GB2450907A

GB2450907A - Reducing stray losses in a laminated rotor

Info

Publication number: GB2450907A
Application number: GB0713469A
Authority: GB
Inventors: William Brian Turner; Philip David Bend; Andreas Biebighaeuser; Andreas Kellerer
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2007-07-11
Filing date: 2007-07-11
Publication date: 2009-01-14
Anticipated expiration: 2027-07-11
Also published as: GB2450907B; GB0713469D0

Abstract

The rotor comprises a plurality of salient poles 14, 16 comprising a main pole body 18, 20 and a pole shoes extending from the body overlying the windings. At least one part of each pole shoe of pole 14 is recessed 40,44 in a circumferential direction with respect to another part of the same pole shoe 42,46, and the recessed part of a pole shoe is aligned with a non-recessed part of an opposing pole shoe 16. This can increase the apparent air gap between adjacent poles, which may allow magnetic flux leakage from one pole to another to be reduced, while still enabling the pole shoe to retain the windings and to distribute magnetic flux across the air gap between the rotor and stator. The laminations may be identically stamped and stacked rotated by 90{ to provide the required recesses. Amortisseur bars (fig 4) may be present and a castellation effect may be achieved (fig 3). Figs 5 and 6 show a coil wedging arrangement.

Description

1 2450907

REDUCING STRAY LOSSES

The present invention relates to rotating electrical machines of a salient pole design, and in particular to techniques for reducing stray losses in such machines.

Rotating electrical machines, such as motors and generators, generally comprise a rotor and a stator, which are arranged such that a magnetic tiux 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 tend to force the windings outwards in a radial direction. For this reason many salient pole machines have pole shoes at the pole tip, which overlap the rotor windings. The pole shoes thus assist in retaining the windings against the centrifugal forces developed as the rotor rotates. The pole shoes also distribute additional magnetic flux across the air gap between the rotor and the stator.

In a rotating electrical machine there are various losses which may reduce the efficiency of the machine. Such losses include windage and friction, iron losses in the rotor and stator, and copper losses in the rotor and stator windings. Part of these losses is attributable to what are commonly called "stray" losses. Some of these stray losses are caused by magnetic flux leakage from one pole to the adjacent and opposite polarity pole. This magnetic flux leakage manifests itself when load is applied, and is dependent on rotor current. If these losses can be lowered then the efficiency of the machine will rise.

According to a first aspect of the present invention there is provided a rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles, at least one of the poles comprising a main pole body and a pole shoe, wherein at least one part of a pole shoe is recessed in a circumferential direction with respect to another part of the same pole shoe.

By arranging a part of a pole shoe to be recessed in a circumferential direction, it may be possible to reduce the apparent air gap between adjacent poles. This may allow magnetic flux leakage from one pole to another to be reduced, while still enabling the pole shoe to retain the windings and/or to distribute magnetic flux across the air gap between the rotor and stator.

Preferably a pole shoe comprises at least two parts which are not recessed. This can allow the windings to be retained in at least two places, while reducing the apparent air gap between adjacent pole shoes. The pole shoe may also comprise at least two parts which are recessed. This may allow the pole shoe to be castellated, which may provide an effective way of retaining the windings while reducing the apparent air gap between adjacent poles.

Preferably a recessed part of a pole shoe is aligned with a non-recessed part of an opposing pole shoe, on an adjacent pole. This may assist in increasing the apparent air gap between adjacent poles, which may allow magnetic flux leakage from one pole to another to be reduced.

In any of the above arrangements, the amount of recess may be, for example, up to 20%, 30%, or 50% or more of the cantilevered part of the pole shoe in a circumferential direction. In a preferred embodiment, the amount of recess may be up to or around 66% of the cantilevered part of the pole shoe. The part or parts of the pole shoe which is or are recessed may be, for example, up to 20%, 30% or 50% or more of the pole shoe in an axial direction. Different parts of the pole shoe may be recessed by different amounts. Preferably the amount of recess in a circumferential direction and/or the percentage of the pole shoe which is recessed in an axial direction is sufficient-to-increase the-apparent-air gapbetween two poles; while retaining sufficient strength in the pole shoe to retain the windings and/or retaining sufficient magnetic flux across the air gap between the rotor and stator.

Rotors in electrical machines are usually formed from a plurality of laminations (e.g. laminated sheets of metal), in order to reduce eddy currents flowing in the rotor.

Where the rotor is formed from a plurality of laminations, a part of one lamination forming a pole shoe may be recessed with respect to the corresponding part of another lamination forming the same pole shoe. This may allow one or more recesses to be provided as part of the manufacturing process. l0

Preferably the laminations are arranged in groups of two or more successive laminations, and the pole shoe in one group of laminations may be different from the pole shoe in another group of laminations. For example, the laminations in one group may be recessed with respect to the corresponding laminations in other group. This can allow the non-recessed parts of the pole shoe to comprise two or more adjacent laminations, which may help to ensure that the pole shoe has sufficient strength to retain the windings. This can also allow the recessed parts of the pole shoe to comprise two or more adjacent laminations, which may be effective in reducing the apparent air gap between adjacent poles. For example, in at least one group, all of the laminations may be recessed.

It has been discovered that the most vulnerable part of a pole shoe may be the two ends, in an axial direction, of the pole shoe. These parts of the pole shoe are adjacent the winding out hangs, and thus may suffer additional centrifugal forces due to the winding out hangs. Thus the ends of the pole shoes in an axial direction may be non-recessed, in order to ensure the pole shoe has sufficient strength at these points. For example, an Outer lamination, or an outer group of laminations, may be non-recessed.

This may be the case for either or both ends of the pole shoe, and for some or all of the poles shoes in the rotor. Alternatively or in addition, parts of the pole shoes towards or at the ends may be recessed by less than other recessed parts of the pole shoes.

In one-embodiment of the invention; a group of Iaminationsaranend-ofa-pole shoe in an axial direction has at least one lamination which is recessed, and at least one lamination which is not recessed. This may enable the apparent air gap between adjacent poles to be reduced, while helping to ensure that the pole shoe has sufficient strength at the ends adjacent the winding out hangs.

In a preferred embodiment of the invention, the rotor comprises a plurality of laminations each of the same shape, and at least one lamination is rotated with respect to at least one other lamination. In this case, in each lamination, at least one of the pole shoes may have a different profile from at least one of the other pole shoes. For example, in each lamination, one pole shoe may recessed with respect to another pole shoe in the same lamination. This can simplify the manufacturing process, by allowing all of the laminations to be the same, and the recess or recesses to be formed simply by rotating the laminations. The at least one lamination may be rotated by the angle between the poles, or a multiple thereof (e.g. an odd multiple), with respect to the other laminations.

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. Rotating some laminations with respect to other laminations may also produce the advantage that 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.

Preferably windings are wound on the main pole body, and the pole shoe is arranged at least partial to overlap the windings in a circumferential direction so as to retain the windings.

In a rotating 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 usual 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.

It has been discovered that the traditional wedging arrangement may also contribute to stray losses, due to the magnetic bridging effect of the wedge. It has been discovered that a radial wedging arrangement may be effective in reducing these stray losses.

Thus the rotor may further comprise a radial wedging arrangement for wedging the windings of two adjacent poles. For example, the radial wedging arrangement may comprise a wedge located between two adjacent windings, and a radial member which connects the wedge to the rotor body, so as to retain the wedge radially. The wedge may be at least partially formed from a non-conducting material. Alternatively an axial bolting wedge design could be used, as described in United Kingdom patent publication number GB 2381389, the contents of which are incorporated herein by reference.

The present invention extends to a rotating electrical machine (such as a generator or a motor) comprising a stator, and a rotor in any of the forms described above.

According to another aspect of the present invention there is provided a method of manufacturing a rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles, at least one of the poles comprising a main pole body and a pole shoe, the method comprising manufacturing the rotor so that at least one part of a pole shoe is recessed in a circumferential direction with respect to another part of the same pole shoe.

The method may comprise manufacturing a plurality of laminations each of the same shape, and rotating at least one lamination with respect to at least one other lamination in order to form the rotor.

Any of the apparatus features may be provided as method features and vice versa.

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 parts of a rotating electrical machine; Figure 2A shows a plan view of the poles of Figure 1; Figure 2B shows an end section view of an exemplary pole lamination; Figure 3 shows a plan view of poles with an alternative arrangement to support end windings; Figure 4 is a plan view of a part of one pole; Figure 5 shows parts of a rotor with a radial wedging arrangement; and Figure 6 shows parts of a rotor with an axially bolted wedging arrangement.

Figure 1 shows parts of a rotating electrical machine. Referring to Figure 1, the machine comprises a rotor 10 and a stator 12. The rotor is formed from a plurality of laminated sheets of metal stacked together into the plane of Figure 1 to form a rotor of the required thickness. The rotor 10 comprises a plurality of salient poles 14, 16. For simplicity only two poles are shown in Figure 1, but it will be appreciated that the rotor has a number of poles disposed symmetrically about a central axis. Similarly the stator 12 extends circumferentially around the rotor 10.

In Figure 1, each pole comprises a pole main body 18, 20 and pole shoes 20, 22, 24, 26. Each pole main body is wound with a respective winding 28, 30. In operation an electrical current is supplied to the wmdings 28, 30. The windings of adjacent poles usually carry a current of the opposite sense, so that adjacent poles are of opposite polarity. The current in the windings causes a magnetic flux 32 to develop between the rotor and the stator.

In operation the rotor rotates about the central axis. The pole shoes 20, 22, 24, 26 are arranged to assist in retaining the windings 28, 30 against centrifugal forces as the rotor rotates. The pole shoes also help to distribute the magnetic flux 32 across the air gap 25 between the rotor 10 and the stator 12. A wedging arrangement 34 is provided to wedge the windings 28, 30 in place.

In rotating electrical machines various losses may reduce the efficiency of the machine. These losses include fan and rotor resistance (windage) and friction in the bearings; iron-losses-in the rotor and-the stator; and copper losses in the rotor windings and the stator windings.

Another part of these losses is made up from what are commonly called "stray" losses. These stray losses are not easily defined and are generally considered together. Some of these losses are caused by magnetic flux leakage 36 from one pole 14 to an adjacent pole 16 of opposite polarity. This magnetic flux leakage manifests itself when a load is applied, and is dependent on rotor current.

Figure 2A shows a plan view of the poles 14, 16 of Figure 1. Each pole is formed from a plurality of laminations which run from left to right in Figure 2A. Each pole main body 18, 20 is wound with respective windings 28, 30.

In Figure 2A, the laminations are arranged in four groups 40, 42, 44, 46. The pole tips of the laminations in one group are cut away or otherwise recessed with respect to the pole tips of the laminations in an adjacent group, to create a castellation effect.

For each group of laminations 40, 42, 44, 46, the pole tips in one of the poles 14, 16 are cut away, while the pole tips in the other pole are not cut away. This causes the castellation of one pole shoe to be offset with respect to the castellation of the opposing pole shoe, so that cut-away parts of one pole shoe are aligned with non cut-away parts of the opposing pole shoe.

Figure 2B shows an end section view of an example lamination 21 in a four pole machine. It can be seen that in this lamination the pole tips in the pole 16 are cut away, while the pole tips in the pole 14 not cut away.

The castellated pole shoes shown in Figures 2A and 2B cause the apparent air gap 19 between adjacent pole shoes to be increased, in comparison to the case where no parts of the pole shoes are cut away. This helps to reduce magnetic flux leakage between adjacent poles. Thus the castellation may help to reduce stray losses, and thereby increase the efficiency of the machine.

The amount of pole shoe which is retained must be sufficient to support the windings so that they do not bend significantly into the unsupported space under centrifugal forces. If the windings were to bend significantly; this could cause electrical shorts, turn to turn, or layer to layer in the windings. Investigations carried out by the present applicant into how much of the pole shoe is required to retain the winding without overstressing the pole shoe cantilever have shown that, in some circumstances, up to around 66% of the pole shoe circumferentially can be removed. Investigations have also shown that this may not significantly affect the magnetic flux path between the rotor and the stator. The potential benefits have been found to be the greatest in machines having six poles or more, although the invention may also be used with machines of fewer poles, such as four pole machines.

In practice, when designing a pole shoe with partially cut-away pole tips, it may be desirable to test the mechanical proprieties of the pole shoe, for example using finite element (FE) analysis. FE analysis can be used to check the loading on the cantilever part of the pole shoe under centrifugal forces, with the outer portion of the windings being only partially supported by the pole shoes. The inner portion of the winding is constrained by the inner portion of the full pole shoe cantilever. The area of high stress is normally at the base of the pole shoe cantilever.

The stresses in the cantilever of the pole tip section at each end of the winding must also be considered. Here there will be additional centrifugal loading due to the winding Out hang. It may be desirable not to have full castellation features at the ends, where the impact of the conductor mass out hang has greatest effect.

Figure 3 shows a plan view of poles shoes with an alternative castellation arrangement. In Figure 3 the laminations are arranged in four groups 50, 52, 54, 56, as examples. The middle groups of laminations 52, 54 are castellated in the same way as the laminations 42, 44 in Figure 2A. However the end groups of laminations 50, 56 are arranged such that individual laminations are alternately cut-away and not cut-away. A lamination which has a cut-away pole tip in one pole has a non cut-away pole tip in the other pole, so that cut-away parts of one pole shoe are aligned with non cut-away parts of the opposing pole shoe.

The arrangement of Figure 3 provides additional cantilever support for the windings at the ends of the poles where additional centrifugal loads due to winding out hang are maximum: The arrangement of Figurei may thus be more robust than that of Figure 2, while still reducing magnetic flux leakage between adjacent poles.

In Figure 3, rather than individual laminations in the groups 50, 56 being cut away, two or more successive laminations may be Cut away. In generally, any arrangement of cut-away and non-cut-away laminations is possible.

In order to minimise the number of rotor lamination designs, each lamination may be of the same design with the pole tips removed from every other pole. The castellated effect may then be created by rotating some groups of laminations by an amount equivalent to the angle between adjacent poles, or an odd multiple thereof. To provide addition cantilever support at the end of the windings, single laminations, or smaller groups of laminations, may be rotated by the appropriate amount in the outer laminations.

The arrangement described above can allow each lamination to be the same, and the rotor to be formed simply by rotating some groups of laminations through the appropriate angle. This can reduce the manufacturing costs compared to the case where laminations with different profiles are used. Furthermore, rotating groups of laminations with respect to other laminations may also compensate for crowning effects in the steel, and thus may help to ensure that the mass of steel in each pole is uniform. If required, different laminations or groups of laminations may be indexed by different amounts. For example, in a four pole machine, different groups of laminations may be rotated by 90 , 180 , and 270 respectively.

Figure 4 is a plan view of a part of one pole. In Figure 4, groups of laminations are arranged in a similar way to that described above with reference to Figure 3. Also shown in Figure 4 are amortisseur bars 60, 62, 64, 66. These bars are used to assist with electrical disturbances and machine design characteristics.

In operation, as the rotor starts to rotate and the loading on the pole shoe increases, the weakest point initially may be under the amortesseur bar. As the loading increases, for example during overspeed operation, plastic deformation of the lamination cantilever shoes takes place, and the pole shoe outer section cantilever deflects until the clearance between the amortesseur bar and the hole for the bar is closed Then the effective depth of the cantilever increases and since top half of this cantilever beam is in compression, this now shifts the maximum stress area to the corner of the pole shoe at the cantilever point 23 shown in Figure 2B.

Figure 5 shows parts of a rotor in which a radial wedging arrangement 70 is used in place of the wedging arrangement 2, 34 shown in Figure 1. The radial wedging arrangement 70 may be any of those described in GB 2425663, the contents of which are incorporated herein by reference. The use of a radial wedging arrangement such as that shown in Figure 5 may further reduce the stray losses in the machine, by reducing any magnetic bridging between the windings of adjacent poles. l0

Figure 6 shows parts of a rotor in which an axially bolted wedging arrangement 38, 39 is used. This wedging arrangement may be as described in GB 2381389, the contents of which are incorporated herein by reference. Such a wedging arrangement may also be effective in reducing stray losses.

In any of the above arrangements, the castellations may be formed with a specially adapted blanking tool when manufacturing a rotor with laminated steel, or by flame or laser cutting or otherwise profiling sheet steel laminations, or by having suitable shapes machined in the pole tips of solid rotors, or by any other method.

Claims

1. A rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles, at least one of the poles comprising a main pole body and a pole shoe, wherein at least one part of a pole shoe is recessed in a circumferential direction with respect to another part of the same pole shoe.

2. A rotor according to claim 1, wherein a pole shoe comprises at least two parts which are not recessed.

3. A rotor according to claim 1 or 2, wherein a recessed part of a pole shoe is aligned with a non-recessed part of an opposing pole shoe.

4. A rotor according to any of the preceding claims, wherein the rotor is formed from a plurality of laminations, and a part of one lamination forming a pole shoe is recessed with respect to the corresponding part of another lamination forming the same pole shoe.

5. A rotor according to claim 4, wherein the laminations are arranged in groups of two or more successive laminations, and the pole shoe in one group of laminations is different from the pole shoe in another group of laminations.

6. A rotor according to claim 5 wherein, in at least one group, all of the laminations are recessed.

7. A rotor according to claim 5 or 6 wherein a group of laminations at an end of a pole shoe in an axial direction has at least one lamination which is recessed, and at least one lamination which is not recessed.

8. A rotor according to any of the preceding claims, wherein the rotor comprises a plurality of laminations each of the same shape, and at least one lamination is rotated with respect to at least one other lamination.

9. A rotor according to claim 8 wherein, in each lamination, at least one of the pole shoes has a different profile from at least one of the other pole shoes.

10. A rotor according to claim 8 or 9 wherein, in each lamination, one pole shoe is recessed with respect to another pole shoe in the same lamination

11. A rotor according to any of claims 8 to 10, wherein at least one lamination is rotated by the angle between the poles, or a multiple thereof, with respect to the other laminations.

JO

12. A rotor according to any of the preceding claims, wherein windings are wound on the main pole body and the pole shoe is arranged at least partial to overlap the windings in a circumferential direction so as to retain the windings.

IS

13. A rotor according to any of the preceding claims, further comprising a radial wedging arrangement for wedging the windings of two adjacent poles.

14. A rotor according to claim 13, wherein the radial wedging arrangement comprises a wedge located between two adjacent windings, and a radial member which connects the wedge to the rotor body.

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

16. A method of manufacturing a rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles, at least one of the poles comprising a main pole body and a pole shoe, the method comprising manufacturing the rotor so that at least one part of a pole shoe is recessed in a circumferential direction with respect to another part of the same pole shoe.

17. A method according to claim 16, the method comprising manufacturing a plurality of laminations each of the same shape, and rotating at least one lamination with respect to at least one other lamination: - 18. Apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.

19. A method substantially as described herein with reference to the accompanying drawings.

Amendments to the claims have been filed as follows

1. A rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles comprising a main pole body and a pole shoe, wherein at least one part of each pole shoe is recessed in a circumferential direction with respect to another part of the same pole shoe, and wherein the recessed part of a pole shoe is aligned with a non-recessed part of an opposing pole shoe.

3. A rotor according to claim 1 or 2, wherein the rotor is formed from a plurality of laminations, and a part of one lamination forming a pole shoe is recessed with respect to the corresponding part of another lamination forming the same pole shoe.

4. A rotor according to claim 3, wherein the laminations are arranged in groups of two or more successive laminations, and the pole shoe in one group of laminations is different from the pole shoe in another group of laminations.

5. A rotor according to claim 4 wherein, in at least one group, all of the laminations are recessed.

6. A rotor according to claim 4 or 5 wherein a group of laminations at an end of a *:*::* pole shoe in an axial direction has at least one lamination which is recessed, and at least one lamination which is not recessed. S...

7. A rotor according to any of the preceding claims, wherein the rotor comprises * a plurality of laminations each of the same shape, and at least one lamination is * ** rotated with respect to at least one other lamination. * S.

8. A rotor according to claim 7 wherein, in each lamination, at least one of the pole shoes has a different profile from at least one of the other pole shoes.

9. A rotor according to claim 7 or 8 wherein, in each lamination, one pole shoe is recessed with respect to another pole shoe in the same lamination 10. A rotor according to any of claims 7 to 9, wherein at least one lamination is rotated by the angle between the poles, or a multiple thereof, with respect to the other laminations.

11. A rotor according to any of the preceding claims, wherein windings are wound on the main pole body and the pole shoe is arranged at least partially to overlap the windings in a circumferential direction so as to retain the windings.

12. A rotor according to any of the preceding claims, further comprising a radial wedging arrangement for wedging the windings of two adjacent poles.

13. A rotor according to claim 12, wherein the radial wedging arrangement comprises a wedge located between two adjacent windings, and a radial member which connects the wedge to the rotor body.

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

15. A method of manufacturing a rotor for a rotating electrical machine, the rotor comprising a plurality of salient poles comprising a main pole body and a pole shoe, s the method comprising manufacturing the rotor so that at least one part of each pole *:::* 25 shoe is recessed in a circumferential direction with respect to another part of the same pole shoe, and the recessed part of a pole shoe is aligned with a non-recessed part of an opposing pole shoe.

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* * 16. A method according to claim 15, the method comprising manufacturing a plurality of laminations each of the same shape, and rotating at least one lamination *** with respect to at least one other lamination.

17. Apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings. * .

18. A method substantially as described herein with reference to the accompanying drawings. e * * * * .1 * *. * S 0* I.. S.

S * S. * S * S..

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