AC drives are AC motor speed control systems. The speed is controlled by changing the frequency of the electrical supply to the motor. The three-phase voltage in the national electrical grid connected to a motor creates a rotating magnetic field in it. The rotor of the electrical motor will follow this rotating magnetic field. An AC drive converts the frequency of the network to anything between 0 to 300Hz or even higher, and thus controls the speed of motor proportionally to the frequency.
The types of motors that AC drives control are normally operating at constant speed. Enabling the user to control the speed of motor potentially gives him various benefits in terms of process control, system stress and energy savings.