GB2165400A - Beam support for bar windings of large electric generators having a stator core with dual dovetail slots for engaging wedges - Google Patents

Description

1

SPECIFICATION

Beam supportfor bar windings of large electric generators having a stator core with dual dovetail slots for engaging wedges Background of the Invention

The present invention relates to large dynamoelectric machines and, more particularly, to the stators of the large electric generators which are commonly the armatures of the generators.

As is well known, large electric generators employ a stator built up by stacking thin laminations of magnetic material to form a generally annular assembly. An array of axial slots are formed in the radially inner surface of the annular assembly. Each axial slot contains at least one and preferably two armature barswhich form part of the armature winding in which the electric output is induced.

A rotor is arranged to rotate coaxially within the stator. The rotor conventionally containsfield windings which are excited by a DC excitation source of relatively lowvoltage (from about 300 to about700 volts) in orderto produce a magneticfield rotating at the same speed as the rotor. Prior artexcitation sources include shaft-mounted DC generators, shaftmounted AC alternators feeding rectifiers and external sources of excitation power. The excitation power from external sources isfed to the rotating field windings through slip rings. One populartype of external source makes use of the AC output of the generator itself which, after passing through a transformerto provide the desired excitation voltage, is rectified and fed through slot rings to the f ield windings.

A more recent excitation system employs potential windings in, for example, three statorslots spaced 120 degrees apart. The three potential windings, which are hereinafter referred to as -P- bars, are exposed to alternations of the magneticfield which induces an AC voltage therein. The three "P" bars, being spaced 120 mechanical degrees aparttherefore comprise a threephase source whose output can be transformed as necessaryto establish a desired excitation voltage which maythen be rectified and the resulting DC fed through slip rings to thefield windings. A self-excited system of this type is described in a paper by Cotzas et al entitled "GENERREX-PPS (Potential Power Source). Excitation System forWisconsin Power& Light, Edgewater W, presented at the Forty-Third Annual

American Power Conference, Chicago Ill., April 27-29, 1981. The GENERREXPPS system provides a compact equipment giving reduced space requirements and simplified maintainability of a large generator. Control of the generatorfield voltage is accomplished by high-speed thyristor action. Further details of the GENERREX system do not concern the present disclosure and aretherefore omitted.

The stator armature bars are conventionally clamped in the statorslots using dovetail grooves near the radially inner ends of the slots into which wedges are firmly engaged. The wedges impose radial forces on the armature bars for resisting radial forces on the armature bars due to normal and GB 2 165 400 A 11 abnormal magnetic and electrical forces imposed on them.

The "P" bars are conventionally disposed radially inward of the wedges between the wedges and the surface of the rotor and are tied in place using, for example, afiber roving pre-impregnated with a heat-curable resin which is cured afterthe roving is installed. One such method for securing -P- bars in place is disclosed in U.S. Patent No. 4, 385,252.

Securing the "P" bars atop the wedges imposes certain penalties. The need to manually wrap ties everyfew inches aboutthe "P" bars and the wedge and to then cure the resin in the ties adds substantially to the labor required for assembly of the generator. In addition, after a generator has been in service for a substantial period, the stator may require refurbishing. Since the three "P" bars are atop the wedges, replacement of the wedges in the three slots containing the "P" bars requires cutting the roving ties and removing the "P" bars before removing and replacing thewedges. In orderto removethe "P" bars,their coolant piping and end turns must be disconnected. Sincethe "P" bars are. on the order of an inch ortwo in cross section and as much as 25 or more feet long, the chance of damaging them du ring removal is quite high. After new wedges are in place, the "P" bars must again be installed, tied in place and cured. These penalties all equate to adverse labor cost andlor schedule impact.

It is an object of the invention to provide a technique for securing "P" bars in a generator statorwhich overcomes the drawbacks of the prior art.

It is a further object of the invention to provide a technique for securing "P" bars in a generator stator in which the ends of the stator core are provided with an outwardtaper.

It is a still fu rther object of the invention to provide supporting bridges to maintain radial force on outer ends of "P" bars and armature bars whose central portions are secured beneath wedges in dovetail slots.

Briefly stated, the present invention in one aspect thereof provides apparatus for securing three potential, or'P' bars, atop armatu re bars in a stator of a large generator. At leastthe stator slots containing the "P" bars include a pair of dovetail slots spaced radially closerto the air gap than the dovetail slots in the remaining statorslots used forsecuring armature conductor bars in statorslots. An outward taper atthe ends of the stator intersects and weakensthe axially outer portions of the dovetail slots in the stator slots containing the "P" bars thereby weakening and reducing the effectiveness of the support provided by those dovetail slots. The inner end of a beam is fitted into the dovetail slot overthe "P" bar. Ayoke is secured overthe outer end of the beam. The yoke is secured to studs in space block extensions positioned adjacentthe stator slot containing the "P" bar. A ripple spring between the beam and the "P" bar applies a radial force to securely retain the "P" bar and the armature conductor bars in the slot.

The invention in another aspectthereof provides apparatusfor securing conductors in statorslots in a The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

2 GB 2 165 400 A 2 stator core fora dynamoelectric machine. Each stator slot includes two pairs of opposing dovetail slots. All of the statorslots contain ararnture bars. Afew equally spaced stator slots also contain "P" bars atop the armature bars. Wedges are installed in the radial ly outerdovetail slots of all stator slots containing only armature bars. The wedges are installed in the pairs of dovetail slots atthe radially inner pair of dovetail slots in the stator slots containing both the armature bars and the "P" bars- Onetechniqueforsecuring'p'bars in stator slots according to a further aspect of the invention is as follows: Each stator slot includes an inner pair of the dovetail slots and an outer pairof dovetail slots. In all statorslots exceptthose containing "P" bars, the wedges are installed in the inner pairof dovetail slots to securethe armature bars in place. In statorslots containing the "P" bars, the "P" bars are installed on top of the armature bars insidethe slots and the wedges are installed atop the "P" bars in the outer pair 85 of cl oveta i 1 s i ots.

In orderto reduce heating of the ends of the stator core dueto stray axial magneticfields, it is desirable to outwardly taper the lastseveral inches of the stator core. The outward taper intersectsthe outer pair of dovetail slotsthus reducing oreliminating supportof the -P- bars and underlying armature bars in the outer several inches of the statorslots containing the -Pbars. The outwardtaper may not intersectthe inner pair of dovetail slots andthusthe armature bars contained in the statorslots notcontaining "P" bars may notbe satisfactorily supported tothe axial ends of thestator. - The above, and other objects, features and advan- tages of the present invention will become apparent - from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.

Brief Description of the Drawings

Fig. 1 is a simplified axial cross section of a 105 dynamoelectric generatorto which referencewill be made in describing the invention.

Fig. 2 is an end view of a dynamoelectric generator showing the positioning of "P" bars in the stator thereof.

Fig. 3 is a cross sectional view of a statorslot showing the external mounting of a "P" bar according to the prior art.

Fig. 4 is a cross section of an adjacent pair of stator slots taken along]V-1V of Fig. 1 showing mounting of115 an armature bar both with and withoutthe presence of a "P" bar.

Fig. 5 is a viewof an end portion of a statorslot illustrating howthetapered portion atthe end of the stator intersects and eliminatesthe outer pairof 120 dovetail slots. - - Figs. 6A-6E, respectively, are cross sectionstaken alongA-Athrough E-E, respectively, in Fig. 5. - Fig. 7 is a topviewof the portion of thestator of Fig. 5 which includes abeam support according to an 125 embodiment of the invention.

Fig. 8 is an e.tploded view of the beam support according to an embodiment of the invention.

Fig. 9 is across sectiontaken along IX-1X of Fig. 7.

Fig. 1 OFis an end view of the beam support of Figs. 130 7-9.

Detailed Description of the Preferred Embodiment Referring to Fig. 1, there is shown, generally at 10, a dynamoelectric generatorto which the present inven- tion may be applied. Dynamoelectric generator 10 includes a stator 12 which has a stator core 14 built up by stacking a large number of laminations of a magnetic material. The stacked laminations are axially compressed by axial forces applied to the endsthereof through coreflanges 15.

A plurality& armature bars (notshown in Fig. 1) are axially disposed in slots (notshown) in the radiallyinnersurface of stator 12.The armature bars are interconnected byendturns 16 extending beyondthe axial extremities of stator core 14. Stator 12forms an annular body having a cylindrical opening 18 on its axiswithin which a rotor20 is rotatably mounted using, for example, bearings 22 in a generatorframe 24 rotatably supporting shafts 26 of rotor20.

In orderto avoid heating of theends of stator 12 by strayaxial magneticfields, an outwardtaper28 is giventothe outerfew inches of cylindrical opening 18 at both ends of stator 12.

Referring nowto Fig. 2, a simplified transverse cross section of dynamoelectric generator 10 is shown. Rotor 20 isexcited by DC excitation applied to itto produce a magneticfield indicated by an arrow 30. A large number of slots are present in the radially inner_ surface of stator 12, of which onlythe three statorslots

32 containing "P" bars 34 are shown. As rotor20 is rotated in the direction shown by a prime mover (not shown), voltages are induced inthe "P" bars bythe rotation of the magneticfield. The induced vottages are phased 120 electrical degrees apart corresponding to the physical spacing of "P" bars 34. Itwould be clear to one skilled in the artthat the above illustration of rotor 20 with a single pair of magnetic poles does not exhaust the possible options. For example, rotor 20 may alternatively be wound to provide two or more sets of angularly spaced magnetic poles. In such a generator, the one-to-one correspondence between electrical degrees and physical degrees is not maintained. The disclosure herein is unaffected by such difference.

Before continuing with the disclosure of the present invention, a brief description of the technique for mounting a "P" bar 34 according to the above referenced U.S. Patent No. 4,385,252 is includedfor providing additional background to aid in understand-ing the present invention.

Referring to Fig. 3, stator slot 32 includes a pairof armature bars 36 (only the upper, or radially inner, oneof which is shown). The opposed sides of stator slot 32 include dovetail slots 38 which receive a wedge 40 to rigidly secure armature bars 36 in stator slot32.1twill be noted thatstator slot32 extends radial ly inward a substantial distance beyond atop surface 42 of wedge 40. This space is vacant in all of stator slots 32 except for those containing "P" bars 34. "P" bar 34 is appropriately insulated and blocked in place in stator slot32, and, as fully explained inthe referenced patent, istied in place atoptop surface42 using a plurality offles44whiph go aroundthetop and sides of "P" bar34 and undera portion of wedge40.Ties 44 are preimpregnated with a heat-curable resirywhich is- 3 cured after tying.

Referring now to Fig. 4, there is shown a stator slot 32'according to an embodiment of the invention which does not contain a -P- bar 34. An inner pair of dovetail slots 46 are disposed in the same radial positions as were occupied by dovetail slots 38 (Fig. 3). An outer pair of dovetail slots 48 are disposed radially inward of inner pair of dovetail slots 46. Wedge 40 is shown installed in inner pair of dovetail slots 46 and securing armature bar 36 in place in stator slot 32. The figure also shows a stator slot 32 containing a "P" bar 34 as well as armature bars 36. "P" bar 34 is disposed atop armature bar36 beneath wedge 40. In orderto provide the necessary additional depth in stator slot 32, wedge 40 is installed in outer pair of dovetail slots 48. This provides secure retention of "P" bar34 in position withoutthe cost of installing ties and without the need to remove "P" bar34 if wedge requires replacement.

Referring mornentarilyto Fig. 1, the embodiment of the invention illustrated in Fig. 4 is satisfactory throughoutthe axially central portion of stator 12, but a problem develops in the region of outward taper 28. Since outer pair of dovetail slots 48 are closerto the radially inner extremity of stator slot32 than are inner pair of dovetail slots 46, soon afterthe beginning of outward taper 28, outward taper 28 cutsthrough outer pair of dovetail slots 48 and eliminates the support given to wedge 40. This is illustrated in Fig. 5 and the related cross sections shown in Figs. 6A-61E. As is conventional, stator core 14 is assembled in sets of core packages 50 separated by gas passages 52 for the radial passage of coolant gas therethrough. Gas passages 52 conventionally contain a plurality of 1-shaped.members maintained in compression between theflanges thereof bythe compressive forces applied to the stack of laminations in stator core 14. Outward taper 28 is accomplished over a plurality of core packages 50. For example, core package 50a is a full-depth core package whose cross section A-A, shown in Fig. 6A, exhibits a normal amount of supporting material above outer pair of dovetail slots 48a. The next core package 50b begins outward taper 28 by being stepped down to a slightly shallower depth. As seen in cross section B-B in Fig. 6B, the 110 amount of material above outer pair of dovetail slots 48b is reduced but may still contain enough material to provide substantial support for a wedge 40. The nextcore package 50c is stepped down even furtherto the extentthat little or no material is left above outer 1 pair of dovetail slots 48c (Fig. 6C) for supporting wedge 40. The next core package 50d is stepped down even further so that no su pport for wedge 40 is provided. The extreme outer core package 50e is stepped down at its abutment with the adjacent gas passage 52 and continues additional diverging steps until itterminates at a space block 54 clamped against core package 50e by coreflanges 15.

Inner pair of dovetail slots46, being furtherfrom the extremity of stator core 14, is relatively unaffected by outwardtaper 28 in core packages 50a-d. Inner pair of dovetail slots 46e is intersetted by outward taper 28 in co re p a cka g e 5 o ccu rs cl o s e enough to tHgW3%1111y' 6u'te"'r extremity of stat6rcore 14 so thatexternal support of and.turns (nbi,-'for-- GB 2 165 400 A 3 an armature bar 36 in a stator slot 32'not containing a "P" bar 34, may be sufficientto overcome the relatively short unsupported run within stator core 14 beyond the intersection of inner pair of dovetail slots 46c by outward taper 28. As previously noted, however, the ability of outer pair of dovetail slots 48to support wedge 40 is reduced and then eliminated a substantial distancefrom the axially outer end of stato r core 14.

A space block extension 56 is included on spaceapart ones of space blocks 54for supporting studs 58 which are normally used for attachment of a gas seal (not shown) in dynamoelectric generator 10. Use is made of space block extension 56 and stud 58 in the present invention as wil 1 be explained.

Referring nowto Fig. 7, a top view of a stator core 14 is shown in which a "P" bar 34 and the u nderlying armature bars 36 are held in place by a beam support 60 according to an embodiment of the invention.

Wedge 40 isterminated at a point in core package 50a, or core package 50b, (a point in core package 50b is illustrated) where adequate supporting material remains in outer pair of dovetail slots 48 (see Fig. 5 and Fig. 6A or 613). Awedge beam 62, having the same cross-sectionai shape as wedge 40, is inserted into outer pair of dovetail slots 48 and into abutmentwith the end of wedge 40. A space block extension 56 is disposed at each side of stator slot 32 with an inward-extending portion 64 of each stud 58 extend- ing axially inward towardthe center of dynamoelectric generator 10. A yoke 66 includes a pair of holes 67 which are fitted onto inward- extending portions 64. Yoke 66 straddles an outer end of wedge beam 62. A trapezoidal locking device 68 is angularlyfitted into an angled slotand locked in place by a pin 72. Atie 74is wrapped abouteach stud 58 andthrough a hole76 in each side of yoke 66to retain yoke 66firmly against space blockextensions 56. The axially inner end of wedge beam 62 is held down against"P" bar34 by engagementwith outerpairof dovetail slots48 in core package 50b. The axially outer end of wedge beam 62 is held down against "P" bar 34 by yoke 66. Thus, "P" bar 34 is fully secured in place under wedge beam62.

Referring nowto Fig. 8, in which the elements beam support 60 are shown in an exploded viewyw purposes of illustration, and Fig. 9, in which thi., 'elements are shown assembled, wedge beamj62includes an upper portion 78 of generally recid6dtilar cross section and a lower portion 80 which (hc'iddbs the angled flangesfor engaging outer pai,r 6f dovetail slots 48 atthe axially inner end of wedge IYam 62. A tongue 82 having a flat upper surface 840)te'n"u-s axiallyfrom wedge beam 62. An angledfa(:586 joins an upper surface 88 of upper portion-7 - 8 - idilatupper surface 84. A ripple spring 90 is dispo'sedb'eiow wedge al d' beam 62 for providing a substanti ' r W1f6rce tending to secure "P" bar34 in posi tiogAlople spring i 6M g-- is shown in its relaxed condition i 8.When installed, ripple spring 90 is prefera6f'co'mpletely flattened between wedge beam 621Ahd -P" bar 34 as shown in Fig. 9.

In orderto prevent a high-voitagg insulation layer 92 on "P" bar34 from being damagedlycontact with ripple spring 90, a hard, non-:abraf\e'ffiler strip 94 is positioned between ripole'sefng 90"and "P" bar 34.

4 As particularly shown in Fig. 9, an angled face 96 on yoke 66 lies parallel to angled face 86 on wedge beam 62. Trapezoidal locking device 68 isthus enabled to slide between angled face 96 and angled face 86 and, when pinned in place by pin 72, prevents removal of trapezoidal locking device 68.

When statorcore 14 is assembled, studs 58 are normallywelded or otherwise premanently installed in space blocks 54 before beam support60 is mounted. During installation of beam support 60, wedge beam 62,with ripple spring 90 and non abrasive filler strip 94 below it, is inserted into outer pairof dovetail slots48. In orderto clearthe inner ends of inward-extending portions 64, yoke 66 must be positioned to occupythe space latertaken by trapezoidal locking device 68 and then holes 67 are slid onto inward-extending portions 64. Inward-ex tending portions 64 preferably contain an uncured resin during assemblywhich is later cured to firmly affix yoke66 in place against space block54. After 85 yoke 66 is slid onto inward-extending portions 64, the trapezoidal space left vacant bythis assemblytechni que isfilled by trapezoidal locking device 68which thereupon provides axial supportfor helping to maintain yoke 66 and wedge beam 62 intheirfinal positions.

Referring nowto Figs. 8 and 10, yoke 66 includes a pairof legs 98joined by a bridge 100 spanning tongue 82. Legs 98 includes angled surfaces 102 whose angles generally coincide with the angles of edges 104 95 of flat upper surface 84. Thus, when tongue 82 is held down by bridge 100, angled surfaces 102 and edges 104 tend to maintain the axially outer end of wedge beam 72 properly centered over stator slot 32.

Beam support 60 is preferably made of a non- 100 magnetic material and most preferably of a non metallic material. With the exception of studs 58 which are stainless steel, all parts of beam support 60 are preferably made of a hard, high-strength matrix of resin and fabric. Wedge beam 62 and yoke 66 are preferably-of glass fiber in a cured epoxy resin. Ripple spring 90 is also preferably a glassfiber and epoxy resin chosen from types which retain their resilient properties overtime. Ripple spring 90 may alternative ly be replaced by corresponding meansfor applying radial force to "P" bar34 such as, for example, a body of elastomeric material or a spring having a lunate cross section. Filler strip 94 is chosen for its abilltyto prevent damage to the high-voltage insulation on the surface of "P"-bar34. Forthis purpose, an organic fiber-resin system may be employed, such as conve niently a cotton-phenolic laminate.

The orientation of thefabric in wedge beam 62 is preferably parallel to the axis of wedge beam 62 for best resistance to bending forces. The orientation of thefabric in yoke 66 is preferably normal to the axis of wedge beam 62. In afurther embodiment of the invention,wedge beam 62 and yoke 66, instead of beingtwo separate pieces, may be cast, or otherwise formed, in a single unitary assembly. In a castversion of the invention,a chopped fiber is preferably used in orderto deveiGp adequate strength in both the axial and the normal directions.

Having described preferred embodiments of the invention with reference to the-accompanying draw- GB 2 165 400 A 4 ings, it is to be understood thatthe invention is not limited to those precise embodiments, and that various changes and modifications may be affected therein by one skilled in the artwithout departing from

Claims (20)

the scope or spirit of the invention as defined in the appended claims. CLAIMS

1. Apparatusfor securing a "P" bar in stator slot of a generator stator atop at least one armature bar disposed in said slot, said stator slot fncluding at least one pair of dovetail slots radially disposed to permit fitting said at least one armature bar ancfsaid "P" bar under a wedge fitted into said dovetail slots, said stator including a taper at an outer end thereof, said taper intersecting said pair of dovetail slotscomprising:

a beam; said beam including angled edgesata first axially inward endthereof fittable into said dovetail slot; a yoke effective for stradling a second axially outward end of said beam; means for securing said yoke to an axially outward end of said stator; said means for securing being effective for applying a radially outward force to said second end; and resilient means between said beam and said "P" bar for applying a substantially radial force to said "P" bar.

2. Apparatus according to claim 1 wherein said beam includes a tongue at said second end and said yoke includes first and second legs joined by abridge, said bridge being fittable over said tongue.

3. Apparatus according to claim 2 wherein said tongue includes second angled edges and said legs include angled surfaces effective to engage said second angled edges and to urge said tongue into a centered position in said stator slot.

4. Apparatus according to claim 2 or3 wherein said beam includes a first angled surface adjacent said tongue, said bridge includesa second angled surface parallel to said first angled surface, said first and second angled surfaces being separated byan axial separation when said beam and said yoke are installed in their operational positions.

5. Apparatus according to claim 4Jurther com- prising blocking piece inserted between saidfirst and second angled surfaces and effective for maintaining said beam andsaid yoke in their axial locations.

6. Apparatus according io claim 5 wherein said locking piece Includes a pin passing through said locking piece and into at least one of said beam and saldlyoke.

7. Apparatus according to any preceding claim wherein said beam, said yoke and saidmeansfor applying a radial force are of non-metallic material.

8. Apparatus according to cWtm7wherein said non-metallic material is afiberin a cured resin.

9. Apparatus according t(i claim 8 wherein said fiber includes a glass fiber.

10. Apparatus according to claim 9 wherein said fiber in said beam is oriented parallel to an axis of said beam and said fiber in said yoke is oriented perpendicularto said axis of said beam.

11. Apparatus according to claim 1 wherein said yoke i ncludesfirst and second legs joined by abridge, said meansfor securing said yoke includes first and second space blocks adjacent said stator slot, said first and second space blocks including first and second space block extensions respectively thereon, first and second studs in said first and second space block extensions respectively, said first and second space 70 blockextensions including an inward-extending por tion extending axially inward a predetermined dis tance, said yoke including first and second holes therein fittable upon said first and second inward extending portions and means fortieing said first and 75 second legs to said first and second space block extensions.

12. Apparatus according to claim 1 or 11 wherein said resilient means includes a ripple spring.

13. Apparatus according to claim 12 wherein said ripple spring is of non-magnetic material.

14. Apparatus according to claim 1 or 11 wherein said resilient means includes a ripple spring and a non-abrasive fillerstrip between said ripple spring and said "P" barwhereby damage to said "P" bar from contact with said ripple spring is avoided.

15. A generator stator comprising:

a plurality of stator slots; at least one armature bar in each of said stator slots; a "P" bar atop said at least one armature bar in at 90 least one of said stator slots; first and second opposed dovetail slots in said at least one of said stator slots; a wedge fitted into said first and second opposed dovetail slots, said wedge being effective for applying 95 a radial force to said "P" bar and said armature bar; a taper at an outer end of said stator, said taper intersecting said first and second dovetail slots and at least reducing an ability of said wedge to apply said radial force on said "P" bar and said armature bar; 100 a beam; said beam including angled edges at a first axially inward end thereof fittable into said first and second dovetail slots at an axial position in said statorwhere a substantial portion of said first and second dovetail slots remain; a yoke effective for stradling a second axially outward end of said beam; means for securing said yoke to an axially outward end of said stator; said means for securing being effective for applying a radially outward force to said second end; and resilient meansbetween said beam and said "P" bar for applying a substantially radial forceto said "P" bar.

16. A lamination for stacking to form a statorof a dynamoelectric machine comprising - aslot-shaped opening in said lamination which, when assembled into said stator, forms a stator slot; a first dovetail slot in said opening at afirst radius from an axis of said stator; a second dovetail slot in said opening at a second raditisfrom said axis of said stator; said first radius being effective for receiving a wedge in said first dovetail slotfor securing at least one armature bar in said slot; and said second radius being effective for receiving a wedge in said second dovetail slotforsecuring a "P" bar and at least one armature bar in saidslotwhereby eitherarmature barsa lone ora "P" bar and at least one armature bar maybe secured irT said slot by GB 2 165 400 A 5 selection of which dovetail slot receives said wedge.

17. A stator fora dynamoelectric machine comprising:

an annular stator core of magnetic material; a plurality of axially disposed stator slots in an inner surface of said statorcore; at least one armature bar in each of said statorslots; at last one of said stator slots containing a -P- bar atop said at least one armature bar; at least a second of said statorslots containing only said at leastone armature barand notcontaining a "P" bar; firstandsecond pairs of opposed dovetail slots in sides of said first and second statorslots; said first pairof dovetail slots being at a first radius from an axis of said core, said first radius being effective for positioning a wedge therein in securing relationship with said at least one armature bar in said second stator slot; said second pair of dovetail slots being at a second radius smallerthan said first radius, said second radius being effective for positioning a wedge therein in securing relationship with said "P" bar atop said at least one armature bar in said first stator slotwhereby either armature bars alone orarmature bars with a "P" bar atop them may be secured in a stator slot by selective positioning of saidwedges in said first or said second pairs of dovetail slots.

18. Apparatus according to claim 1 substantially as herein described with reference to and as shown in the accompanying drawings, except Figure 3 thereof.

19. Astator according to claim 15to 17, substantially as herein described with reference to and as shown in the accompanying drawings, except Figu re 3 thereof.

20. A lamination according to claim 16, substantially as herein described with reference to and as shown in the accompanying drawings, except Figure 3 thereof.

Printed in the United Kingdom for Her majesty's Stationery Office, 8818935, 4186 18996. Published at the Patent Office, 25 Southampton Buildings, London WC2A lAY, from which copies may be obtained.