7/28/2019 Construction and Working of 3 Phase Induction Motor

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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 differentapplications with various speed and load requirements.

The ship’s generator  3 phase AC supply can be connected to the AC induction motor via a starter or anyother arrangement like auto transformer to improve the torque and current characteristics.

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 circuitwhich 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

7/28/2019 Construction and Working of 3 Phase Induction Motor

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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 theshort circuiting end rings.

The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed for thefollowing 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.

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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 currentgeneration; 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.

You may also like to read-Single Phasing in Electrical Motors & Safety Devices for Main Switch Board onShip 

References- 

Book on Marine electro-technology by Mc George

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