The construction
of a 3 phase induction motor can
be divided into two two parts – stator and rotor. Further, rotors can be sub-
divided into squirrel cage type rotor and slip ring type rotor.
Lets understand each one by one easy way.
(1) Stator
:- It consists of a steel frame which encloses a cylindrical core made up of
thin laminations of silicon steel to reduce hysteresis and eddy current loss.
A number of evenly spaced slots are provided on the
inner periphery of the stator. The insulated conductors are placed in these
slots and are suitably connected to form a balanced star or delta
configuration.
The winding is wound for a specific number of poles as per the speed requirement of the motor. For greater speed, lesser number of poles are used and vice-versa.
When the stator winding is energised by a 3-phase supply, a magnetic field of constant magnitude is produced which revolves around the stator with synchronous speed. The synchronous speed is inversely proportional to the number of poles.
Ns=120f/P
(2) Rotor :- It is the rotating part of the machine and is of two types.
Squirrel cage rotor
It consists of a laminated cylindrical core having parallel slots
on its outer periphery. One bar is placed in each slot. All these bars are joined
at each end by metal rings called end rings.
This forms a circuit with rotor
bars permanently short-circuited on themselves, hence any external resistance
cannot be added in series with the rotor winding. This is a big drawback of the
Squirrel cage type rotor.
Note that rotor bars are not quite parallel
to the shaft but are slightly skewed. This skewing helps in
1. Reducing
the locking tendency of the rotor i.e the tendency of the rotor teeth to remain
under the stator teeth due to the attraction between them.
2. Running
the motor quietly by reducing magnetic humming.
Slip ring or wound type rotor
The one end of three terminals is connected internally and the other three winding terminals are brought out and connected to 3 insulated slip rings mounted on the shaft with carbon brushes pressed against them. External resistances can be connected in series with the rotor through these stationary brushes and this is a big advantage of slip ring rotor over a squirrel cage rotor.
Induction Motor which uses squirrel
cage rotor is called squirrel cage induction motor and which
uses wound type rotor is called slip ring induction motor.
That was all about construction of three phase induction
motor.
