sir can any body tell mee how to control three phase sin voltage using single varaic . my aim is to generate 50hz 15 v three phase whose magnitude can be varied between 0-15v please help me

 

While there are three phase variacs available - http://www.stacoenergy.com/variable_transformers.htm#GangedUnits - they are an expensive and unsafe method.

What is your application? How much power is required?

 

Could be done safely electronically say, using a phase-locked loop locked to a low voltage single phase 50Hz input. A 'crude' 3-phase generation could be done within the loop using a 3-stage Johnson counter. The 3-phase square waves could be filtered to produce a reasonable sine wave in each case. Amplitude control could probably be accomplished using an appropriate technique - the trick being to do it concurrently for all three phases.

 

Three-phase variac + three mains/15V transformers(or one three-phase transformer).

 

For a education demo. I once used Labview, sound card, and a external amplifier. Just then I was writing this the idea of using a allpass filter (Google it) came to me. After some fine tuning it may work for you(or not). But it is easy test on bread board