Squirrel cage motors are the most commonly used type of Three Phase Induction Motors and those motor has got 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 my 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 short-circuited, currents start flowing in die rotor conductors.
- The current-carrying rotor conductors are placed magnetic field produced by die stator. Consequently, a mechanical force acts on the rotor conductors.
- The sum of dying 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 of producing them. Now, the cause producing the rotor currents is the relative speed between the motor rotating field and me stationary conductors.
- Hence to reduce this relative speed, the rotor starts running in me same direction as that of the 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
A 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.
Construction of Induction Motor |
The stator of Induction Motor
The 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 my lamination. The insulated conductors are placed in the stator slots.
Stator of Induction Motor |
The rotor of Induction Motor
It is mounted on the shaft, is a laminated core having slotted its outer periphery. There are two main types of rotors available in the Induction motor.
- Squirrel cage rotor
- Wound rotor.
Squirrel cage rotor Three Phase Induction Motors:
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 rotors are called squirrel cage induction motors. It suffers from the disadvantages of a low starting torque. The squirrel cage rotor is as shown below.
Squirrel cage rotor |
Wound rotor:
It consists of a laminated cylindrical core and carries a 3-phase winding, similar to the one on the 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 the starting period only.
Wound rotor |