Electricals 9

Relation between torque and slip

torque / slip curve is shown in figure:

fig. Torque-slip characteristic

1) working principle of three phase induction motor

2) Working principle of dc generator

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.

Construction And Working Of  DC motor
A motor is an electrical machine which converts electrical energy into mechanical energy.




    CONSTRUCTION:
A DC machine consists two basic parts - stator and rotor. Basic constructional parts of a DC machine are described below.

Yoke:
The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field winding.

Poles and pole shoes:

Poles are joined to the yoke with the help of bolts or welding. They carry field winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and (ii) spread out the flux in air gap uniformly.

Field winding:

They are usually made of copper. Field coils are former wound and placed on each pole and are connected in series. They are wound in such way that, when energized, they form alternate North and South poles.

Armature core: Rotor
Armature core is the rotor of the machine. It is cylindrical in shape with slots to carry armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to the shaft.

Armature winding:

It is usually a former wound copper coil which rests in armature slots. The armature conductors are insulated from each other and also from the armature core. Armature winding can be wound by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally used. A double layer winding means that each armature slot will carry two different coils.

Commutator and brushes:

Physical connection to the armature winding is made through a commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the armature conductors. A commutator consists of a set of copper segments which are insulated from each other. The number of segments is equal to the number of armature coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite. They rest on commutator segments and slide on the segments when the commutator rotates keeping the physical contact to collect or supply the current.

   DC Motor Vs DC Generator
  Construction of DC Motor and DC Generator is same
  And both are called as DC machine.

 Working Principle of DC Motor :

Working Principle Of A DC Motor

A motor is an electrical machine which converts electrical energy into mechanical energy.
The principle of working of a DC motor is that "whenever a current carrying conductor is placed in a magnetic field, it experiences a mechanical force". The direction of this force is given by Fleming's left hand rule and it's magnitude is given by F = BIL. Where, B = magnetic flux density, I = current and L = length of the conductor within the magnetic field.

Fleming's left hand rule: If we stretch the first finger, second finger and thumb of our left hand to be perpendicular to each other AND direction of magnetic field is represented by the first finger, direction of the current is represented by second finger then the thumb represents the direction of the force experienced by the current carrying conductor.

  Working of DC Motor

 When armature windings are connected to a DC supply, current sets up in the winding. Magnetic field may be provided by field winding (electromagnetism) or by using permanent magnets. In this case, current carrying armature conductors experience force due to the magnetic field, according to the principle stated above.

Commutator is made segmented to achieve unidirectional torque. Otherwise, the direction of force would have reversed every time when the direction of movement of conductor is reversed the magnetic field.


 

Clssification of Motors

Basic of Electric Motor

Define Electric Motor :

• First electric motors were simple electrostatic devices created by the Scottish monk Andrew Gordon in the 1740.

• The theoretical principle behind production of mechanical force by the interactions of an electric current and a magnetic field, Amperes force law, was discovered later by Andre-Marie Ampere in 1820.

• The conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821.  

⇒Read Classificatin of Electrical Motors