Various insulation materials are used in electric motors and each has its own function. The most important materials are:
- Wire insulation
- Slot and phase insulation materials (insulation between the winding and the stator lamination pack and phase insulation between the windings heads).
- Winding impregnation.
- Insulating sleeve used to cover wire/lead connections.
- Insulation of winding leads (between the winding and the terminal board).
All these insulation materials are specified in thermal classes that are referenced using a letter (Y—A—E—B—F—H—C). Every thermal class has its own temperature limit specified (see table). An insulation material of a specific class needs to retain its mechanical and electrical properties within the temperature limit and have a reasonably long service life. The maximum permissible temperature rise (see table) of the winding is determined based on the thermal class temperature limits. Continuous duty (S1) at the rated power output at an ambient temperature of 40˚C is specified for indoor/outdoor installations. The temperature of the winding increases as a result of the copper and iron losses in the electric motor during operation. The winding temperature rise is determined through measuring the resistance of the winding(winding resistance increases with increasing temperature).
Hot spots in windings cannot be determined using the winding resistance method. To allow for any hot spots in winding, lower temperature limits are specified for the used insulation materials. It is now a common practice to produce motors with insulation class F with winding temperature rise in accordance with class B (max. 80 K). This means that the motors have a temperature reserve of 25 K. This reserve can be utilized for short-term overload, a higher ambient temperature (above40°C), for supply voltage/frequency fluctuation etc. Should you know that if the thermal reserve would be utilized ,it is advisable to discuss the application requirements with the motor manufacturer.