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load efficiency

In induction motor, the efficiency is maximum for rated load. But for lighter loads, efficiency decreases. I wonder if the same principal applies for BLDC motor also?

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The efficiency vs. load curves for any types of electric motors will be generally similar. When the motor is turning with no load, there are still some losses, but the output mechanical power is zero. Since zero output divided by any input is zero, the no-load efficiency is always zero. The effect of the no-load losses has some effect at all loads, but the effect varies as the large differences in efficiency below 20% of rated load for different motor ratings in your example curves shows. Since permanent magnet motors have no excitation losses, the no-load losses will probably be less, but not zero.

When comparing one type of motor with another, it important to consider the losses in the control system. If the control system is considered, the shape of the curves will be similar, but the overall efficiency will be lower for all types of motor.

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  • \$\begingroup\$ What if BLDC motor use much less power for light loads (less armature current) and more power for heavy loads. Then efficiency over varying loads would be like equal right? \$\endgroup\$ – Alper91 Dec 15 '16 at 18:55
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    \$\begingroup\$ Efficiency would tend to remain constant over much of the load range just as it does with the motors shown in your example. However it must drop to zero at no-load. It is not going to suddenly drop to zero at no load. There will always be a curve of the shape shown, but most of the drop could occur below 10% or even 5% load. However, with very small motors, bearing friction and windage losses may not scale down as much as winding resistance losses do, so those losses may predominate at higher percent loads than you might expect. \$\endgroup\$ – Charles Cowie Dec 15 '16 at 19:12

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