WO2016156885A1

WO2016156885A1 - Winding construction for an electric machine

Info

Publication number: WO2016156885A1
Application number: PCT/GB2016/050956
Authority: WO
Inventors: Robert Camilleri; Malcolm Duncan Mcculloch
Original assignee: Oxford University Innovation Limited
Priority date: 2015-04-02
Filing date: 2016-04-04
Publication date: 2016-10-06
Also published as: GB201505759D0

Abstract

A winding construction for an electric machine comprises a winding formed by a plurality of turns of an electrical conductor wound around a winding axis, wherein the electrical conductor is in the form of a continuous conductive strip with at least one fold, folded to form the plurality of turns around the winding axis of the winding, with the turns of the conductive strip lying in planes perpendicular to the winding axis, the successive turns of the conductive strip forming a stack. The at least one fold of the conductive strip is in a region that projects out from the turns of the winding away from the winding axis into a coolant flow path.

Description

WINDING CONSTRUCTION FOR AN ELECTRIC MACHINE

The present invention relates to a winding construction for an electric machine. In known electric machines, such as electric motors, coils of insulated copper wire are provided around an iron core. A resin may be injected to fill the air gaps between the copper windings to increase the mechanical rigidity.

There is a demand to increase the current density in the coil winding, for example to increase the torque density of an electric motor. However, the intrinsic resistance of the copper winding generates ohmic heating, which increases in proportion to the square of the current. Therefore simply increasing the current causes additional heating that raises the temperature of the winding which results in an increase in the resistance of the copper, which in turn increases the heating losses which are directly proportional to the resistance. This results in several problems. Firstly, the efficiency of the machine is reduced because a larger proportion of the input energy is lost by conversion to waste heat. Secondly, if the machine operates at a higher temperature, the rate at which the winding insulation degrades is also increased, and so the life expectancy of the winding before failure is reduced.

Thirdly, the peak performance of the machine is not limited by the average temperature of the winding, but by the temperature of the winding insulation at the hottest part, and hot spots can form in the winding, so limiting the torque density of the machine.

The present invention has been made in view of the above problems.

According to the present invention, there is provided a winding construction, for an electric machine, comprising a winding formed by a plurality of turns of an electrical conductor wound around a winding axis, wherein the electrical conductor is in the form of a continuous conductive strip with at least one fold, folded to form the plurality of turns around the winding axis of the winding, with the turns of the conductive strip lying in planes perpendicular to the winding axis, the successive turns of the conductive strip forming a stack.

According to a further aspect of the present invention, there is provided a method of forming such a winding construction by cutting a continuous conductive strip from a sheet of conductive material and folding the conductive strip.

In such a winding construction, the fill-factor of the winding is high because the turns of the conductor stack very closely. This means that a greater cross-sectional area of conductor is present than with conventional wire windings. Therefore a greater current density can be achieved.

Optionally, at least one fold of the conductive strip is in a region that projects out from the turns of the winding away from the winding axis. In some embodiments where the winding construction is disposed within a coolant flow path for cooling the winding construction by a coolant, the region may project into the coolant flow path.

As such, the heat generated within the winding can be removed and dissipated more easily. This can enable the machine the run at a cooler temperature if it is desired to provide a longer life time. Alternatively, for the same operating temperature as

conventionally used, the current can be increased to provide increased torque, i.e.

effectively a more powerful electric machine can be provided for a given size, or a more compact machine can be provided for the same power output as conventionally obtained. It is, of course possible to obtain a combination of these benefits, rather than optimising just one, for example more compact size, increased torque, and longer life time.

The winding construction may be applied to advantage in electric machines of various types, for example, in traction motors for vehicles.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figs. 1 and 2 are axial views of an electric machine;

Fig. 3 is a perspective view of a single winding construction for use with the stator of

Fig. 1;

Fig. 4 is a schematic vertical cross-section through the winding construction of Fig.

3;

Figs. 5 to 9 are plan views of a flat conductor prior to folding to form the winding of Fig. 3;

Figs. 10, 12, 14, 16 and 18 are perspective views of alternative winding constructions for use with the stator of Fig. 1;

Figs. 11, 13, 15, 17 and 19 are plan views of a flat conductor prior to folding to form the windings of Figs. 10, 12, 14, 16 and 18, respectively;

Figs. 20 and 21 are plan views of two ways of arranging on a sheet several flat conductors according to the embodiment of Fig. 5; Figs. 22 and 23 are plan views of two ways of arranging on a sheet the flat conductors according to the embodiment of Fig. 1 1;

Figs. 24 and 25 are plan views of arrangements on a sheet of the flat conductors according to the embodiments of Figs. 6 and 13 respectively;

Fig. 26 is a detailed plan view of a fold region of a flat conductor according to a further embodiment of the invention; and

Figs. 27 and 28 are detailed plan views of the fold region of a flat conductor, before and after folding, respectively, of a conductor which has a slit.

The present invention relates to a winding construction for any electric machine, such as a motor, generator, transformer or choke. Merely as an example, embodiments will all be described for use in a rotary machine with concentrated windings, such as illustrated in Fig. 1.

Fig. 1 illustrates an example of a stator 1 in an axial flux Yokeless and Segmented Armature (YASA) electric machine. The rotational axis of the rotor (not shown) is at the centre of the figure, perpendicular to the plane of the figure. Although the axis of the rotor is at the centre, the rotor itself is displaced axially on one side or the other side of the stator 1, or can be on both sides of the stator 1. The stator 1 is generally cylindrical, though in this embodiment with an axial thickness much less than the radius. The stator 1 is provided with a housing formed by inner and outer rings 10, 12 to define a space 14 for a coolant, such as a liquid coolant, typically cooling oil. The space 14 defines a coolant flow path. Within this space 14, the rounded trapezium shapes are each the segmented pole pieces 16 of an electric coil winding. In this example there are twenty four concentrated windings.

The winding axis W of each winding 52, and thus the magnetic flux, is axial with respect to the rotational axis of the rotor of the machine, perpendicular to the plane of the figure. This embodiment utilises direct cooling in which the coolant is in intimate contact with the windings. Coolant enters through a port 18 and exits through a port 20 to remove heat generated in the windings. Flow stoppers 22 ensure that the coolant flows not just circumferentially around the stator 1, but is also forced radially between the windings. In the upper right of Fig. 1, two of the pole pieces are shown cut-away to show the iron core 50 (or bobbin) around which the winding 52 is wound. An alternate embodiment of an electric machine is illustrated in Fig. 2. Like parts are denoted with like reference numerals, so will not be described again, in order to avoid repetition. Fig. 2 shows the stator 1 for a radial flux electric machine. Again, the rotational axis of the rotor (not shown) is at the centre of the figure and is perpendicular to the plane of the figure. The space 14 defining the coolant flow path is simply a ring-shape around the periphery of the stator 1. The stator 1 comprises an iron ring 53 from which project radially inwardly iron teeth. Each iron tooth comprises a core 50. As shown in the cut-away partial section in the upper right of Fig. 2, a winding 52 is provided around each core 50. The winding axis W of each winding 52, and thus the magnetic flux, is radial with respect to the rotational axis of the rotor of the machine. Again, in this example, twenty four concentrated windings are provided in total. In this case, the liquid cooling is indirect because heat generated by the windings 52 is conducted through the core 50 and the stator iron ring 53 to reach the coolant flow path in the space 14.

Fig. 3 shows a winding construction according to an embodiment of the invention, taken from the stator 1 of Fig. 1. It comprises a core 50 around which multiple turns of an electric conductor 51, are wound to form the winding 52 of an electro-magnet. The pole piece 16 is an extension of the core 50. The turns of the conductor 51 are wound around a winding axis W of the winding 52, being a notional axis that is always perpendicular to the path of the conductor. The conductor 51 is in the form of a conductive strip. In the stator 1 of Fig. 1, the winding axis W is in the axial direction of the electric machine, being the direction of the rotational axis.

The winding construction in the stator 1 of Fig. 2 may similarly be formed from turns of a conductor in the form of a conductive strip wound around a winding axis W of the winding, but in this case the winding axis W is in the radial direction of the electric machine, being a direction radial to the rotational axis.

Fig. 4 shows a vertical cross-section through the winding construction of Fig. 3. The winding axis W of the winding is the up and down direction in the plane of the drawing. As can be seen, the electric conductor 51 is a thin, flat strip with a substantially rectangular cross-section. The thickness is much less than the width. The turns of the electric conductor 51 lie in planes perpendicular to the winding axis W and the successive turns of the electric conductor 51 form a stack. As can be seen from Fig. 4, the fill-factor of the winding 52 is high because the turns of the conductor 51 stack very closely. This means that a greater cross-sectional area of conductor is present than with conventional wire windings; therefore a greater current density can be achieved. Also, as can be seen in Fig. 3, the conductor 51 provides a heat- conduction path from the interior of the winding 52 that enables efficient heat removal and the avoidance of hot spots. The direction of heat conduction can be outwards away from the core 50 in the case of a direct-cooled arrangement, such as Fig. 1, or could be inwards towards the core 50 in the case of indirect cooling as with the arrangement of Fig. 2.

In this preferred embodiment, the conductor 51 is a copper strip with thickness preferably in the range 0.01 mm to 1 mm and is coated with an insulating material to prevent short-circuits between turns of the winding. Further details and options for the material and construction of the conductor 51 are explained below. The space between the turns of the winding 52 can, optionally, be injected with a resin to increase the mechanical integrity of the winding 52, and to thermally bond the winding 52.

A method of forming the winding 52 of Figs. 3 and 4 that is not in accordance with the invention is to start with a stack of flat loops which are each then cut and joined so that one end is connected to the loop beneath and the other end is connected to the loop above. However, this requires a joint to be made at each turn of the winding which can be difficult to manufacture and can increase the electrical resistance of the winding.

Another method of forming the winding 52 which embodies the present invention is as follows. In this case, the winding 52 is formed by cutting out a continuous conductive strip 51 from a single sheet of conductive material which is then folded to form the turns of the winding 52. One embodiment of the flat conductor 51 cut from a sheet is illustrated in Fig. 5, which shows the flat conductor 51 in a meandering or serpentine form which is cut (e.g. by a laser or by stamping) from a sheet. The resulting conductor 51 is shown shaded; the other lines in this and other similar figures are merely construction lines to assist illustrating the folding to form the winding. By folding the conductor 51 along the dashed fold-lines 60, in alternate directions in the form of a concertina, it can be seen that turns are formed that form a loop that can surround a magnetic core, such as illustrated in Fig. 3.

The fold lines 60 of the embodiment of Fig. 5 are on each side of the winding 52.

Fig. 6 shows an alternative embodiment of a flat conductor 51 cut from a sheet which is folded to form the winding 52 illustrated in Fig. 3 along fold lines 60 which are at the ends of the winding 52.

The flat conductor 51 before folding does not have to extend in just one dimension, but can, for example, include turns to cover more of a two-dimensional sheet. Figs. 7 to 9 illustrate examples of alternative embodiments of portions of the flat conductor 51 cut from a sheet and the fold lines along which it is folded to form the winding 52 illustrated in Fig. 3. Configurations of this type can enable a longer continuous conductor 51 to be obtained from a particular sheet of starting material with given finite dimensions.

With the embodiment of Fig. 5, the conductor 51 next to the fold line 60 becomes doubled in thickness. This can result in the winding being less compact.

By way of contrast, Fig. 10 illustrates a different embodiment in which the folds are formed in respective regions 62 of the conductor 51 that project out from the stack of turns in the completed winding 52 away from the winding axis W.

Fig. 11 illustrates a flat conductor 51 cut from a sheet which is folded to form the winding illustrated in Fig. 10 along fold lines 60, showing in particular how the fold lines 60 are located in the regions 62 of the conductor 51. Thus, the shape of the flat conductor 51 of Fig. 11 corresponds to the shape of the flat conductor 51 of Fig. 5, except for the additional provisions of the regions 62.

The regions 62 can extend into the coolant flow path in which case they act has heat- exchange portions. They can increase the surface area for contact with the coolant fluid, and they can be shaped, if desired, in order to direct the flow of coolant to provide different or more even distribution of coolant flow around the or each winding 52.

The folds and projecting regions 62 of the embodiment of Fig. 10 and 11 are on each side of the winding 52. Figs. 12 shows an alternative embodiment in which the folds and projecting regions 62 are at the ends of the winding 52. Fig. 13 illustrates a flat conductor 51 cut from a sheet which is folded to form the winding 52 illustrated in Fig. 12 along fold lines 60 .The windings 52 described above each have a rounded trapezium shape, but other shapes are possible. Some examples of windings 52 with other shapes are shown in Figs. 14 to 19. Figs. 14, 16 and 18 show the winding 52. Figs. 15, 17 and 19 show a flat conductor 51 cut from a sheet which is folded to form the windings 52 illustrated in Fig. 14, 16 and 18, respectively, in a similar manner to Fig. 13. In the example of Figs. 14 and 15 and in the example of Figs. 16 and 17, the winding 52 has a rectangular shape, although the regions 62 are in different locations in each example.

In the example of Figs. 18 and 19, the winding 52 has a circular shape.

It can also be seen in Figs. 4 to 7 that for each embodiment, the fold lines 60 are all parallel which makes the folding operation much simpler, whether done manually or whether automated by machine. In these and other embodiments there are also only two folds per turn of the winding 52.

Fig. 20 shows how two flat conductors 51 can be arranged on a sheet (the boundaries of the sheet are not shown) in a partially intermeshed fashion that can be repeated to cut multiple conductors from a single sheet. With this shape of conductor 51 having alternating wider and narrower arches, Fig. 21 shows another lay out for cutting two flat conductors 51 from a sheet. By inverting one of the conductors and shifting it along, as illustrated, it can be seen that the meandering paths of two conductor patterns intermesh more completely, and so more conductors can be cut from a single sheet and with less wastage of material.

Fig. 22 and 23 essentially correspond to Figs. 20 and 21, but using conductor patterns according to the embodiment of Fig. 11.

Figs. 24 and 25 illustrate intermeshing arrangements for cutting out multiple conductors 51 corresponding to the embodiments of Fig. 6 and Fig. 13, respectively.

Although not illustrated in Figs. 20 to 25, perfect tessellation, in which the successive conductors 51 exactly fit next to each other with no gaps (wastage), may be possible depending on the shape of the magnetic core around which the winding is to be wound, and on the design of the conductor.

Fig. 26 shows in more detail the conductor 51 in the region of a fold line 60 according to a further preferred embodiment (illustrated on a portion of conductor 51 from Fig. 5 before folding, but applicable to any shape of conductor 51). When the fold along the fold line 60 is made, there is an overlap region 70 where two portions of the conductor 51 are on top of each other. According to this embodiment of the invention, the material of the conductor 51 in the overlap region 70 is thinner than the primary thickness of the conductor so that there is less extra thickness of material, or preferably no extra thickness of material, in the overlap region of the fold. In other words, after the fold is made, the thickness in the overlap region is less than twice the primary thickness, and can be substantially the same as the primary thickness. The area over which the material is thinned is not restricted to precisely the overlap region 70, but can extend beyond it. The thinning of the region 70 of the conductor 51 can be done by any suitable means, such as laser ablation, chemical etching, rolling, stamping or pulling.

In any of the preceding embodiments of the invention, the insulating material on the conductor 51 can either be left on or removed in the vicinity of the fold line 60. However, for the embodiment of Fig. 26, it is preferable to remove the insulating material from the thinned overlap region 70 where the surfaces will meet when folded because otherwise there would be increased resistance where the conductor is thinner. By not having insulating material at the fold on the surfaces that go face to face, the thickness of the conductor is effectively restored in the region of the fold, but has the advantage of not increasing the thickness of the stack, and so the density of the winding is not reduced.

Fig. 27 also illustrates the vicinity of a fold line 60 of a conductor 51 according to any preceding embodiment in which a cut or slit 80 is formed along the direction of the conductor 51 in the region of the fold line 60, and spanning the fold line 60. A suitable cut or slit 80 can be formed by mechanical cutting or by laser cutting. Fig. 28 illustrates the conductor 51 after the fold has been made. In this embodiment it is preferred that the insulating material is present on the surfaces of the conductor 51 that meet face to face after folding. As can be seen from inspection of Figs. 27 and 28, the region al that is on the inside of the turn of the winding is connected to the region a2 that is on the outside of the turn of the winding, and, similarly, the outside region bl is connected to the inside region b2. The presence of the slit or cut 80 reduces skin effects and eddy currents in the region of the fold. This improves the uniformity of the time-varying current flow in the conductor 51 and avoids the current being predominantly confined to the edges of the conductor 51. The slit 80 can be used in combination with the thinner overlap region of the embodiment of Fig. 26, if desired. The cut or slit 80 is not restricted to being in the region of a fold. One or more cuts or slits along the direction of the conductor 51 can be provided elsewhere on the conductor instead of, or as well as, at a fold.

It is, of course, understood that the features of the winding constructions illustrated in the accompanying drawings can be used in any desired combination or sub-combination appropriate to the topology of the electric machine for which the winding construction is to be provided.

The material from which the winding is made, is substantially non-ferromagnetic, but with good thermal conductivity and electrical conductivity. In preferred embodiments the thermal conductivity is greater than 10 W/mK. Suitable examples include metals such as copper, aluminium, and their alloys. A preferred embodiment employs a thin sheet or foil of copper which can easily be cut/shaped and folded to the desired configuration. The sheet thickness of whichever material is preferably in the range 0.01 mm to 1 mm, and is substantially uniform. This sheet thickness defines the thickness of the conductor, also referred to as the primary thickness.

In all embodiments of the invention, the conductor 51 is preferably unjointed.

Except as explained above, the surfaces of the conductor 51 in all of the

embodiments are preferably provided with an electrically insulating coating layer or varnish. This can be applied by conventional techniques, such as spraying, painting or dipping. The insulating material can be applied while the conductor is still in sheet form before cutting out the specific shape to form the winding, or can be applied after cutting out. Preferred insulation materials, with suitable electrical and thermal properties, include polyamide-imide coatings or polyimide films, for example Kapton (trade mark).

Embodiments of the winding construction described herein could be applied to every pole piece winding 16 in a machine such as illustrated in Figs. 1 and 2, to generally lower the temperature/improve the cooling efficiency throughout, or could just be applied to particular windings at particular locations to reduce hot-spots and/or to modify the coolant flow characteristics at particular locations to improve cooling effectiveness.

In a conventional winding, the winding layers next to the coolant can generally be kept at a low temperature. However, the heat from the other winding layers in the interior of the winding has to be conducted through outer layers before reaching the coolant. The poor thermal contact between the windings, even when encapsulated in resin, results in a low thermal conductivity of approximately 3 W/mK. Consequently, there is a thermal gradient across the winding, and some portions of the winding can experience significantly elevated operating temperatures which degrade the performance. According to the embodiments of the invention, an improved heat path is provided out of the interior of the winding, thus creating a shunt or bypass for the heat generated by the windings. For example copper has a conductivity value of 400 W/mK. Initial experimental results show that for a direct-oil -cooled YASA machine, the temperature in the interior of the winding could be reduced by 20 degrees C or more, for the same current density, by using an embodiment of the invention. Similarly, for the same operating temperature as a conventional winding, the current density could be increased by about 60-70% compared to a conventional winding construction, providing a significant increase in power output, from the same size machine, i.e. an increase in power density.

These improvements in performance alone can justify the use of embodiments of the present invention, even though the construction is possibly more complex and more costly in materials.

The above-described embodiments of the invention relate to so-called permanent magnet (PM) machines (only the stator 1 comprises electro-magnets). However, the invention can also be applied to non-permanent magnet machines, such as switch reluctance (SR) machines, especially ones which make use of concentrated windings. Non-permanent magnet machines are typically less expensive to manufacture, so are being used in some fields that are very sensitive to costs, such as the automotive industry, even though their power density is only around half that of PM machines. Embodiments of the invention applied to non-permanent magnet machines can help to bridge this gap in power density, because they enable the power density to be increased, and even though some costs will be added to the winding process of the machine (such as a SR machine), it will remain cheaper than a PM machine.

According to a further aspect of the present invention, there is provided a winding construction, for an electric machine, comprising: a plurality of turns of an electrical conductor comprising a winding defining an axial direction; wherein the electrical conductor is in the form of a continuous conductive strip with at least one fold, folded to form the plurality of turns around the axial direction of the winding, with a plane perpendicular to the axial direction lying in the plane of the conductive strip, and the successive turns of the conductive strip forming a stack.

According to a yet further aspect of the present invention, there is provided a method of forming a winding construction, comprising: cutting a continuous conductive strip from a sheet of conductive material; folding the conductive strip to form a winding comprising a plurality of turns of the conductive strip defining an axial direction of the winding, with a plane perpendicular to the axial direction lying in the plane of the strip, and the successive turns of the conductive strip forming a stack.

Claims

1. A winding construction, for an electric machine, comprising a winding formed by a plurality of turns of an electrical conductor wound around a winding axis, wherein the electrical conductor is in the form of a continuous conductive strip with at least one fold, folded to form the plurality of turns around the winding axis of the winding, with the turns of the conductive strip lying in planes perpendicular to the winding axis, the successive turns of the conductive strip forming a stack.

2. A winding construction according to claim 1, wherein at least one fold of the conductive strip is in a region that projects out from the turns of the winding away from the winding axis.

3. A winding construction according to claim 2, wherein said region projects into a coolant flow path.

4. A winding construction according to any one of claims 1 to 3, wherein there are two folds of the conductive strip per turn of the winding.

5. A winding construction according to any one of the preceding claims, wherein each fold of the conductive strip has a fold line, and the fold lines are parallel to each other.

6. A winding construction according to any one of the preceding claims, wherein the conductive strip has a primary thickness, and wherein the portions of the conductive strip at a fold are thinner than the primary thickness, so that the total thickness of the overlapped portions of the conductive strip at a fold is less than twice the primary thickness.

7. A winding construction according to any one of the preceding claims, wherein the conductive strip forming the winding is coated with an electrically insulating material.

8. A winding construction according to any one of the preceding claims, wherein, at a fold, the portions of the conductive strip that overlap face to face are not covered by an electrically insulating material.

9. A winding construction according to any one of the preceding claims, wherein at least one slit is provided in the conductive strip extending in a direction along the length of the strip, and preferably crossing the fold line of a fold.

10. An electric machine comprising at least one winding construction according to any one of the preceding claims.

11. An electric machine according to claim 10, being a rotary electric machine.

12. A electric machine according to claim 11, wherein the winding axis is in the axial direction of the rotary electric machine.

13. An electric machine according to any one of claims 10 to 12, further comprising a housing that houses the at least one winding construction, the housing defining a coolant flow path around the at least one winding construction.

14. A method of forming a winding construction, comprising:

cutting a continuous conductive strip from a sheet of conductive material;

folding the conductive strip with at least one fold to form a winding comprising a plurality of turns of the conductive strip around a winding axis, with the turns of the strip lying in planes perpendicular to the winding axis, the successive turns of the conductive strip forming a stack.

15. A method according to claim 14, wherein the at least one fold of the conductive strip is in a region that projects out from the turns of the winding away from the winding axis.

16. A method according to claim 15, further comprising arranging the winding construction with said region projecting into a coolant flow path.

17. A method according to any one of claims 14 to 16, wherein the conductive strip cut from the sheet of conductive material has a meandering path.

18. A method according to claim 17, wherein multiple conductive strips are cut from a single sheet of material by intermeshing their respective meandering paths.

19. A method according to any one of claims 14 to 18, further comprising reducing the thickness of the portions of the conductive strip at a fold to be thinner than the primary thickness of the conductive strip, so that the total thickness of the overlapped portions of the conductive strip at a fold is less than twice the primary thickness.

20. A method according to any one of claims 14 to 19, further comprising providing a slit in the conductive strip, the slit extending in a direction along the length of the strip, and preferably crossing the fold line.