High frequency filter techniques for inverter-fed motors

Inverter-fed electrical drives generate premature aging of the machine isolation, damage of the bearings and high frequency currents. The motor cable stray elements increase these effects as a consequence of the overvoltage and the voltage rise high rate (dv/dt) that are generated when the propagated inverter pulse along the cable is reflected at the motor terminals. In order to reproduce the high-frequency behaviour of an electrical drive, it is necessary to take into account the appropriate models for every drive element. That comes in a very complex system that makes difficult to analyse the main factors that are relevant in each problem. This paper proposes a methodology to analyse the high frequency filtering of the motor voltage. This methodology makes use of two simple circuits, one for the differential-mode and another one for the common mode, and is based on three filtering options derived from the transmission line theory: incident voltage wave attenuation at the beginning of the cable; cable termination network to modify the high frequency impedance at motor terminals; rise time increasing of the incident wave.