Dear Mr. M. Kandpal,

Here is a circuit diagram from my (in coop) book..

You can replace thyristors VS1 and VS2 by one IGBT.

In this case all parts, named by U2 are not needed.

If you have other questions, please do not hesitate.

Yury

Dear Yury,

Thanks for the reply.

I am having bit difficulty in understanding the circuit.

What I get is  ,the rotor terminals are connected to uncontrolled bridge which has been connected with Ld to reduce ripple content across an external resistance R105.

Through switching IGBT rotor resistance can be varied.

Net external  resistance would be Rnet= Rs+R105(1-k)   where k is the duty cycle

Please correct me if I going in the wrong direction.

Few confusions that I have are:

1)Why has the capacitor Ckdm been used?Do I need to use it with IGBT too?To what voltage rating an IGBT can be used?

2)I can not understand the function of VD7 , VD8, Lkom,TK,CY,...Please explain about them..

3)How is the controller judging the speed of rotor in  order to change the duty cycle to vary resistance?Do we need to employ a micro controller for it?Can you please explain the logic.