current of the incoming line. The semi-converter, due to the diodes in the negative bus, has an output frequency of 180 Hz (except in the full ON state). For these reasons, larger units may use a full converter or, in some cases, a 12 SCR converter.


Figure 45: Converter Waveforms

Figure 46: Converter Circuits

Triac

A triac is a three-terminal device similar in construction and operation to the SCR. The triac controls and conducts current flow during both alternations of an AC cycle, instead of only one. The schematic symbol is shown in Figure 47.


Figure 47: Triac Schematic Symbol
Both SCRs and triacs have a gate lead. However, in the triac, the lead on the same side as the gate is main terminal 1, and the lead opposite the gate is main terminal 2.

The triac is able to conduct a flow of current in either direction with negative or positive gate voltages. It is evident that a Triac can be switched to a high conduction state when MT2 is positive or negative. Current can flow through the triac in either direction (from MT1 to MT2 or from MT2 to MT1). For this reason, it is called a bilateral or bi-directional device.

Diac

A Diac is basically a two-terminal, parallel-inverse combination of semiconductor layers that permits triggering in either direction. Figure 48 shows the schematic symbol representing a diac. The two terminals of the diac are labeled main terminal 1 and main terminal 2. A diac is used primarily in trigger circuits for SCRs or triacs. It will not conduct a current flow until its threshold voltage is reached. Threshold voltages from 15 to 50V are commonly available. Once the threshold is reached, the device fires and allows a current flow in either direction, thus reducing its voltage drop significantly.


Figure 48: Diac Schematic Symbol

Logic Gates

More and more, digital logic gates are being employed in regulating or protective circuits. The use of these gates provides speed, low power consumption, and space savings. Logic gates are usually two-state devices. The schematic symbols shown in Figure 49 are all shown with one or two inputs, but, logically, these devices can have multiple inputs. The truth tables for the gates are also shown in the figure.

 

An inverter is a device that will change an input level to the opposite level, for example, a high to a low. The AND gate is a device that will produce an output high only when both inputs are high.

The NAND gate is an AND with the output inverted. The OR gate will produce a high any time there is a high at one or both input leads. The NOR provides the inverted output of the OR. The LOR gate is a device that produces a high output when either of the input leads is high, but not when both input leads are at the same level.

A gate can be drawn different ways and will still have the same truth table. Often in schematic diagrams, a particular gate will be drawn using either of its two versions.


Figure 49: Logic Gates

Operational Amplifiers

The operational amplifier is used to perform mathematical operations on analog voltages. The symbol for the op-amp is shown in Figure 50. There are two power supply connections: +Vcc and -Vcc. The voltages at these inputs will establish the upper and lower limits for the output voltage of the op-amp. There are also two signal inputs: an inverting input (-) and a non-inverting input (+). There is only one output.

 


Figure 50: OPAMP Schematic Symbol

Op-Amp Parameters

The parameters of the ideal op-amp are:

  • Differential voltage gain = |infinity
  • Common-mode voltage gain = zero
  • Bandwidth = infinity
  • Input Impedance = infinity
  • Output Impedance = zero
  • Output voltage = zero (if input voltage is 0)
  • Parameter drift with temperature = zero
  • Equivalent input noise = zero

There are two important rules to be remembered when working with op-amps. Op-amp input terminals draw no current, and the voltage across input terminals is zero for an op-amp with feedback. By understanding these parameters and the two rules for analysis of op-amps, these devices can be easily understood.

Op-Amp Circuits

An op-amp comparator circuit is shown in Figure 51. This is a (+) biased, non-inverting comparator. When the input waveform rises above the bias voltage provided by the battery, the output goes to its maximum (+) voltage. If the input is less than the bias, then the output goes to the maximum (-) voltage.


Figure 51: op-amp Comparator Circuit and Waveform
Figure 52 shows the feedback and circuitry of some common op-amp circuits. Note that, in each circuit, the basic op-amp is the same. The input, output, and feedback circuits control the behavior of the op-amp.


Figure 52: op-amp Circuits

Human Interface Module

When the drive-mounted human interface module (HIM) is supplied, it will be connected as adapter 1 and visible from the front of the drive. The HIM can be divided into two sections: display panel and control panel. The display panel provides a means of programming the drive and viewing the various operating parameters. The control panel allows different drive functions to be controlled. Figure 58 shows the HIM.


Figure 54: 1305 HIM

NOTE: The operation of some HIM functions will depend upon drive parameter settings. The default parameter values allow full HIM functionality.

When power is first applied to the drive, the HIM will cycle through a series of displays. These displays will show drive name, HIM ID number, and communication status. Upon completion, the Status Display will be shown. This display shows the current status of the drive (i.e., "Stopped," "Running," etc.) or any faults that may be present ("Serial Fault," etc.).

Display Panel

The display panel consists of the following components:

  • Display
  • Escape key
  • Select key
  • Increment/Decrement keys
  • Enter key

Escape Key

When pressed, the ESC key will cause the programming system to go back one level in the menu structure.

Select Key

Pressing the SEL key alternately moves the cursor to the next active area. A flashing first character indicates which line is active.

Increment/Decrement Key

These keys are used to increment and decrement a value or scroll through different groups or parameters.

Enter Key

When the Enter key is pressed, a group or parameter will be selected, or a parameter value will be entered into memory. After a parameter has been entered into memory, the top line of the display will automatically become active, allowing a parameter (or group) to be chosen.

Control Panel

The control panel contains keys programmed to control the operation of the drive. The following information is correct for an as shipped from the factory drive. Reprogramming of mask parameters can disable control of some of these functions. Reprogramming of masks is sometimes done to prevent operation of the drive from the control panel. The following functions are found in the control panel section of the HIM:

  • Start
  • Stop
  • Jog
  • Change Direction
  • Direction LEDs
  • Increment/Decrement arrows
  • Speed Indicator LEDs

Start

The Start key will initiate drive operation if no other control devices are sending a Stop command. This key can be disabled by the (Logic Mask) or (Start Mask).

Stop

If the drive is running, pressing the Stop key will cause the drive to stop using the selected stop mode. If the drive has stopped due to a fault, pressing this key will clear the fault and reset the drive.

Jog

When the JOG key is pressed, jog will be initiated at the frequency set by the [Jog Frequency] parameter if no other control devices are sending a Stop command. Releasing the key will cause the drive to stop using the selected stop mode.

NOTE: If the drive is running prior to issuing a jog command, the jog command will be ignored. A start command from another source will override the jog command.

Change Direction

Pressing the change direction key will cause the drive to ramp down to 0 Hz and then ramp up to set speed in the opposite direction. The appropriate Direction Indicator will illuminate to indicate the direction of motor rotation.

Note: The factory default for control of the reverse function is the reverse input at the TB2 control terminal block. To enable the HIM control of the reverse function, change Bit 0 of the [Direction Mask] parameter to "0" to disable the reverse function at TB2.

Direction LEDs

Figure 59 for an explanation of the direction LEDs.
  • When the left LED is off and the right LED is on, the motor is rotating in the forward direction. If the left LED is on and the right LED is OFF, the motor is rotating in the reverse direction.
  • When the left LED is FLASHING and the right LED is on, the motor is decelerating REVERSE and will begin accelerating in the FORWARD direction.
  • If the left LED is on and the right LED is FLASHING, the motor is decelerating FORWARD and will begin accelerating in the REVERSE direction.


Figure 59: Direction LEDs

Increment/Decrement Arrows

The Increment/Decrement Arrows are only available with digital speed control. Pressing these keys will increase or decrease the HIMfrequency command. An indication of this command will be shown on the visual Speed Indicator LEDs. The drive will run at this command if the HIM is the selected frequency reference.

Pressing both the Increment and Decrement arrows simultaneously stores the current HIM frequency command in HIM memory. The Speed Indicator LEDs will flash momentarily to indicate a successful save if the speed is above 20%. If the speed is less than 20%, the HIM will not store the current speed into memory. Cycling power or connecting the HIM to the drive will set the frequency command to the value stored in HIM memory.

Speed Indicator LED

The Speed Indicator LED is only available with digital speed control. This LED illuminates in steps to give an approximate visual indication of the commanded speed.

Analog Speed Potentiometer

If the Analog Speed Potentiometer option has been ordered, the Increment/Decrement keys and Speed Indicator LEDs will be replaced by this potentiometer.

  • Rotating the Analog Speed potentiometer clockwise will increase the HIM frequency command.
  • Rotating the Analog Speed potentiometer counter-clockwise will decrease the HIM frequency command. The position of the Analog Speed potentiometer provides an approximate visual indication of the commanded speed.

HIM Removal and Installation

The HIM may be mounted directly on the drive, used as a hand-held programmer, or it may be mounted in the front of an enclosure.

HIM Removal

The HIM can be removed from the drive in one of two methods:

Disconnect power from the drive and remove the HIM as outlined in the following steps:

  • Lower the hinged panel located below the HIM as shown in Figure 60.
  • Press the retaining lever located directly beneath the HIM, slide the HIM downward, and remove it from the drive.


Figure 60: HIM Removal and Installation
Remove the HIM from the drive with the drive running by masking out the (LOGIC MASK) bit that identifies the adapter address of the HIM. Refer to Figure 9 to identify the adapter address for the HIM or view the HIM ID# on the display as the drive is powered up. The ID# corresponds to the adapter address.

NOTE: If the (Logic Mask) bit of the adapter is not masked out, set to a 0, and the HIM is removed a communication fault will occur and the drive will be disabled. However, if the HIM is the active frequency source, the drive will issue an Hz Error fault F29.

After correcting the cause of a fault, the fault must be cleared. To clear a fault, perform one of the following actions:

  • Cycle the power to the drive.
  • Cycle the stop signal to the drive.
  • Cycle the Clear Fault parameter.

NOTE: The stop signal will not clear a fault if the Logic Mask or Fault Mask bit of that adapter has been disabled or the Fault Clear Mode parameter is disabled.

NOTE: When the (Logic Mask) bit for a HIM adapter is changed from "1" to "0", it disables all command functions for that adapter with the exception of the Stop command and Frequency reference.

  • Lower the hinged panel located below the HIM as shown in Figure 61.
  • Press the retaining lever located directly beneath the HIM, slide the HIM downward, and remove it from the drive.


Figure 61: HIM Adapter Mounting

HIM Installation

The HIM may be installed onto the drive in one of two methods.

Disconnect power from the drive and install the HIM as outlined:

  • Reinsert the HIM by placing the top edge of the HIM about inch from the edge of the cover. Push inward on the bottom of the HIM and slide the HIM up into position.
  • Apply power to the drive.
  • Verify the (Logic Mask) bit for the adapter is set to a 1.

Installing the HIM onto a running drive.

  • Reinsert the HIM by placing the top edge of the HIM about inch from the edge of the cover. Push inward on the bottom of the HIM and slide the HIM up into position.
  • Verify the (Logic Mask) bit for the adapter is set to a 1.

HIM Modes

The HIM has up to seven different modes. Figure 62 shows a flow chart of the HIM programming modes. This flow chart provides a visual of how each of the seven different modes may be accessed. The seven HIM modes are as follows:

1. Display

2. Process

3. Program

4. EEPROM

5. Search

6. Control Status

7. Password


Figure 62: HIM Programming Flow Chart

Display

When the Display mode is selected, the Display mode allows any of the parameters to be viewed. However, parameter modifications are not allowed. The HIM programmer displays the statue display, also known as the operator level, when powered-up. Figure 63 shows the key strokes required to enter the Display mode.

Program

Program mode provides access to the complete listing of parameters available for programming.Figure 63 shows the key strokes required to enter the Program mode.

ACTIONDESCRIPTIONHIM DISPLAY
 1. The Display and Program modes allow access to the parameters for viewing or programming. 

 

 

a. From the Status Display, press Enter (or any key). "Choose Mode" will be shown.

 

 

 

 

b. Press the Increment (or Decrement) key to show "Program" (or "Display").

 

 

 

 

c. Press Enter. 

 

 

d. Press the Increment (or Decrement) key until the desired group is displayed.

 

 

 

 

e. Press Enter. 

 

 

f. Press the Increment (or Decrement) key to scroll to the desired parameter.

 

 

 2. With Series A HIM software versions 3.00 and above or Series B HIM software version 1.01 and above, you have the ability to access and modify each individual bit or digit.

 

Note: This procedure assumes the Password is not set, you have already logged in, or the device has been set to Defaults.

 

 

 

 

a. Select a parameter with Increment (or Decrement) keys.

 

 

 

b. Press the SEL key to view the first bit. Pressing this key again will move the cursor to the left one bit or digit.

 

Individual bits of a Read/Write parameter can be changed. Pressing the SEL key will move the cursor (flashing character) one bit to the left. Pressing the Increment/Decrement keys can then change that bit. When the cursor is in the far right position, pressing the Increment/Decrement keys will increment or decrement the entire value.

 

 

 

 

Bit ENUMs

 

3. With Series A HIM software versions 3.00 and above or Series B HIM software version 1.01 and above, bit ENUMs (16 character text strings) will be displayed to aid interpretation of bit parameters.

 

 

 

 

a. From the Choose Group menu, use the Increment/ Decrement key to select the Masks group. Press Enter.

 

 

b. Press the SEL key to view the ENUM of the first bit. Pressing this key again will move the cursor to the left one bit or digit and view the next bit's ENUM.

 

 

Process

The Process mode allows a "configurable" display to be programmed. One user-selected parameter can be displayed with programmed text and scaling. Figure 64 shows the key strokes required to enter the Process mode.

ACTIONDESCRIPTIONHIM DISPLAY
 1. When selected, the Process mode will show a custom display consisting of information programmed with the Process Display group of parameters. 
 a. Complete steps a though c from Figure 63 to access the Program mode.

 

 

 

 

b. Press the Increment/Decrement key until "Process Display" is shown. Press the Enter key.

 

 

 

 

c. Using the Increment/Decrement keys, select [Process Par] and enter the number of the parameter you wish to monitor. Press the Enter key.

 

 

 

 

 

d. Select [Process Scale] using the Increment/ Decrement keys. Enter the desired scaling factor. Press the Enter key.

 

 

 

 

e. Select [Process Text 1] using the Increment/Decrement keys. Enter the desired text character. Press the Enter key & repeat for the remaining characters.

 

 

 

f. When process programming is complete, press ESC until "Choose Mode" is displayed. Press Increment/Decrement until "Process" is displayed. Press the Enter key to get process value.

 

 

 

 

g. With Series A HIM Software Versions 3.00 and above or Series B HIM Software Versions 1.01 and above, the user has the ability to save the Process Display for power-up. To do this, simultaneously press Increment and Decrement keys on programming panel.

 

 

EEPROM

This mode allows all parameters to be reset to the factory default settings. For Series B HIM Software Version 1.01 and above, uploading and downloading of drive parameters may be performed. Figure 65 shows the key strokes required to reset the drives factory defaults. Figure 66 shows the key strokes required to upload drive parameters. Figure 67 shows the key strokes required to download drive parameters.

Search

(Series A HIM, software V3.00 & up or Series B HIM, V1.01 & up only.) This mode will search for parameters that are not at their default values. The Search mode is a read-only function. Figure 68shows the key strokes required to enter the Search mode.

Control Status

Series A HIM software V3.00 and up (or Series B HIM, V1.01 and up only) permits the [Logic Mask] parameter to be disabled/enabled, allowing HIM removal while drive power is applied. This menu also provides access to a fault queue that will list the last four faults that have occurred. "Trip" displayed with a fault indicates the actual fault that tripped the drive. A clear function clears the queue. Figure 69 shows the keystrokes to disable the logic mask, thus preventing a Serial Fault when the HIM is removed. Figure 70 shows the keystrokes required to view the fault queue and clear it when desired.

NOTE: Clearing the Fault Queue will not clear an active fault.

Password

The Password mode protects the drive parameters against programming changes by unauthorized personnel. When a password has been assigned, access to the Program and EEPROM modes can only be gained when the correct password has been entered. The password can be any five-digit number between 00000 and 65535. Refer to Figure 71 for setting a password. Figure 72 shows the key strokes required to log in to the drive. Login is used to enter the password for access to the Program, Control Logic, Clear Fault Queue, and EEPROM modes. Figure 73 shows the key strokes required to log out from the drive. Logout is used to disable access to the Program, Control Logic, Clear Fault Queue, and EEPROM modes.

1305 Drive Startup

This section describes the steps needed to start up the 1305 drive. Included in the procedure are typical adjustments and checks to ensure proper operation. The information contained in previous sections of this text must be read and understood before proceeding.

Startup Procedure

The following startup procedure is written for a drive with a Human Interface Module (HIM) installed in the drive (Port 1). For drives without a HIM, external commands and signals must be substituted.

NOTE: The parameters in the Setup group (pages 5-8 of the 1305 User Manual) should be reviewed and reprogrammed as necessary for basic operation.

Power must be applied to the drive when viewing or changing parameters. Previous programming may affect the drive status when power is applied.
  • Confirm that all circuits are in a de-energized state before applying power. External supplied voltages may exist at TB2 even when power is not applied to the drive.
  • Programming

    This section describes the 1305 parameters, which are divided into groups for ease of programming and maintenance access. All of the parameters required for any given drive function are contained within a group, eliminating the need to change groups to complete a function.

    Parameter Flow Chart

    The parameter flow charts shown in Figure 76 and Figure 77 highlight each group of parameters and list all parameters for each of the 13 groups. Some parameters appear in more than one group; these parameters are shown in bold. Immediately after the parameter name, the parameter numbers are shown in parenthesis. In this section, we will only cover some of the parameters to help you understand how to use the 1305 User Manual and change parameters. All of the parameters are located in the 1305 User Manual.
    Figure 76: Parameter Flow Chart One

    Figure 77: Parameter Flow Chart Two

    Metering

    The Metering group of parameters consists of commonly viewed drive operating conditions. Viewing these parameters can be useful in troubleshooting or just checking the drive's operating condition. All of the parameters in this group are read only. This means that they may be viewed but not changed. The following parameters are contained in the Metering group:

    • Output Current
    • Output Voltage
    • Output Power
    • DC Bus Voltage
    • Output Freq
    • Freq Command
    • MOP Hz
    • Drive Temp
    • Last Fault
    • % Output Power
    • % Output Curr

    Output Current

    The Output Current parameter displays the output current present at TB1 terminals T1, T2, and T3. These terminals are shown in Figure 78. The units displayed are in amps. The minimum output current is 0.00 amps, and the maximum current is two times the drive's rated output current.


    Figure 78: Terminal Block TB1

    Output Voltage

    The Output Voltage parameter displays the output voltage present at TB1 terminals T1, T2, and T3. These terminals are shown in Figure 78. The units displayed are in volts. The minimum output voltage is 0.00 volts, and the maximum is the drive's maximum rated output voltage.

    Output Power

    The Output Power parameter displays the output power present at TB1 terminals T1, T2, and T3. These terminals are shown in Figure 78. The units displayed are 0.01 kW. The minimum output power is 0.00 kW, and the maximum power is two times the drive's rated output power.

    DC Bus Voltage

    The DC Bus Voltage parameter displays the DC bus voltage level. The units displayed are in volts. The minimum DC bus voltage level is 0 volts, and the maximum voltage is 410 VDC for a 230 volt drive and 815 VDC for a 460 volt drive.

    Output Freq

    The Output Freq parameter displays the output frequency present at TB1 terminals T1, T2, and T3. These terminals are shown in Figure 78. The units displayed are 0.01 Hz. The minimum output frequency is 0.00 Hz, and the maximum output frequency is + 400 Hz. With the motor operating at a steady state, the Output Freq parameter should match the Freq Command parameter. If these two parameters do not match, you should check the Minimum Freq and Maximum Freq parameters located in the Setup Group. If the Minimum and Maximum Freq parameters are not preventing the Output Freq from matching the Freq Command, there is a problem within the drive.

    Freq Command

    The Freq Command parameter displays the frequency that the drive is commanded to output. This command may come from any one of the frequency sources selected by Freq Select 1, Freq Select 2, or any of the seven preset frequencies Freq 1-7. The seven preset frequencies, Freq 1-7, are selected by the inputs to SW1, SW2, and SW3 on TB2. These terminals are shown in Figure 79. Figure 80 shows the preset frequency, acceleration, and deceleration times selected by switches SW1, SW2, and SW3. The units displayed are 0.01 Hz. The minimum frequency is 0.00 Hz, and the maximum frequency 400 Hz.


    Figure 79: Terminal Board TB2

    Figure 80: Frequency Set Table

    MOP Hz

    The MOP Hz parameter displays the frequency reference commanded by the Motor Operated Potentiometer (MOP). The MOP frequency command can be adjusted from TB2 terminal 16 and TB2 terminal 17, provided the appropriate Input Mode is selected (Figure 79). The MOP frequency may also be changed through serial communication. This value is displayed regardless of whether or not the MOP is the active frequency command. The units displayed are 0.01 Hz. The minimum frequency is 0.00 Hz, and the maximum frequency 400 Hz.

    Drive Temp

    The Drive Temp parameter displays the internal temperature of the drive. The units displayed are in degrees C. The minimum drive temperature displayed is 0 degrees C, and the maximum temperature displayed is 100 degrees C. High internal drive temperature indicates a problem developing within the drive. This could also indicate poor cabinet ventilation. A high drive temperature must be resolved quickly to prevent damage to the drive.

    Last Fault

    The Last Fault parameter displays the fault code for the present drive fault. If there are no active faults, the value will be zero. Using this parameter, you can determine what has caused the drive to stop. For example, if the value displayed in this parameter is F8, the drive has an Overtmp Fault. The internal drive temperature has risen above the disserted operating temperature level.

    % Output Power

    The % Output Power parameter displays the percent of drive rated output power. The units displayed are in percent of drive rated power. The minimum % Output Power is 0%, and the maximum % Output Power is 200% of rated drive output power.

    % Output Curr

    The % Output Curr parameter displays the percent of drive rated output current. The units displayed are in percent of drive rated current. The minimum % Output Curr is 0%, and the maximum % Output Curr is 200% of rated drive output current.

    Setup

    The Setup group of parameters consists of parameters that define the basic operation of the drive. Some of these parameters were covered in the 1305 drive setup section. These parameters should be set before initial operation of the drive. Viewing these parameters can be useful in troubleshooting or just checking the drives operating condition. All of the parameters in this group are Read/Write. This means that they may be viewed or changed. The following parameters are contained in the Setup group:

    • Input Mode
    • Freq Select
    • Accel Time 1
    • Decel Time 1
    • Base Frequency
    • Base Voltage
    • Maximum voltage
    • Minimum Freq
    • Maximum Freq
    • Stop Select
    • Current Limit
    • Overload Mode
    • Overload Current
    • Sec Curr Limit
    • Adaptive 1 Lim

    Input Mode

    The Input Mode parameter selects between Three-Wire and Run Fwd/Rev control. The drive must be stopped before this parameter can be changed. After the Input Mode parameter is changed, power to the drive must be cycled. The factory default for the Input Mode is Three-Wire. Figure 81 shows the wiring connections for Three-Wire mode. Three-Wire mode requires a normally open momentary pushbutton to start the drive and a normally closed momentary pushbutton to stop the drive.
    Figure 81: Terminal Board TB2 Three-Wire Mode

    Figure 82 shows the wiring connections for Two-Wire mode. Two-Wire mode requires a jumper between the stop and common input and a maintained switch between common and start. Direction is controlled in both modes by a maintained switch between common and reverse. Additionally, the second acceleration rate and MOP frequency reference can be selected from this parameter. These selections are available for both Three-Wire and Two-Wire modes.

    Figure 82: Terminal Board TB2 Two-Wire Mode

    Freq Select 1

    The Freq Select 1 parameter is the factory default parameter for selecting the frequency source that will supply the Freq Command to the drive. The factory default for the Freq Select 1 parameter is Adapter 1. Adapter 1 is the HIM when it is mounted on the drive. It is important to remember that if an adapter selected as the active frequency source is removed, the drive will fault on "Hz Sel Fault" (F30). The following settings are available for the Freq Select 1 parameter:

    • Remote Pot
    • 0 to 10 Volt
    • 4-20 mA
    • MOP
    • Adapter 1
    • Adapter 2
    • Adapter 3
    • Adapter 4
    • Adapter 5
    • Adapter 6
    • Preset 1
    • Preset 2
    • Preset 3
    • Preset 4
    • Preset 5
    • Preset 6
    • Preset 7

    Accel Time 1

    The Accel Time 1 parameter is the factory default parameter for determining the time it will take the drive to ramp from 0 Hz to Maximum Frequency. Figure 83 shows an example of the acceleration time. The rate is linear unless the S Curve is "Enabled." The Accel Time 2 parameter can be selected in place of this parameter. The units for the Accel Time 1 parameter are 0.1 seconds. The minimum acceleration time is 0 seconds, and the maximum acceleration time is 3,600.0 seconds. The factory default setting for Accel Time 1 is 10.0 seconds. That is, the drive will accelerate from 0% speed to 100 % speed in 10 seconds.
    Figure 83: Acceleration/Deceleration Time

    Decel Time 1

    The Decel Time 1 parameter is the factory default parameter for determining the time it will take the drive to ramp from maximum frequency to 0Hz. Figure 83 shows an example of the deceleration time. The rate is linear unless the S Curve is "Enabled" or Stop Select is set to S-Curve. The Decel Time 2 parameter can be selected in place of this parameter. The units for the Decel Time 1 parameter are 0.1 seconds. The minimum deceleration time is 0 seconds, and the maximum deceleration time is 3,600.0 seconds. The factory default setting for Decel Time 1 is 10.0 seconds. That is, the drive will decelerate from 100% speed to 0 % speed in 10 seconds.

    Base Frequency

    The Base Frequency parameter should be set to the motor nameplate rated frequency. The factory default for this parameter is 60Hz. The minimum setting is 40 Hz, and maximum setting is 400 Hz. Setting this parameter at a value other than the motor nameplate may cause unpredictable drive/motor operation.

    Base Voltage

    The Base Voltage parameter should be set to the motor nameplate rated voltage. The factory default for this parameter is the maximum drive rated volts. The minimum setting is 25% of the drive's rated volts, and the maximum setting is 100% of the drive's rated volts. Setting the base voltage below the motor's nameplate voltage will cause the motor to draw excess current and may damage the motor. Setting the base voltage greater than the motor's nameplate voltage may cause the motor to operate faster than required and damage equipment.

    Maximum Voltage

    The Maximum voltage parameter sets the highest voltage the drive will output. The factory default for this parameter is the maximum drive rated volts. The minimum setting is 25% of the drive's rated volts, and the maximum setting is 110% of the drive's rated volts. The Maximum voltage parameter does not have to be set greater than the base Voltage parameter. However, the maximum drive output is limited to the Maximum voltage parameter.

    Minimum Freq

    The Minimum Freq parameter sets the lowest frequency the drive will output. The factory default for this parameter is 0 Hz. The minimum setting is 0 Hz, and the maximum setting is 120 Hz. This parameter may not be programmed while the drive is running. All analog inputs to the drive (4-20 mA, 0-10 V, and Remote Pot) are scaled for the range Minimum Freq to Maximum Freq. That is, for a minimum setting of any of the analog inputs, the output frequency will be whatever this parameter is set for.

    Maximum Freq

    The Maximum Freq parameter sets the highest frequency the drive will output. The factory default for this parameter is 60 Hz. The minimum setting is 40 Hz, and the maximum setting is 400 Hz. This parameter may not be programmed while the drive is running. All analog inputs to the drive (4-20 mA, 0-10 V, and Remote Pot) are scaled for the range Minimum Freq to Maximum Freq. That is, for a maximum setting of any of the analog inputs, the output frequency will be whatever this parameter is set for.

    Stop Select

    The Stop Select parameter sets the stopping mode when the drive receives a valid stop command. The factory default for this parameter is Ramp. When Ramp is selected as the Stop Select parameter, the drive decelerates to 0 Hz then turns off. The Ramp selection requires a value in Decel Time 1 or Decel Time 2. Ramp time depends on which parameter (Decel Time 1 or Decel Time 2) is selected and the setting of that parameter. Other selections for Stop Select parameter are Coast, DC Brake, and S-Curve.

    • When Coast is selected, the drive will turn off immediately when stop is commanded.
    • If DC Brake is selected, the drive injects DC braking voltage into the motor. This will cause the motor to stop much faster. The DC Brake selection requires a value in both Decel Hold Time and Decel Hold Level parameter.
    • When S-Curve is selected, the drive ramps to stop using a fixed S-Curve profile. The stop time is twice the selected Decel Time.

    Current Limit

    The Current Limit parameter sets the maximum drive output current that is allowed before current limiting occurs. The units for this parameter are in percent. The factory default is 150% of the drive's rated current. The minimum Current Limit setting is 20% of the drive's rated current, and the maximum setting is 150% of the drives rated current. These settings are based on three-phase power input; for single-phase input ratings, refer to the 1305 User Manual Section 5 - Single Phase Input Ratings.

    Overload Mode

    The Overload Mode parameter selects the derating factor for the I2T electronic overload function. This is designed to meet NEC article 430. The factory default is no derating. See Figure 84 for a graphic view of the no derating selection. In this mode, the overload is set to 100% for an output speed of 0% to 100%.


    Figure 84: Overload Mode No Derating
    Figure 85 shows a graphic view of the Min Derate selection for the Overload Mode parameter. When Min Derate is selected, the overload is lowered at base speeds less than 25%. As the base speed increases above 25%, the overload is increased to 100%. This is used to prevent high current draw at speeds less than 25%.


    Figure 85: Overload Mode Min Derating
    Figure 86 shows a graphic view of the Max Derate selection for the Overload Mode parameter. When Max Derate is selected, the overload is lowered at base speeds less than 50%. As the base speed increases above 50%, the overload is increased to 100%. This is used to prevent high current draw at speeds less than 50%.
    Figure 86: Overload Mode Max Derating

    Overload Current

    The Overload Current parameter selects the amount of current at which the drive will trip on over-current. This value should be set to the motor nameplate full load Amps (FLA). The units for this parameter are 0.1 amps. The factory default is 115% of the drive's rated current. The minimum Overload Current setting is 20% of the drive's rated current, and the maximum setting is 115% of the drive's rated current.

    Sec Curr Limit

    The Sec Curr Limit parameter selects the value for current limit to taper down to at frequencies between the Base Frequency and 1.5 times the base frequency. Figure 87 shows a graph of how the Current Limit and Sec Curr Limit parameters operate together. When the Sec Curr Limit parameter is set to 0, the Current Limit setting is used throughout the frequency range. When the Sec Curr Limit is set to a value other than 0, the Current Limit value will be active up to the Base Frequency setting, and then taper down between the Base Frequency setting and 1.5 times the Base Frequency setting. The units for this parameter are in percent. The factory default is 0% of the drive's rated current. The minimum Current Limit setting is 0% of the drive's rated current, and the maximum setting is 150% of the drive's rated current. These settings are based on three-phase power input; for single-phase input ratings, refer to the 1305 User Manual Section 5, Single Phase Input Ratings.
    Figure 87: Current Limit and Sec Curr Limit Interaction

    Adaptive 1 Lim

    The Adaptive 1 Lim parameter may be enabled or disabled. When enabled, this parameter allows tripless commanded accelerations into medium to high sluggishness conditions and delivers maximum performance when the drive load conditions change with time. For most circumstances, this is the correct selection. When the Adaptive 1 Lim parameter is disabled, it allows quicker acceleration times from stopped to commanded speed with low system sluggishness. The factory default for this parameter is enabled.

    Advanced Setup

    The Advanced Setup group of parameters consists of parameters that define the advanced operation of the drive. Some of these parameters were covered in the 1305 drive setup section. These parameters should be set before initial operation of the drive. Viewing these parameters can be useful in troubleshooting or just checking the drive's operating condition. All of the parameters in this group are Read/Write. This means that they may be viewed or changed. The following parameters are contained in the Setup group:

    • Minimum Freq
    • Maximum Freq
    • Base Frequency
    • Base Voltage
    • Maximum voltage
    • Break Frequency
    • Break Voltage
    • DC Boost Select
    • Start Boost
    • Run Boost
    • PWM Frequency
    • Analog Invert
    • 4-20 mA Loss Sel
    • Stop Select
    • DC Hold Time
    • DC Hold Volts
    • Motor Type
    • Compensation

    Minimum Freq

    The Minimum Freq parameter sets the lowest frequency the drive will output. The factory default for this parameter is 0 Hz. The minimum setting is 0 Hz, and the maximum setting is 120 Hz. This parameter may not be programmed while the drive is running. All analog inputs to the drive (4-20 mA, 0-10 V, and Remote Pot) are scaled for the range Minimum Freq to Maximum Freq. That is, for a minimum setting of any of the analog inputs, the output frequency will be whatever this parameter is set for.

    Maximum Freq

    The Maximum Freq parameter sets the highest frequency the drive will output. The factory default for this parameter is 60 Hz. The minimum setting is 40 Hz, and the maximum setting is 400 Hz. This parameter may not be programmed while the drive is running. All analog inputs to the drive (4-20 mA, 0-10 V, and Remote Pot) are scaled for the range Minimum Freq to Maximum Freq. That is, for a maximum setting of any of the analog inputs, the output frequency will be whatever this parameter is set for.

    Base Frequency

    The Base Frequency parameter should be set to the motor nameplate rated frequency. The factory default for this parameter is 60Hz. The minimum setting is 40 Hz, and maximum setting is 400 Hz. Setting this parameter at a value other than the motor nameplate may cause unpredictable drive/motor operation.

    Base Voltage

    The Base Voltage parameter should be set to the motor nameplate rated voltage. The factory default for this parameter is the maximum drive rated volts. The minimum setting is 25% of the drive's rated volts, and the maximum setting is 100% of the drive's rated volts. Setting the Base Voltage below the motor's nameplate voltage will cause the motor to draw excess current and may damage the motor. Setting the Base Voltage greater than the motor's nameplate voltage may cause the motor to operate faster than required and damage equipment.

    Maximum Voltage

    The Maximum Voltage parameter sets the highest voltage the drive will output. The factory default for this parameter is the maximum drive rated volts. The minimum setting is 25% of the drive's rated volts, and the maximum setting is 110% of the drive's rated volts. The Maximum voltage parameter does not have to be set greater than the Base Voltage parameter. However, the maximum drive output is limited to the Maximum Voltage parameter.

    Break Frequency

    The Break Frequency parameter sets a midpoint frequency on a custom Volts/Hz curve. Figure 88 shows a standard volts/Hz pattern. In this figure, the Maximum Voltage and Base Voltage are both set to the motor rated voltage. The Maximum Frequency is set to some value greater than Base Frequency. This produces a linear voltage ramp up to the Base Voltage setting. After the Base Voltage setting has been reached, increase in frequency will not cause an increase in voltage.


    Figure 88: Standard Volts/Hz Pattern
    When the Break Frequency and Break Voltage parameters are combined, they determine the Volts/Hz ratio between 0 and the Break Frequency parameter. The units for this parameter are 1 Hz. The factory Default is 30 Hz. The minimum setting is 0 Hz, and the maximum setting is 120 Hz. The DC Boost Select parameter must be set to Break Point for this parameter to be active.

    Break Voltage

    The Break Voltage parameter selects the voltage the drive will output at the Break Frequency. Combined with Break Frequency, the Brake Voltage parameter determines the Volts/Hz ratio between 0 and the Break Frequency. The DC Boost Select parameter must be set to Break Point to activate this parameter. The factory default for the Break Voltage parameter is 115 volts for 230-volt drives and 230 volts for 460-volt drives. The minimum setting for this parameter is 0 volts, and the Maximum setting is 50% of the drive rated volts. Figure 89 shows a graph of a custom volts/Hz pattern. This pattern is active only when DC Boost Select is set to Break Point.


    Figure 89: Custom Volts/Hz Pattern
    The following guidelines must be followed when setting up a custom volts/Hz curve:

    • Base Voltage must be greater than Start Boost
    • DC Boost Select set to Break Point
    • Base Voltage must be greater than Break Voltage
    • Break Voltage must be greater than Start Boost

    DC Boost Select

    The DC Boost Select parameter selects the level of DC boost at low frequencies. It also selects special Volts/Hz patterns. This allows the drive to operate various motors connected to a wide verity of devices. The factory default is Break Point. Break Point is shown in Figure 89. Break Point requires Break Frequency and Break Voltage to be configured. Additional selections for DC Boost are:

    • No Boost
    • 6 Volts
    • 12 Volts
    • 18 Volts
    • 24 Volts
    • 36 Volts
    • 42 Volts
    • 48 Volts
    • Run Boost
    • Fan Set #1
    • Fan Set #2

    For settings of No Boost through 48 Volts Boost, see Figure 90. A setting of No Boost produces a linear voltage output from 0 volts to the Maximum Voltage parameter. A setting of 6 Volts Boost will provide 6 volts of boost at the low end of the frequency output and then produce a linear voltage output until the Maximum Voltage parameter is reached. Each of the additional voltage boosts produces a similar voltage output.


    Figure 90: Standard Boost Volts/Hz Pattern
    When the DC Boost Select parameter is set to Run Boost, the Start Boost parameter selects the value of DC voltage applied during the start of the drive. Figure 91 shows the Start/Run Boost pattern. This provides a higher start voltage at startup, allowing the motor to develop more torque.


    Figure 91: Start/Run Boost
    Fan Sel #1 and Fan Sel #2 provide a Volts/Hz curve designed to operate motors connected to fans and pumps. Figure 92 shows a graph of these selections. Fan Sel #1 provides a break point at 35% of Base Voltage and 50% of Base Frequency. Fan Sel #2 provides a break point at 45% of base Voltage and 50% of Base Frequency. [[Image:AC Fan Pump_edit.png|framed|center|caption|

    Figure 92: Fan/Pump Volts/Hz Pattern

    Start Boost

    The Start Boost parameter sets the DC boost level for acceleration when the DC Boost Select is set to Run Boost or Break Point. Figure 91 shows a graphic of the Start/Run Boost. The units for this parameter are volts. The factory default setting is shown in the table below. The minimum setting is 0 Volts, and the maximum setting is 25% of drive rated volts.

    Run Boost

    The Run Boost parameter selects the DC Boost level for constant speed level when DC Boost Select is set to Run Boost. Run Boost must be set at a value less than Start Boost. Figure 91 shows a graphic of the Start/Run Boost.

    PWM Frequency

    The PWM Frequency parameter selects the center frequency for the PWM output Waveform. PWM Frequency parameter units are 0.1 kHz. The factory default is 4.0 kHz. The minimum setting for this parameter is 2.0 kHz, and the maximum setting is 8.0 kHz. It is recommended for most installations to use the factory default. Changing the PWM parameter may result in changes in Startup and Holding Current if start boost and DC Holding voltages are in effect. These parameters should be checked if the PWM parameter is changed.

    Analog Invert

    The Analog Invert parameter selects the inverting function for the 0 to 10 volt and 4 to 20 mA analog input signals at TB2. The default setting is disabled. When this parameter is disabled, 0V or 4mA will cause the drive to produce minimum frequency (Figure 93). If the Analog Invert parameter is Enabled 0 volts or 4 mA, it will cause the drive to produce maximum frequency.
    Figure 93: Analog Invert Graph

    4-20 mA Loss Sel

    The 4-20 mA Loss Sel parameter selects the drive reaction to a loss of a 4-20 mA signal when the active Frequency Source is 4-20 mA. A loss of signal is defined as "a signal less than 3.5 mA or a signal greater than 20.5 mA." The factory default for this parameter is Stop/Fault. When Stop/Fault is selected, the drive will stop and issue an Hz Err Fault (F29) anytime it detects a loss of signal. Other selections for the 4-20 mA Loss Sel parameter are as follows:

    • Hold/Alarm
    • Max/Alarm
    • Pre1/Alarm
    • Min/Alarm If Hold/Alarm is selected, the drive will maintain the last output frequency and set an alarm bit. The drive output contacts can be used to issue an alarm signal by setting Output 1 Config or Output 2 Config to alarm. When Max/Alarm is selected, the drive will output Maximum Freq and set an alarm bit on a loss of signal. If Pre1/Alarm is selected, the drive will output Preset Freq 1 and set an alarm bit. When Min/Alarm is selected, the drive will output Minimum Freq and set an alarm bit.

    Stop Select

    The Stop Select parameter sets the stopping mode when the drive receives a valid stop command. The factory default for this parameter is Ramp. When Ramp is selected as the Stop Select parameter, the drive decelerates to 0 Hz then turns off. The Ramp selection requires a value in Decel Time 1 or Decel Time 2. Depending on which parameter (Decel Time 1 or Decel Time 2) is selected, the setting of that parameter will cause the ramp time. Other selections for Stop Select parameter are Coast, DC Brake, and S-Curve.

    • When Coast is selected, the drive will turn off immediately when stop is commanded.
    • If DC Brake is selected, the drive injects DC braking voltage into the motor. This will cause the motor to stop much faster. The DC Brake selection requires a value in both the Decel Hold Time and Decel Hold Level parameters.
    • When S-Curve is selected, the drive ramps to stop using a fixed S-Curve profile. The stop time is twice the selected Decel Time.

    DC Hold Time

    The DC Hold Time parameter selects the amount of time that the DC Hold Level voltage will be applied to the motor when the stop mode is set to either DC Brake or Ramp. When the Stop Select parameter is set to Coast mode and the drive is stopped and restarted within the DC Hold time setting, the speed will resume at the output frequency prior to the Stop command. The units for DC Hold Time are 0.1 seconds. The factory default is 0.0 seconds. The minimum setting for this parameter is 0.0 seconds, and the maximum setting is 150 seconds. Figure 94 shows the DC Hold Time parameter applied to a Stop Select of Ramp. In this example, the DC hold time starts after the voltage has ramped to 0 and ends after the DC Hold Time parameter value.
    Figure 94: DC Hold Time with Ramp
    Figure 95 shows the DC Hold Time parameter applied to a Stop Select of DC Brake. In this example, the DC hold time starts when the Stop Command is issued and ends after the DC Hold Time parameter value.
    Figure 95: DC Hold Time with DC Brake

    DC Hold Volts

    The DC Hold Volts parameter selects the DC voltage applied to the motor during braking when the Stop Select parameter is set to Either DC Brake or Ramp. The factory default value is 0 volts. The minimum setting is 0 volts, and the maximum setting is 25% of the drive rated volts. When setting this parameter, begin at a low voltage and continue increasing until sufficient holding torque is achieved and the drive output current rating is not exceeded. Setting the DC Hold Volts parameter to a voltage higher than required to achieve sufficient holding torque may damage the drive and motor.

    DB Enable

    The DB Enable parameter enables and disables the use of an external dynamic brake resister by disabling the internal ramp regulation. The factory setting for this parameter is Disabled.

    Motor Type

    The Motor Type parameter selects induction motor or synchronous permanent magnet motor. The factory default for this parameter is Induc/Reluc. This is the type of motor used for most installations.

    Compensation

    The Compensation parameter selects between Comp and No Comp. Some drive/motor combinations have inherent instabilities that are exhibited as non-sinusoidal current feedback. The Compensation, when enabled, will correct this condition. The factory default for this parameter is Comp. This is the setting that should be used for most installations. Enabling compensation when it is not required will have little to no effect on the drive/motor's operation. However, disabling compensation on a drive/motor that requires compensation will have a large effect on its operation.

    Drive Parameters

    Additional drive parameters may be found in the 1305 User Manual. This course has covered most of the more commonly used parameters. Section 5 of the 1305 User Manual provides a complete listing of drive parameters and how to program them.

    Troubleshooting

    This section provides information to guide you in understanding drive fault condition and general troubleshooting procedures for the 1305 drive. Section 6 of the 1305 User Manual provides a complete listing of all fault codes and possible solutions.

    Fault Information

    Drives equipped with a HIM will display a brief fault message on Line 1 of the LCD display when a fault occurs. Line 2 of the display indicates the corresponding fault number. Figure 96 shows a HIM display with an Overvolt Fault (F5).


    Figure 96: Fault Display

    NOTE:

    For Series A HIM software version 3.00 and above or series B HIM software version 1.01 and above, faults are displayed as soon as they occur. Earlier HIM versions only display faults when the HIM is in the Status Display Mode. Fault Buffer 0 through Fault Buffer 3 display previous faults.

    Fault LED

    The 1305 Drive comes equipped with a fault LED. When the fault LED is illuminated, it is an indication a fault condition exists. Refer to Figure 97 for the location of the fault LED. Once the fault is properly cleared, the LED will return to an off state.


    Figure 97: 1305 Drive

    Diagnostic

    As seen in Figure 97, there are two indicators provided to display the drive's status condition. The DC Bus Charge Indicator is a neon bulb that will be illuminated when power is applied to the drive. The Fault Indicator is an LED that will be illuminated if a drive fault condition exists. Under normal operating conditions, the DC Bus Charge Indicator will be lit and the Fault Indicator will not be lit. Always use caution, as Terminal Block TB1 and TB2 contain dangerous voltages.

    Clear a Fault

    Remember that resetting a fault will not correct the cause of the fault condition. Corrective action must be taken prior to resetting a fault. After correcting the cause of a fault, the fault must be cleared. To clear a fault, perform one of the following actions:

    • Cycle the power to the drive.
    • Cycle the stop signal to the drive.
    • Cycle the Clear Fault parameter.

    NOTE: The stop signal will not clear a fault if the Logic Mask or Fault Mask bit of that adapter has been disabled or the Fault Clear Mode parameter is disabled.