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The purpose of this paper is to propose a simplified thermal circuit to estimate the temperature rise in the winding of a totally enclosed fan cooled electric motor for different loads and/or cooling conditions since the motor has already been tested for a known condition.

The determination of the convection resistance is based on classical Nusselt correlations and the value of conduction resistance is provided from a known load condition and the corresponding temperature rise in the winding.

Predicted temperature values showed good agreement with the experimental results, demonstrating that the hypothesis of simplification to a punctual source of thermal energy is acceptable.

It is necessary that the motor has already been tested for a known condition (losses, temperature, and ventilation). Although the basic idea of this methodology is based on the use of a reference test condition of the same motor, as a suggestion, with small modifications the same methodology can be used to estimate the thermal behavior of different sized motors, provided a similar motor has already been tested.

This approach results in a fast methodology to estimate the thermal behavior of an electric motor in different loads and/or cooling conditions.

The differential of this circuit is the use of only two thermal resistances, one for the whole conduction and the other for the convection. This approach is a way to overcome the difficulties related to the determination of the thermal contact resistance and the equivalent conduction resistance between the winding and the different isolation systems inside the slots.