The shaft speed of an electric motor is most commonly expressed in which unit?

• A. Feet per second per second
• B. Feet per second
• C. Revolutions per second
• D. Revolutions per minute

Answer: (D)

Rotational speed describes how many full turns the shaft makes in a given time. In practice, this is most commonly expressed as revolutions per minute because it aligns directly with mechanical interfaces like gears, belts, and couplings, and with how motor ratings are specified. An RPM value is easy to relate to the motion of machinery: it tells you how many times the shaft completes a circle each minute, which you can then convert to other useful forms as needed (for example, to radians per second with ω = 2π × RPM/60, or to revolutions per second by dividing by 60).

The other units describe different kinds of motion that aren’t the shaft’s rotation. Feet per second or feet per second squared refer to linear speed or linear acceleration, which depend on radius if you tried to apply them to a rotating shaft, making them inappropriate for describing shaft speed itself. Revolutions per second is a valid measure, just less common in industry, since RPM is the standard for naming speeds and matching with gear ratios and performance specs.