An Adjustable-Speed Induction Motor Drive with a Cycloconverter-Type Thyristor-Commutator in the Rotor

An adjustable-speed drive comprising a slip-ring induction motor and a cycloconverter-type thyristor-commutator in its rotor circuit has been developed. The thyristor-commutator acts as a frequency-changer like a conventional commutator in a stator-fed ac commutator motor, converting line-frequency injected voltages directly to slip-frequency at the rotor terminals. The circuit permits a reversible power flow naturally, and speed-control is possible for subsynchronous as well as supersynchronous operation by controlling the injected secondary voltage. A rotor position-detector is used to switch the thyristor configuration in a sequential manner to generate an output voltage having a predominant slip-frequency component. The control logic and triggering circuit developed for the experimental drive are described in some detail. Experimental results as obtained with a laboratory model are presented together with some results of computer simulation of the drive system. Major limitations of the scheme and the scope for improvement are discussed.