Squirrel cage motors are the most commonly used type motors and those motor has get their name from the appearance of rotors. Being rugged and requiring neither a separate DC power source nor slip-rings. They are essentially constant speed devices when energized by a constant frequency AC supply, however electronic speed control is available. Over 90% of all motors are AC Three Phase Induction Motors in the world.
Operation of Induction motors
- When Three Phase Induction Motors starter winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round die stator at some type of speed.
- The rotating field passes through me air gap and cuts me rotor conductors, which as yet, are stationary.
- Due to the relative speed between the rotating flux and the stationary rotor, Elector Magnetic Force is induced in the rotor conductors.
- Since the rotor circuit is shot-circuited, currents start flowing in die rotor conductors.
- The current carrying rotor conductors are placed magnetic field produced by die stator. Consequently, mechanical force acts on the rotor conductors.
- The sum of die mechanical forces on all the rotor conductors produces a torque, which tends to move the rotor in the same direction as the rotating field.
- The direction of rotor current will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between motor rotating field and me stationary conductors.
- Hence to reduce this relative speed, the rotor starts running in me same direction as that of stator field and tries to catch it.
Applications of Three Phase Induction Motors
· Air conditioners
· Washers
· Dryers
· Industrial machinery
· Fans
· Blowers
Construction of Three Phase Induction Motors
Three-phase induction motor has a stator and rotor. The stator carries a 3-phase winding
or stator winding and rotor carries a short-circuited winding or rotor winding- The rotor
and stator is separated from a small air gap.
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| Construction of Induction Motor |
Stator of Induction Motor
Stator consists of a steel frame, which encloses a hollow, cylindrical core made up of thin lamination of silicon steel to reduce hysteresis and eddy current loss. A number of even space slots are provided on the inner periphery of me lamination. The insulated conductors are placed in the stator slots.
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| Stator of Induction Motor |
Rotor of Induction Motor
It is mounted on shaft, is a laminated core having slots its outer periphery.There are two main types of rotors available in Induction motor.
- Squirrel cage rotor
- Wound rotor.
Squirrel cage rotor:
It consists of a laminated cylindrical core having parallel slots on its outer periphery. One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings. Those induction motors that employ squirrel cage rotor are called squirrel cage induction motors. It suffers from the disadvantages of a low starting torque. Squirrel cage rotor is as shown below.
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| Squirrel cage rotor |
Wound rotor:
It consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on Motor stator. The rotor winding is uniformly distributed in the slots and is usually star connected There are some resistances which are gradually reduced to zero as the motor runs up to speed. The external resistances are used during starting period only.
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| Wound rotor |
