CONTROL EQUIPMENT
Selecting the right squirrel-cage motor for the machine in question is but one step toward meeting the motor-application problem. Selection of the proper control apparatus is just as important as the selection of the motor itself. The intelligent choice of motor control involves a complete knowledge of the types of control apparatus available, as well as their functions.
The functions of the control apparatus for squirrel-cage motors are:
1. Starting the motor on (a) full voltage, or (b) reduced voltage,
2. Stopping the motor,
3. Disconnecting the motor upon failure of voltage,
4. Limiting the motor load,
5. Changing the direction of rotation of the rotor,
6. Starting and stop-ping the motor (a) at fixed points in a given cycle of operation, (b) at the limit of travel of the load, or (c) when selected temperatures or pressures are reached,
Where multispeed motors are involved, the following functions may be added:
7. Changing the speed of rotation r/min,
8. Starting the motor with a definite speed sequence.
Stopping the Motor
Control devices permit motors to be stopped as follows:
1. Under the direction of an operator,
2. Under the control of a pilot -circuit device, such as a thermostat, pressure regulator, float switch, limit switch, or cam switch,
3. Under the control of protective devices which will disconnect the motor under overload conditions detrimental to the motor, or upon failure of voltage or a lost phase.
Protecting the Motor
To protect motors against damage during severe momentary or sustained overloads, overcurrent devices are employed to disconnect them from the line. These devices are of two general types:
1. Thermal overload relay,
2. Dashpot overload relay.
The characteristics of the thermal-type overload relay are illustrated in Fig. 21, which indicates an inverse relationship between current and time-that is, the greater the current, the sooner the relay operates to disconnect the motor from the line. When the relay operates, it opens an independent, or pilot circuit, which causes the main line contacts to drop out, thus disconnecting the motor from the line. Dashpot overload relays have a similar inverse time-current relationship. Thermal and dashpot overload relays are of two types:
1. . Hand reset,
2. Automatic reset.
As the names denote, the hand-reset type must be reset by hand after having tripped (usually by pressing a button projecting through the enclosing case), whereas the automatic reset types reset themselves automatically. Dash pot overload relays are generally of the automatic reset type, but they can be made of the hand-reset type by providing a hand-reset attachment which will prevent automatic resetting.
Standard polyphase motor control devices, almost without exception, are equipped with two thermal-overload relays of the hand-reset type.
rhermal-overload relays of the automatic-reset type are also used. Their purpose is to prevent burnout of the motor windings; hence, they should be selected carefully to accomplish that end. Thermal relays must be of the proper rating, and dashpot relays must be properly set to disconnect motors from the line before operating conditions tax the motor windings to the point of breakdown.
Overload relays protect polyphase motors against phase failure due to a blown fuse or some other power interruption in one line of the supply circuit. If phase failure occurs while the motor is at rest, the motor will trip the overload relay, thereby disconnecting the motor from the line. If phase failure occurs while the motor is running, the motor will continue to run, provided the load is not too great. The current taken by the motor when running single-phase will be from two to three times the normal three-phase value, and unless the motor is very lightly loaded, will be sufficient to trip the overload relays before the motor windings can be damaged. If the load is very light, the overload relays will not trip, and the motor will continue to operate without damage until shut down, but will refuse to start the next time an attempt is made to operate it.
After hand-reset overload relays have tripped, they must be manually reset by an operator before the control will again connect the motor to the line. The operator, noting the failure of the motor to start, will presumably investigate and remove the trouble.
Automatic-reset overload relays in magnetic starters controlled by two-wire pilot-circuit devices, such as float switches, pressure regula tors, thermostats, snap switches, etc., will cause the motor to be connected to the line again and again, and each time a heavy load current will be drawn from the line which will reheat the motor windings until they burn out.
Voltage failure also requires protection to operators and motors. Control apparatus designed so that the main-line contacts are held in by a magnet coil operated across one phase of the supply circuit can have their control or pilot circuits so arranged that, upon failure of the voltage, the motor will not restart until the operator goes through the necessary starting operations. Three-wire pilot-circuit control is re quired for such control devices to cause and maintain the interruption of power to the main circuit. Motor applications involving control appa ratus which starts and stops the motor automatically by means of two-wire pilot-circuit devices, such as thermostats, float switches, and pressure regulators, cannot be arranged for low-voltage protection.
To reverse squirrel-cage motors, it is only necessary to interchange two leads-
(a) any two leads to three-phase motors;
(b) two leads of either phase to two-phase four-wire motors, leaving one phase intact;
(c) the two outside leads to two-phase three-wire motors, leaving the common lead intact. Reversal is accomplished by means of either a smalt drum controller for small motors, or a reversing magnetic con tactor for larger motors like the one shown previously in Fig. 15.
Control for Multispeed Motors
Multispeed motors involve all the devices heretofore mentioned to effect starting and stopping under the six conditions listed. Additional control devices are required, however, for changing their speeds, which is accomplished by regrouping the motor windings. Two types of controls used are:
1. Manually operated speed-setting drums,
2. Combinations of magnetic contactors.
Magnetically operated controllers can be so arranged that the mul tispeed motors will always start up through low speed, subsequently being transferred to a higher operating speed. The auxiliary control device accomplishing this is known as a compelling relay. It may be operated either manually or automatically by means of a time-delay device.
APPLICATION
As pointed out in the beginning of this chapter, the squirrel-cage motor, because of its simplicity of construction and because it can be built with electrical characteristics to suit almost any industrial require ment, has made it one of the most widely used machines. As noted from the characteristics and classification, squirrel-cage motors as a rule are not suitable where a high starting torque is required, but are most suitable where the starting-torque requirements are of a medium or low value.
In selecting a motor for a certain application, there are, in addition to torque and starting-current considerations, a large number of factors affecting an economical and efficient operation. The selection is deter mined in whole or in part by the user on the basis of available data, such as speed range and regulation, mechanical arrangement, available voltage, direction of rotation and reversing, operating schedule, method of control, surrounding atmosphere, etc.
Since the power factor and efficiency of any induction motor are lower at light loads than at heavy loads, it is obvious that in selecting a motor for a definite load, the size should be such as to permit . the operation of the motor as nearly as possible at full load. Also, low speed motors have a lower power factor and weigh more per horse power than high-speed motors. Therefore, in choosing a motor for a certain duty, size and speed should be carefully considered so as to give the most economical and satisfactory service.
For the largest group of applications, such as for fans, pumps, compressors, conveyors, etc., which are started and stopped infre quently and have low inertia loads so that the motor can accelerate in a few seconds, the conventional general-purpose NEMA Class-A motor can be used.
If a motor is to be installed in a location where there is a limitation on the starting current, the modified general-purpose NEMA Class-B motor can be used. If the starting current is still in excess of what can be permitted, then reduced-voltage starting is employed.
On those applications where reduced-voltage starting does not give sufficient torque to start the load with either NEMA Class-A or -B motors, Class-C motors with their high inherent torque, reduced starting current, and reduced-voltage starting may be used.
Conveyors and Compressors
For applications such as conveyors and compressors, which some times require a starting torque of at least twice full-load torque, NEMA Class-C motors may be used with full-load voltage starting.
Large Fans
This type of drive is one which requires special consideration. These drives, once they are accelerated, run continuously at full load; there fore, it is desirable to have the best possible efficiency and power factor. Some of the fans, however, have extremely high values of moment of inertia (U1/2) and motors with normal starting-torque characteristics may require from 30 seconds to 1 minute to accelerate. With starting current flowing in both the rotor and stator for this long period of time, it may generate sufficient heat to damage the windings.
To meet this application with a squirrel-cage motor, special NEMA Class-B motors are used to reduce the starting current to a minimum. The rotors of these special motors are designed with a large mass of material, especially in the end rings, so that it is possible for the motor rotor to absorb the tremendous losses during the accelerating period without reaching excessive temperatures. Once the motor reaches its full-load speed, the losses return to normal and the rotor rapidly cools down to its normal operating temperatures.
Flywheels, Presses, and Bolt Headers
Another continuous-running motor application which requires special consideration is on drives where there is high external inertia, quite often in the form of a flywheel, and where the load, instead of being of a continuous full-load torque, is pulsating in nature. Typical examples of this type of application are presses and bolt headers. In this type of application, no work is being done most of the time, and then a peak load occurs which may require torques of many times the full-load torque of the motor.
Under these conditions, the running efficiency of the motor at full load is not important, because the motor never operates at that point. Therefore, it is deliberately designed with more than normal secondary resistance so that it has a tendency to slow down as the load comes on the drive. This tendency for the motor to slow down permits the flywheel to give up energy to absorb the peak load, with the motor exerting no more than full-load torque. However, the energy that is taken out of the flywheel during the work stroke must be returned to it by the motor. Thus, the motor must have sufficient torque to accelerate the flywheel before the next work stroke is made.
Since presses work at widely varying rates, say from 2 or 3 to as high as 100 to 150 work strokes per minute, the correct amount of rotor resistance will vary, depending on the number of strokes per minute. On extremely large presses making only a few strokes per minute, there is ample time for the flywheel to slow down 10 to 15% during the work stroke and give up a considerable part of its kinetic energy. Therefore, motors used on this type of press should have high slip. One of the standard ratings listed by the motor manufacturers is a motor having 8 to 13% slip.
As the number of strokes per minute increases, the length of time of the working stroke decreases, and there is not much time available for the flywheel to slow down. Neither is there much time for the motor to reaccelerate the flywheel between strokes, so that the amount of slowdown of the flywheel is usually between 5 and 10%. The standard line of press motors with 5 to 8% slip has been designed for this application. These presses usually make 10 to 40 strokes per minute.
On the smaller presses, making 100 to 150 strokes per minute, there is very little time for the flywheel to accelerate or decelerate, and a standard motor which has approximately 3% slip is entirely adequate and there is no object in supplying a high-slip motor. Quite often, someone tries to use NEMA Class-C motors on these high-torque applications, and if there is not trouble due to overheating, trouble may develop in the form of mechanical failure of the rotor due to the unequal heating in the double-deck winding, as previously explained.