AU500552B2 - Commutators - Google Patents

Description

819 03/n5 Form 0 AUSTRALIA 37

Paeta Act 192-1962

DCLARATION IN SUPPORT OF A CONveWTIOW APPLICATION FOR A PATIhNT ON PATENT OF ADDITION

In 3upport of the Cconvontion a Plicatian rvada for a patent4*i+ o~-#~for an invariti -i egntitlad COMMUTATORS

1 I1$HEAHT WTL.'jU1THSY GRACE Of Gb-74 V Ittevia Street, London SWA, England do salwanly and zincealy declare afe foliowa.

1. 1 a0mauthois~ed by NATIMAL PESEARCH DEVELOIPMENT CORPORATION th ppiat o teptaiift to-make this decartion 6 on~ its 130hO-f,

2,r~ The baic application ais dtfinted uy Section 141 of the Act was maein the United Kingdomr on i ne 1974 6 by .JAiS JOHN~ UATE 6:3. WES JOH 014ATES3 of 17 ParX Avenu6, Shi-ivonhma Wiltshirs, tLngland is tth' atoak! inventor of the invention and thB facts ujpor which thea torp ionts ave fe-ntitled o makia the application ot'e as follmis;- 0: to6Th appicants 4re thle asignso of the actual inventor.

64.. The Dealc application rforred toa in pragraph 2 of this beclartition thti f~rat application m~da in a Conveontion country In respect of tho invantlot the autiject OT the application.

Declared at London, England this 27th 0Y1* ~a

Tu: The Cammirsioner of Patents

~-6JUN 19T

A;J- 3/75 jo t(11)

(12) PATENT SPECIFICATTON 1 I A WA.$S14- 1..t

(19) AU (21) 81903/75 500,552 (22) 6.6.75 (23) 6.6.75 (24) 10-6.74 (31) 25623/74 (32) 10.6.74 (38) OB (43) 9.12.76 (44) 24.5.79 (51)2 HURj 39/04 HO1R 39/24 E09L 13/10. (54) COMiUTATORS. (71) NATIONAL RSEARCH DEELOPM-NT CORQRATI0N. (72) BATES, J.J, (74) WM (56) 77105/75 02.2 60253/65 403461 02.2 (57) CLAIM 1, A comuator comprising a pluxali ty of conductive segments electrically isolated from each other and each having a first loundary portion, extending in a, direction su.b- statitially nomal to the direction of rotation and. a further boundary portion extendizg from one end of t he first portion so that when t'he commutator rotates in a predetermined divaction and a brush bears uniformly thereon, the brush being of -'.1longate rectangpular form and extendJ'.Lng in a direction normal to the direction of rotation, the area of c between the brush and a segment irncreases initially in a subs ta'tially stepise nan'aer and increases subuently at a mean rate which progressively declines.

18.2

i at's

'Form M~MMONW19ALTH OF AIJS1RALIAT *'O PATENT19 A(:T 1452,69

CODMPLETE SPE'CIFICA-TIJON JORtINALI

8 19 0 3/7 Chin Inu. Cls Application Nuwr. L'odqed;

7' .Comiplete Specification Lodf Accepts4'

4 a A.USt"A14A 4

4 4 4 L .7, Ralatod Art P'ATENT oQr:Cj

4. 477 4 4

4. 4

Ntmo of Applic2ant: MIT'XONAL RUMBAROTH DEV7OM GL1JI OROPATIONT

Addreti of Applicant.. 66-74. Vicori~a Streot, L~ondoni, Bnalaid.

Actual Inventon JAIES j0Uim BATLtEs

Ad'esm for Sotvtca: BOWfl. WAT)~Rs SON$, 5 0 WE ER

Compleae Specifkio for the. Invention entitied:

00O1WhTATOPS 4

Tbefollo~s~tnen f l riptioo ofthiti wnion, lcluInO thO best method of performing, it kinowni to:t-

This invention relates to commrvutators for uso in electrical

machipes and has particfalar applirnation in fractionel-horse-power

machines empl1oyitig co-fibre brusheig.

The purposo of commutation is to reverse the direction of

currei flow in the successive co!,ls of an armature -winiding as these

P coils move past a certain6 point. and so maintain constant direction of eDcurwrent flow in that part Ox -rhe armature winding that lies to the C>right of.-this point -and conotant direction of current :Clow in that

V-4 part OT the arniAcure wintding that lies to the left of this poinlt.

The Armature windings are connected to a, series of closely-spaced

cmiductive segments mounted on the armature shaft so that sliding contact is made -with a btush in the form of a~ carbon block connected

*to tile external, circuit. The segment with which the brush. is in.

contact at Any moment then becomes the point of reference Xor the

15 process of ciirrent reversal, The segments are usually mounted on. -thle cylindrical isorace ot a druim in Which case the insulating

:.::boundrjez- oC 'tile segments are straight lines parrallel 'to the axis of rotation Altern)ativey the commutator may comprise a face-

plate lying in a plano normal to the axis o~f rotation in w-hich the

boundaries of tho seqtments ara radiAl ~lines. As 'the armkature rotat the brush maken contact altenatl

:::with a sinole 4epment and thiet vith that segment and th succeeding, OIT SIMultftokifly, Puiring the second of these periods, it should ideally be Arranget4 that the current dornsity at the trailing edge of the brush is glways held beIQ*w a citical level at which sparking will occur and falls to zero before contact is hrioken, One of the factors determining these ;o-adition.6s is 'the rate of chianxe: of totitact resiatauce between the brush and, each of the two segments. In the use of a conventional solid carbon brush the resistanice depends on contact at a few raadtlitly distributed poirncs on tfte brush surface and it is not possible to control the rate of change with any precision. A carbon fibre br-ush however (or other type of brush having flexible elements) has

'-4 many uniformly distributed points of contact so that the resistance of the brush varies to a much m~ore precise degree in inverse proportion to its area of contact, it then becomes

pt~cticable and is an object of the invention to control the change of resistance so as to optimnise the current density at the brush face.

According to a first aspect of the invention a. confimutator comprises a plurality of conductive segments electrically isolated from each other and each having a first boundary portion extendinrg in a direction substan-tially normal to the, direction .0 of rotation and a further bounday portion extendinag -fromt onre end of the first portion so that -when the couumute'tor ;eotates in a predetermined direction aitd a brush bears uniformly therecn., the brush beiag of elongate rectangular form -and extending itt a direction nor-mal to the direction of rotation, the area of

contact between the brush and a segmnent increases initially in a, s~bstanti~lly stepwise manner and increases subsequently at a mean rate vhich progressively declines.

According to a second aspect of the invention a connutator

c comprises a plurality of conductive segments electrically

r-4 isolated from each other and each having a first boundary 00

portion extending in a direction substantially normal to the

direction of rotation and a further boundary portion

extending from one end of the first portion so that when the

comuintator rotates in a predetermined direction and a brush

bears uniformly thereon, the brush being of elongate rectangular

S S

3a form and sectending in a direction no-nal to the direttion of rotation, the area of contact between the brush and a segmeant increases initially in a substantially stepwise manner and subsequently in a substantially linear manner.

Preferably in each case the initial s tepwise increase in

the area of contact lies in the range 0.25 to 0.75 of the 4 0 0 maximum area of contact. 4 C) 0000 cv:, An electrical machine incorporating a commutator according *004 0*

*0 r-4 to the first aspect of the invention may have a brush arranged 00 9: to bear uniformly thereon, the brush being of elongate 0* rectangular form and extending in a direction normal to the S*

direction of rotation, the arrangement being such that, for a

designated speed of rotation, the mean current density over

9 the area of contact between the trailing edge of the brush and

a segment during commutation is substantially constant. 9.

An electrical machine incorporating a commutator according 4

to the second aspect of the invention may have a brush arranged i 2 20 to bear uniformly thereon, the brush being of elongate

rectangular form and extending in a direccion normal to the

direction of rotation, the arrangement being such that, for a

designated speed of rotation, the initial and final values of

current density over the area of contact between the trailing

edge of the brush and a segment during commutation are

substantially equal.

Embodiments of the invention and its manner of operation

will now be described with reference to the accompanying

drawings in which:-

Figure 1 is a diagrammatic representation in end-elevation of a face-plate comutator according to one aspect of the

V invention;

C 1

C) Figure 2 is a diagramnmatic representation of a portion

or. of a face-plate conmutator of the kind shown in Figure 1,

connected to an associated winding;

Figure 3 is a graph showing the variation of brush

current-density during commutation in relation to commutator

segment shape;

4a rigur-e 4 is a diagrammatic representation of the form of comutatr segments accordin-g to a further aspect of the invention;

Figure 5 is a diagrammatic representation of modifications to

the segment forms of Figure 1 or Figure 4; and

tFigure 6 is a diagrammatic representation of a carbon-fibre

brush for use with commutators of the kinds shown in Figures 1 to

Referring to Figure 1 a face-plate commutator 1 comprises a

-4 disc 2 of insulating material arranged for mounting by a central 00 hole 3 on a spindle of a fractional horse-power motor (not shown).

Five segments 4 of conducting material applied to the surface of the

disc 2 are arranged uniformly about the central hole 3 between the

inner and outer edges 5 and 6 of the effective area of the disc 2. 9999

The segments 4 Pre electrically isolated from each other but are

5 closely spaced and complementary in profile, each adjacent pair of

segments 4 being separated by an insulating boundary 7. Each

boundary 7 extends radially outwards from edge 5 for half the

distance between the edges 5 and 6 and continues smoothly in a clockw.ise direction on a curve which terminates at edge 6 at a

point 8 displaced by 600 from the initial radial direction. The

radial position of any point on the curve is linearly related to

its angular displacement from the initial radial direction. From

a point 9 on the edge 6, which is 100 anticlockwise of the point 8,

a radial cut is made to remove a small area of conductor between

8 the boundary 7 and edge 6. Each shaded area 10 represents

strictly only the contact area of ant inclined carbon fibre brush

n in il li

(not ohown)v having its major dimension in a radial adiecti.ol

Detween thte 1dges 5 and iut fokr cnveniece will be ,Et-r -Aed to

to the brush itself. The brush dimensions are such that during the

rotationi of the commatator 1 the contact area of the brush 10 is

confined momedtrily to each of the segments 4 in turn and at no

point during rotation extends to more than two of the segments 4.

0 For a commutator 1 of 2.5 cnts diameter the dimensions of the

contact area of the brush 10 are 0.8 cm x 0.2 cm. The form of the

segments 4 prevents the use of the commutator 1 other than in OO anticlockwise rotation but it will be understood that the form of

the segments 4 may similarly be designed to accommodate clockwise

rotation.

The f Ce-plate commutator is particularly convenient in that

S its dimensions are not determined by the size of the motor and may

S1 1 he b chosenl to provide any required degree of resolution of the s* 4

segment pattern in relation to the size of brush.

A cylindrical comniutator may equally well have a segment

pattern modified in a manner analogous to that for the face-plate

'type. In each case it is envisaged that the pattern would he

20 prepared by techniques known in ,the manufacture of printed-circuit

boards.

It is necessary in order to appreciate the derivation of the

segment patterns described in this specificotion to consider

briefly the mtechanism of commutation. Referring now to Figuae 21

m basic machine is sbov'n in which two adjacent segments 44 and 4b

of a commutator 1 are connected to ac tion 1% of a closed armature w winditko. A radially mounted carbon-fibre br~ush 10 is showrn in Aii.~

Oont+A^t withi the g~oqg~nt h4a during antialockwise rota:Linn of the

commtator I at the instant before the brush 10 makes contacat with

segmnent 4~b, The curr~ent In the segment 4a at that instant idlll be

refer~red to as I an~j the steady current density over the full areo,

of the brush 1.0 as Jo. The brush 10 is in full contAct with

~'sog?-ent 4a only within a riarrow sector of the commutator 1) the timne for the brush, 10 to pass from this position to tht c(rrespondinag

position. in segment 4b being the, commrutation time 11c. The var-ying

t0 current donsity over a keducing contact area between the bruoh 4 and segrn~nt 4 during the commutation tf.;ie To in referred to asf Ja.

:99.The inducteice of the wind-ino 11 is reLerred to as L and the

onotacot x siztancae for the Sull area of the brush 10 as R.

It is understood throughoot that the term 'resistance' as 'L pplied -to the, contatct between a carbon brush and a conducting

surface in a crent-caxrryinq circuit refers to the incremental ratio of voltage drop to outrentt and takes no account of7 any

standingq voltage drop.

i~t is abaracteristio oX a *wterial such as caybon Li~.e usei

20 for tho brush 10 that uniform, contact is niaintaifted on a microscopic

scale, which is loss likely to be achieved with a conventional zolid.

:bruish. As the commutator rotates the resistance IR of the brmsh *is therefore redistributed between the segments lia and 4b. ill

Iiverso px'cportion to the respective -areas of contact And If the

inductancea L were absent the currant I would be redistributed

correspondingly betweeni the segmeits 4a and 4,b in dirett proportion to the re peotiveareas oze co,1tact. The efct Of 'the i~dt&:n~ce L

4#*Alrav hnth +hA eoP of the current to seanen't 4a and tbe

growt~h of current to segm~ent 4b. Clearly if tiha rate of rotation

is 11i1h the aea of contact with seglfent lia will decrease rap~idly

while the current to that segment is decreasing slowly and

Ja will result P, underix-ably high val.ues of the current density CC Generally th rate of rotation will be chosen so that the

__commnutation title Ta in at lekst Oornparable with the inductive time

V4constant T whi.,'h is equal to the raltio L/R, the reoistance of the

winding It. beintg aasumed negligible.

Conmputat ionio have been mixde of the val'ues of Ja/Jo during the

commutation. period Tc for various shapes of segment and 'the results

are shown graphically in F'igur-e 3. Curve A refers to the shape of

IS sogment in the emboam~ent of Figures I and 2 in which the initial radi# l step is half the total radial distance between edges 5 and 6.

::If this rati!o is denoted as r -0.5 r a thexi by the same cohyention~t curve _0refer4 to thje condition r 0. 75 r;i

curve C refer.- 'to the Conit ion r -0.25 r and 1 0

.curve 0 refers to the condit.01 r 0 in Which there is no

initial step and the boundary' beoi" to curve immneditely.

The curved portion, of the boundary in each case tollows the

to laxw by which radial and angjular displacement are linearly rolated and in ench case the time constanit T is equal to Tc.

Referring again to Vigjure 2 we consicear the conditionis farising

whon the brush 10 is carried from its initially shown position to

a pasitjlon -12 011:1y 3jurt AcOross the 'boundary 7 frc selqmeat 4a, to segment 4b,. One half of the area oC the brus~h 1.0 (the leading edoe) edge) remIn~ns in contact with segment Iia. Instantianeovasly whilG there is no growth of current mn segment 4b, th(e cutrrent I in segin~t ka remains at its full value and -the current density Ja therefore rises to 2 Jo. This condl'.tiona corresponds inL Figure 3 to the initial value

CV (Ja/Jo) 2 for curve 4. The derivation of the ini'tial values of

__Ja/Jo for oth.er segment profiles may be similarly demonstrvated, -the

reclAtionship being Simply epesda J/o 1 l( 0 .I

will, be appreciated that the cur.-ent deonsity at the leading edge of

the brush 10 cwi never' exceed Jo, the problem to be solvNed is to

reSdLrict thle Value of J.A/jo at the trailing edge to a safe I eve.

It ig olear that in general A slow change in segment profile allowim Jix/Yo to remain low oarlV in the commutation period but,: at

15 the cost of higher, values at the point of breaking contact; a

.*stoep radial ste~pt for eXaemple r 0 r r75 must cause on inltia.lly

higher, value o,,r Ja/JYo but aoslow values later in the period and

is therafore, to be preferred to eitherL of the conditions r 0 o

r =0#25 ro Even when the -rati.o V/i-4 rises to 2 or excessive

current density is avoided by using the condition r 0.75 ro

It can be shown that the current densiity at the end of the

communtation period is defined ~by the expression

F (VTOs hi~hvalus a Jrr /t d,:therefore tends to Ver~y appoa~es uit igher values

oT the radial step are, thpwo;Coro appropriate to higher vausof

(JSa/Jo_) Vcini order to limit the value 0ci'

For a va.1~ie ot T/To which does not exceed unity however thre

P gmwvnt hape in which r 0. r prOVIdAe 4-1- moat- r-1

performatace of The designs considered, tho initial and terminal

values of JA/Jo b tIng equal at a Satisfactorily law level. It has

bedn aemonstrated in experimental trials that the power rating and

U) the rotational speed of a. p rticulor size of motor oar) be doubled

r~z without in oeasing the teriniP.l current by employino, a segment

doqign ot -the cufve A type inistead of either a conventionaml radial

Stype or-amrv D type.

It will be understood that although some members of a partiocxlar

family of segment shape have been shown ats preferabl~e to others in

dicsaig the eurves of Fiqure 3, that Aspect of the invention which

hAS been deaorlbed ith refe)-ence to Figuros 1, 2 and 3 is not

a.limited to thome members or to this family; it extends 'to all cases 0*14 in whith the cortact orea between the brush 4nd a segment initially

incr-ease tn a Stepise manter, and then continues -to increase in

o. aY maine'r which,~ having regard -to -the scale of construction1 is

Oerctlvely linear.l

:This oltass of sagment has hoori fotund to give Substantial

40 advwagos iii operation over the cnventional form While being simple" in b vrin The preoeding discussion h~wever provides

the basi- for conslderiing a broader ~f -,et the invention in *relation~ to Figures 4(a) ad As Can t"nalFogue OX theo race-

platte, commutator of Figixre 1, part of the developed ~Surtaco of aX S cylindric al oommtator 14 is shown, in Fivure U the Axial.

distance bttween the end races :t5 and 1I riou taken to be r tho portion 17 of the insulating houndary between adjacent

Se,,72-68 4A -d A Ift fi pnt 20 at the entd 15 in an

axial direction for a distance r. The boundary then follows a

linear path 21 at an angle to the axis to reach the- el -ac-e 16 at

a point 22 within the rbkt width of the segment 19. If i-fr

Sand the speed of rotation is such that T/Tc 1 then curve A of

Figure 3 is applicableo This curve indicates that initial and

terminal values th current density ratio Ja/Jo are equal but

that at intermediate points Ja/Jo is Ieduced.

'he possi litty therefore arises thaft the riachine efficiency

could be improved by orraglng that Ja/Jo should ramain uniform

over the whole period oC commutation. This result is achieved U~

as is indicatod iti Yigure 4(b) by locally reducing the area of the

segmuent 18 to be swept by the brush 23 after passing the boundary 17

so that the currant dernsity must be increased. The necessary

roduction In area io obtained by changing the path of the linear

boundary 21 -to a nojeic Irvx 24. It cani be shovrn that the

csoidir~es of any point on the required curve 24, y being

i measured axially from tn origin at point 20, are given by the

expression 2 Y ir/ro E,

::Assuming, as before, that the commutating w-nding 11 (Figure 2) is

of negligible resistance a saevxent having a step 17 :Collowed by a

curve 24 gives constant curretnt density during Commutation at a

speed simh that

-(2r/r C)/ 7 rrj

Thns for r/r, 0.5 the segment form of Figure 4(a) Vould follow

,P F4 gutrn 1 t6 curve ft of 6,Vt A naximinj current density JEa/J 2

for a speed such that T/Tc 1- for the namovalue of r'/r th4e

boundary 24 of F'igure Yb) would give constant current density

j T Ja/Jo rs 2 at a speed such that T/re 4/3c hence eneblfrng speed

Sto be increased saely by As an indication of the dilmensional

c difaeron- e between the paths 21 and. 2.4t for an axial length of

25 mri the mxinmum deviation D4 about I mm corresponding to a

T-4 reduction in the axial dimension of the segment 18 at this point

by

The designer iA theretor6 enabled, as a first approximation, to

derive the valuo of r/r £pr vehich current density is constant by

Inserting in equation the value of the time constant T'for the part of the maL-hine winding being comuiitated, and a designhted

5 speud of rotation, From this value of r/r the corresponding curve 24 t can be constructed by applying eqorntion

*the regIstance of the ldinq Ui. Wureo 2) has so far been

asumed negligible but in practice the resistance is Higniicant arid

a correction must be made for the cohoequential decrease in the time

too ao constant T. It can he thown tthat the efteot in to eAblo the

narrowing portion of the segment 18 to be f:urther reduced, while

nmaintaini constant current density at a selecttd speed.

A genral design procedure not hitherto available hag been

derivod and is set out in the following stops I to 9. The

pcocdure onables a revised boundary 25 to be determined taking into

count windlrii resistance and other 'machine circuit parameters.

The oegtnents 18 and 19 are assumed -to be conneatod acros csa

wihdiiia 1i as was shown in F'iure 2.

I. A valla is nominated for the current density Yii at the tra4116g

edge of the brush, to lie maintained in the segment 18 during,

:ommutation at a level jut below the denisity whicoh ib found to

69) ausoje fsipakin

Y 2, A sto ratio r/r a is chooen for the preliminary comuutetion.

$3 In the segment 19, duiring commutation. on segment 18 the Initial

current density at the leading edge is zero and the final

oreht density Jo am' (I The voltage drop V betWeen a

64: bruah (of width r Anid uniform thickness) and the segment

a miaterial is established exporimentally kor the current

dennitites O(W )i JO(V' and Ja(V A factor R analogous to

4 w dirfarontial resistatnc is then determined graphically as 1 (V N.52 VIV/J o.r.

4~ ~The induotance L of tne winding 11 is determined to give the

time onstant T L/fk.

u 5 The resistaice Rw of the windixig 11 is detexiiined, to give a

ratio fl RW/R.

ac0 6. The voltages whieb arise to assist or oppose commutation are

z V-V

(ii) A vltage of net valicU E acting in the commutating tiil 11.

This value may be positive if the opposing effect of

self-induction is overcome by r'oking the brusbes into the

magnetic field of a machine but may be negative for some

modes of Operation wd some typess of m chlne.

I_

If -the curront to bG comrnitated 3s I t value is derlvea1 for

C E)/

7. To simplify the relationships we set:

c (1 a)R i2" (1 a)e i

d (C 4- 4aR1)

Then the coordinates x"1y, of a point on the cu"VO 25 are

given by the expresion; C 2 xi log e d 2RY 1 c)(d 4R [oge 2 ey I~d 2RY1 c)(d 2R a ca a

o log, d 2Ft1 o)(d 2Ra L c) -l ;dZ1." 7RA f i

whiah may be :'eprsented by the f'orm:

x lo.g log. EBI 15 iog, CC log [i :8 a Eution defines tile segment shape for which Ja is held

constant at a ipead fot which:

To T/jd) lJoqOjiU- log. LJ

It mtry be found that for initially chosen val 'cc of ro and a

there is no solution, to equations And and it is necessary

to repeat the calculation on a trial and error basis until

tisi-actory vlues of ro and a have been found.

I L i d X

L r I r

A further possibility Which may be considered in step is tht

the seament material may be made non-uniform in resistance.

Alternatively, the normally desirable property of a brush constructed

from carbon-fibre or other flexible elements, that its contact

resistance varies in precise inverse ratio to contact Irea, may be

modified. It,is considered to be a further advantage if the resistance

CM3 of the brush is made as low as possible in the leading edge which makes

the first contact with a segment and is relatively higher elsewhere. C Referring to Figure 6 a carbon-fibre brush 30 comprises a row of CK) Uniform tufts 32a to )ge of fibre secured in a conductive brush- .9 99 *i 9 9 casing 33 having a mounting atid connecting point 34 for external 9 sees wiring, Th us the brush 30 is mounted so that the fibre tufts near 99 9* 99 the and,"tutt 32i form the leading edge and those near the end 32e the

trailing edge. The fibres in the portion of the brush 30 nearest the 9.

9 9 99 45 end 32a are arranged to make contact directly with the conductive

99 brush casing 33. The fibres near the end 32e are however prevented,

by an insuiating layer 35 which lines that end of the casing 33, from .9. 9

9.~ 9. making direct contact with the casing 33. The current path through

.9 the brush 30 from the area near the end 32e to the opposite end of the 9 9 9 .9e 20 casing 33 is therefore dependent oin lteral cox'tat between fbres, 9 9. for which the resistance is much higher than that along the tibree. 9 9 9

999 A segment shape hao been derived for a cylindrical coimmutator,

and illustrated in Figure which permits the highest possible

speed at a uniform current density. The boundary opUation may of

ourse be transfaormed in radial coordinates for a. face-plate

comutator, a basic machine incorporating the commutator 'then appearing closely similar to that shown in Figure 2 with the boundary 7 modified to sptisfy the equation of the boundary ahown in Figure I has been found that the sarface condition of the segment 19 differs between the area swept by the brush 23 after crossing the inclined insulating boundary 25 and the area

Sswept by the brush 23 after crossing the step boundary 17. The

C surface condition appears to be correlated with brush-wear and

this can be made nmore uniform and brush-life extended by

Sconstructing the bdundary 2 ae a series of smnall step-boundaries. CO Figure 5 indicates such a. stpped boundary 26 for which the curve

S 0 represents a mean position, 0000

Figure 5 also shows as an extension of the segment 18 a narrow sees atrip 27 extendine Axialy from the edge 16 to form a spur from the *0

00000 boundary 25. The purpose of the strip 27 is to reduce the effect

a* 15 of any tnderpy for sparking to occur as the trailing edge of the

brush 23 leaves the segment 18 by distributing the current at this

Sposition over a larger are. The strip 27 may be typically equal

in width to tti brush 23 And betwaen one-third and one half of the

0 length of the brush 23.

Modifications similar to the stepped boundary 26 and the 00

strip 27 of Figure 5 could equally be applied to the boundaries,

such as the inear part aof the boundr.ry 7i of the segments of the

cotmnutater of Figure 1.

In the most general terms the invention is directed to the

construction of forms of Uomtmutator and of machines incorporating

them hWving the ability to 1reverse a given current in the minimum

s~ time, and hence at the highest speed, withoat sparking, This

Ability' in secured According to one Aspect of the inVentioi, by

shaping the cottmutator segments so thAt the current densitly over

the trailing edge of the brush is maintainied At o.t ubstantiall,

constant value just below the limiit at which sparking commences,

SA x469ent shape -which -enables this ideal, condition to be cosely

Sapproached has been descr~ibed with, referen~e to Figure 4. and, a

C shape of practical value for which the ideal condition may be

V4 approximated has been descriLbd Vwith reference to Figures 1, 2 and 3 Con

Claims (1)

1. 4M. 7 'T'

   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

   1 A miie~vr p~nrL~n nl'urglitv of coiductive

   segments electrically isolated from each other and each

   having a first boundary portion extending in a direction

   substantially normal to the direction of rotation and a

   further boundary portion extending from one en! of the first

   D portion so that when the commutator rotates in a predetermined

   direction and a brush bears uniformly thereon, the brush being of elongate rectangular form and extending in a direction

   normal to the direction of rotation, the area of contact

   between the brush and a segment increases initially in a

   substantially stepwise manner and increases subsequently at a

   mean rate which progressively declines,

   *Poo 2, A commutator comprising a plurality of conductive

   segments electrically isolated from each other and, each

   having a first boundary portion extending in a direction

   00 00 substantially normal to the direction of rotation and a 96 further boundary portion extending from one end of the first V..

   portion so that when the commutator rotates in a predetermined direction and a br-,sh bears uniformly thereon, the brush being

   of elongate rectangular form and extending in a direction

   normal to -the direction of rotation, the area of contact between

   the brush and a segment increases initially in a suabstantially

   stepwise manner and subsequently in a substantially linear

   manner.

   3. A conutator according to Claim 1 or Claim 2 in which

   F,

   the initial stepwise increase in the area of, contact betweert

   ,the brush and a segment lies in thc- range tW 0.75 lof the

   -maximum area of contact.

   4. A cormntator according to Claim 1 or Claim 2 in which the initial stepwise increase in. the area of cvatacx M between the brush and .9 segment is one half or- the 'maxlimum area of contact,

   A commtatoxr according to any preceding claim in

   which the fuirther p)oriLon c-f each 6egment boundary comprises a succession of small stepwise excursions about th-e desired mean position of the boundary, each step rising pc-rpendiculavly to the direction of rocation.

   6. A conmmtator according to any of the preceding claims inwich the segmeiats lie in the suirface of a disc.

   &VG! 7. A cummutator according to any of the claims I to 5 in

   which the segmnents lie in the irufenalsurfa ce Of a. cylinder.

   8. A commmtator subs tandi-lly as described hereinabefore

   with r-ference to Figure 1.

   9. A comutator substantiallyv as described hereinbefore with ref-erenoe to Figtire 4,

   counutatox substantial.ly as describad herei'rhefore with refereace to VF~gu c

   An electrical machine incorporating a coiffutator

   4according to Cl~aim I and a brush arranged to beat uniformly thareor', the brush being of elongate reatangular for 'and

   extending in a direction normal to the direction of rotation, the arrangement being such that, for a desIg'nated speed of rotation, the mean current denisity over the area of contact: M between the trailing edge of the brush and a segmient during C>comirutation is s'ubstantially constant. 0

   09~

   M An electrical machine incorporatinag a comuutator

   according to Claim 2 and a brush arranged to bear uniformly thereon, the brush being of elongate rectangular form and extending in a direction normal to the direction of rotation, 00

   the arraftgement being such that for a designated speed of rotation,~ the initial and final values of mean current density over the area, of contact between the trailing edge ovf the brush, and a segment during comttation are substantially equaal.

   114, An electrical machine incorpoxatinag a commutator

   according to any of thve Claims 1 toA4. and having brushes, adapted to react flexibly to contact pressure at each elemenatal ~reat of contact with a sagment,

   An electrical machine according co Claim~l or Claimn~ ha'$.ng bxushes adapted to react flexibly to contact pressure a.t each. elemental area, of coutact -with. a segmenL.

   416. Ani 4lectrical Mac),ine according to ClaimYJ4 or 61'arft in which each brush, compriset cartonised fib'rou-s or lamaT LX tatril

   An eleattical machine according to GCiui1,6 in, whidi each brush compriLea a stack of carboa fibres so arranged for extersna current conneation that the curren't path to the portion of the brush which forms a trailing edge contact 'A) 0

   4 surrface during commutation is of greater resistance than that 0

   00 to the remai-ning portionLi of the brush.

   S~

   *9 9

   9 0 g

   Dated this 23rd day of August. 1978,

   09 9 *9

   .9. N1ATIONAL 4 USEARCff DEVELQOPMENT CORPORATIOQN 99

   9 S

   .9 59 r 9

   59 0 99 EXDWD. WATERS SONS, Patet Attoneys, Queeft Street, Nelbourne, Vic,, 300O Australia.

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