Speed control of 3 phase induction motor using thyristor
Is it possible to operate all thyristor by applying same pulse?


No. For induction motor voltage control, the pulses need to be timed for each phase to occur at a point delayed from the zero crossing point the same number of degrees for each phase. The same portion of each cycle is passed. Stopping or passing selected numbers of cycles is too crude for motor control.

In addition, controlling the speed of an induction motor by controlling the voltage is very limited in its application. With three-phase motors, a high-slip design is required. The only suitable loads for this approach are fans and centrifugal pumps.

Look at:

Calculating the value of the resistance for controlling the speed of AC fan

Additional Details

The diagram below shows the basic synchronization and control range required for the gate pulses. Delay angles are referred to point A. The gate drive circuits require electrical isolation from one another and from the control circuit. The gate pulses must be continuous or a series of short pulses exceeding the load power factor angle. Synchronization needs to take into account load power factor.