A 4O-Kva 4OO-Cycle Aircraft Alternator

H. E. KENEIPPASSOCIATE AlEE

Synopsis: A-c auxiliary-power systems foraircraft date back to World War I. Re­cently numerous technical articles have indi­cated an intense revival of interest in thissubject. Heart of any electric system is thegeneratdr which supplies electric power toall parts of the system. An alternator,especially designed and built for 208-voltthree-phase systems is described in thispaper. Light weight and reliability domi­nate the design,whichinvolved a large num­ber of new and unusual problems.

ALTERNAT ING current has re­placed direct current in large com­

mercial power systems because largeamounts of power can be transmitted longdistances more economically; alternatingvoltages can be stepped up or down atwill by a transformer; and the polyphasesquirrel-cage induction motor, which em­ploys no brushes, is so simple and reli­able. These advantages, which also ap­ply in aircraft use, have caused a long­continued interest in a-c systems. r.eWhile this interest is particularly strongnow, it is to be remembered that the useof a-c systems in aircraft dates back toWorld War I when wind-driven alternatorswith built-in spark gap were used to sup­ply power for radio transmitters. Morethan ten years ago 6OO-watt alternatorsdriven by a main engine through a con­stant-speed drive, were used by the Navy.Prior to the present war two different a-csystems were tried experimentally indifferent planes: one of these was asingle-phase 800-cycle system, the other,a three-phase 120-volt 400-cycle system.Rectified a-c systems with 30-volt docoutputs of 200-800 amperes have alsobeen proposed. J

Vernon Grant and Melville Petersshowed in 1939 that the optimum volt­age for the electric system of airplanesof 20,000 pounds gross weight or largerexceeds the present universal standard of28 volts. to Because higher system volt­ages are desirable for large airplanes, al­ternating current appears preferable to

Paper 44-190. recommended by the AlEE com­mittee on air transportation for presentation at theA1EB Los Angele' technical meeting, Los Angele,.Calif., Augu,t 29-September I, 1944. Manuscriptsubmitted Juue 22, 19·14; made available for print­ing July 11, 1944 .

H. E . KaNBIPP and C . G . V"'NOTT are both in theupneerlng depart_nt of Westinghou.. Electricand Manuf""turing Compauy. Lima. Ohio, whereMr. ·Veinott is special development engineer. smallmotor enginttring department .

The authon acknowledge the assistance of J. C .Cwmiagbam. L. A. K ilgore , and E. C. Whitney, ofthe East Pittsburgh Work.s, and J . D . Miner of theLima W...ks, of Westinghouse Electric and Manu­f~nringCompany.

816 T~SACTlONS

c.e. VEINOnMEMBER AlEE

direct current for high-altitude operation.Alternating current is easier to interrupt ;moreover, the motors have no brushesand are generally lighter and more re­liable. A-c power can supply 85 percent of the total load and, in addition, ithas many advantages as a power supplyfor aircraft radio,"

Weight, of course, is always a primaryconsideration in aircraft equipment. Ata meeting of the AlEE Dayton Sectionin May 1944 an interesting tabulation ofcomparative weights of different electricsystems was presented. The weight com­parison was made for a hypotheticalbomber of assumed dimensions, and anassumed gross weight of 150,000 pounds.This comparison is given in Table 1.

Viewed in any light, the advantages ofalternating current are so great that thedevelopment of the alternator describedin this paper was initiated.

Alternator Rating and Requirements

Alternators of any desired rating canbe built for aircraft service. This paperhowever, is limited specifically to thedescription of an alternator having acontinuous rating of 40 kva, 208 volts,three-phase, 400 cycles, i5 per cen! powerfactor. Overload ratings for this alter­nator are as follows :

60 kva at 100 per cent of rated voltage forfive minutes.80 kva at 90 per cent of rated voltage forfive seconds.

From a heating standpoint, 150 percent load for five minutes is a more severerequirement than full load continuously,but overload capacity, of course. is neces­san' to meet emergency conditions. The80-kva rating was set up as a requirementfor two reasons: to provide capacityfor starting a number of motors simul­taneously, and to ensure greater stability

Figure 1. Front view of 4O-Icva alternator,howing air connector and mounting bend

Keneipp, Veinott-Aircraft Alternator

when carrying a load of 60 kva. Theseload ratings are based upon 250 cubic feetper minute of cooling air being suppliedfrom a blast tube at not less than sixinches water total pressure. An internalfan has been built into this alternator toprovide cooling sufficient for a smallamount of output-about 25 per cent ofrated load-while operating on the groundwithout the benefit of blast cooling . Thealternator is designed to supply a three ­phase system with grounded neutral.Six leads are brought out to permit theuse of differential protection, which dis­connects the alternator from the systemin the event of a short circuit, either at itsterminals or inside the alternator.

Leads are tagged according to AmericanStandards. TI, T2 , T3 are line leads,whereas T4, T5, T6 are neutral leads.Alternator and exciter connections areshown in Figure 10. The terminal blockis stepped to facilitate wiring connectionsand to avoid interference of leads . With­out the stepped arrangement, the leadseasily could be short-circuited at theblock. A cover is provided for the ter­minal block to protect the leads and ter­minals.

Damper or amortisseur windings areincorporated to provide stability whensynchronizing and when operating inparallel with other machines.

Careful consideration was given toselection of the operating speed, and6,000 rpm finally was selected as thehighest speed feasible for a machine of40 kva, But the machine is designed towithstand an overspeed requirement ofsuccessful mechanical operation at 9,000rpm, to correspond with the maximumspeed of the drive shaft.

Alternator Size and MountingArrangement

All important outline dimensions ofthis 40-kva alternator are shown in Figure3. It will be noted that the length is \ginches and the diameter of the bodv isnine inches. The 19-inch dimension' in­cludes the air connector shown on theright, but does not include the shaftextension and pilot fit on the left . These

Figure 2. .Re. r view of 4O-kva aircraft Illcr'nltor ,howing br.cket .nd shaft exten,ion

ELECTRICAL ENGlNEERI:\G

Figure 3. Outline dimensions, 4O-Icva alternator

Sr-tem

Components

Airplane is a long-range bomber, weighing 150,(XX)pounds; continuous electric load of 60 kw. WiDCspread, 170 feet ; length of fuselage, 110 feet; 45feet to outboard engine ; 28 feet to inboard engine.Generator specification :

30 kw continuous, 45 kw for five minutesSpeed range, 3.000-9,000 rpmNot engine-mountedBlast-cooledAltitude, 0 to 50 ,000 feet

Th is table ..as developed at a rouad-table confer­ence conducted by Major W . A. Barden, UnitedStates Army Air Forces. equipment laboratOl'Y.Wright Field. Members of conference were:George W . Sherman, Wright Field; L . G . Levoy,R . H. Kaufmann, General Electric Company; 1. C.Cunningbam, D. E . Fritz , C . G . Veinott, Westing­bouse Electric and Manufacturing Company.

1. Generator weight,one unit 150 tb .. 1201b.. 80 Ib

2. Voltage regulator,one unit .. . . .. .. .. 61b.. 61b.. 91b

3. Main generatorbreaker, orswitch.. .. .. .. . .. 61b .. 61b .. SIb

4. Drive shaft fromengine 151b.. 151b.. ISlb

5. Constant - speeddrive ... . . .. .. . ... . . ... .. OOlb

6. Total, .items I to 5 . . . 177Ib.. 147 lb.. 199 Ib7. Total ..eight of

four units 7081b .. 588lb .. 796lb8. Batteries, total

..eight.. • .. .. .... 55 tb .. 55 lb . . 55 Ib9 . T ..o 28-volt 6·k..

d-e sources of. po..er .. .. .. . .. .. . Olb .. J251b .. JOOlb

10. T ..o 12-kvasources of single­phase 400..,yclepo..er 300lb.. 280lb.. Olb

II . Four sets of auto-~~ic synchro-IllZlng control. .. .. . . .. . • .. . . • .. • .. 40lb

12. Weight of mainpo..er wirin/t ... .. . 420 Ib.. 551b.. 6SIb

13. Total ..eight ex-cluding utiliza- .tion equipment.. . . 1483 lb . . 1I03lb.. l056Ib

Figure 6. A-e stator coils

Table I. Comparative Weighb of ElectricSystems

ductor alternator is heavier than a docmachine and requires the use of a recti­fier, which would need to be mounted inor on the alternator in order to makethe latter interchangeable with anotherunit which might use a doc exciter. Suchan arrangement would be awkward and'heavy.

Excitation might have been furnishedfrom batteries or from the d-e power

SHAFT SPUN[ DATA24 TEETH-

~?~ mg: DIA.:SO' PA[SSUA[ MGLE

Figure 5. Magnesium-llJloy frame of alter­nator showing internal ribs

inherently means loss of excitation in theevent of a fault, and the alternator willnot provide enough current to burn clearmany types of faults which might Occur.This arrangement had the further disad­vantages of added weight, and also ofappreciable warm-up time for the elec­tronic tubes. Use of an inductor alter­nator to supply excitation power was con­sidered because of the advantage of elimi­nating brushes. However, an in-

Figure 4. Wound-stator before impregnation

Excitation is furnished by an integrald-e exciter of conventional aircraft­generator construction, an arrangementchosen after a careful study of all possiblemethods of excitation. This arrange­ment is lightest in weight, is less compli­cated, requires the smallest regulator, andismore stable than any other method con­sidered. On the XB-19 airplane, excita­tion power was obtained from the a-coutput , which was rectified and controlledelectronically .! The latter arrangement

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a.e ,

Method of Excitation

outline dimensions as ..a whole conformto specifications of the United StatesArmy Air Forces. The alternator is de­signed to bolt to the constant-speed driveto make, in effect, a single unit. Pilottit and splined shaft of the alternator en­~age corresponding parts of the drive.

Figure 3 also shows an adjustable two­piece mounting ring. This ring with itsfour projecting ears serves to support thealternator end of the combined unit inthe airplane framework. Adjustabilityofthe ring over a range of four inches axiallyfacilitates installation. The body of thealternator , over which the ring is adjust­able, is machined to a smooth, accurate,cylindrical contour, so that the ring maybe secured firmly in any position withinthe limits of adjustment.

Coupling to the constant-speed driveis through a replaceable 24-tooth driving ..spline. Some radial freedom, to allowior a small amount of misalignment be­tween the alternator and constant-speedunit , is provided by use of a stub shaft,the opposite end of which is spline-fittedto the inside of the main shaft of thealternator . A shear section is providedin the stub shaft for protection of theconstant -speed drive in case of combatdamage or mechanical failure of the al­ternator. Removal and replacement ofthe stub shaft does not require dismant­ling the alternator. If broken, the stubshaft can be driven out easily by insertinga rod in the hollow shaft at the air-con­nector end of the machine.

~OVEMBER 1944, VOLUME 63 Keneipp, Veinott-Aircraft Alternator TRANSACTIONS 817

Asure 7. Complete pole and field coil ofalternator

system of the airplane. Such an arrange­ment has three principle disadvantages :

1. A fault in the doc system would causefailure of the a-c system.2. A larger and heavier voltage regulatorwouldbe required, involving a major devel­opment. as such a regulator is 'not nowavailable.3. There would be an excessive waste ofpower in the regulator .

Brush life at high altitude-the prin­cipal objection to the use of the doc ex­citer-was no longer considered theproblem that it once might have been,since a successful altitude treatment forbrushes is now available.• Based onflight experience with doc generators andaltitude-chamber tests on the alternator,(he expected brush life at :1;;. 000 feet isapproximately 500 hours.

Electrical Design Considerations

Fundamental principles employed inthe design of aircraft alternators and docgenerators are the same as those used inthe design of standard industrial ma­chines. The elements of an aircraft electric 'machine resemble those of an industrialmachine of far greater physical size.However. in the design of aircraft electricmachines, extreme effort is put forth toobtain a given rating at the lowest pos­sible weight ; thus, many refinements arerequired in the electrical and mechanicaldesign which are unwarranted in standardmachines.

Electric conductors of aircraft alterna­tors, as well as those of engine-mounteddoc generators, are operated at currentdensities far in excess of standard ma­chines; 10,000 amperes per square inchis not unccmmons--u figure four times ashigh as used in continuous-rated indus­trial machines. Magnetic circuits of this400-cycle aircraft alternator' are workedat about the same densities as conven­tional 50-cycle machines. Because of themuch higher frequency: however, thewatts loss per pound of magnetic ma­terial is from 15 to 20 times as high as ina standard 6O-cvcle machine. Althoughthe copper and iron are worked harder inaircraft than in industrial machines, theefficiencies of the former are as high orhigher for comparable ratings. This is

818 TRANSACTIONS

possible because of greater refinements indesign, higher operating speeds, andbetter ventilation of aircraft machines.

Insulation-of the windings of aircraftalternators must be suitable for thetemperatures encountered. Because in­definite life is not expected and could notbe obtained without a great increase inweight, class A and class B insulatingmaterials usually are operated at highertemperatures in aircraft than for indus­trial service. Thus, for the life expectedof an aircraft alternator, class-A insula­tion can be operated at 136 degreescentigrade and class-B insulation can beoperated at 150 degrees centigrade."Each type of insulation has been used inthis alternator only where its maximumpermissible temperature will not be ex­ceeded.

D-c generators for aircraft use arerated at 28.5 te 30 volts, whereas the 208­volt alternator described in this paperis operated at 120 volts to ground. Be­cause of the higher dielectric stresses, thea-c winding is given a potential test toground of 1,500 volts (nns), and a poten-

FiSUle 8. Complete rotor showing aitcrMtorReId, slip rings, exciter armature, and com­

mutator

tial test between phases of 500 volts;alternator field and exciter armature aretested at 500 volts to ground.

As in the docsystem, greatest reliabilityis obtained with parallel operation, t andthe alternators are designed accordingly.As mentioned before, a damper windingis placed in the pole faces to provide morestable operation of the alternators underany operating condition which mightcause hunting. This winding, made ofcopper, is similar to that used in a largesynchronous machine.

Stator Construction

Figure 4 shows the alternator statorwith laminations and winding in place,and Figure 5 illustrates the lightweightframe. Internal r ibs support the lamina­tions and allow axial passage of ventilat­ing air over the stator core.

Circular laminations without studs orrivets are used , permitting skewingwhich improves the wave form . To en ­sure exact alignment, laminations arestacked on an accurately ground man ­drel, which has guide bars for aligning

Keneipp, Veinott-Aircrajt Alternator

the punchings and obtaining the correctskew. While still on the mandrel, thepunchings are compressed axially, and aslot is machined in the outer peripheryparallel to the axis of the stator. A cor­responding slot is machined in one of theribs of the magnesium-alloy fram e, and [,rectangular key fits into both slotsthereby locking the punchings.

Stator laminations are held in cornpression between a shoulder in the frameand a spacer ring held in place by mean>of six axial through bolts. Openings areprovided in the ring and shoulder so thatventilating air can enter the axial duct,between laminations and frame.

A-c stator coils are formed of double ­glass-covered rectangular copper wire.Two of these coils are shown in Figure 6Accurate forming assures that the coilswill not touch each other at the ends andthat they will have the strength neces­sary to withstand short-circuit stressesClass-B insulation is utilized in the a-(winding. Paper-and-mica slot cells an­used, the paper backing merely providingstrength during assembly. Glass sleeving is slipped over the exposed portion ofeach stator coil. Coils are anchored inslots by means of trapezoidal wedgesAfter winding, the a-c stator is irnpregnated with an alkyd resin varnish to resist the moisture, dirt,and fungus growth ,which an aircraft alternator may encounter.

Rotor Construction

Two obvious methods for constructingthe rotating field of alternators such (I '

described in this paper are one-piece laminations and separable poles . Wittone-piece laminations, the poles and polefaces are integral with the yoke; this ty peof construction has been used with suc­cess in many small machines and in J

few large four-pole industrial machine,Since the yoke and poles are continuousa good magnetic circuit is obtainedthrough the rotating field. However, theone-piece rotor punching has the disadvantage of requiring a random field winding, which is satisfactory in small 10\\

speed machines, but seldom if ever, is usedin large high-speed machines. This COJI

Figure 9. Altemator with air connector re­mond, showing front braclcet and roclcer rin!

ELECTRICAL E:-rGINEERI~G

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Figure 12. Saitur.tion curves and short-circuitcuracteristic of 4O-Icva .Ite..... tor at 6,000 rpM

steel bushing, which later is pressed ontothe exciter shaft.

Mechanical Design Considerations

Aircraft generators must operate withminimum attention and trouble. No partof a generator is more important to re­liable operation than the bearings, andcareful consideration must be given totheir selection. Sealed or shielded anti­friction bearings offer excellent possi­bilities for aircraft generators, becausethey require no attention. Bearings arelubricated at the factory with grease ofproper grade and quality to assure longlife. When the lubrication is properlyapplied, a bearing life of several hundredhours may be expected. Because it is inthe path of the incoming air, the frontbearing is unlikely to become overheated.However, special precautions have beentaken to make certain that the rear hear­ing is adequately cooled. The excitershaft is hollow, and. the front end is opento the incoming air, while at the rear endfour holes are provided in the shaftflange. Cooling air passes through therotor, and then over the rear bearing,thereby preventing excessive temperaturerise . This ventilating path serves a sec­ond purpose, that of helping to cool therotor yoke.

To obtain minimum weight. both thefront and rear brackets are cast of mag­nesium alloy. The rear bracket has sixarms, channel-shaped for increasedstrength. The cone-shaped front brackethas four ribs which serve to support thethe exciter stator and rocker ring, as wellas the front bearing.

Engine-mounted doc generators aresubjected to more severe linear andtorsional vibrations than encountered inany other type of service. History of theengme-mounted doc aircraft generatorrecords a great amount of progress in thesolution of the problems involved. Be-

EllClTl:IIF'Ino

field. A shorter flanged shaft is boltedin a similar position to the opposite endof the yoke. Thus, the complete rotorutilizes only two bearings. This con­struction greatly simplifies assembly ofthe alternator and prevents any possibleinternal misalignment.

The front bracket of the alternatorsupports the stator of the exciter, as wellas the rotor, bearing, and brush assem­blies, This construction is illustrated inFigure 9, a view of the alternator withthe air connector removed, showing thearrangement of the exciter in the bracket.Exciter brushes and slip-ring brushes areall carried on the rocker ring, which ismounted in the front bracket. By refer-

Figure 11. Terminel boaIrd.nd studs, showingstepped .rr.ngement

Exciter Construction

ence to the schematic wiring diagram,Figure 10, it can be seen that the slip­ring brushes are connected directly to theoutput of the exciter, thus requiring mini­mum wiring between the exciter and al­ternator field: To provide maximum ex­citation for the alternator 'when re­quired, a compensated exciter with inter­poles is used. Two collector rings aremounted at the front end of the rotor,adjacent to the exciter commutator.The rings are shrunk over an insulated

TEIMIIUII. IlllAAO

ALTERNATORF'IUD

I I I I I I,I , I , , ' A+0---6' 6' '0u: T2 I :T3

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OJ 0" L{)T~ T5 T' A- 0--".

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Figure 10. Intem.1 wiring diagr.m .nd termi­Aalarr.ngement of 4O-kva .Itern.tor, including

exciter

'Xlhen rotation is clockwise:. (cSt comrnutetorend), phese sequence is T1, T2, T3

struction was not adopted in the machinedescribed in this paper because of lowmechanical strength and poor space factorinherent in a random winding.

Figure 7 illustrates the separable-poleconstruction used in this machine. Polelaminations have a rectangular hole inthe body, by means of which they areassembled onto a snug-fitting rectangularbar of iron having good magnetic proper­ties. These laminations are clamped to­gether by means of the damper bars, asteel through rivet, and aluminum­alloy pieces which also serve to supportthe coil ends. Holes are now drilled andtapped through the punchings into therectangular bar previously described.This construction permits the field coilsto be wound directly on the poles.

A heavy layer of fish paper, glass, andmica insulates each coil from the body ofthe pole. Each coil is wound in two:sections of thin copper strap insulatedbetween turns with glass tape. Micartawashers insulate the upper section of thecoil from the pole and the coil sectionsfrom each other.

After the coils have been wound, theyare bolted to a cylindrical yoke and thenconnected between poles at this time inorder to avoid disturbing them later.Wedges are now inserted between adja­cent coils in both upper and lower sec­tions; Mter connecting the coils andinserting wedges, the rotor is thoroughlyvacuum-impregna ted with an alkyd resinvarni sh and then baked. Thus, a solidrotor winding is obtained, free of airpockets and impervious to moisture.

Alternator field, exciter armature, com­mutator, and slip rings form an integralunit. The exciter armature is pressedonto the long shaft, flanged at one endand bolted to the yoke of the alternator

~OVHMBBR 1944, VOLUME 63 Keneipp, Veinott-Aircraft Alternator TRANSACTIONS 819

Ventilation and Cooling

Performance

Saturation curves of the alternator an'given in Figure 12. These curves include ;a no-load saturation, a zero-power-factorsaturation, and a three-phase short-cir­cuit saturation. Dynamometer tests arerecorded in Figure 13, which showsefficiency, torque, kilowatts output, andamperes output, plotted against kilovolt­ampere output. Efficiencies reportedhere include losses in the exciter, voltageregulator, and alternator itself; theywere computed from direct measure­ments of input and output.

to the main engine, an exhaust shroudaround the outlet ducts of the alternatorhas to be provided; because the entirealternator is surrounded by air at a rela ­tively high pressure ; this shroud mustdischarge to a region of lower ambient airpressure.

Air ducts have been provided betweenthe outside diameter of the stator punch­ings and the frame. Air which passesthrough these ducts also passes directlyover the end windings, picking up heatfrom them as well as from the stator'laminations, and from the frame which isa good conductor of heat. Cooling airalso flows axially between the field coilsof the rotor. The front bearing is cooledby the main blast of air as it enters th ealternator, whereas the rear bearing iscooled by air which flows through thehollow shaft, as explained previously.

No effort has been spared to providethe best cooling obtainable with th especified total head of six inches, and th etemperature rise has been held down topermissible limits. But it must not beoverlooked that, however effective theventilating system may be , for any givenpressure, better results will be obtainedfrom the alternator if higher pressure ormore cooling air is made available. Bybetter results are meant lower tempera­tures of the alternator, with consequentlon ger life of insulation and bearings, andability to carry higher overloads of shorttime duration.

signer use restricted ducts in his machine,or shall he make them as wide open aspossible? Few problems are more diffi­cult to solve than the precise answeringof this question. In general, the gen­erator designer knows that, with blast­tube cooling, the less air his machine uses,the more total pressure head is availableat the generator; and, conversely, that a .wide-open generator which uses more airresults in less available total head acrossthe generator. Total available head andvolume of air could be determined by thegenerator designer, as pointed out in .arecent AlEE paper. ! from a pressure­volume curve of the blast-tube installa­tion, if the latter were available.

Some general considerations on thequalitative difference s in the coolingproblem of wide-open and restrictedgenerators will assist in an understand­ing of the ventilation problem. The sig­nificant factor is total temperature rise ofthe winding hot spot above ambient,which is the sum of two temperature rises:I . Rise of cooling air passing through themachine.2. Rise of hot spot above the cooling air .

A wide-open generator passes more air;hence , rise of cooling air is invariably less.But, in the wide-open generator, the dis­sipating area of the cooling ducts is less,and air velocities in the ducts tend to belower; the net effect is to cause a higherrise of hot spot above the cooling air.Since total temperature rise is the im­portant factor, it can be seen that theminimum sum of the two factors occurswhen the generator is neither toe wideopen nor too restricted. A completeanswer to the ventilation problem requiresan analytical or empirical evaluation ofthe hot-spot rise for different degrees ofrestriction, taking into account the pres­sure-volume characteristic of the blasttube.

For the 40-kva alternator discussed inthis paper, a total head of at least sixinches must be maintained between theinlet and outlet. If the alternator is in­stalled in a large duct which supplies air

800

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cause the alternator of this paper is notengine-mounted, the problem of linearvibrations and accelerations is less severe ;however, the machine is subjected to allthe vibrations and accelerations occurringin the nacelle structure. Torsional vibra­tion presents a severe problem of unde­termined magnitude, because torsionalimpulses may be transmitted through aswell as developed in the constant-speeddrive. If the generator is mounted on anauxiliary engine, severe vibrations, bothlinear and torsional, may he encountered.

oo 10 20 30 40 50

OUTPUT -KVA

Fie'" 13. Efflciency, torque, and outputCInft of 4O-Icva alternator at 6,000 rpm, from

dynamometer tests

Ventilation is unquestionably one ofthe principal factors influencing theweight of any aircraft generator. With­out blast-tube cooling," 'th e extremelyhigh outputs per pound of modem air­craft generators would not have beenpossible. Aircraft-generator buildershave been hampered by lack of accurateinformation on the pressure-volume char­acteristics of practicable aircraft blast­tube systems. Shall the generator de -

Figure 14. Com­parison of 4O-Icvaalternator and 300­ampere type R-1 d-e

generator

Conclusions

A number of experimental 40-kva 40U­cycle aircraft alternators has now beeubuilt, and extensive tests, in addition tvthose reported in the paper, have demon­strated that such machines are ready foruse in aircraft as soon as a constant-speeddrive or other suitable driving means ismade available. (Some of the tests notreported in the paper include: short-circuit tests, three-phase, line-to-line, andline-to-ground; altitude-chamber testsof heating and brush wear ; parallel:operation tests; overspeed tests ; startinxa heavy load, such as a motor generatorset.) A weight of 85 pounds for this 40·kva alternator, compared with a weight of47 pounds for a nine-kilowatt doc air-

820 TRANSACTIONS Keneipp, Veinou-i-Aircraft Alternator ELECTRICAL ENGlNEERn'G