Inverters are power supply devices that control the frequency (Hz) of three-phase induction motors to vary speed continuously. Speed can be controlled by changing frequency because the speed of rotation of induction motors is determined by the number of poles and the frequency. For example, a four-pole induction motor will rotate at a constant speed of 1500 min-1 at a power frequency of 50 Hz or 1800 min-1 at 60 Hz. Here, the speed of the motor can be freely varied, regardless of power frequency, by installing an inverter between the power supply and the motor.