AC drives are AC motor speed control systems.
Slip controlled drives control the speed of an induction motor by increasing a motor's slip, either by reducing the voltage applied to the motor, or increasing the resistance of the rotor windings. Because they are generally less efficient than other types of drives, slip controlled drives have lost popularity and have recently been used only in special situations. See eddy current drives above.
In larger ratings (more than a few kilowatts), a wound-rotor motor has its rotor connected to a converter that returns energy to the power system, converting it from low slip frequency to the line frequency. This reclaims the energy that would otherwise be wasted in rotor circuit resistors. These are called "slip energy recovery drives" and are used on such applications as forced-draft blowers for boilers. An electromechanical version using a rectifier, DC motor and AC generator is called a Kramer drive.