Saturation Factors for Leakage Reactance or Induction Motors

The leakage reactance of an induction motor that determines the locked-rotor current at full voltage is generally much smaller than the reactance that determines the full-load power factor or the pullout torque. Anything that can be done to bring these extreme values of the reactance together is helpful, as the designer can thereby either reduce the locked-rotor current or increase the power factor and pullout torque, whichever he prefers. A chief cause of reduced reactance at locked rotor, i.e., under starting conditions, is magnetic saturation of the stator and rotor tooth tips, due to the combined effects of zigzag and slot-leakage flux. In this report, the way in which this saturation occurs is analyzed, and a method to account for it is developed. A chief conclusion of the paper is that the reactance saturation factor is sharply reduced if the angle between the air-gap periphery and the slope of the teeth overhang is small. For motors to be started on full voltage, this angle probably should be greater than 30 degrees, especially for rotors with closed-slot bridges. It is concluded also that the current versus voltage curve of an induction motor with locked rotor can be represented by a straight line, with slope fixed by the nonsaturating portion of the leakage reactance, that starts from a point on the zero-current axis at a voltage V0, where V0 is the voltage induced by the flux in the saturating portion of the reactance when these paths are saturated.