Low Speed Overheating

With constant torque applications, there is a possibility that the motor will overheat during low speed operation. A constant torque load draws the same amount of current regardless of the motor's operating speed. This creates a problem at very low speeds, because the drive reduces motor frequency in order to decrease motor speed. If the motor produces a high level of heat while operating at low speed, overheating results.

As a rule of thumb, a fully loaded motor with Class B insulation running at 50% rated speed on a constant torque load will not overheat. If the motor is run below 50% speed continuously, it will overheat. Generally, the speed of a fully loaded motor with Class F insulation may be decreased to approximately 20% speed without overheating. The lower the continuous operating speed below the motor overheating point, the more the motor and drive must be derated.