Speed control

For the inverter-driven induction motor, the speed control is effectively achieved by means of variable frequency. However, apart from the frequency, the applied voltage also needs be varied so that the air gap flux can be maintained at a constant value without letting it to saturate. It is well known that in order to maintain the

air gap flux constant, the ratio between the phase voltage and the supply frequency is to be maintained to a constant value.

Therefore, whenever stator frequency is changed to obtain speed control, the stator input voltage has to be changed accordingly to maintain the air gap flux at a constant value.

The requirement of keeping the ratio between the stator voltage and the stator frequency constant, actually compounds the speed control problem in an induction machine. This, in fact, is the difference between the speed control problem of an induction motor and a DC motor, which requires only the voltage control for the purpose, and the simplicity of the control problem made it preferable machine in many applications until early 1980s.

      Various speed control strategies have been formulated for the induction machine, depending upon how the voltage-to-frequency ratio is implemented. The more important and commonly employed speed control strategies are precisely revisited here

The commonly employed speed control strategies for the induction machines are:

1.     Constant Volts/Hz Control

2.     Constant Slip-speed Control

3.     Vector Control or Field-Oriented Control