Variation in Motor Cable Dieletric Properties Caused by Temperature and the Effect of Variation on Active Motor Overvoltage Mitigation Methods

Cable reflection is a known and well understood phenomenon in electric drives. Due to the finite propagation velocity of the reflected waves along the motor cable, oscillating voltage and current waves are induced, which can be seen as an oscillating overvoltage voltage at the motor terminals. The overvoltage has adverse effects to the system, such as stator winding insulation failure. Several active output filtering methods have been introduced to mitigate the overvoltage problem by taking advantage of the time delay introduced by the finite propagation velocity on the motor cable. In this kind of filtering approaches, the filtering result depends directly on the propagation delay. In this paper, it is demonstrated that temperature variation that can occur in regular use has a significant effect on the propagation delay and will therefore affect the filtering outcome. The cable temperature may change with the ambient temperature or with variation of the load current amplitude. As the inverter, cable, and motor form a transmission line resonator, the system can be used to characterize the cable. In addition, the variation of the cable properties due to changes of environmental properties, such as temperature, can be determined.