Slot-Harmonic Fields in Closed-Slot Machines

This paper is concerned with the development of an analytical method for calculating the air-gap harmonic fields produced by current flowing in the bars of a cage rotor with closed rotor slots. The method defines harmonic correction factors which are used to modify the magnitudes of the flux density waves produced by the rotor slotting magnetomotive force harmonics to account for their interaction with the air-gap permeance harmonics introduced by the saturation of the rotor slot bridges. The correction factors themselves depend on harmonic air-gap permeance coefficients, which must be calculated by taking slot-bridge saturation into account. This is accomplished by using a slot-pitch finite-element model for the rotor. The method, which has been developed for routine design office use, is verified by using careful measurements made on a particularly constructed test rig.