Description

Technical Field

The present invention relates generally to rotor construction, and more particularly to rotors for use in polyphase permanent magnet machines.

Background Art

Prior permanent magnet machines have typically included permanent magnet rotors which rotate within a wound stator. The rotor includes one or more magnets each having a pair of spaced magnetic poles which develop flux in paths that intercept the stator windings. Typically, the magnets have only limited tensile strength and hence a retaining ring or other strength imparting apparatus must be provided to prevent the rotor from fracturing at high rotational speeds.

Permanent magnet machines which utilize rotors of this type typically exhibit an equivalent reactance which comprises the combined effect of several reactive compo- neηts. One of these components is the "subtransient reac¬ tance" which predominates the machine equivalent reactance when a load is applied or removed in step fashion over a predetermined short period of time. In the case of a poly¬ phase permanent magnet generator (PMG) this reactive compo- nent primarily governs the voltage balance between the phase voltages of the generator.

Prior wound field polyphase machines have utilized amortisseur circuits to improve voltage balance. These amortisseur circuits have typically included conductive bars disposed in portions of the rotor. The bars are electrical-

ly connected together at their ends to form a circuit which reduces the equivalent reactance of the machine under tran¬ sient or unbalanced conditions.

Kanovalov et al U.S. Patent No. 3,683,220 disclos- es a rotor for a wound field machine having dampers each comprising a ring of copper disposed in annular slots in the rotor.

Each of Bacardi et al U.S. Patent No. 4,339,874, Bergmeier et al U.S. Patent No. 4,296,544 and Silver U.S. Patent No. 4,260,921 discloses the use of damping bars dis¬ posed in ferromagnetic support members located between mag¬ nets in a permanent magnet rotor. These bars act in con¬ junction with a damping hoop disposed about the rotor to provide electrical damping. Further, it is contended that these bars can be used without the hoop to intercept and diminish the flux harmonics caused by the stator.

A publication of the General Electric Company dat¬ ed May, 1983, entitled "Permanent Magnet Generator Rotor Container Study" prepared for the Air Force discloses the use of amortisseur bars in magnetic segments adjacent the magnets of a permanent magnet rotor.

Disclosure of Invention

In accordance with the present invention, a rotor for a polyphase permanent magnet machine accomplishes a re- duction in the subtransient reactance of the machine to thereby improve the voltage balance thereof.

More specifically, the rotor includes a magnet structure including at least two magnetic poles for develop¬ ing magnetic flux in flux paths which intercept windings disposed in a stator of the machine, at least one electrical conductor disposed in a flux path between each pole and the winding and means for electrically connecting the conductors

together to provide an amortisseur circuit whereby a reduc¬ tion in equivalent reactance of the machine is accomplished to improve the voltage balance thereof.

In specific embodiments of the invention, the electrical conductors comprise amortisseur bars which are rectangular or circular in cross-section wherein a plurality of such bars are disposed in channels in the face of each pole of the magnet structure or in pole pieces disposed atop the poles. The amortisseur bars are shorted at their ends by conductive plates which are disposed at axial ends of the magnet structure. The entire magnet structure is surrounded by a nonmagnetic retaining ring which imparts strength thereto to prevent fracturing at high rotational speeds. The amortisseur bars may all be of the same cross- sectional size or may be of different cross-sectional sizes wherein one or more bars located near the center of the pole faces are larger than the remaining bars to further lower the reactance of the machine. In a further embodiment of the invention, the mag¬ net structure includes amortisseur bars disposed in channels in the pole faces, thin magnetically permeable cover plates covering the pole faces, high reluctance supports extending between and joined to the cover plates and a retaining ring surrounding the amortisseur bars, the cover plates and the supports. The combination of the cover plates and the sup¬ ports lend added strength to the magnet structure so that the retaining ring may be reduced in thickness as compared with the previous embodiments. This reduction in thickness together with the thin magnetically permeable cover plates permit the effective air gap length of the machine to be decreased and result in lowered amortisseur leakage reac¬ tance. As a consequence, better voltage balance is achieved

and the size of the machine with which the rotor is used can be reduced as compared with prior machines.

Brief Description of the Drawings

Fig. 1 is a perspective view illustrating the gen- eral construction of each of the rotor embodiments of the present invention;

Fig. 2 is a diagrammatic elevational view of a first embodiment of the invention with the shorting plates of Fig. 1 removed in conjunction with a wound stator of a permanent magnet machine; and

Figs. 3-7 are views similar to Fig. 2 with the stator omitted illustrating further embodiments of the in¬ vention.

Best Mode for Carrying Out the Invention Referring now to Fig. 1, there is illustrated the general or overall construction of each embodiment of the present invention. Each of the rotors is particularly adapted for use in a permanent magnet machine, such as a generator, which exhibits a subtransient reactance and which includes a stator 10, shown in Fig. 2, that includes wind¬ ings 12 therein.

The rotor includes a magnet structure 13 including at least two magnetic pole regions 14a,14b of opposite mag¬ netic polarity for developing magnetic flux in flux paths that intercept the windings 12. The magnet structure is cylindrical in shape and includes axial faces 16a,16b (only the face 16a being shown) at axial ends of the magnet struc¬ ture 13.

As noted more specifically below the pole regions 14a,14b may comprise poles 15a,15b of the magnet structure itself or pole pieces disposed atop the poles.

A retaining ring 18 of high strength, high reluc¬ tance material surrounds the magnet structure to impart strength thereto. The retaining ring 18 prevents the magnet structure 13 from fracturing at high rotational speeds which might other-wise occur due the low tensile strength of the magnet material. In the preferred embodiment, the retaining ring 18 is fabricated from high strength glass or other ma¬ terial which minimizes eddy current losses therein to im¬ prove the efficiency of the permanent magnet machine. The magnet structure 13 rotates about an axis of rotation, illustrated by the line 20 in Fig. 1, within the stator 10.

Referring now to Fig. 2, there is illustrated a first embodiment of the invention. The magnet structure includes at least one, and preferably a plurality of elec¬ trical conductors 22 disposed in the flux paths between or otherwise in radial alignment with each pole 15a,15b and the polyphase windings 12. In the embodiment illustrated in Fig. 2, the conductors are disposed directly in channels 24 formed in pole faces 26a,26b of the poles 15a,15b, respec¬ tively.

Means are also provided for electrically connect¬ ing the conductors together to provide an amortisseur cir¬ cuit whereby the subtransient reactance is reduced to mini- mize imbalance among the polyphase voltages in the windings 12. Such means comprises first and second electrically con¬ ductive shorting plates 28a,28b, Fig. 1, which are disposed adjacent the end faces 16a,16b, respectively. The shorting plates are electrically connected to the electrical conduc- tors 22 by brazing or by any other suitable method.

As seen in Fig. 1, each of the shorting plates 28 includes opposing portions 30,32 having slots or channels therein for receiving the conductive bars 22. The plates 28

are otherwise identical in cross-section to the magnet structure 13 exclusive of the retaining ring 18.

In the preferred embodiment, each electrical con¬ ductor comprises a bar of copper or other electrically con- ductive material. In the embodiment illustrated in Fig. 2, the bars are rectangular in cross-section and extend along the axial dimension of the magnet structure. Moreover, the cross-sectional area of each conductor is constant along the length thereof and all of the conductors are of the same cross-sectional size.

Referring now to Fig. 3, there is shown a first alternative embodiment of the invention wherein the channels comprise rectangular slots 40 formed in each of the poles 15a,15b. The amortisseur bars 22 are disposed within the slots 40. The magnet structure further includes overhanging portions 42,44 associated with each of the slots 40 to as¬ sist in retaining the bars 22 in the slots 40.

In all other respects, the rotor shown in Fig. 3 is identical to that shown in Fig. 2. Illustrated in Fig. 4 is a further embodiment of the invention which is identical to that shown in Fig. 3 with the exception that the rectangular slots 40 are re¬ placed by circular slots 50 having circular conductive bars 52 disposed therein. Also, the regions 30,32 of the short- ing plates 28 include circular slots identical to and in registry with the slots 50, rather than the rectangular slots shown in Fig. 1.

It should be noted that the embodiment illustrated in Fig. 4 is generally more difficult to construct than the previous embodiment owing to the difficulty of machining round slots in the magnet structure material.

In the previous embodiments illustrated in Figs. 2-4, the electrical conductors are disposed in poles of the

agnet structure. Illustrated in Fig. 5 is a still further embodiment of the invention wherein electrical conductors are disposed in pole regions comprising pole pieces 60a,60b disposed on the poles 15a, 15b, respectively. The pole pieces 60a,60b are fabricated of magnetic steel or other magnetically permeable material. The electrical conductors, in this case the bars 22 illustrated in Figs. 2 and 3, are disposed in rectangular slots 70 similar to the slots 40 shown in Fig. 3 which are in turn formed in the pole pieces 60a,60b.

It should be noted that the slot 70 may alterna¬ tively be circular in cross-section, similar to the slots shown in Fig. 4 or may comprise the channels 24 illustrated in Fig. 2. In all instances, the construction of the rotor is simplified over those illustrated in Figs. 2-4 since it is generally easier to machine slots or channels in steel than in magnetic material.

Referring now to Fig. 6, there is illustrated an embodiment similar to that disclosed in Fig. 5 with the ex- σeption that the slots 70 and conductors 22 all having the same cross-sectional size are replaced by a plurality of slots 80 and conductors 82 of various cross-sectional sizes. More particularly, and with reference to the pole region comprising the pole piece 60b, rectangular conductive bars 80a-80e are disposed within rectangular slots 82a-82e, re¬ spectively. The conductors 80a,80b,80d,80e are all of the same cross-sectional size as are the slots 82a,82b,82d,82e. The conductive bar 82c, however, is larger in cross-sec¬ tional size than the remaining bars 80 as is the slot 82c. Of course, the rectangular bars and slots 80,82 may be replaced by circular bars and slots, if desired. Furthermore, the relative sizes of the bars and slots may be varied from that shown in Fig. 6, if desired.

By providing one or more relatively large conduc¬ tive bar in each pole region, the overall resistance of the amortisseur circuit can be lowered, in turn resulting in a desirable decrease in the subtransient impedance of the ma- chine.

It should be noted that the embodiments illustrat¬ ed in Figs. 5 and 6 involve a lesser reduction in the sub- transient reactance than the embodiments shown in the pre¬ ceding figures due to the placement of the magnetic steel pole pieces in the path of the magnetic flux. However, this is outweighed by the increased ease in forming the channels or slots in the pole regions.

Referring now to Fig. 7, there is illustrated an¬ other embodiment of the invention which results in improved efficiency and reduced reactance. The embodiment illustrat¬ ed in Fig. 7 is similar to the embodiments illustrated in Figs. 2-4, with the exception that means are provided sur¬ rounding the rotor for imparting strength thereto so that the thickness of the retaining ring 18 can be reduced. Such means comprises first and second magnetically permeable cov¬ er plates 90a,90b which extend over and cover the pole faces. 26a,26b of the poles 15a,15b, respectively. These cover plates may be fabricated from magnetic steel or any other high permeability material. A pair of. high reluctance supports 92,94 extend between the first and second cover plates 90a,90b and are joined thereto in any suitable fashion, such as by welding.

The high reluctance supports may be of any type of material which imparts strength to the magnet structure yet which resists the passage of magnetic flux therethrough.

The retaining ring 18 surrounds the rotor includ¬ ing the conductors, in this case the round conductors 52 similar to those shown in Fig. 4, the cover plates 90 and

the supports 92,94. The retaining ring, as before, is fab¬ ricated from high reluctance metallic or nonmetallic mater¬ ial, such as high strength glass.

Each of the cover plates 90 has a substantially constant thickness di over the width (i.e from one support 92 to the other support 94) and the length (i.e. in the ax¬ ial direction) thereof. This thickness, in the preferred embodiment, is on the order of the thickness d2 of the re¬ taining ring 18. The retaining ring thickness, in the em- bodiment illustrated in Fig. 7, is substantially less than the thickness of the retaining rings illustrated in the pre¬ vious embodiments. This reduction in thickness is possible due to the extra strength imparted by the cover plates 90 and the supports 92,94. In effect, magnetically permeable material, i.e. the cover plates 90, replaces what would nor¬ mally be nonmagnetic material required to hold the rotor together. This reduction in the thickness d2 of the retain¬ ing ring 18, coupled with the use of the magnetically per¬ meable cover plates 90 permits an overall reduction in the ai gap length of the machine. The reduction in air gap length means that the magnet structure can be made smaller for specified output characteristics as compared with ma¬ chines utilizing a relatively thick retaining ring.

It should be noted that the thickness d^ of the cover plates 90 is selected to be sufficiently small to pre¬ vent substantial leakage flux between the conductors or bars in each pole or pole piece. That is, the thickness i is selected so that saturation occurs at a relatively low flux level whereby as much flux as possible is coupled to the stator windings rather than from conductor to conductor.

It should be noted that the embodiment of Fig. 7 may be modified, if desired, by substituting rectangular bars or conductors of the same or varying cross-sectional

sizes and/or by using pole pieces as the pole regions of the magnet structure, if desired.

Any of the foregoing embodiments may be modified by the addition of shorting plates or conductors, illustrat- ed in dotted line form in Fig. 7 as elements 100 and 102, which extend axially between the shorting plates 28a,28b, Fig. 1. The conductors 100,102 are electrically shorted either by the end plates 28 or by additional conductors or bars extending across the axial faces and joined to the con- ductors 100,102. Each of the conductors 100,102 may com¬ prise a bar of copper or other conductive material or to¬ gether may be formed by a series of electrically conductive windings wound about the axial extent of the magnet struc¬ ture. The conductors 100 and 102 and the shorting plates 28 together define a conductor path which extends about the axial extent of the rotor and which has an axis coincident with the "direct axis" of the magnet structure. The "direct axis" is the line shown in Fig. 7 extending through the poles and the axis of rotation 20 of the magnet structure. The addition of these shorting plates reduces the

"direct axis transient reactance" of the machine, which is defined as the reactance thereof along the axis of the rotor magnetic field. This reduction of the direct axis transient reactance in turn results in a reduction in the rise or sag of terminal voltage of the machine when a load is removed or applied, respectively. Consequently, the size and/or weight of the machine can be reduced for a given transient response requiremen .

A greater understanding of the effect of the shorting plates or conductors 100,102 may be obtained by reference to Vaidya et al U.S. Patent Application Serial No. , filed , entitled (Sundstrand Docket

B02189-AT3-USA) , the disclosure of which is hereby incorpor¬ ated by reference.

The addition of the conductors 100,102 to any of the embodiments described above results in a permanent mag¬ net machine having greatly reduced reactance and improved transient response and voltage balance characteristics.