In this method the speed of squirrel cage induction motor control by varying the stator voltage, which is possible with the help of three sets of SCR’s connected back to back(parallel but opposite in direction ) with each phase of input supply as shown in figure below.
The stator voltage can be reduced by delaying triggering pulses of the SCR, if we delay these pulses the voltage may be reduced from rated i/p voltage, which causes to decrease the speed of motor. On increasing the triggering pulses of the gate terminal the action will be vice versa to the previous case. In this method there are more losses are produced in the voltage and power due to the change in power factor, to recover theses loses in power a special SCR is used called gate turn of SCR (GTO’s ), which force the current to flow almost in phase with voltage.
“The gate terminals are shorted and a common control pulse is used to control the speed of the motor”