Three Phase motor control using SCR

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”