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.

DC Generator are classified on the basis of their winding assembling. Means how to stator and armature winding connected it may be series or parallel or both. here you learn and understand Types of DC Generator | series dc generator | shunt dc generator | compound dc generator.

Types of DC Generator with diagram

Generators are classified as follows:
1) Parmanent Magnet DC Generator
2) Sepratly Excited DC Generator
3) Self Excited DC Generator

Self Excited DC Generator are 3 types:
1. Shunt wound generator
2. Series wound generator
3. Compound DC Generator

Compound DC Generator are 2 types:
(1) Short shunt
(2) Long shunt


Detail of the Types of DC Generator | series dc generator | shunt dc generator | compound dc generator:


1) Permanent Magnet DC Generator:
   Parmanent magnet is used for flux.
   (you can read detail we already posted)

2) Sepretly Excited DC Generator:

DC Generator in which field winding is supplied from an external DC source (EX. battery) is called a separately excited DC generator.
Figure shows the connections of a separately excited generator. The voltage output depends upon the speed of rotation of armature and the field current (Eg = φZNP/60 A). Larger the field current and speed, larger is the generated e.m.f.

Separately excited DC generators are rarely used in practice.Normally of self excited type  used.



Armature current, Ia = IL
Terminal voltage, V = Eg - IaRa
Electric power developed = EgIa
Power delivered to load = EgIa - I R = I E - I R = VIa





3) Self Excited DC Generator

3.1) Shunt-Wound DC generator

When the field winding of a generator is connected in parallel with the generator armature, the generator is called a shunt-wound generator .
The excitation current in ashunt-wound generator is dependent upon the output voltage and the field resistance. Normally, field excitation is maintained between 0.5 and 5 percent of the total current output of the generator.




Shunt field current, Ish = V/Rsh
Armature current, Ia = IL + Ish
Terminal voltage, V = Eg - IaRa
Power developed in armature = EgIa
Power delivered to load = VIL

There are four internal Types of Losses in DC Generator that decrease efficiency of a DC generator.
1) Copper losses
2) Eddy-current losses
3) Hysteresis losses
4) Mechanical losses

Types of Losses in DC Generator

1) Copper Losses:

Copper loss is the power lost as heat in the windings; it is caused by the flow of current through
the coils of the DC armature or DC field. This loss varies directly with the square of the current
in the armature or field and the resistance of the armature or field coils.

Armature: Ia^2*Ra (armature curent square into armature resistance)

Field: If^2*Rf (field current square into field resistance)


2) Eddy-Current Losses:

As the armature rotates within the field, it cuts the lines of flux at the same time that the copper
coils of wire that are wound on the armature cut the lines of flux. Since the armature is made
of iron, an EMF is induced in the iron, which causes a current to flow. These circulating
currents within the iron core are called eddy-currents

How to reduce:
To reduce eddy-currents, the armature and field cores are constructed from laminated (layered)
steel sheets. The laminated sheets are insulated from one another so that current cannot flow
from one sheet to the other.

3) Hysteresis losses :

Hysteresis losses occur when the armature rotates in a magnetic field. The magnetic domains
of the armature are held in alignment with the field in varying numbers, dependent upon field
strength. The magnetic domains rotate, with respect to the particles not held in alignment, by
one complete turn during each rotation of the armature. This rotation of magnetic domains in
the iron causes friction and heat. The heat produced by this friction is called magnetic hysteresis
loss.

How to reduce:
To reduce hysteresis losses, most DC armatures are constructed of heat-treated silicon steel,
which has an inherently low hysteresis loss. After the heat-treated silicon steel is formed to the
desired shape, the laminations are heated to a dull red and then allowed to cool. This process,
known as annealing, reduces hysteresis losses to a very low value.

4) Mechanical losses :

Rotational or mechanical losses can be caused by bearing friction, brush friction on the
commutator, or air friction (called windage), which is caused by the air turbulence due to
armature rotation. Careful maintenance can be instrumental in keeping bearing friction to a
minimum. Clean bearings and proper lubrication are essential to the reduction of bearing friction.
Brush friction is reduced by assuring proper brush seating, using proper brushes, and maintaining
proper brush tension. A smooth and clean commutator also aids in the reduction of brush
friction.

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.

Working principle of dc generator
Theory of operation of dc generator and construction and working principle of dc generator.

A basic DC generator has four basic parts:
(1) a magnetic field (2) a single conductor or loop
(3) a commutator and (4) brushes.

A basic DC generator working