Which aspects of an AC motor are determined by its frame size?

• A. Foot hole mounting pattern, shaft diameter, and shaft height.
• B. Color of the motor.
• C. Horsepower rating.
• D. Mounting bracket type.

Answer: (A)

Frame size defines the motor’s physical interface with the equipment it drives. It fixes the mechanical dimensions you must match in a drive system: where the mounting holes on the base are located (foot hole mounting pattern), the diameter of the shaft (what couplings or pulleys must fit), and the shaft height (to keep the driven shaft in proper alignment with the belt or gear train). These dimensions are standardized so motors of the same frame size can interchangeably bolt into the same bases and align correctly with driven components.

Color is purely cosmetic and has no bearing on the motor’s mechanical fit. Horsepower rating comes from the electrical design, insulation, and cooling of the windings; while a larger frame can accommodate higher power, that rating isn’t set by frame size alone. Mounting bracket type is a mounting accessory choice and, while related to how a motor is installed, isn’t a defining attribute of the frame size itself.