Motors and transformers
Motor efficiency and losses
- where
- efficiency
- mechanical output power (W)
- active electrical input power (W)
- motor losses (W)
Explain this equation
Gives a motor's efficiency and the power it loses, from its shaft output and electrical input. Use it to compare motors or estimate the heat a motor gives off.
P_out is the useful mechanical power at the shaft and P_in the active electrical power drawn, both in watts. η is their ratio, a decimal below 1. P_loss is the difference, the power lost inside the motor.
Energy is conserved: whatever goes in but does not come out at the shaft is lost, mostly as heat in the windings, heat in the iron core, and friction and fan losses. The ratio shows the useful fraction.
800 W out from 1000 W in gives an efficiency of 0.8 and 200 W of losses. Small motors are often 70–85% efficient; large modern ones exceed 95%. One per cent on a big motor running all year is a real saving.
Watch out: use active input power in W, not apparent power in VA, or power factor gets mixed into the efficiency. Both readings must be taken at the same load.
Efficiency compares useful shaft power with active electrical input. The difference becomes losses in the motor, including winding, iron and mechanical losses.
Use the same power units on both sides. Power factor and efficiency are separate: power factor links active and apparent input power, while efficiency links input and output.
Enter your values to see the result.
Common mistake
Do not divide shaft kW by input kVA and call it efficiency.
Limits
Values must refer to the same operating point. Nameplate efficiency at rated load is not necessarily part-load efficiency.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.