Induction motors speed changes

These methods provide continuously variable control of the speed over part or all of the torque-speed characteristic of the motor. One of the simplest ways of causing an induction motor speed to change is by altering the magnitude of the applied voltage to its stator. This will cause the motor torque to change in proportion to the square of the voltage, i.e. TaV. ... 

In the preceding discussion of multispeed ac motors note that only induction motors are considered. These have no discrete physical rotor poles, so that only the stator-pole configuration need be modified to change speed. To operate multispeed, a synchronous motor would require a distinct rotor structure for each speed. Thus multispeed is practical only for squirrel-cage induction motors. 

A variation on the theme of pole changing is a particular type of squirrel-cage induction motor, called the Pole Amplitude Modulated (PAM) motor. PAM motors should be used for low speed applications thereby requiring many poles e.g. 10, 12, 16. In addition, the various speeds required should not be widely different. This means that the number of effective poles will not be too dissimilar, e.g. 

The familiar equivalent circuit for the induction motor will be developed from (20.24). The oil industry occasionally uses variable frequency power supplies to start and run variable speed pumps and compressors. The nominal frequency applied to the motor is The inductances in (20.24) can be changed to their nominal reactances by using the nominal frequency The steady state variables replace the instantaneous variables and the differential operator p is replaced by the steady state frequency in conjunction with the j operator. 

It is noteworthy that ac induction motors can be slowed down without losing much torque, with the use of this circuit. A simple resistor or a variable transformer can not do that, since they work by lowering the peak height of the ac waveform. Even better control over motor torque can be obtained with more complex tiiac circuits, which sense the motor impedance and correct for its changes at different speeds, using feedback. 

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