An induction motor is an AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. The application of induction motor differs by the type of rotor which is being used:
- Squirrel Cage Rotor
- Slip Ring Rotor or Wound Rotor Type
Squirrel Cage Rotor
A squirrel type rotor is used in a squirrel cage motor. It has a cylindrical laminated core with slot parallel to the shaft, uninsulated conductor bar of aluminum or copper in each slot of the rotor and the rotor conductor are short-circuited by heavy end ring at the end of the rotor.
The end ring and the conductor bar looks same as a cage of squirrel so it is named as squirrel cage rotor.
Squirrel cage rotor have high efficiency and high power factor, require less maintenance and is cheap.
The Torque Slip characteristics determines the use of squirrel cage motor which is stated as under,
Generally, it is classified into four classes:
- Class A Motor: Normal Starting torque, high starting current with Low Slip is the required operating criteria of this type of motor with high full load efficiency.
Examples: Fan, Blowers, Machine tools, etc.
- Class B Motor: Normal Staring torque, low Starting current, and low operational slip are the required operational criteria of this type of motor. Applications are the same as class A Motor.
- Class C Motor: High Starting torque, low Starting current and applicable for a constant speed-load condition is the required operational criteria for this type of motor. Examples: Compressor, Crushers, Conveyors, etc.
- Class D Motor: Highest Starting torque of all the class motor, low starting current and high operating slip. This type of motors has low efficiency suitable for intermediate loads. Examples: Bulldozers, Die stamping, etc.
Slip Ring Rotor
Slip Ring Rotor consists of a sloted armature, insulated conductor arranged in slots forming a three-phase double layer distribution winding similar to that of the stator winding. The rotor is star connected and the open ends of the rotor are connected with brushes to slip ring mounted in the shaft whereas the other end of slip ring is connected to a variable resistor which is totally used for speed control purpose as stated in the below diagram.
Slip ring inductor motor have High Starting Torque and low starting current so the load which require this operational condition uses slip ring type rotor induction motor.
Ex :- Conveyors, Cranes, Compressors, Elevators, Hoist etc.