Which factor in motor ratings indicates the ability to operate above its rated horsepower?

• A. Service factor
• B. Efficiency
• C. Power factor
• D. Torque rating

Answer: (A)

Service factor denotes how much overload a motor can handle above its rated horsepower under specified cooling. It’s a multiplier applied to the rated horsepower to give the maximum allowable output in short-term overload conditions. For example, a 10 HP motor with a service factor of 1.15 can deliver up to 11.5 HP for a limited period, provided cooling and duty conditions meet the specification. This directly reflects the motor’s ability to operate above its rated horsepower.

Efficiency, on the other hand, tells you how well the motor converts electrical power into useful mechanical work but not how much extra horsepower it can safely produce. Power factor relates to how effectively the motor’s current and voltage are aligned, affecting current draw and electrical losses, not overload capability. Torque rating indicates the maximum torque the motor can produce, which relates to load capability but not indicating how much above the rated horsepower can be used for a short time.