GB2509738A

GB2509738A - Integral slot wedges and stator sleeve

Info

Publication number: GB2509738A
Application number: GB1300484.1A
Authority: GB
Inventors: Ellis Fui Hen Chong; Geraint Jewell; Maurice Patrick Orwen Shortte
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2013-01-11
Filing date: 2013-01-11
Publication date: 2014-07-16
Also published as: GB201300484D0

Abstract

A stator sleeve 40, 80, arranged to be positioned between a rotor cavity and a stator core 50 of an electrical machine, comprising integral slot wedges 72 formed by ridges 56 closing the stator core slot opening. The sleeve may be formed with integral slot liners 82. A portion 52 of greater radius which may be radially thicker than the section containing the troughs 54 and ridges 56 may be formed at one or both ends of the trough/ridge section and may form an abutment wall 58 which engages the end of the stator core. Rebate portions 62 beneath the top surface 64 or each rib may be provided so as to interlock with the core teeth. Alternatively or additionally, the ramp surfaces 89 may define rebated portions (90 fig 4a). The precision fit of the sleeve in the stator prevents radial and circumferential displacement and the wall 58 prevent axial displacement. The arrangement of troughs and ridges improve resistance to slot wedge failure and provide reinforcement such that the troughs 54 can be thinner than prior sleeve arrangements improving electromagnetic performance by allowing the air gap to be reduced.

Description

STATOR SLEEVE

The present invention relates to a stator sleeve, a stator core, an electrical machine and a method of assembling an electrical machine. The invention may have particular application to electrical machines having stator cores that are flooded with cooling liquid, but its utility is not limited to such applications.

Prior art electrical machine stator windings are typically contained in stator core slots. Each slot is provided with a slot wedge which blocks the slot opening and so retains the windings in the slot. The slots may be provided with notches in their side walls in which the slot wedge is seated. Alternatively the side walls of the slot may be provided with an extending rim onto which the slot wedge is seated. The wedge typically has a coefficient of thermal expansion that is different to the stator core. The wedge may for example be glass-fibre reinforced resin board, soft magnetic powder combined with elastomer (semi-magnetic wedge) or fibre-board strips, whereas the stator core may be silicon Iron. The different thermal expansion rates may allow for a slot wedge to be dislodged, especially where the slot is subject to vibration.

A further issue regarding slot wedges is that they must generally be relatively thick in order to withstand the rigours of assembly and to be sufficient to retain the windings without failing. The extent to which the slot wedge extends into the slot is therefore increased, reducing the slot fill factor (the number of windings in the slot).

A further feature of some prior art designs is the use of a stator sleeve extending from one end-cap of the machine casing to the other and separating the stator core from the rotor cavity. This may be useful where it is desirable to prevent the ingress of fluid and/or material from the stator core to the rotor chamber and/or vice versa. It may be for example that the rotor cavity utilises exhaust gas containing contaminants which should be kept away from the stator core and windings. Additionally or alternatively the stator core may be flooded with coolant liquid in intimate contact with the stator windings and core. This may facilitates higher power densities, but generally requires that liquid is contained within the stator region and not allowed to flood the rotor cavity. A further potential use of a stator sleeve is to preserve where present a pressure differential between the stator core and rotor cavity (as may occur in vacuum pumps). In each case the stator sleeve provides the necessary barrier but also passes through the main working air gap of the electrical machine. This reduces the magnitude of flux density in the air gap and at the windings as produced by the rotor magnets.

Consequently the torque density of the machine (and so its electromagnetic performance) is reduced. From an electromagnetic performance perspective it is therefore desirable to reduce the stator sleeve thickness. Nonetheless a particular stator sleeve thickness will be required in order to maintain the mechanical rigidity of the stator sleeve under a particular fluid pressure differential.

According to a first aspect of the invention there is provided a stator sleeve optionally arranged in use to be positioned between a rotor cavity and a stator core of an electrical machine, the stator sleeve optionally comprising at least one integral slot wedge optionally arranged in use to block an opening of a stator core slot in the stator core.

By making the slot wedges intrinsic with the stator sleeve, the component count may be reduced, there now being a single component rather than a stator sleeve and one or more slot wedges. Specifically there may be no other slot wedges provided, independent or otherwise. The reduction in component count may reduce manufacturing cost, facilitate simplified assembly, replacement and repair, and improve reliability.

In some embodiments the stator sleeve is rigid and is dimensioned so as in use it is a precision fit inside the stator core.

In this way the intrinsic wedges may be more resistant to dislodging and radial displacement. In prior art systems such displacement may be caused by the force exerted by windings in the slots on independent slot wedges. Dislodging may be made more likely by thermal expansion discrepancies between the stator core and slot wedge and/or vibration. As will be appreciated the rigidity of

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the stator sleeve may mean that except in a case where it fails, axial radial displacement of an integral slot wedge would require radial displacement of the whole sleeve. This would be prevented by the precision fit inside the stator core.

Therefore radial migration of slot wedges in either direction may be prevented and the radial forces on the stator sleeve may be balanced by windings positioned around its circumference.

The slot fill factor (the number of windings that can be provided in the slot) may also be increased as the support provided by the stator sleeve may mean that the slot wedges can be thinner than would otherwise be necessary to prevent failure. A further reduction in the volume of the slot that is occupied by the slot wedge may also arise because there may be no need for the wedge to engage notches in, or a rim extending from, the side wall of the slot. Encroachment of the slot wedge into the slot may therefore be reduced.

In some embodiments the sleeve comprises a wall with interior and exterior surfaces, the interior surface defining a cylindrical rotor cavity and the exterior surface incorporating the integral slot-wedges.

In some embodiments the integral slot wedges comprise one or more axially extending ribs which locally increase the radial extent of the exterior surface.

In some embodiment an axially extending trough is provided intermediate each adjacent pair of ribs, the troughs locally reducing the radial extent of the exterior surface relative to the exterior surface at the location of a rib. This may allow engagement of the rib with the slot to circumferentially locate the stator sleeve (relative rotation of the sleeve and stator core are prevented when the two are engaged).

In some embodiments the wall is thicker where there are ribs than where there are troughs. The thinner trough areas may therefore reduce the working air gap of an electrical machine in which the stator sleeve is used, therefore improving electromagnetic performance and torque density. In concert the thicker ribs serve not only as slot wedges but also as structural bodies which add rigidity and strength to the sleeve. In this way the integral slot wedges may facilitate the thinness of the troughs (with the attendant performance benefits) while also maintaining stator sleeve rigidity. This may be particularly advantageous where the stator core is flooded and the stator sleeve must withstand the fluid pressure loading. Without the ribs a thicker stator sleeve might be required.

In some embodiments ribs and troughs alternate around the entire circumference of the sleeve.

In some embodiments a rebate is provided into each circumferential side of each rib, each rebate being arranged in use to engage a stator core tooth to resist radial separation of the sleeve and stator core. Through the interlocking of several such rebates with corresponding teeth, the rebates may be shallower and the teeth shorter than would otherwise be required for independent slot wedges. Therefore magnetic flux leakage through the teeth may be reduced.

In some embodiments the sleeve further comprises an end region beyond the axial extent of the ribs and troughs to one or both sides, the radial extent of the exterior surface being greater in the end region than at the location of the troughs. The radial extent of the exterior surface may also be greater in the end regions than at the location of the ribs. In this way the or each end region may define an abutment wall which may in use abut parts of the stator core. In this way the stator core may axially locate the stator sleeve and thereby the slot wedges (preventing their axial migration).

In some embodiments the wall is thicker in the end regions than at the location of the troughs. The wall may also be thicker in the end regions than at the location of the ribs. This may increase the strength and rigidity of the stator sleeve without increasing the main working air gap in the region of the ribs and troughs.

In some embodiments the end regions extend around the full circumference of the sleeve.

In some embodiments the sleeve is arranged to create in use a seal between the rotor cavity and stator core. This may prevent coolant liquid ingress into the rotor cavity where the stator core is flooded with coolant liquid to cool the windings.

Additionally and/or alternatively it may prevent contamination of the stator core and windings where the rotor cavity contains contaminants (e.g. where the rotor is exposed to exhaust gases).

In some embodiments the sleeve further comprises one or more integral stator core slot liners. In prior art systems separate liners would normally be cut and folded to fit, comprising for example glass fibre reinforced sheet. Integral liners may provide better protection for the windings and improve long term stability, especially when the liner is exposed to coolant fluids.

In some embodiments each integral liner comprises a loop extending radially outwards from the exterior surface and with the rib forming a base wall.

In some embodiments each integral liner defines at least one rebated portion arranged in use to engage a tooth of the stator core and thereby resist radial displacement of the sleeve and stator core.

In some embodiments rebated portions of adjacent integral liners define a dovetail shaped orifice arranged in use to receive a pair of stator core teeth so that they resist radial displacement of the sleeve and stator core.

In some embodiments the liners extend axially beyond the slots when the sleeve is engaged with the stator core. The axial extension beyond the slots may be in one or both axial directions. These extensions may serve to protect more of the windings, especially in their end regions (the end windings).

According to a second aspect of the invention there is provided a stator core arranged to engage a stator sleeve according to the first aspect of the invention.

According to a third aspect of the invention there is provided an electrical machine comprising a stator core, a rotor and a stator sleeve according to the first aspect. The electrical machine may be of any configuration, e.g. a generator, a motor or both, a magnetic bearing or a rotary position sensor.

Example areas of application include liquid cooled generators for unmanned aerial vehicles, tunnel thrusters for marine applications, automotive liquid cooled traction machines and vacuum pumps.

In some embodiments the stator core of the electrical machine is arranged in use to be flooded with liquid coolant. The sleeve may then form a barrier against ingress of coolant into the rotor chamber.

According to a fourth aspect of the invention there is provided a method of assembling an electrical machine comprising the step of sliding one or both of a stator sleeve and a stator core into engagement optionally so that one or more slot wedges intrinsic with the stator sleeve are aligned with and optionally block openings of corresponding slots in the stator core The skilled person will appreciate that a feature described in relation to any one of the above aspects of the invention may be applied mutatis mutandis to any other aspect of the invention.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying Figures, in which: Figure la shows a cross-section through a portion of a prior art stator core; Figure lb shows a cross-section through a portion of an alternative prior art stator core; Figure 2 shows a cross-section through a prior art electrical machine; Figure 3a is a perspective view of a stator sleeve according to an embodiment of the invention; Figure 3b is a perspective view of the stator sleeve of Figure 3a engaged with a stator core according to an embodiment of the invention; Figure 3c is a perspective view of the stator sleeve and stator core of Figure 3b with stator windings according to an embodiment of the invention; Figure 4a is a perspective view of a stator sleeve according to an embodiment of the invention; Figure 4b is a perspective view of the stator sleeve of Figure 4a engaged with a stator core according to an embodiment of the invention; Figure 4c is a perspective view of the stator sleeve and stator core of Figure 4b with statorwindings according to an embodiment of the invention; Figure 5 is a block diagram showing a method of assembling an electrical machine.

Referring first to Figure la a portion of a prior art stator core is generally provided at 1. The stator core 1 is cylindrical in shape (not all shown) and has stator core slots 3 spaced at fixed intervals around its circumference. The slots 3 are bounded by slot side walls 5, which are provided with notches 7. The notches 7 retain a slot wedge 9 which sits within the slot 3, blocking an opening 11 at the radially innermost end of the slot 3. Unless the slot wedge 9 fails (structurally or in its engagement with the notches 7) it will retain stator windings (not shown) within the slot 3.

Figure lb shows a portion of an alternative prior art stator core generally provided at 13. As before the stator core 13 is cylindrical in shape (not all shown) and has stator core slots 15 spaced at fixed intervals around its circumference.

The stator slots 15 are bounded by slot side walls 17 which project circumferentially towards each other at the radially innermost end of the slot to produce a pair of extending rims 19. The extending rims 19 retain a slot wedge

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21 which sits within the slot 15, blocking an opening 23 at the radially innermost end of the slot 15. Unless the slot wedge 21 fails it will retain stator windings (not shown) within the slot 15.

As will be appreciated the slot and slot wedge arrangements of Figures la and lb have disadvantages.

In the embodiment of Figure 1 a the use of notches 7 mean that the slot wedge 9 must sit further into (radially outwards) the slot 3, thus reducing the slot field factor. Further the thickness of the wedge 9 must alone be sufficient to withstand the forces exerted by the windings (not shown). The resulting relatively thick wedge 9 may further reduce the slot field factor. Additionally the slot wedge 9 may be prone to failure by disengagement from the notches 7 as a result of differing thermal expansion rates of the stator core 1 and slot wedge 9 (especially where the slot wedge 9 is subjected to vibration).

In the embodiment of Figure lb the use of the extending rims i9 to seat the slot wedge 21 again reduces the size of the slot 15 and therefore the slot field factor.

Furthermore the relatively large extending rims 19 provide a leakage path for magnetic flux.

In accordance with prior art electrical machines, where separation of the stator core 1, 13 and a rotor chamber (positioned radially inward of the stator core) is required, a stator sleeve is used. Figure 2 shows a cross-section through an electrical machine 25. A stator sleeve 27 is shown positioned between a stator core 29 and a rotor chamber 31. The stator sleeve 27 is cylindrical in shape and extends from one end-cap 33 of the machine 25 to the other. The stator sleeve 27 therefore spans and extends axially beyond a main working air-gap 35 of the machine 25 into an area where the stator end windings (not shown) are located.

The stator sleeve 27 provides a fluid seal between a rotor 37 in the rotor chamber 31 and the stator core 29. As will be appreciated in order to perform this function adequately the stator sleeve 27 must be of sufficient thickness to withstand fluid pressures that it may encounter. In particular the stator core 29 may contain cooling fluid that is pumped around the stator core 29 and particularly into contact with the stator end windings (not shown). As will be appreciated however stator sleeves 27 having greater thicknesses are disadvantageous from an electromagnetic performance perspective because the stator sleeve 27 passes through the main working air-gap 35 of the machine 25, reducing the magnetic flux density experienced by the windings (not shown).

Referring now to Figures 3a-Sc a stator sleeve according to an embodiment of the invention is generally provided at 40. The stator sleeve 40 is cylindrical, rigid and unbroken in that it does not have through bores or other passages through its wall 42. The stator sleeve 40 may be made from a high strength non-conducting polymer (e.g. polyether ether ketone), a carbon fibre or glass fibre composite or a non-magnetic metal (e.g. titanium). The wall 42 has an interior 44 and exterior 46 surface. The interior surface 44 defines a rotor chamber 48 and is arranged in use to receive the rotor (not shown) of an electrical machine. The exterior surface 46 is arranged to engage with a stator core 50 positioned radially outward of the stator sleeve 40.

The wall 42 has an end region 52, troughs 54 and ribs 56. The end region 52 extends axially from one end of the sleeve 40 to approximately half its length.

The troughs 54 and ribs 56 extend axially from the end of the end region 52 to the other end of the sleeve 40. The troughs 54 and ribs 56 alternate around the full circumference of the sleeve 40 so as to form a castellated pattern. The wall 42 in the end region 52 is thicker than at the troughs 54 and ribs 56. The wall 42 is thicker at the ribs 56 than at the troughs 54. At the ribs 56 the exterior surface 46 has a greater radial extent than in the troughs 54. Further in the end region 52 the exterior surface 46 has a greater radial extent than at the ribs 56.

Consequently the exterior surface 46 defines an abutment wall 58 extending radially at an interface between the end region 52 and troughs 54 and ribs 56.

Each rib 56 has two circumferential sides 60, each of which has a rebated portion 62 beneath a top surface 64 of the rib 56. Each rebated portion 62 is arranged to receive a tooth 65 of the stator core 50. Teeth 65 of the stator core extend circumferentially in both directions from radially extending legs 66.

The base surface 67 of each tooth 65 arranged to contact a trough 54 are radiused to match the curvature of the trough 54 in the circumferential direction.

As will be appreciated however in other embodiments the base surface 67 may be flat and may co-operate with a local flat provided in the exterior surface 46 at the trough 54. The radially extending legs 66 of the stator core 50 are joined by an outer annulus 68. When the stator core 50 and the exterior surface 46 of the sleeve 40 are slid together (see Figure 5) with the legs 66 aligned with the troughs 54, the teeth 65 interlock with the rebated portions 62. The precision fit of the sleeve inside the stator core 50 prevents relative radial and circumferential movement between the sleeve 40 and the stator core 50. Contact between the abutment wall 58 and the legs 66 also prevents axial migration of sleeve 40 in one direction.

With the stator core 50 and sleeve 40 engaged, the ribs 56 act as slot wedges 72, blocking openings in stator core slots 70 defined by the legs 66 and outer annulus 68. As can be seen in Figure Sb, stator windings 74 are provided in the slots 70 and are retained by the ribs 56.

The integral nature of the slot wedges 72 and sleeve 40 may improve the resistance of the slot wedge 72 to failure and especially radial displacement.

Further the reinforcement provided by the slot wedges 72 may mean that that parts of the sleeve 40 (i.e. the troughs 54) can be thinner than in prior art systems without compromising the sealing capabilities or strength of the sleeve 40. This may improve electromagnetic performance.

Referring now to Figures 4a-c, a stator sleeve is generally shown at 80. The stator sleeve 80 is similar to the stator sleeve 40 and like reference numerals are used for like features. The only difference is the additional provision of stator core slot liners 82 that are integral with the sleeve 80. The liners 82 extend radially outwards from the exterior surface 46 and with the rib 56 forming a base wall 86. The liner 82 defines an internal space 88 which is substantially cone frustrum in shape, modified by a pair of ramp surfaces 89 which extend from the base wall 86. The ramp surfaces 89 define a pair of rebated portions 90, one either side of the rib 56, between the liner 82 and a respective trough 54.

As can be seen in Figure 4b each liner 82 is a precision fit inside the slot 70 defined by the legs 66 and outer annulus 68 of the stator core 50. Engagement between the stator core 50 and sleeve 80 may therefore be achieved as previously by sliding the two 50, 80 relative to each other. Further the ribs 56 continue to act as slot wedges 72. The teeth 65 of the stator core 50 engage the rebated portions 90, negating the need for rebated portions 62, although these may still be provided. As can be seen in Figure 4c, stator windings 74 are provided in the slots 70 inside of the liners 82, and are retained by the ribs 56.

It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the various concepts described herein. Any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein in any form of stator sleeve, stator core and electrical machine.

Claims

Claims 1. A stator sleeve arranged in use to be positioned between a rotor cavity and a stator core of an electrical machine, the stator sleeve comprising at least one integral slot wedge arranged in use to block an opening of a stator core slot in the stator core.
2. A stator sleeve according to claim 1 which is rigid and is dimensioned so as in use it is a precision fit inside the stator core.
3. A stator sleeve according to claim 1 or claim 2 where the sleeve comprises a wall with interior and exterior surfaces, the interior surface defining a cylindrical rotor cavity and the exterior surface incorporating the integral slot wedges.
4. A stator sleeve according to claim 3 where the integral slot wedges comprise one or more axially extending ribs which locally increase the radial extent of the exterior surface.
5. A stator sleeve according to claim 3 or claim 4 where an axially extending trough is provided intermediate each adjacent pair of ribs, the troughs locally reducing the radial extent of the exterior surface relative to the exterior surface at the location of a rib.
6. A stator sleeve according to claims 4 and 5 where the wall is thicker where there are ribs than where there are troughs.
7. A stator sleeve according to claims 4 and 5 or claim 6 where ribs and troughs alternate around the entire circumference of the sleeve.
8. A stator sleeve according to claim 4 or any of claims 5 to 7 when dependent through to claim 4 where a rebate is provided into each circumferential side of each rib, each rebate being arranged in use to engage a stator core tooth to resist radial separation of the sleeve and stator core.
9. A stator sleeve according to claims 4 and 5, or claim 6 or 7 or claim 8 when dependent through to claim 4 and 5 where the sleeve further comprises an end region beyond the axial extent of the ribs and troughs to one or both sides, the radial extent of the exterior surface being greater in the end region than at the location of the troughs.
iO.A stator sleeve according to claim 9 where the wall is thicker in the end regions than at the location of the troughs.
11.A stator sleeve according to any preceding claim where the sleeve is arranged to create in use a seal between the rotor cavity and stator core.
12.A stator sleeve according to any preceding claim where the sleeve further comprises one or more integral stator core slot liners.
13.A stator core arranged to engage a stator sleeve according to any of claims ito 12.
14. An electrical machine comprising a stator core, a rotor and a stator sleeve according to any of claims 1 to 12.
i5.A method of assembling an electrical machine comprising the step of sliding one or both of a stator sleeve and a stator core into engagement so that one or more slot wedges intrinsic with the stator sleeve are aligned with and block openings of corresponding slots in the stator core.
16.A stator sleeve of the kind set forth substantially as described herein with reference to and as illustrated in the accompanying drawings.

i7.A stator core of the kind set forth substantially as described herein with reference to and as illustrated in the accompanying drawings.