Motor Efficiency
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The difference between input and output is heat: stator and rotor I²R losses, core losses, windage and friction. A 10 hp motor at 88% efficiency swallows 7457/0.88 ≈ 8474 W to deliver 7457 W of shaft power, dumping about a kilowatt into the room — which is why motor rooms need ventilation and why efficiency shows up twice in an energy audit, once as electricity and once as cooling load.
Efficiency is not constant: it peaks near 75–100% of rated load and falls off a cliff below about 40%, which is the strongest argument against habitually oversizing motors. Since the 1990s, minimum efficiencies have been legislated — IE3 "premium" class in the IEC world, EPAct and NEMA Premium in the US — and the gain from an IE1 to an IE3 machine, a few points, usually repays the price difference within a year of continuous running.
- = Efficiency
- = Output power
- = Input power
- Efficiency — Three-Phase Motor Full-Load Current, Boiler or Furnace Output from Input
- Output power — Decibel Power Gain, Three-Phase Motor Full-Load Current
- Input power — Decibel Power Gain, Coefficient of Performance (COP)