Permeance of closed-slot bridges and its effect on induction-motor-current computation

A procedure is described for the calculation of the specific permeance of the bridge of a closed slot as a function of the slot m.m.f. It is an improvement on other published procedures as it includes the changing saturation in each section of the bridge, using the actual magnetisation curve of the material to calculate the m.m.f. drop. The influence of the slot-top and airgap leakage fluxes on the saturation of the bridge is also included. For cast-aluminium rotors, where the rotor slot is completely filled by the conductor, account is taken of the changes in m.m.f. producing the slot-top leakage fluxes. The theory is applied particularly to slots having semicircular tops, but it is sufficiently general to be capable of application to any shape of slot. Further, the method employed can readily be adapted to similar problems involving parallel magnetic circuits having changing cross-sectional areas and varying degrees of saturation. Computed and experimental results of the specific permeance of slot bridges, and the variation of the input currents with the rotor skew of a cage induction motor, confirm the validity of the theory.