The slip energy recovery system for wound rotor induction motors
The overall principle of slip energy recovery systems (or 'static Kramer systems') is to insert a variable back-EMF into the rotor circuit in such a way that the resultant energy can be recovered and fed back into the AC mains network which is feeding the stator of the induction motor. The result can then be an efficient method of reducing the speed of the motor. These principles were initially established using motor generator sets to achieve the energy recovery and feedback, with a DC motor to absorb the energy and an AC generator to return the power to the mains network. However, due to high cost and the relatively high power losses in such schemes, the principles of slip energy recovery are now universally applied using static convertors. This system requires the use of a wound rotor induction motor with slip rings to connect into the rotor circuit. It therefore tends to be used in custom designed systems where the motors and convertors are specifically chosen for the application. It is used for drives in the hundreds or thousands of kilowatts ratings where the cost of a specially designed system can be justified.