Cable aging influence on motor diagnostic system

It is now established that the turn-to-turn capacitance variation of electrical motor windings can be used as an insulation system aging indicator: turn insulation aging causes a significant increase of the corresponding capacitance and a decrease of several resonance frequencies in the upper part of the impedance spectrum of the motor windings. To set up a monitoring system based on this indicator, it is necessary to consider the whole system including the cable used to connect the motor to the electronic converter. The paper highlights the cable influence on the whole system. Aging tests were made on samples of a three-wire shielded cable used in the aircraft industry in order to analyze the cable aging influence on the monitoring system dedicated to get data on the motor insulation aging. This analysis shows that the series resonance situated at higher frequencies gives better results. It should be used by the monitoring system in order to get useful data on the motor aging, independently of the slight variations of the cable capacitances.