Multi-Stress Endurance Tests on High-Voltage Motor Insulations, With Equal Acceleration Of Each Stress

An ageing test was performed on two high-voltage motor insulation systems using three principal ageing stresses: temperature, electric field, and mechanical stress. An equal acceleration of each stress was used with the goal of an ageing process identical with that at service conditions. For the determination of the true acceleration factor a parallel "ageing" test at service conditions was run, up to so long testing times that changes in characteristic properties (flexural modulus, tanδ, and capacitance) could be determined. Humidity tests and high-voltage withstand tests were also performed after each ageing cycle. The observed acceleration factor for both systems was lower than predicted. In one of the systems the changes in characteristic properties were measurable and of the same type at both test conditions, so that the test acceleration could be evaluated reliably. Both systems show good class F performance connected with appropriately defined severe service conditions.