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Motor

Motor Input/Output Efficiency

Calculate motor efficiency, power losses, and heat dissipation from electrical input and mechanical output measurements.

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Formula

η = P_out / P_in × 100%, P_loss = P_in − P_out

η— Efficiency (%)
P_in— Electrical input power (W)
P_out— Mechanical output power (W)

How It Works

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 (I²R) losses in the windings, iron (hysteresis and eddy current) losses in the stator core, mechanical friction and windage losses, and stray load losses. Efficiency is not constant — it peaks at a specific load point (typically 75–85% of rated torque) and falls off at both light load and overload.

Worked Example

A 48 V BLDC motor draws 8.5 A while delivering 320 W of mechanical power. Step 1 — Electrical input power: P_in = V × I = 48 V × 8.5 A = 408 W Step 2 — Efficiency: η = (P_out / P_in) × 100 = (320 / 408) × 100 = 78.4% Step 3 — Total losses: P_loss = P_in − P_out = 408 − 320 = 88 W Step 4 — Temperature rise estimate (assuming natural convection, thermal resistance R_θ = 2 °C/W): ΔT = P_loss × R_θ = 88 × 2 = 176 °C above ambient Step 5 — Conclusion: The motor requires forced-air cooling or a heat sink. With a fan reducing R_θ to 0.5 °C/W: ΔT = 88 × 0.5 = 44 °C — acceptable for a motor rated to 85 °C rise.

Practical Tips

  • ✓Run the motor at 70–85% of rated torque for best efficiency; design your gear ratio and load to place the operating point in this range
  • ✓For battery-powered systems, measure total system efficiency (battery → controller → motor → load) rather than motor efficiency alone — controller losses of 5–15% significantly affect runtime
  • ✓BLDC motors typically achieve 85–95% efficiency vs. 70–85% for brushed DC motors of similar size — the tradeoff is controller complexity and cost

Common Mistakes

  • ✗Using rated efficiency at all operating points — motor efficiency at 10% load can be 20–30% lower than the peak-efficiency figure
  • ✗Measuring only DC input power for a brushless motor controller — switching losses in the inverter are part of the system loss and must be included
  • ✗Ignoring power factor for AC induction motors — a poor power factor means higher apparent power (kVA) even if real power (kW) efficiency looks good

Frequently Asked Questions

Measure DC input power (V × I at the motor terminals), and measure mechanical output power using a torque sensor and tachometer (P_out = T × ω). For small motors, a dynamometer or brake test with a calibrated load cell is common. Ensure the motor has reached thermal steady state before recording measurements.
The IE (International Efficiency) classification system rates AC induction motors from IE1 (standard) to IE5 (ultra-premium). Each class requires a minimum efficiency at rated load. IE4 and IE5 motors are synchronous reluctance or permanent-magnet types with efficiencies exceeding 95% at rated load.
Yes — operating below rated voltage increases current for the same torque, raising I²R copper losses and reducing efficiency. Operating above rated voltage increases iron losses. Most motors peak within ±10% of their rated voltage.

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