Active motor terminal overvoltage mitigation method for parallel two-level voltage source inverters

Inverter-fed motor drives can be subject to transient overvoltage problems caused by switching operations. Motor terminal overvoltages may occur if a motor is fed through a long motor cable. These overvoltages may result in a premature motor insulation failure. The phenomenon exists whether the motor is fed by a single inverter or parallel inverters. A modulation strategy and a parallel inverter topology that mitigate the motor terminal overvoltages are presented in this paper. The topology consists of parallel inverters with a common point of connection at the motor terminals. The proposed method does not use any passive filters. Experimental measurements were conducted with standard industry frequency converters to verify the method. Modulation sequences resulting in smallest possible overvoltage for two and three parallel inverters applying the method were found.