3 phase induction machine

Construction
A 3-phase induction motor has two main parts (i) stator and (ii) rotor. The rotor
is separated from the stator by a small air-gap which ranges from 0.4 mm to 4
mm, depending on the power of the motor.

1. Stator

It consists of a steel frame which encloses a
hollow, cylindrical core made up of thin
laminations of silicon steel to reduce
hysteresis and eddy current losses. A numberof evenly spaced slots are provided on theinner periphery of the laminations [See Fig.]. 
The insulated connected to form a
balanced 3-phase star or delta connected
circuit. The 3-phase stator winding is wound for a definite number of poles as
per requirement of speed. Greater the number of poles, lesser is the speed of the
motor and vice-versa. When 3-phase supply is given to the stator winding, a
rotating magnetic field of constant magnitude is produced. This
rotating field induces currents in the rotor by electromagnetic induction.


2. Rotor

The rotor, mounted on a shaft, is a hollow laminated core having slots on its
outer periphery. The winding placed in these slots (called rotor winding) may be
one of the following two types:

(i) Squirrel cage type 
(ii) Wound type

(i) 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 . This forms a permanently short-circuited windingwhich is indestructible. The entire construction (bars and end rings)
resembles a squirrel cage and hence the name. The rotor is not connected
electrically to the supply but has current induced in it by transformer action
from the stator.
Those induction motors which employ squirrel cage rotor are called
squirrel cage induction motors. Most of 3-phase induction motors use
squirrel cage rotor as it has a remarkably simple and robust construction
enabling it to operate in the most adverse circumstances. However, it
suffers from the disadvantage of a low starting torque. It is because the
rotor bars are permanently short-circuited and it is not possible to add any
external resistance to the rotor circuit to have a large starting torque.