What device is primarily used to control the speed of an electric motor?

The variable frequency drive (VFD) is primarily used to control the speed of an electric motor by adjusting the frequency and voltage of the electrical power supplied to the motor. By changing the frequency of the current that drives the motor, a VFD allows for precise speed control, which is essential in many applications that require variable speed operations. This capability not only enables more effective use of energy but also enhances the overall performance and longevity of the motor.

In contrast, a motor starter primarily serves to provide power to the motor and protect it from overcurrent conditions. While it does start and stop motors, it does not control their speed. Similarly, a programmable logic controller (PLC) is used for automation and control of complex processes and can interact with a VFD, but it does not directly control motor speed on its own. A relay switch functions as a simple on/off control mechanism and lacks the capability to vary speed, making it unsuitable for applications requiring speed modulation. Therefore, the VFD stands out as the most effective device for controlling an electric motor's speed.