Electrical Motor Efficiency
Reference data and engineering information about electrical motor efficiency for electrical applications.
Overview
Electric motor efficiency is the ratio of mechanical power output at the shaft to the electrical power input. This key performance metric indicates how effectively a motor converts electrical energy into useful work. Losses occurring within the motor—including copper, iron, mechanical, and stray losses—prevent the efficiency from reaching 100%.
Key Formulas
Efficiency (Power in Watts): When shaft output power is measured in Watts (W), efficiency (η) is:
η = (P_out / P_in) × 100%
Where P_out is mechanical power output and P_in is electrical power input.
Efficiency (Power in Horsepower): When shaft output power is measured in horsepower (hp), the formula incorporates the unit conversion:
η = (P_out × 746) / P_in × 100%
Copper Losses: Power lost in the windings due to resistance, which varies with the square of the load current.
P_cl = I² × R
Variables
η: Motor efficiency (%)P_out: Mechanical power output at the shaft (W or hp)P_in: Electrical power input to the motor (W)P_cl: Copper loss in windings (W)I: Current (A)R: Winding resistance (Ω)
Efficiency Reference Data
Motor Power Range | Min. Nominal Efficiency(%) |
|---|---|
| 1 - 4 hp | 78.8 |
| 5 - 9 hp | 84 |
| 10 - 19 hp | 85.5 |
| 20 - 49 hp | 88.5 |
| 50 - 99 hp | 90.2 |
| 100 - 124 hp | 91.7 |
| ≥ 125 hp | 92.4 |
Source: engineeringtoolbox.com
Calculators
Motor Efficiency (Watts)
Motor Efficiency (Horsepower)
Unit Converter
Motor Efficiency Unit Converter
Motor Loss Categories
Primary and Secondary Resistance Losses
Primary stator and secondary rotor resistance losses are copper (I^2R) losses. They increase with the square of load current and usually form the largest loss component at high load.
Iron Losses
Iron losses occur in the magnetic core from hysteresis and eddy currents. They are mainly set by voltage, frequency, and core material and are present whenever the motor is energized.
Stray Losses
Stray load losses are additional electromagnetic losses caused by leakage flux, harmonics, slot effects, and non-ideal current distribution. They are difficult to measure directly and are often estimated by standard test methods.
Mechanical Losses
Mechanical losses include bearing friction, windage, and cooling fan power. They depend on speed, bearing condition, lubrication, and enclosure/cooling design.
Restored Original Source Tables
The following tables are restored from the original source page to preserve the complete reference data.
Electric Motors - Efficiency
Power (hp) | Minimum Nominal Efficiency1) |
|---|---|
| 1 - 4 | 78.8 |
| 5 - 9 | 84 |
| 10 - 19 | 85.5 |
| 20 - 49 | 88.5 |
| 50 - 99 | 90.2 |
| 100 - 124 | 91.7 |
| > 125 | 92.4 |
Source: engineeringtoolbox.com
Engineering Notes
- Load Dependence: Motor efficiency is not constant; it varies with the load. Maximum efficiency typically occurs near 75-100% of rated load.
- Loss Components: The main losses are copper (I²R), iron (core), mechanical (bearings, fan), and stray (harmonics). NEMA standards set minimum efficiency requirements for common motor designs.
- Standards: The provided efficiency values are minimums for NEMA Design B motors under specific operating conditions. Actual motors often exceed these values.
- Application: For continuous operation (e.g., pumps, fans), selecting a high-efficiency motor can significantly reduce lifecycle energy costs.