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Thermal time constants of motor

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Hello merajab

Welcome to the forum


Induction motors have thermal mass, and they also have thermal resistance. The thermal mass determines the temperature rise per joule of energy absorbed by the motor and the thermal resistance is the ability of the motor to dissipate that energy. If the motor has a High thermal mass, it will take longer to heat up and cool down than a motor with a low thermal mass.

Likewise, a motor with a low thermal resistance will cool quicker than a motor with a high thermal resistance.

Under steady state conditions, with a constant power being dissipated by the motor, the thermal resistance determines the temperature rise of the motor.

The time constant of the motor gives a measure of both the thermal mass and thermal resistance of the motor. If the motor is subjected to high but short overloads, then it does not need to have a high time constant, but if the overloads are longer, then the thermal time constant needs to be longer to prevent the motor being over heated.


Best regards

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dear Mark Empson


Thank u very much for the beautiful expanation. I would appreciate if u kindly tell me the application of the thermal time constants with regards to the thermal withstand curves. Now, the thermal time constants that I have come accross are given in minutes and after reading your reply it makes sense to me. a bit of elaboration on this or any helpful link is welcome as I am not able to find it clearly in any text book and I have got to test a motor protection relay soon.


Thank u in advance



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