Influence of Ambient Conditions on the Qualification Tests of the Interturn Insulation in Low-Voltage Electrical Machines

An increased dc-link voltage and fast-switching semiconductors constitute new challenges on the insulation system of low-voltage electrical machines, especially in automotive applications. During the design process, the different components of the insulation system can be tested according to IEC 60034-18-41. This industrial standard prescribes certain test voltages for type-I insulation systems based on empirical values. However, important ambient influence factors such as humidity and air pressure are not considered. Therefore, this work focuses on the influence of ambient conditions on the insulation strength of low-voltage electrical machines. Partial discharge tests are performed on specimens representing the interturn insulation which is usually regarded to be the weakest part of the insulation system. To represent the electrical stress during inverter operation, a bipolar test voltage is applied. The results of the measurements show that the standard is not applicable for the studied conditions in this work. It is concluded that the humidity and especially the ambient pressure have to be considered. The standard method using safety factors to calculate the test voltage is discussed. The method is extended to consider ambient conditions which are not covered by the standard.