Three Phase Induction Motor
Three phase induction motor can be used for
different applications it is
Construction of three
phase induction motor :-
Three phase induction motor 85% of
the installed capacity of the industrial purposes because of good operating
characteristics, low maintenance, easy construction , good speed regulation and
absence of commutator.
Three phase induction motor consist
of two main parts--
1 1. Rotor
2. Stator
Rotor -> Rotor of three phase induction motor are two different
type one is squirrel cage rotor and
other is wound rotor or slip ring rotor . wound rotor made up of aluminium or
copper bars and the shaft of three phase motor supported on two bearings each
ends. A slip ring or wound rotor is made up of three phase winding are jinxed at
one end and other end connected to slip
rings mounted on the rotor shaft.
Squirrel cage rotor consist of cylinder of
steel lamination where copper and aluminium conductors embedded in the surface of rotor.squirrel cage rotor
is skewed to avoid cogging.
Stator:- as the name suggested
stator is the stationary part of three phase induction motor and the three
phase supply deliver to it. Stator of three phase induction motor consist of
stator frame, stator core, stator winding or field winding. Stator winding
connected star or delta, it depends what type of starting method is used. When winding is
excited by three phase supply it produces a rotating magnetic field. Three
phase winding phases placed at 120 degree apart from each other.
A small distance between the rotor and stator is called air
gap. Distance between the stator and rotor varies from 0.4 mm to 4 mm.
Formulas
:-
Ns =120f/p
Ns - Synchronous speed
f - Frequency
p - no. of poles
Synchronous speed is inversely
propotional to the no. of poles
Slip :- it is the difference between the synchronous speed and operating speed
S = Ns - Nr/Ns
Ns - stator speed
Nr - rotor speed
Percentage slip :-
Ns – Nr/Ns X
100
Principle of operation :-
When three
phase supply provided to the stator winding, it produces a rotating magnetic
field and flux generates due to induced
rotor current. According to the Faraday’s laws of electromagnetic induction. As these rotor
conductors are shorted, the current starts to flow through in these conductors. direction of the rotor can be given by Lenz law -
the direction of induced current will be in the opposite of the motion, This result the rotor starts rotating in the same
direction of the stator rotating magnetic field.
Advantages
of three phase induction motor :-
- Simple construction
- Low cost
- Easy to maintain
- Characteristics to suit most industrial requirements
- Rugged
Disadvantages of three phase
induction motor :-
- At heavy load it draws high initial starting current
- Three phase induction motor speed difficult as compare to dc machine
- It operates lagging power factor which result to increase I2R losses
applications
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