# Why is an electric motor more efficient at higher loads?

My question is driven by the plot below. We see that acceptable operating range of a motor is between 50-100% of the rated load. Below 40% or so the efficiency of the motor drops off dramatically.

What is the cause of this phenomenon?

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Because at low loads you're still spending the same energy just getting the armature to turn, but you are drawing out less energy for useful work. – Robert Harvey Dec 6 '12 at 18:21
It depends on the type of electric motor. AC, DC, and there are plenty of subtypes of both. You'll be more likely to get a good answer if you can provide this information. – Alan Rominger Dec 6 '12 at 20:07

The other way to ask your question is why is efficiency low at low loads? Friction is the main cause of inefficiency at low loads. Losses due to friction are essentially constant with respect to load so at low loads, the majority of your input power may be used to overcome friction. As the load increases, friction plays a smaller and smaller roll in the overall efficiency. Granted, other inefficiencies begin to occur at larger loads ($I^2R$ losses, copper losses, stray load losses, etc.) but in a well-designed motor the efficiency will peak in the 80-100% load range.