How is speed governed in an electric motor?

The speed of an electric motor can be governed by controlling the applied voltage or using a variable frequency drive (VFD). This is because the speed of most AC motors, particularly induction motors, is predominantly determined by the frequency of the power supply. By adjusting the frequency through a VFD, you can effectively control the speed of the motor. For DC motors, varying the applied voltage directly affects the speed; as the voltage increases, the motor speed increases, and vice versa.

Using a VFD not only allows for precise control of speed but also improves energy efficiency and reduces wear on the motor. This control method is widely used in various applications where speed regulation is important.

The other methods outlined in the incorrect options do not provide effective means for governing motor speed in comparison to VFDs or voltage adjustment. Varying resistance or limiting current flow can affect motor torque and operation, but they do not offer the same direct control over speed as adjusting voltage or frequency does. Adjusting the physical size of a motor also does not serve as a method for adjusting speed; it would only change the motor's capability and performance characteristics without allowing for dynamic speed regulation.