US3753180A - Rotary solenoid - Google Patents

Abstract

A rotary solenoid comprises a ferromagnetic core assembly flux linked to an electromagnetic coil. The core assembly comprises confronting pole pieces having flat face portions spaced apart to define an air gap. The rotary solenoid includes a rotary assembly comprising a shaft adjacent said pole pieces journalled to said core assembly for rotation about an axis perpendicular to said pole faces. The rotary assembly includes an armature plate fixed to said shaft and projecting outwardly from said shaft for entry into said air gap. The rotary solenoid includes non-magnetic spacer members, one disposed on each side of said armature plate, said spacer members sized with said aramture plate for a close fit between said pole faces, said spacer members cooperating with said pole faces to restrain said rotary assembly against axial movements.

Description

United States Patent [191 [111 3,753,180

Sommer Aug. 14, 1973 ROTARY SOLENOID Primary Examiner-George Hams [75] lnventor: William F. Sommer, Brookville, Attorney-H Talman Dybv'g Ohm 57 ABSTRACT [73 1 Assign: Dayton Ohm A rotary solenoid comprises a ferromagnetic core as- [22] Filed: May 23, 1972 sembly flux linked to an electromagnetic coil. The core assembly comprises confronting pole pieces having flat [21] App! 256173 face portions spaced apart to define an air gap. The rotary solenoid includes a rotary assembly comprising a [52] US. Cl. 335/272, 310/46 Shaft j n said pol pieces joumalled to said core 51 Int. Cl. H01: 7/14 assembly for rotation about an axis pcrpendicular to [58] Field of Search 335/228, 272; Said P faces- The rot ry embly includes an arma- 310/37, 46; 74/63 ture plate fixed to said shaft and projecting outwardly from said shaft for entry into said air gap. The rotary [56] R m- Cit d solenoid includes non-magnetic spacer members, one UNITED STATES PATENTS disposed on each side of said armature plate, said 3 147 4 9/1964 pp et al 335/272 spacer members sized with said aramtureplate for a 3I435I394 3/l969 Egger 335 272 fit between and members cooperating with said pole faces to restrain said rotary assembly against axial movements.

9 Claims, 6 Drawing Figures 1i}: ROTAR 'soLENolm BACKGROUND'ZOWTHE INVENTION 1. .Field rofithez-lnventiom Thisa'invention: relateszto rotary v solenoids and 3 more: '5.

largetrotarwstrokesr 2; Description :ofathe' Prion-Art- Arrotary; solenoid capable oflexes:utinggailargearotary, stroke-is illustratedin US. Pat: No. 3,4 3 523942 This-pa, tent teaches the :USC;:? of i an; armature whose apparent: centersofimassrcoincidesaw ithzthesaxis.ofarmaturerotaa tion. The. armaturei is-fithus gravitationally; balanced; about rtheiaxis aofirotation: To achieve:thisrgrayitationali balance, ,theearmature:istmade substantially symmetric; on; diametrically oppositee sides: of 1 its-,axisof rotationz The. arm atureprotateszin a. solenoid Lenergized r core-las-c. I sembly; which has two diametrically opposite; pairsxofi polespiecesz. Tonprovide upzto .l.80'armature;rotation; diametrically; opposite.sidszof-Zthe 1 armature-eare 'oppositely; canted; and; diametrically; opposite: ones of I the; polw pieces ;are" also; oppositely, cantedii, The; canting; gives the:- armature aposition 20f highxmagne-tic-t reluctancespacedxrotationallyql803"from:a'ipositionwof slow magnetic reluctance @and, hence, all 80- stroketcapabil ity in -..response-i,to :energizationwofithe,solenoid :coil. As the armature --moves-to tits-position"; of? low; reluctance; however; the; prior; art, construction is; suchw that a the; gravitationally;-balanced :armature-moves into a region" of :magnetic:im'balancetin whichzahigh ,magneticallyim duced torque 'couplewisdeveloped which :tends to twist:

the armature out of its plane/of irotationz; This; couple, su bljects the bearings. whic h-srotationallyt mount ithGLZI-fmature to a: substantial radialiloading'iwhich leads;;toiai 3 premature-bearing failure;

SUMMARY" OF; THE RRESENTJINVENTION,

Inthe; present: invention, canted ;surfaces--,0n:-the arr mature andipolepieces-have. beeneliminated and finstead. an armature plate: with. flat' parallel surfaces is permitted to move between 'confr'ontingpolefaces-hawing flat parallel surfaces This: constructionpermitsthe:

armature. to move; into. an air; gap between: the. pole pieceswithout atherdevelopment :of a rmagneticallyf induced force, couple such; as would radially: load i the bearings whichimoiuntathe shaft for-rotation; To assure. that the armature will move freelytbetween the aforementioned 5 pole faces, without: seizing; against; either} pole, face, non-magneticvspacers which engage: theopposite surfaces of :the armaturexare-sized Ito snugly; fiti into the airzgap,alongwithnthearmature. so asto. support the armature, against axialimotion while: progressing through ther'airgap;

Stated; briefly, the; prior; art' .hassacrificed,magnetic;

balance to; obtain it gravitational; balance, The, present;

60 thereof. The-tendplate116 has a similarly located .borez 30 Thebores 28;and 30.,receive sleeve-bearings 322and, respectively, which journal a-rotarv assembly comprising; a anon-magnetic shaft 1 36. .Fixedlymounted: to a theshaft 36.-by means ofrk-nurling -37.isLa-fe'rromagr ie-1tic;. 65, armaturey38r .The shapeof then-armaturea38 appearsdnw FIG; 4 where it canwbenoted :that ,thetarmature.38-is;a-v generally: semi-cylindrical vaplateorzannulus z-projecting 1'.

ect: Such a construction" iscnot feasible.when-thesarmaa tures is tovbee gravitationally balancedr Anobject:of'mepresent*invention'istozproyide axnew andlimprovedxrotary; solenoidideyice: 4

Another-objectofthezpresentzinvention iszto providex particularly; to rotary, solenoids capable: of executing, aarotaryysolenoidfin which magneticpzforcesetendingato.

deflect an-- armature. from: its: allowedv path 2 of rotary movementsareiminimizedr Axfurther:object ofitheipresentinventionis:to:provide l 0 a-frotary solenoidixtilizing anarmature provided withianz arcuate notch intoz-whichvstopzmeansztoalimit hth e :rotaetionwoftheearmaturermay projeet;

Q'theri objects andxadvantagesresidezin the::construc tionqof: parts, thestcombination thereof," the: method. of 1 manufacture; andfithe: mode; of operation; as. will E be:-.-

comeemorea apparent from:theirfollowingdescription;

BRIEE-DESCRIPTIION-ZOF"THE."DRAWINGS? FIG; 11 is. a.rperspective;view withe a portionibrolten" away; illustrating; a; rotary solenoid 11 devices embodyingthe present invention;

FIG 2Iiis -a :section :viewtaken substantially in ;the..-di+ rection indicated :by -thedine '2.;-2-(of=-FlG; l;

thecoil housing show'n;

FIG. 4 is'a section ;viewtaken .-substantially,in.theadi1 rectiorr indic-ated.by the-line.-,4 -4;of FlG. 2-iwith' the soi *lenoid;coil iremovedaandtheafull circumference ,-offthe coil- :housing slrown;

FIG ,5*isa.;section view: taken substantially in'zthe di-. rection-indicated.bythealine 5:5fofFlG lgwithvthetsolenoid :coil removed Zand;,the-;full 'circumferencesof the-,

coilihousing shown;

FIG; 6. :is. amfragrnentary perspe a. modification-.

DETAILED DESCRIPTION PREFERRED? EMBODIMENT? to the. opposite: ends ofqthewall member-12k, i

Aperturessuch asshown at ,l9l bored radially through'v the, wall member 1 2; there being; two 2 such apertures, receive tconductors,.20' 'wandn 22?: which connect 1 electrically; to theooppositetends of .thevcoil IBLEach-QfIthe. conductor-5:20. and22is surrounded with-insulation 24 and; is-.further protected from electricaLcontact-with; the-wall :member 12'; by; an insulating; sheath 26, The;

conductors-20 and;22;;providewaameansrtorzconnecting;

arsourceiiof power to the-s coil 118 l'so 8810; energize the coilf 18. The powertsource; not shown, is -:preferably 4 a directzcurrent source: Thetend, plate l4 has. anaxial,bore;28-:at-the-.center laterallyoutwardly from theshaft 36. The armature has ctiveeview: illustrating The :rotary. solenoid-illustratediimFlG: ,1 'rcomprisesa unitary ferromagnetic housing; 10 "which :enclosesvandm isflux-vlinkedixto anelectromagnet coil -il8'.-'.The ,-housing; 10.?comprisesa tubular wall Imember ;12'I.and;.end ;plates--t flat sides and an arcuate outer margin 39 connecting between a trailing radial margin 40 and a leading radial margin 42. The open area of ,the armature between the leading and trailing margins is, in reality, a l80 notch which, as will be described, receives a stop pin 54 used to define the starting and stopping positions for the armature. It can be noted in FIG. 4 that the armature is not an exact semicircle because the central portion of the armature defines an annular hub 44 which siezes upon the knurling 37 on the shaft 36.

The side of the end plate 14 which confronts the coil 18 appears in plan view in FIG. 3. Projecting axially into the coil 18 from the end plate 14 is a pole piece 46 formed integrally on the end plate 14. It can be seen in FIG. 3 that this pole piece 46 is generally of semicylindrical shape but has its diametric margin 48 interrupted by a first semicircular recess 52 receiving the stop pin 54 and a second semicircular recess 50 which is a margin of the bore 28.

The end plate 16 is constructed similar to the end plate 14 and has a generally semi-cylindrical pole piece 60 integrally formed thereon. The end plate 16 can be seen in plan view in FIG. 5.

The surface configuration of the end plate 16 can be seen in FIG. 1 and the surface configuration of the end plate 14 is the same. The end plate 16 has an outer, axially thin, annular section 61 'which is sized to receive the radial dimension of the coil 18. The end plate 16 also has a slightly thickened pad 63 which cooperates with the pole piece 60 to define the interior margin of the annular section 61 and thus support the coil 18 in concentric relation to the shaft 36. The pole pieces 46 and 60 have flat confronting generally semi-annular face portions which are perpendicular to the shaft 36, and which define an air gap between the pole pieces.

As best seen in FIG. 5, the pole piece 60 also has its diametric margin 65 interrupted by a first semicircular recess 67 receiving the aforementioned pin 54 and a second semicircular recess 69 which is a margin of the bore 30. The semicircular recess 52 described with reference to the end plate 14 and the semicircular recess 67 described with reference to the end plate 16 are produced by axial bores through the end plates 14 and 16 which are sized to receive the pin 54. The pin 54 is press fitted into the end plates 14 and 16 during the same operation by which the end plates 14 and 16 are press fitted into the wall member 12. Prior to this press fitting, however, annular spacers or bearings 62 and 64, which are non-magnetic, are positioned about the shaft 36 and in contact with the opposite sides of the armature plate 38. Also, prior to the time the aforedescribed press fitting is accomplished suitable mounting studs 71, which may be threaded, are countersunk in suitably located apertures in the end plate 14, these mounting studs appearing in FIG. 1.

The extent of axial movement of the end plates 14 and 16, one toward the other, permitted during the press fitting thereof into the wall member 12 is controlled so that the air gap between the confronting pole pieces 46 and 60 will remain sufficient to receive the combined axial thicknesses of the armature plate 38 and the spacers 62 and 64 without frictionally restraining free rotation of the armature plate 38. This control may be achieved by assigning proper tolerances to the axial thicknesses of the pole pieces 46 and 60 and to the axial thickness of the wall member 12 and allowing the press mechanism which presses the end plates l4 and 16 into the wall member to bottom against the end surfaces of the wall member 12.

Affixed to the outer surface of the end plate 14, as by spot welding, not shown, is a cage member 74 which has an annular ring of upright posts 76 struck upwardly therefrom. The posts 76 form a cage for receiving a spiral spring 66. The inner end 68 of the spring 66 is fitted into a suitably shaped notch, not shown in detail, formed in the shaft 36. The outer convolution 70 of the spring 66 has a tongue72 struck outwardly therefrom for hooking engagement with any one of the posts 76. The particular post 76a selected for hooking engagement by the tongue 72 is so selected that a desired bias will operate on the shaft 36 to press the leading margin of the armature 38 against the pin 54. Having reference to FIG. 1, the bias operates to press the armature plate 38 in the counterclockwise direction against the pin 54.

As best shown in FIGS. 4, 5 and 6, the central axis of the pin 54, which serves as a stop pin positioned in a 180 armature notch, is aligned with the diametric edges 48 and 65 of the pole pieces 46 and 60. The diameter of the pin 54 is sufficient that when the trailing margin 40 of the armature plate 38 engages the pin 54 the leading margin 42 is displaced rotationallya small angle into the air gap between the pole pieces 46 and 60. By way of illustration, this angle may be 10 with the result that the leading margin 42 of the armature 38 is able to rotate in the counterclockwise direction as appears in FIG. 4 to engage the diametrically opposite side of the pin 54.

It can be noted that the leading margin 42 of the armature 38 never leaves the air gap between the pole pieces 46 and 60. Likewise, it can be noted that the trailing margin 40 never enters the air gap between the pole pieces. Since the leading margin 42 remains always in the air gap, it is apparent that the leading margin 42 will encounter substantially the same magnetic flux, when the coil 18 is energized, regardless of the rotational position of the armature 38. Accordingly, with the armature 38 in its starting position, as illustrated in FIG. 4, an energization of the coil 18 will cause the armature 38 to progress in the clockwise direction, delivering always a substantially uniform torque to the shaft 36 until ultimately coming to rest when its leading margin 42 engages the stop pin 54. Such motion can be characterized as a movement from a starting position of high reluctance toward a position of lower reluctance to the magnetic flux produced by the coil 18. One of the particular-benefits to-the construction described is that the preferably, but not necessarily, non-,

magnetic spacers 62 and 64 coact with the flat faces of the pole pieces 46 and 60 to support the armature 38 centrally in the air gap. This assures that the magnetic forces tending to pull the armatureaxially in the air gap are substantially opposite and balanced and accord- I ingly, the armature 38 exerts no appreciable force couple tending to twist the shaft 36 and thereby load the bearings 32 and 34. There is a gravitational imbalance to the armature 38 because its mass is disposed to one side of the shaft 36, however, this gravitational imbalance is a minor component of force readily supported by the spacers 62 and 64 with only a nominal radial loading being applied to the bearings 32 and 34.

Those skilled in the art will realize that, absent the pin 54 or some alternative means to control the starting and stopping positions of the armature 38, a armature plate exactly aligned with the substantially course, wherr the load attached to "the shaftfibimposes Nn' obvious'ad antageoffered by the embodimentof FIG; 6" is that'; asthe'armattire38 approachesthe end of it's' power stroke durihg' the' time the electromagnet coihisenergized; theleading'fmargin'of the armature approaching the Wings 80* and 82 is v magnetically attracted to these "wings: The-wings thusenable' the arm ature to deliver-a h'ighertor uere' the shaft=3 6 as the armatureapproa'ches the*end ofits powerstr'oke'; As aresul't; thearmature 38 is betterequipped to overcome the bias=of the return"spring 'and' isable to completeits p'ower strolte=with' greater certainty than isthe case for the preferred embodimentaj I't will occur*tothose skilled in the art that the same benefits'asde'seribed-With'reference to the modification a substantia'l resistancef to'iretitrn ofthe shaft' aften the offFIGi 'scimayfie obtain-(11B! etectingthe wings rotary solenoid hast-accomplished it's -poweri'stroke; the returns spring: 661* mustbe: proportionately strong; and one can encounter operating requirements which taxi execute a" full power" the: ability ofthe: armature to) stroke;

T-he modificationshown' in filG G-disclbsesone expe dient-that'can be employed in cases when the armature' spring requirements overtax nthe capability-ofithe arma ture to executeits full "power stroke:

In 1 the modification of I FIG. 6; the primarychanges:

reside in rthe i end plate constructiomand more particu-* larly thepole piececonstruction: Elementsof'theimodi fication which are :the .same as a corresponding-element described with respect'& to the" preferred embodiment bear: the L same reference numbers as employed inwde+ scribing the: Prefrredi embodiments Elements" of *the modification wh'icharedifftent that correspondingiel ements described in reference .to the-preferred embodi ment are distinguished in FIG; fibyrtl'leisuffiit-a; A*'com-- parison of i? lGS: 1 and :6 will reveal that th'emodifiedi pole pieces 46a' and a. are rotated 180"with respect:

to the corresponding polezpiecesillustratediin'FlG; 12 This-change in :thes-manneriofiillustrating themodifica.-- tion was employed 'so asto better illustrate the differ encesbetween thepreferredembodiment and theanod ification';

As apparent in FIG. 6 theupole pieces-46a-and 602i have been modified b'y' addingiga swing-s to thepole' piece 46a andr the wing 8'2 tothe-pole piece 60a; The:

wings-80sand 82 1Tproject axially on'ei towardlthe other;

without touchingioneranother; and "occupy'a small pottion of what was air'gap in the' preferred embodiment: It can be noted that:th'e wings-:80 "and 823areigenerally pie shaped whenviewed indirections parallel to theaxis of the shaftr36zlt"can':also benoted that the-pole pieces 46a and 6011 have flat 2 confronting, generally, semi-annular face portionsexcept where-interrupted by the wings 80-and:82.- I

In the -modification of FIG.- 6t ther' stop" pin 54 'is located adjacent pole. pieces I 460 :and 60a -and essentially in contact with such pole.pieces.Thus, thestoppinSd of the modification is-not received partially within: one

of the radial margins of each polepieceaswas the case in the preferredembodiment. Thevonly other differ-' ence betweenthepreferredsembodiment andthemodification resides in thedirection in whichthe armature:

38 is biased. Thus; the armature- 38 'of themodification is biased in the.clockwiseJdirection-asappearsinFlGi 6 and accordingly, FIG. 6 illustrates a the: armature :38 after the modification of FIG. 6I'h'as-been energizedand has taken its power stroke.

82 o'n the trailing" edge of" the armature plate 38'; In

some applicationsi this alte'rnate construction, whichjis not shownin:the drawings; will otferthe fiirtlier ad ani' tage that the armature need not*physically' contact any part of tlie'pole 'pie'ces and thusthere will be no need to overcome' a magnetic." retention between i the arm ature' andthe pole piecesw 1 Although" the preferred embodiment of the device has been: described} it will be: understood that within" the purview of 'thisinve'ntion variousch an'ges may bemadein the-form; details,- proportio'n' andarrangement offparts; the combinationthereofand modeof operation; which generally'stated'consist in a devi ce'cap'able 'of carrying; out 'theobjects set 1 forth, as disclosed and definedin' the-appended claimsz' Havingthusdescribed my intention; l claimz l2 lma 'rotary, 'sol'enoidi thecombination comprising a -coilj a ferro'magnetic core assembly' flux linked with said coil; saidl core assembly; including a'- pair of conftontihggplatel members eachh'aving a' pole piece 'projectinggtherefrom'; said core a'ssembly' includingmeans' supporting'said 'platememb'ersin spaced apart relation;

said pole pieces projectingjone toward 'the other and havingflat'parallel face' portions spaced "apjart'todefine an:air 'gapi'therebetween; a rotary} assembly; including a shaftand an 'ann'ature fiited to said shaft; means mounting said shaft to said coreassembly"fonrotation about an -axis' perpendicularto 'saidffa'ce portions, said arma-' ture' comprising a ferrom'a'gnetic plate portion havingflatsides perpendicular tosaid axis; said plate portion projectingioutwardly; from-said shaft for movem'ent'in I ing-vmargin,- said arcuate margin" subtending an angle between said leading and "trailingnot exceeding margins.

3. The rotary solenoid of claim' 2 "in which'said arcuate'rmargin subtendsan' angle of l8 0 between said leading and trailing margins.-

4S The 'rotary; solenoid 'ofelaim 3 wherein each said" 4 face portion is an approximate 180 concentrictosaid rotary axis; and includingstop'mean'scoacting between'said coreassembly and "said rotary,

assem'bly'toconfine said rotary assembly to rotary posiwherein said plate annular segment tions in which a part of said plate portion remains in said air gap.

5. The rotary solenoid of claim 4 wherein said stop means comprises a pin member extending between said end plates, a portion of said pin member projecting into said air gap for engagement with the leading margin of said plate portion.

6. The rotary solenoid of claim 5 including resilient means coacting between said rotary assembly and said core assembly to bias said rotary assembly to a position in which the trailing margin of said plate portion engages said stop means, said plate portion, upon energization of said coil, being urged magnetically through said air gap against the bias of said resilient means.

7. The rotary solenoid of claim 6 wherein said pole pieces have wing means to augment the magnetic urging of said plate portion.

8. In a rotary solenoid of the type comprising a coil, a ferromagnetic core assembly flux linked with said coil, said core assembly including a pair of pole pieces having flat parallel face portions spaced apart to define an air gap therebetween, and a rotary assembly including an armature supported for rotation about an axis perpendicular to said face portions, said armature including a ferromagnetic plate portion having flat sides perpendicular to said axis and movable in said air gap, the improvement comprising non-magnetic bearing means disposed in said air gap and coacting with said pole pieces to support said plate portion centrally in said air gap and away from contact with said face portions.

9. In a rotary solenoid of the type comprising a coil, a ferromagnetic core assembly flux linked with said coil, said core assembly including a pair of pole pieces having flat parallel face portions spaced apart to define an air gap therebetween, and a rotary assembly including an armature supported for rotation about an axis perpendicular to said face portions, said armature including a ferromagnetic plate portion having flat sides perpendicular to said axis and movable in said air gap, the improvements wherein said plate portion is an annular segment concentric to said axis of rotation, said plate portion having a radially disposed leading margin for movement through said air gap and having a radially disposed trailing margin, and wherein said armature or one of said pole pieces includes wing means projecting axially therefrom to augment magnetic attraction between at least one of said pole pieces and said armature.

k III I!

Claims (9)

1. In a rotary solenoid, the combination comprising a coil, a ferromagnetic core assembly flux linked with said coil, said core assembly including a pair of confronting plate members each having a pole piece projecting therefrom, said core assembly including means supporting said plate members in spaced apart relation, said pole pieces projecting one toward the other and having flat parallel face portions spaced apart to define an air gap therebetween, a rotary assembly including a shaft and an armature fixed to said shaft, means mounting said shaft to said core assembly for rotation about an axis perpendicular to said face portions, said armature comprising a ferromagnetic plate portion having flat sides perpendicular to said axis, said plate portion projecting outwardly from said shaft for movement in said air gap, non-magnetic spacer members encircling said shaft and contacting opposite sides of said plate portion, said plate portion and said spacer members received between said face portions, said spacer members contacting said face portions to support said plate portion against axial movement relative to said pole pieces.

2. The rotary solenoid of claim 1 wherein said plate portion has an arcuate margin connecting from a radially disposed leading margin to a radially disposed trailing margin, said arcuate margin subtending an angle not exceeding 180* between said leading and trailing margins.

3. The rotary solenoid of claim 2 in which said arcuate margin subtends an angle of 180* between said leading and trailing margins.

4. The rotary solenoid of claim 3 wherein each said face portion is an approximate 180* annular segment concentric to said rotary axis, and including stop means coacting between said core assembly and said rotary assembly to confine said rotary assembly to rotary positions in which a part of said plate portion remains in said air gap.

5. The rotary solenoid of claim 4 wherein said stop means comprises a pin member extending between said end plates, a portion of said pin member projecting into said air gap for engagement with the leading margin of said plate portion.

6. The rotary solenoid of claim 5 including resilient means coacting between said rotary assembly and said core assembly to bias said rotary assembly to a position in which the trailing margin of said plate portion engages said stop means, said plate portion, upon energization of said coil, being urged magnetically through said air gap against the bias of said resilient means.

7. The rotary solenoid of claim 6 wherein said pole pieces have wing means to augment the magnetic urging of said plate portion.

8. In a rotary solenoid of the type comprising a coil, a ferromagnetic core assembly flux linked with said coil, said core assembly including a pair of pole pieces having flat parallel face portions spaced apart to define an air gap therebetween, and a rotary assembly including an armature supported for rotation about an axis perpendicular to said face portions, said armature including a ferromagnetic plate portion having flat sides perpendicular to said axis and movable in said air gap, the improvement comprising non-magnetic bearing means disposed in said air gap and coacting with said pole pieces to support said plate portion centrally in said air gap and away from contact with said face portions.

9. In a rotary solenoid of the type comprising a coil, a ferromagnetic core assembly flux linked with said coil, said core assembly including a pair of pole pieces having flat parallel face portions spaced apart to define an air gap therebetween, and a rotary assembly including an armature supported for rotation about an axis perpendicular to said face portions, said armature including a ferromagnetic plate portion having flat sides perpendicular to said axis and movable in said air gap, the improvements wherein Said plate portion is an annular segment concentric to said axis of rotation, said plate portion having a radially disposed leading margin for movement through said air gap and having a radially disposed trailing margin, and wherein said armature or one of said pole pieces includes wing means projecting axially therefrom to augment magnetic attraction between at least one of said pole pieces and said armature.

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