Double Cage Rotor - A brief understanding

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A double cage rotor is a specialized design used in certain types of induction motors to improve their performance characteristics. It consists of two separate rotor windings, each with its own set of rotor bars and end rings. The windings are placed concentrically with one another, forming two distinct cage structures.

The construction of a double cage rotor involves using two different types of rotor cages: outer cage and inner cage. The outer cage comprises high-conductivity bars, typically made of copper, while the inner cage consists of lower-conductivity bars, often made of aluminum or copper alloy. 

The outer cage consist of bars of small cross section hence the resistance is high. However, since the same is nearer to the air gap, the leakage reactance is very low. On the other hand, the inner cage consist of rotor bars of large cross section, hence the resistance is low and since the same is connected with the air gap with a long narrow slit, the leakage reactance is high.


The Working Method:

During starting of the motor, the frequency of the rotor current is same as the supply frequency. Hence, the current will be distributed in both the cage depends on the impedance of each cage. Another effect of the frequency is that the reactance will be higher than the resistance, hence the current will try to flow to that cage whose reactance is lower than the other cage which is the outer cage.

Under load condition, when the motor runs near the synchronous speed, the reactance of both the cage is negligible . On that time the current distribution is dependent on the resistance value of each cage. Since the resistance value is lesser for the inner cage than the outer cage, carries most of the current.

Therefore, during starting , due to the high resistance of the outer cage , the higher starting torque with lower starting current is generated and during running of the motor, higher efficiency is ensured because of the lower resistance of the inner cage.

The double cage rotor design offers several advantages over a conventional squirrel cage rotor. It provides higher starting torque, improved efficiency, better stability, and enhanced control over the motor's performance characteristics.

It's important to note that the specific design and characteristics of a double cage rotor may vary depending on the motor's size, application requirements, and manufacturer specifications. Consulting the motor's technical documentation or working with a qualified engineer will provide more detailed information about the specific double cage rotor design and its working principle for a particular induction motor.



Comments

  1. AnonymousJune 12, 2023 at 9:32 AM

    Hello. I feel two more information should have been handy on this type of motor. First due to double cage the motor will have a lower locked rotor time than equivalent rating motor as only the lowest value out of the two rotor will be considered. Second I am little apprehensive about the efficiency part as alloy rotor will lead to higher rotor losses and lowering the efficiency. Please clarify

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  2. Low Voltage AC Induction MotorJune 15, 2023 at 8:17 PM

    Hi, the Locker Rotor Withstand time or LRWT is very important factor to select a motor against a specified load. The manufacturer define the LRWT of a normal induction motor depend on the stator criticality or the Rotor criticality. It may be possible that for a specific case the stator criticality is less than the rotor criticality, hence more LRWT value considering stator, although this is absolutely not recommended. So we need to check always whether the measurement parameters are same or not. Even if the LRWT of dual cage rotor is smaller than normal rotor, then also it should not be a problem if the starting suitability is checked. On top of it the higher starting torque may reduce the starting time than the normal motor.
    Now coming to efficiency part, it is obvious that the additional reactance of the inner cage lowers the full load power factor and the high resistance of the outer cage increase the I^2R loss which will definitely lowering the efficiency. However, if you compare the normal induction motor with the same starting torque deliverability like the the double cage rotor, you will find better efficiency for double cage rotor only. This is as per my knowledge only. Kindly correct me if I am wrong. this blog is created to discuss about various topics on Induction Motor only.

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