[Al Skierkiewicz]

Brendan,
Steppers are a particularly useful device for certain applications. They are effectively a multiphase controlled positioner. You can find them from three phase on up depending on the accuracy you need for an application. They also require feedback of some sort, usually an accurate tach wheel or in the case of video tape recorders both a tach wheel and position info derived from the recorded video signal and a control track on tape. The phase difference between drive signals is dependent on the number poles of the motor and the angular relationship. There are many integrated circuit solutions for stepper control that can take in error signals, tach and power and output high current phase control. How you develop the error signal is dependent on what you are using the motor for. In a tape recorder, the motor is first brought up to speed using tach wheel, then the motor is phased using the control track recorded on tape, then fine phase is achieved using the video track or track start and RF sampling to stay locked.