Nevin:
Dirves are designed to be controlled independently, like what NJwirenut is stating. There have been vast improvements over the drives from even 5 years ago. Newer IGBT's, faster commutation..etc.
Sizing comes down to application mostly. The application depicts the derating (if necessary) and accessories. You may need line reactors, filters, and dynamic braking. If you want some more help, put down the size of the motor (HP, FLA, speed, voltage, freq.)and the application.
I do this stuff all day. I'm in tech support not sales...so I won't try to sell you something...promise.
Carl
