Deep Bar Rotors are used in induction motors to increase the torque during starting. Deep bars indicate that the bars which comprise the cage in the rotor are deeper than those in normal rotors.
When an induction motor is started, the slip between the rotor and the stator is high. Thus the frequency of the rotor current is high.
This high frequency results in high reactance in the lower layers of the deep bar. Hence, most of the current flows in the surface of the rotor bars. This results in high current density and increased resistance. This resistance produces high torque during starting.
When the motor reaches its rated speed, the slip frequency drops and the reactance reduces. The current now, flows uniformly across the entire cross section of the rotor bar. The resistance in the rotor drops and the motor runs normally.