Section 15.7: Starting of Induction Motors cont.
Deep-bar rotor depth is 2-3 times its width. At low slip (starting), the rotor-current frequency is high (60 Hz.) and as a result of skin effect, the rotor’s AC currents crowd towards the tops of the bars (rotor’s periphery) and thus flow through a smaller cross-sectional area (\ higher resistance) than they do at high slip (when fr is a few Hz.).
Double-cage rotor employs two sets of rotor bars (one deeper than the other) to employ skin effect in a fashion similar to that of the deep-bar rotor scheme
Starting Techniques Based on Rotor-resistance and Reduced Stator Voltage cont.