1.0 - dc generator student

7/29/2019 1.0 - DC Generator Student

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Electricity can be madeusing a very simple

generator as shown inthis video.

around within a coil ofwire, electricity is made inthe wire.

If we connect a light bulbto the wires that comeout of the generator thiselectricity will light it up.

In an oil, coil or

To make electricity for everyone, we need to use very largegenerators. We make these generators turn in different ways.

gas re powerstation, we

burn fuel tomake waterturn to steam.

This steam is then used to turn a big set of wheels called a steamturbine. This then turns the generator.

In a hydro electric power station, lots of water is dropped through abig wheel making it turn. This is connected to the generator.


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Nuclear power station use radioactive fuel called Uranium.

Uranium atoms release small particles called neutrons which hit otheruranium atoms and split them into two creating heat, radiation andmore neutrons.

The heat is usedto make thesteam whichruns a steamturbinegeneratingelectricity.

In a biomass power station biomass fuel from trees, shrubs andanimal poo is burned in a boiler to produce high pressure steam.

The steam rotates the turbines which turn the generator to produceelectricity.

Geothermal energy uses the steam from hot water reservoirs deepbeneath the earth. Engineers drill down to these reservoirs.

As the water rises to the surface it begins to boil.

The steam isused to spinturbines whichturn thegenerator andproduceelectricity.


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The energy of sunlight can be used generate electricity using specialsolar panels called photovoltaic cells.

These panels usually contain silicon produce electricity when sunlightfalls on them.

Wind turbines use a very big blade attached to a generator.

The blades of the wind turbine catch the wind, causing it to turn,generating electricity.

The electricity from the generator travels along underground power lines to asubstation. Inside the substation, a transformer changes the electricity to ahigher voltage.

This makes it easier andmore efficient to move alonglon distances of overheadpower lines and undergroundcables.

Overhead lines allow electricity to be transported at high voltage over longdistances.

Another transformer turns the high voltage electricity into a lower voltage,making it safe to be used in the house.

In a city, the electricity is sent to your house from the substation along anunderground cable that you cant see.


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The symbol used to represent flux is ( ).

Magnetic lines of flux do not flow

it is assumed they are in a direction north ( N ) to south ( S ).

Magnetic field of a magnet bar Direction of Magnetic field

The filings will align themselves in a pattern as show as figure below.

Flux lines can be seen by placing a piece of cardboard on a magnet andsprinkling iron filings on the cardboard.

Magnetic lines of repel each other and never cross.

Flux lines of a single magnet bar Flux lines of two magnets bar


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A basic law of physics state that whenever an electric current flowsthrough a conductor, a magnetic field is formed around the conductor.

Electromagnets depend on electriccurrent flow to produce a magneticfields.

Magnetic field of a Single Wiretraight Wire and Current

Direction of the magnetic fields can be determine by using right handrule. ( by assumed that the current flow from +ve to ve source )

Direction of current flow and magnetic fields

If the conductor is wound into a coil as shown below, the magneticlines of flux add to produce a stronger magnetic field.

A coil with ten turns of wire will produce the times as strong as themagnetic fields around a single conductor.

Coiled Wire around Iron CoreCoiled Wire, Current andMagnetic Field


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Another factor that effects the strengthof an electromagnetic field is the amount

of current flowing through the wire..

= Magnetic Flux= Current

So that the two factors that determine the number of flux linesproduced by an electromagnet are:i. Number turns of wire.ii. The amount of current flow through the wire.

Movement of Magnetic field in acoil Relation of Direction betweenCurrent and Magnetic field in a coil

Faradays law of electromagnetic

induction, it revealed a fundamental

relationship between the voltage and

flux.

Whenever a conductor cuts magnetic

flux, an

induced in that conductor.


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Faraday found no evidence when the current was steady, but did see acurrent induced when the switch was turned on or off.

The phenomenon whereby an

emf and hence current is induced

in any conductor which is cut

across or is cut by magnetic flux

is as know as

.

The of induced electromotive force ( emf ) in aclosed loop is

Mathematically,

Faradays Second Law state that:

. [ 1 loop ]

. [ N loop ]

Where: ( volt )( Weber Wb )

:The was introduced The sign indicatesthe direction of emf induced.

When the magnet is moving up towardcoil, the induced voltage will causeelectrons flow in the direction indicatedby the arrows.

If the magnet is moving downward fromcoil, the polarity of induced voltage will bereversed and the current will flowo osite direction .

But when nomovement ofmagnetmeaning thereare nomagnetic fieldcuts through aconductor,

there are novoltage induce.


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an induced electromotive force generates acurrent that induces a counter magnetic field that opposes themagnetic field generating the current

The polarity of the induced voltage is determined by the polarity ofthe magnetic field in relation to the direction of movement.

So that, reversing the polarity of the magnetic field will reverseshe polarity of the voltage induce.

From that mathematically equation, there are three factors

determine the amount of voltage induced in a conductor:

i. The number turns of wire,

ii. The strength of the magnetic field (flux density), and

iii. The speed of the cutting action (velocity).


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In order to induce 1V in a conductor, the conductor must cut100,000,000 lines of magnetic flux in 1s.

In measurement, 100,000,000 lines of magnetic flux are equal toone Weber (Wb).

When conductors are woundinto a loop of 20 turns, thevoltage induced into eachconductor will add.

Therefore, if a conductor cuts magnetic lines of flux at a rate of1Wb/s, a voltage 1 volt will be induced.

The total induced voltage will be20 volt.

The second factor is the strength of the magnetic field.

Flux density ( B ), is a measure of the strength of a magnetic field.

If the number of turns of wire in the armature remain constantand the speed remain constant, the output voltage can be

Increasing the lines of flux will

increase the number of fluxlines cut per second.

.

Therefore, the voltage output willincrease so.

The magnetic field strength canbe increased until the iron of thepole pieces reaches saturation.

Induced voltage is proportional to the number of flux lines cut persecond.

If the strength of the magnetic field have reach saturated valueand the number of turns of wire in the armature is hard toincrease, the out ut volta e can also be determined b the s eed at

Increasing the speed of thearmature will increase thespeed of the cutting action,which will increase the outputvoltage.

which the conductors cut the flux lines.

And it work in otherwise

direction.


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use to determine the relationshipbetween the motion direction of the conductor, direction of themagnetic field and to the direction the induced current.

To use the , place thethumb, forefinger, and center finger atright angles to each other as shown in

.

i. direction from north to south of the

The direction of each finger representthe direction of each parameter involvein the generator, which are:

ii. direction of the motion or , and

iii. direction of voltage induced or .

i. direction from north to south of the

ii. direction of the motion or , and

iii. direction of voltage induced or .

For the is usefor the , that willdiscuss in the next chapter.

An electrical device, which convertsmechanical energy into electrical energy. AC generator producesalternating current.

Figure ( a ) construction of an elementary / basic AC Generatorand ( b ) Voltage induced in the AC Generator as a function ofthe angle of rotation.


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An electrical device, which convertsmechanical energy into electrical energy. DC generator producesdirect current.

Figure ( a ) construction of an elementary / basic DC Generatorwith a mechanical rectifier called a commutator.

Figure ( b ) The elementaryDC Generator produces a pulsatingDCCVoltage.

The main different of construction between and Generatoris at the end of wire terminal connection. Refer figure below.


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In figure shown, The leg of the loop is connected to theof the commutor, and the leg to theof the commutator.

The two segments are , so that no electricalcontact is possible.

The two areon opposite SIDES of

the SPLIT RING,mounted in such amanner that each

The loop in is moving in a

, parallel to the flux. Hence, no

emf is generated. Notice that the

is just coming in contact with

the , and the

.

In , the flux is being cut at a

. The is

contacting the and

the the

. And the

needle is deflected to the .


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At , the loop has completed

Again, no flux is being cut, so

the emf is . The is

off the andthe . At the same instant,

the is leavin the

, and going on to the

.

In , commutator action

in the external

circuit, and the second half cycle.

A graph for of a d.c. generator is shown in figure . Thegeneration of the emf for is the

But at the brushes, in moving from onecommutator segment to the other,

rather than becoming negative.

,to maximum, and falls

back to zero for

. To produce ad.c.,


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The main different of construction between AC and DC Generatoris at the end of wire terminal connection. Refer figure below.


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A construction of DC generator has

1. Bearing

2. End shield

104. Armature winding

5. Brush holder

9. Mounting


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The outer frame / yoke is thestationary part of machine.

Stator is made by Cast Iron (smallmachine) or Cast Steel (largemachine .

Functions of the stator is to :

i. Provides mechanical support(mounting) for the poles andacts as a protecting cover forwhole machine.

ii. It carries the magnetic fluxproduced by the poles.

Produced the magnetic field (flux) in the machine.

Consist of permanent magnet (small machine) or turns of wirealso know as field coil (large machine)

Field coils, mounted on the poles, carry the DC exciting current.

Main parts of DC Generator in Cutaway view

Armature is a rotating parts of the DCGenerator.

It consists of a commutator, an iron core, and aseveral coil of wire wound on an armature andalso know as armature coil or armaturewindings.

The armature conductors carry the load currentdelivered by the generator.


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The armature is keyed to a shaft and revolves

between the field poles.

Armature is a cylinder of laminated iron mountedon an axle.

In other word, it is a houses of the armatureconductor or coils and causes them to rotate andhence cut the magnetic flux of the field magnets.

For that reason, voltage produced in the armature..

The voltage produced in all rotating armatures isalternating voltage.

ROTOR / ARMATURE The axle is carried in bearings mounted in the external structure of

the generator.

Torque is applied to the axle to make the rotor spin.

Since direct current generators mustproduce DC current instead of ACcurrent, some device must be used tochange the alternating voltageproduced in the armature windingsinto direct voltage, this job isperformed by the commutator.

In other word, the commutator actsas a mechanical rectifier, which isconvert alternating current (AC) to

direct current (DC)


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Commutator mounted on the shaft of the machine, meaning that

it is a moving part.

The commutator is constructed from copper ring spilt into a

the segments.

The different connection between AC and DC Generator

i. Commutator / Split Ring for DC Generator while,

ii. Slip Ring for AC Generator

and of DC Generator withof commutator


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The number of segments id depending to the number of armaturecoils.

and of DC Generator withof commutator

The number of commutator segments is increased in directproportion to the number of loops; that is, there are two segmentsfor one loop, four segments for two loops, and eight segments forfour loops.

Output waveform and diagram of DC Generator with 3 coil and 6segments of commutator

As the number of loops is increased, the variation betweenmaximum and minimum values of voltage is reduced and theoutput voltage of the generator approaches a steady dc value andthe output voltage pulsates but never falls to zero.


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Figure (a) and (b) below show the different of output waveform ifthe commutator segments is increases.

Figure show a numbers of commutator which are electricallyinsulated from one to another with mica sheets insulation.

To carry the induced current from the loop to the outside circuit.

A two-pole generator has two brushes fixed diametricallyopposite to each other.

The slide on the commutator and ensure ood electrical contactbetween the revolving armature and the stationary external load.

The brushes are made of carbon because it has good electr icalconductivity and its softness does not score the commutator.


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The brush pressure is set bymeans of adjustable springs.

If the pressure is to great, thefriction produces excessiveheating of the commutator and

,imperfect contact may producesparking


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We have learn from previous lesson that one of the factor toincrease the voltage induces in generator is increase the turns ofwire in the armature.

The windings of armatures are connected in different waysdepending on the requirements of the machine.

The of armature windings are the , and

The difference is merely due to the different arrangement of the endconnection at the front or commutator end of armature.


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Before we discuss further about these three basic types ofarmature winding, let see what is a coil.

Turn of a single wire two conductors connected each other as

shown in figure ( a ). While a coil is several turns of conductor refer figure ( b ).

Figure ( c ) show that several coil connected together in seriesconnection.

The windings are connected in parallel.

This permit the current capacity of each winding to be added andprovides a higher operating current.

This windings are used in machines designed for low voltage andhigh current.

These armatures are generally constructed with large wire becauseof high current.

A good example of where lap wound armature are used is in the

stator motor of almost all automobiles.

Commutator bar


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no. of parallel paths, a = No. of poles, p,= no. ofbrushes

If this types of windings are use, they will have asmany pairs of brushes as there are pairs of poles.

Figure shown a polar windings

diagram of a 4 pole, 12 coil,

lap wound DC machine.

Figure (b) shown a developed diagram of a 4 pole, 8 coil, lapwound DC machine.

Figure (a) shown a polar windings diagram of a 4 pole, 8 coil, lapwound DC machine.

(a) (b)


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This windings are connected in series.

When the windings are connected in series, the voltage of each

This type of windings are used in machines designed for high voltageand low current.

, .

A good example of where wave wound armatures are used is in thesmall generator in hand-cranked megaohmeters.

Wave wound armatures never contain more than two parallel paths

for current flow regardless of the number of pole piece, and they

never contain more than one set of brushes.

number ofbrushes positions

number of parallel paths,a = 2

number ofbrushes isincreased in largemachines tominimize thecurrent density in

brushes.

=


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These armatures are used in machines designed for use withmoderate current and moderate voltage.

The windings of frog leg are connected in series-parallel.

This type of windings are probably the most used.

Most large machines use frog leg wound armature.

For this type of winding, we are not going to discuss further In thischapter.


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1) Number of parallel paths equalsthe number of poles.

2) The number of brush positionson the commutator equals thenumber of oles.

1) Number of parallel paths arealways two.

2) A minimum of two brushposition are required.

3) The two ends of an armature coilare connected to the twoadjacent commutator segments.

4) The winding forms a continuousclosed circuit.

5) The lap wound generators areused for supplying low voltage,high current loads.

3) The two ends of an armature coilare connected to the twocommutator segments..

4) The winding forms a continuousclosed circuit.

5) The wave wound generators areused for supplying high voltage,low current loads.

In such windings, there are several sets of complete ly closed andindependent windings.

If there is only one set of close winding, it is called simplex wavewinding.

If there are two such windings on the same armature, it is calledduplex winding and so on.

The multiplicity affects a number of parallel paths in the armature.

If the multiplicity increases, the number of parallel paths in thearmature increases.

Simplex, m = 1

Plex of the winding is label by m.

Duplex, m = 2

Triplex, m = 3


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Thecan be expressed as:

)(60 volt

apxNZEG = = number of coil in the armature

= number turns of wire

,

Generated voltage

Speed (rpm)

Total number of conductor

Flux per pole (Weber Wb)

Number of pole

number of current path

i . For windings:

ii. For windings

iii. For windings:

number of poles on thearmature

plex of the windings


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We already discuss the basic windings in the armature

Now we going to learn the basic connection of the windings toproduced the magnetic field.

Instead of using permanent magnets to create the magneticfield, pairs of electromagnets called field poles are employed.

Magnetic field produced byPermanent magnet

Magnetic field produced byElectromagnetic conductor

When a dc voltage is applied to the field windings of a dcgenerator, current flows through the windings and sets up asteady magnetic field. This is called

Figure ( a ) Magnetic field around a coil and ( b ) the direction ofmagnetic field


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This excitation voltage can beproduced by the generator itselfor it can be supplied by an

outside source, such as abattery.

Therefore excitations in DCGenerator can be divided intotwo major types which are

andgenerator.

generator hasthree basic types such as

, andthat we will discuss

later.

In other word, a DC Generator is a

When the DC field current in such a generator issource (such as a battery storage) the

generator is said to be

a

b

IX

EO

x

y

..

There are three basic types ofgenerator such as ,

and generator.

For now, we will focus a shuntgenerator an example of the

generator.

So that the generator can be self

excited as shown in Figure

The shunt field coil is connected inwith the armature

terminals,


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In other word, a self-excited generator is a generator thatby having its field connected

directly across the terminals of the machine. When the generator starts rotating, the weak residual

magnetism causes a small voltage to be generated in thearmature.

This small voltage applied to the field coils causes a small fieldcurrent. Although small, this field current strengthens themagnetic field and allows the armature to generate a highervoltage.

The higher voltage increases the field strength, and so on.

This process continues until the output voltage reaches therated output of the generator.

a

b

IX

EO

x

y


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Your group will be given 10 minutes to discuss among your groupmembers the questions below and I will call a name of your group topresents it.

Sketch and describe the Lap winding.

- Sketch and describe the Wave winding

State two method of field excitation and how to differentiate

between them?

Sketch the equivalent circuit for the two method of fieldexcitation and describe it.

State the all formula related to calculate the internal generatedvoltage in DC Generator and list down all parameter involved.


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Before we discuss further the types of generator, let discuss firsttwo types of field windings are common used which are and

windings.

The terminal leads of the

windings are made with relativelyand have a very .

series fie are a ee an

While winding aremade by

, it has a muchthan the series

winding.

The terminals leads of the

shunt field are labeled and.


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From previous discussion, we know that generator are usuallyclassified according to the way in which their fields are excited.

Generators may be divided into separately-excited and self-excited.

i. Separately-excited generator is a generator whose fieldcurrent is

ii. Self-excited generator is a generator thatby having its field connected directly across

the terminals of the machine.

a

b

IX

EO

x

y

There are three types of self-

excited generators named

according to the manner in

windings) are connected to

the armature. which are

, and

generator.


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The field windings areconnected in series with thearmature

The external circuit connectedto the generator is called theload circuit.

When the field winding isconnected in series thearmature current, load andfield current are the same.

The field windings areconnected in parallelwith the armature.

Shunt generator can beeither self-excited orseparately excited as wediscuss before.

1) Number of turns of the fieldwinding is large and varies from300 to 1000 turns..

2 Resistance of the field windin is

1) Number of turns of the fieldwinding is small, and variesbetween 2 and 10 turns.

2 Resistance of the field windin ishigh and varies from 200 to400 .

3) The shunt field current is usuallyless than 2A, and therefore thecross-sectional area of the fieldwinding used is small.

very small, generally less than0.10.

3) The series field winding carriesthe same current as thearmature and therefore the sizeof the conductor used is verylarge.


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Contain

. Most large DC generator are

compound wound.

The series and shunt fieldcan be connected in twoways.

One is calledwhich is shunt field

as shown in the figure.

The second connection is callas shown in the

figure.

The short shunt connectionhas the shunt field

connected in parallel withthe armature.

And the series field isconnected in series with thearmature.


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VT =EG - IARA VT =EG - IA ( RA+RS) VT =EG - IARA

VT =EG - IA ( RA+RS) VT =EG - IARA - ISRS

VSVA Generated VoltageTerminal Voltage = VLLoad CurrentArmature Resistance

VT =EG - IA( RA+RS)

Voltage Drop across thearmature resistance

Series Field ResistanceVoltage Drop across theseries winding

Armature Current

Series Current

VA

Generated VoltageTerminal Voltage = VLLoad CurrentArmature Resistance

VT =EG - IARA

Vo tage Drop across t earmature resistance

Shunt Field ResistanceVoltage Drop across theShunt winding

Armature Current

Shunt Current


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VA

Generated Voltage

Terminal Voltage = VLLoad CurrentArmature Resistance

VT =EG - IARA

VF



Armature Current

Shunt Current

VA VSGenerated VoltageTerminal Voltage = VLLoad CurrentArmature Resistance


VT =EG - IA( RA+RS)

VF


Armature Current

Shunt Current

Series Field ResistanceVoltage Drop across theseries windingSeries Current

VA VSGenerated VoltageTerminal Voltage = VLLoad CurrentArmature Resistance


VT =EG - IA( RA+RS)

VF

Shunt Field ResistanceVoltage Drop across the

Shunt winding

Armature Current

Shunt Current

Series Field ResistanceVoltage Drop across theseries windingSeries Current


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The losses taking place in the motor are the . There aremajor losses in , which are:

When an electric current, I ( ampere )flows in a resistance, R ( ohms ), heat is lost at the rate of

, and the loss is .

Motors and generators have one or more field circuits and an armaturecircuit in which such losses occur. All resistance losses of kind are classed asco er loss.

where RA = resistance of armature. Thisloss is about 30 to 40% of total full-load losses.

this losses cause by field winding, for shunt fieldwinding this losses equals , while for series field winding equals

. This loss is about 30% of total full-load losses.

The loss due to resistance of brush contact. Thevoltage drop at the brush is almost independent of For carbon brush

the voltage drop around 1 volt per brush. The power loss due to brushescontact resistance is , where is the voltage drop at one brush.


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Iron lossesare a function of both flux and speed.

the watt loss in form of heat that occurs in the

iron of a magnetic fields are constantly reversed. current that are produced in the iron of a

magnetic circuit. The current flow within the iron as it is cut by.

There is friction loss in the machine bearings, atthe surface of the commutator due to the rubbing of the brushed.These losses depend on the speed but are independent of the load onthe machine.

at bearings and commutator.

of rotating armature

1) H steresis loss.1) Armature copper loss (IA2 RA)2)Field co er loss (PCU = I 2 R

known as variable losses cause itvaries with the load current

known as constant , rotational or straylosses.

1) Friction loss(bearings and

commutator)2)Air friction loss

2) Eddy currentloss

and PCU = IF2 RF)3)The loss due to brush contact

resistance..


1) Friction loss(bearings andcommutator)

2)Air friction loss

2) Eddy currentloss


resistance..


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By voltage regulation of a generator is meant the change in itsterminal voltage with the change load current when it is running ata constant speed.

If the change in voltage between no-load and full-load is small, thenthe generator is said to have good regulation and the way the

Voltage Regulation, = x 100%( no load Voltage - full load Voltage )

full load Voltage

generator has poor regulation.

The voltage regulation of a DC Generator is the change in voltagewhen the load is reduced from rated value to aero, expressed as

percentage of the rated load voltage..


1) Friction loss(bearings andcommutator)

2)Air friction loss

2) Eddy currentloss


resistance..


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By the term torque is meant the turning or twisting moment of aforce about an axis. It is measured by the product of the force andthe radius at which this force acts.

2 N

60 Power developed, P = x T

The torque which is available for doing useful work is known asshaft torque, .

It is so called because it is available at the shaft.

Shaft Torque, = N-mOutput in watts

2 N / 60

Where is in .

The equation of shaft torque, as shown below


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1.0 - dc generator student

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