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The influence of several environmental conditions on the partial discharge characteristics and on the lifetime of magnet wires under inverter pulse operation

During the operation of three-phase asynchronous motors by pulse frequency inverters, overvoltages occur, which places considerably more stress on the insulation system than under sinusoidal conditions. Once the inception voltage is exceeded, partial discharges come into being and damage the winding insulation of the motor coils. Below the inception voltage, there is only a negligible deterioration in comparison with that occurring under discharge conditions. For the evaluation of the endurance of enamelled magnet wires under partial discharge conditions it is necessary to know the influences of several parameters on these discharges such as temperature, humidity, pressure or the nature of the surrounding gases. On the basis of experimental investigations, the paper deals with the dependence of partial discharge characteristics such as inception voltage or frequency upon environmental parameters and the lifetime. Possible connections between discharge characteristics and lifetime are discussed. The results are based on standard enamels. During all of these investigations, a new measurement technique was used allowing us to acquire the discharge parameters under inverter pulse conditions and not, as previously done, under sinusoidal conditions with mains frequency.