Motor Efficiency

Formula

Description

Motor efficiency is the ratio of useful mechanical output power to electrical input power. The difference is lost as heat due to copper losses (I²R in windings), iron losses (hysteresis and eddy currents in the core), friction and windage losses, and stray losses. Efficiency varies with operating point: it is typically low at no load (where iron and friction losses dominate) and at stall (where copper losses dominate), peaking at an intermediate load. Large industrial motors achieve 90-97% efficiency, while small hobby motors may only reach 50-70%.

Variables

• η — Motor efficiency (0 to 1)
• Pmech — Mechanical output power (W)
• Pelec — Electrical input power (W)

Practical Notes

IE efficiency classes for industrial motors: IE1 (standard), IE2 (high), IE3 (premium), IE4 (super-premium). Variable speed drives improve system efficiency by matching motor speed to load requirements instead of throttling. BLDC motors are generally 10-20% more efficient than brushed DC motors at the same power level due to eliminated brush friction and optimized commutation.