Polyphase Induction Motors with Unbalanced Rotor Connections

This paper investigates the application of symmetrical component analysis to the problem of predicting the characteristics of the wound-rotor induction motor when used with a speed-control scheme in which rotor resistors are commutated one at a time. General equations are developed for determining performance with the foregoing mode of operation and also for more extreme cases of rotor unbalance which results in the Görges phenomenon. Specialized equivalent circuits are presented as a means toward simplifying calculations and physically interpreting the phenomena associated with rotor unbalance. These circuits are shown to be valid for any case of symmetrical rotor unbalance. The assumption that rotor and stator resistances remain constant is shown to lead to large errors in computed speed-torque curves. It is also shown that these errors can be reduced considerably if a suitable linear approximation of the resistance-versus-frequency relationship is used not only with respect to the rotorfrequency variation but also with respect to the variation in the subsynchronous frequency component of stator current. Extensive comparisons of computed results with experimental results are presented in order to illustrate the range of applicability of the symmetrical component analysis when the foregoing approximation is used.