Rotor cage anomalies and unbalanced magnetic pull in single-phase induction motors

This paper consists of an experimental and theoretical investigation of the effect that rotor cage defects have on the unbalanced magnetic pull (UMP) present in a single-phase motor. The paper presents a detailed development of a mathematical model which can be used to examine both porosity and soldering defects in cage rotors. In the experimental investigation, the UMP developed by a number of nominally identical motors that were known to have rotor defects was measured using a specially designed test rig. The rotors were then divided into two equal groups. The rotor bars in the first group were removed from the rotors and their resistance measured to determine the range and distribution of porosity defects around the periphery of the rotor. The ovality of the rotors of the second group was carefully measured, and their end rings were turned off to enable the bar-to-bar resistance to be measured. This was done for all pairs of adjacent bars in the rotors of the second group, giving again an indication of the range and circumferential distribution of soldering defects.