CA1329471C - Device for pre-tautening of the annular stator core laminations of a large electric generator - Google Patents

Abstract

ABSTRACT
This invention refers to a device for the pretautening of the stator core laminations (20) of a large electric generator.
A suitable number of anchoring devices (40) according to the invention is positioned between the shell (10) and the stator core laminations (20), evenly dis-tributed in height and around the circumference.
Each anchoring device (40) by means of which said pre-tautening is achieved, has a baseplate (41) secured to the shell (10) for example by welding, and a bracket (51) equipped with tong-like arms (52) by means of which it is able to grip the fins (32) made on tension rod keys (30) having a dovetail-shaped rib (31) capable of being inserted into a corresponding seat (21) formed in the extrados of the stator core laminations (20).
Pre-tautening of the stator pack is achieved by exer-cising centrifugal traction on the brackets (51) and securing them by means of screws (57) arranged between the baseplate (41) and the bracket (51) (figure 3).

Description

t 329471 Devi~e for pre-tautening o$ the annular st~tsr c~re lami~ations Df a larg~ electric generator The present invention relates to a device f~r pre-tautening the annular stator oore la~tions of a large electric generator.
In designin.~ lar.~e 510w, vertical-a;<is electric ~ener-ators it is necessary to a~opt suitable constructional solutions in srder to overcon,e the pro~lems linl:e~ witt, the large geometrical dimensions typical of this fan,ily .
of .3enerators. The stator and rotor compunent designer n.ust keep in mind not only their state of tautness ~ut :
also their state of ~eformation.
One typical an~ fun~an,ental pro~len, of this nature is the problem concerning waviness ~ue to peak loa~s ~uckling) of the stator core lan-inations. The in-crease~ geometrical dimensions consequent to lncreased output and to the pnlarity num~er renders the stator core laminations o~ these machines more an~ more sensi~
tive to this phenomenon, since the increase of the a~erage ~iam`eter is not accon.panied ~y a correspon~ing increase in the radial ~imensiuns. The stator is made up af a pack of thin laminations arrange~ in superim-pose~ layers, each similar to an annular crown. Durin~
operation they are su~jected to a system o4 forces ' .
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1 32947 ~

which give ris~ to ~mpressive str~sses. Since stresses even of a limite~ nature may ~ecome ~angerous for the laminations from the point of view of buckling, the mechanical design of the stator must pay particular at-tention to the state of stress o4 the laminations under all operating conditions.
The followin3 is a ~escription of the forces which cause ~uck:ling of the stator core laminations.
The different heat conditions which occur in the s~ator core laminations and in the shell during operation may cause huck.ling of the laminations. It i5 well-known that the stator oeing the seat of electro~agnetic lossesr the stator core laminations have a higher average running tempærature than the- shell, which is heated only in~irectly ~y the air which has pass~
through the ventilation ~ucts. The stator core laminations~ therefore, tend to expand ra~ially more than the shell, and are therefore free 4rom stresses only i4 the latter does not create a constraint preventing it from expanding freely. If, on the other hant, the shell ~oes stop it ei~pan~ing freely, that is to 5ay if it forms a hoop aroun~ it, the reciproral force on the lamin~tions-shell constraint, which acts in a centrifuqal ~irection on the shell and in a centripetal direction on the laminations~ leads to a `
, A
, tan,3ential compressive ~tress on the latter. This stress is all the great~r ag the ~ifference in tempera-ture is gre~ter~ an~ as the ra~ial stiffness o4 the sheIl increases.
It stands to reason, also, that this hoopin,g action is increased if the shell is not free to eKpand ra~ially on Its foundations~ that is to say if the shell-foun~ation constraint partially or totally prevents the heat expansion ~roul~ht a~out by the temperature.
Without interfering with the other con~itions~ the state of compressisn of the stator core laminations may be reduced hy ~esi~ning the shell so as to reduce its ra~ial ~tiffness. This contri~ution, howe~er7 may not exceet certain limits since the shell must be a~le to withstan~ without serious deformations and transmit to the foun~ations the tangential force due to the electromagnetic couple ~uring normal running an~ due to transient electrom~gnetic couples cause~ ~y short cir-cuits or 4al5e par~lleling.
The "magnetic pull" between the rotor and the stator is another cause of undulation of the laminations. This acts in a centripetal direction on the intra~os of the laminations an~ therefore gives rise to a tangential compressive stress~ From this point of view, the laminations-shell constraint acts in a ~,anner opposite 1 32q47 1 to that ~escribe~ above, that i5 to say it attenuates the state of compression of the laminations; tl~e reciprocal action between the laminations an~ the shell has a centrifugal direction on the eKtra~os of the laminations .
It is not easy to determine by theoretical calculations the tangential compressive stress which initiates the bucklin~. Indeedi in addition to the geometrical ~imen-sions of the rore the phenomenon may be conditioned even ~eci~vely ~y other factsrs, such as - for example - the thickness of the laminations, the pressure under whirh they are held together an~ which may however vary during the life-span of the machine, the circumferen-tial pitch of the pr~ssing tie ro~s. The serious nature o4 the phenomenon is obvious ~ the core loses its radial compactness and damages may ~e cause~ to the win~ing.
The stator shoul~ there40re ~e designed aimin~ at eliminating the cause at the origin of the phenomenon, and there$ore with th~ purpose o$ keeping the state of compres~ion of the core laminations ~own to a low level. It is therefore necessary to ensure free thermal eKpansion of the stator on it~ foundations, to cancel out or at least minimi~e the hooping effect e~erted on the core by the shell, and to re~uce the stre~s due to ~-- ~ . - ~ . - . : . . :
- - . : ~ . : . . . -magnetic pull.
Accordin3 to this invention"the reduction in the conl-pressive stress ~ue to n,a,3netic pull an~ the elimina-tian of the hooping effect of the shell on the stator core due to the difference in temperature between the core and the shell have been achieved by "pre-tautening" the stator core laminations. This arrange-ment is possihle if the stator core laminations are as-sembled on site in a continuous process~ so that there is no tangential discontinuity as found in workshop-assem~led stators shipped in sectors.
Once the different sectors of the shell have been as-semble~ on ~ite and coupled together so as to provide continuity of the ~tructure, the stator core lamina-ti~ns may ~e assembled. During this oper~tion, carried out in the traditlonal manner, the reference point for torrect radial and tangential positionin~ of the laminatlon~ of the core is en~ured by previously fa~-tened tensian rod-keys, evenly distributed around the circumference on the inner ed~e of the ring shaped walls o$ the shell by means of the anchoring devices accor~in~ to this invention~ -Each of the anchoring devices comprises a plate welded to the wall of the shell and a ~racket which slides hori~ontally, in a radial direction in respect ~f said .'~' plate.
The bra~ket n,ay be made fast to the plate ~y means of suita~le screws.
Each ~racket also t,as tong-like arms able to retain the fins o4 the tension rod-key whi~h, on the si~e opposite to said fins~ has a dovetail type rib capable of bein3 inserted into the corresponding seat on the extra~os of the stator core laminationsu Suita~le mechanical means able to exert an outward pull on the brackets allow a state of traction to be created in the laminations o4 the core.
The extrados of the core laminations has a series of axial grooves evenly distributed aroun~ the circumference. ~ tension ro~-key of the type described aoove i5 inserted into each of these keyways.
Each tension rod-key i5 sei2ed and pulled towards the outside of thè core. that is to say in a centrifugal dir~ction, by a suitable number of anchoring ~evices according to the invention, and of which for each ten-sion rod-key there are a number equal to the nun,ber o~
sheet metal annular rims formin~ the horizontal ri~s of the shell of the stator.
The number of anchoring devices according to the inven-tion will ~e there40re equal to the number o4 tension rod-keys n,ultiplied by the number of sheet metal an-~ .

nular rims forming the stator shell~
The scope, advantages and characteristics uf the inven-tion are also given in the following de~cription, referred to forn,s of em~odiment chosen, ~y way of eK-ampl~ only, with particular referen~e to tt,e attached trawings, in which :
- figure 1 shows a vertical cross-section of the shell and the stator core laminations of a large electric generator e~uipped with the anchoring devices ac-corting to the inven~ion:
- figure Z shows one of the anchoring devices already illustrated in figure 1, but on a larger scale; in figure Z the thickness of the lamin~tions is not in scale, but enlarged to show them more clearly;
- flgure 3 is a plan of the anchoring dev~ce of figure 2:
- figure 4 shows on a plan the mobile e~uipment to be U5 ed for pre-tautening the stator core lan,inations, ant applica~le, in a suita~le consecutive order, to the anchoring devices claimed:

~ .
- figure S shows a diagram of the forces which develop between the shell and the st~tor core laminations ~ue to the ten,pera~ure differénce ~etween the ; laminations and the shell~

- figure 6 is similar to the previous figure but 1 32q47 1 refe~s to the force~ develoF,ed between the lamina-tions and the shell due to the effect o4 the ma,3-netic pull exerted on the stator core laminations by the rotor.
With particular r.e4eren~e to the a~ove figures~ 10 in-dicates a radial cros~-seltion o$ the stator shell made up g4 a series o4 annular plates 11A-11F connected to one another by means of a cylindri~l ring 1Z along their extra~os. ~or the sake of simplicity, no radial stiffening plates are not shown in the figure.
T~e annular shell 10 is supported ~y ~ base 13r il-lustrated only schematically~
20 repre~ents the stator made up of a series of super-imposed laminations, e~uippet, at regular circumferen-ti~l intervals, with grooves havin~ a dovetail-shaped cross-section 21 ~see figure 3), and into which ~om-plementary ri~s ~1 of ten~ion rod-keys 30 are inserted, Each tension rod-key 30 is supported and pulled out-wards in a lentrifu~al dirertion ~y a suitable number of anchoring devices 40, the sub~ect-matter of this invention, in turn fastenet over the annular plates 11 o4 the shell 10, and protruding radially towards the inside of the inner edge of s~id annular pl~tes 11.
~ccordlng to the preferred emhodiment of the invention, as illustrated in the figures~ for each tension ro~ key :-there are ~s many anchorin3 ~evi~es 40 a5 there are annular plates 11 of the shell 10. In the case il-lustrate~ in the figure the annular plates of the sh~ll 10 are six, 11A, 11~, 11C, 11D, 11E and 11F.
Reference shall ~e made ~elow tG this parti~ular ex-an,ple of embo~iment in or~er to illustrate the pre-tautenin,3 metho~ to be followe~.
As known, each tension rod-key terminates at its two ends in threa~e~ spigots 3Z onto which nuts ~3 are screwed. By means of radial brackets 34 an~ annular sectors 35 the~e nuts 33 make it posçihle to tighten toyether axially the laminations of the stator core.
Each anchoring tevice 40 is made up of a ~ase plate 41, welded to one of the annular plates 11 of the shell 10.
The base pla~e 41 has a vertical flange 4Z located on its outer end in respect of the axis o$ the stator. The ~ase plate 4~ i5 e~uipped with a vertical threated hole 43. In turn, the vertical flange 42 is equipped with two through holes 44.
Qn the ~ase plate 41 a U-shaped ~racket 51 rests, which has, in a centripetal direction with regard to the shell, two tong-like arms 52 which ~efine an area 53 a~le to retain ra~ially the tension rod-key 30 hy means of the two $in~ which the latter has in a direction op-posite to the tail 31.

1 32~471 The bra~et 51 has a vertical hole 54 with a ~iameter suitably larger than the diameter of the vertic~l hole 43 of the ~ase plate 41.
The radially eKternal face of the bracket 51 is also equippe~ with two hori2antal tappe~ holes 56~ threaded, an~ placed radially in respect of the shell and aligned with the holes 44 of the vertical flange 42 of the ~ase plate 41.
vertical 5crew 55 passes through the hole 54 in the bracket 51, with a certain clearance, and is screwe~
into the threade~ hole 43 of the ~ase plate 41.
pair of radial screws 57 passes through the holes 44 .~................... .. .
in the vertical flan~e 4Z of the b~e plate 41, with a certain clearance, and is screwed into the threaded screws 56 of the ~racket 51.
The ~racket S1 i5 e~uipped with fins 5~ which protrude circumferenti~lly in respect of the stator, and these permit the application of a tool for pulling in centrifugal direction as compared to the stator.
This tool may ~e configure~ ~5 illustrated in figure 4~
~ U-shaped ejector is fitted with wards 61 facing .~. .~
towards the inside of its arms 62, suitable 40r engag-ing the fins ~8 of the bracket 51.
jack 64 positioned ~etween the cross-member 63 of the e~ector 60 and the outer face of the ~ertical flange 42 .
,, ,. ,~, ~. ... : -of the base plate 41 ma~es it F~ossible to exert a centrifugal $orze on the series of laminations 20. This force, exercised ~y the jack 64 ~urin3 the pre-tautening operation, is transmitte~ to the laminations 20 by the chain of elements 5~-51-52-3Z-30-31-21.
The pre-tautening o~ the anchoring ~evices accor~ing to the invention must be carrie~ out after axial pressing of the laminations 20 and tightening of the tension ro~-key~ 30 ~y means of the nuts 30, and n,ust ~e carried out according to the following se~uence~
1) The screws 55 ~re loosene~ until a suitable vertical clearance, for example 3mm, is o~taine~ between the bracket 51 and the head of the screw 55.
Z) Using a torque wrench, an initial tightening torgue is applie~ to each screw 57 of all the anchoring ~evices 40.
During this opæration the stop screws 59 must ~e loosened. The operation must be rarrie~ out by a suitable num~er of persons, for example 5iX, evenly distri~uted aroun~ the circumference, whn tighten the screws 57 simultaneously.
Each operator must take care o4 tightening the an-chorin~ ~evices 40 of the circumferentlal fraction o4 the stator with which he is concerne~. If the operalors are six, as suggested above, each of them ; ~ :

will take care of tightenin,3 one 5iKth of the ~lock:
ing ~evic~s.
The tightening se.~uence is carried out by r~dial planes, and on each plane it will envisage the con-secutive tightening of the screws 57 of the devices associate~ with the annular plates 11A, 11~, 11B, 11E, 11C and 11D~
The group of operators will then proceed to deal with the anchoring devices of the adjacent tension rod-key~
~fter the preliminary tightening of the screws 57 using tor~ue wrenches, final pre-tautening of the stator core laminations 20 is carried out using the equipment 60 shown in figure 4.
If, as proposed a~ove, the ti.3htening is done by si~
men actin~ simultaneously and evénly ~istri~uted around the circumference, tautening of the tighten-ing deviccs ~y means of the equipment shown in figure 4 will follow the same seguen~e descri~ed for the pre-tautening desori~ed under point (Z) a~ove~
~ suitable load will be applied to the ja~k 640 While the ~ack is loaded as above, the screws 57 and the stop nuts 5~ will he tightened definitively using a torque wrench.
Finally, still using a torque wrench, the screws SS

,~

" . . , ., ,, :: .. ~ . , ., ... : ,~ ,.. ,, :,, . ,,, ,- . . . .. . .

t 32947 will ~e tightQned~
Once this operation has ~een carried out on all the an-chorin3 equipmentJ the core ZO is elastically ~eformed outwards and therefore subject to a state of tangential tensile stress. The shell 10, the radial stiffness of which is exploited in order to ensure pre-tautening of the core laminations 20, remains on the other hand, elastically deformei inwards, and is therefore subiect to a tan~ential compressive stress.
The ~ehaviour while the machine i5 in operation o$ the laminations and shell as a whole, pre-stressed as described ~y means of the anchoring systems, is ex-plainet more clearly i4 the aspects related to the tem-perature (see fig. 5) and to the magnetic pull Isee 4igure 6) are exa0ined separately, takiny into account that the principle of superimposition of the effects applies.
When the stator starts to warm up, the stator core laminations ZO, the temperature o~ which is higher than that of the shell 10, would tend to eYpand to a greater eY~tent than the latter~ This different radial e~pansion generates a radial force (F) which is a function of the temperature dif$erential ..t~
The initial elastic deformation of the stator core laminations due to the pre-tautening, and consequently 1 32q47 1 the initial reciprocal force ~etween the laminations and ~,e shell, decrease proportionally to the increase of th~ difference in their temperatures. The stressed state of the care laminations and shell decreases as the force exerted by the an~horin~ syste~,s ~ecre~ses~ -It is possible to keep the core laminations inert still in a state of tensile stress even when the stator reaches its highest runnin3 temperat~re ~y choosing suitable ~alues for the pre-tautening force. lt stands to reason that the value of this force must equal or e~ceed the force (F) originated, in the a~sence of pre- ~ -tautening, ~etween the laminations and shell connected stiffly to one another as is the case in traditionally ~uilt stators (figure 5).
From the point of view of the action exerted ~y the n,agnetic pull (T~), a stiff constraint between the shell and the stator core laminations h~s the effect of re~ueing the state of compression which would originate in the core in the opposite condition~ that is to say with the core laminations and shell totally uncoupled from one another. Indee~ the magnetic pull, acting on the core in a centripetal direction, tends to deform it inwards and is there~ore sust~ined ~y the laminations and shell as a whole if these are stiffly connected to one another. That is to say that with its own radial , '' :, . ' , ' ' . . . ' . ' . .: ~ -. '' . . ,': : ' ~ , '':

t 329471 stiffness the shell limits the elastic contraction Df the laminations, consequently reducing the state of compression~
The traditional stator design, with tension rod-keys wel~ed directly to the annular walls of the shell is therefore a~vantageous from this point of view.
The an~horing systems give rise hetween the core la~,inations an~ the shell to a condition of infinitely stiff constraint for radial forces acting in a centripetal direction on the intra~os of the stator ~ore. The constructional solution adopted therefore limits the compressive stress, even without taking the pre-tautening into consideration, as in tra~itional designs.
If the initial pre-tautening maintains in the core a stress which is still tensile even when the stator has reached its runnin~ temperature, the tensile stress i~,-posed ~y the magnetic pull i~ the traditional solution would superimpose itsel4 on this state of tra~tion. The overall state of compression of the core is therefore decrease~ and may even ~e cancelled out ~y a suita~le pre-tautening value.
Although for descriptive reasons this invention has been ~ase~ on the foregoing ~escriptions and illustra-tions ~y way of eKample only, many v~riations and al-terations may be brought about in the embodiment of the invention.

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Claims (6)

1. Device for the pre-tautening of a ring-shaped lamination (20) on a stator for use in a large-sized electric generator, said stator being of the type comprising a supporting shell (10) formed by a cylindrical housing (12) and by a plurality of superimposed annular plates (11) having edges which support tension rod-keys (30) to which a plurality of said stator core laminations (20) are secured axially with nuts (33) and radially with dovetail type keys (31) protruding from the tension rod-keys(30) and engaging corresponding grooves (21) made in the stator core laminations (20), characterized in that said device includes:
a) a base plate (41) secured by welding to the intrados of the annular plates (11) of the stator shell (10);
b) a bracket (51) equipped with tong-like arms (52) capable of gripping suitable fins (33) protruding circumferentially from the corresponding tension rod-key (30); and c) tautening means (57) for exerting and maintaining a centrifugal tensile stress on the bracket (51) and therefore on the laminations (20).

2. Device according to claim 1, characterized in that a plurality of said devices are applied to the core lamina-tions (20), and are distributed evenly both circumferen-tially and in height.

3. Device according to claim 1, characterized in that each base plate (41) has a vertical flange (42) with a pair of through holes (44) through which pass a pair of traction screws (57) having threaded shanks screwable into corres-ponding threaded holes (56) made in the back of the bracket (51).

4. Device according to claim 1 or 3, characterized in that stop nuts (59) are provided on said traction screw (57) between said flange (42) of the base plate (41) and the bracket (51).

5. Device according to claim 1 or 3, characterized in that said bracket (51) is provided with a vertical hole (54) for a retaining screw (55) having a threaded shank screwable into a corresponding hole (43) made in the base plate (41), said hole (54) in the bracket (51) having a diameter greater than the shank of the screw (55).

6. Device according to claim 1 or 3, characterized in that said bracket (51) has circumferentially protruding fins (58) providing a retaining surface for an U-shaped ejector 60 having a central cross-member (63), said retaining surface being useful when a jack (64) is inserted between said central cross-member (63) and the back of the vertical flange (42) of the base plate (41).