CA1047090A - Rotor construction for slow speed, large diameter electrical machines - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A rotor of an electrical machine having a large diameter and which rotates about a vertical axis comprises a central body and rim which form concentric inner and outer rings respectively, joined together by means of a number of spokes spaced uniformly around the circumference. The arrangement of the spokes is such that they are tangent to the surface of an imagined coaxial cylinder the diameter of which is smaller than that of the inner ring, and the axes of two neighboring spokes when imagined as extended in a straight line over both rings intersect only within the inner ring. The spokes are inflexible over their whole effective length in the longitudinal direction corresponding to the applied load.

Description

The invention concerns a rotor of an electrical machine in particular of vertical construction, the central body and rim of the rotor forming two rings joined by means OI a number of spokes spaced evenly around the circumference.
Rotors of this kind are built in particular for slow-run-ning machines of large diameter, for example cement mill drives or hydro power generators. The means of fixing the rotor rim to the rotor spider must be capable of accommodating both the torques arising when the electrical machine is in operation, and also the radial forces resulting from enlargement of the rim brought about by centrifugal stress and the effects of hsating. The aim indeed is that the rim should lift off from the spider only above the operating speed, and as far as possible only at the test overspeed.
A first generaLly known method of fixing is to shrink and/ -or radially wedge the rim to the spider, which is provided with radial arms. This can give rise to very large shrin~age forces between the two components which under certain circumstances may necessitate mechanical reinforcement of both components; this is true especiaLly of vertical-axis rotors, which are severely stressed in compression by the shrinkage of the spider. These very ;
costly methods have the further disadvantage that the rotor rims cannot easily be removed from the spider.
Another known construction (Swiss patent 537 656, published July 13, 1973, issued to S;iemens Aktiengesellschaft and invented by Alfred Valencsak et al) provides for fixing the rotor rim to the spider,

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~4~709~ -which is furnished with radial arms, by means of resilient adapter pieces which are fixed to the arms and at the same time engage snugly in slots in the rim. These adapter pieces transmit the tangentially directed forces arising from the torque. In order to ensure that the adapter pieces engage fully in the radial direction under all operating conditions they are radially oversize prior to assembly, to create a pre-stress opposing the centrifugal force.
The rim, together with the adapter pieces, is heated and drawn on to the spider. After cooling, the adapl:er pieces are welded ;
to the arms. An arrangement of this kind prevents the rim from lifting off at overspeed, but owing to the use of resilient com- ;
ponents it is not possible to guarantee either the circularity or concentricity of the rotor, particularly in the event of asymmetri-cal load.
A further rotor concept has been proposed in connection with a cement mill rotor (U. S. Patent 3, 272, 444, issued on September 13, 1966 to and in~rented by Rich et al). Resilient -spokes run eccentrically from the central body to the rotor rim, ~ ~ ;
but because they are resilient they can also stretch radially.
Alternatiaely, the spokes can be replaced by discs, provided :.: : :
these are thin enough to give the required resilience. If the supporting structures is in the form of a rigid disc, the rotor hub must be of resilient construction. -~

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With all the solutions described hitherto, relative expansion of the various components is made possible either by radial flexibility or by the ability of the arms to move radially.
With large machines especially, for which the ring-shaped components are designed to be highly resilient, the problems shift to the stiffness of the rings and concentricity, and this leads to very expensive solutions.
Because the radial rotor spokes are subjected to bending stress when transmitting the torque, the cross-section of the spokes must be sharply curved in order to absorb the forces created by a dead short at the generator terminals, for example.
The object of the invention is to avoid the disadvantages . .: :: :
stated above and devise a rigid construction which can transmit `;~
axial, radial and tangential forces, allows concentric expansion ~ ~;
of the ring in the event of both symmetrical and asymmetrical loads, and is ~ubject to only small expansion forces .
This object is achieved in that the spokes are tangents to the surface of an imagined coaxial cylinder the diameter of ,.:'.' ;'' ~ ',. .

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which is smaller than that of the concentric inner ring, that the axes of two neighbouring spokes in the circumferential direction when imagined as extended in a straight line over both rings intersect only within the inner ring, and that the spokes are inflexible over their whole effective length in the lingitudinal direction corresponding to the applied load.
The advantage of the invention lies particularly in the fact that all components are completely free to expand con-centrically without the use of resilient radial elements, such as springs for example. This allows simpler and less expensive constructions, as the shrink-fitting technique is also eliminated.
If the spokes are fixed to at least one of the rings by means of hinged joints, the centrifugal and thermal expansion forces exerted on the rings by the spokes are almost completely ~-eliminated, and so the rings can be of lighter construction.
Since the spokes are not subjected to bending stress their cross-section can also be made smaller, yielding a significant saving in material. In addition, a hinged form of attachment has important ;
advantages over a rigid fixing as regards assembling and dis-mantling the components.
It is convenient to make the spokes adjustable in length. In this way, initial eccentricities can be kept small and the circular shape of the ring-shaped elements can be adjusted exactly.
Particularly effective is an arrangement whereby one of the rings consists of a number of segments linked together by hinged joints, thus forming a kind of chain. It is then possible, especially in the case of very low-speed machines with a large number of poles, to create a standardized pole shoe which con-sequently can be made in quantity and at low cost.
In one aspect of the present invention there isprovided a rotor of an electrical machine of the vertical axis S~ , ,,, ~
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~0470~0 type, the central body and rim of the rotor forming two concentric rings joined by means of a number of spokes spaced uniformly around the circumference, in which the spokes form ~
tangents to the surface of an imagined coaxial cylinder, the . :
diameter of which is smaller than that of the concentric inner ring, the axes of two neighbouring spokes when imagined as :~
extended in a straight line over both rings intersect only within :~
the inner ring, and the spokes are inflexible over their effective :
. ~ .
length in the longitudinal direction corresponding to the applied load. ~ :

The invention is explained in the following by means of examples with reference to the accompanying drawings, ln which~
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~ ig. 1 ~hows part of a radlal section through the rotor of an electrlcal machine, Fig. 2 is a partlal cros~-section o~ the arrangement ~hown in Fig. 1, Fig. 3 is a ~chematic repre~entation o~ a rotor to illu~-trate the principle of the invention, and Fig. 4 ~hows part of a radial section through the rotor o~
a horizontal-axi~ mill drive.
Identical components are provided in the ~igure~ with the ~ame reference ~ymbols.
The example shown in Fig. 1 and 2 i~ a laminated~rim rotor of a low- peed vertical machine and i~ a composite construction -o~ spokes and di3cs. Components not essential to the lnvention, .
~uch as pole shoe~, tie bolts, machine shaft and the like, are not hown. The concentric lnner ring 1 comprise~ a laminated assembly, pre~erably incorporating discs 2 which are arranged axially in two planes and are each welded at their inner circum-ference 6 to the machine shaft, which is only indicated. The outer ring 3 is formed by a laminated rim made up o~ blanked and ; ~;
stacked ~egments which are as long as pos~ible and contains a number o~ poles, the segment3 being ~oined by tie bolts to form a complete ring. The laminated rim and inner ring ara Joined by ~ -spokes 4 which form tangent~ to the surface of an imaginad coax-ial cylin~er ~, the diameter o~ which is smaller than the out~ide diameter Or the inner rlng. The fixlng points o~ the spokes 4 in the two rings 1,3 are ln the form of hinged ~oints, indicated in the example ~hown by hinge rod~ 5, which are ~itted in guides not ;~
shown. The ~poke~ 4 themselve~ are plates~welded to the hinge ~ ;
rods and extendlng over approximately the entire axial length Or the rotor.
The principle o~ the invention can be de~cribed with re~er-, . ~ , ........................ ..
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~04709~
ence to the diagra~ of Fig. 3. The two concentric rings and the spokes are identirled by the same reference ~ymbols as the corres-ponding parts ln Fig. 1 and 2. The lines of the ~pokes 4, when produced, form tangents to the lndlcated cylinder 7. Starting from the centre 8 Or the system, RI lq the radiu~ of ring 1, RA
that of ring 3, and the two radli form an angle ~ denotes the angle contained between RA and the considered spoke, which has a length of L. For ~implicity the angle ~ is introduced;
this i8 the arithmetic sum of ~ and ~
Let one first consider symmetrical expansion ~ L of the spokes 4, ~ue to heat, for example. This elongation is converted ~
into a relative rotation of the two ring~ 1J3J which is moæt con- -~ ~ -veniently expressed as a change in angle ~ ~ _ L

According to the obJect o~ the invention, expansion of the ~ ~-rlngs is required to be free and concentric under all load con-ditions. The following generally valid relationship can be writ- -ten for both separate and comblned changes of L, RI and RA:

L + ~RI RA . cos o~
RI sin ~ RI . tan~ RI . sin This relationship is valid when the fixings at both ends ~
of spokes 4 are hinged. ;
When the assembly ls loaded symmetrically the expanslon or .-shrinkage of the ring-shaped parts ls always concentric.

If the ~ixing points on both rings are provided with hinges, none of the ~orces caused by the ~pokes 4 are transmitted to the rings 1,3. Per~ect transmission o~ torque i~ possible, however.
As the!spokes are stressed only in tension and compression they need be dimensioned only with respect to these normal ~orcesJ

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~04~09~) which are derined as follows:

F _ M
N . sin C RA
where, F = normal ~orce acting on an lndividual spoke M = torque to be transmitted N = number o~ spoke~
R~ and ~in o~ are as in Fig. 3.
Another example of the lnvention is shown in Flg. 4, Thi~
ls the rotor o~ a horizontal-axls split machine, a design used particularly ~or cement mill drives. Very low-speed rotors of this kind are known to be subJected in operation to little cen~
tri~ugal ~orce, but instead to extreme, at time~ asymmetrical thermal expanaion ~orces. The latter can induce stresses in the rotor rim o~ such magnitude that the Joints at the split can rupture. Owing to the low speed, rotors o~ thi~ kind are usually fed via ~requency changers with a current of low frequency ( ~ 5 Hz) in order to keep the number of poles wikhin reasonable limits. Not shown in Fig. 4 is the drum; the concentric innn~r ring 1' is formed by the drum flange which carries the laminated rim via forty spokes 4' evenly spaced ~ound the circumference.
The spokes 4' are anchored in hinged ~oints 5' in both the rim and the drum flagge, and their lines when produced form tangents to the sur~ace of a coaxial cylinder 7. The spokes 4 are round bars. It is not shown that they are ad~ustable in length; this can be done by providing the ends with left and right-handed threads. With this length adJustment it is possible to reduce to a minimum any initlal eccentricities caused during erection, and ensure that the rotor i8 exactly circular. The laminated rim, with twenty pole pair~, consists of ~orty segments 10 ~oin-ed together by hinges 9 and ~orms the concentric outer polygon 3 - 7 - ~

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10~90 o~ the invention. This arrangement, whlch may be termed a chaln rotor, together wlth the drum flange and the spoke~ 4', which are hinged at least on the rim itself, forms a rigid structure;
firstly, bec2use no ben~ing takes place in the rim owing to the . ~ .
hinged Joints, and secondly because the Porces and moments acting - ;~
on the rotor are separated into components which act in the dir-ectlon of the spoke axes and so cause relative rotation between rim and drum flange. Thi~ is of particular advantage Por the drum manu~acturer who, in designing the drum, no longer need take into account any rorces exerted on the drum by the rotor. A
-..-further advantage o~ the chaln rotor is that, despite changes oP
geometry, the same type of pole can always be used~ it being nec~
essary only to vary the number o~ pole pairs and match the supply Prequency to the desired speed by way oP the frequency changer.
The ~nvention is Or course not re~tricted to what is ~hown in the drawings. As an alternative to the hinged spoke ~oints, these could also be rigid, ~or example by means o~ bolts or weld~
ing, Owing to the relative rotation of the rings the spokes will ~ ;
be subjected to slight bendingJ and the change Or angle ~ ~ will be somewhat smaller than with the hinged configurationO Since the spokes are pre~erably o~ shaped pr~Plle with a relatively high moment Or inertia~ owing to the risk of buckling, the ends can be solid, but must have a moment o~ inertia as small as pos3-ible about the bending axi~. In this wayJ bending can be local~
ized to the ends Or the spokes. A particularly slmple way Or making these end~ easily bendable is to use hollow sectio~ for the spokes. Over their eP~ective load-bearing length these have a thin-walled cross-~ection with a relatively high moment Or lnertia; at the polnts o~ attachment the hollow proPile i9 pres~- ;
ed to a rlat proPile. The load-bearing cross-~ection then remains the sameJ while the moment oP inertia about ~he bending axis is .

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sharply reduced.
The rings Or the inven~ion can be o~ polygonal or circular :
~hape, provided the anchoring point~ of each ring lle on a circle. -~: ' ' ' '~ ~-. .

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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rotor of an electrical machine of the vertical axis type, the central body and rim of the rotor forming two concen-tric rings joined by means of a number of spokes spaced uniformly around the circumference, in which the spokes form tangents to the surface of an imagined coaxial cylinder, the diameter of which is smaller than that of the concentric inner ring, the axes of two neighboring spokes when imagined as extended in a straight line over both rings intersect only within the inner ring, and the spokes are inflexible over their effective length in the longitudinal direction corresponding to the applied load.

2. A rotor as claimed in Claim 1, in which the spokes are attached to at least one of the rings by means of hinged joints.

3. A rotor as claimed in Claim 1, in which the spokes are adjustable in length.

4. A rotor as claimed in Claim 1, in which one of the rings consists of a number of segments linked together by hinged joints.