squirrel cage induction motor || Advantages

On three phase induction motor, it consist of rotor and stator. The stationary part is stator and rotating part is rotor. The rotor is made of high grade steel thin lamination in cylindrical shape (i.e. hollow inside). The rotor winding (bars) is placed on these slots on outer periphery. On the basic of slot winding, rotor can be divided into 2 type. They are squirrel cage induction motor and wound rotor (slip ring rotor). As we know this is three phase induction motor, it requires 3 phase connection to stator. Both delta and stator winding is used in stator connection. Below small motor can use any of these type but for large motor, it need to done star-delta connection i.e. first star and after motor gain 80% of its rated speed then delta connection so that at starting it provide less starting current by using star connection and on delta, it provide high torque.

Squirrel cage induction motor

The motor which consist of squirrel cage rotor is called squirrel cage induction motor. This rotor consists of high grade silicon steel laminated cylindrical shaped core with parallel slots on outer periphery where winding is placed on these slots. The laminated steel is used to the reduce losses (iron loss & eddy current loss).  Also, it provide mechanical protection. Slots used on stator and rotor may be open, semi-closed and closed. Here uninsulated bar of aluminum or copper on these slots are used as winding. There bars are short circuited to each other by the help of end ring made of copper or aluminum and nearly parallel also with certain equal distance. The bar and end rings form a cage of round shape. The shape is like cage which is used to  keep squirrels so it is named squirrel cage rotor and the motor is called squirrel cage induction motor. 

Rotor is not connect with electricity but current is induced on it by mutual induction where no need of direct connection to transfer power as like in transformer. On this rotor, starting current is 5/6 times greater than full load current and current increase with increase with load i.e. at no load, current is less. Also, its starting torque is less so it is prefer to use double cage induction motor. Its speed is decreasing with increasing the load and at no load, speed is nearly equal to synchronous speed.

Advantages of squirrel cage induction motor

Mostly found motor on market is squirrel cage inductor motor because it has simple construction and robust construction so easy to made and repair. It is very useful in case of required uniform torque and less noise because due to this rotor, it produce uniform torque and its produce less noise during operation. Due to magnetic action (there is mutual induction to transfer current from stator to rotor), sometimes rotor and stator teeth attract each other because they are near and only airgap is present between stator and rotor. This is called magnetic locking. In squirrel cage induction motor, there is less chance of magnetic locking. Due to its robust construction and cheaper in price, it is much more popular. Not only these, it require less maintenance and it has higher efficiency and higher power factor as compare to wound type rotor. There is no slip ring (there is end ring) so it reduce the risk of sparking.

Disadvantage of squirrel cage induction motor

Its main disadvantage is that there is no option to addition of resistance. In induction motor, it is very easy and economical method to control speed but in this motor, we can't add additional resistance to control speed. As we know, rotor windings (bar conductors) are short circuited on end ring permanently, we can't add external resistance. Also, it has high starting current and low torque as compared to wound type rotor.