Commissioning Experience with Dynamic Termination of Transmission Line Effects in an Extended Magnet Cable Application

Active magnetic bearing controllers generate current and voltage commands in response to axial and radial forces acting on rotating shaft. The subsystems that respond to these commands are known as power amplifiers. Due to advancements in power electronics and rapid industrial trend towards higher efficiency, compactness and lower cost, recent AMB’s utilize switched mode PWM power amplifiers which regulate current in magnets by varying the ratio of on duration of switching device to switching period. With increasing no of AMB installations in hazardous areas for e.g. volatile gas compressors, it is imperative to provide a safe separation distance between the bearing and the control system itself. Transmission line effects can result in power cables connecting the PWM power amplifier to the bearing in such installations. These effects give rise to voltage and current reflections due to a mismatch of source, line and load impedance at high switching frequencies. They can cause insulation failures, overvoltage, ringing and improper operation of parts subjected to these voltages and currents. A dynamic termination circuit was developed at Waukesha Magnetic Bearings to terminate these reflections on a strategic basis according to instantaneous PWM pulse amplitude and return most of the reflected energy to the DC link power supply. This paper discusses actual results seen during commissioning on site in which an AMB system was successfully employed to suspend a volatile gas compressor rotor shaft by mitigating transmission line effects in the power cables. It is also shown that this circuit can improve dynamic performance of PWM power amplifier and meet requirements of industrial AMB’s in hazardous area applications.