How to Select an AC Motor: A Motion Engineer’s Guide

AC motors are among the simplest of motors amid the common choices for motion control systems. There are a few basic criteria for selecting an AC motor. These involve common variables such as the operating speed and load torque, in addition to the power supply voltage and frequency.

First of all, calculate the operating speed. Induction and reversible motor speeds cannot be adjusted. This is why the motor speed must be reduced with gearheads to match the required machine speed. It’s therefore necessary to determine the correct gear ratio.

Next, calculate the required torque. In a single-phase induction motor, starting torque is always lower than the rated torque. Therefore, to drive a frictional load, select the speed on the basis of starting torque. This will cause the actual speed to exceed the rated speed. Also, motors are designed so that increases in motor temperature are at their lowest when operating close to the rated speed of rotation.

Then, knowing the required torque and speed, select a motor and a gearhead. Select a motor wattage and gear ratio based on the required torque and speed. Then confirm the permissible inertia. The permissible inertia of the selected motor and gearhead combination should be greater than the load inertia. Refer to the manufacturer’s guide of the permissible inertia for each size of motor and gearhead.

As a common consideration for all motor types, an electromagnetic brake type motor is recommended for any vertical operation to provide holding force for the load when the power is down. Regeneration units are also recommended for vertical or large inertial operation to handle the back EMF generated from the motor.

In addition to the selection procedure, accurate calculation of the speed, torque, and inertia based on the motion requirements and mechanisms are also critical. These calculations take time and experience. Online calculators such as this motor sizing program from Oriental Motor ( can help. And because this tool is designed as an engineering tool, it does not recommend products after calculation. Product selection requires other considerations such as input voltage, required functions, features, and user preferences.

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