In this article we will discuss about how to control the speed of electric motors.
Speed Control of Single-Phase Induction Motors:
The stator winding of a single-phase induction motor can easily be arranged to give two synchronous speeds, one double the other. In Fig. 1.88 (a) the connections for two pole motors are shown in which two coils A and B have been connected in series with their mmfs helping each other. If the connections to coil B are reversed so that both the coils are again in series but their mmfs opposing each other, as shown in Fig. 1.88 (b), 4 poles are produced and the synchronous speed is reduced to half. Similarly a four-pole motor can be reconnected to give 8 poles. The change from one speed to the other is easily and quickly accomplished by means of a two- pole double-throw switch.
Furthermore, because of the comparatively small power handled by single-phase induction motors, excessive slip to give a slight reduction in speed is not serious, from the stand point of waste of power, as in case of a 3-phase induction motors. For that reason in various cases, such as for fan and blower applications where speeds of 1,100 or 1,300 rpm might be required, it is not unusual to employ 4-pole motor with high slip.
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This can be accomplished:
(i) By using high rotor resistance, in which case no variation in speed is introduced, but the motor displays a drooping speed curve; or
(ii) By reducing the voltage applied to the stator winding.
For this purpose variable resistance or tapped reactance coil in series with the motor can be employed for speed regulation. This method is inefficient, gives large variations in speed with the variations in load, but it is much used inspite of these drawbacks. Alternatively, the voltage applied to the motor stator winding can be varied either by means of a transformer with taps on its secondary or by means of a variac, which provides the maximum possible number of taps and thus gives finest speed adjustment.
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This is one of the simplest methods of speed control of fractional kilowatt motors. The speed of the single phase-induction motor can also be controlled by employing tapped exciting winding, so that the constant voltage source may be impressed across the entire winding or some part of it. By this means, a normal speed and a single reduced speed are provided.
Speed control because of its more general use, has brought about the standardization of fan speeds, allowing the use of high slip motors. A squirrel cage motor with 8-10 per cent slip and with low pull-out torque operates satisfactorily not only for constant speed drives, but also for adjustable speed drives through the use of voltage control on the stator winding. With this method of speed control, the torque-speed characteristics resemble with those of a wound rotor motor with different number of external resistances in the rotor circuit.
Speed Control of 3-Phase Synchronous Motors:
The speed of a synchronous motor depends upon two factors viz., number of poles, P and supply frequency, f. The construction of the rotor of a synchronous motor being fixed, the number of poles on the rotor is likewise fixed and cannot be changed during operation. However, when a single synchronous motor is supplied power from an alternator, as in case of ship propulsion, the speed of the motor can be changed by changing the speed of the alternator-the speed of the motor changes exactly in the same proportion as that of the alternator supplying power to it.
It is to be noted here that the voltage and frequency are directly proportional to the speed at which the alternator is driven. So no special control is required for maintaining V/f ratio constant. Field excitation control, however, is required as voltage applied to the terminals of stator is changed. If not, power factor is greatly affected.
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Assuming the motor is operating at near unity power factor and normal voltage, increase in speed causes increase in applied voltage across the motor stator terminals and the motor is badly under- excited (i.e., low lagging power factor) for fixed dc excitation. On the other hand, lowering of speed causes over-excitation (i.e., low leading power factor). Thus field excitation control is essential.
Speed Control of AC Commutator Motors:
Speed control of ac commutator motors can be had either by varying the applied voltage or by movement of the brushes round the commutator. The former method makes use of tappings on a transformer or, occasionally, a series resistor or inductor so as to provide variation of voltage in steps, while brush shifting provides a gradual control throughout the range.
In case of a single phase ac series motor, the speed control can be accomplished, when required, only by voltage variation because it operates with fixed brush position. Transformer tappings provide variation between zero and maximum speed, as depicted in Fig. 1.89, without any appreciable additional loss such as would occur with series resistance. The latter method is employed, only when speed control is required very occasionally, and where capital cost is the prime importance.
In case of repulsion motors, brush shifting is the simplest method of speed control for a speed range between about 0.5 and 1.1 times synchronous speed, but beyond this range voltage variation control is preferred due to commutation difficulties. Speed-torque characteristics with different brush positions are shown in Fig. 1.90.
In case of 3-phase series motors, brush shifting is the most economical method of speed control and provides a range of 3 or 4 to 1 with characteristics similar to those of repulsion motors.