1 Generator Fundementals

8/17/2019 1 Generator Fundementals

1/17

GENERATOR FUNDAMENTALS

INSTALLATION& SERVICE

ENGINEERING DIVISION

NATURE AND ASSOCIATION OFELECTRICITY AND MAGNETISM

Most people a re famil ia r with the general nature ofelectrical energy as transmitted and distributed by

power companies and, perhaps through regular

distribution of elec tri c bills at the sta rt of eachmonth .

Electr ical cur ren t can be thought of as a concertedmotion of electrons in a substance (conductor)caused by the presence of an electric force o r po-tential, generally referred to as a voltage. Elec-tr ic voltage is a kind of electrostatic tension or pressure which exci tes elec trons to motion in a

concerted fashion in many substances.

It is necessary that an electric potential or voltageexist in a substance before current flow is possible;however, current will not necessarily flow even ifan electr ica l voltage does exist. Assuming that anelect rical potential is applied, two conditions arenecessary to establish a current flow. The elec-tric al path must be through an electrical"conductor", and it must be a complete, unbroken,o r closed path which eventually returns to thevoltage source.

Regarding the first condition, depending upon the

Copper is second only to silver as a conductor ofelect rical currents, and, since it is available atreasonable cost, is used almost exclusively asconductor material in the manufacture of electr ica lapparatus.

Certain definite relationships exist between volt-

age, current, and resistance in a given electricalcircuit such that elec trical energy lends itselfreadily to accurate calculation and measurementof it s various part s.

All mas s is basically composed of charged elec-trical particles although the net charge exhibited by one molecule is generally of a random naturerelative to the charge exhibited by another mole-

cule resulting in a general over -all neutralizationo r cancellation.

If the electrical nuclei of a given substance can becaused to align themselves uniformly such thattheir basic electrical charges reinforce each otherand are additive, a strong overall charged condi-tion will be formed, o r the substance is said to bemagnetized. Thus a magnetic force is establishedand the material act s much as a magnetic batterywith a high positive charge at one end compared toa negative charge at the opposite end dependingupon the basic alignment of the electrically chargednuclei. Simila r to electr ica l energy previously


2/17

ega d g t e st co d t o , depe d g upo t e gy p y


There are many materials which are far betterconductors of magnetic flux than air, and,accordingly magnetic flux will s eek t o flowthrough such material , and furt her, will actually

exert mechanical for ce to align the mat eria l inthe shortest and least resistance path possible.Thus, the familiar " pull" of a magnet upon mag-netic materials placed in its flux field as the mag-net attempts to align o r move the materia l to formthe easies t path fo r magnetic flux. Thi s can bevisualized as a bending or distortion of magneticflux lines to pa ss through highly magnetic ma-terials with the f lux lines functioning somewhat as

springs attempting to straight en themselve s toform the shortest straight line by " pulling" themagnetic material into the shortest path.

One of the best ways to form a magnet is to pass astrong direct cu rrent through a suitable material .Electric curr ent c ause s the charged nuclei of asubstance to align o r orient themselves, thus pro-ducing net magnetic cha rge s as in a pe rmanent

magnet. Thi s charge will be found to exist as longas elec tri c current flows and, of course, will be proportional to the strength of the el ectric cu rrent.Permanent magnets may be formed by using highcurrents , special mate ria ls and methods. Referto Figure 1 showing a sketch depicting each mole-cule of a mater ia l as a very small magnet which,when aligned uniformly similar, results in anover all positive magnetic charge being exhibited bythe material.

The preceding broad conceptions are intended toemphasize the ge nera l nature of elec tri cal voltageo r potential, current , and resistan ce; as relatedto simi lar conceptions of magnetic potential o r

charge, "current" o r flux, and " resistance" orreluctance. Of perhaps even more importance isthe interrelation between the familiar electricalentities such as elec tri c c urr ent with magneticeffects due to the e lec tr ica l nature of all matter.Thus, a magnetic field exi sts simultaneously withcur rent flow, which will have a definite directi onand strength depending upon the amount of currentflowing and the nature of the conducting material.

A similar and very important manifestation of theinterr elatin g qualities of elec tri cal and magneticeffects o ccur s when a conductor is moved through amagnetic field of force, or a magnetic field of forcemoves across a conductor i.e,there is relativemotion between an e lectr ic al conductor and magneticflux. When this occu rs, electron s are set in motionin the conductors, an elect rical voltage is generated,

o r induced, and an electrical curr ent wil l flow if acomplete el ectr ical conducting loop o r circu itexists. The voltage induced will have a definitemagnitude and di rect ion which will be dete rmined bythe strength and direc tion of the magnetic field andthe speed and directi on of rel at ive motion betweenthe conductor and the magnetic field; and, a s might be expected, the numb er of conduc tors within themagnetic field which may be connected in series;

o r connected so th at the induced voltage in each addto r esult in much higher and stron ger voltages.

The principle of induced voltages due to relativemotion between el ec tr ic al conductors and magnetic


3/17

GENE RATOR FUNDAMENTALS

RELATIONSHIPS OF ELECTRICALVOLTAGE, CURRENT AND POWER

A direct current is a current produced by a steadynon-oscillating, or uni-directional voltage. In suchan electric circuit the power developed in watts wil l

be equal to the product of the current and the voltagemeasured in amps and volts respectively, or :

w = VI

Where: W = watts, V = volts, and I = amperes

Also, in such dc circuit s the voltage across anyselected portion of the ci rcui t will be equal t o the

product of the current flowing and the resistan ceof that portion of the circuit, or:

V = R I

Where: V = volts, R = ohms of resistance andI = amperes

These two relationships lead mathematically toother equally valid relationships. One of the mostimportant is shown following:

2W = I R

Where: W = watts, I = amperes, and R = ohms ofresistance.

Th is relationship indicates that the power absorbedor developed in any portion of an electr ical circuitwill be in proportion to the resistance of the circuit

d h f h fl i h i

Consequently, all rotating generators are essen-tially alternating current gene rators with theirvoltage produced by the effects of alternating mag-

netic fields and the laws of magnetic induction.The so-called dc generators are, therefore, reallyac generators with provisions for rectifying or con-verting the ac voltages to dc in removing themfrom the generator. This function is performed bythe commutator and its associated voltage gather -ing circuits in dc generators.

Again, due to the lack of continuously varying mag-netic fields, dc current and voltage does not lenditself to many functions desirab le in transmittinglarge blocks of electrical power. For instance,transformers may be used to convert the ac poweroutput of large generators to extremely high voltageswith corresponding reductions in current with rela-tively small losses. Power in thi s form may then be transmitted for long distances with only a f rac-tion of the losses at normal voltage and currentsince the actual current flow is very small. Youwill reca ll that power or heating loss will be proportional t o the square of the current flowing.

Similarly, alternat ing current energy adapts itselfto many other conversions allowing ease in handling power and minimizing losses, all by utilizing the principles of electromagnetic induction.

Thus, alternating current or voltage continuously

varies and undergoes repetitive reversals in posi-tive and negative cycles. The usual frequency atwhich ac power is generated and distributed in thiscountry is 60 cycles per second. A s has previouslybeen stated most modern genera tors are two pole


4/17


In power generation and distribution the mean ef -fective value is the only one of inte rest as this isthe value measured by instruments, used in cal-

culations, etc. All referenc es to ac amps o r voltsautomatically refer to th is mean heating value. Note ac wave form shown on Figu re 3.

The eff many adaptions or uses to facilitate the handlingand use of ac power. Its effects also introducesome slight complications in power measurementand calculation which for many years was a majormystery and impediment to the advancement of theelectrical industry.

CURRENT & VOLTAGE IN PHASE

TIME

CURRENT LAGGING VOLTAGE BY e DEGREES

An amp of ac power is equivalent in most all re-spects to an amp of dc power and the sam e may besaid of an ac volt re latively to a dc volt. However,

the cycle of ac voltage which produces a givencycle of cur ren t may o r may not reach its maximumand minimum values a t the same instant as the cur -rent cycle. If the two waves, current and voltage,did reach their positive and negative peaks atexactly the same instant, or were exactly "in

phase, "the power produced would be equal to thevolts indicated by a voltmeter multiplied by theamps indicated by an ammeter, just as in a dccircuit.

In ac circuits the use of coils, condensors, andother types of devices in the continuously variablemagnetic fields will have the effect of causing slightlags in the buildup of current compared to theapplied voltage in a given cycle due to the effect ofinduced voltages, sometimes re fer red to as"counter voltages" since they will be in a directionto oppose the cu rrent flow causing them.

Similarly, some devices such as capacitors o r con-densors will have the effect of causing the voltageto slightly lag the cur ren t in an ac circui t due tothe tendency of such devices to sto re or "hold " acharge of voltage yet offer no resistance to cu rrentflow. In any case, t he curren t would be said to beout of phase with the voltage and would, therefore,lag or lead the voltage slightly in a given timecycle. In the case of voltage and curren t not in

phase, the product of the voltage and curren t nolonger gives the tru e power o r wattage since theeffective value and current and voltage do not occur


5/17


Note Figure 4 for a visual analogy of phase angleand power factor in relation to the power produced.

It should be noted, however, that the express ionfor power dissipated in heat as shown in (3, for dccircui ts will sti ll be valid in ac cir cui ts since oneampere of ac by definition will produce the sameamount of heating as an ampere of dc. This isrepeated as follows:

2W = I R

Where: W = watts, I = amperes, and R = ohms ofresistance.

Consequently, operators of alternating curren tturbine-generators carefully monitor the powerfactor at which ac power is supplied. They are paid by the kw-hour and many of their cir cu itlos ses will be in proportion to the cur ren t flowsquared. Since power output is low for a given

curre nt flow, and losse s high at low power factorsth is type of operation is avoided and may be adjusted

0

to some extent by controlling the excitation cir cui tsof the generator.

Also of importance is the fact that low power factoroperation tends to produce severely distorted mag-netic fields in the generator. This is part icularlytr ue in the case of leading power factors which tendto cause overheating in some pa rt s of the generatorwindings, or a loss in "synchronizing" power ofthe generator; i.e. , it may tend to drop out ofsynchronism with the system if sudden loads are

imposed.

Alternating current generators are always manu-factured and their ratings, guarantees, e tc.established based upon lagging power factor opera-tion over a relatively small range from unity powerfactor of 1.0 to 0.8 or 0.85 lagging power factor.These values are consistant with the load charac-te ri sti cs of most user s and offer no particular di f -

ficulties or limitations to operators in supplyingdistribution systems except in unusual ca ses.

0

90' 180o 270

o 360

o


6/17


MAJOR GENERATOR PARTS ANDTHEIR FUNCTION

A. Refer to Figure 5 for reference in consideringthe major generator par ts.

B. These a re defined further as follows:

1. The generator fra me provides the st ructuralstrength and rigidity for the generator andserves a s a housing to guide cooling air or

gas flow.

2. The inner air shield is a baffle used to forma path for cooled air or gas.

3. The generator fan, mounted on the rotatingfield causes continuous circulation of coolingair or gas.

4. The rotating field fo rms a strong polarized,rotating magnetic field when energized by anexternal source of dc power.

5. The stator core carries the stationary highvoltage windings and forms a magnetic patchfor magnetic fields.

6. The a ir gap is the rad ial cl earance betweenthe rotating field and the stator core.

7. Stator core spring bars act as somewhatflexible support f or the st ato r core assembly.

8. Stator coil end tu rn s are formed when coilsleave one slot in the stator core and are re-turned to a different slot.

9. The end turn support s tructu re provides for bracing and t ies to secure the stator coi l endturns against magnetic forces.

10. The high voltage terminal leads serve to con-duct the three phase voltage and current flowfrom the generator stator to the externalsystem.

11. Collector rings are used to provide a con-nection and path for dc power into the ro-tating field windings.

12. The outboard end stub shaft is sometimesused to drive a small dc generator used tosupply dc power to the rotating field.

13. Field conductor end turns are securely blocked and serve as connection points for thedc power applied to the field windings.

14. The main coupling is bolted to the drivingturbine shaft.

15. Generator coolers serve to remove heat fromthe generator cooling ai r or gas after it has passed over or through the stator and rotating

field.

16. Cooling water connections are supplied to thegenerator a ir coolers.


7/17


FUNDAMENTALS OF GENERATORS

Alternating current generators, as has been indi-

cated, are designed to utilize the principles ofelectromagnetic induction to generate electr ica lenergy. From the previous discussions throughoutthis co urse it will be clear that alternating curren tgenera tors may be, and frequently are, built inlarge siz es and ratings, and generally operate athigh rotating speeds . Furthe r, such genera torsoperate with very high efficiency in the neighbor -hood of 98 to 99 percent.

Also, it will have been seen that a lternating currentgenera tors utilize definite, carefully designed mag-netic paths o r ci rcuits a s well as the electric cir -cuits which would normally be expected. In con-side ring the operation of generators, the path andfunction of magnetic fields involved should bevisualized as closely as possible . Magnetic pathslink both the rotating field windings and the statorcoils and therefore, must cr os s the air gap. It

should be kept in mind that leakage flux from thedes ired magnetic path tends to reduce efficiencyand may cause heating of some generator pa rts suchas structura l sections. Abnormal magnetic fluxleakage or abnormal f lux pa tt erns may also becaused by unusual operating conditions on thegenerator.

Considering the importance and key function of mag-

netic circuits, it would be well to carefully studythe construction and intended operation of the gen-erator co re where the intensified magnetic field is

purposely confined as much as possible to reducegenerator los ses and to eliminate stray o r ex

sulated from adjacent sheets to allow unrestrictedflow of magnetic flux while stopping the r ight angleflow of Eddy currents.

Two elect ric cir cuits are involved in alternatingcurrent generators. The firs t is the externallysupplied dc circuit through the-rotating field coils .After the field has reached running speed the ro-tating field circui t is gradually energized or, "fieldexcitation" is establ ished which effectively convertsthe generator field into a huge and powerful electro-magnet. It will, therefore, have a north and southmagnetic pole and a st rong flow of magnetic fluxwill exist from field poles, across the air gap, andthrough the generator core; thereby linking orcrossing the stator windings. Note Figure 6.

Since the field rotates at 3600 rpm (usually), a ro-tating magnetic field traverse s the stator windingsand cor e. Each stator coil will have an alternatingvoltage induced each rotation of the generator fieldwhich wil l be essentially as shown on Figure 4,

with respect to time and to positive and negativehalf -cycles.

Generator stator windings wil l be imbedded in slotsin the s tat or co re, usually two coi ls or winding barsto the slot. Each bar or winding must be suitablyinsulated for the rather high terminal voltages ofthe stator winding. All bars will be symmetricallyconnected to form three phase belts in the genera-

to r windings. Thus, three windings are formedwhich are symmetrically displaced from each other by 120 F and which have symmetrically similar


8/17

GENE RATOR FUNDAMENTALS

terminal voltages due to elec tr ical connections of bars in each phase belt.

For each revolution of the energized rotating field,

there will, therefore, be three symmetrical ter -minal voltages induced in a three phase sequencewhich may be suitably connected to furnish e lec tri-cal energy to an external system.

Improved magnetic materials, electrical-insulatingmaterials, and cooling methods have allowed theratings of single alternating current generators to

approach 500,000 kw. Generator termin al voltageshave also tended to increase and may be around20,000 volts for some utility turb ine-generators.Most industrial power generation is done at 2400 to13, 800 volts.

Since the alternating curre nt generator suppliesthree phase power into existing systems, it isnecessary to carefully "match" electr ica l conditions

of the generator to the system before closing thegenerator output bre ake r to electrically connect thetwo. The terminal voltages of the generator shouldexactly match the voltage of the system at everyinstant to avoid large inrushes or exchanges ofelect rical power, severe distortion of generatormagnetic fields, and high instantaneous torques ongenerator and turbine shafts. Accordingly, theturbine governor is adjusted until the genera torelectrical frequency, and the generator field exci-tation is varied until genera tor termina l voltageexactly matches the system voltage.

Finally an instrument known as a synchroscope is

erato r in electrical "step" synchronism through themedium of the genera tor magnetic circu its.

The actual force or torque nece ssary to cause thegenerator to depart from electrical synchronismwill depend upon the electro-magnetic character -ist ics built into the generator circuits, and,the operating conditions imposed upon the generator.Capability curves are furnished by manufacturersto define abnormal opera ting conditions. For in-stance, the abnormal condition of generatorsfurnishing substantial power at high "leading" powerfactors (current leading voltage with respect to

phase relationship) causes a weakening of the gen-erator magnetic fields and results in reductions insynchronizing power or stability, and abnormalmagnetic field patterns.

If the turbine governor se tting is now increased, amomentary speed or frequency increase occu rswhich causes the phase relationship of the generatorvoltages to advance with respect to the phase re -

lationship of the system and cu rren t will flow intothe system; or power may be delivered to t he sys-tem . Thus, the generator may be "loaded " or"unloaded'' by adjusting the turbine governor.

The generator, while operating at a fixed govenorsetting, will also share with other connected gen-er at or s in supplying any additional elect rical load

placed on the system up to the limit of the turbinegenerator s et s capacity.

Finally, when the generator begins to deliver cur -rent, new magnetic fields are established due to thelarge cur ren ts flowing in the stato r or armature


9/17


1. ROTATING FIELD2. STATOR CORE3. ROTATION4. MAGNETIC FLUX PATTERN DUE TO FIELD CURRENT5. MAGNETIC FLUX PATTERN DUE TO ARMATURE CURRENT

Figure 7. Relationship of Magnetic Fields Due toField Current and Armature Current

(a) Lagging power factor operation causes highfield currents and may cause heating of thegenera tor field to be the limiting factor in highload operation

'A

I

I

SUPPLYINGCURRENTINPHASE

IR

SUPPLYINGLAGGING

- - CURRENT

FR

Eo

SUPPLYING

CURRENT

R

V

I


10/17


EXCITERFIELD-

GENERATOR,FIELD BREAKER

GEN ERATOR

ARM GEN.

IPOT ENTI AL

COLLECTOR RING TRANSFORMERI

I

I

VOLTAGEREGULATOR

Figure 9. Basic Excitation System

A s an aid in understanding typical operating limi tsfor generators, Figure 10 is shown which gives therecommended operating limi ts in terms of kva out-

put, field amps, and power factor of the generator.These curves ar e called "V" curves due to theirshape. Also shown on Figure 10 is a typical gen-era tor capability curve defining recommendedoperating conditions on the generator in terms of

kw output, kvar output, and power factor .

OPERATING CHARACTERISTICS ANDLIMlTATIONS OF GENERATORS

GENE RAT0R "V" CURVES

LIMITED BY ARMATURE

HEATING

140

FIELD AMPERES


11/17


gas cooling, indust rial applications have used airor hydrogen cooling exclusively and generally findhydrogen cooling justified in modern units with

ratings over 15000 kw.

Elect rical insulation of generator windings limitsthe effectiveness of the cooling medium because ittends to also serve as the rmal insulation. Genera-tor winding temperatu res are generally measuredat two or mor e locations in each phase belt by in-stalling thermocouples or other temperature mea-suring devices between top and bottom coi ls in agiven sta tor s lot . These devices do not measureactual copper temperatu re but will always indicatea significantly lower temperature due to the tem-

perature drop across winding insulation. Th isreading will be 15 C to 20 C lower (27 F to 36 F)than copper temperature fo r normal operation inmany generators.

It is normally recommended that the generator

coolers be operated by throttling discharge waterflow from the cooler to resul t in maintaining cooledair from the coo ler s at approximately 40 C (104 F).This will cause cooler water pre ssur e to be higherthan atmospheric pressu res f or all operation andavoid the possibility of air locking coole rs to pre-vent cooling water flow, as well as insure thecooler is completely filled on the water side. Ventsar e usually provided near the very top of generator

coolers to allow visual indication of vent flow toverify that c oolers ar e properly filled.

Generator field winding temperatu res are evendiffi l j d d hi h i d

and longer fields. When excessive growth of wind -ings occurs afte r the generator reac hes ratedspeed, scuffing of field coi l insulation could occur ,

or mechanical stresses could be developed inwindings restr icted fro m fre e expansion.

Similarly, generator forging tempera tures will beallowed to increase substantially before high cen-trifugal forces at normal operating speeds, orduring overspeed tria ls, are experienced. Gener -ator field forgings will exhibit better propertieswith respect to ductility at temperatures higherthan ambient, particularly in the ca se of lowerthan normal ambient temperatures,

The generator field, functionally an electrical de -vice, is nevertheless a marvelous combination ofelectrical and mechanical design to achieve suit-ability for high rotating speeds. For instance, asingle field coil fo r a 10,000 kw generato r weighs300 pounds at rest, but exer ts a force of over1,000,000 pounds when operating at 3600 rpm . The

mechanical rest raint necessa ry and the qualities ofthe field forging must be of the highest order pos-sible to result in safe operation and perform itselectr ica l function reliably fo r the life of the tur -

bine-generator. I f a generator field coil is allowedto shift or move at all, the effect upon the field

balance will be immediate and high vibrations mayresult.

Similarly, the effect of differential temperaturesfr om one side of the field to the opposite side, evenas small as one or two degrees centrigrade maycause a bow in the generator field sufficient to un-b l h fi ld h l d d Si i h


12/17


essentially cylindrical sleeves installed with veryhigh shrink fits to centering rings on the generatorshaft which serv e to secure end turns and blocking

while positioning this assembly in a permanent,symmetrical, balanced assembly.

Generator field retaining rings, simi la r to fieldforgings, are given the utmost in car e and testingthroughout their manufacture. Retaining rings are

perhaps the key struc tural pa rt of generator fieldsand are the most highly s tressed in operation. Sim-ilar to large turbine wheels, a scratch on a retain-

ing ring surface would be a matter of concern. Theutmost in mechanical strength is designed for andno sharp breaks in contour or surface conditionare allowed due to possibilities of introducingst re ss concentrations.

Terminal connections of generator field windingsare brought through the shaft bore to connect elec-trically to collector rings. Terminal leads are

carefully insulated from the shaft or forging as arethe collector rings. The collector r ings thus serv eas input. terminals fo r the dc power providing ex-citation for the generator field.

Excitation is introduced into the collector rings,which a re essentially accurately machined andhighly polished sleeves, by means of carbon

brushe s which are fitted to ride the su rface of the

rings accurately.

Generator fan rings ar e installed to the generatorshaft t o provide a means of circulating cooling airor gas The power absorbed by the fans along

excessive wear, vibration, o r contamination maycause serious trouble such as heavy sparking,flashover, o r loss of generator field excitation.

Stator windings, simila r to field coils, are subjectto appreciable growth due to differential expansion

between the copper and the stator c ore. Coil in-sulation is of the highest order to i solate the highalternating current voltages in their desired paths.Stator windings are not subject to centrifugalstresses as are field coils and slot wedges andfiller material will not be as substantial as forfield coils.

Stator windings are subject, however, to magneticforces which will be somewhat dependent upon thecurrent density in the windings. Thi s could causestator bar vibration and damage to insulation ifstator ba r wedges became excessively loose.

Generator stator windings cannot generally be

monitored or their condition judged during normaloperation. Thei r good condition can best beassured by ca reful observance of good operatinghabits with respect to the generator. Normal tem-

pe ratures can be maintained by proper operation ofthe generator cooling system and by reference totemperature indications at the stator windings andin the cooling air path.

Also, operation of generators outside recommendedlimits furnished by the manufacturer involves cer -tain r is ks and the possibility of decreasing life ofgenerato r windings. Manufacturers normallyfurnish generator capability curves which define


13/17


14/17

GE Industr ial & Power Systems

12 ”

Figure2 AC Generator Three Phase

As our two pole rotor turns the field will induce volt-age in one set of coils after the other. Thus, in one

revolution the rotor will have been active in threewindings 120” apart. That is, there will be threeidentical generating episodes, with one episode ineach set of armature windings.

Since the coils are 120”apart and the field passes one

after the other, the voltages in the coils will be timed120” apart and we have three phase voltages 120”apart as shown in Figure 3.

As mentioned above there are three distinct arma-

ture coils. When isolated they would be representedas shown in Figure 4 and 5 .I 1

B

Wye (left) and Delta (right) Orientationof Coils

I

I A I

Figure4

IE

Figure5 Delta and Wye Connections

It can be seen that the power for the three isolatedcoil generator will be the sum of the power in eachcoil. That is,


15/17


16/17


17/17

Documents

1 Generator Fundementals