GB2425663A

GB2425663A - Wedging arrangement

Info

Publication number: GB2425663A
Application number: GB0508332A
Authority: GB
Inventors: William Brian Turner; Philip David Bend
Original assignee: Newage International Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2005-04-25
Filing date: 2005-04-25
Publication date: 2006-11-01
Also published as: GB0508332D0

Abstract

A rotor for a rotating electrical machine is disclosed, the rotor having a plurality of salient poles for carrying windings. A wedging arrangement (45) is provided for wedging the windings of two adjacent poles (12,14). The rotor comprises a channel (52) passing through the rotor in an axial direction, and connecting means (42) for connecting the wedging arrangement to the channel. By providing such a channel and connecting means, the invention can allow the wedging arrangement to be fixed to the rotor body. This may avoid the need for a wedge stud assembly, and may reduce the stress on the pole shoes when the rotor is rotating.

Description

1 2425663 WEDGING ARRANGEMjNT The present invention relates to a wedging

arrangement for retaining the windings on the rotor of a rotating electrical machine. The present invention has particular application in rotating electrical machines of a salient pole design.

Rotating electrical machines, such as motors and generators, generally comprise a rotor and a stator, which are arranged such that a magnetic flux is developed between the two. In a rotating machine of a salient pole design, the rotor has a plurality of poles which extend radially outwards, on which a conductor is wound. An electrical current flowing in these windings causes a magnetic flux to flow across the air gap between the rotor and the stator. In the case of a generator, when the rotor is rotated by a prime mover, the rotating magnetic field causes an electrical current to flow in the stator windings, thereby generating the output power. In the case of a motor, an electrical current is supplied to the stator windings and the thus generated magnetic

field causes the rotor to rotate.

In a salient pole machine, as the rotor rotates, centrifugal forces develop on the windings, which tends to force the windings outwards in a radial direction. For this reason many salient pole machines have pole shoes at the pole tip. The pole shoes overlap the rotor windings, and thus retain the windings against the centrifugal forces developed in a radial direction as the rotor rotates.

In a salient pole machine forces will also develop on the windings in a circumferential direction as the rotor rotates. In order to counteract such forces and centrifugal forces it is known to provide a wedge between the windings of adjacent poles. The wedge is bridged between the pole shoes, and presses against the windings on either side on a circumferential direction so as to retain the windings in place. Typically the wedge comprises two wedge parts each of which abuts a respective winding, and a stud assembly between the two wedge parts. The stud assembly forces the two wedge parts against their respective windings.

In the known wedging arrangement the wedge stud assembly is in bending moment, and is held in place by the ends of the pole shoes. However such an arrangement creates extra loading on the pole shoes. Since the pole shoes are cantilevers, this can create a large stress at the support corners of the pole shoes, which leads to a requirement for larger pole shoes than might be desired.

According to a first aspect of the present invention there is provided a rotor for a rotating electrical machine having a plurality of salient poles for carrying windings, the rotor comprising: a wedging arrangement for wedging the windings of two adjacent poles; a channel passing through the rotor in an axial direction; and connecting means for connecting the wedging arrangement to the channel.

By providing such a channel and connecting means, the present invention can allow the wedging arrangement to be fixed to the rotor body. This may avoid the need for a wedge stud assembly, and may reduce the stress on the pole shoes when the rotor is rotating.

The present invention may also provide the advantage that it is not necessary to drill and tap into the rotor body. A rotor is usually formed from sheets of laminated steel, and such laminations do not form a good substrate for drilling and tapping.

Furthermore the part of the rotor body between the poles may have poor accessibility for drilling and tapping.

In addition, the present invention may allow the cross-section of the wedge to be smaller than would otherwise be required, which may increase axial airflow, thereby improving cooling of the rotor.

The channel may be located between two adjacent poles and below the windings.

This can allow the connecting means to extend radially between the wedging arrangement and the channel. Such an arrangement may be effective in countering centrifugal forces. Preferably the channel has an opening in a radial direction through which the connecting means can pass.

The channel may be formed by a bridging member which bridges the two adjacent poles. Such a bridging member can be made from a continuous piece of metal, which in turn may ensure that it has the strength necessary to resist any tension which is applied to the connecting means. Forming the channel from a bridging member can also allow air to pass through the channel, which may assist with the cooling of the rotor.

The bridging member may be intermittent in an axial direction through the rotor. This can allow an opening in the bridging member through which the connecting means may pass for engagement with the retaining piece.

Alternatively the channel may be a slot passing through the rotor in an axial direction, the slot having an Opening in a radial direction. The opening is preferably smaller than the maximum dimensions of the slot. Preferably the opening is large enough for the connecting means to pass through. The opening may be continuous in an axial direction through the rotor, or it may be intermittent.

Rotors in rotating electrical machines are usually formed from a plurality of laminated sheets of metal, in order to reduce the eddy currents flowing in the rotor. In order to provide an opening in the channel through which the connecting member can pass, where the rotor is formed from a plurality of laminations, in one lamination or a group of laminations the channel may be open in a radial direction, while in one or more other laminations the channel may be closed.

When manufacturing a rotor in accordance with the present invention, the rotor may be formed from a plurality of laminations each of which is the same shape, and one or a group of laminations may be rotated with respect to the other laminations. The laminations may have an asymmetrical profile. For example, in each lamination, the channel may open in a radial direction between some adjacent poles but not others.

For example, where the channel is formed from bridging members, in each lamination, a bridging member may be provided between some adjacent poles but not others. The effect of rotating the laminations is then to create openings in the channel when the laminations are placed together. This technique can allow openings for the connecting member to be formed without the need for manufacturing laminations with different shapes, which may make the manufacturing process more efficient.

Laminations for rotors are usually made from rolled sheet steel. During manufacture of the rolled steel there may be a slight crowning across the width of the roll due to deflections in the roller. This crowning effect may lead to a rotor having slight differences in the mass of steel in different poles. By rotating groups of laminations with respect to other laminations the mass of steel in each pole can be made more uniform. This may help to reduce vibration, and may also have the effect of raising the maximum flux density before saturation on the complete rotating field, thereby raising the rating of the machine.

The connecting means may comprise a connecting member extending from the wedging arrangement into the channel, and a retaining piece accommodated in the channel for engagement with the connecting member.

The retaining piece may have an internal thread for engagement with the connecting member. For example, the connecting member may be a bolt, and the retaining piece may be a nut or a bar with a drilled and tapped hole therein. Alternatively the connecting member may be a rod, and the retaining piece may be a pin or a bolt which is received by the rod on the inside of the channel. Other arrangements for connecting the wedging arrangement to the channel will be apparent to the skilled person.

The channel and the retaining piece may be arranged such that the channel can hold the retaining piece in place before the retaining piece has engaged with the connecting member. To achieve this, the retaining piece may have a cross section at least part of which substantially corresponds to at least part of the internal dimensions of the channel. For example, the retaining piece may have a cylindrical cross-section, or some other cross-section such as hexagonal, and at least part of the inside of the channel may have substantially the same cross-section.

If the retaining piece has a non-cylindrical cross-section then it may be possible to insert it into the channel without the need for it to be rotated to achieve the correct orientation with respect to the connecting member. Such an arrangement may assist when connecting the wedge to the rotor.

In one embodiment the channel itself has dimensions which substantially correspond to the cross-section of the retaining piece. In another embodiment the channel is larger than the cross-section of the retaining piece, and part of the channel is formed into a closed or semi-closed section formed to take the retaining piece. Where the channel is formed by a bridging member, this closed or semi-closed section may be formed in the bridging member. This arrangement can allow a cooling fluid (such as air) to pass through that part of the channel that is not obscured by the retaining piece, thereby improving the cooling.

Preferably the connecting member is in tension. This may be achieved, for example, by screwing the connecting member into the retaining piece until sufficient tension is obtained. By arranging the connecting member to be in tension, the wedging arrangement may be pressed securely against the windmgs thereby holding the windings in place.

In any of the arrangements described above the connecting member may be a bolt or rod, or anything else such as part of the wedge itself. Two or more connecting members may be provided for each wedge if desired.

In known wedging arrangements, metals are used for the wedge stud assembly in order to provide the required strength when the stud assembly is in bending moment.

In such wedging arrangements, magnetic bridging may occur between the poles due to the metallic stud assembly. This may cause a partial short circuit of the magnetic flux path, thus reducing the amount of magnetic flux and hence the efficiency of the machine.

The present arrangement does not require a wedge stud assembly which is in bending moment. Thus the wedging arrangement may be formed (partially or wholly) from a non-magnetic material. This may ensure that the wedging arrangement does not cause magnetic bridging between the poles.

In any of the arrangements described above the wedging arrangement may be finned, in order to assist with cooling of the rotor. The wedging arrangement may be a single piece, or more than one piece.

The invention also provides a rotating electricaj machine comprising a stator, and a rotor in any of the forms described above.

The present invention may be used in Conjunction with the rotor Cooling techniques described in the co-pending application entitled "Rotor Cooling" in the name of Newage International Limited and having the same filing date as the present application (representative's reference MIW/SS/43 158), the entire contents of which are incorporated herein by reference.

Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a rotor in a salient pole machine; Figure 2 shows parts of a rotor in more detail; Figure 3 shows parts of a rotor in accordance with a first embodiment of the present invention; Figure 4 shows a side view of the rotor with the windings and the wedge in place; Figure 5 shows how a wedge is inserted between the windings; Figure 6 shows a cross section through part of the rotor; Figure 7 shows parts of a rotor in accordance with a second embodiment of the invention; Figure 8 shows parts of a rotor in accordance with a third embodiment of the invention; and Figure 9 shows parts of a rotor in accordance with a fourth embodiment of the invention.

Figure 1 shows an overview of a rotor in a salient pole machine having a conventional wedging arrangement. Referring to Figure 1, rotor body 10 is formed from a plurality of sheets of metal which are laminated to create a rotor of the required thickness. The rotor body 10 comprises a plurality of salient poles 12, 14, 16, 18 each of which has on each side a pole shoe 12b, 14b, 16b, 18b. Each pole is wound with a respective winding 22, 24, 26, 28. In operation the rotor rotates about central axis 20.

As can be seen in Figure 1, wedges 32, 34, 36, 38 are provided between the windings of adjacent poles. The wedges 32, 34, 36, 38 press against the windings 22, 24, 26, 28 in order to hold the windings in place. Each of the wedges 32, 34, 36, 38 consists of two wedge parts which are pressed against respective windings, and a stud assembly which extends between the two wedge parts. The stud assembly is in bending moment in order to apply pressure to the windings. The wedges themselves are held in position by the tips of the pole shoes 12b, 14b, 16b, 18b.

When the rotor is rotating, the pole shoes 12b, 14b, 16b, 18b act to retain the windings in place against centrifugal force, as well as to retain the wedges in place. The combination of the bending moment of the wedges, and the centrifugal force of the windings can place a considerable stress on the pole shoes. In addition, differential thermal expansion forces and vibrations will also act on the pole shoes when the machine is operated. Hence the stresses in the pole shoes at running speed conditions can be very large. In a cantilever, the maximum stress occurs at the support corner of the cantilever, and therefore it is important when designing the rotor to ensure that the pole shoes have sufficient strength to withstand the various forces which may act upon them. In practice this may mean making the pole shoes thicker than would otherwise be desired, leaving less space for the windings, and hence reducing the amount of flux which can be generated by the rotor.

Figure 2 shows in more detail parts of a rotor having a conventional wedging arrangement. The wedging arrangement consists of a first wedge part 34a, a second wedge part 34b, and stud assembly 34c. In Figure 2 the first and second wedge parts 34a, 34b are provided with fins which act to cool the windings when the rotor is rotating.

Figure 3 shows parts of a rotor in accordance with a first embodiment of the present invention. In Figure 3, for the sake of clarity, the rotor has been shown without the windings in place. As can be seen from Figure 3, a plurality of bridging members 40 are provided between two adjacent poles of the rotor. The bridging members 40 are located below the windings in a radial direction, and act to support the windings from below.

In Figure 3, each of the bridging members 40 is formed from a group of laminations, each of which has a piece of metal extending between the poles as shown. Between the laminations which form the bridging members 40 are groups of laminations which have no bridging member. The effect of this is to create gaps 41 between the bridging members 40. These gaps can be used to secure a wedge to the bridging member, as will be explained below.

Figure 4 shows a side view of the rotor with the windings and the wedge in place. In Figure 4, bridging member 40 extends between rotor poles 12, 14 and abuts the bottom of the windings 22, 24 in order to retain the windings in a radial direction. A wedge 45 abuts the side of the windings 22, 24 and retains the windings in a circumferential direction. The wedge 45 is provided with a bolt 42 which extends between the windings 22, 24 and through a gap 41 between the bridging members 40 (see Figure 3).

Referring back to Figure 4, the bridging members 40 are provided with a semi-closed slot 43 which extends in an axial direction through the bridging members. The slot 43 accommodates a tapped bar 44. The tapped bar 44 has a cross section which substantially matches the profile of slot 43. The bolt 42 is screwed into the tapped bar 44 such that the wedge 45 is pulled by the tension of the bolt 42 towards the bridging members 40. In this way the wedge 45 is pressed against the windings 22, 24 so as to retain the windings against the various forces which the windings may experience when the rotor rotates.

By providing a sufficient tension on the bolt 42, the stresses which are applied to the pole shoes 12b, 14b can be significantly reduced.

Figure 5 shows in more detail how the wedge 45 is inserted between the windings 22, 24. Before the wedge is inserted, the tapped bar 44 is slid into the slot 43 in an axial direction. The relative dimensions of the slot and the tapped bar are such that the tapped bar is held in place by the slot 43. If the tapped bar is cylindrical then it may need to be rotated so that the tapped hole faces outwards, for example by using a magnetic screw driver. However, if the tapped bar has some other cross-section, such as square or hexagon, and the slot has a corresponding cross-section, then the tapped bar can be inserted with the correct orientation. Axial positioning may be achieved, for example, using a magnet fixed on a rod.

The bolt 42 is then inserted between two adjacent bridging members and screwed into the tapped bar 44, thereby drawing the wedge 45 towards the bridging members 40.

The tapped bar 44 is of sufficient length to be supported at least by the bridging members on either side of the bolt.

Figure 6 shows a cross section through part of the rotor with the bolt 42 screwed into place into the bar 44.

If required, two or more bolts may be inserted through respective adjacent bridging members, and screwed into respective holes in the tapped bar.

Also shown in Figures 4, 5 and 6 are cooling vents 50 which pass through the windings. These cooling vents are described in the co-pending patent application entitled "Rotor Cooling" referred to above. A channel 52 passing through the bridging members 40 in an axial direction provides the air supply for these cooling vents.

Figure 7 shows parts of a rotor in accordance with a second embodiment of the invention. In this embodiment the windings do not have cooling vents. Referring to Figure 7, rotor 60 comprises a plurality of poles 62, 64 of which two are shown in Figure 7. Each pole has a respective pole shoe 62b, 64b, and is wound with a respective winding 66, 68. A wedge 70 is located between the windings 66, 68 and retains the windings in a circumferential direction.

The rotor 60 has a semi-closed slot 72 which passes through the rotor in an axial direction at the junction between the poles 62, 64. This slot 72 is arranged to accommodate a tapped bar (not shown in Figure 7), which can be slid into the slot 72.

A bolt 74 extends in a radial direction between the wedge 70 and the slot 72. The bolt is screwed into the tapped bar, thereby pulling the wedge 70 towards the windings 66, 68.

In the arrangement of Figure 7, the slot 72 has an opening which is Continuous through the rotor in an axial direction. This opening is large enough to receive the bolt 74, but small enough to ensure that the tapped bar is held in place within the slot 72. In this arrangement all of the laminations can be made the same, and there is no need to index the laminations.

In an alternative arrangement the slot 72 is closed on either side of the bolt 74, which may give the slot greater strength. This arrangement can be achieved by appropriate indexing of the laminations.

Figure 8 shows parts of a rotor in accordance with a third embodiment of the invention. In Figure 8 the cooling channel and the channel for receiving the tapped bar are both part of a bridging member, but do not open into each other.

Figure 9 shows parts of a rotor in accordance with a fourth embodiment of the invention. In Figure 9 the channel for receiving the tapped bar is square shaped. It will be appreciated that in any of the other embodiments the corresponding channel may also be square shaped, or indeed any other appropriate shape.

The various embodiments described above provide the advantage that stress is removed from the pole shoes and transferred to the tension in the bolt. This can allow part of the pole shoe to be removed, compared to a conventional design, which can allow room for extra turns in the windings. This in turn may improve the power density of the rotor, or may increase the peak efficiency of the machine.

The nature of the construction removes the need for precise axial location of the wedging arrangement and this can allow for randonmess due to tolerances or manufacturing variations.

The wedging arrangement described above allows the entire windings to be constrained while permitting thermal expansion. This can minimise micromovement of the windings, which can reduce failure rates due to turn to turn or layer to layer short circuits in the windings.

While preferred features have been described with reference to specific embodiments, it will be appreciated that variations are possible within the scope of the invention. For example, the wedging arrangement may be used with a rotor having any number of poles. The bolt may be inserted in the opposite direction and tightened with a nut on the other side of the wedge. Various other modifications will be apparent to the skilled person.

Claims

1. A rotor for a rotating electrical machine having a plurality of salient poles for carrying windings, the rotor comprising: a wedging arrangement for wedging the windings of two adjacent poles; a channel passing through the rotor in an axial direction; and connecting means for connecting the wedging arrangement to the channel.

2. A rotor according to claim 1, wherein the channel is located between two adjacent poles and below the windings.

3. A rotor according to claim 1 or 2, the channel having an opening in a radial direction through which the connecting means can pass.

4. A rotor according to any of the preceding claims, wherein the channel is formed by a bridging member which bridges the two adjacent poles.

5. A rotor according to claim 4, wherein the bridging member is intermittent in an axial direction through the rotor.

6. A rotor according to any of claims 1 to 3, wherein the channel is a slot passing through the rotor in an axial direction, the slot having an opening in a radial direction.

7. A rotor according to claim 6, wherein the opening is continuous in an axial direction through the rotor.

8. A rotor according to any of the preceding claims, wherein the rotor is formed from a plurality of laminations, and in one lamination or a group of laminations the channel is open in a radial direction, while in one or more other laminations the channel is closed. S 13

9. A rotor according to any of the preceding claims, wherein the rotor is formed from a plurality of laminations each of which is the same shape, and one or a group of laminations is rotated with respect to the other laminations.

10. A rotor according to claim 9 wherein the laminations have an asymmetrical profile.

11. A rotor according to claim 9 or 10 wherein, in each lamination, the channel is open in a radial direction between some adjacent poles but not others.

12. A rotor according to any of claims 9 to 11 wherein, in each lamination, a bridging member is provided between some adjacent poles but not others.

13. A rotor according to any of the preceding claims wherein the connecting means comprises a connecting member extending from the wedging arrangement into the channel, and a retaining piece accommodated in the channel for engagement with the connecting member.

14. A rotor according to claim 13, wherein the retaining piece has an internal thread.

15. A rotor according to claim 13 or 14, wherein the channel and the retaining piece are arranged such that the channel can hold the retaining piece in place before the retaining piece has engaged with the connecting member.

16. A rotor according to claim 15, wherein the retaining piece has a cross section at least part of which substantially corresponds to at least part of the internal dimensions of the channel.

17. A rotor according to any of claims 13 to 16 wherein the channel has dimensions which substantially correspond to the cross-section of the retaining piece.

18. A rotor according to any of claims 13 to 16 wherein the channel is larger than the cross-section of the retaining piece, and part of the channel is formed into a closed or semi-closed section formed to take the retaining piece.

19. A rotor according to any of claims 13 to 18, wherein the connecting member is in tension.

20. A rotor according to any of claims 13 to 19, wherein the connecting member is a bolt or rod.

21. A rotor according to any of the preceding claims, wherein the wedging arrangement is formed from a non-magnetic material.

22. A rotating electrical machine comprising a stator, and a rotor according to any of the preceding claims.

23. A rotor or a rotating electrical machine substantially as described herein with reference to and as illustrated in the accompanying drawings.