Construction and Working of 3 Phase
Induction Motor on Ship
The
popularity of 3 phase induction motors on board ships is because of their
simple, robust construction, and high reliability factor in the sea
environment. A 3 phase induction motor can be used for different applications
with various speed and load requirements.
3 phase induction motors are used in almost all machinery system of the ship such as crane motor, propulsion motor, blower motor, sea water pump motor, and even small synchronous motor.
Construction
The main body of the
Induction Motor comprises of two major parts:
Stator
Stator is made up of
number of stampings in which different slots are cut to receive 3 phase winding
circuit which is connected to 3 phase AC supply.
The three phase
windings are arranged in such a manner in the slots that they produce a
rotating magnetic field after AC supply is given to them.
Usually, windings are
kept at different pitch circle with 30 % overlap to each other.
The windings are wound
for a definite number of poles depending upon the speed requirement, as speed
is inversely proportional to the number of poles, given by the formula:
Ns= 120f/p
Where Ns= synchronous speed
f = Frequency
p = no. of poles
Rotor
Rotor consists of cylindrical
laminated core with parallel slots that carry conductor bars.
Conductors are heavy
copper or aluminium bars which fits in each slots. These conductors are brazed
to the short circuiting end rings.
The slots are not
exactly made parallel to the axis of the shaft but are slotted a little skewed
for the following reason:
§
They reduces magnetic hum or noise
§
They avoid stalling of motor.
Principle and working
When 3 phase supply is
given to the motor, the resulting current generates a magnetic flux “Ø”.
Due to the switching
sequence of 3 phase current in R, Y and B, the generated flux rotates around
the rotor conductor.
According to Faraday’s
law which states that –“an emf induced in any closed circuit is due to the rate
of change of magnetic flux through the circuit”.
Emf is induced in the
Copper bar and due to this, current flows in the rotor. The direction of rotor
can be given by Lenz law which states that – “the direction of induced current
will be in the opposite of the motion causing it”
Here the relative
velocity between the rotating flux and static rotor conductor is the cause of
current generation; hence the rotor will rotate in the same direction to reduce
the cause i.e. the relative velocity, thus rotating the rotor of the induction
motor.
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