In a squirrel cage induction motor, the rotor has conductive bars embedded in slots in the laminations, running the length of the rotor. These bars are shorted at each end with shorting rings. There is no electrical connection between these bars and the motor terminal box. The current in the rotor bars is a function of the motor slip, and we have no direct control over it.
Squirrel cage induction motors have caged rotor with skewed long aluminium bars. This makes inbuilt system in the motor to reduce starting current and achieve maximum starting torque.
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Squirrel Cage Induction Motor
Russian inventor Mikhail Dolivo Dobrovolsky developed first three Phase Squirrel Cage Induction Motor in 1889.
Squirrel Cage Induction Motor consists of following parts:
It consists of three phase winding with core along with metal housing. Windings are such placed that they are electrically and mechanically 120o apart from in space. The winding is mounted on the laminated iron core to provide low reluctance path for generated flux by AC currents.
It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy. The rated output of the motor is mentioned on the nameplate in horsepower. It consists of a shaft, short-circuited copper/ aluminum bars, and a core.
The rotor core is laminated to avoid power loss from eddy currents and hysteresis. Conductors are skewed to prevent cogging during starting operation and gives better transformation ratio between stator and rotor.
A fan is attached to the back side of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit. Bearings are provided as the base for rotor motion, and the bearings keep the smooth rotation of the motor.
Rotation of Magnetic Field in Space
A three phase squirrel cage motor works on the fundamental principle of electromagnetism. When a three phase supply is given to the stator winding it sets up a rotating magnetic field in space. This rotating magnetic field has a speed which is known as synchronous speed.
This rotating magnetic field induces the voltage in rotor bars and hence short-circuit currents start flowing in the rotor bars. These rotor currents generate their self-magnetic field which will interact with the field of the stator.
Now the rotor field will try to oppose its cause, and hence rotor starts following the rotating magnetic field. The moment rotor catches the rotating magnetic field the rotor current drops to zero as there is no more relative motion between the rotating magnetic field and rotor.
Hence, at that moment the rotor experiences zero tangential force hence the rotor decelerates for the moment. After deceleration of the rotor, the relative motion between the rotor and the rotating magnetic field reestablishes hence rotor current again being induced.
So again, the tangential force for rotation of the rotor is restored, and therefore again the rotor starts following rotating magnetic field, and in this way, the rotor maintains a constant speed which is just less than the speed of rotating magnetic field or synchronous speed.
Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed. The frequency of the rotor current = slip × supply frequency.
Squirrel Cage Induction Motor Conclusion
After going through the above portion of squirrel cage induction motor we can now establish parts squirrel cage induction motor. I hope you enjoy when reading this article, thank you.