UNIT - I

D.C. GENERATORS

D.C. GENERATORS-CONSTRUCTION & OPERATION

• DC Generators • Principle of operation • Action of Commutator • Constructional details of DC Machine• Types of DC generators• EMF Equation

DC Generator

DC motor

D.C. GENERATORS PRINCIPLE OF OPERATION

DC generator converts mechanical energy into electrical energy. when a conductor move in a magnetic field in such a way conductors cuts across a magnetic flux of lines and e.m.f. produces in a generator and it is defined by faradays law of electromagnetic induction e.m.f. causes current to flow if the conductor circuit is closed.

First Law :

  Whenever the magnetic flux linked with a circuit changes, an e.m.f. is always induced in it. 

                                        or

  Whenever a conductor cuts magnetic flux, an e.m.f. is induced in that conductor. 

Second Law :

  The magnitude of the induced e.m.f. is equal to the rate of change of flux linkages.

Faradays laws

Faradays Law of Electromagnetic Induction

A changing magnetic flux A changing magnetic flux through a  loop or loops of wire through a  loop or loops of wire induces an electromotive force induces an electromotive force (voltage) in each loop(voltage) in each loop..

Lenz’s Law

“The induced currents in a conductor are in such a direction as to oppose the change in magnetic field that produces them..”

“The direction of induced E.M.F in a coil (conductor) is such that it opposes the cause of producing it..”

OR

Fleming's Right Hand Rule

• The Thumb represents the direction of Motion of the conductor.

• The First finger (four finger) represents Field.

• The Second finger (Middle finger) represents Current

E.M.F

Fleming's Right Hand Rule

The following are the basic requirements to be satisfied for generation of E.M.F

• Magnetic field :- Permanent Magnet (or) Electro Magnet (practical) • Conductor :- Copper (or) Aluminum bars placed in slots cut around the periphery of cylindrical rotor• Relative motion:- By Prime Mover Turbine I.C Engine (Internal combustion)

1.A uniform Magnetic field

2.A System of conductors

3.Relative motion between the magnetic field and conductors

Simple loop generator

Basic Generator

Generators

Simple loop generator with slip ring

Generators

Basic operation of the generatorBasic operation of the generatorAs the loop rotates, the magnetic As the loop rotates, the magnetic flux through it changes with timeflux through it changes with timeThis induces an e.m.f and a current This induces an e.m.f and a current in the external circuitin the external circuitThe ends of the loop are connected The ends of the loop are connected to slip rings that rotate with the loopto slip rings that rotate with the loopConnections to the external circuit Connections to the external circuit are made by stationary brushes in are made by stationary brushes in contact with the slip ringscontact with the slip rings

Simple loop generator with split ring

Simple loop generator with split ring

Working Principle of D.C Generator

Schematic diagram of a simple DC Generator1st half cycle(00 to 1800 ) Path of current ABR1B1MLR2B2CD

2st half cycle(1800 to 3600) Path of current DCR2B1MLB2R1BA

DC Generators, cont• The output voltage always

has the same polarity• The current is a pulsating

current• To produce a steady

current, many loops and commutators around the axis of rotation are used– The multiple outputs are

superimposed and the output is almost free of fluctuations

Unidirectional current wave shape

Resultant current wave shape when number of conductors used result current wave shape

Constructional Details Of DC Machine

Yoke: Rotor:  Stator:  Field electromagnets:  Pole core and pole shoe: Brushes: Shaft: Armature:  Coil:  Commutator: Bearings:

Cross section view of dc machineConstruction details of DC generator

N

S

shaft

Main parts of a 4-pole d. c machine

Practical Dc Machine

1)Yoke1)Yoke:-1)Yoke:- - Acts as frame of the machine- Acts as frame of the machine - Mechanical support- Mechanical support - low reluctance for magnetic flux - low reluctance for magnetic flux - High Permeability- High Permeability -- For Small machines -- Cast iron—low cost-- For Small machines -- Cast iron—low cost -- For Large Machines -- Cast Steel (Rolled steel)-- For Large Machines -- Cast Steel (Rolled steel)

Large DC machine Small DC machine

2)pole cores and pole shoes2)Field Magnets:-2)Field Magnets:- a) Pole core (Pole body) :- --Carry the field coilsa) Pole core (Pole body) :- --Carry the field coils --Rectangle Cross sections--Rectangle Cross sections -- Laminated to reduce heat losses -- Laminated to reduce heat losses --Fitted to yoke through bolts --Fitted to yoke through bolts b) Pole shoe:- Acts as support to field poles b) Pole shoe:- Acts as support to field poles and spreads out flux and spreads out flux Pole core & Pole shoe are laminated of annealed steel Pole core & Pole shoe are laminated of annealed steel (Of thickness of 1mm to 0.25 mm)(Of thickness of 1mm to 0.25 mm)

2)pole cores and pole shoes 2)Field Magnets:-2)Field Magnets:- c) Field coils (Magnetizing coils):- -- Provide excitation c) Field coils (Magnetizing coils):- -- Provide excitation (exciting coils) I . e field flux(exciting coils) I . e field flux --Number of poles depends speed of armature on and the --Number of poles depends speed of armature on and the output for which the machine designed output for which the machine designed --Frame to used for design for exciting coils --Frame to used for design for exciting coils Different types of fields Different types of fields i) Separately Excitingi) Separately Exciting ii) Self Exciting ii) Self Exciting

3)Armature core

3)Conductor system:- 3)Conductor system:- a) Armature core (Armature):- a) Armature core (Armature):- -- To support armature windings -- To support armature windings --To rotate conductors in a magnetic field--To rotate conductors in a magnetic field -- it is cylindrical or drum shaped is built -- it is cylindrical or drum shaped is built --Made of high permeability silicon steel --Made of high permeability silicon steel stampings (of 0.5 mm thick) stampings (of 0.5 mm thick) -- Each stamping is separated from its -- Each stamping is separated from its neighboring one by thin varnish as insulationneighboring one by thin varnish as insulation --Laminated to reduce eddy current losses--Laminated to reduce eddy current losses

-- A small air gap between pole pieces and -- A small air gap between pole pieces and armature so that no rubbing between themarmature so that no rubbing between them

-- High grade silicon steel used to reduce-- High grade silicon steel used to reduce i) Hysteresis lossi) Hysteresis loss ii) Eddy current lossii) Eddy current loss -- Ventilating ducts are provided to dissipate -- Ventilating ducts are provided to dissipate heat to dissipate heat generated by above lossesheat to dissipate heat generated by above losses b) Armature Winding:-b) Armature Winding:- Main flux cuts armature and hence E.M.F is inducedMain flux cuts armature and hence E.M.F is induced --winding made of Copper (or) Aluminum--winding made of Copper (or) Aluminum --windings are insulated each other--windings are insulated each other

4)commutator4) Commutator:--Hard drawn copper bars segments insulated from each 4) Commutator:--Hard drawn copper bars segments insulated from each other by mica segments (insulation)other by mica segments (insulation) -- Between armature & External circuit-- Between armature & External circuit -- Split-Rings (acts like Rectifier AC to DC ) -- Split-Rings (acts like Rectifier AC to DC )

5&6 Bearings and Brushes 5)Brushes and brush gear:- 5)Brushes and brush gear:- Carbon, Carbon graphite, copper used to Collects currentCarbon, Carbon graphite, copper used to Collects current from commutation (in case of Generator)from commutation (in case of Generator)

6)Shaft and bearings:- 6)Shaft and bearings:- Shaft-- Mechanical link between prime over and armatureShaft-- Mechanical link between prime over and armature Bearings– For free rotationBearings– For free rotation

DC Machine Construction

Rotor of a dc machine

DC Machine Construction

Cutaway view of a dc machine

DC Machine Construction

Armature Winding

Armature Winding is classified into two types:

Lap winding

Wave windings

Armature windings

Lap Winding:

are used in machines designed for low voltage and high current

armatures are constructed with large wire because of high current

Eg: - are used is in the starter motor of almost all automobiles

The windings of a lap wound armature are connected in parallel. This

permits the current capacity of each winding to be added and provides a

higher operating current.

No of parallel path, A=P ; P = no. of poles

Wave winding:

are used in machines designed for high voltage and low current

their windings connected in series

When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same

are used is in the small generator.

No of parallel path, A=2,

Commutation process in D.C Generator

Commutation is the positioning of the DC generator brushes so that the commutator segments change brushes at the same time the armature current changes direction.

Generated EMF or EMF Equation of a generator

Let = flux/pole in Weber

Z =Total number of armature conductors

=No. of slot × No. of conductors/slot

P= No. of generator poles

A =No. of parallel paths in armature

N= Armature rotation in revolutions per minute (r. p. m)

E= e.m.f induced in any parallel path in armature

Generated e.m.f Eg= e.m.f generated in any one of the parallel paths i.e E

Average e.m.f generated/conductor = d volt

dt

Now, flux cut/conductor in one revolution d = P wb

No. of revolutions/sec=N/ 60Time for one revolution , dt= 60 /N sec According to Faraday’s Law of electro magnetic induction E.M.F generated/conductor = d= PN volts dt 60No. of conductors (in series) in one parallel path= Z / A

E.M.F generated/path= PN × Z Volts 60 A Generate E.M.F, Eg= Z N × P Volts 60 AFor i) Wave winding A = 2 ii) Lap winding A = P

Generators

D.C Generators A.C Generators (Alternators)

Cummulatitave differentially Cummulatitave differentially

1)Separately excited generators

2)Self excited generators

i) shunt wound

ii) series wound

iii) compound wound

a) long shunt

b) short shunt

Types of Generators

Clasifications of Generators

Separately excited generators

Ia=IL

E=Vt+ IaRa +BCD

shunt wound

G

L

VL

series wound

G VL

L

compound woundlong shunt short shunt

G VL

L

L

G VL

L

L

The Practical DC Generator

The actual construction and operation of a practical dc generator differs somewhat from our elementary generators

Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator

The main advantages of using electromagnetic poles are: (1) increased field strength and (2) possible to control the strength of

the fields. By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled.

Four-pole generator (without armature)

D.C. Generator Characteristics

The following are the three most important characteristics in a D.C. generator:

1. Open Circuit Characteristics (Eo/IF)

2. Internal Characteristics (E/Ia)

3. External Characteristics (V/Ia)

Critical Resistance for shunt Generator

Critical field resistance is a term that is associated with a DC Shunt generator. The value of resistance of shunt field winding beyond which the self generator fails to build up its voltage is known as " critical resistanceat a given speed it is the maximum field resistance with which the shunt generator excite. Shunt generator will build up voltage only if field circuit resistance is less than critical field resistance.

How to Draw O.C.C. at Different Speeds?

If we are given O.C.C. of a generator at a constant speed N1 then we can easily draw the O.C.C. at any other constant speed N2.Fig (3.11) illustrates the procedure. Here we are given O.C.C. at a constant speed N1.It is desired to find the O.C.C. at constant speed N2 (it is assumed that n1 < N2)For constantexcitation, E α N.

E2/E1=N2/N1

As shown in Fig. (3.11), for If = OH, E1 = HC. Therefore, the new value of e.m.f. (E2) for the same If but at N2i.

E2=HC ×( N2/N1) = HD

Critical Speed (NC)

The critical speed of a shunt generator is the minimum speed below which it fails to excite.

Therefore , Speed α Critical resistance

In order to find critical speed, take any convenient point C on excitationaxis and erect a perpendicular so as to cut Rsh and R’sh lines at points B andA respectively. Then,

BC/AC =NC/N

or NC = N ×(BC/AC)

Conditions for Voltage Build-Up of a Shunt Generator

The necessary conditions for voltage build-up in a shunt generator are:

(i) There must be some residual magnetism in generator poles.

(ii) The connections of the field winding should be such that the field current strengthens the residual magnetism.

(iii) The resistance of the field circuit should be less than the critical resistance. In other words, the speed of the generator should be higher than the critical speed.

Open circuit characteristics of Separately Excited D.C.Generator

Internal and External Characteristics

Characteristics of Shunt Generator

Characteristics of Series Generator

Compound Generator Characteristics

Armature Reaction

The effect of magnetic field set up by armature current on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:(i) It demagnetizes or weakens the main flux

(ii) It cross-magnetizes or distorts.

Commutation

It is the process of converting A.C generated voltage in the armature conductors to D.C for external load.

Commutation process in interpoles in DC machine

Applications of D.C Generators

Separately excited generators

i) These are used for speed control of D.C motors over a large range.

ii) These are used in areas where a wide range of terminal voltage is required

Self excited generators

i) shunt generators :-

i) These are used as exciters for exciting the field of synchronous machines and separately excited D.C generators

ii) These are used for battery charging because it’s terminal voltage are almost constant or can be kept constant.

iii) Commonly used in ordinary lighting purposes and power supply purposes.

ii) series generators:-

i) These are used for series arc lighting

ii) Series incandescent lighting

iii) As a series booster for increasing the voltage across the feeder to compensate the resistance drop of the line. because of their rising characteristic.

iv) Special purposes such as supplying the field current for regenerative breaking of D.C locomotives (railway service).

v) Constant current for welding.

iii) compound generators:-

i) Compound generators are used where constant terminal voltages have to be maintained for different loading conditions.

ii) Cumulatively compound generators:-These are for domestic lighting purposes and to transmit energy over long distance and for heavy power service such as electric railways.

iii) Differential compound generator:- The use of this type of generators is very rare and it is used for special application like arc welding.

Total losses in a D.C Machine

Armature windings

Armature windings

Total losses in a D.C Machine

The total losses in a dc machine are

1.Cu losses

2.Iron losses

3.Mechanical losses

Cupper losses are mainly due to the current passing through the winding. 1.Armature cu losses (30 to 40% of full load losses)

Cu losses 2.Shunt field cu losses(20 to30% of full load losses)

3.Series field cu losses

Armature cu losses=Ia2 Ra

Ra=Armature resistance

Ia= Armature current

--Losses due to brush contact resistance is usually include in

armature cu losses

Shunt field cu losses=Ish2Rsh

Rsh=Shunt field resistance

Ish=Shunt field current

Series field cu losses=Ise2Rse

Rse=Series field resistance Ise=Series field current

Iron losses (Magnetic losses) (20 to 30% of full load losses)

1)Hysteresis losses

2)Eddy current losses

Hysteresis losses (Wh):-The losses is due to the reversal of magnetisation of the armature core

Every portion of the rating core passes under N and S poles alternately. There by attaining S and N polarity respectively. The core undergoes one complete cycle of magnetic reversal after passing under one pair of poles.

P=No. of poles

N= Armature speed in rpm

frequency of magnetic reversals

f=NP

120

The losses depends upon the volume and B max and frequency of reversals.

Hysteresis losses is given by steinmetz formula

Wh=η B1.6maxf V wats

V=Volume of the core in m3

η= Steinmetz hysteresis coefficient

Eddy current losses:-(We) when the armature core rotates, it cuts the magenetic flux hence an

e.m.f induced in in the body of the core according to faradays law of electro magnetic induction. This e. m.f through small sets up large current in the body of the core due to its mall resistance. This current is known as “Eddy Current”

-These core laminations are insulated from each other by a thin coating of varnish. Due to the core body being one continuous solid iron piece (fig a)

The magnitude of eddy current is large. As armature cross sectional area is large it’s resistance is small. hence eddy current losses is large.

In (fig b) The same core has been split up in to thin cross section has very high resistance, hence magnitude of eddy currents is reduced considerably there by reducing eddy current losses.

We=k B2 maxf2t2v2 watts

Bmax=maximum flux densities f=Freequency of the magenetic reversals v=volume of the armaturecore t=Thick ness of lamination

we∞t2 hence t should be kept as small as posible.Eddy current losses is reduced by laminated core but hysteresis losses can

not be reduced by this way.

Mechanical losses ( 10 to 20% of full load losses)

1.Friction losses

2.Windage losses

Friction losses:-

Frictional losses due to bearings

Windage losses:- Windage losses due to air gap between armature and pole shoe

Stray losses(Rotational losses):-

magnetic losses and mechanical losses are collectively known as stray losses

Losses are classified in to two types:-

i) Constant losses (standing losses)(Wc)

--Field cu losses is constant

--for shunt and compound generator are constant losses

so, stray losses+ shunt cu losses are combined called

“constant losses”

ii) Variable losses:-The losses which varies with the load called

“variable losses”

-- Armature cu losses is know as “variable losses”

-- In series generator shunt field cu losses also

variable losses (IL=Ise=Ia)

So, Total losses=Armature copper losses + WC

=Ia2Ra+Wc=(I+Ish)2Ra+Wc

Total losses=Variable losses+ Constant losses

Efficiency of D.C GeneratorEfficiency of generator is defined as the ratio of output power to input power

Efficiency (η) =output ×100

input

input=output+ losses (or) output=input-losses

For D.C generator input mechanical & output electrical

Variation of η with load current