Saturated leakage reactances of cage induction motors

Abstract

The paper presents a method of predicting the saturation of the leakage reactances of cage induction motors. It commences with a discussion of leakage fluxes and reactances and the manner in which they are affected by magnetic saturation. The calculation of the components of leakage reactance under unsaturated conditions is described. The theory then progresses to the calculation of the magnitude and phase of the leakage-flux components likely to be affected by saturation, and to the determination of the leakage-flux paths. A general method is given for computing reduction factors, or saturation factors, to be applied to those components of leakage reactance with saturable associated fluxes. A logical system for dealing with complex magnetic circuits which contain combinations of fluxes in a number of saturable areas is described and explained by the use of examples. Saturable areas in the magnetic circuits of induction motors are defined for a number of alternative slot shapes. The variation of skew-leakage flux, and therefore of the saturation, as a function of axial position is treated by subdividing the core length into short sections and summing their reactances. Results of computations are compared with test results on 18 medium and large motors, and good correlations are shown for both unsaturated and saturated locked-rotor impedances.