# Speed control of 3 phase induction motor using thyristor

I am working on project of controlling speed of 3 phase induction motor. To control the speed of motor I am using thyristor TYN16-600CT. As I am searching for different research papers and study material on speed control of 3 phase induction motor I found this MATLAB simulation...

In this simulation, I found that there is individual pulse required for each thyristor. This firing pusle is generated using this equation,

t = alpha/omega= alpha/2*pi*f

For, simplicity if alpha = 30 degree then t= 30/50*360

My question is, Is it possible to operate all thyristor by applying same pulse??

Can I use below schematic to control speed of motor?? But as I said I have to use thyristor and in this schematic there is use of triac.

So if this schematic can work fine, what changes I have to do to replace triac with thyristor??

• You should use additional support tyristor which short capacitor to close them. Sep 30, 2019 at 13:18
• Connect the inputs of optocouplers in series Apr 29 at 19:05

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

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. • ....thanks for the answer. So can I use second schematic, as in that same pulse applied to each thyristor individually?? Apr 5, 2017 at 13:58
• No. The timing is different for each phase because the zero crossings occur at different times. In addition, MOC3061 provides a gate pulse at the zero crossing, not delayed after the zero crossing.
– user80875
Apr 5, 2017 at 14:43
• I am changing firing angle using buttons, so how can i apply different pulse to different thyristor?? Apr 5, 2017 at 14:59
• You need to search for an appropriate 3-phase SCR firing circuit. You can generally get answers here only for specific questions. Likely no one will supply a complete circuit.
– user80875
Apr 5, 2017 at 15:28
• I am not asking for complete circuit. I have posted my work which I have done and studied. After that I have a question and I am asking just that question, not whole project or circuit. Apr 6, 2017 at 4:21