Formula & Calculator
Motor Efficiency
The fraction of electrical input power a motor converts into useful mechanical output power.
Interpretation
Motor efficiency η = P_out / P_in is the ratio of mechanical output power to electrical input power.
It is always less than 1 (or 100%) due to losses.
Example: P_in=2000W, P_out=1700W → η = 1700/2000 = 0.85 (85%).
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| η | Motor efficiency | dimensionless (0 to 1) |
| P_out | Output power (mechanical) | W |
| P_in | Input power (electrical) | W |
What it means
Motor efficiency η is the ratio of mechanical output power to electrical input power, expressed as a percentage. η = (P_out / P_in) × 100%. Losses include copper losses (I²R in windings), core losses (hysteresis and eddy currents), mechanical losses (friction and windage), and stray losses. High‑efficiency motors are essential for energy savings. Motor efficiency varies with load; it is highest near full load. The efficiency is used to size motors and to compare their performance. Example: An electric motor draws 2000W of electrical power and delivers 1700W of mechanical power. Its efficiency is (1700/2000)*100 = 85%. The remaining 15% is dissipated as heat, which must be removed by cooling.
Worked example
Motor Efficiency – Practical Example
Real‑World| Parameter | Value |
|---|---|
| Pin | 500 W |
| Pout | 400 W |
| Formula | η = (Pout / Pin) × 100% |
Common mistakes
Watch unit consistency and the assumptions behind the formula; misapplying it outside its valid conditions is the most frequent error.Applications
Motor efficiency η = P_out / P_in is the ratio of mechanical output power to electrical input power. It indicates how much of the input energy is converted to useful work. Engineers use it to select motors, to reduce energy costs, and to comply with efficiency standards. By measuring efficiency, they can identify losses and improve motor design. This formula is essential for energy management and sustainability.
- Motor selection for energy‑efficient applications
- Energy audits and cost reduction projects
- Compliance with motor efficiency standards (IE codes)
- Loss analysis and motor design improvement
- Educational understanding of motor efficiency
Frequently Asked Questions
Efficiency is the ratio of mechanical output power to electrical input power: η = P_out / P_in, usually expressed as a percentage.
Copper losses (I²R in armature and field), brush losses, core losses (hysteresis and eddy currents), mechanical losses (friction and windage), and stray losses.
Efficiency is low at very low loads (fixed losses dominate) and at very high loads (copper losses dominate). Maximum efficiency occurs when variable losses equal fixed losses.
Small motors: 50‑70%; large industrial motors: 85‑95%.
Common errors: 1) confusing efficiency with power factor, 2) using the wrong input/output (e.g., mechanical vs electrical), 3) not accounting for all losses, 4) using peak efficiency for all loads.
Motor selection, energy cost estimation, and performance comparison.
By measuring input power (with a wattmeter) and output mechanical power (torque × speed), or using a dynamometer.
Efficiency is the ratio of output power to input power; power factor is the ratio of real to apparent power in AC circuits.