A simplified thermal model for power derating prediction of TEFC induction motors

In this article, a simplified thermal model for variable speed total enclosed fan cooled (TEFC) induction motors is proposed and experimentally verified. The thermal model is based on simple equations that are compared with more complex equations reported in literature. The proposed thermal model allows to predict the over temperature in the main parts of the motor, starting from the measured or the estimated losses in the machine. The description of the thermal model set up is reported in detail. Finally, the model is used to define the correct power derating for a variable speed PWM induction motor drive. In fact, if variable speed PWM drive employ general purpose self cooled induction motors, a torque derating is required due to motor efficiency and cooling effect reduction. This article presents in detail a procedure to preview the derating of self cooled induction motors, based on the proposed simplified thermal model, but accurate enough for practical applications. The obtained results are experimentally verified on five standard industrial motors of the same series (4-7.5-15-30-55 kW, 4 poles, 380 V, 50 Hz). The previewed overtemperatures match the measured ones with an average accuracy better then 10% (absolute error less than +/-5degreesC).