HomeApplicationVFD output 100% torque at low speed

VFD output 100% torque at low speed

Case: I have an application that require running an 75Kw high efficiency induction motor at rated torque at any rpm from 5% to 100% of the rated rpm. It will be controlled by a VFD with FOC.
This motor has independent cooling fan running with a separate motor that is capable of removing 5Kw heat from the motor. It will be controlled by a VFD with FOC. The motor is a good 95% efficient with cast copper rotor / not the regular cast aluminum. Can an induction motor give out full torque at 5% of its rated rpm with a VFD?? My concern is it may over heat at low frequency - not due to cooling fan because that is independent, but due to the other reasons.

The motor is going to run hot, but as long as the cooling fan does the job, I think you're on the safe side. What I'd recommend is to look at the fsw switching frequency in the variable frequency drive. Try to work with the minimum possible setting (2kHz,4kHz), more than that you're certain increase the losses into the motor (and the VFD drive). Are you going to run in encoder or encoderless vector? Not all the variable frequency drive manufacturers can work in an optimized way at low speed, I'd check that first. But as you're going to cool down the motor with the fan, I expect the stator and rotor resistance to remain stable, which should help the drive to control it at this low speed.
I am not as concerned about heat. Today's motors (yes this is a general statement) especially motors with copper rotor are designed to operate with VFDs. They are designed to move the heat to the outer skin of the motor for dissipation, the addition of the separate fan for cooling adds to the life of the motor, the exception is if your ambient environment temperature exceeds 40 degree C.

To ensure best performance 100% torque at low speeds, is achieved if you operate in close loop or vector control, with some form of performance feedback, ie... encoder.

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