This method of induction motor speed control varies the stator voltage while keeping the supply frequency constant. Reducing the stator voltage by 10% reduces the developed and starting torque by 19% and narrows the speed range at low slip. The motor efficiency is highest at rated speed and decreases for very low speeds. This method is suitable for speed control below rated speed as it reduces starting current and improves light-load efficiency but cannot exceed the rated terminal voltage to prevent winding insulation damage. Stator voltage can be controlled using an autotransformer or primary resistance in series with the stator winding.

1. BY GANESH BEHERA

2. By varying the stator voltage the induction motor
speed can be controlled and this is called as stator
voltage control.
Rotor power can be varied by the varaiable
resistance or by feeding back to the mains through
appropriate power conditioning circuits.
In the case of variable resistance the power
associated with the rotor circuit is wasted.
This method is utilized in rotor resistance control
and slip power recovery.

3.  Here the supply frequency is constant,as
we know that torque is directly
proportional to the square of stator
voltage therefore 10% of change in stator
voltage results in 19% of reduction of
developed torque as well as starting
torque and maximum torque.


5.  From the graph we can see there are two curves
with different values of stator voltage. Slip at
maximum torque remains unchanged since its
not a function of speed, for a lowest slip the
speed range is narrow.
 This method can only be used for low starting
torque and narrow speed range at relatively low
slip.
 If the stator copper loss,windage,core losses are
neglected from the equation then the motor
efficiency is given by
 Pm/Pag=(1-S)

6.  For very low speeds the motor efficiency is poor
 It is an excellent method for reducing starting
current and increasing the efficiency during light
load conditions. The starting current is reduced
,since it is directly proportional to the input
voltage. The losses are decreased, mainly core
losses, which are proportional to the square of
the voltage. The terminal voltage cannot exceed
rated value to prevent the damage of the
winding insulation. Thus, this method is only
suitable for speed control below the rated
speed.

7.  Stator voltage can be controlled by two
methods.
 1.using auto transformer.
 2.primary resistance connected in series with
stator winding.
 Using auto transformer:
speed of the induction mototr can be controlled
by using auto transformer.by varying it we can
get a variable ac output voltage without any
change in supply frequency.
Variable voltage is fed to the induction motor
then the induction motor speed also changes.