Stator Voltage Control Methods
By controlling the applied voltage the speed of an induction motor can be controlled. Since the output torque of 3-? induction motor is given by T sI/2 and current is proportional to voltage . therefore as voltage is reduced to reduce speed for the same current motor develops lower torque.
If stator copper loss core loss and friction and wind age loss are ignored then motor efficiency is given by
Η = Pm /Pg = (1-s)
This equation shows that the efficiency falls with decrease in speed. The speed control is essentially obtained by dissipating a portion of rotor power in rotor resistance. Thus not only the efficiency if low the power dissipation occurs in rotor itself which may overheat the rotor. Variable voltage for speed control is obtained using ac voltage controllers as shown in figure.
By changing the conduction period of the SCR the magnitude of stator voltage can be controlled from zero to full supply voltage. Thus we can obtain the speed variation. But this methods is suitable for small and medium size induction motors because supply current contains harmonic components. The another disadvantage of this method is that a heavy current flows through stator if speed is reduced therefore there may be undue heat generation is stator windings.