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 = P_m /P_g = (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.