GB2500040A

GB2500040A - Cooling of electrical machines

Info

Publication number: GB2500040A
Application number: GB1204079.6A
Authority: GB
Inventors: Steven Paul Randall
Original assignee: Nidec SR Drives Ltd
Current assignee: Nidec SR Drives Ltd
Priority date: 2012-03-07
Filing date: 2012-03-07
Publication date: 2013-09-11
Anticipated expiration: 2032-03-07
Also published as: GB2500040B; GB201204079D0

Abstract

A cooling system for an electrical machine comprises a stator, having windings, and a rotor. At the ends of the stator, a sealing arrangement 26 is provided which prevents communication between a void 38 in the stator and the space 40 in which the rotor turns. Fluid can be made to flow from inlet 50 through the voids in the stator to the outlet 52 to cool the windings and is at least partially prevented from flowing into the rotor space. Any fluid leaking from slot wedges into the rotor space is removed via the drain 56. The pressure of the fluid is regulated by allowing it to flow over a weir 54 at a high point in its path along the cooling system.

Description

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COOLING OF ELECTRICAL MACHINES

This invention relates to the cooling of electrical machines, in particular electrical machines which are cooled by a liquid.

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Electrical machines typically have windings made from insulated conductors in which current flows and which, unless the material from which the conductor is made is superconducting, have resistive loss. Unless this resistive loss (the so-called I R loss) is removed, it heats up the conductors. The continuous rating of 10 the electrical machine is generally restricted by the temperature limit of the insulation material of the winding. Often these limits conform to internationally recognised values, e.g. Class F (145°C), Class H (180°C) etc., and the expertise of the designer is brought to bear on the problem of removing the heat associated with the I R loss at a rate sufficient to keep the temperature of the 15 winding below the chosen limit.

The problem is made more difficult because of the generally conflicting requirements to provide both good electrical insulation and good thermal conduction. The conductors of the winding are typically coated with an 20 electrically insulating coating, and the completed winding assembly is typically impregnated with resin to give a chosen thickness of resin film. These measures contribute to ensuring that the electrical insulation of the winding is of a high quality. However, most good electrical insulators are also good thermal insulators, so the use of these materials generally makes more difficult the task 25 of providing paths of good thermal conductivity for the heat associated with the I R loss from the winding to a heat sink where the heat may be dissipated.

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The problem of heat removal is further compounded on electrical machines which have windings in which a coil spans a single tooth, for example brushless dc machines and switched reluctance machines. While the coils of a conventional distributed winding are generally dispersed across a relatively 5 large number of slots and have intimate contact with the iron of the stator laminations, the short-pitched windings of a salient pole machine generally contact the stator only on two sides of the coil, resulting in a heat removal path which has a smaller cross-sectional area.

10 Various methods are known for improving the efficiency of the dissipation path for the heat from the winding. For example, the coating of impregnating varnish is sometimes made very thick, so as to eliminate virtually all the air pockets around the winding and provide a path through the varnish for the heat. This improves the heat transfer, but is often a messy and time-consuming 15 process, involving the use of compounds which constitute health and safety hazards.

Liquid cooling is a known technique. The liquid flow rate can be set to keep the liquid at a relatively low temperature compared with the winding so that the 20 consequential thermal gradient gives high heat transfer from the winding.

It is also known to provide cooling jackets surrounding the stator core. For example, US5859482 (Crowell) discloses a cooling jacket made from conduits which are cast into the frame of the electrical machine. Such a technique is 25 often useful when the machine is relatively long compared with its diameter but, as the length/diameter ratio reduces, this method is less and less successful. Using serpentine paths through the frame, for example as shown in EP 1719236

(Bostwick), is a technique sometimes used to improve the efficiency of the jacket. However, while these methods have the advantage of keeping the cooling liquid well separated from the electrical conductors, there is inevitably a significantly long thermal path between the conductors and the coolant.

To attempt to shorten these paths, some have introduced coolant pipes into spaces directly adjacent to the windings. For example, US 3109947 (Thompson) discloses coolant pipes adjacent the end windings and WO 00/01053 (Sjoberg) discloses coolant pipes in the slots of the stator, adjacent the sides of the coils. However, these techniques typically involve many connections between the coolant pipes and their supply manifolds, which can prove very costly in manufacture and unreliable in service. In addition, if the coolant pipes are made from an electrically conducting material, there are the added problems of insulating them from the electrical conductors and of avoiding conducting loops into which circulating currents can be induced to flow.

Another proposed method is to use liquid coolant passing through the winding conductors themselves. This technique is more applicable to large machines in the multi-megawatt range, where the great expense of the complex arrangements needed to provide both fluid seals and electrical insulation on the individual conductors is offset by the large saving in the running cost of the machine. Though this technique has been proposed for much smaller machines in very specialised applications, for example, as described in US 5489810 (Ferreira), the saving has failed to justify the complexity.

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There is therefore a need for a cooling arrangement for electrical machines which avoids a long thermal path between the conductors and the coolant but does not compromise the integrity of the insulation of the machine.

5 Aspects of the invention are set out in the independent claims. Further, optional, features of embodiments of the invention are set out in the dependent claims.

In some embodiments, an electrical machine assembly comprises a stator, a 10 movable part for movement relative to the stator and at least one energising winding mounted on the stator. In the case of a rotating machine, the movable part is a rotor mounted for rotation relative to the stator. The rotor rotates either inside the stator in a typical arrangement, or around the stator in an inverted machine. In the case of a linear machine, the movable part moves linearly 15 relative to the stator.

The electrical machine further comprises a divider for separating a first space, in which the movable part is arranged to move, from a second space surrounding at least a portion of the at least one winding. Inlet and outlet ports 20 are in communication with the second space so that liquid can flow from the inlet port to the outlet port through the second space to cool the at least one winding.

The divider may comprise a dividing ring at an end of the stator, and the 25 dividing ring may be sealed to the stator. Further, the dividing ring may be sealed to an end portion of a housing surrounding the stator. The housing may comprise a frame and the end portion may comprise an end plate secured to the

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frame. The seal between the end portion and the dividing ring may comprise an O-ring. Further, a dividing ring may be provided at each end of the stator. The divider may comprise slot wedges disposed in pole slots between the first and second space.

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Where the divider comprises a single dividing ring at one end of the stator, the dividing ring can be used to separate the first and second spaces in the region of the coil overhangs at the corresponding end of the stator to directly cool these coil overhangs, and as a result the whole winding. Where the divider comprises 10 a dividing ring at each end and slot wedges therebetween, cooling liquid can be made to flow across the entire length of the winding, thus increasing cooling efficiency. Instead of using slot wedges, the seal between the two spaces in between the dividing rings can be formed by alternative means, for example the windings themselves may be shaped and disposed to seal the mouth of the pole 15 slots between the dividing rings. In some embodiments, the seal may be improved in the regions of any potential gaps between the dividing rings and slot wedges and/or windings by using an overlapping arrangement of slot liners lining the slots between stator poles.

20 In any case, where the divider comprises a plurality of components, these may be sealed together by a sealing coating, for example, a vanish typically used for sealing stator windings.

In some embodiments, the outlet port is open to atmospheric pressure, thereby 25 reducing the pressure differential between the spaces (assuming that the space occupied by the movable part is also at atmospheric pressure). As a result, the risk of leakage of cooling liquid between the spaces is reduced. To deal with

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any residual leakage that may occur, the space occupied by the movable part may be in communication with a drain port for draining such liquid from this space. In embodiments where the cooling liquid flows at a positive pressure relative to atmosphere, the space occupied by the movable part may be 5 pressurised to reduce leakage or the risk thereof.

In some embodiments, a method of cooling an electrical machine comprises flooding a space surrounding at least a portion of an energising winding with cooling liquid without flooding another space in which a movable part is 10 arranged to move relative to a stator of the electrical machine. In some embodiments, there is provided a corresponding electrical machine assembly comprising means to implement this method.

It will be understood that, in embodiments of the invention where the inlet and 15 outlet are adjacent opposed aspects of the machine assembly, the cooling liquid can rise through all or nearly all of the winding containing space, where the outlet is disposed at a higher potential relative to gravity than the inlet (or alternatively the liquid may fall with gravity if the arrangement is the other way round). In any case, it will be appreciated that embodiments of the invention 20 include arrangements in which the inlet and outlet ports are disposed at intermediate positions. In any event, only a portion of the space surrounding the phase winding may be flooded in some embodiments, for example only a portion in the region of a dividing ring adjacent a coil overhang of the machines winding.

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The invention can be put into practice in various ways, some of which will now be described by way of example with reference to the accompanying drawings

in which:-

Fig.l shows an electrical machine with dividing rings;

Fig.2 shows a cross section of an electrical machine assembly including a machine as illustrated in Figure 1;

Fig.3 shows a detailed view of a portion of Figure 2; and

Fig. 4 shows a perspective view of the machine assembly of Figure 2.

Figure 1 shows side and end views of a stator for an electrical machine which has salient poles on the stationary member. Such stators are typical of, eg, switched reluctance, brushless DC and permanent magnet machines. The stator comprises a stator bodylOwhich is made of a stack of laminations of a suitable magnetisable steel. The stator body 10 comprises a back iron part 12 and a set of radially inwardly projecting stator poles 14. Energising windings include coils 16, which are arranged around some or each individual one of the poles 14. These coils have end portions 18 which overhang the ends of the stator, generally referred to as coil overhangs, and side portions 20, generally referred to as coil sides, which extend axially along the stator poles 14. By connecting the coils together in groups, a stator having a number of phases according to the number of groups is created. The connection of the windings into groups will depend on the type of motor that is being constructed and is well-known in the art.

A rotor 22 is arranged on bearings (not shown) to rotate around the central axis of the stator. The rotor 22 is constructed similarly to the stator in that it is formed from laminations of electrical sheet steel. It has a number of rotor poles 24. The number of poles and the types of rotor that can be used in conjunction with the various numbers phase windings will be well known to the person of

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ordinary skill in the art and will not be described in further detail here.

Also shown in Figure 1 are two dividing rings 26, one on each end of the stator. As will be described below, the primary function of these rings is to divide the 5 space occupied by the end portions 18 from the space in which the rotor operates. This division is achieved in conjunction with the endplates of a machine assembly, as discussed below with reference to Figure 2. It will be appreciated that the dimensions used in Figure 1 are chosen for illustration rather than necessarily being representative of a practical embodiment.

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Figure 2 shows a cross-sectional view of a machine assembly comprising a frame 28 surrounding a stator body 10 made up of a stack of laminations with coils 16 wound on the stator poles 14. Dividing rings 26 are secured to the stator body 10. A rotor 22 is mounted for rotation inside the stator body 10 and 15 comprises laminations secured to a shaft 30. The assembly comprises at each end of the frame 28 a respective endplate 32, which each carries a bearing 34. The shaft 30 is mounted in the bearings 34 in a conventional manner. Hidden detail has been omitted from Figure 2 in the interests of clarity.

20 In this illustration, the dividing rings 26 are set into the ends of the stator body 10. The end laminations of the stator body 10 are made with a larger internal bore than the others in some embodiments, so as to form a step 36 in the bore diameter. Alternatively, the lamination pack is machined at the ends after the pack is formed in some other embodiments. The difference in bore size used to 25 form the step 36 may be around twice the wall thickness of the dividing ring 26, so that the internal diameter of the ring matches the main bore diameter of the stator body 10 as shown in Figure 3. In some embodiments the dividing rings

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26 are mounted to the end laminations of the stator body 10 and secured by adhesive.

The dividing rings 26 divide the space at each end of the machine into a space 5 38 in which the end windings of the coils are situated and a space 40 adjacent the rotor 22 and shaft 30. The dividing rings 26 are fitted to mating faces on the endplates 32 so as to provide seals which prevents fluid passing between the dividing rings and the endplates 32. This sealing can be achieved in a number of ways, e.g. by using an interference fit between the ring and the endframe, by 10 using sealing compound, or any other known method.

A particular embodiment is shown in Figure 2 and in greater detail in Figure 3, where the dividing rings 26 have a groove 42 sized to receive an O-ring 44. The O-ring seals against a mating face 46 of the endplate 32 and prevents the 15 passage of liquid between the two spaces 38 and 40 on either side of the dividing ring 26. The O-rings are made from a material which is suitable for the temperatures experienced in the electrical machine and is resistant to the cooling liquid. A suitable material is Viton®, a fluoroelastomer material from DuPont Performance Elastomers L.L.C. and many others are known in the art.

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The dividing rings 26 are preferably made from a material which is not an electrical conductor, i.e. is an electrically insulating material. This avoids introducing any insulation problems to the stator and also avoids any circulating currents being induced in the dividing rings 26. A suitable material is a glass-25 reinforced epoxy composite, e.g. one of the Vetronite® range and many others are, again, known in the art

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The dividing rings 26 can be dimensioned so that they fit tightly into the step 36 in the stator body 10, or can be a looser fit and be bonded in with a suitable adhesive. A particular method is to use a relatively tight fit and assemble the rings to the stator before the whole assembly undergoes the conventional 5 varnish impregnation process. The penetration of the impregnating varnish into and around the ring then seals it to the stator.

In some embodiments, the design of the dividing ring 26 includes a feature which cooperates with a further feature on the stator body 10 such that the ring 10 is easily assembled to the stator but requires much greater force to disassemble it. This can be achieved by the use of clip or bayonet features. In some embodiments, a detent on the dividing ring 26 mates with a corresponding feature of the stator body 10, or vice versa. The benefit of this arrangement is that if the machine is subsequently disassembled the ring remains securely 15 attached to the stator, preserving the seal between the ring and the stator, and the parting line is between the ring and the endframe.

As can be appreciated with reference to Figure 4, slot wedges 48 close the mouth of the slots between stator poles 14 (i.e. the regions near the air gap and 20 between the sides of adjacent stator poles 14) and are sealed into place by the impregnating resin used to seal the coils 16. Together with the dividing rings 26, this completes the sealing separation between the space 38 and the space 40.

In some embodiments an insulating liner surrounds each or both coil sides in 25 each slot and the end of these liners are overlapped behind the respective slot wedge 48 and dividing ring 26, further contributing to the seal.

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An inlet passage 50 provides a liquid inlet port and an exit passage 52 provides a liquid outlet port to allow liquid to flow into and out of the space 38. In a particular arrangement, the outlet passage 52 defines a weir at a corner 54. A drain passage 56 provides a liquid drain port in communication with the space 5 40 to drain liquid that has leaked into the space 40. Liquid flow and drainage is discussed in more detail below.

In some embodiments the endplates 32 are sealed to the stator frame 28 by O-rings 58, as shown in Figure 2. These prevent leakage of the cooling fluid 10 between the frame 28 and the endplates 32. In another aspect of the invention, one of the endplates 32 is made integral with the frame 28.

The use of the dividing rings 26 in cooling the machine will now be explained. Liquid coolant is pumped through the inlet passage 50 and floods the space 38 15 surrounding the end portions 18 and the side portions 20. Figure 4 shows an oblique view of the machine assembly and it will be clear from Figures 2 and 4 that the space 38 comprises a region around the end portions 18 at one end of the stator which is connected, by axial spaces generally between the coil sides which share a slot, to the region around the end portions 18 at the opposite end 20 of the stator. As the fluid is pumped into the inlet passage 50, it floods the bottom of the machine and then the level rises until space38 is filled and the fluid reaches the outlet passage 52.

Because the dividing rings are sealed to both the stator body 10 and the 25 endplates 32, the cooling liquid is prevented from entering the space 40 surrounding the rotor. The cooling liquid is further prevented from reaching the space 40 from the areas of the slots because the mouths of the slots stator poles

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14 are closed by the slot wedges 48 which are sealed to the stator laminations by the impregnating resin.

Because the dividing rings 26 are sealed to the endplates 32 and the stator body 5 10 and because the slot wedges 48 seal the mouths of the slots, any liquid pumped in at the inlet passage 50 is intended to eventually find its way to the outlet passage 52. This arrangement provides direct liquid cooling for the windings without introducing cooling liquid into the rotor space 40, so there is no extra drag on the rotor due to the rotor operating in contact with liquid. The 10 use of cooling liquid directly on the windings is most efficacious in removing heat from the winding as the thermal path between the conductor of the coil and the liquid is very short, so the heat transfer is maximised.

If, however, there are any residual small gaps between slot wedges 48, dividing 15 ring 26, endplates 32 and stator body 10, small amounts of cooling liquid may leak into the rotor space. These small amounts of liquid can be drained out of the rotor space through the drain passageway 56 shown in Figure 2. Further drains may be provided at other positions to accommodate any motor orientation.

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When sufficient cooling fluid has been pumped into the machine to fill the space 38, the fluid exits the machine via the outlet passage 52. A preferred arrangement is shown in Figure 2, where the shape of the exit passage 52 forms a weir at the corner 54 and the fluid exits over the weir. Because in this 25 embodiment the space 40 is at atmospheric pressure and the weir is open to the atmosphere, this arrangement can usefully limit the pressure exerted by the fluid on the seals and thus reduces the likelihood of leaks from the system. When the

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fluid exits over the weir it is collected in a receiver 60 and can then be discarded or can pass through a heat exchanger before being re-used.

In some embodiments it may be useful to increase the pressure of the cooling 5 liquid to increase the flow. It may then be useful to also increase the pressure in the space 40 to reduce the likelihood of leaks from space 38 to space 40.

The above disclosure is also equally applicable to linear machines^ and to inverted machines, in which the rotor is arranged on the outside of the stator; 10 the dividing rings 26 are accordingly designed with suitable shapes dictated by machine geometry. It applies equally to machines operated as motors or generators. Further, while the above disclosure has been made in terms of a machine having a laminated stator and rotor, one or both of the stator and rotor may equally be made in a non-laminated construction, for example machined or 15 cast from solid material or using soft magnetic composites (SMC) such as Somalloy (™).

It will be apparent to the person of ordinary skill in the art that variations and modifications can be made without departing from the invention. Accordingly, 20 the above description of embodiments is made by way of example and not for the purposes of limitation. The present invention is intended to be limited only by the scope of the following claims.

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Claims

CLAIMS:

1. An electrical machine assembly comprising: a stator; a moveable part mounted for movement relative to the stator; at least one energising winding

5 mounted on the stator; a divider for separating a first space, in which the movable part is arranged to move, from a second space surrounding at least a portion of the at least one winding; an inlet port in communication with the second space; and an outlet port in communication with the second space, wherein liquid can flow from the inlet port to the outlet port through the second 10 space to cool the at least one winding.

2. An electrical machine assembly as claimed in claim 1, in which the divider comprises a dividing ring at an end of the stator.

15 3. An electrical machine assembly as claimed in claim 2 in which the dividing ring is sealed to the stator.

4. An electrical machine assembly as claimed in claims 2 or 3 in which the dividing ring is sealed to an end portion of a housing surrounding the stator.

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5. An electrical machine assembly as claimed in claim 4, in which the housing comprises a frame and the end portion comprises an endplate secured to the frame.

25 6. An electrical machine assembly as claimed in claim 4 or 5, a seal between the end portion and the dividing ring comprising an O-ring.

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7. An electrical machine assembly as claimed in any one of claims 2 to 6 in which the dividing ring comprises a feature cooperating with a feature of the stator such that a greater force is required to separate the dividing ring and the stator than to assemble the dividing ring and the stator.

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8. An electrical machine assembly as claimed in any one of claims 2 to 7, in which a dividing ring is provided at each end of the stator.

9. An electrical machine assembly as claimed in any preceding claim, the 10 stator comprising salient stator poles defining slots for accepting the at least one energising winding therebetween, the divider comprising slot wedges disposed in the slots between the first and second space.

10. An electrical machine assembly as claimed in any preceding claim, 15 comprising a drain port in communication with the first space for draining liquid from the first space.

11. An electrical machine assembly as claimed in any preceding claim, in which the outlet port is open to atmospheric pressure.

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12. An electrical machine assembly as claimed in any preceding claim, the divider, stator and at least one energising winding being sealed together by a sealing coating.

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13. An electrical machine as claimed in claim 12, the sealing coating comprising a varnish.

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14. A method of cooling an electrical machine having a stator, a movable part mounted for movement relative to the stator and at least one energising winding mounted relative to the stator, the method comprising flooding a space surrounding at least a portion of the at least one energising winding with

5 cooling liquid without flooding another space in which the movable part is arranged to move.

15. An electrical machine assembly comprising a stator; a movable part mounted for movement relative to the stator; at least one energising winding

10 mounted relative to the stator; and means for enabling cooling liquid to flood a space surrounding at least a portion of the at least one energising winding without flooding another space in which the movable part is arranged to move.

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Amendments to the claims have been filed as follows:

CLAIMS:

1. An electrical machine assembly comprising: a stator; a moveable part mounted for movement relative to the stator; at least one energising winding mounted on the stator; a divider for separating a first space, in which the movable part is arranged to move, from a second space surrounding at least a portion of the at least one winding; an inlet port in communication with the second space; and an outlet port in communication with the second space, wherein liquid can flow from the inlet port to the outlet port through the second space to cool the at least one winding, wherein the stator defines a bore and a step of increased bore diameter at an end of the stator and the divider comprises a dividing ring set into the step at the end of the stator .

2. An electrical machine assembly as claimed in claim 1 in which the dividing ring is sealed to the stator.

3. An electrical machine assembly as claimed in claim 2 in which the dividing ring is sealed to an end portion of a housing surrounding the stator.

4. An electrical machine assembly as claimed in claim 3, in which the housing comprises a frame and the end portion comprises an endplate secured to the frame.

5. An electrical machine assembly as claimed in claim 3 or 4, a seal between the end portion and the dividing ring comprising an O-ring.

6. An electrical machine assembly as claimed in any one of claims 1 to 5 in which the dividing ring comprises a feature cooperating with a feature of the

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stator such that a greater force is required to separate the dividing ring and the stator than to assemble the dividing ring and the stator.

7. An electrical machine assembly as claimed in any one of claims 1 to 6, in 5 which a dividing ring is provided at each end of the stator.

8. An electrical machine assembly as claimed in any preceding claim, the stator comprising salient stator poles defining slots for accepting the at least one energising winding therebetween, the divider comprising slot wedges disposed

10 in the slots between the first and second space.

9. An electrical machine assembly as claimed in any preceding claim, comprising a drain port in communication with the first space for draining liquid from the first space.

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10. An electrical machine assembly as claimed in any preceding claim, in which the outlet port is open to atmospheric pressure.

11. An electrical machine assembly as claimed in any preceding claim, the 20 divider, stator and at least one energising winding being sealed together by a sealing coating.

12. An electrical machine as claimed in claim 11, the sealing coating comprising a varnish.

13. An electrical machine as claimed in any preceding claim, in which laminations at the end of the stator have a bore diameter larger than other laminations so as to form the step.

• ♦ • • •

« •

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i • • • • •

• •

• I

• #

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14. An electrical machine as claimed in any one of claims 1 to 12, in which the end of the stator has been moulded to form the step.

15. A method of cooling an electrical machine as claimed in any preceding claim, the method comprising flooding the second space with cooling liquid without flooding the first space.

16. A method as claimed in claim 15, including pressurising the cooling liquid and the first space.

17. An electrical machine assembly comprising an electrical machine as claimed in anyone of claims 1 to 14 and means for enabling cooling liquid to flood the second space without flooding the first space.

18. An electrical machine assembly as claimed in claim 17, comprising means for pressurising the cooling liquid and the first space.