Squirrel-Cage Rotors with Split Resistance Rings

The characteristics of a squirrel-cage induction motor are investigated when the resistance rings are provided with cuts 360 electrical deg. apart and the cuts in the front ring are displaced against those in the back ring by 180 electrical deg. A theoretical investigation is given, which leads to the conclusion that splitting the rings results in an equivalent ring resistance which varies with double-slip frequency in the ratio of one to three; and the average ring resistance is twice what it was before the rings were cut. The variable rotor resistance effects a periodic fluctuation of the line current and rotor speed. The fluctuations have double-slip frequency. The theoretical conclusions are checked by tests and oscillograms. Test results show that the splitting of the rings is followed by increased rotor leakage, which results in a starting torque smaller than that corresponding to the increased resistance and original leakage.