Motorette aging test for insulation materials of inverter-fed motors

Inverter Driven Motors (IDM) with high speed switching and advanced Pulse Width Modulation (PWM) techniques have been widely used for variable speed applications. Adjustable Speed Drives (ASD) with high speed switching devices provide very accurate control of speed and torque to a system. However, ASD give electrical stress such as high peak voltage, fast rate of voltage change (dV/dt) and extremely high carrier frequency to the insulation materials of a motor. The degradation of electrical insulation used in Inverter Driven Motors can lead to premature failure of motors sometimes days or months after startup. This paper presents the evaluation process of insulating materials used for IDM. Thermal aging tests were carried out on motorettes with enameled wires, impregnating resin and different insulating materials. Partial Discharge Inception Voltage (PDIV) measurement and Diagnosis test, based on IEC 60034-18-41 TS, were conducted on the motorettes. It could be demonstrated that the PDIV of the motorette decreased during aging and was very much dependent on the types of insulating materials being used