Single Cage & Double Cage Induction Motors

In this article, we will see the difference between Single Cage & Double Cage Induction Motors.

Single Cage Double Cage
1. Starting performance As compare to double cage rotor, the single cage has less starting torque so at starting less heating loss occur. A double cage motor develops higher starting torque for low starting current and its rotor heating loss is more.
2. Full-load performance Full load slip is higher than double cage rotor. It has higher efficiency. Slip and efficiency are lower than the single cage rotor.
3. Torque slip characteristics It is not feasible in a single cage induction motor. A wide range of torque-slip characteristic is obtained.

Excercise

Q. Why do you use Double Cage Induction Motor? Explain disadvantages if any.

Answer. A rotor design, which though more expensive gives still better starting and running performance than the deep-bar design, is the double-cage rotor. The squirrel-cage winding in this design consists of two layers of bars short-circuited by end rings. The upper bars have a smaller cross-sectional area than the lower bars and consequently a higher resistance. The slots accommodating the two sets of bars are joined by a constriction which also shows the slot leakage flux pattern for the double cage rotor.

By arguments similar to those presented for the deep-bar rotor construction, it is seen that the upper bars have a much lower leakage flux linkage and therefore a much lower reactance. Furthermore, the self-leakage flux linking the upper/lower bars can be controlled by the dimension of the air constriction. The constriction is also necessary because of the fact that in its absence the main flux will return via the iron path between the two slots thereby “missing” the inner bars which then would not contribute to torque development.

It is found that the outer cage has high resistance and low reactance while the inner cage has low resistance and high reactance. Therefore, in the starting the current is mainly confined to the outer cage with a consequent decrease in starting current and an increase in starting torque. Under the running condition, the reactance difference between the two cages evens out because of the low frequency of rotor current such that these act to conduct current in proportion to their d.c. resistances and as a group present a low-resistance rotor to the air-gap flux giving an excellent running performance.

Disadvantage: Rotor design is more expensive.

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