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When a TEFC induction motor fed by static frequency converter drives a load which demands constant torque throughout the operation range, the low speeds are thermally critical, because the motor losses (heat sources) do not vary much as a function of the speed, but the ventilation efficiency decreases as the operation speed falls down, since the fan is installed on the very motor shaft. In such cases, when operating at low speeds, the temperature rise often exceeds the limits of the motor thermal class due to the lack of cooling. In order to prevent this problem the industry has traditionally adopted one of the following solutions: independent ventilation (a small auxiliary motor is used to exclusively drive the fan that provides the main motor cooling) or oversizing (the motor used in the application provides a higher torque than the rated load demand). However, neither one nor the other of these two options are attractive, as both, besides increasing the space required for the installation, still increase the motor price [1]. Frequency converters usually apply to the motor a constant voltage/frequency ratio throughout the operation range, so that no loss control is provided. But the study on the motor losses composition and its relation with voltage, frequency, magnetic flux and current, allied with the study on the influence of the ventilation on the temperature rise of the motor, has led to an optimal voltage/frequency ratio, which minimizes the total motor losses at each speed [2]. This way, by implementing the automatic control of the voltage/frequency ratio in the converter, the motor loss minimization can be automatically obtained throughout the frequency range, so that the motor temperature rise is kept within the thermal class limits even at low speeds with reduced ventilation.

Minimization of Losses in Converter-Fed Induction Motors – Optimal Flux Solution