Power Energy
Motor Efficiency
Motor efficiency is the ratio of mechanical output power to electrical input power, expressed as a percentage. The difference between input and output is lost mainly as heat in the windings, the magnetic circuit and the bearings. Efficiency is not constant: it changes with load, so a value quoted at rated load should not be applied to a lightly loaded motor.
Motor Efficiency
η = P_out ÷ P_in × 100%
Variables and units
| Symbol | Quantity | Unit |
|---|---|---|
| η | Efficiency | % |
| P_out | Mechanical output power | W |
| P_in | Electrical input power | W |
Conditions of application
Steady-state operating point. Input and output must be measured or specified at the same load and conditions; efficiency varies with load, speed and temperature.
Worked example
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Efficiency, losses and input power for an electric motor.