Winding tests for "Drive duty" induction motor stators

Adjustable speed drives (ASD) are being widely used at this time. These drives, particularly those utilizing pulse width modulation, impose severe stress on the winding insulation of the motors being fed. Motor manufacturers are increasingly aware of new failure mechanisms associated with "drive duty". Several tests have been adopted to assure reliable motors even under these more stressful conditions. Much of the increased stress is centered on the turn and phase insulation of the motors, so the tests on these insulations are the focus of this paper. The surge test is widely used to assure the quality of these insulations. New techniques have been developed to simulate the stresses imposed by PWM drives and measure the ability of the insulation to withstand that stress, particularly inter-turn discharging and corona. Several measurement tools are now available to detect and quantify these discharges during the surge test. The detection circuit is described and test results presented for one technique being used to measure a discharge inception voltage and determine the suitability of a motor winding for "drive duty".