An AC motor drive is underpowered or overloaded when the difference between full-load voltage and no-load voltage at the output terminals exceeds what percent?

• A. 10%
• B. 5%
• C. 1%
• D. 3%

Answer: (D)

The key idea is voltage regulation of the motor drive under load. An AC motor drive must hold the output voltage fairly steady as load changes. When the motor is loaded, current increases and voltage drops occur across the drive’s internal impedance and wiring, so the output voltage at the terminals falls from the no-load value. The difference between the full-load voltage and the no-load voltage is a direct measure of how well the drive maintains voltage under load. A small sag is expected, but if this difference exceeds about three percent, the motor isn’t getting enough voltage to develop the required torque, so it feels underpowered or appears overloaded. That’s why three percent is the threshold used. Larger tolerances like five or ten percent would mask real underperformance, and one percent is often too tight for practical drives.