Yes it does look like that things start detriorating above 5Khz or more. The motor then starts loosing out on torque. In our actual trials ( pending a calculated proof) the 4 to 5Khz did seem to be working out well. Anyway now a 'pure" electrical guy is on the job to find out things like the inductance, time constant etc of the motor and come out with his recommendations. Shall revert with the findings if they are worth sharing.

Ans in between someone has started a new thread within my thread !! OK on the face of it , it looks like he just has to look around for a 3Phase AC to DC rectifier circuit and get going. This is the safest.

Unless he directly plans to use thyristors with phase angle control , zero crossing and the works. Any slip anywhere in this and all the other sensitive instruments in the vicinity will go for a spin, before they know what hit them.

Thyristor control drives are just not for everyone or the weak in heart.

Raghu