A DC electric motor is a motor which uses electricity from a direct current (DC) electrical source to produce mechanical energy. As with any mechanical object that converts one form of energy to another, some energy will be lost during the conversion, in the form of heat, sound, friction and other forms of loss. The efficiency of a motor is how much of the input energy (or power) is converted to usable mechanical energy. Find the efficiency of a DC electric motor in a few short steps.
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Determine the shaft power produced by the motor. Find this by multiplying the speed and the torque of the motor, and dividing the product by 5252, which gives the power in horsepower (hp). For example, if the speed is 1500 revolutions per minute (rpm) and the torque is 5 foot pounds (ft lbs), the product of this is 7500 ft lbs. Dividing this by 5252 gives 1.43 hp.

Multiply the power output in horsepower by 746 watts per horsepower (W/hp), which will give the output power in units of watts (W). In our example, multiplying 1.43 hp by 746 (W/hp) gives 1064.9 W. Call this number result B.

Divide result B by the electric power supplied to the motor. For example, if an input power of 1500 W is provided to the motor, dividing result B (1064.9 W) by this input power gives 0.71, which is the efficiency of the motor.
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