The physical design of a brushless DC motor can have a major effect on how easily it can be integrated into a machine. Electrical performance may determine whether the motor can produce the required output, but dimensions and mounting characteristics determine whether it can actually fit the equipment. Shaft diameter, shaft length, mounting holes, motor body dimensions, connector position, and overall weight can all become important during mechanical design.

NEMA frame sizes provide a useful reference when a standardized mounting arrangement is required. Different sizes are intended for different physical and performance requirements, with configurations such as NEMA 23, NEMA 32, NEMA 34, NEMA 42, NEMA 52, and NEMA 72 offering different mechanical envelopes. However, the NEMA designation should not be treated as a complete description of motor performance. Two motors with similar frame dimensions can have different torque, speed, voltage, winding, and thermal characteristics.

The internal construction of the motor can provide additional flexibility. Inrunner and outrunner designs place the rotating and stationary components in different arrangements, which can affect speed, torque, and mechanical integration. A frameless BLDC motor can be useful when the motor needs to become part of a larger mechanical assembly rather than being installed as a conventional enclosed unit. Sensored and sensorless configurations provide further choices depending on the control requirements.

Mechanical integration should also take the surrounding drive system into account. Couplings, gears, belts, pulleys, wheels, and other transmission components can change the torque and speed requirements seen by the motor. The motor shaft must be compatible with the transmission while the mounting structure must be able to handle the expected mechanical forces. These considerations are particularly important when a brushless motor is being integrated into equipment with tight dimensional constraints.

For this reason, selecting a BLDC motor should involve more than checking the motor's rated power. A successful installation requires the electrical specifications, torque-speed characteristics, frame size, shaft configuration, mounting arrangement, and surrounding transmission to work together. Considering mechanical integration at the beginning of the design process can prevent costly modifications later.