Simple and reliable model for the thermal protection of variable speed self-ventilated induction motor drives

Self-ventilated, variable speed, induction motors cannot be protected using standard overload relays. At reduced operating speeds, the heat loss dissipation capability is deteriorated, and the motor could be subjected to overheating, even with operating currents below the rated value. The practical solution generally employed is to oversize the motor in order to avoid overheating at reduced speeds.