aech chapter 15

7/30/2019 AECH Chapter 15

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Section V Electromagnetic and Other Applications of Aluminum

Chapter 15

Capacitor Foil

Capacitors both fixed and variable are used todayin almost every electrical system. From great powergenerating and distributing networks to electric organs,including telephone and radio systems, computers andmotors, elevators and x-ray apparatus and so on, theuse of a capacitor almost always is a fundamentalnecessity.Any arrangement of electrodes whatsoever upon whichelectric cbarges accumulate or move will exhibit the incidence of capacitance. Where the electrode geometry isextensive in space such as a wire or cable, the charges aredistributed likewise and we speak of such a structure asbeing a distributed capacitance. When the electrodes aredeliberately concentrated in space, the charges are concentrated and this is termed a lumped capacitance. All electrical components used specifically as capacitors are lookedupon as providing lumped capacitance.Aluminum has been and is the preferred metal for

capacitor electrodes whether used in rigid plate form orin varying thicknesses of foil for d-., a-c low voltage,high voltage, high frequency, high or low power, impulsedischarge, etc.

Capacitor Design ConsiderationsUnder given conditions of electrical, physical and environmental factors, a capacitor may be called upon to provide a precise amount of capacitance, a required timeconstant of charge and discharge (with proper circuit resistance) or a specified impulse release of stored energy.

The selection of electrode and insulation materials and thedesign of their electrical and mechanical arrangementscan be optimized to produce an economical capacitoI" thatwill perform properly in its intended service.

In Section Ill, Chapter 8, the general conditions governing the relation of potential and charge to capacitancewere discussed as well as the influence of the dielectricmedium. The equations for capacitance relating to a widevariety of electrode geometries were given and the natureof the dielectric polarization of the insulation discussed.All of this is applicable to capacitor design.

A common economic consideration in capacitor designis to obtain the largest amount of capacitance per unit volume of material used. This is, of course, obtained by usingthe thinnest electrodes and the thinnest insulating materialpossessing the highest dielectric constant. How far onecan go in these directions depends upon the circuit voltageto be withstood, the conductance and!or dielectric lossthat can be tolerated and the stability of the assemblyunder the operating conditions.Equivalent Network of a Capacitor

All capacitors possess a certain amount of series resistance and inductance as well as shunt capacitance and conductance. Fig. 15-1 shows a simple, unrolled foil capacitor and its equivalent electrical circuits. The series impedance (r + jwL) is made up of the resistance and inductance of the capacitor leads plus those inherent in theelectrode material, shape and extent. Usually the inductance is negligibly small as compared to the other factors.Actually, the inductance is approximately the same as awire loop equal in area to that formed by the two leadsand the capacitor unit itself. The foil electrodes appearas a uniformly distributed series resistance.

The overall impedance of the equivalent network shownin Fig. 15-1 (c ) is:.,C- ~ ~ - + j w Lg2 +.,2 C' g2 +w'C' (Eq. 15-1)where:

w=2".ffi n HzginmhosCin faradsLin becrysr in ohms

or since g2 is usually very small compared to "PC',g j (w 'CL- I )

Z ~ ( r + - - - )wC (Eq. 15-2)

15-1


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electromagnetic and other applications of aluminum

FO'l elECTRODES

lEADS "" ,....,'''' ".. . D.elECTRIC

LEAD INDUCTANCE IL)LEAD INDUCTANCE II I

lEAD RESISTANCE PLUSEFFECTIVE FOIL RESISTANCE ldlEAD RESISTANCE

EQUiVALENT PARALlelIDEAL RESISTANCE CORRESPONDINGAPACiTANCE TO CONDUCTANCE (g )OF DIELECTRIC1


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This is because the effective resistance undergoes largechanges with changing frequency and because of the widefrequency-range which circuits are often required to cover.Loss in Foil and Leads: At a first approximation, theeffective impedance of the foil and leads of a capacitor appears as a straight-line factor over a wide range of frequency. At higher frequencies, impedance increases due

to eddy-current and other losses including skin effectwhere only the outer portion of the metallic componentscarry the current.From the watt loss (Eq. 15-3) above, it is seen thatthe heat loss in the foil and leads increases as the squareof the frequency for constant applied voltage. In general,this condition applies over the operating frequency-rangeof many capacitors.

EIJeClive Resistance of Foil ElectrodesIn the case of wound paper capacitors, there is a simplerelationship between the effective resistance of the foilelectrodes and their de resistance. With reference to Fig.

15-1, it is clear that alternating current entering the foilelectrodes at the lead-in wires decreases as it spreads ordistributes along the foil, and the current flowing at pointsremote from the lead-in wires may be only a small fractionof the entering current. It may be shown theoretically anddemonstrated experimentally that for the long, narrow

100

10

o.1V ) o.I~:x:o

~ 1wUZ~ 10- 100iiSZIOOO

capacitor foilelectrodes of wound paper capacitors the effective foilresistance is approximately equal to 1/3 of the loop dcresistance obtained by adding the dc resistance values ofthe two foils. In other words, due to current attenuationalong the foils only 33 percent of the total doc foil resistance is effective with respect to alternating current.Fig. 15-3 shows the effect of several laid-in terminals inreducing effective foil resistance especiaUy at the higherfrequencies.

Where, as is more usual in practice, the terminals arelaid-in at approximately the middle of the foil electrodes,the current spreads in opposite directions along the foils.The effective resistance of the loop in each direction isthen R/6 and, since the two loops are in parallel, the totaleffective resistance becomes R/12. When "n" terminalsare laid-in on a foil of length "L", it may be shown thatthe lowest resistance is obtained by spacing the terminalsat intervals of L/n, with each end terminal located L/2nfrom the end of the foil. With this arrangement, the effective resistance is inversely proportional to the square ofthe number of terminals,In the limiting case, the edge of the foil is connected together along its entire length. This, kno,,\'n as "extendedfoil" or "overlapped foil" construction, gives the lowestattainable effective foil resistance for a foil of given material and dimensions. In addition, by providing an efficient

~ A P A C I T A N C E . C=1MF !

\1-

INDUCTANCE.l =0.2)J.HSERIES

~ i - "~

INDUCTIVEEFFECT

R E A C T A ~ C ./'7 IVE RESISjANiE ,,I y :1,' \Q


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electromagnetic and other applications of aluminum0

w 10Uz 20~u;"w 30a: Z 40 Z 0 50 i=u:::l 60 0 wa: 70I -Zw 80Ua:w 90" 234 5

NUMBER O f LAID- IN TERMINALS PER FOIL ELECTRODEFig. 15-3. Impedance at high frequency is reduced by adding terminals.

~ CAPACITANCE = lM f\\ -......--~ AT 1 kHz\\.\ \ EFFECTIVERESISTANCE

\\,....

..----- AT kHz

.... .... .. de RESISTANCE OFFOIL ELECTRODES------ 1--- ______

conduction path for heat from the inside to the outside ofthe unit, extended foil construction is advantageous inhigh-power capacitors having large heat dissipation. Fig.15-5 illustrates the relation between heat loss and frequency in a waxed paper capacitor.Electrolytic Capacitors

The electrolytic capacitor provides the most capacitancein a given space at the lowest cost per microfarad. Primarilya filtering capacitor, this type is largely used in connection with de circuits at working voltages less than 500volts. For example, at low voltage, several thousandmicrofarads may be contained in a one cubic inch electrolytic capacitor using etched aluminum foil electrodes.Fig. 15-4 shows a typical eleCtrolytic capacitor design.The high capacitance per unit votume of electrolyticcapacitors comes from the extreme thinness of the dielectric which is an anodic oxide film previously built up by anelectrolytic process on one of the foil electrodes, known asthe anode (capacitance per cubic inch is inversely proportional to the thickness of the dielectric). The thickness of this insulating film is but a few millionthsof an inch and the working voltage gradient can be of theorder of 10 million volts per inch. Etching the anode increases the effective area so as to increase the capacitanceas much as 7 to 30 times.

With the voltage applied in one direction. the film hasa high resistance to the flow of current and behaves like adielectric. With the voltage reversed, the film behaves likea relatively low resistance and.. i f the voltage is highenough, it passes large currents, heats 1!tp and soon breaksdown. Because of this unidirectional property, the film is154

suitable only for direct voltage in a single direction anthe anode terminal is usually marked "positive" to indicatin which direction the voltage shall be applied.ElectrOlytic capacitors are used extensively in lowvoltage ac applications. One type consists virtually o

two capacitors with their cathodes connected togetheso that the two capacitors operate in series but in oppositdirections. One capacitor absorbs the applied voltage oone halfof the ac cycle and the other capacitor comes int

ANODECATHODE

..J..h ki ,':,;',.... .'. J'.!, c-', "f'

FOILPAPER

Fig. 15-4. Cross-sectional view o j a typical capacitor.


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capacitor foil105

2


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electromagnetic and other applications of aluminumTABLE 15-1

Chemical Composition-Maximum Allowable Impurities in Weight Percent

I Iron & ! r MinimumAlloy Silicon Iron Silicon Copper Manganese Titanium Magnesium i Zinc Other Aluminumi1235 ! I .65 .05 I .05 99.351145 .55 .05 .05 .03 99.451180 .09 .09 .01 .02 .02 99.801188 .06 .06 .005 .01 .01 .01 .02 .01 99.881193 .04 .04 .006 .01 .01 99.931199 .006 .006 .006 i i .006 .006 .002 99.99

Anodized Foil: High purity aluminum foil is speciallytreated to provide a very thin oxide film on its surface.This film acts as a dielectric and results in high capacitance as compared to paper capacitors. It can be etchedto increase the surface area 1 to 30 times. thereby providing even greater capacitance in a given volume.A typical group of product data tables of one aluminumfoil manufacturer is reproduced here.

Table 1 5 ~ 1 gives chemical composition of the aluminum allomost used in condenser foil production. It is to be recalled, in thconnection, that the addition of other metals to aluminum usuallowers its electrical conductivity, Also that heat treatment puttinother metals in solid solution with the aluminum also lowers c oductivity,Table 15-2 gives typical properties.Table 1 5 ~ 3 gives thickness and width limitations,TabJe 15-4 gives welght-area conversion factors.Table 1 5 ~ 5 gives typical splice data.Table l56 gives foil toll sizes and weights.

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capacitor foil

TABLE 15-2 TABLE 15-4Typical Physical Properties-O Temper WeightArea Conversion Factors

Alloy Gauge Tensile-psi1199 .003" 5,0001193 .003" 8,1001188 .003" 6,3001180 .OO3 tt 6,5001145 .003" 10,0001235 .003" 10,500

TABLE 15-3

% Elongation Thickne. . lin.)3.2 .000176.6 .00025.7 .000236.0 .000257.0 .000308.1 .00035

.00040

.00045

.00050

.00055Thickness And Width Limitations .00060

.00065

I Sq In./lb ISq Ftllb !lb/432,000 Sq In."60,300 418.75 7.1651,300 356.25 8.4244,600 309.72 9.6941,000 264.72 10.5434,200 237.50 12.6329,300 203.47 14.7425,600 177.78 16.8822.800 156.33 18.9520,500 142.36 21.0718,600 129.17 23.2317,100 118.75 25.2615,800 109.72 27.34

Alloy Gauge1235; 1145 .00017 . .0002 .1235; 1145 .0002"-.00023"1235;1145 .00025"1235; 1145 .000311

. 0 0 0 0 0 f ~ _ . O O O 4 ~1235;11451235; 1145 .00045".001"1235; 1145 .0015".0059"1235; 1145 .002 . .0059"1180; 1188 .0004"-.0015"1180; 1188 !.002 ...0059 .1193; 1 1 9 ~ ,.001 ....0015..1193; 1199 .002",0059""MIS des.gn.tes Matte one side.2SB designates Two sides bright.

Finish" Widths 14,6000007 101.39 29.59MISMISMISMISMISMISMIS,2SB2SBMIS2SBMIS2SB

3/8 I ~ 2 6 t 13 / 8 u ~ 3 1 u3/.43"

.3 /g " ~ 5 0 u3 I a u ~ 6 4

3 / 8 " ~ 7 2 "1/4 N.72"I /4 ~ 5 2 "3/s".36"1/4 "36"3 /8 "36'*1 /4 u36"

.00075

.00060

.00085

.00090

.00095

.0010

.0015

.0020.0025

.0030

.0035

.0040

.0045

13,66712,80012,05811,40010,78910,2506,8305,1304,1003,4202,9302,5602,2802,0500050

.0055 1,860

94.91 31.6188.89 33.7583.74 35.8379.17 37.8974.92 40.0471.18 42.1547.43 63.25

84.21_5.6328.47 105.37126.323.75

20.35 147.4417.78 168.7515.83 189,4714.24 210.7312.92 232.26

432,000 sq. in. signifies one ream (500 sh ts) of 24TABLE 15-5 in. x 36 in. sheets.

Splices (Annealed Foil-Dry or Slick)G a B ~.00017"-.0004".00017"-.0015".002 t1 -.OO5"(Electrolytic Foil)

Width23u MaximumAll WidthsAll Widths

SpliceKnurlFoil Tape(Electric Weld)(Ultrasonic Splice)

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electromagnetic and other applications of aluminum

Width114 11 3113n -311#

1 /4 "_3 813"_72##

171i_72u

TABLE 15-6(a)Roll Size

Type of Core1 SI I , ' Aluminum1 5/ 16 #1 Aluminum3" Aluminum 3"" Aluminum 3U Iron

Maximum 00 6"

12H 8"

13"30"

TABLE 156(b)Roll Weight Data-Unmounted Foil

SPOOLED ROLL WEIGHT OF FOIL PER INCH OF WIOTH-(POUNOSIOutside ~ i a m e t e r ALUMINUM CORE IRON CORE

(lnehes) 10-1-5/16" 10-3" 10-21/2" 10-3"00-11/2" 00-3-3116" 00-3" 00-3-1/4"

2" 0.31b - - -2%'" 0.3 - - -3" 0.53%" 0.7 0.21b 0.31b 0.21b4" 1.0I

0.4 0.5 0.44%U 1.3 0.7 0.8 0.75"!

1.7!

1.1 1.2 1.15%" 2.1 1.5 1.6 1.56" 2.6 2.0 2.1 2.06%" 3.1 2.5 2.6 I 2.57" 3.6 3.0 3.1 3.07%" 4.1 3.5 3.6 3.58" 4.7 4.1 4.2 4.18y"" 5.3 i 4.7 4.8 4.79" 6.0 i 5.4 5.5 5.49%" 6.7 6.1 6.2 6.1lOU 7.5 6.9 7.0 6.910%U 8.3 7.7 7.8 7.71,." .., 8.5 8.6 8.511 Y,," 10.0 9.3 9.4 9.312" 10.9 10.2 10.3 10.212%" 11.8 11.2 11.3 11.2131"1 12.8 12.2 12.3 12.21314" 13.8 13.2 13.3 13.214n 14.8 14.2 14.3 14.214%U , 15.9 15.3 15.4 15.315" 17.0 16.4 16.5 16.415%U 18.2 11.6 17.7 17.6

NOTES: 10 and 00 dimensions represent the Inside and Outside Diameter, respectively, of the metal COre.The above figures are approximate and do not include core weight.For approximate net weight of Foil per roll, exclusive of core weight, multiply the figure under the applicable roll00 and type of core by the inches of roll width.

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aech chapter 15

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