Preliminary evaluation of motor insulation for variable speed applications

Variable speed drives for electric motors produce surge voltages or voltage spikes with rates of rise to crest value that are equivalent to the rates of switching surges. Although the magnitude of voltage surges on a per unit basis for drives is less than that for switching surges, they occur on a continuing basis and therefore can accelerate aging of the motor insulation system. To reduce the impact of surges on a prolonged basis, insulating materials with increased resistance to deterioration caused by partial discharges (PD) or corona should be used and materials known to be sensitive to PD should be minimized or eliminated.