Influence of variable frequency drive on motor winding temperature rise

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Courtesy of Variable Frequency Drives

Induction motors may heat up more when fed by variable frequency drive (VFD) than when fed by sinusoidal supply. This higher temperature rise results from the motor losses growth owing to the high frequency components of the PWM signal and the often reduced heat transfer resulting from speed variation.

The voltage harmonic distortion contributes to increase the motor losses, once that creates minor hysteretic loops in the lamination steel, increasing the effective saturation of the magnetic core and giving rise to high frequency harmonic currents, which bring about additional Joule losses. Nevertheless, these high frequency components do not contribute to the production of torque at steady operation of the motor, since they do not increase the airgap fundamental flux, which rotates at synchronous speed. The operation at low speeds causes the ventilation over the (self-ventilated) motor frame to decrease, consequently lowering the motor cooling and raising in this way the thermal stabilization temperature.

Therefore, when operating with VFDs, both the effects mentioned above must be considered. There are basically the following solutions to avoid excessive overheating of the variable frequency drive fed motor:

  • Torque derating (oversizing of the self ventilated motor frame);
  • Utilization of independent cooling system (separate ventilation);
  • Utilization of the “Optimal Flux Solution” (exclusive to applications using Gozuk VFDs and motors).

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