Control of wound-rotor induction motor with rotor impedance variation

It is proposed to modify the performance of a three-phase induction motor, fed from a conventional three-phase supply of constant voltage and frequency, by the use of a network of general, slip-dependent impedances connected externally into the secondary circuit, via slip rings. The impedances to be connected into the secondary circuits of the motor are not resistors but passive impedances of a more general kind. It is shown that certain steady-state motor performance criteria can mostly be represented by circular loci in the secondary impedance plane and that these loci may be used to design appropriate external secondary circuits for optimum power factor, efficiency, and efficiency-power factor product, with or without capacitance circuits on the primary side. For both series-connected and parallel-connected external secondary circuits, the necessary characteristics are given for variation with frequency of the resistive and reactive components