MOTOR CONTROL FUNDAMENTALS
The main functions of a motor controller are to start and stop the motor and to protect the motor, machine, and operator. The controller may also be called upon to provide other functions that could include reversing, jogging or inching, plugging, operation at various speeds or at reduced current levels, and controlling motor torque.
The purpose of controller enclosures is to provide protection of operating personnel by preventing accidental contact with energized components. In certain applications, the controller is protected from a variety of environmental conditions including water, rain, snow, sleet, dirt, non-combustible dust, oils, coolants, and lubricants. Motor control centers are designed to meet the requirements of the National Electrical Code (NEC), specifically Article 430 for motors and motor control centers and Article 500 for electric equipment in hazardous locations.
The National Electrical Manufacturers Association (NEMA) and other organizations have established standards of enclosure construction for control equipment. Common types of enclosures, per NEMA classification numbers are:
This type of enclosure is primarily intended to prevent accidental contact with the enclosed apparatus. It is suitable for general purpose applications indoors where it is not exposed to unusual service conditions. A NEMA 1 enclosure serves as protection against dust and light indirect splashing but is not dust-tight.
This enclosure type is designed to provide suitable protection against specified weather hazards. A NEMA 3 enclosure is suitable for application outdoors, on ship docks, canal locks, construction work, and for application in subways and tunnels. It is also sleet-resistant.
This type protects against interference in operation of the contained equipment due to rain and resists damage from exposure to sleet. It is designed with conduit hubs and external mounting as well as drainage provisions.
Watertight enclosures are designed to meet the following hose test: "Enclosures shall be tested by subjection to a stream of water. A hose with a one-inch nozzle shall be used and shall deliver at least 65 gallons per minute. The water shall be delivered on the enclosure from a distance of not less than 10 feet and for a period of five minutes. During this period, it may be directed in any one or more directions as desired. There shall be no leakage of water into the enclosure."
These types of enclosures are generally constructed along the lines of NEMA 4 enclosures, except they are made of a material that is highly resistant to corrosion. For this reason, they are ideal in applications, such as paper mills and chemical facilities, where contaminants could destroy an enclosure over a period of time.
This type of enclosure is designed to meet the application requirements of the NEC for Class I hazardous locations. Class I hazardous locations are those in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixtures.
Class II locations are those that are hazardous because of the presence of combustible dust. The letter or letters following the type number indicate the particular group of hazardous locations as defined by the NEC for which the enclosure is designed. The designation is incomplete without a suffix letter or letter. Example, NEMA 9, Class II, Group F.
This enclosure is designed for use in those industries where it is desired to exclude such materials as dust, lint, fibers, oil seepage, or coolant seepage. There are no conduit openings or knockouts in the enclosure, and mounting is by means of flanges or mounting feet.
These generally are of cast construction, gasketed to permit use in the same environments as NEMA 12 devices. The basic difference is that, due to the case housing, a conduit entry is provided as an integral part of a NEMA 13 enclosure, and mounting is by means of blind holes rather than mounting brackets.
A manual controller is one having its operations controlled or performed by hand at the location of the controller, as shown in Figure 1. Perhaps the most popular single type in this category is the manual, full-voltage motor starter in the smaller sizes.
Figure 1: Manual Control for a Motor
A manual starter is used frequently where the only control function needed is to start and stop the motor. The manual starter generally provides overload protection for the apparatus being powered. Manual control, which provides the same functions as those achieved by the manual full-voltage motor starter, can be had by the use of a switch with fusing of the delayed-action type, which provides overload protection for the motor.
Examples of this type of control are very common in small metalworking and woodworking shops that use small drill presses, lathes, and pipe-threading machines. Another good example is the exhaust fan generally found in machine shops and other industrial operations. In this installation, the operator or maintenance person generally pushes the START button for the fan in the morning when the plant opens, and it continues to run throughout the day. In the evening, or when the plant is shut down, the operator then pushes the STOP button, and the fan shuts down until needed again.
A manual controller is easily identified because it has no automatic functions of control. This type of controller is characterized by the fact that the operator must move a switch or push a button to initiate any change in the condition of operation of the apparatus being operated. A manual controller must, therefore, have two components: a manually operated switch and a circuit protective device.
A semiautomatic controller uses a magnetic starter (a switch operated by an electromagnet) and one or more manual devices such as pushbuttons and other similar equipment. Figure 2 shows a simple semiautomatic control scheme for a motor. Semiautomatic control provides flexibility of control by allowing remote and multiple control locations in installations where manual control would otherwise be impractical.
Figure 2: Semiautomatic Control for a Motor
The key to classification as a semiautomatic control system lies in the fact that all the pilot devices are manually operated and that the motor starter is the magnetic-type. There are probably more machines operated by semiautomatic control than by either manual or automatic. This type of control requires the operator to initiate any change in the attitude or operating condition of the machine. Using the magnetic starter, however, this change may be initiated from any convenient location, as contrasted to the manual control requirement that the control point be at the starter.
An automatic controller is a magnetic starter in which functions are controlled by one or more automatic control or pilot devices. Figure 3 shows an automatic control scheme for a motor. As shown, an automatic pilot device is some type of control device, such as a limit switch or float switch, that functions independent of operator action to initiate a change in the operating condition of a motor or machine.
Figure 3: Automatic Control for a Motor
Some systems may use a combination of manual and automatic devices in the control circuit. When a control system uses one or more automatic devices, it is classed as an automatic controller.
Consider, for example, a tank that must be kept filled with water between definite limits and a pump to replace the water as it is needed. If we equip the pump motor with a manual starter and station a person at the pump to turn it on and off as needed, we have manual control. Now, let us replace the manual starter with a magnetic starter and put a pushbutton station at the foreman's desk. If we ring a bell to let them know when the water is low and again when it is high, they can do other work and just push the proper button when the bell rings. This would be semiautomatic control. Now, suppose we install a float switch that will close the circuit when the water reaches a predetermined low level and open it when it reaches a predetermined high level. When the water gets low, the float switch will close the circuit and start the motor. The motor will now run until the water reaches the high level, at which time the float will open the circuit and stop the motor.
Although the automatic system is more expensive to install, it requires less operator attention and functions more reliably and accurately.
Automatic control systems are found in many applications, such as large power plants, where they are used to control many mechanical systems in machine shops, where precision machines, such as drill presses and lathes, are automatically controlled for better accuracy and efficiency, and in the home, where automatic control systems are used to control such common household machines as dishwashers and washing machines.
Understanding, troubleshooting, and repairing control systems requires a knowledge and understanding of the physical devices that are used in control circuits and the symbols and terminology that are used to designate those devices on wiring diagrams. Most symbols used have been standardized throughout the industry to assure uniformity. Figure 4 shows American National Standard Graphical Symbols for Electrical Diagrams. The chart shown in Figure 5 shows standard symbols used in motor control circuits.
Figure 4: American National Standard Graphical Symbols for Electrical Diagrams
Figure 5: Standard Wiring Diagram Symbols
All components used in motor control circuits may be classed as either primary control devices or pilot control devices. A primary control device is one that connects the load to the line, such as a motor starter, whether it is manual or automatic. Pilot control devices are those that control or modulate the primary control devices. Pilot devices are things such as pushbuttons, float switches, pressure switches, and thermostats.
An example (Figure 6) would be a magnetic contactor controlled by a toggle switch used to energize and de-energize the contactor, or M coil. To start the motor, the toggle switch is switched ON, which energizes the contactor coil and closes the main line contacts, which energizes the motor. Switching the toggle switch OFF de-energizes the contactor coil, opens the main line contacts, and de-energizes the motor.
Figure 6: Basic Motor Control Circuit
In this example, the contactor, in that it connects the motor or load to the line, would be classed as a primary control device. The switch does not connect the load to the line, but is used to energize and de-energize the coil of the starter. Therefore, it would be classed as a pilot control device.
For any given controller, there are generally two primary control devices used. These are the disconnecting means, or circuit breaker (usually a manual device), and the magnetic contactor. There may be many pilot devices used in parallel and series combinations to control the function of starting and stopping performed by the primary control device. The overload relays, for instance, which are included in the motor starter, are actually pilot devices used to control the primary device whenever the motor is overloaded.
Pilot devices vary greatly with their function and intended use. From manual switches to automatic control devices, pilot devices are what make a motor controller adaptable to fit a multitude of applications.
Symbols 1 and 2 represent electrical contact devices (see Figure 7). They may represent line contacts on a starter, contacts on a limit switch or relay, or any other type of control device that has electrical contacts. Recall that circuit diagrams are shown in their de-energized condition. Therefore, Symbol 1 is a normally open (NO) contact, and Symbol 2 is a normally closed (NC) contact.
Figure 7: Basic Symbols Used on Motor Control Circuits
The designations "a" or "b" associated with a set of contacts are used to identify the state of the contacts (open or closed) in reference to the main operating device. An "a" contact will normally be closed when its associated coil is energized and its main contacts are closed. These same "a" contacts will open when the associated coil is de-energized. A "b" contact will normally be open when its associated coil is energized, thus operating the opposite of an "a" contact. Remember, however, that not all electrical drawings will indicate the state of contacts with this designator. If not, the drawing notes should annotate whether the circuit is in the energized or de-energized state. This "a" and "b" notation holds true for auxiliary contactors and relays, as well as the main contactor. Note that all circuits are shown in the de-energized, or shelf, condition unless stated otherwise.
Two other terms often used in conjunction with contacts, either relay contacts or switch contacts, are make and break. When a contact goes closed, it is said to "make," and when the contact opens, it is said to "break."
Symbols 3 and 4, shown in Figure 7, represent manually operated pushbutton switches with normally open and normally closed contacts, respectively. This spring-returned type switch will return to its normal position when released by the operator. Because the switch returns to its original position, and its contacts are only closed or open for the moment (however long) the switch is pushed, these contacts are referred to as momentary.
Symbols 5 and 6, shown in Figure 7, represent manual contacts of a toggle type of switch. Symbol 5 contacts are normally open, and Symbol 6 contacts are normally closed. This type of switch has maintaining contacts; that is, once switched to a different position, the switch will stay in that position. When found in a circuit diagram, the switch positions should be labeled as OFF or ON, FAST or SLOW, or other appropriate labeling. Symbol 7 is a toggle switch of the single-pole, double-throw (SPDT) type, where one contact is normally open and the other normally closed.
When more than one set of contacts are operated by moving one handle or pushbutton, they are generally connected by dotted lines, as in symbols 8 and 9. The dotted lines represent any form of mechanical linkage that will make the two contacts operate together. One other method that is used frequently to show pushbuttons that have two sets of contacts is shown in symbols 10 and 11. Symbol 10 has two normally open contacts, and symbol 11 has one normally open and one normally closed contact.
Switches can be designed to operate in one of two ways. The first, and most common, is referred to as break-before-make contact arrangement. In an arrangement of this type, one set of contacts opens before the next set of contacts closes.
The second arrangement is referred to as make-before-break. In this arrangement, when the switch is being switched from one position to the next, shorting or bridging of the first and second set of contacts occurs, for a short period, during the contact transfer. This arrangement is used when it is necessary to ensure continuity of power to a circuit during the switching evolution.
Symbol 12, shown in Figure 7, is a pilot or indicating light that is indicated chiefly by the short lines radiating out from the center circle. Normally, the color of the light is designated by the appropriate letter in the circle, such as RL for red or GL for green.
Symbols 13 and 14 shown in Figure 7 represent a coil. It may be a relay coil or a main line contactor. Relays and contactors are electromagnetic devices in the sense that magnetic forces are produced when electric currents are passed through coils of wire; in response to such forces, contacts are closed or opened by the motion of plungers or pivoted armatures. Symbols 15 and 16 are discussed later in this text.
As defined by the National Electrical Manufacturers Association (NEMA), a relay is "a device that is operated by a variation in the conditions of one electric circuit to effect the operation of other devices in the same or another electric circuit." A contactor, on the other hand, is "a device for repeatedly establishing and interrupting an electric power circuit." It is important to recognize the difference between the two, noting particularly that the relay, serving a secondary role, causes other devices to function, whereas the contactor is the primary unit, doing its work in the main power circuit.
A drawing showing the basic construction of a relay is shown in Figure 8. Note the relay coil and coil terminals.
Figure 8: Relay Construction
This relay has one set of NO (C1) and one set of NC (C2) contacts. The path for current flow on each set of contacts is through the movable contacts to the common terminal. In the de-energized state, contacts C2 are closed and C1 are open. When the coil is energized, the coil attracts the movable contacts to closed contacts C1 and open contacts C2.
Figure 9 shows a contactor assembly drawing. Note the solenoid assembly, moving armature and contact assembly, and the stationary contacts with terminal connections for line and load wiring. Also, note from the picture that the contacts are normally encased to protect the contact assemblies.
Figure 9: Contactor Construction
Contactor assemblies are frequently made with main contacts that serve to connect and disconnect the main power circuit and auxiliary contacts (both normally open and normally closed) that operate with the main contacts. The auxiliary contacts are then available for use in the control circuit of this or another machine.
Auxiliary contacts are frequently used to seal in a coil. "Sealing in" is when a parallel path for current flow is formed to keep a coil energized after the original path of current flow is interrupted. Auxiliary contacts of the coil being sealed in are commonly used to complete the parallel path for current, but this is not always the case. Figure 10 shows a simple control circuit using a magnetic contactor to illustrate sealing in.
Figure 10: Simple Control Circuit Showing Seal-In Contact mA
Magnetic contactors are electromagnetically operated devices that serve to provide a safe, convenient way to connect and disconnect circuits. The magnetic circuit of this type of contactor consists of a magnet assembly, a coil, and an armature. The current flowing through the coil causes a magnetic flux to be set up in the iron the coil is physically wrapped around. The alternating magnetic flux (if it is an AC contactor) produces heat, which is reduced by the use of laminated cores.
The magnet assembly is simply the stationary part of the contactor. The coil is supported by, and surrounds part of, the magnet assembly to induce magnetic flux in the iron when the coil is energized. Figure 11 shows the essential parts, including the magnet, coil, and armature.
Figure 11: Magnetic Contactor Assembly
The armature is the moving part of the magnetic circuit. When energized, the coil induces a magnetic flux in the iron core and attracts the armature, which moves toward it. When the armature has been attracted to its sealed position (closed), it completes part of the magnetic circuit.
When the armature has sealed in, it is held tightly against the magnet assembly. Notice in Figure 11 that an air gap exists even when the armature is in the sealed position. This is because, when the coil is de-energized, residual magnetism is inherent in the magnet assembly. The air gap in the iron circuit prevents the residual magnetism from being strong enough to keep the armature held in its sealed-in position.
There are four basic types of electromagnetic contactors (Figure 12):
Figure 12: Magnetic Contactors
When a magnetic controller is in its OPEN position, a large air gap exists between the armature and the magnet assembly. The impedance of the coil is low, and thus, when the coil is energized due to the air gap, it will draw a high inrush current. As the armature moves closer to the magnet assembly, the air gap gets smaller and smaller. The coil current drops off until the armature seals into its CLOSED position. This inrush current is typically 6 to 10 times the sealed-in value.
Magnet coils that are energized by AC voltage should never be connected in series. This is because, if one contactor seals in ahead of the second, the increased impedance of the circuit will reduce the second coils current so that the second device will either not pick up or will pick up but not seal. AC magnetic coils should, therefore, be connected in parallel.
The ratings of magnetic coils are usually given in volt-amperes, or VA. An example would be a coil rated at 600 VA inrush current and 60 VA sealed-in current. The inrush current would then be 600/120 =5 amps.
If the applied voltage to a magnetic contactor is too high, the coil will draw more than its designed current. Excessive heat will result, and this will cause early failure of the insulation of the coil. In addition, the magnetic pull will be higher. This will cause the armature to close with excessive force. This, in turn, will result in a wearing of the contact faces,contact bounce, and shortened contact life.
When the applied voltage is too low, similar effects occur. There will be a low coil current applied that will reduce the magnetic pull. On some types, especially vertical action, this may result in a contactor that picks up but does not seal, resulting in a continuous draw of inrush current. It will quickly burn up. Another effect is chattering as the coil strains to pick up and seal in its armature.
AC magnetic contactors have a certain hum associated with their operation. This noise is mainly because of the changing magnetic pull due to the alternating flux in the magnet. This humming, and changing magnetic fluxes, will cause small mechanical vibrations.
Excessive chattering and loud humming can result when:
Some larger magnetic contactors, especially older clapper models, have arc chutes installed. Inside these arc chutes are heavy copper coils, or blowout coils. These are mounted above the main contacts and are in series with them to provide arc suppression. Blowout coils are installed for contacts opening under AC and DC loads. The electric arc is similar to that found during the welding process.
Contacts that are subject to frequent interruption of large currents suffer a destructive burning if the arc is not suppressed or extinguished. Magnetic blowout coils work on the principle of motor action; that is, the arc is lengthened and extinguished by the magnetic field setup due to a current-carrying conductor. Since the blowout coil is in series with the main line contacts, the strength of the magnetic field setup and the resultant extinguishing action will be in proportion to the size of the arc.
Figure 13 shows a section of a magnetic blowout coil with an arc conducting between the contacts. Figure 14 shows the process of lengthening the arc. At first, the arc begins to deflect due to the blowout coil field. Next, as the contacts open further, the magnetic field lengthens the arc, and it moves near the tip of the horns. Finally, the arc is so lengthened that it is extinguished and unable to conduct.
Figure 13: Magnetic Blowout Coil
Figure 14: Lengthening the Arc
Although the power circuit can be single-phase or three-phase, the control circuit to the magnet coil is always single-phase. The control circuit includes:
Figure 15 shows a Size One starter control circuit.
Figure 15: Size One Starter Control Circuit
Published charts list identification numbers, ratings, and operating characteristics of magnetic coils. These charts list the rated voltage and the coil volt-amperes for both inrush and sealed conditions. AC magnetic coils, in general, are designed to operate on line voltages fluctuating as much as 15% below, and 10% above, nominal rating. DC coils have corresponding limits of 20% below, and 10% above, nominal rating.
The holding circuit interlock is a normally open (NO) auxiliary contact provided on standard magnetic starters and contactors. It closes when the coil is energized to form a holding circuit for the starter after the START button has been released. These are typically mounted on the upper-left portion of magnetic contactors.
Auxiliary contacts are frequently used to seal in a coil. Sealing in creates a parallel path for current flow to keep a coil energized after the original path of current flow is interrupted. Auxiliary contacts of the coil being sealed in are commonly used to complete the parallel path for current, but this is not always the case. Figure 16 shows a simple control circuit using a magnetic contactor to illustrate sealing in. The starting sequence for Figure 16 is shown below.
Figure 16: Simple Control Circuit
Starting sequence is a series of events that occurs to energize a machine once the sequence has been initiated by a pilot device, either manual or automatic. When the START button is pushed, the M coil is energized, which will close the M contacts, thus keeping the M coil energized when the START button is released. The M coil is now sealed in. The M coil also closes the M contacts, energizing the motor. The "M" designation used here is frequently used in control circuits to designate the main contactor that controls the switching of line power to the device being controlled. Multifunction controllers frequently do not use M but rather more specific designations such as F or R for forward and reverse.
Control circuits frequently control more than one contactor, such as in a two-speed motor control circuit or a control circuit for controlling the direction (forward or reverse) on a motor. In many situations such as this, equipment damage could result if both contactors were closed at the same time.
Two methods are used to provide an interlock to prevent this from occurring. First is an electrical interlock. A "b" contact from each contactor is in series with the operating coil from the other contactor. Thus, if contactor A was energized, its open "b" contact would prevent energizing contactor B. The opposite would be true if contactor B was energized.
The second method employed is a mechanical interlock. To accomplish this, the two contactors are physically mounted side-by-side in the control box. A mechanical linkage that prevents both contactors from being closed at the same time connects them. If one contactor was closed and something occurred to energize the other contactor, the coil would be energized, but motion of the contact assembly would be physically blocked.
Symbol 15, shown previously in Figure 7, represents the heating element of an overload relay. Overload relays are devices found on all motor controllers in one form or another. The current that a motor draws while running is directly proportional to the load on the motor. An overload condition, whether caused by mechanical or electrical fault, will result in increased current flow.
Overload protection is achieved in almost all controllers by placing heating elements in series with the motor leads on multiphase motors. These heater elements activate electrical contacts, which open the coil circuit when used on magnetic controllers. When used on manual starters or controllers, the heating elements release a mechanical trip to drop out the line contacts. Older controllers use two overloads, while newer units are required to have three overloads in accordance with changes in the National Electrical Code.
The overload relay is sensitive to the percentage of overload; therefore, a small overload will take some time to trip the relay, whereas a heavy overload will cause an almost instantaneous opening of the circuit. The overload relay does not give short-circuit protection, however. It is quite possible that, under short-circuit conditions, the relay might hold long enough to allow considerable damage to the motor and other equipment.
Short-circuit protection is provided by installing either a fused disconnect or a circuit breaker ahead of the motor in the main feeder lines.
There are three types of overload relays in general use today. The first uses a low-melting-point metal that holds a ratchet assembly, as shown in Figure 17.
Figure 17: Melting-Pot Relay for a Thermal Overload
When the metal is heated beyond the melting point, the ratchet releases, causing a set of contacts to open in the control circuit and open the main line contactor.
The second type of overload device, shown in Figure 18, uses a bimetallic element. The bimetallic element is made of two different metals bonded together. When heated, the metals expand at different rates, and the element bends. The resultant motion releases a trip mechanism that opens contacts in the control circuit, and the main line contactor trips open.
Figure 18: Bimetallic Type of Thermal Overload
Figure 19 shows the third type of overload relay the magnetic type. A magnetic trip element uses an electromagnet in series with the circuit load. With normal current, the electromagnet is not affected. As load current increases above the setpoint, the relay opens a set of contacts in the control circuit, and the main line contactor trips open.
Figure 19: Magnetic Overload
Overload relays must be reset after each tripping, either automatically or manually. The automatic reset type should not be used except on equipment that is so designed. There can be no danger to life or equipment from the restarting of the motor. After the overload relay has been tripped, it requires a little time to cool so that there is some delay before resetting can be accomplished.
Factors that determine the overload relay thermal units or overload heaters are:
Motors with the same speed and horsepower do not necessarily have the same full-load current. Refer to the motor nameplate for full-load current and not to published charts. Charts tend to show averages of normal full-load currents. The full-load current of a specific motor may be different. Selection tables are usually based on continuous duty motors with a service factor of 1.15 operating under normal conditions.
Bimetallic overload relays that are ambient-compensated are designed for one particular situation: when the motor is at a constant temperature and the controller is located somewhere else where the temperature varies. If standard overload relays are used, it may not trip consistently at the same level or motor current if the temperature of the controller has changed. The surrounding temperature affects standard thermal overload relay.
To compensate for temperature variations that the controller may be subjected to, an ambient-compensated overload relay should be selected. Its trip point is not affected by temperature, and so it will consistently trip at the same value of current.
The last symbol in Figure 7, symbol 16, is a rotary selector switch. A rotary switch is a multicontact switch with the contacts arranged in a full or partial circle. Instead of a pushbutton or toggle, the mechanism used to select the contact moves in a circular motion and must be turned.
Rotary switches can be manual or automatic switches. An automobile distributor and the ignition switch on a motor vehicle are rotary switches (Figure 20). Some rotary switches are made with several layers or levels. This arrangement makes possible the control of several circuits with a single switch.
Figure 20: Rotary Snap Switch
Switches can be either automatic or manual. A manual switch is a switch that is turned on or off by an operator. Examples of common manual switches, such as the toggle, pushbutton, or rotary switches already covered, are a light switch, a dryer start button, and a TV channel selector switch. Each of these requires operator action to initiate a change in a control system.
An automatic switch is a switch that is controlled by a mechanical or electrical device, there is no need to turn an automatic switch on or off. Two examples of automatic switches are a thermostat and the distributor in a motor vehicle. The thermostat will turn a furnace or air conditioner on or off by responding to the temperature in a room. The distributor electrically turns on the spark plug circuit at the proper time by responding to the mechanical rotation of a shaft. Even the switch that turns on the light in a refrigerator when the door is opened is an automatic switch.
Automatic switches are not always as simple as the examples given above. Limit switches, which sense some limit such as fluid level, mechanical movement, pressure, or an electrical quantity, are automatic switches that are sometimes quite complicated.
Any switch that turns a circuit on or off without operator action is an automatic switch. Figure 21 shows the symbols for various automatic switches commonly used.
Figure 21: Symbols of Various Automatic Switches
Symbols 1 and 2 shown in Figure 22 represent normally open and normally closed liquid-level or float switches. Float switches take many forms in their physical or mechanical construction. They consist of one or more sets of contacts, either normally open or normally closed, operated by a mechanical linkage. Many float switch units, as well as other pilot devices, use a mercury switch in place of metallic contacts. The simplest mechanical arrangement for a float switch, shown in Figure 22, would be a pivoted arm having the contacts fastened to one end and a float suspended from the other end.
Figure 22: Float Switch
As the water level rises, it would lift the float, thus moving the contact end of the level downward and either making or breaking the contact, depending on whether the stationary contact were mounted above or below the arm. If a single-pole, double-throw action of the contacts were desirable, then one stationary contact could be mounted above and one below the center of the arm. If the float were all the way up, it would make the lower set of contacts, and if the float were all the way down, it would make the upper set of contacts.
Symbols 3 and 4, shown in Figure 21, represent normally open and normally closed vacuum or pressure switches. This mechanical motion is used to operate one or more sets of contacts. A typical pressure switch design using a bellows as the pressure-sensing element is shown in Figure 23. Two other common sensing elements used are the diaphragm and the bourdon tube. The type of detector is determined by the system requirements. Most devices of this type have a means to adjust the setpoint of the sensing device.
Figure 23: Pressure Switch, Bellows Type
Symbols 5 and 6 shown in Figure 21 represent temperature-activated switches, more commonly called thermostats. Many different types of thermostats are available that employ different methods of sensing temperature. The two most common are bellows and bimetallic strips. As in the pressure switch, the mechanical motion of the sensing elements is used to operate a set of contacts. A typical thermostat is shown in Figure 24.
Figure 24: Thermostat, Bellows Type
Symbols 7 and 8 represent flow switches that are used to sense the flow of liquid, air, or other gas through a pipe or duct and to transform this flow or lack of flow into the opening or closing of a set of contacts.
One type of flow switch, shown in Figure 25, uses a pivoted arm that has contacts on one end and a paddle or flag on the other end. The end with the paddle or flag is inserted into the pipe so that the flow of liquid or gas causes a lever to move and open or close the contacts.
Figure 25: Flow Switch, Paddle Type
Symbols 9, 10, 11, and 12 shown in Figure 21 represent timer contacts that are normally operated by a timing relay. This type of relay and contact arrangement provides two important advantages of automatically controlled circuits: sequencing and delaying events in a control system. Many types of timing relays are available that can be adjusted to give time delays of as little as a fraction of a second to as much as several minutes. Moreover, extremely long time delays, up to several hours, are possible with timing relays that are motor-driven. Since most industrial control systems do not run through unattended repetitive cycles, the timing relays for such installations are generally non-cyclical. Thus, timers are used to separate events in a control-starting sequence that occurs instantaneously from those that are delayed. Instantaneous events are those that occur as soon as a start circuit is initiated, the only delay being the time it takes coils to operate or contacts to open or close. Delayed events are those that have some type of controlled delay provided by a pilot device.
Common designs are pneumatic, dashpot, and motor-controlled timers. Motor-controlled timers are generally used for operations that are repeatable, such as traffic signal controllers and sequentially operated, motor-starting circuits. A simple motor timer found in many homes is used to control the wash cycles of automatic washing machines.
A dashpot timer, shown in Figure 26, consists of a plunger which, when the coil of the timer is energized, moves slowly through a bath of oil and closes a contact at the end of its stroke. The dashpot is usually provided with a bypass near its upper limit of travel so that the contact is permitted to close with a snap action. Snap action allows quick-closing contacts to minimize arcing during the closing cycle. In addition, a valve is included in its construction to allow the oil to flow freely as the plunger falls when the relay is de-energized. Many of these relays also have an adjustment that can be varied to change the time delay.
Figure 26: Time Delay Relay, Dashpot Type
Another popular timer is the pneumatic timer. It uses the restricted airflow across a diaphragm to create the time delay. The airflow passes through an adjustable orifice so the time delay is adjustable.
The time delay of a timing relay can be applied when the relay is energized or when it is de-energized. Symbols 9 and 10, shown in Figure 21, represent timer contacts that have timed closing after energization (TCAE) and timed opening after energization (TOAE), respectively. Symbols 11 and 12, also shown previously, represent timer contacts that have timed opening after de-energization (TOAD) and timed closing after de-energization (TCAD), respectively. Some timers may be equipped not only with contacts that are delayed but with contacts that operate instantaneously.
Symbols 13, 14, 15, and 16, presented in Figure 21, represent direct-actuated limit switches. Limit switches use an arm, lever, or roller protruding from the switch that will be bumped or pushed by some piece of moving equipment (Figure 27). This movement is then used to operate a set of contacts. Limit switches vary widely in size and design. There are large, rugged devices for heavy industrial use, such as that shown in Figure 27 , and smaller, more accurate and precise units that use micro switches that can operate on very minute movements of the operating lever.
Figure 27: Limit Switch
Symbols 13 and 14 of Figure 21 show limit switches in their normally open or closed condition, and symbol 15 represents a normally open limit switch, which is held closed; symbol 16 represents the opposite.
Symbols 17 and 18 of Figure 21 represent foot switches. Switches of this type are often used in applications that require the machine or process cycle to be started at a time when the operators hands are both engaged in loading or handling the materials. Foot-operated switches are frequently employed for such purposes. Typical examples of foot switches are punch presses, drill presses, and sewing machines. Foot switches are actually limit switches enclosed in a convenient and rugged casing for foot operation and are available in a variety of contact arrangements such as single-pole double-throw, two-pole double-throw, or other arrangements to suit a specific need. Figure 28 shows a typical foot switch.
Figure 28: Industrial Foot Switch
Simple control circuits are sometimes referred to as ladder diagrams in that they are drawn to resemble a ladder. Figure 29 shows a simple control circuit and its components.
Figure 29: Simple Control Circuit and Components
Wiring diagrams will not have wires jumping one another. Wires are shown as crossing each other and, unless specified with a node or dot, are not connected. Mechanical connections, such as those found on double-pole switches and between mechanical interlocks, are shown as broken lines.
Electrical circuits are generally shown by one of two types of diagrams: line diagrams and wiring diagrams. A wiring diagram includes all the components in the circuit and shows the physical relationships between them. Wiring diagrams give the needed information for actually wiring the circuit and allow a troubleshooter to physically trace the wires. However, wiring diagrams often look like an enormous maze of parallel and crossing lines that make it difficult, if not impossible, for someone to recognize and understand the operation of the circuit.
Line diagrams simplify the circuit to a degree necessary to understand the operation of the circuit. Line diagrams, also known as elementary diagrams, do not show the components in their actual physical locations. Control devices, such as relays, contacts, and pushbuttons, are shown on horizontal lines between two vertical lines. The vertical lines always represent the power source. The connections of line diagrams are drawn such that both the function and sequence of operation can be readily determined.
There are two basic types of control circuits: three-wire and two-wire. These designations stem from the fact that for three-wire circuit control, only three wires are required from the ordinary across the line motor starter to the control components. In two-wire control, only two wires are required.
A three-wire circuit uses momentary contact START-STOP buttons and a holding circuit interlock, or maintaining contact across the push-button START switch, to keep the circuit energized after the push-button has been released. This type of scheme provides low-voltage protection. A low-voltage condition or loss of incoming power will cause the starter to "drop out." Figure 30 shows a three-wire LVP control circuit. When power is restored, the starter connected for three-wire control will not pick up automatically since the maintaining contact around the start switch is now open. To restart the motor after a power failure, the pushbutton must be pressed. In this way, a deliberate action must be performed, ensuring a measure of safety.
Figure 30: Three-Wire LVP Control Circuit
A two-wire control circuit is, by its nature, a low-voltage release circuit. A reduction or loss of voltage stops the motor, but when power returns or comes back up to nominal value, the motor will restart. This type of restart can be a safety hazard to both personnel and machinery since power may return without warning. This type of circuit is shown in Figure 31.
Figure 31: Two-Wire LVR Control Circuit
The coil circuit of a magnetic starter or contactor is distinct from the power circuit. The coil circuit can be connected to any single-phase power source and the controller would be operable, provided the coil voltage and frequency match the service to which it is connected.
When the control circuit is tied back to lines 1 and 2 of the starter, the voltage of the control circuit is always the same as the power circuit, and the term common control is used to describe this relationship. Other variations include separate control and control through a control power transformer.
It is sometimes desirable to operate pushbuttons or other control circuit devices at some voltage lower than the motor voltage. For example, if the main service is 480 volts, this voltage may need to be reduced to 120 volts. A fuse is often used to protect the X1 side of the transformer secondary while the other side is grounded.
NEC Section 250 states the requirement for grounding the secondary of 120-volt control transformers. According to the rule, any 120-volt, two-wire circuit must normally have one of its conductors grounded. Other systems are also required to be grounded, although they have no bearing on this aspect of motor control centers. This specific requirement has caused some difficulty when applied to control circuits derived from the secondary of a control transformer that supplies power to the operating coils of motor starters, contactors, and relays. For example, there may be cases where a ground fault on the hot leg of a grounded control circuit can cause a hazard to personnel by blowing the protective fuse or operating a circuit breaker and thus shutting down the entire industrial process in a sudden, unexpected way. This may result in excessive loss of production time and/or damage to equipment that is stopped abruptly. A sudden shutdown to a ground fault in the hot leg of a grounded control circuit would be objectionable in this instance.
NEC 250.21(3) provides an exception to this rule. A 120-volt control circuit may be operated ungrounded provided ALL of the following conditions are met:
When it is desired to select the function of a motor controller either manually or automatically, a hand-off-automatic switch is used. Figure 32 shows a typical control circuit with a standard duty, three-position selector switch.
Figure 32: Typical Control Circuit
When the switch is turned to the HAND position, the M coil is energized continuously and the motor runs. In the AUTOMATIC position, the motor will run whenever the contact in line with the M coil is closed. A timing relay, float switch, or any other type of control device can control this contact.
Figure 33 shows a three-position, double-break selector switch. This is used for manual or automatic control in much the same way as the previous circuit.
Figure 33: Three-Position Double-Break Selector Switch
Simply interchanging any two of the three incoming leads can reverse a three-phase motor. When magnetic starters are used, reversing starters reverse the motor direction, as shown in Figure 34.
Figure 34: Reversing Starter
Reversing starters in conformity with NEMA standards interchange lines L1 and L3 or phases A and C. To accomplish this, two starters are needed, one for the forward direction and one for the reverse direction (Figure 35).
Figure 35: Reversing Contactor Line Connections
Interlocking is used to prevent both contactors from being energized simultaneously or closing at the same time. This would cause a short circuit. Three basic methods of interlocking are:
Mechanical interlocks are assembled at the factory and are physically located between the forward and reverse contactors. The interlock locks one contactor out at the beginning of the stroke of either contactor to prevent both from closing simultaneously.
A broken or dotted line indicates a mechanical interlock. Often, the dotted line will be broken in the middle and angled with a solid bar at the middle junction.
This method is an electrical method of preventing both starter coils from energizing together. Figure 36 shows an example of pushbutton interlocking.
Figure 36: Pushbutton Interlocking
When the forward pushbutton is pressed, the F coil is energized, and the normally open F auxiliary contact closes to maintain the circuit to operate the motor in the forward direction. Pressing the reverse pushbutton automatically breaks the circuit in line with the F coil, dropping the forward coil out and energizing the reverse (R) coil.
Reversing the direction of motor rotation on a repeated basis is not recommended, since this may cause the overload relays to overheat and disconnect the motor from the circuit. NEMA specifications require a starter to be derated or to select the next larger size starter whenever it is going to be used for plugging or reversing at a rate of more than five times-per-minute.
This method is also an electrical interlock. It consists of normally closed auxiliary contacts on the forward and reverse contactors, as shown in Figure 37.
Figure 37: Electrical Interlocking
In the forward direction, the normally closed contact (F) on the forward contactor opens to prevent the reverse contactor from being energized.
A method by which starters are connected so that one cannot be started until another is energized is called sequence control. This is required whenever auxiliary equipment associated with a machine, such as a priming pump for a drain pump, must be operating to prevent damage to the main machine. Figure 38 shows a standard starter wired for sequence control.
Figure 38: Sequence Control
The control circuit of the M2 coil is wired through the maintaining contacts of the M 1 coil. The result is the second starter is prevented from starting until after the M 1 coil is energized.
Many motors can be started automatically with one START-STOP button, as shown in Figure 39.
Figure 39: Automatic Sequence Control
The power supplies to motor control centers are usually circuit breakers located in switchgear. Familiarity with the symbols and conventions of single-line diagrams for both switchgear and motor control centers is necessary to understanding the overall conception of the motor control center as a unit. Standard symbology was discussed in a previous section. Figure 40 shows some standard electrical symbols and conventions.
Figure 40: Standard Electrical Symbols and Conventions
The Rotating Apparatus column of Figure 40 shows one-line elementary and plan symbols for various motors. Note that the number inside the circle indicates horsepower, and the number at the lower right of the circle indicates speed in RPM. The absence of a number indicates 1,800 RPM.
Under the Switching and Protective Apparatus column of Figure 40, notice the numbers located to the left of the circuit breakers. An example is:
The top number, in this instance, indicates the trip rating of the breaker, and the lower number indicates the frame size. This is not true in all one-line diagrams. Many times, these two numbers will be interchanged, and the top number will indicate the frame size, whereas the bottom number indicates the trip rating of the breaker.
This annotation denotes that the trip rating is adjustable. Many modern breakers have this feature. It can consist of either an adjustable setting on the breaker faceplate or a rating plug that is inserted into a special socket. These rating plugs are often shipped separately from the breakers and must be checked upon installation to ensure that they are installed according to the specifications and prints.
The static trip devices shown on the motor starters rated above 600 volts, as well as some circuit breakers, are denoted by the abbreviation ST. These are micro-logic (digital) units that are adjustable over a wide range of available parameters including ground fault pickup, ground fault delay, instantaneous overcurrent, long time delay, long time pickup, motor-starting current, phase failure, and others.
Figure 41 shows other apparatus and devices associated with motor control centers, conduit and raceways, wire and terminal connection location symbols, and PLC input/output symbols.
Figure 43 is a one-line diagram of a 480-volt substation powering many motor control centers. This diagram indicates that this substation is a main-tie-main bus scheme. This means there are two main breakers and a tie breaker. The K symbol located in the box above the tie breaker and in the circle beside the main breakers indicates that these breakers are mechanically interlocked with a key system. The dashed lines connect both mains and the tie. This indicates that only two breakers can be shut at any one time. Other significant items to be seen:
Figure 41: Standard Symbology
Figure 42 and Figure 43 are one-line diagrams of 480-volt, three-phase motor control centers. Be certain to become familiar with all symbols on these diagrams, as well as all abbreviations.
The upper left portions of both prints show the incoming power supply. MCC 3 is powered from 480-volt substation number 2. The motor control centers in-breaker is a 1,600-amp frame, 1,200-amp trip circuit breaker. An ammeter and a voltmeter monitor the voltage and current drawn by the motor control center. Current transformer and potential transformer ratios are indicated as well as the number of each one required.
Each starter is labeled by size. MCC 3 indicates this with "Size 5," for example, written next to the starter. MCC 12 simply places a number beside the starter.
The number and letter combinations shown at the lower edge of the dotted lines indicate which position each starter bucket is located in the motor control center. For example, 2D indicates that this starter is in section 2, position D. Manufacturers vary in regard to this labeling, so refer to the drawing that pictorially shows the motor control center and all bucket position labels.
Figure 42: Three-Phase, 480V One-Line Diagram
Figure 43: Three-Phase, 480V One-Line Diagram
Refer to Figure 44, an elementary diagram, for several starters located in motor control center 3. The first thing to notice is the absence of a start switch. Instead of a start switch, an input from a programmable logic controller (PLC) is inserted in the control circuit. This symbol, for motor number 301M1, is at coordinate 08E. Coordinate numbers are located vertically down each diagram, and the letters run horizontally across the top of the page. Notice how some devices listed at the right sides of each diagram have a reference number. For example, for motor number 301M1, the M coil is described as a contactor and has the numbers 1, 2, 3, 6, 11, and SP. These numbers indicate which lines (vertical numbers) have contacts or electrical connections to this coil. SP indicates a spare.
The PLC input at coordinate 08E has the number 0:072/01 above it.
0 indicates a PLC output
072 indicates the PLC rack and slot
01 indicates the point
PLC input signals, such as those shown at coordinates 11J and 12J, have a similar address number.
Figure 44: PLC Elementary Diagram
To simplify electrical diagrams, many switchgear devices are not labeled with reference to their function. Standard numbers are commonly used instead of standard abbreviations. These standard numbers, like abbreviations, allow the designer to produce an uncluttered drawing by minimizing the amount of writing.
The following is a list of standard numbers for labeling switchgear devices. This list can be used for quick reference; memorizing the numbers is not necessary. Numbers commonly used will become as familiar to you as common abbreviations.
Standard numbers for switchgear devices:
The following gives a brief description of the function of each of the switchgear devices in the previous list.
When the standard numbers are used on an electrical diagram, they are sometimes preceded (or followed) by an additional number or letter. This is used for more precise identification. For example, there may be differential relay protection on two different buses. The designation of a differential relay is 87. However, if this number is used for both buses, confusion could result. If the two buses are the 6,900-volt bus and the 4,160-volt bus, the 6,900-volt bus relay could be 687, and the 4,160-volt bus relay could be 487.
Having been introduced to the common component parts of a control circuit, the learner should now be able to understand the development of a control circuit. Control circuits are usually developed in one of two ways:
In the following example, we will improve upon an installed circuit in a step-by-step manner, such as a series of improvements performed at different times. This controller is being used to start and stop a motor, but be aware that a control circuit can be used to perform the function of any electrical device, such as turning on and off lights, opening and closing a motor-operated valve, or energizing and de-energizing a heater.
The basic circuit, shown in Figure 45, controls a pump that pumps water from a storage tank into a pressure tank.
Figure 45: Pump Control Circuit
Figure 46 shows the physical arrangement of the pump and the two tanks, along with the final control components. As the original circuit stands, it is a manual operation requiring that the START button be pushed whenever the water is too low in the pressure tank. The pump is allowed to run until the tank is observed to be full. The operator then pushes the STOP button, securing the pump and stopping the flow of water into the pressure tank.
Figure 46: Water System Configuration
The owner decides that a float switch should be installed in the pressure tank near the top so that the operator need only push the START button, thus energizing the pump and starting water to flow into the tank. When the level of the water has reached float switch 1(FS1), its contacts will be opened, stopping the pump and the flow of water. The function to be performed by the float switch is that of STOP. Therefore, it must be a normally closed contact and must be connected in series with the original STOP button, as shown in Figure 47.
Figure 47: Pump Control Circuit - Phase 1
After operating with this control for some time, the owner decides that it would be more convenient if the pump is started automatically as well as stopped automatically. They installed another float switch to maintain the lower level of the tank. This section of the control circuit requires that the pump be started whenever the water reaches a predetermined low level. The control function desired is that of START. The float switch (FS2) must have a set of normally open contacts that will close whenever the water drops to the lowest desired level. These contacts must be connected in parallel with the original START button to perform the function of start for the motor, as shown in Figure 48.
Figure 48: Pump Control Circuit - Phase 2
After some time of operation, it is discovered that, occasionally, the storage tank drops so low in water level that the pump cannot pick up water. This requires a control to prevent the pump from starting whenever the storage tanks water level is low and to stop the pump if it is running and the water reaches this low level in the storage tank. The new control will perform the function of STOP for the pump.
This function can be obtained by the installation of a float switch to sense the extreme low level of water in the storage tank. The float switch (FS3) was installed and adjusted to open a set of contacts whenever the water in the storage tank reached the desired low level. Because the control function to be performed is that of STOP, float switch 3 must have normally closed contacts, which will be opened whenever the water level drops to the set level of the float switch. It is, therefore, wired in series with the other stop components, as shown in Figure 49.
Figure 49: Pump Control Circuit - Phase 3
Later, it is decided that the pressure placed on the line by the pressure tank when it is full is insufficient for the needs of the plant. The owner requests the installation of the necessary components and controls to maintain a pressure on the tank by the addition of the proper amount of air to the top of the tank. In order for the proper balance of water level and air pressure to be maintained at all times, air must be let into the tank only when the water level is at its highest position and the pressure is below the desired discharge pressure of the tank.
To achieve this, we will install a solenoid valve in the air supply line that will allow air to flow into the tank when the coil of the solenoid valve is energized. Now, we can install a pressure switch in the top of the tank that will sense the pressure in the tank at all times. This pressure switch will perform the function of START for the solenoid valve. When the pressure is lower than the setpoint of the pressure switch, its contacts must close and complete the circuit to the solenoid. If the water is below its top level when the pressure drops, we do not want the solenoid valve to open; therefore, we require the function of stop in regard to water level to prevent air being put into the tank when it is not desired.
If float switch (FS1) is of the double-pole variety, having one normally-open and one normally-closed set of contacts, we can wire it into the circuit, as shown in Figure 50.
Figure 50: Pump Control Circuit - Phase 4
The circuit for the solenoid valve is a two-wire control requiring that both FS1 and pressure switch (PS1) be closed in order that air will be placed into the tank by the energizing of the solenoid valve. When the water level reaches its highest point, FS1 is activated. The normally closed contact in the pump circuit will open, and the normally open contacts in the solenoid circuit will close. If the air pressure is low, the contacts of PS1 will be closed until the pressure increases to normal and opens PS1, satisfying the requirements of the circuit as specified.
The circuit in Figure 49 gives a degree of hand operation because the pushbuttons were left in the circuit. It will be preferable to have either a definite automatic operation or hand operation as desired by the operator. The necessary changes to give hand, off, and automatic operation are shown in Figure 51.
Figure 51: Pump Control Circuit - Final Arrangement
The starting sequence for Figure 51 above would be as follows:
If the learner had been charged with the responsibility of developing the final circuit of Figure 51, the learner would have had certain specifications or requirements as to the proper functions or operation of the completed circuit as indicated below:
To develop this circuit properly from this set of specifications, the procedure would be the same as that we have followed, if we assume that the circuit was built up a little at a time by going back and adding control components to the original manual circuit.