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How do you control the speed of an AC electric motor?

To control the speed of an AC motor, use a VFD to change the power supply frequency or adjust the number of poles in the motor. The choice depends on the motor type and application requirements.

AC motors are widely used in various industries. Because different applications have different requirements for motor speed, controlling the speed of AC motors is a factor that needs to be considered in many industries.

Method for controlling the speed of the AC motor

Change the power supply frequency

The most direct method is to change the frequency of the AC supply voltage. Because the synchronous speed of the motor is proportional to the power frequency, you can do this with a variable frequency driver (VFD), which converts the input power into variable frequency and voltage outputs. However, this method requires additional equipment and may produce harmonics.

Change Poles

The speed of the motor is inversely proportional to the number of poles. By increasing the number of poles, the speed of the motor will decrease, and vice versa. However, this approach is not as flexible as using a VFD, as it requires rewinding the motor and may not be suitable for all applications.

Change the load torque

The regulated load torque can be used to control the speed of the synchronous motor. When the motor is operating at near synchronization speed, a change in the load torque causes the motor to adjust its speed to maintain synchronization. However, this method is generally only applicable to applications where the load torque is variable, such as pumps or fans.


Controlling the speed of AC motors is critical for a variety of applications and can be accomplished in a variety of ways. The choice of method depends on the type of motor, the level of speed control required, and the specific application. By understanding the advantages and disadvantages of each technology, you can choose the solution that best suits your motor control needs.

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