What is the need of Deep-Bar and Double-Cage Rotor?
Conventional Squirrel Cage motors suffer from the disadvantages of low starting torque because of low rotor resistance. The starting torque can be increased by using the bar material of higher resistivity. However high rotor resistance reduces the full-load speed, increases rotor ohmic loss and lower efficiency. Therefore in order to achieve high starting torque without effecting the efficiency, the rotor resistance is made higher at the time of starting & low under normal operating conditions. In wound rotor induction motors these conditions are met by connecting external resistance in rotor circuit at the time of starting & resistances are cut out in steps as the motor attains its normal speed. In squirrel cage motor this is not feasible as the conductors are short-circuited by end rings. In order to attain the above desired conditions following types of rotors are used:
- Deep bar rotor
- Double cage rotor.
Deep Bar Rotor:
Figure below shows a cage rotor with deep and narrow bars. A bar may be assumed to be made up of number of narrow layers connected in parallel. It is seen that the top most layer element ‘A’ is linked with minimum leakage flux and therefore its leakage inductance is minimum. On the other hand, the bottom layer ‘C’ links maximum flux, therefore its leakage inductance is maximum.
At starting the rotor frequency is equal to the supply frequency. The bottom layer element ‘C’ offers more impedance to the current flow then upper layer ‘A’. Therefore the maximum current flows through the top layer and minimum through 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 are 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.
Double Cage Rotor:
An induction motor with two rotor windings or cages is used forn obtaining high starting torque at low current. The stator of a double cage induction motor is similar to that of ordinary induction motor. In double cage rotor there are two layers of bars as shown. The outer cage bars have a smaller cross-sectional area than the inner bars and are made of high resistivity materials like brass etc.
The inner cage bars are made up of low resistance material like copper. Thus the resistance of outer cage is greater than that of inner cage. There is a slit between the top and bottom slot. Thus the leakage flux linking the inner cage winding is much larger than that of outer cage. The inner cage winding, therefore has a greater self inductance. At starting voltage induced in the rotor is same as the supply frequency. Hence the leakage reactance of the inner cage winding is much larger than of the outer cage. Therefore most of the starting current is flowing in the outer cage winding which offer low impedance to current flow. The high resistance outer cage winding, therefore develops high starting torque. As the rotor speed increases, the frequency of rotor emf decreases.
At normal speed, the leakage reactance of both the winding becomes negligibly small. The rotor current division between the two cages is governed mainly by their resistances. Since the resistance of the outer cage is about 5 to 6 times that of inner cage, most of the rotor current flows through the inner cage. Hence under normal operating speed, torque developed mainly by low resistance inner cage.