An induction also called asynchronous motor is an AC electric motor in which the electric current in the rotor required to generate torque is obtained by electromagnetic induction from the magnetic field of the stator winding.
Induction motor, like any other motor, is mainly composed of rotor and stator. Stator, as its name shows, is a static part of induction motor. A three phase supply is given to the stator of induction motor. The rotor is a rotating part of induction motor which is connected to the mechanical load through the shaft. The rotor of the 3 phase induction motor are further divided into: squirrel cage rotor and slip ring rotor or wound rotor or phase wound rotor. According to the type of rotor used in the motor, the 3 phase induction motor are classified as: squirrel cage induction motor and slip ring induction motor or wound induction motor or phase wound induction motor.
The construction of stator for all types of 3 phase induction motor is the same and is discussed in the following parts. The stator of the 3 phase induction motor consists of 3 main parts: stator frame, stator core and stator winding or field winding.
It is the most outer part of the 3 phase induction motor and has the main function 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 3 phase induction motor. The frame is made up of either die cast or fabricated steel. The frame of 3 phase induction motor needs to be very strong and rigid as the air gap length is very small, or rotor will not remain concentric with stator which will cause unbalanced magnetic pull.
The main function of the stator core is to carry alternating flux. The stator core is laminated so as to reduce the eddy current losses. This laminated structure are made up of stamping about 0.4 to 0.5 mm thick. All the stamping are stuck together to make stator core, which is then housed in stator frame. The stamping are generally made up of silicon steel, which reduces the hysteresis loss.
The slots on the periphery of stator core carries 3 phase windings. This 3 phase winding is supplied by 3 phase ac power supply. The 3 phases of the winding are linked either in star or delta depending on the type of starting method used. The squirrel cage motor is mostly started by star – delta stater and thus the stator of squirrel cage motor are delta linked. The slip ring 3 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 the motor is also called field winding and when this winding is excited by 3 phase ac supply it generates rotating magnetic field.