[Al Skierkiewicz]

Marshall,
Three phase (or polyphase) motors operate in relation to the way they are wound and the input frequency. Their greatest torque is at the speed for which they are designed. There is a distinct relationship between the number of windings/poles and the operation of the motor. When these motors are manipulated for speed, the controllers are adjusting frequency. The controller is essentially a large power amplifier. And they don't change speed quickly. In general when a person speaks of three phase motors, they are referring to power line operated devices used in locations where three phase power is available.