Sending PWM Down Extended Magnet Cables

With a high power active magnetic bearing system, rapid changes in the magnet coil current are achieved by switching a high DC link forcing voltage onto the inductive load for short periods of time using Pulse Width modulation (PWM). In the steady state the on time of the voltage is a very small proportion of the cycle period such thatthe average voltage is sufficient to maintain the DC current through the DC resistance of the load. In some situations where it is necessary to have a large separation between the PWM driving amplifier and the load, the cable between behaves as a badly terminated transmission line. Significant reflections on the PWM pulse waveform are able to cause potentially damaging voltage overshoots up to around twice the original voltage. Conventional source and or load termination impedances would introduce unacceptable power losses. Using filters to limit the bandwidth goingdown the cable is both costly and has broadly similar effect to reducing the forcing voltage. This paper describes tests of a dynamic termination method where most of the power that would otherwise be lost is returned to the source of the DC link, giving dramatic improvement in the waveform quality.