Electricals 9

Working principle of three phase induction motor base on the electromagnetic induction.In which stator rotating field cuts the rotor conductor, due to this EMF induced in rotor which produce opposite current in rotor .This effect causes rotation of three phase induction motor.

 Explain working principle of three phase induction motor

When three phase supply is connected to the three phase stator winding A rotating magnetic field (flux) is produced.
The magnetic fields rotates at the synchronous speed (Ns) given by

This rotating flux passes through the air gap and also flows over the motor surface.
As the Rotor is  stationary,This rotating flux cuts the stationary rotor conductors.
Due to the relative velocity between rotor conductor and rotating magnetic field, An EMF will be induced in the rotor conductors.

The magnitude of this in the induced EMF will be Proportional to the Relative speed between rotor and magnetic field.
As the rotor conductors forms the closed circuit,A current start flowing in the rotor circuit.



By lens law the direction of this current is such that is tries to oppose the reason producing it.

As a rotation of magnetic field is the cause, the rotor conductor will experience A force which start rotating the rotor in the same direction As that of flux and tries to reach the speed of magnetic field To reduce the relative velocity and has a induced EMF is zero.

In induction motor stator field rotates at the synchronous speed And thus produces rotor current (or Rotor torque).It is because of relative difference in speed of rotor and stator field.

The rotor current or torque rotates the rotor in the direction of the stator field (RMF),So that the cause which is producing the current or torque can be opposed.
But if the rotor reaches the synchronous speed then the relative speed between rotor and stator field will become zero.

And hence no rotor current or torque Will be produced to maintain the rotation of Rotor.Hance a rotor always lags behind the status field.That mean rotor speed is always less than synchronous speed.

construction of dc generator and dc motor is similar . A dc motor can be used as dc generator without any constructional changes or vice versa.There are some main part such as  Armature, Rotor, Stator, Field, commutator, brush and yoke.

Construction of dc machine
Construction of dc generator
Construction of dc motor

Construction and parts of dc motor or dc generator:

1. Armature
2. Rotor
3. Stator
4. Field
5. commutator
6. brush

fig.1 DC Generator construction

Armature:

Armature is rotating part of machine, It is conductor coil which cuts magnetic flux of magnet and generate electrical energy at output terminal
The purpose of the armature is to provide the energy conversion in a DC machine (refer to Figure ).

In a DC generator, the armature is rotated by an external mechanical force, such as a steam turbine and wind turbine . This rotation induces a voltage ( as per faraday law) and current flow in the armature. Thus, the armature converts mechanical energy to electrical energy.

In a DC motor, the armature receives voltage from an outside electrical source and converts electrical energy into mechanical energy in the form of torque.

Rotor:

Rotor is moving part.
The purpose of the rotor is to provide the rotating element in a DC machine (refer to Figure 2). In a DC generator, the rotor is the component that is rotated by an external force. In a DC motor, the rotor is the component that turns a piece of equipment. In both types of DC machines, the rotor is the armature.

Stator:

stator is fixed part.
The stator is the part of a motor or generator that is stationary (refer to Figure 1). In DC machines, the purpose of the stator is to provide the magnetic field. The stator in Figure 1 is provided by permanent magnet (generally electromagnets are used).



Field:

The purpose of the field (winding) in a DC machine is to provide a magnetic field for producing either a voltage (generator) or a torque (motor) (refer to Figure 1). The field in a DC machine is
produced by either a permanent magnet or an electromagnet. Normally, electromagnets are used
because they have an increased magnetic strength, and the magnetic strength is more easily varied
using external devices. In Figure 2, the field is provided by the stator.

Yoke:
Yoke is frame and outer covering body  made up of iron metal.

Comutator:

AC to DC Voltage converter.
The commutator converts the AC voltage generated in the rotating loop(ARMATURE) into a DC voltage. It also serves as a means of connecting the brushes to the rotating loop(ARMATURE).
In a simple one-loop generator, the commutator is made up of two semi-cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material, as shown in below Figure . Each half of the commutator segments is permanently attached to one end of the rotating loop(ARMATURE), and the commutator rotates with the loop(ARMATURE).

commutator with carbon brush

Commutator is the electrical device which perform mechanically conversion of electric current from AC to DC. Commutator is made up of two semi-cylindrical pieces of a smooth conducting material, usually copper, separated by an insulating material.

Commutator Action is called as commutation. Means AC to DC conversion.



Commutation Action : In DC Generator

• The commutator converts the AC voltage generated in the rotating loop into a DC voltage. It also serves as a means of connecting the brushes to the rotating loop.
• The purpose of the brushes is to connect the generated voltage to an external circuit. In order to do this, each brush must make contact with one of the ends of the loop.
• Since the loop or armature rotates, a direct connection is impractical. Instead, the brushes are connected to the ends of the loop through the commutator. which the brushes make contact with each end of the loop.