SOLUTION

Deep bar rotor construction is used in three-phase induction motors mainly to Increase the Starting Torque.

There is no constructional difference between the stator of the deep bar motor and that of an ordinary induction motor. The rotor consist of deep bars, short-circuited by two end rings one on each side. A bar may be assumed to be made up of the number of narrow layers connected in parallel. It is seen that the topmost layer element is linked with minimum leakage flux and therefore its leakage inductance is minimum. On the other hand, the bottom layer links maximum flux, therefore its leakage inductance is maximum.

At starting the rotor frequency is equal to the supply frequency. The bottom layer element offers more impedance to the current flow in the upper layer hence the maximum current flows through the top layer and minimum through the bottom layer. This unequal current distribution causes the effective rotor resistance to increase. With a high rotor resistance at starting condition, the starting torque is relatively high & starting current is low as desired.

Now under normal operating conditions, the slip and the rotor frequency are very small. The reactance of all the layers is small compared to their resistances. The impedances of all the layers are nearly equal, so current flows through all the parts of the bar equally. The resulting large cross-section area makes rotor resistance small, resulting in good efficiency at low slips.