Which equation correctly defines AC motor efficiency?

• A. Eff = Pin / Pout × 100
• B. Eff = (Pout / Pin) × 100
• C. Eff = (Pin − Pout) / Pin × 100
• D. Eff = (Pout − Pin) / Pin × 100

Answer: (B)

Efficiency tells you how much of the electrical power put into the motor actually becomes useful shaft work. It’s the ratio of mechanical output power to electrical input power, expressed as a percentage. That means the correct formula is η = Pout / Pin × 100, where Pin is the power drawn from the electrical supply and Pout is the shaft power delivered to the load (torque times speed).

This ratio can never exceed 100% because losses in copper, core, windage, and friction reduce the shaft output below what’s drawn in. The other expressions invert the ratio, represent losses rather than efficiency, or would give nonsensical results (potentially negative or greater than 100%).