Rating of High-Temperature Induction Motors

The advent of new high-temperature insulating materials, such as the silicones, gives new freedoms to the motor designer. It is probable, however, that factors other than insulation will limit the desirable motor-temperature rise to values well below the endurance limits of these materials. It is, therefore, desirable to recognize that the size of an induction motor is measured electrically by its breakdown torque, and mechanically by the size of shaft and bearings, the temperature rise being only a secondary factor. Calculations are presented, based on idealized assumptions, which indicate that the optimum temperature rise for continuous rated induction motors is below 80 degrees centigrade, whatever the insulating materials used.