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Alternators

• Are the primary source of allelectrical energy

• Commonly used to convertthe mechanical power outputof steam turbines, gasturbines, reciprocatingengines, hydro turbines intoelectrical power for the grid

• Are known as synchronous

machines because theyoperate at synchronous speed(speed of rotor alwaysmatches supply frequency)



Alternator Overview

• The alternator contains:

• A rotating field winding calledthe rotor.

• A stationary induction windingcalled the stator.

• An excitation system for thefield winding.

• A control device called thevoltage regulator.

• Two internal fans to promote aircirculation.





Advantages of Stationary Armature

Better insulation to the armatureconductors.

Number of slip rings are reduced

Improved ventilation arrangement

Rotor construction becomessimpler which makes the machine

to attain higher speeds.



Stator WindingsStator Windings

• The laminated stator core iscut into slots on its innerperiphery.

• The stator is made with three

sets of windings.

• Each winding is displaced byan angle of 120 degrees..

• Stator leads are brought out

and connected to the load

Laminated Iron

Frame

Stator Lead Ends Three Windings

Neutral Junction in the Wye design can

be identified by the 6 strands of wire



Rotor

Two types of rotorsare

used in alternators

(i) Salient-pole rotor

(ii) smooth-cylindrical

type rotor



Salient –Pole Rotors

Used for low speedapplications and are calledhydro generators

The poles are salient(projecting) and are laminated

Field winding is fed from a lowvoltage dc supply

Flux distribution is not uniform



Smooth –Cylinderical Type Rotor

Used for high-speed applications

(steam/ gas turbines) and are calledturbo alternators

The laminated rotor core is cut into

slots on its outer periphery toaccommodate the field coils

Field winding is connected to lowvoltage dc supply through 2 slip rings

High speed of rotation produces strong

centrifugal forces, which impose upperlimit on the rotor diameter.



Field Excitation and Exciters

DC field excitation is an important part of the overall design of asynchronous generator

For both stationary and revolving fields, DC supply is normallyproduced by DC generator mounted on same shaft as rotor.

Permanent magnets can also produce DC field – used increasinglyin smaller machines as magnets get cheaper

Main and pilot exciters are used in common

Brushless excitation systems employ power electronics(rectifiers)to avoid brushes / slip ring assemblies.



Synchronous Generatorcross section view of a 500MW synchronous generator and the

excitation systems



Principle of Operation

• As the rotor assembly rotateswithin the stator winding.

• The alternating magnetic fieldfrom the spinning rotor inducesan alternating voltage into the

stator winding.

• Both the strength of themagnetic field and the speed of the rotor affect the amount of voltage induced into the stator.



–Stator of a 3-phase, 500 MVA, 0.95 p.f, 15 kV, 60Hz, 200r/min

generator



Synchronous Generator –Rotor36 pole rotor is being lowered in to the stator as shown below.

2400v dc exciting current is supplied by a 330V electronic rectifier

S h G t R t



Synchronous Generator –RotorRotor with its 4 pole dc winding. The dc exciting current of

11.2kA is supplied by a 600V dc brushless exciter bolted to theend of the main shaft.



Short pitch winding & Pitch Factor/ChordingFactor

Short pitched coils are deliberately used because of the following

advantages:

• They save copper of end connections

• They improve the waveform of the generated e.m.f

• Reduces Eddy current & hysteresis losses

But the total voltage around the coil is some what reduced, So

Pitch Factor/Chording Factor, kc=

= cos(α/2), Where α is knownas chording angle.

Vector sum of the induced e.m.fs. per coil

Arithmetic sum of the induced e.m.fs. per coil



Distribution or Breadth Factor/ SpreadFactor

Comes into picture when the coils in each phase are distributedinstead of concentrated in one slot

e.m.fs. induced in coil sides of a polar group differ by an angleequal to the angular displacement of the slots

Distribution factor ,kd

=

kd =

Vector sum of coil e.m.fs.

Arithmetic sum of coil e.m.fs.

2sin

2

β

β

m

mSin



Number of Poles

The number of poles on a synchronous generator depends upon the speed of rotation and desired frequency

Where f = frequency of the induced voltage (Hz)

p = number of poles on the rotor

n = speed of the rotor (rpm)



Induced E.M.F

R.M.S value of e.m.f per phase = 4.44fØT Volt

Actually available voltage/phase = 4.44kdkcfØT Volt

Where,

f = frequency of the induced voltage (Hz)

p = number of poles on the rotor

n = speed of the rotor (rpm)Ø = flux/pole in webers

T = No. of turns /coils per phase (Z=2T)

kd = distribution factor

kc = pitch factor



Alternator on load

Terminal voltage of the alternator varies with the load due to thefollowing reasons:

Voltage drop due to armature resistane Ra

Voltage drop due to armature leakage reactance XL

Voltage drop due to armature reaction



A armature leakage reactance XL

When current flows through thearmature conductors , fluxes areset up which do not cross the airgap, but take different paths. Suchfluxes are known as leakage flux

Leakage flux depends on currentand phase angle with terminalvoltage & sets up reactance e.m.f

Hence armature winding is

assumed to posses leakagereactance XL )( La jX R I V E ++=



Armature Reaction

• Effect of armature flux on the

main field flux

• Power factor of the load has aconsiderable effect on thearmature reaction

• The drop in voltage due toarmature reaction is accountedby means of a fictitious reactanceXa in the armature winding

• Synchronous reactance Xs = XL +Xa

• Synchronous impedance Zs =Ra+j

Xs



Armature Reaction

• Unity P.F

Armature flux is cross magnetizing,armature reaction is distortional

• Z.P.F Lagging

Armature reaction is whollydemagnetizing, lesser induced e.m.f

• Z.P.F Leading

Armature Reaction is whollymagnetizing, greater induced e.m.f

• 0.7 P.F LagFor intermediate p.f , the effect ispartly distortional & partlydemagnetizing

Armature Flux

Unity P.F

Main Flux

Zero P.F

Lagging

0.7 P.F

Lag

Zero P.FLeading



Voltage Regulation

• It is defined as the change interminal voltage, expresses aspercentage of the rated voltage ,when the load at a given powerfactor is removed, with speed andfield current remaining

unchanged. Therefore,

• Lagging power factor load

E0 always increases & voltageregulation positive

• Leading power factor load

E0 decreases & voltage regulation

negative

( )100Re

0×

−=

V

V E gulationVoltage



ψ δ Ø

I d

I a

I qO E o

I q X q

I d X d

I q R

q

V



Armature Reaction

ArmatureFlux

Unity

P.F

Main Flux

ZeroP.F

Lagging

0.7 P.F

Lag

ZeroP.F

Leading

a)

b)

c)

d)



Synchronous Reactance



Parallel Operation of Alternators



Synchronous Reactance



Voltage Regulation

P .F . U n i t y P .F

. L a g g i n g

Load Current

TerminalVolts

P.F. Leading



Temp

N S

Ø

I X L

I R a

V

E b

0

I

ØI X L

`

I R a

V

E

0

I

E 0 I Z S

I X a

I X s

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