The rated speed of the motor is selected based on the requirements of the production machinery. When determining the rated speed of the motor, the transmission ratio of the mechanical reduction mechanism must be considered. The two cooperate with each other and can only be determined after a comprehensive technical and economic comparison. Usually, the motor speed is not less than 500r/min, because when the power is constant, the lower the motor speed, the larger the size, the more expensive the price, and the efficiency is lower. If a high-speed motor is selected, the mechanical reduction mechanism will inevitably be enlarged. The transmission ratio makes the mechanical transmission part complicated.

For some high-speed and medium-speed machinery that does not require speed regulation, such as water pumps, blowers, air compressors, etc., motors with corresponding speeds can be used for direct transmission without mechanical reduction mechanisms. For machinery that requires speed regulation, the maximum speed of the motor should be adapted to the speed of the production machinery. If a DC variable speed motor with changing excitation is used, in order to make full use of the motor capacity, the base speed of the excitation speed regulation should be selected. Another example is some steel rolling machinery, etc., which work at a low speed and are often in frequent forward and reverse operation. In order to shorten the forward and reverse transition time, improve production efficiency, reduce consumption, reduce noise, and save investment, appropriate selection is required. For low-speed motors, it is more reasonable to use direct transmission without reducer.

Intermittent cycle working machines that require fast and frequent starting and braking can usually achieve the fastest starting and braking effects when the product of the motor rotor's moment of inertia and the square of the rated speed is the minimum.