No Load Power consumption of different Induction Motors

Dear Niazi,

You are right. Power is required to run the motor even at no-load against

bearing power loss, windage loss, plus the electrical losses like magnatising loss,

no-load copper loss etc. At lower range of motors the percentage (%) of this loss

to the total power may be high .In high power motors the % may be less when

compared to the total power. This is the reason to the best of my knowledge.

Manroop.Chennai

As a general rule, electromagnetic machines (transformers, motors, and generators) all benefit from economies of scale. The larger they are the more efficient they will be.

While efficiency should be expected to be lower in the smaller motors, I bet most of that no-load current in your first example was reactive and the efficiency wasn't quite as bad as you think.

That's why looking at current alone is never a good method of determining motor loading. At low load, you have high current, very low Power Factor, so the POWER consumption is actually low. The smaller the motor, the lower the PF is at low loading, because a grater relative percentage of the incoming power is being used just to create the magnetic field.

there is also the kinetic energy stored in the rotor of larger motors that assists the field in rotating the rotor in the larger motor.

This can be observed by the run down time once the power is switched off, a larger motor will take longer to stop than a smaller one.

Also if you're really interested, measure the air gap between rotor and stator on both motors and see if the gap is relative to the frame size/rotor diameter