Skin-effect bars of squirrel-cage rotors

IN ORDER TO INCREASE the starting torque, larger induction motors are built usually with skin-effect bars in their rotor. Fig. 1 shows various types of skin-effect bars. Types A, B, and C, mostly type C, are used in induction motors; type D is used in synchronous motors. The analytical investigation of the increase of resistance and decrease of slot-leakage reactance of the skin-effect bars is possible under the following assumptions: (1) the vertical component of the slot flux is negligible; (2) the permeability of the iron is infinite; (3) the resistivity of the conductor is constant over the depth of the slot; and (4) the current density along any line parallel to the bottom of the slot is constant.