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Three Phase Induction Motor : Working Principle Construction and Other Detail

What is a three-phase induction motor?

A three-phase motor is a type of electrical machine That is operated by a three-phase power supply. It is a very popular and common type of AC motor. In most industries, three-phase induction motors are used as electrical drives. The main reason for the excessive use of induction motor in the industry is: -
  • It is cheaper as compared to other drives.
  • It is cheap as well as very efficient.
  • It has a higher starting Torque.
  • Its speed regulation is very good.

 Construction of Three-Phase Induction motor

Any type of rotating electrical machine has two main parts. (1) Stator and (2) Rotor. The stator is a static part while the rotor is a rotating part. The stator is built up of high-grade alloy steel laminations to reduce eddy current losses in the stator.

Stator 

The stator of the three-phase induction motor is a static part. it is further divided into three categories
  • Stator frame
  • Stator core
  •  field winding.

Stator frame

it is the outer part of the induction machine. Its main function is to provide mechanical support and protection for field winding and other inner components of the stator. To Construct stator frame Cast iron is used as material. For a Big size and high capacity three-phase induction motor the outer part of the stator should be mechanically hard and rigid.

Stator Core

The stator Core is the second inner component or part of a three-phase induction motor. its main function is to provide a path for carrying magnetic flux .since these are connected with magnetic flux hence to constructs it such type of material is used which has minimum Reluctance to minimize the eddy current loss.

Except this to minimize the Eddy current loss The stator Core is laminated as well. These laminated types of structures are made up of stamping which is about 0.3 to 0.5 mm thick. All these stampings are stamped together to form stator core and which is then fixed in the stator frame.

Field Winding

it is the third main part of the stator for a three-phase induction machine. The main function of the field winding is to produce alternating magnetic flux in the stator. Thus Produce magnetic flux is linked with the rotor. 

These field windings are put in the slot of the inner part of the stator frame. These winding are Connected either in Star or Delta. The connection of Field windings depends on what type of Rotor has been used for a three-phase induction motor.

Two types of rotor are used in Induction Motor which is as follows: -

  • Squirrel cage rotor
  • Phase wound rotor

Squirrel cage rotor

This type of rotor has a cylindrical laminated core on which parallel slots are built parallel to each other. In this slot, aluminum or copper rods are inserted. In this way, the end of each rod is connected to each other by a ring of the same material. In this way, due to the joining of the sticks, it looks like a squirrel's cage, so it is called Squirrel Cage Rotor. As shown in the figure below.
three phase induction motor

Phase wound rotor

It is coarse cylinder-shaped cast iron and a rod made of high-quality silicon alloy metal with slots parallel to each other on the upper periphery of the cylinder. The coil is made by inserting copper or aluminum wire into these slots.

These slot conductors are connected with each other to form a three-phase winding. The windings of the rotor are always connected to the star. Thus the end of the wire connected in star connection is put out with the help of a slip ring.
three phase induction motor

Three ends of star-connected winding after putting out is connected with the terminal of three variable resistors which are also connected in star connection. The speed of the motor can be easily controlled by changing the Resistance value of these three Variable Resistors.

Types of Three Phase Induction Motor

There are mainly two types of three-phase induction motors. A three-phase induction motor is classified based on the rotor used in it. These two main induction motors are as follows: -
  • Squirrel Cage Induction Motor.
  • Slip Ring Induction Motor or Wound Rotor Induction Motor
EMF Equation of Generator.

Working Principle of Three Phase Induction Motor

The working principle of any type of three-phase induction motor is the same, which is based on the electromagnetic induction principle. When a three-phase AC supply is provided in the stator of the three-phase induction motor, then a three-phase AC current starts flowing in it.

A magnetic flux is produced in the stator due to the magnetic effect of the current. The magnetic flux thus generated has alternating nature, When this alternating magnetic flux is cut by the Coil of the Rotor, it induces an EMF according to Faraday's electromagnetic theory.
What is separately Excited DC Motor?
Since the end of the wire of the coil of the rotor is shorted, thus the induced EMF causes a current to flow in the rotor conductor. Due to this rotor current, a magnet is also generated in the rotor.

When the magnetic flux generated in the rotor interacts with the main flux of the stator, it tries to shift itself into a region where the magnetic flux is not available and during this process, a force is experienced on the conductor which are slotted on the periphery of the rotor. Due to this force, a torque is developed on the rotor which causes the rotor to rotate.

 The main reason for the continuous rotating of the rotor is the availability of the main magnetic field in the stator. Interaction of the magnetic flux produced in the rotor and the magnetic flux produced in the stator is called armature Reaction.

What is a slip in Induction Motor?

We have seen above that when AC supply is provided in the stator, a magnetic field is generated in it. This magnetic field is of Rotating Nature. which rotates with Angular Velocity. The speed with which this magnetic field rotates inside the stator is called Synchronous Speed.

The synchronous speed of an induction motor depends on the frequency of the magnetized pole in the motor stator and the frequency of the AC supply. It is always measured in RPM and denoted by Ns.

 Ns = 120F / P
where
F = supply frequency
P = number of Pole


The rotor starts rotating due to the magnetic field in the induction motor. The rotor always rotates less than the speed of the magnetic field. The difference between magnetic field speed and rotor speed is called Slip. There is very little difference between the magnetic field and the speed of the rotor. The slip is denoted by the S of the English letter. If the speed of the rotor is indicated by N then the slips
S = Ns - N

As the slip is very small, it is always displayed in percentage. Whatever happens like this.

% S = {(Ns - N) / Ns} X 100
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