Induction-motor normalised stray loss with or without central rings in rotor cage

An expression is derived for the bar plus interbar stray loss of a cage induction motor when conducting rings are equispaced between the endrings of the rotor. The parameters of the rotor are normalised on the basis of the per-unit bar reactance. In consequence, it is shown that a single family of characteristics can be derived for the stray loss so that it is applicable to any number of central conducting rings, including none, and any values of core-stack length, harmonic order, rotor slip, skew, bar reactance, induced e.m.f. and interbar resistance for a given value of bar resistance. The forms of the characteristics can be changed to assess more readily the effect of the variation of a particular parameter or parameters. By such means, the limits of the interbar-resistance values for which the stray-loss torque reduces or increases a torque dip, owing to the addition of one or more central rings, can be shown directly for a given value of skew, and its practical implications are discussed.