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Let us say there is a constant load which is driven by a geared 3-phase ACIM (AC Induction Motor) of 5kW power rating. The mechanical load and its speed is constant.

If I would need to replace this ACIM motor and gear with a BLDC motor and gear so that it could drive the same load with same speed as before then would I need to use a BLDC motor of 5kW power or is it possible that a lower power BLDC motor will also be able to drive the same load with same speed? The reason I think that a lower power, say 3kW BLDC motor will also be able to drive the same load is that BLDC are more efficient but I am not sure if my understanding is correct or not.

If my mechanical load is same then is it possible that a different type of motor will have different electrical power to drive the same load with same speed?

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Motors are generally rated by their output, mechanical, power. So if you want to replace a 5 kW ACIM by a motor of any other type, then you should choose a 5 kW motor.

It may be that your load does not need 5 kW to drive it, and the original motor is over-sized. In which case you could investigate your load, find out what it does need, and then choose a smaller motor to provide just that.

Different types of motor are likely to have different efficiency, so will require different input electrical power to provide the same mechanical output power. At the 5 kW level, you could expect to find the efficiency differences to be fairly small.

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Induction motors are very efficient - it should be stated in the datasheet for the motor. It is more likely the BLDC motor will be less efficient. The BLDC motor will most likely be smaller and lighter though. Other considerations might be starting torque, but it is more likely the BLDC motor will have superior starting torque to the induction motor.

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