Stray losses of polyphase cage-induction motors with particular reference to the condition of imperfect rotor-bar—iron insulation

The paper presents an analysis of stray losses in polyphase cage-induction motors, under linear and saturation magnetisation conditions, with particular attention being given to the effects of circumferential currents flowing in the rotor as a consequence of imperfect bar-iron insulation. Such effects are included in a modified equivalent circuit of the motor, and are expressed in terms of factors readily determined from a set of normalised curves, not previously available. For example, the consequent reduction in the effective skew of the rotor bars can be determined, and thus a more correct evaluation obtained for those factors and losses dependent upon the resultant skew. Also, a simple graphical method is given for determining the effective increase of the air gap due to main-flux saturation, which assists in demonstrating that favourable agreement between the predicted and experimental torque/slip characteristics can be obtained with some reduction of the empiricism employed hitherto.