Horsepower ratings given to electric motors relate to the motor's mechanical output.

• A. Electrical input power
• B. Mechanical output power
• C. Efficiency
• D. Voltage

Answer: (B)

The horsepower rating is a measure of the shaft’s mechanical output power. It tells you how much useful work the motor can deliver to the load through its rotor, at a specified speed and torque. Since a motor converts electrical energy into mechanical energy, the shaft power is related to the electrical input by efficiency: output power = input power × efficiency. That means the electrical input needed to deliver a given horsepower is higher than the shaft horsepower when there are losses.

Voltage itself isn’t the horsepower. It influences current and input power, but the rating describes the actual mechanical power available at the shaft, which is torque times rotational speed. If you know the speed and torque, you can compute horsepower directly, and if you know the efficiency, you can relate it to the electrical input.