Well... i tried with the optocoupler as u said and i was successful in triggering the igbt accordin to the output from micro.... but wen the chopper circuit was connected to the motor another problem stood up... The motor startd rotating without the gate pulses which is not supposed to happen... after consulting with professors i removed the filter part of my circuit and removed the collector and emitter and connected just the gate of igbt but didnot apply the gate pulses and gave a 3phase supply and the motor didnot run which was right.... but when the collector and gate was removed and only emitter was connected and when the 3phase supply was given the motor started to rotate at 250v and that is supposed to happen... i tried changing the igbt(ct60 am 18b) and replaced with a mosfet (irf250n)... but the same result....please give me a solution ASAP in how to prevent the motor from rotating or if anything wrong in the connection i did.... so ryt nw in my circuit... a 3phase motor , rectifier, chopper circuit, the controller... theses are the main parts....