I worked with industrial drives for a while, they were all AC except what was less than 1/2 HP. All were controlled by $200-$1000 (depending on power) speed controls mainly ABB. The control of the variable speed control was where the locally designed equipment came in, I could not match the functionality and capability of the ABB units for less than 5 * the price. When buying or designing AC motor variable speed drives, make sure the IGBTs have a breakdoen voltage of no less than 3 * the power they are fed, I had a lot of problems with brands using 1200V IGBTs for 480V drives and 1500V worked well. If you decide to design your own, do not forget the skip speeds or the darn thing will shake apart.

Erik

Erik