CONTROLLING MOTOR STARTING
Motor starters generally fall into two categories: full-voltage or reduced-voltage. The choice of which type of starter to use depends on various factors, such as current-carrying capacity of plant wiring, ability of plant power supplies to absorb power transients, size of the motor, and other special control requirements. The first type to be covered is the full-voltage starter.
Full-voltage starters, sometimes referred to as ''across-the-line'' starters, are the most widely used type of starter. A starter of this type, shown in '''Figure 1''', directly connects the motor leads and line leads together through either a manual starter or the main line contacts of a magnetic starter and does not provide any means of reducing the applied voltage or limiting the starting current.
'''Figure 1: Motor Controller Wiring Diagram'''
The full-voltage starter is used on almost all three-phase, squirrel-cage, and single-phase machines. The use of this type of starter for squirrel-cage motors is limited by the strain imposed on the installed wiring systems and by the starting current and torque when the motor is first energized.
Whenever the starting of a motor at full voltage would cause serious and unacceptable voltage transients or excessive torque when unneeded, reduced-voltage starters are used to reduce the starting transients. There are, however, other reasons for using this type of control. The effect on the equipment must be taken into account in the selection of motor starters. When a large motor is started across the line, it puts a tremendous strain or shock on such things as gears, fan blades, pulleys, and couplings. When the load is heavy and hard to bring up to speed, reduced-voltage starting may be necessary. Belt drives on heavy loads are apt to have excessive slippage unless the torque is applied slowly and evenly until full speed is reached.
Overload devices have become increasingly reliable, giving the motor every opportunity to make a safe start. Using reduced voltage in an attempt to prevent overheating during starting and acceleration is a wasted effort. The acceleration time will increase, and the overloads will still trip.
Reduced-voltage starting lowers the applied starting torque, thus minimizing the shock on the driven machines. Regardless of the method used to reduce the voltage to a motor, it must be kept in mind that the current will also be reduced, as will the torque. With this in mind, it becomes apparent that a motor that will not start on full voltage at a given load will also not start under the same load at a reduced voltage and subsequent reduced current. Any attempt to use a reduced-voltage starter will not be successful in accelerating troublesome loads.
As voltage is reduced, current is reduced proportionally; however, torque decreases with the square of voltage.
Example: At 70 percent applied voltage, find the percentage of starting current for a standard motor that draws six times normal full-load running current.
(.70) (600 percent) (FLA) = 420 percent FLA
If the voltage were reduced to 80 percent of its nominal value, the torque would be:
(.80) (.80) = 64 percent
One of the most common functions of reduced-voltage starting is to reduce the starting current of induction motors. The rate-of-change of starting current is confined to prescribed limits. In other words, the motor presents a predetermined, current-time picture to the supply system.
This current-time picture for a given area is maintained and regulated by the utility company serving the area. The power company will attempt to maintain a reasonably constant voltage at points of supply so that lights do not dim or flicker noticeably. The success of the power company in this attempt depends on the generating capacity to the area, including transformer and line-loading conditions and adequacies as well as automatic voltage regulating equipment. Transient overloading of the power supply may be caused by sudden high surges of reactive current from large motors on starting and by pulsations in current taken by electrical machinery driving reciprocating compressors or X-ray equipment as well as other devices.In this regard, difficult loads that may present a starting current problem, when started singly or in combination, are regulated by the power company. Some type of reduced voltage and, therefore, reduced current will be required.
Power company rules and regulations vary. Installations may be under some of the following regulations, though this list does not include all the possible restrictions.
The most common methods of starting three-phase induction motors are:
Regardless of the method used to provide reduced-voltage starting, it must be kept in mind that the starting torque of the motor is also reduced. If a motor is not capable of starting its load under across-the-line conditions, the application of reduced-voltage starting only aggravates the situation because of the reduced starting torque. The torque of an induction motor is a function of the square of the rotor current, or approximately the square of the line current. If the starting voltage is reduced by 50 percent, the motor current will be reduced to 50 percent of normal, but the torque will be reduced to 25 percent of normal.
There are four basic ways to accomplish reduced-voltage starting. Each method is described below and is graphically shown with the required motor connections and contact arrangements to accomplish the reduced-voltage starting.
Line-resistance starting, as shown in '''Figure 2''', uses suitable high-current, low-ohmic value resistances in each line. It is designed to connect the machine initially to the power source through the three S contacts and resistors. When the motor comes up to speed, the resistors are short-circuited by the R contacts, after which the S contacts are opened. Should there be a power failure or a large voltage dip, the undervoltage relay UV will drop out to disconnect all other relays and contactors and stop the motor. Upon the return of power, the motor is started in the usual way.
'''Figure 2: Line-Resistance Starting Configuration'''
The starting sequence for this control circuit would be as follows: when the START button is pressed, the UV relay is energized and is sealed by its own UV contact. Another UV interlock also closes to permit relay 1CR to operate. When the latter is actuated, the S contactor picks up through a 1CR contact, whereupon the motor starts at reduced voltage since line resistors are inserted. After the S contactor times out, contact S-TC closes to permit relay 2CR to pick up, the operation of which causes the R contactor to close its main contacts and short-circuit the line resistors. At the same time, the NC R contact in the 1CR relay circuit opens to disconnect the R contactor. When the latter drops out, all associated S contacts open, but the 2CR relay remains energized through its own sealing contact.
Line-reactance starting, as shown in '''Figure 3''', uses suitable iron-core reactors in place of the resistances used in '''Figure 3 '''to accomplish the same result.
'''Figure 3:Line Reactance Starting Configuration'''
When a three-phase motor is started at reduced voltage by the line-resistance or the line-reactance method, the line and motor currents are, of course, the same. The starting current will be less than that existing with full-voltage starting only to the extent that line resistors or line reactors incur voltage drops. The autotransformer method of starting, on the other hand, supplies the motor with reduced voltage by transformer action, and this implies that the line-side, or primary, current will be reduced during the start sequence just as the secondary voltage is reduced.
Autotransformer starting uses tapped autotransformers in open delta connection to reduce the motor voltage. Standard voltage taps are at points that yield motor voltages of 50, 65, and 85 percent of rated value.
A wiring diagram of an autotransformer starting circuit and its control circuit is shown in '''Figure 4'''. Note that the START-STOP station and its two relays are operated from the secondary of one control transformer, while a DC RUN contactor R is wired through a bridge rectifier from the secondary of another control transformer. The S contactor and the CR relay are actuated from the full-voltage source.
With the pressing of the START button, the UV and TR relays are picked up and sealed-in by their interlocks, and another UV contact is closed to energize the coil of the S contactor. When the S contactor operates, it immediately opens the NC contact in the primary of the lower control transformer (thus electrically preventing the simultaneous operation of contactors S and R, which are also mechanically interlocked), closes an interlock in the CR relay circuit, and closes five main contacts at the autotransformer. The motor, therefore, starts on reduced voltage (65 percent rated voltage in the diagram).
After relay TR times out, its TR-TC contact closes to energize the CR relay. This instantly opens the NC contact in series with the S contactor, which drops out, and closes an interlock in the R contactor circuit, with the de-energization of the S interlock in the R contactor circuit. With the de-energization of the S contactor, the five main contacts are opened, and the motor is momentarily disconnected from the power source. The NC S contact closes next, permits the R contactor to pick up, and causes the three main R contacts to close. The motor now proceeds to run from the full-voltage source.
'''Figure 4:Autotransformer Starting Configuration'''
Wye-delta, or star-delta, starters function on the principles of voltage and current relationships in wye and delta systems. A quick review of these relationships is needed to fully understand the underlying principle.
'''Figure 5''' shows a delta connection. The connections to the three-phase distribution system are made at points A, B, and C.
'''Figure 5: Delta Connection'''
The voltages across the three loads are called ''phase voltages'' and are designated E1, E2, and E3. The currents through the loads are called ''phase currents'' and are designated I1, I2, and I3. By comparing the distribution system shown in the previous figure to the connected loads, we see that the phase voltage and the line voltage are equal.
The current in each phase of the load can be found by dividing the phase voltage by the impedance. Once the current in the phase is found, the line current can then be determined. Because of the 120-degree phase difference, the line current is equal to the square root of three multiplied by the phase current.
'''Figure 6 '''shows graphically why this is so. In keeping with Kirchhoff’s Current Law, the line current in a delta-connected system is the sum of the current in the two sides of the delta to which the line connects. Because of the phase difference, however, the currents cannot be combined by simply adding them.
'''Figure 6: Three-Phase Vector Addition'''
This shows three phases 120 degrees apart. The lines represent individual phases called ''vectors''. Their lengths represent the magnitude of the phase current. The angle of the lines is the phase angle. When combining these current vectors to satisfy Kirchhoff’s Current Law, we have to use vector addition. Doing so, the combined phase currents that comprise the line current will be equivalent to the distance between the ends of the current vectors, or the line current.
'''Figure 7 '''shows the wye connection. In this case, the line currents will be equal to the phase currents, but this time the voltage in each line will be equal to the phase voltage multiplied by the square root of three. This can be proven in much the same way as the current relationships in a delta system were proven.
'''Figure 7: Wye-Connected Load'''
Wye-delta starting, as shown in '''Figure 8''', is used when the motor is designed for delta operation at its rated voltage. The motor phase windings are reconnected by contactors for a wye circuit at starting. As a result, each phase will see the normal line voltage divided by the square root of 3. Reduced-voltage starters are generally designed for a particular motor and for a particular application, and there may be many variations in control circuits used to achieve one of the basic methods of reduced-voltage starting.
'''Figure 8: Star-Delta Starting System'''
To achieve the effect of reduced-voltage starting requires the use of a time delay in the control circuit to achieve the desired delay between energizing the motor in a start configuration and switching to a run configuration. This is accomplished by the use of a time-delay relay. In the case of primary resistance or reactance starters, this relay is required only to energize the coil of the run contactor. When used on an autotransformer starter, this relay must break the circuit to the start contactor and then make the circuit to the run contactor. The use of a time delay relay in this service gives fixed time control. Another method that is sometimes used employs a current relay that opens the start contactor and closes the run contactor when the motor current drops to a predetermined level. This is called ''current limit control''.
'''Figure''' 8''' '''shows a basic diagram of the motor power circuits of a star-delta starting system. The controller connects the motor in a wye configuration on start and then shifts it to a delta configuration for run. In this case, the overload relays are connected in the motor winding circuit, not in the line.
''Open transition'' for a wye-delta starter is so called because there is a moment where the motor circuit isopenbetween the opening of the S contacts and the closing of the 2M contacts. This is shown in '''Figure 9'''.
'''Figure 9:Open Transition'''
A closed transition starter, as shown in '''Figure 10''', has the addition of resistors that function to keep continuity to the motor and provide closed transition from wye to delta. This may be required to prevent power line disturbances. Tracing the power flow through the control circuit and referring to '''Figure 11''', you can see how closed transition is provided.
'''Figure 10: Closed Transition'''
'''Figure 11: Control Circuit'''
When first started, the S and 1M coils are energized, connecting the motor stator in a wye configuration. After a preset time delay, the time delay contact for 1M closes to complete a path for current to flow through the 1A coil. 1A connects resistors around the wye-connected phases as shown in part 2. When coil 1A picks up, it opens a contact to de-energize the S coil, disconnecting the center of the wye configuration. When de-energized, the S contact in line with the 2M coil recloses, allowing that coil to energize and connect the motor to a delta configuration with the transitional resistors in line with the phases. 1A now opens to complete the delta connection.
Overloads for wye-delta starters are selected based on the winding current, not the delta-connected, full-load current. So, if the nameplate on the motor indicates the delta-connected, full-load running current, you will have to divide this value by 1.73 to get the winding current needed to select overload relays.
Part-winding motors are similar to standard squirrel-cage induction motors, except that they have two identical windings that are connected to the supply in sequential order to provide reduced starting current and torque. These two windings operate in parallel, and since only half of the windings are connected to the supply during startup, it is called ''part-winding.''
Part-winding starters are used with motors having two separate and parallel windings in the stator, connected either wye or delta.
This type of starting does not necessarily reduce the starting current but rather starts the motor incrementally, thereby splitting the inrush current in sections. The windings are not designed to operate alone for more than a few seconds. They do not have that sort of thermal capacity. Six overload relays are used, three per winding. The start winding phases carry the identical current that the corresponding phase in the run winding carries. This means that when three overload relays are connected in the three-run phase, windings and the other overloads are connected in the start windings, and full protection for three phases is ensured.
It is important to note that each of the overload relays carries half of the motor current. The overloads, then, should be selected based on half the full-load current of the motor unless otherwise specified.
'''Figure 12 '''shows a part-winding starter with two main contactors that provide two-step starting. Each contactor connects one winding of the motor. The motor is first energized with one winding; then, the second winding is energized in parallel.
'''Figure 12: Part-Winding Starter'''
Advantages of two-step, part-winding starters:
Disadvantages of this method are:
Three-step starting uses a resistor in each phase. The resistors are connected in line with the first winding. After a suitable time delay, the resistor is shunted out, and the first winding will now be at full voltage. Another time delay passes, and the second winding is energized in parallel with the first. '''Figure 13 '''shows a three-step, part-winding starter.
'''Figure 13:Three-Step, Part-Winding Starter'''
Each circuit that we have examined to this point and, in fact, every control circuit of any machine that is controlled, either semi-automatically or automatically, actually has two separate power circuits. One circuit supplies power to the machine being controlled (motor, heater, etc.) and is called main power, and the second circuit supplies power to the control devices that make up the controller. This second power source is normally called control power. When using a schematic or one-line diagram, the control power and main power portions of a circuit should be readily and easily distinguished.
The most convenient method of supplying control power is also the most common: tapping the main power either directly or through a transformer to supply the control power. '''Figure 14''' shows an example of a control power supply that uses a transformer. '''Figure 15''' is an example of a control-power supply that uses one phase of the applied power connected directly to the control circuit at line voltage.
'''Figure 14:Controller with Separate Control Power Supply'''
'''Figure 15:Motor and Motor Control Circuits Using a Common Power Source'''
In both examples, one can see where the main power connection is made. The control power taps into the main power after the manually operated circuit breaker that supplies power to the load from the power supply and before the main line contacts that are operated by the control circuit. This allows the control circuit to be energized anytime power is available to supply the load. It also allows power to be isolated from both the load and the control circuit when necessary for preventive or corrective maintenance. In most cases, the control power is supplied through a set of control power fuses, which provides protection for control devices and allows for de-energizing the control circuit without opening the main breaker.
Control transformers are generally used to supply control power when it is necessary to step down the line voltage to a suitable safe value (115 volts) for the control circuit. For system potentials in excess of 600 volts, this must always be done; apart from the element of safety to personnel, it is impractical and difficult to design such control components as operating coils and pilot devices for the higher voltages.
Typical dry-type control transformers with two primary coils that can be connected in series or parallel for 460- or 230-volt service are used. Such transformers are constructed in sizes up to about 5 kVA and for primary potentials as high as 4,600 volts.
The second method to supply control power is the use of a power supply altogether separate from the one that supplies the main power. An example of this type of control power configuration is shown in figure 15.
Control power supply schemes such as this are used when it is desirable to have a more reliable control power supply than the power supply to the load. Another situation when this might be used is when it is desired to use the same control power supply for several different controllers.
Basic maintenance tools include a flashlight, an air hose or vacuum cleaner, and a small brush. Debris, dirt, and dust can be spotted and removed from contacts and other areas of a motor control center. Light rust and dirt can be vacuumed out. Vacuuming is preferable to an air hose in that an air hose simply redistributes the dirt to other areas of the enclosure. A file or abrasive should never be used on the pole faces of magnets. This could misalign the precise fit between the armature and core and result in heating and noise production.
Dirt and dust on the pole faces should be removed since this produces an excessive air gap, which causes it to hum as the AC changes the magnetic flux of the core.
Terminal connections should be checked to ensure they are tight. This ensures a low resistance path for current flow. Deterioration of a loose terminal connection occurs at a rapidly increasing rate, because the heat resulting from increased resistance of loose connections causes corrosion and greater resistance in the joint until the connection is subjected to a resistance that is too high and the circuit burns.
Oil should never be used on the moving parts of motor starters unless specifically suggested by an individual manufacturer. Oil forms a film over magnet pole faces, attracts dirt, and increases air gaps between the pole faces and air armatures. This could result in excessive noise. Another cause of excessive noise is broken shading coils. These are not always obvious to the naked eye and sometimes may require the use of a magnifying glass if the noise cannot be attributed to anything else. Improper alignment is another possible cause of excessive noise production.
Electrical surges, abrasion from vibration and door openings, and other factors can result in insulation breakdown to ground either in the coil or in the control circuit. This can cause low holding power on the contacts, erratic operation, failure to operate at all, or even hazardous voltage conditions if equipment grounds are not in good condition.
To test the insulation, disconnect the starter from the line at visible disconnect and measure resistances from both power and control circuits to ground with a megger.
If the control circuit is supplied without a control power transformer, a single test to ground from one of the power source leads supply both motor and control circuits would produce an adequate check of both circuits (XXX UNCLEAR). However, if this test shows low resistance, it will be necessary to isolate the two sections to locate the trouble.
Improperly sized overload elements of any of the several types available could cause unnecessary tripping of the contactor and shutdown of the motor if the elements are undersized. It could result in motor burnout if the elements are oversized. Ratings of overload elements should always be checked during periodic maintenance to ensure they exactly match the requirements of the motor.
To meet NEC®code safety requirements, motor starters should always be connected in combination with fuses and a disconnect switch or circuit breaker on the source side. The starter will remain closed if a fuse blows on one phase leg that isn’t used for the control circuit, and the motor will single phase. It is often said that a single-phasing motor cannot be restarted; there is one instance in which it may be restarted, and that is if any other motors are running on the circuit beyond the blown fuse.
This situation contributes to, or results in, a burned-out motor or seized bearings if the load is of any appreciable size due to over-saturation of the iron core, excessive hysteresis, eddy currents, and large ampere flows in the un-faulted phase. Checking fuses for continuity and proper rating should always be an important part of any maintenance procedure for starters.
Contactors are subject to both mechanical and electrical wear during their operation. In most cases, the mechanical wear is insignificant. The erosion of the contacts is due to electrical wear. During arcing, material from each contact is vaporized and blown away from the useful contacting surface.
Critical examination of the appearance of the contact surfaces and a measurement of the remaining contact overtravel will give the user the information required to get the maximum contact life. '''Figure 16 '''shows both new and used contacts.
'''Figure 16:Contact Surfaces'''
Although most controllers are designed with reliability and long life as primary design factors, it is inevitable that trouble will occur. Sometimes, when a controller fails to operate or signs of trouble occur (i.e., smoke or smell of burning insulation are signs of heat), the cause of the problem can be quickly identified and corrected. On other occasions, however, locating the cause of the problem will involve more detailed investigation and analysis to identify and correct the problem.
The secret to efficient and accurate troubleshooting lies in determining the section of the control circuit that contains the trouble component and then selecting the proper component to be checked. This can only be accomplished by efficient and accurate circuit analysis, not by trial and error, long, extended wire tracing, or indiscriminate checking of components at random.
In this section, we will assume that the troubleshooting is being done on an existing system that was functioning properly, but for causes unknown has failed.
If the circuit was a new installation that failed to work on the first attempt, the troubleshooting procedure would include analyzing the circuit design to ensure it was designed properly to achieve the desired effect and possibly to verify that the system was properly installed.
When considering troubleshooting an existing circuit, we can generally eliminate the possibility of improper connections. If the circuit had been improperly wired, it would not have operated originally.
The first step in troubleshooting an existing circuit that has developed trouble is to understand that circuit and to understand the operation of the machine it controls. On complex circuits, time generally does not allow the troubleshooter to study in-depth the complete circuit to determine how it operates. With the help of the operator, however, one can quickly analyze and identify the various parts of the control circuit on the schematic. The operator should also be able to help determine how much of the machine is operating. You can also depend on the operator to help locate and identify controller components and pilot devices that may be hidden by the machine or located at a remote location.
One concept that should be understood is that a control circuit is made up of two things: contacts, which make and break the circuits, and coils, or other pilot devices, which operate these contacts. If the contacts close and open as they should, then the proper voltages will be applied or disconnected from the coils. If this is true, then the malfunction must lie in the coil itself. If the contacts do not operate properly, then the trouble must be in the contacts, in the associated wire that carries this current from the contact to the coil, or in the device that operates the contacts.
The second step is to follow the machine through its cycles until it reaches the point where it does not function properly. Having determined this point, you can analyze the circuit, starting with the section that does not operate. A careful check of this circuit and locating the components involved in this section of the circuit will generally lead you to the source of the trouble you are seeking. The malfunction of some control component must be the cause of the circuit failure.
To effectively troubleshoot a control circuit, the troubleshooter must first ensure he has the proper tools available. The basic tools that are needed are the multimeter and clamp-on ammeter. The multimeter will be used for measuring voltage and resistance. The ammeter can be used to measure current flow to the load.
'''Figure 17''' is used to show how a voltmeter can be effectively used to isolate a fault to a specific portion of a control circuit.
'''Figure 17: Troubleshooting Three-Phase Magnetic Line Starter'''
Let us begin the scenario by stating that the operator reports that the motor does not start when the START button is pushed. Troubleshooting should begin by pushing the START button and listening for the holding coil to operate. In most controllers, one should be able to hear the main contactor operating. If the contactor does not operate, troubleshooting should be directed at the control circuit. If the contactor does operate and the motor is not running, the controller should be shut off to prevent damaging the motor. The motor should then be checked to see if it has seized, has an open winding, or has some mechanical or electrical malfunction.
In this situation, the contactor does not appear to be operating and an electrical failure is suspected in the power circuit. One must first check the line voltage and fuses as shown in figure 17. The voltmeter probes should be placed on the hot side of the line fuses as shown at position A. A line voltage reading shows that the voltmeter is operational and that there is voltage to the source side of the line fuses L1-L2. One should also check between L1-L3 and L2-L3. To check the fuse in line-1, the voltmeter should be placed across the line fuse, as shown at position "B" between L1-L2. A voltage reading shows a good fuse in L1. Likewise, the other two fuses between L1-L3 and L2-L3 should be checked. A no-voltage reading would show a faulty fuse.
If the line fuses check good, then the voltage between terminals T1-T2, T2-T3, and T1-T3 should be checked. The controller is faulty if there are no voltmeter readings on all three of the terminal pairs; one would then proceed to check the power contacts, overloads, and lead connections within the controller. However, if voltage is indicated at all three terminals, then the trouble is in either the motor or lines leading to the motor.
If the failure is suspected to be in the control circuit, and the overload devices have been reset, the following voltage checks are typical of those that could be made to identify the faulty component if the main contacts are not closing (holding relay not energizing):
1. Check for power available at L1, L2, and L3 as previously instructed.
2. Place the voltmeter probes at points C and D shown in. There should be a voltage reading when the STOP switch is closed and zero voltage when the STOP switch is open. These conditions would indicate a good STOP switch. If there was zero voltage in both cases, it could indicate a blown fuse.
3. Next, check the voltage between points C and E. If there is a no-voltage reading when the START switch is open and a voltage reading when the START switch is closed, then the START switch is good.
4. Place the voltmeter probes at C and F. A voltage reading with the START button closed would indicate a good OL1 but would also indicate an open OL3, an open relay coil, or an open connection to line 3.
5. Place the voltmeter probes at points C and G and close the START switch. A no-voltage reading locates the trouble in the control circuit; the OL3 is faulty.
If it is certain that the overload is not overheated and causing the OL3 contact to open, the overload would be replaced.
This example used a simple circuit to demonstrate the technique of using a voltmeter to isolate a problem in a control circuit. One should be aware however, that most control circuits are not this simple, and there may exist parallel paths that make it difficult to use a single voltage measurement to make a sound conclusion about whether a component is faulty or good. Sometimes, it is necessary to disconnect a circuit at various points to take a voltage or resistance reading (resistance readings must be taken with the circuit de-energized).
Care must be taken to check all parts of a de-energized circuit, because control circuits frequently have cross-connected interlocks that bring power into the controller from another source.
The following list is a brief summary of some common problems and their causes. There can be many variations to the many faults listed, and, as experience is gained, one will find that there will be other faults that could be added to the list.
Reduced-voltage, soft-start starters are used to limit the starting current that a motor draws from the utility when it first is started. This avoids large starting currents that may cause voltage to dip, impacting other loads that are sensitive to low voltages. The graph shows the curves for the full-voltage and soft-start (solid-state starter) starting currents. The full-voltage starting current starts at 600 percent, or six times the motor’s full load current rating. The soft-start starting current starts at 300 percent, or three times the motor’s full load current rating.
Reducing motor-starting current reduces the motor’s starting torque. With full-voltage starting, the starting torque is about 180 percent with a peak torque of over 600 percent. However, with the soft-start starting, the starting torque is 70 percent with a peak of about 180 percent. Limiting the starting current reduces system’s mechanical stress.