NASA Technical Memorandum 83656

Development of High FrequencyLow Weight Power Magnetics forAerospace Power Systems

Gene E. SchwarzeLewis Research CenterCleveland, Ohio

Prepared for theNineteenth Intersociety Energy Conversion Engineering Conferencecosponsored by the ANS, ASME, SAE, IEEE, AIAA, ACS, and AIChESan Francisco, California, August 19-24, 1984

NASA

https://ntrs.nasa.gov/search.jsp?R=19840014824 2018-07-14T03:21:29+00:00Z

DEVELOPMENT OF HIGH FREQUENCY LOW HEIGHT POWER MAGNETICS FOR AEROSPACE POWER SYSTEMS

Gene E. SchwarzeNational Aeronautics and Space Administration

Lewis Research CenterCleveland, Ohio 44135

ABSTRACT

A dominant design consideration in thedevelopment of space-type power magneticdevices is the application of reliable ther-mal control methods in order to prevent

5 device failure due to excessive temperature° rises and hot spot temperatures in critical^ areas. The resultant design must also yield

low weight, high efficiency, high reliabil-ity and maintainability, and long life. Atransformer weight equation is developed toshow that the weight is directly propor-tional to the 3/4th power of the kVA ratingand inversely proportional to the 3/4thpower of the product of the variables f,Bm, Js, a, (FF),.and (SF)C. A familyof curves, obtained when the normalizedspecific weight is plotted as a function ofthe transformer's kVA rating with frequencyas the parameter, show insignificant weightsavings for frequencies above 50 kHz. Theweight savings and high efficiency thatresults by going to high frequency andunique thermal control techniques is demon-strated by the development of a 25 kVA, 20kHz space type transformer under Lewis'power magnetics technology program. Earlierwork included heat pipe cooled inductors andtransformers in the 3 kVA range. Presentwork in the area of power rotary trans-formers is also discussed.

INTRODUCTION

The power circuits in future high powerspace systems w i l l in many cases require theuse of power transformers. The power trans-former, whether fixed or rotary, providesthe means to transfer power at source volt-age levels to load voltage requirements.The requirement for isolation between sourceand load is inherently provided by the powertransformer.

In the space environment, rejection ofthe transformer's core and winding lossesmust be accomplished almost solely by con-ductive heat transfer to a radiator. Thedevelopment of thermal control methods whicheliminate potential hot spots and also givepredictable core and winding temperaturerises thus becomes a dominant considerationin the design strategy of space-type powermagnetics. The resultant design must alsobe such that the device has low specific

weight, high efficiency, high reliabilityand long life.

Over the past decade, the NASA LewisResearch Center has been actively engagedin developing the technology of lightweight,high efficiency and high reliability powermagnetic devices for aerospace powersystems. The characterization and testingof these magnetic power components throughtechnology development programs is a pre-requiste for consideration of the com-ponent's use in future aerospace powersystems.

One of the first magnetic devices de-veloped was a conduction cooled high volt-age transformer for the cathode/collectorpower supply of the 200 watt TWT used on theCommunications Technology Satellite (1).This 500 VA, 11.3 kV output transformeroperated at 10 kHz at an efficiency greaterthan 98 percent and weighed 0.82 kg for aspecific weight of 1.6 kg/kVA.

Another magnetic device development wasa conduction cooled 2.3 kVA, 10 kHz trans-former for a 30-cm ion thruster power supply(2,3). This 1.1 kV output transformer hasan efficiency of 98.6 percent, a weight of1.8 kg and a specific weight of 0.78 kg/kVA.

A heat pipe cooled transformer havingidentical electrical specifications of theabove conduction cooled transformer has anefficiency of 98.2 percent, a weight of 1.2kg,'and a specific weight of 0.52 kg/kVA,(4,5). A significant difference betweenthese electrically identical transformers isthat the conduction cooled transformer has atemperature rise of 41° C while the heatpipe cooled transformer has a temperaturerise of only 20° C. Thus, the heat pipetransformer would have a higher rating than2.3 kVA and a lower specific weight than0.52 kg kVA if it were operated at the sametemperature rise as the conduction cooledtransformer.

The specific weight and efficiency ofpower magnetic components is of primary con-cern to space power systems' designers. Inthis paper a transformer weight analysis isperformed and the results are used to plot afamily of curves of normalized specificweight versus the kVA-rating. The paperalso describes a recent addition to lowweight, high efficiency power magnetics

technology which was the development of a~conduction cooled 20 kHz, 25-kVA, 200 Vinput and 1500 V output, continuous duty,single phase transformer (6,7). Rotarypower transformers for photovoltaic spacepower systems are also briefly addressed.

Equation (6) can be written in the form

Aw =

ft)(7)

WEIGHT ANALYSIS : -

The weights of the magnetic devices inspace power supplies are generally thegreatest percentage of the total weight.Thus, every effort must be made to reducethe magnetics weight and still achieve highefficiency, high reliability and permissibletemperature rises in the core and windings.The design variables which affect .the weightwill now be investigated by means of the,following weight analysis.

The kVA rating of'a single phase trans-former is

kVA = IsVsxlO-3

-Sr-l XlO-3

(1)

The secondary RMS current densityJs i s by definition

= i A/m

The substitution of Eq. (2) intoEq. (1) gives

(21

From this last equation it should benoted that for a specified kVA rating, thewindow area decreases for increases .in o,Js, and (VS/NS). High voltage trans- _ , ,formers will nave small values of a andthus it is seen that they will requirelarger window areas for the same kVA ratingsince more electrical insulation isrequired. The limit, to which Js can beincreased is dependent on the maximum allow-able temperature rise of the-windings.

If it is assumed that the voltage dropsdue to the primary and secondary windingimpedances are negligible respectively tothe primary input and secondary output volt-ages, then by means, of Faraday's Law we canwrite

V Vs/= TT = 4(FF)Bmf (SF)c Ac • V/turn

Solving Eq. (8) for AC give's

.(8)

Ac = W7ft) (9)

kVA = N.sbsJ, (^

The window area required for theprimary and secondary turns is

(3)

, _ (Vp 1W \ a (4!

If it is assumed thatthen Eq. (4) can be written as

aANpbp = Nsbs = (5)

By the use of this last result, Eq. (3)then becomes

kVA = ft)xlO-3 (6)

A comparison of Eq. (9) with Eq. (7)shows that for a specified kVA rating thatas (VS/NS) increases, then the windowarea decreases but the core 'cross-sectionalarea increases. Since the weight and sizeof a transformer are dependent on Aw andAc, it would appear that increasing ordecreasing (VS/NS) does not contributeto any weight savings. An increase in Accauses an increase in core weight, generallyan increase in the mean-length of turn ofthe primary and secondary winding, and thusan increase in winding weight. A decreasein Aw results in reduced core weight,reduced mean-turn lengths and hence reducedwinding weight.

The substitution of Eq. (8) intoEq. (6) gives

kVA = 2 a (SF)C (FF) JsBmfAcAwxlO-3

(10)

The total transformer weight, W-j-includes the weight of the windings, Ww,the weight of the magnetic core, Wc, and

2

the weight of the mechanical structuralelements, insulation, and cooling hardware,Wx. The total weight is then defined as

W Wx (ID

The dimensions of the core and windingscan all be expressed in terms of a basiclinear dimension, d, and dimensionless scal-ing constants, aj_, &2> a3> • • •> an-That is, all the linear dimensions of thecore and windings can be normalized withrespect to a particular linear dimension ofeither the core or winding. This normalizedor reference dimension is then the basiclinear dimension d, and the ratio of a part-icular core or winding dimension to thebasic dimension is the scaling constant forthat dimension.

For example, the basic dimension couldbe the length of the core window. The win-dow area would then be

Aw = (d)(aid) = aid2 m2 (12)

where ai is the scaling constant of thewidth of the window and a^d is thewidth.

The weights of the core and windingscan thus be written in terms of the basicdimension and the appropriate scaling con-stants along with the density of the core orwinding. For the winding conductor,

W,, Dw(U U kg (13)

where

where

Ww = kwOw d3 kg

(a2 + a3)

(19)

kw = aai(

For the magnetic core,

Wc = DCAC Tc kg (20)

where the geometrical cross-sectional areaof the core Ac is given by

A =p- (21]

and where the core path length, Tc, isgiven by

lc (22)

The substitution of Eqs. (21) and (22) intoEq. (20) gives

Wc = kcDc d3 kg (23)

where

kc = 343535

The weight Wx can be represented as

w =x fth-kxWT kg (24)

where

U = b 1 = b N (MLT)p = "P P P P P

(14;

(15)

The substitution of Eqs. (14) and (15) intoEq. (13) gives

aD Aw,, = -TT-* (MLT)r

Let

(MLTL kg

(MLT)p = a2d

(MLT)S = a3d

m

m

(16)

(17)

(18)

Now by means of Eqs. (12), (17), and (18)Eq. (16) becomes,

The substitution of Eqs. (19), (23) , and(24) into Eq. (11) gives,

= kTDw d3 kg (25 )

where

kT =

Trfe substitution of Eqs. (12) and (21)into Eq. (10) gives

kVA = kva (FF ) (SF) C JsBmf d4 xlO'3

(26)

wherekv = 2

Solving Eq. (26) for d and substituting thisresult into Eq. (25) gives

r 3 -I1/4kVAxKTk a (FFTTST) J B fL v c s m j

d =

"T-»,». k ™*10_

c JsBmf

(27)

(28)

where

kR 'I k 3/4.̂.

Equation (28) provides the means to-examine the variables which affect the totaltransformer weight. The first significantthing to note is that the weight is not alinear function of the kVA rating but ratherthe weight varies as the 3/4th power of thekVA rating. Thi's fact leads to the con-clusion that as the kVA rating increases,then the specific weight should decrease,i.e., the higher the kVA-rating, the lowerthe specific weight.

To reduce the weight of a transformerwith a specified kVA rating requires thatthe denominator of the right member ofEq. (28) increases. An examination of eachdesign variable in this denominator and itsupper limit w i l l now.be investigated.

(1) .The conductor space factor, a, isa fraction and thus its limit is one. Inreality though a can only approach abouthalf its upper limit since the insulatingand cooling systems of the windings deter-mine the value of this design variable. Ingeneral, as the voltage rating increases,then a decreases.

(2) The stacking factor, (SF)C, ofthe magnetic core is also a fraction andits upper limit is one. The,upper limit isapproached by increasing the laminationthickness but this leads to higher specificcore losses. For this variable then atrade-off must be made between transformerefficiency and weight.

(3) The form factor, (FF), is fixedonce the voltage waveform is selected. Itshould be noted though that a sine wavevoltage transformer having a form factor of1.11 will yield a lower weight transformer

than a square wave voltage transformerhaving unity form factor.

(4) An increase in the current den-sity, Js, will cause an increase in thewinding losses since the winding losses areproportional to the square of the currentdensity. .The upper limit to which Jscan be increased is determined by the allow-able temperature rise of the windings. Theuse of high thermal conductivity insulatingmaterials, multiple thermal paths for thewinding and core loss heat rejection, andminimum thermal interfaces and joints allpromote reduction in temperature' rise andthus allow for increases in Js.

(5) The upper limit of the magneticflux density, Bm, is its saturation value,which is dependant on the choice of magneticmaterial used for the core. The specificcore loss of a magnetic material is a non-linear function of Bm and increases asBm increases. Thus, increases in Bmresult in core loss increases and thuschanges in this variable likewise require aweight versus efficiency trade-off.

(6) The frequency upper limit in asense is unbounded and thus increases inthis variable provide the largest weightsavings. However, the specific core loss,also a function of the frequency, increases ;as the frequency increases. In order tohave manageable core losses at high fre-quencies it is required that the magneticflux density be decreased. Thus, decreasesin Wj achieved by increases in f canbe offset to a certain extent by the neces-sary decreases in Bm. Also, as the fre-quency increases, the ac winding resistanceincreases due to skin and proximity effectsand thus causes increased winding losses.

From this weight analysis it is seenthat savings in weight can be achieved byincreases in several design variables, mostnotably the frequency but that these savingsin weight can be at the expense of thetransformer's efficiency. However, byproper choice of the variables involved, itis possible to achieve a transformer thathas both a very low weight and a very highefficiency.

From Eq. (28) the normalized specificweight is defined as,

(WT/kR)/(kVA)3/4

kg/kVA (29)

In Fig. 1 a family'of curves of nor-malized specific weight plotted against thekVA rating are drawn for specified valuesof Js, a, (FF), DW, and (SF)C. Each

curve of the family represents a given fre-quency and for this frequency a value ofBm is selected so as to avoid excessivespecific core losses. The curves clearlyillustrate that for a given frequency, thenormalized specific weight decreases as the .kVA-rating increases. Also, as the fre-quency is increased, the normalized specificweight decreases but the rate of decrease,diminishes as the frequency increases.

A comparison of the 20 kHz and 50 kHzcurves shows that minimal weight savings arerealized by going to 50 kHz. Thus, beyond50 kHz the weight savings become insigni-ficant.

An identical family of curves can beobtained for different specified values ofJs. «> (FF), DW and (SF)C. Forexample, if only Js is changed, thenincreases in Js cause a downward shiftof the curves while decreases in Jscause an upward shift of the curves.

25-kVA SPACE-TYPE TRANSFORMER '

The significant weight and size re-ductions which can be achieved by going tohigh frequency operation and unique thermalcontrol techniques is clearly demonstratedby the transformer shown in Fig. 2. This25 kVA, 20 kHz, 200 V input and 1500 V out-put, conduction cooled, continuous duty,single phase transformer was developed underLewis' power magnetics technology program.This transformer will be described in somedetail because of its importance to thespace station, which has power levels con-sistent with 25 kVA modules.

The electrical power loss of this 25kVA transformer is 196 watts for an effi-ciency of 99.2 percent. Its weight is 3.2kg (7 Ib) for a specific weight of 0.13kg/kVA (0.28 Ib/kVA). A comparable 25 kVA,60 Hz commercial power transformer weighsabout 180 kg (400 Ib) for a specific weightof 7.3 kg/kVA (16 Ib/kVA). In Fig. 3 a com-parison of the 25 kVA, 20 kHz space-typetransformer and a commercial 25 kVA, 60 Hzcommercial power transformer are.shown.

Table I gives a breakdown of the trans-former's loss. From the table it should benoted that the losses are fairly evenly dis-tributed between the primary and secondarywinding losses and the core loss. Table IIgives a weight breakdown of the trans-former's major elements. It is significantto note that the sum of the weights of thestructural, and cooling components, insula-tors, and mechanical fasteners comprise justover 50 percent of the total weight.

The mechanical design of the trans-former was based on the premise that itwould be mounted on a constant temperaturesurface. Figure 4 shows the mechanicalstructure of the .transformer. The prin-ciple elements of this structure are thebaseplate, the core clamps, and the coilmounting plates and core supports whichattach to the base plate. Aluminum was usedto fabricate these structural componentssince it has features.that combine goodthermal conductivity, lightweight, and easeof fabrication. The transformer's mechan-ical structure not only provides therequired mechanical support for the core andwindings but it also provides, almost moreimportantly, the mult-ithermal paths for theconduction cooling of the windings and core.

Figure 5 shows a coil plate assemblyalong with the location of the primary andsecondary terminals, the magnetic core, andother structural members. The core arrange-ment is of the shell-type. Supermalloy Ccores with 0.025 mm (0.001 in) thick woundtape material were used for the magneticcores. Four sets of core mounting supportsand clamps encompassing the exposed portionsof the magnetic core hold the core in afixed position. An insulated core tube fitsover the center leg of the core and thistube passes through the center diameter holein all of the coil support plates. Theinsulation on the underside of the exteriorcore legs was removed in order to providemore intimate thermal contact between thecores and the core support surfaces. Aninsulated pressure plate is placed betweenthe core clamp and the upper surface of theexterior core legs. Top mounted screws inthe core clamps are adjusted to apply pres-sure to this insulated plate and the exte-rior core legs. This arrangement of thecore clamps and supports thus provides thecore's mechanical support and the thermalpaths to transmit the core losses to thebaseplate.

Prewound pie or pancake single layercoils are bonded to coil mounting plateswith a thin insulating film placed betweencoil and plate. Each pie coil's I2Rlosses are uniformly transmitted directlyto its mounting plate which conducts theheat directly to the heat sink baseplate.A primary coil is mounted to one face ofthe mounting plate and a secondary coil tothe opposite face. This arrangement leadsto very good magnetic coupling and, hence,low leakage inductance. The equivalentleakage inductance measured at the secondarywinding with the primary winding shorted was57 pH so that the calculated equivalentleakage inductance referred to the primaryis 1.0 wH. The metal mounting plates also

provide a natural electrostatic shield bet-ween the windings. In Figs. 4 and 5 itshould be noted that each mounting plate isslit from its top edge to its center hole toprevent the plate from acting as a shortedturn.

The eight individual primary pie coilsare connected in parallel while the eightsecondary pie coils are connected in series.The turns ratio of the secondary to primaryturns is 7.5 and the volts per turn is 16.7V/turn.

Segmented hollow copper spacers areused to construct both the primary and sec-ondary bus bar assemblies. To reduce thesize and weight of the primary bus-barassembly, the external primary connectionsare made to both ends of the primary ter-minal assemblies.

The transformer was tested under shortcircuit conditions at 60 Hz. The primarywinding was short-circuited and the im-pedance voltage applied to the secondary.Under these conditions full power ratedcurrent flowed through the windings. Forthis test the transformer was instrumentedwith 16 thermocouples to identify hot spottemperatures and also to obtain a tempera-ture profile of the windings. The maximumtemperature rise of the coils was 45- C.

Even though the leakage inductance ofthis transformer is. very low, at 20 kHzunder short circuit conditions the leakagereactance becomes the predominant term inthe .impedance so that the phase angle ap-proaches 90° and the power factor approacheszero. Limited success was achieved in con-ducting 20 kHz short-circuit tests since theamplifier used could not cope with this typeof load.

To conduct full load high frequencycomplex waveform tests will require specialtest circuits and specially designed linearpower amplifiers. Presently under develop-ment at NASA Lewis is a 25 kVA solid stateamplifier with variable output frequency andgain and capable of operating into low powerfactor loads.

time, and low noise. The rotary transformerbecomes an even more attractive candidate if'an ac distribution system is used.

Low power rotary transformers in therange of 1 kVA and operating frequencies upto 20 kHz'have been developed (8 to 11).Design studies have been conducted for a 20kHz rotary transformer with an axial gapgeometry (12) and a radial gap geometry-(13)for 25 to 100 kVA levels. Present studiesinclude the design characterization ofvarious rotary power transformer geometricalconfigurations and how these characteristicsaffect the design of high frequency and highpower inverters. The fabrication of a 25kVA, 20 kHz rotary power transformer forproof of concept tests and device character-ization will begin in the near future.

CONCLUSION

The transformer weight equation showsthat the weight is directly proportional tothe 3/4th power of the kVA rating andinversely proportional to the 3/4 power ofthe product of the variables f, Bm,Js, a, (FF), and (SF)C. The weightequation is also used to define the nor- .malized specific weight. The family ofcurves, obtained when the normalizedspecific weight is plotted as a function ofthe transformer's kVA rating with frequencyas the parameter, shows insignificant weightsavings for frequencies above 50 kHz.

The weight savings and high efficiencythat can be achieved by going to high fre-quency and unique thermal control techniqueswere demonstrated-by the development of a25 kVA space-type transformer developedunder Lewis' power magnetics technologyprogram. Power magnetics technology willcontinue to be developed in the 100 kVArange for the near term and the 1-MVA rangefor the longer term. Anticipated additionalNASA technology to be developed includesboth high temperature (300° to 500° C) andradiation resistant power magnetics. Themagnetic technology developed should beapplicable to both photovoltaic and futurehigh power rotating space-power systems.

ROTARY TRANSFORMER

Future high power spacecraft such asthe space station will require the transferof large quantities of power across a rotat-ing interface. Slip rings and roll ringsare potential candidates as the rotory powertransfer device but the rotary transformeris also a strong candidate since it is con-tactless and thus offers the desirable char-acteristics of high reliability, long life-

SYMBOLS

geometrical cross-sectional area ofmagnetic core, nr

core window area, mscaling constants,dimension!essdimension less

, b

d

FF

kc,kR,kT,kv'kw'kx

Ip, Is

(MLT). (MLT),

(SF)C

, U

V

WeWwWT

Wx

maximum magnetic fluxdensity, T

primary and secondaryconductor wire cross-sectional area,respectively, m 3

core density, kg/m

winding conductormaterial density,kg/m3

basic or reference lineardimension, m

form factor; the ratio ofRMS to average voltage,dimensionless

frequency, hZsecondary RMS current, Asecondary RMS currentdensity, A/m2

constants defined in text

mean path length ofmagnetic core, m

primary and secondarywinding conductorlengths, respectively, in

primary and secondarywinding mean lengthof turn, respectively, m

number of primary andsecondary turns,respectively,dimensionless

core stacking factor;fraction of corevolume filied withmagnetic material,dimensionless

primary and secondarywinding conductor >volume, respectively,nr

primary and secondary RMSvoltage, respectively,V

core weight, kgwinding weight, kgtotal transformer weight,

kgweight of transformer's

mechanical structure,insulation, and coolinghardware, kg

window space factor;fraction of windowfilled with bare wireconductor, dimension-less

REFERENCES

B. FARBER, et al., in PESC '74 Record:IEEE Power Electronics Specialists Con-ference -- 1974, pp. 81-90, InsTrFuTe"of Electrical and Electronics Engi-neers, New York (1974) .M. S. CHESTER and I. G. HANSEN, in PESC' 7 7 Record: IEE E Power El ectron i cs'S'pec i a 1 1s"f s Conference -- 197?, p. 3,Institute of Electrical and ElectronicsEngineers, New York (1977). »I. HANSEN, "Description of a 2.3 kWPower Transformer for Space Applica-tions," NASA TM-79138 (Feb. 1979).I. G. HANSEN and M. CHESTER, "Heat PipeCooling of Power Processing Magnetics,"AIAA Paper 79-2082, (NASA-TM-79270) ,(1979).M. S. CHESTER, "Heat Pipe Cooled PowerMagnetics, Final Report," NASACR-159659, (TRW 33572-6001-RU-OO), TRWDefense and Space Systems Group, PowerConversion Electronics Department(Dec. 1979).G. E. SCHWARZE, in_Ajrcraft ElectricSecondary Power, pp. T4~3-T47, NATS

~7. j; P. WELSH, "Design and Development

of Multi-KW Power Electronic Trans-formers," NASA CR-168082, (TTL ReportNo. 83-1), Thermal Technology Labora-tory, Inc. (Feb. 1983).

8. E. E. LANDSMAN, in Power Condi_tionuig_Specialists Conference--1970, Record,pp. T3T:T5"̂ Institute oF~ETecf?TcaT~andElectronics Engineers, New York (1970).

9. S. H. MARX and R. W. BOUNDS, IEEETrans. Aerosp. Electron. Syst. 7_, 1157TT9TTT.

10. W. AUER, in Photovoltaic Generators inSpace, ESA SP-147, pp. 2"23̂ 2~2lT;European Space Agency, Noordwijk, TheNetherlands (June 1980).

11. Colonel W. T. MCLYMAN and A. 0.BRIDGEFORTH, in 18th IntersocietyEnergy Conversion Ejigi ne'er ingC conference, P roceed ings, vol. 3,pp. lT02-n05, American Institute ofChemical of Engineers', New York,(1983).

12. S. M. WEINBERGER, "Preliminary DesignDevelopment of 100 KW Rotary PowerTransfer Device," NASA CR-165431,(GE-81SDS4215), General ElectricCompany, Space Division (Mar. 1981).

13. S. M. WEINBERGER, "Design Study of aHigher Power Rotary Transformer," NASACR-168012, (GE-82SDS4222), GeneralElectric Company, Space Division (July1982).

TABLE I. - 25 kVA TRANSFORMER LOSS

AND EFFICIENCY

Description

Pri. Wdg I2R

Sec. Wdg I2R

Total I2R

Core Loss

LossW.

74

62

136

60

Percent of totalpercent

37.8

31.6

69.4

30.6

Total transformer Loss - 196 watts.Total transformer efficiency - 99.2 percent

TABLE II. - 25 kVA TRANSFORMER WEIGHT BREAKDOWN

Description

Magnetic coreCoils and bus bar assemblyStructural componentsInsulatorsMechanical fasteners

Total weight

Weight

0.70 (1.53)0.87 (1.92)1.14 (2.50)0.21 (0.47)0.24 (0.53)

3.16 (6.95)

Percent total weightpercent

22.027.636.06.87.6

100

Specific weight - 0.13 k'g/kVA (0.28 Ib/kVA).Specific power - 7.92 kVA/kg (3.6 kVA/lb).

10

oU-

OLLjQ_

U-JM

<t

o

.1

.01

I—/ -'«; 0.i-20; 0.4

I i I i I i l i ii 10 100

kVA RATING, kVA

1000

Figure 1. - Normalized specific weight as a function of trans-former kVA rating, a - 0.25; Js - 2.63x 106 A/m2; (SF)C -0.9; (F. F) - 1.11; Dw • 8.89 x 103 kg/m3.

Figure 2. - 25 kVA, 20 kHz transformer.

TFL/NASA-LeRC I COMMERCIAL

! WEIGHT (IBS)

TEMPEBATUBEIHSEjCn

40097.9

50

t [LOW VOLTAGE (VOLTS) I _ 200 _ i 120/248 |

! HIGH \fdlTAGE (VOLTS) 1500 j 3850/4160/4478 !C-82-2454

Figure 3. - Comparison of space and commercial 25 kVA single-phase transformers.

A; -M C-82-1039

Figure 4. - Mechanical structure of 25 kVA, 20 kHz transformer.

PRIMARY TERMINALS

SECONDARYTERMINAL

COIL SUPPORT PLATE

SECONDARY TERMINAL

CLAMPING SCREW

INSULATEDPRESSUREPLATE

CORE

— CORECOOLINGCLAMP

— CORE SUPPORT

„„— BASEPLATE

Figure 5. - Coil plate assembly for 25 kVA, 20 kHz transformer.

1. Report No.

NASA TM-83656

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle 5. Report Date

Development of High Frequency Low Weight PowerMagnetics for Aerospace Power Systems 6. Performing Organization Code

506-55-72

7. Author(s)

Gene E. Schwarze

8. Performing Organization Report No.

E-204310. Work Unit No.

9. Performing Organization Name and Address

National Aeronautics and Space AdministrationLewis Research CenterCleveland, Ohio 44135

11. Contract or Grant No.

12. Sponsoring Agency Name and Address

National Aeronautics and Space AdministrationWashington, D.C. 20546

13. Type of Report and Period Covered

Technical Memorandum14. Sponsoring Agency Code

15. Supplementary Notes i

Prepared for the Nineteenth Intersociety Energy Conversion Engineering Conferencecosponsored by the ANS, ASME, SAE, IEEE, AIAA, ACS, and AIChE, San Francisco,California, August 19-24, 1984.

16. Abstract

A dominant design consideration in the development of space-type power magneticdevices is the application of reliable thermal control methods in order to pre-vent device failure due to excessive temperature rises and hot spot temperaturesin critical areas. The resultant design must also yield low weight, high effi-ciency, high reliability and maintainability, and long life. A transformerweight equation is developed to show that the weight is directly proportional tothe 3/4th power of the kVA rating and inversely proportional to the 3/4th powerof the product of the variables f, Bm, Js, a, (FF), and (SF)C. A family ofcurves, obtained when the normalized specific weight is plotted as a function ofthe transformer's kVA rating with frequency as the parameter, show insignificantweight savings for frequencies above 50 kHz. The weight savings and high effi-ciency that results by going to high frequency and unique thermal control tech-niques is demonstrated by the development of a 25 kVA, 20 kHz space type trans-former under Lewis' power magnetics technology program. Earlier work includedheat pipe cooled inductors and transformers in the 3 kVA range. Present work inthe area of power rotary transformers is also discussed.

17. Key Words (Suggested by Authors))

Power transformer; High power;Low weight; High efficiency;High frequency

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Unclassified

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