The primary disadvantage of a conventional squirrel cage induction motor is the low starting torque and high starting current because of low rotor resistance.
The starting torque of the motor can be increased by using rotor bar material of high resistivity. The high rotor resistance gives a higher starting torque and lower starting current at a higher power factor. Although the higher rotor resistance decreases the full-load speed, it increases the rotor I2R losses, which results in lower efficiency of the motor. Therefore, a low rotor resistance is required for normal operation of the motor so that the slip is low and the efficiency is high. Thus, for good starting performances, the rotor resistance should be high while under normal operating conditions, the rotor resistance should be low.
In order to obtain high rotor resistance at starting and low rotor resistance at running, two types of rotor connections are used in squirrel cage induction motors −
The squirrel cage rotor with deep and narrow bars is shown in the figure. A bar may be assumed to be made up of number of narrow layers connected in parallel. Such three narrow layers viz. element A, element B and element C are shown in the figure of deep bar rotor.
It can be seen from the figure that the topmost layer, i.e., element A is linked with minimum leakage flux and hence its leakage inductance is minimum. On the other hand, the bottom layer, i.e., element C links maximum leakage flux. Thus, the leakage inductance of element C is maximum.
At starting of the motor, the rotor frequency and the supply frequency are equal. Hence, the element C offers more impedance to the flow of current than the element A. Consequently, maximum current flows through the top layer and minimum current through the bottom layer. Due to this unequal current distribution in the rotor circuit, the effective rotor resistance increases and the leakage reactance decreases. Therefore, with the high rotor resistance at the starting conditions, the starting torque is high while the starting current is low.
Under normal operating conditions, the slip and rotor frequency are very small. Thus, the reactances of all the elements (A, B and C) of the rotor bars are small as compared to their resistances. The impedances of all the layers of the rotor bars are approximately equal, so the current distribution through all the parts of the bars is equal. Consequently, the cross-sectional area of the rotor bars increases, due to which the rotor resistance becomes quite small, resulting in a low slip and high efficiency.
The figure shows a double cage rotor of an induction motor. In the double-cage rotor there are two layers of rotor bars. The double cage induction motors are used for obtaining high starting toque at low starting current. The stator of a double cage induction motor is similar to that of an ordinary squirrel cage induction motor.
Each layer of the rotor is short circuited with the help of end rings. The rotor bars of the outer-cage have a smaller cross-sectional area than the inner rotor bars and are made up of high resistivity materials such as brass, bronze, aluminium etc. The rotor bars of the inner-cage are made up of low resistivity materials like copper. Hence, the outer-cage has greater resistance than the inner rotor cage. There is a slit between the top and bottom slots of the rotor. This slit increases the permeance for leakage flux around the inner-cage rotor bars. Consequently, the leakage flux linking the winding of the inner rotor cage is much greater than that of the winding of the outer rotor cage. Therefore, the inner-cage winding has a greater self-inductance.
At the starting of the motor, the frequency of the voltage induced in the rotor is same as the supply frequency. Thus, the inner-cage winding has a leakage reactance which is much greater than that of the winding of the outer-cage. Hence, most of the starting current is flowing in the outer-cage winding, which offers low impedance to the flow of current. Therefore, the high resistance outer-cage winding develops a high starting torque.
Now, under normal operating conditions, the slip and rotor frequency are very small. Hence, the leakage reactances of both the windings become negligibly small. Now, the current division between the two rotor cages is governed by their resistances. Since the resistance of the outer rotor cage is about 5 to 6 times that of the inner rotor cage, thus, most of the current flows through the inner-cage. Therefore, under normal operating conditions, the torque in the motor is developed by the inner rotor cage which has low resistance.