# 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

First, efficiency of an electric motor is just output power divided by input power. Input power is your electrical input power, which is V*I. Output power is your mechanical output power, which is speed*torque.

Given that, we can see that efficiency for every motor is going to be 0% at no load (i.e., maximum speed at 0 torque). Efficiency will then increase as torque increases until it reaches a maximum and then it will start to drop off until stall torque is reached. At this point, the efficiency is 0% again because the speed will be zero.

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.

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Thanks Eric. It all seems so simple now. Appreciated! – TerryTate Dec 7 '12 at 20:57