Generation of very low frequency beat waves from power converters

In the course of investigation of the EMI emissions from DC/AC power converters controlling synchronous machines in a hybrid electric vehicle (HEV), very low frequency beat waves were picked up in the common mode currents on the AC link. The beat wave patterns have maxima which could be more than ten percent of the phase current amplitude and at frequencies several orders lower than the switching frequency of the converters. These high currents create strong low frequency magnetic fields in the vicinity. The beat waves could potentially perturb the rotor motion and eventually distort the smooth functionality of electric machines if undamped. PSpice models show that PWM switching of converter input voltage, generates common mode current spikes (transients) due to stray capacitances, and the spikes eventually beat up. This paper presents measurements on a HEV showing beat waves, and an experimental verification of the phenomenon using an H-bridge and a DC motor.