An induction motor is also called asynchronous motor as it runs at a speed other than the synchronous speed. Like any other electrical motor, induction motors have two main parts – the rotor and the stator.
As the name indicates stator is a stationary part of induction motor. A three phase supply is given to the stator of the induction motor.
The rotor is a rotating part of induction motor. The rotor is connected to the mechanical load through the shaft. The rotors of the three phase induction motor are further classified as:
- Squirrel Cage Rotor
- Slip Ring Rotor or Wound Rotor or Phase Wound Rotor
Depending upon the type of rotor used the three-phase induction motor is classified as:
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor or Wound Rotor Induction Motor or Phase Wound Induction Motor
The construction of stator for both the kind of three phase induction motor remains the same and is discussed in brief below.
Stator of Three Phase Induction Motor
The stator of the three phase induction motor consists of three main parts:
- Stator frame
- Stator core
- Stator winding or field winding
It is the outer part of the three-phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering and provides protection and mechanical strength to all the inner parts of the machine. The frame is either made up of die-cast or fabricated steel. The frame of three phase induction motor should be strong and rigid as the air gap length of three phase induction motor is very small. Otherwise, rotor will not remain concentric with stator which will give rise to unbalanced magnetic pull.
The main function of the stator core is to carry alternating flux. In order to reduce the eddy current losses the stator core is laminated. This laminated type of structure is made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping are stamped together to form the stator core, which is then housed in a stator frame. The stampings are generally made up of silicon steel, which reduces the hysteresis loss.
Stator Winding or Field Winding
The slots on the periphery of stator core of the three phase induction motor carries three phase windings. This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used.
The squirrel cage motor is mostly started by star-delta stater and hence the stator of squirrel cage motor are delta connected. The slip ring three phase induction motor are started by inserting resistances so, the stator winding can be connected either in star or delta. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic field.