Digital Computer Simulation of an Adjustable-Speed Induction Motor Drive with a Cycloconverter-Type Thyristor-Commutator in the Rotor

This paper deals with a digital computer-simulation study of a complex drive-system which incorporates a thyristor cycloconverter-type frequency-converter in the rotor circuit of a slip-ring induction motor for speed variation in the subsynchronous as well as the supersynchronous region by secondary voltage control The action of the frequency converter is analogous to that of a normal commutator in the stator-fed ac commutator motor while the circuit behavior is similar to that of a cycloconverter. 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. Simulation involves solution of a set of generalized performance equations of an ideal induction machine in an appropriate reference frame under the control conditions imposed by the thyristor-commutator which is simulated using simple logical and limiting statements. Differential equations are solved by the well-known Runge-Kutta numerical integration method. Initial simulation results assuming thyristors as ideal switches and neglecting source impedances show very similar characteristics to the case when a pure sine-wave slip-frequency voltage is injected to the rotor. Rigorous simulation results include the physical thyristor behavior, effect of source impedances, overlap, and logical control of the circulating currents that may occur. Simulation results are presented together with the experimental performance of the drive.