On-line monitoring of Insulation Aging in AC Motors Fed by PWM Inverters

An on-line, nonintrusive monitoring system, able to observe the aging of an AC machine winding insulation is described. The principle of this system, which operates on external magnetic field measurements, consists of detecting small variations of the inter-turn capacitances due to the dielectric aging. The authors have performed experiments on accelerated insulation aging as well as on a system able to measure a high frequency machine winding resonance which depends on inter-turn capacitance. Firstly, twisted pair specimens were submitted to thermal and electrical stresses in order to establish a connection between the inter-turn capacitance with the breakdown voltage. Then, the changes in the resonance frequency of a simplified stator winding were measured. Experiments show that the analysis of the high frequency magnetic field in the vicinity of the machine gives information on the inter-turn capacitance variations hence on the insulation aging