PWM Filter for Active Magnetic Bearings

An example of a PWM filter application in an AMB amplifier is discussed. The design objectives included reduction of both conductive and radiated EMI caused by the PWM pulses, improving system efficiency, reliability, and enabling its use with long AMB cables. The selected filter topology included a “Trap” filter, utilizing a parallel LC resonant circuit to suppress output current harmonics near the circuit resonance, followed by a conventional 2nd-order LCR filter. This choice was dictated by a narrow frequency separation between the 25kHz PWM frequency in the existing amplifier and the required 2kHz bandwidth. Introduction of the filter resulted in the attenuation of the PWM signal at the base frequency ranging from -29dB to -42.5dB depending on the output current, which was varying from 0A to 10A. The filter practically eliminated high-frequency resonant ringing on the amplifier output and reduced output voltage dV/dt by more than 800 times (from 8,360V/µs to approximately 10 V/µs). Both factors are known to cause EMI and shorten the lives of the actuator magnet wires and the power electronics. The filter enabled the control current delivery to the AMB actuators in a commercial 350kW 15kRPM air compressor over 1.2 km (4,000 ft) cable. The compressor was subsequently levitated and spun to full speed.