SYNCHRONOUS GENERATORS.pptx

8/8/2019 SYNCHRONOUS GENERATORS.pptx

http://slidepdf.com/reader/full/synchronous-generatorspptx 1/17

SYNCHRONOUS GENERATORS

(ALTERNATORS)



Synchronous machines are AC machines that have a feld

circuit supplied by an external DC source In a synchronous generator, a DC current is applied to the

rotor winding producing a rotor magnetic feld. The rotor isthen turned by external means producing a rotatingmagnetic feld, which induces a !phase voltage within the

stator winding. In a synchronous motor, a !phase set o" stator currents

produces a rotating magnetic feld causing the rotormagnetic feld to align with it. The rotor magnetic feld isproduced by a DC current applied to the rotor winding.

#ield windings are the windings producing the mainmagnetic feld $rotor windings "or synchronous machines%&armature windings are the windings where the mainvoltage is induced $stator windings "or synchronousmachines%.

Construction of synchronous machines



'ased on the construction o" the machines, it maybe classifed as

(. )otating Armature type *. )otating #ield type

(. )otating Armature type +oos very much lie a dc generator except that there

are slip!rings in place o" commutator )e-uired magnetic felds are produced by dc

electromagnet placed on the stationary member calledstator

The current and generated is collected by means o"brushes and slip!rings on revolving member, called therotor

conomical "or the small low voltage generator Suitable "or small power plants




*.)otating #ield type

/ractically all medium and large machines are alwaysrevolving feld type with these advantages0

asier to insulate stationary armature winding "or very highvoltages

+oad circuit can be connected directly with the fxed

terminals o" stator without passing through slip!rings andbrushes

1nly two slip!rings are re-uired "or the supply o" dc to therotor "or excitation

The exciting current is supplied at low voltages o" (*2 or

*23 v, there is no di4culty in insulating them. Slip rings and brushes have certain disadvantages0

increased "riction and wear $there"ore, neededmaintenance%, brush voltage drop can introduce signifcantpower losses. Still this approach is used in most small

synchronous machines.




Synchronous machine consists essentially o" two parts

(.Armature $or stator% *.#ield magnet system or )otor

(.Armature $or Stator% Iron ring, "ormed o" laminations o" special magnetic iron

or steel alloy $silicon steel% having slots on its periphery

to accommodate armature conductors. Stator core is laminated $thicness 3.2mm or less% and

insulated "rom each other with paper or varnish 1pen slots are more commonly used because the coils

can be "orm!wound and insulated properly also removal

and replacement o" de"ective coils 1pen type o" slots produce ripples in the em" wave

because distribution o" air gap 5ux is into branches Semi!closed type slots are better in this respect but do

not permit the use o" "orm!wound coils




*. #ield magnet system or )otor

$i% Salient pole type

$ii% Smooth cylindrical or non salient poletype


6on!salient!pole rotor0 usuallytwo! and "our!pole rotors.

Salient!pole rotor0 "our andmore poles.

)otors are made laminated to reduce eddy current losses.



$i% Salient pole type 7sed almost entirely "or slow and moderate speed

alternators +east expensive and cheaper than cylindrical rotor

machines when speed does not exceed (333 rpm.

It can cause an excessive windage loss and produce noise i"driven at high speed 7sually made up o" cast!iron or cast!steel and has a heavy

central hub /ole shoes are laminated to avoid eddy current

The air gap is minimum under the pole centers and it ismax. in between the poles

The radial air gap length increases "rom the pole center tothe pole tips so that the 5ux distribution in the air gap issinusoidal




Damper windings are used to damp rotor oscillations during

transient conditions and "acilitate under unbalanced loadconditions +arge numbers o" poles $8 to 93% :ave a large diameter and small length

$ii% Smooth cylindrical or non salient pole type

7sed in very speed alternators Sub;ected to high mechanical stresses due to high speed

and long round rotor 6ormally two!thirds o" the rotor is slotted "or the feld

winding and one!third is le"t without slots

it has "ollowing special "eatures0 They are o" smaller diameter and o" very long axial length )obust construction and noiseless operation +ess windage $ air resistance% loss




'etter in dynamic balancing

:igh operating speed $333 rpm% <ive better em" wave"orm than salient pole feld structure 6o need o" providing damper windings


A synchronous rotor with = salient

poles

Salient pole with feldwindings




Two common approaches are used to supply a DC current to the

feld circuits on the rotating rotor0(. Supply the DC power "rom anexternal DC source to the rotor bymeans o" slip rings and brushes&

*. Supply the DC power "rom a special

DC power source mounted directlyon the sha"t o" the machine.

Slip rings are metal rings completely encircling the sha"t o" a machine butinsulated "rom it. 1ne end o" a DC rotor winding is connected to each o"the two slip rings on the machine>s sha"t. <raphite!lie carbon brushesconnected to DC terminals ride on each slip ring supplying DC voltage tofeld windings regardless the position or speed o" the rotor.




'rush




Slip rings and brushes have certain disadvantages0 increased

"riction and wear $there"ore, needed maintenance%, brush voltagedrop can introduce signifcant power losses. Still this approach isused in most small synchronous machines.

1n large generators and motors, brushless exciters are used. A brushless exciter is a small AC generator whose feld circuits are

mounted on the stator and armature circuits are mounted on therotor sha"t. The exciter generator>s !phase output is rectifed toDC by a !phase rectifer $mounted on the sha"t% and "ed into themain DC feld circuit. It is possible to ad;ust the feld current on themain machine by controlling the small DC feld current o" theexciter generator $located on the stator%.

Since no mechanical contact occurs between the rotor and thestator, exciters o" this type re-uire much less maintenance.




A brushless exciter0 alow !phase current isrectifed and used tosupply the feld circuit

o" the exciter $locatedon the stator%. Theoutput o" the exciter>sarmature circuit $onthe rotor% is rectifedand used as the feld

current o" the mainmachine.




To mae the

excitation o" agenerator completelyindependent o" anyexternal powersource, a small pilotexciter is o"ten added

to the circuit. Thepilot exciter is an ACgenerator with apermanent magnetmounted on the rotorsha"t and a !phase

winding on the statorproducing the power"or the feld circuit o"the exciter.




A rotor o" largesynchronousmachine with abrushless excitermounted on thesame sha"t.

?any synchronousgenerators havingbrushless excitersalso include slip

rings and brushes toprovide emergencysource o" the feldDC current.




A largesynchronousmachinewith theexciterand salientpoles.



Thank you

SYNCHRONOUS GENERATORS.pptx

Documents

Transcript of SYNCHRONOUS GENERATORS.pptx