Induction motors with unbalanced rotor connections

Abstract

THE USEFULNESS of the method of symmetrical components in analyzing the case of an induction motor operating with an unbalanced rotor circuit has been limited by a lack of conclusive evidence as to what assumptions can be made in applying this method. The nature of assumptions useful to this analysis has been established through comparison of numerous experimental and calculated speed-torque curves. An accounting for the variation of both rotor and stator winding resistances with frequency has been found necessary for accurate prediction of these characteristics. It has been found that a linear approximation of the resistance-frequency variation in these windings is sufficiently accurate for calculations on small wound-rotor induction motors. Blocked rotor tests over a range of frequencies have shown the resistance-frequency variation to be essentially linear for this type of motor. It is to be expected that certain cases involving a configuration of conductors in the motor windings such as deep bars or untransposed parallel windings may involve a departure from the linear resistance-frequency variation. In these cases either tests or design calculations can be used to establish a more suitable approximation.