WO2016156886A1

WO2016156886A1 - Winding construction for an electric machine, comprising a heat exchange member

Info

Publication number: WO2016156886A1
Application number: PCT/GB2016/050957
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: GB201505756D0

Abstract

A winding construction for an electric machine comprises: a winding comprising a plurality of turns of an electrical conductor; and a heat-conduction member, having a first portion formed of a sheet material, said first portion being inside the winding. The heat- exchange portion extends into a coolant flow path. The heat-exchange portion is shaped to direct the flow of the coolant in the coolant flow path in a manner that the flow is distributed more uniformly around the winding constructions than if the heat-exchange portion were not present.

Description

WINDING CONSTRUCTION FOR AN ELECTRIC MACHINE, COMPRISING A HEAT EXCHANGE MEMBER

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.

Accordingly, the present invention provides a winding construction, for an electric machine, comprising:

a winding comprising a plurality of turns of an electrical conductor; and

a heat-conduction member, having a first portion formed of a sheet material, said first portion being inside the winding.

By using an embodiment of the invention, 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. These advantages can be desirable, for example, for traction motors for vehicles. 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 heat-conduction member may comprise a heat-exchange portion that extends into a coolant flow path. This facilitates heat transfer from the winding to a coolant in the coolant flow path.

The heat-exchange portion may be shaped to direct the flow of the coolant. This allows improvement of the cooling performance of an electric machine in which the winding construction may be applied. For example, the flow of a coolant in the coolant flow path may be directed in a manner that the flow is distributed more uniformly around the winding constructions than if the heat-exchange portion were not present and/or the temperature distribution of the windings is more uniform than if the heat-exchange portion were not present. As a result, the electric machine may be operated at higher temperatures.

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 electric machines, with concentrated windings, for use with embodiments of the present invention, Fig. 1 showing an axial flux machine, and Fig. 2 showing a radial flux machine;

Fig. 3 is a perspective view of a single winding construction for use with the stator of Fig. 1;

Figs. 4 and 5 show examples of portions of heat-conduction members for use in the winding construction of Fig. 3;

Fig. 6 is a perspective view of a further example of a heat-conduction member; and Figs. 7 to 14 are all cross-sections through winding constructions embodying the invention showing different configurations of the heat-conduction member or members.

Fig. 15 is a coolant velocity map in radial cross-section through the stator flow domain;

Fig. 16 is a temperature distribution map of the windings and coolant in radial cross- section through the stator flow domain, in the same example as the velocity map of Fig. 15;

Fig. 17 is a graph of temperatures of the windings of an electric motor with different ring widths, with an insert showing a schematic view of the ring width;

Fig. 18 is an axial view of an axial flux electric machine showing an arrangement in which coolant flow is controlled; Fig. 19 is four graphs of temperatures of the windings derived by simulation of the electric machine with and without control of the coolant flow as shown in Fig. 18, for a fixed inlet flow rate, inlet temperature and pole piece heat load;

Fig. 20 is a graph of temperature against time showing the thermal behaviour of a concentrated wound winding with three winding layers;

Fig. 21 is a radial cross-section of the temperature profile of the windings with three different constructions derived by a CFD (Computational Fluid Dynamics) model.

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 direct liquid-cooled electric machine, for example a rotary machine with concentrated windings, such as illustrated in Figs. 1 and 2.

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 comprising 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 alternative 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 in the radial direction of the electric machine, being a direction radial 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, such as insulated copper wire, are wound around the winding axis W to form the winding 52, the winding axis W being the direction a notional axis that is always perpendicular to the path of the conductor. Further iron can extend from the core 50 to form a pole piece 16. The winding 52 is built up of multiple concentric layers 52a, 52b, 52c. A heat-conduction member 54 is provided. The heat-conduction member 54 is formed of a solid material in the form of a sheet. The heat-conduction member 54 has a first portion 56 (not visible in Fig. 3), which is interleaved between the winding layers 52a and 52b as an example. There is then a radial portion 58 joining the first portion to a heat- exchange portion 60. The heat-exchange portion 60 extends into a coolant flow path, such as the space 14 shown in Fig. 1.

Figs. 4 and 5 show embodiments of a heat-conduction member in isolation. In both cases, the heat-exchange portions are not shown, for clarity. The heat-conduction member 54 can be in the form of a simple strip or ribbon that is folded into the appropriate shape. It can have one radial portion 58, as shown in Fig. 4, or can have two radial portions 58, one at each end of the first portion 56, as shown in Fig. 5 and leading to multiple heat- exchange portions. The heat-conduction member 54 is not limited to an aspect ratio of a strip that is narrower than its length, but can, for example, be broader.

Fig. 3 only shows a single heat-conduction member 54, but in practice multiple such members may be provided spaced apart around the winding 52.

Fig. 6 shows a further example of a heat-conduction member 54. It has a first portion 56 that extends virtually the whole way around the core 50 or around one of the layers 52a of the winding 52, in this case the inner layer, but it could be any layer.

However, a gap 62 is provided so that if the heat-conduction member 54 is an electrical conductor, it does not form a single-turn coil and so does not affect the electro-magnetic properties of the winding. Multiple heat-exchange portions 60 are provided with a similar shape to the heat-exchange portion 60 of Fig. 3. However, some of the heat-exchange portions, such as the one labelled 60a in Fig. 6 consists of multiple folds to form a serpentine shape to increase the surface area for contact with the coolant fluid, where space is available.

In further embodiments, one or more of the heat-exchange portions can be bent, angled, or shaped in order to direct the flow of the coolant, for example to ensure that the flow is distributed more uniformly around all portions of each winding.

When fabricating the winding construction, according to one embodiment, an inner layer or layers 52a of the winding are wound around the core 50. Then the heat- conduction member 54 is positioned with the heat-exchange portion(s) 60 folded away from the winding to enable the subsequent layers 52b, 52c of the winding to be wound, and then the heat-exchange portions 60 are folded into position in what will be the coolant flow path in the finished machine, in the positions illustrated for example in Fig. 3 and Fig. 6. Optionally, resin can be injected into the spaces between the turns of the winding, and then the resin is set to secure the windings and heat-conduction member 54 in place.

Various further embodiments of the winding construction are now illustrated schematically in Figs. 7 to 14 which are all schematic cross-sections through a portion of the structure, for example as shown in Fig. 2. In these examples, the conductor 51 is in the form of a wire, many turns of which are visible in Figs. 7 to 14.

In these embodiments, the heat-conduction member 54 can be a simple strip or ribbon as explained with reference to Figs. 3 to 5, or the cross-sections of Figs. 7 to 14 could equally be through constructions using a more complex heat-conduction member, such as shown in Fig. 6. In all of Figs. 7 to 14, a portion of the core 50 is shown around which the winding 52 is provided. In the winding 52, the individual wires forming each turn are shown in cross-section. The heat-exchange portions 60 are generally omitted from the figures.

Fig. 7 shows a construction similar to that of Fig. 3 in which a single layer 52a of winding is provided between the first portion 56 of the heat-conduction member and the core 50.

Fig. 8 shows an alternate arrangement in which the first portion 56 of the heat- conduction member is directly next to the core 50 and sandwiched between the core 50 and the winding 52.

Fig. 9 shows an arrangement in which a radial portion 58 of the heat-conduction member is bent towards the core 50; this might be used in a radial-flux machine as shown in part in Fig. 2.

Fig. 10 shows an embodiment in which no radial portion of the heat-conduction member 54 is necessary.

Fig. 11 shows a configuration of a heat-conduction member 54 having more than two radial portions 58, with some of them interspersed within the winding 52.

Fig. 12 shows a configuration in which the first portion of a heat-conduction member

54 does not span the entire width of the winding 52, and multiple heat-conduction members 54 can be provided in a stacked arrangement.

Fig. 13 shows yet a further arrangement in which heat-conduction members 54 are nested within each other within the winding 52.

In the arrangement of Fig. 14, the heat conduction member 54 has first portions 56 interleaved within the winding 52, and the heat exchange portion 60 is adjacent to the core 50. This heat conduction member 54 is for conveying heat to the core 50, and is particularly applicable to machines in which coolant fluid is passed through channels in the pole piece iron at or near the core 50.

It is, of course, understood that the features of the winding constructions illustrated in

Figs. 7 to 14 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 heat-conducting member 54 is made, is substantially non-ferromagnetic, but with good thermal conductivity. In preferred embodiments the thermal conductivity is greater than 10 W/mK. Suitable examples include metals such as copper, aluminium, and their alloys, but could also comprise sapphire or carbon-based materials with high thermal conductivity, such as diamond, graphene or graphite. 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 can be decided based on the material selection and malleability. The sheet material could be perforate, to reduce weight, or could be in the form of a mesh or braid.

In the above examples which relate to a direct liquid-cooled electric machine, the heat-conducting members 54 allow the temperature of the inner layers in the concentrated windings 52 to be lowered. This effect is aided by the heat-exchange portions 60 extending into the coolant flow path, thereby providing for heat-exchange with the coolant.

Additionally, the heat-exchange portions 60 that extend into the coolant flow path may be arranged to control the flow distribution of the coolant providing advantages, for example so that the distribution of the flow of coolant around all portions of each winding than if the heat-exchange portions 60 were not present, and temperature distribution of the stator 1 is more uniform than if the heat-exchange portions 60 were not present as will now be explained further.

The following description relates to an axial flux electric machine of the type shown in Fig. 1 as a test case study to demonstrate the extent of this problem. However, the same considerations apply to other designs of axial flux electric machine and to radial flux electric machines, for example of the type shown in Fig. 2.

In general, the performance of direct liquid-cooled electric machines can be affected by the coolant flow distribution. In particular, that can cause a non-uniform temperature distribution of the stator, for example as illustrated in Figs. 15 and 16 which show results derived from a validated CFD model of a quarter of an electric machine of the type shown in Fig. 1, for a fixed inlet flow rate, inlet temperature and pole piece heat load.

Specifically, Fig. 15 shows a velocity map of the coolant flow and Fig. 16 shows the resultant temperature distribution of the windings and cooolant. It can be seen from Fig. 15 how the coolant takes the path of least resistance through the coolant flow path so that the coolant flow is non-uniform and from Fig. 16 how the non-uniformity of the coolant flow causes a non-uniform temperature distribution in the windings. The performance of the machine is limited by the hottest winding. Therefore it is of interest to increase the uniformity of the temperature distribution of the windings 52.

Fig. 17 shows the results of a simulation of the machine stator temperatures for a fixed flow rate and constant heat load, with different distribution rings widths of 10 mm, 8 mm and 6 mm (the distribution rings width being the dimension between the winding constructions 52 and the inner and outer rings 10 and 12, illustrated schematically between the arrows in the inset). This shows that reducing the flow ring distribution reduces the temperatures of the windings 52, but that the non-uniformity of the temperature distribution still remains.

However, the temperature distribution to increase its uniformity may be controlled by using the heat-exchange portions 60 that extend into the coolant flow path to control the flow distribution of the coolant in the coolant flow path 14. The heat-exchange portions 60 effectively restrict the flow of coolant and so may be provided in strategic places of to increase the uniformity of the coolant flow and hence the uniformity of the temperature distribution. The specific locations where heat-exchange portions 60 are provided to achieve this effect varies depending on the overall machine design, but may be determined by an optimization process for any given design.

By way of example, Fig. 18 illustrates suitable the locations 66 for heat-exchange portions 60 to be provided to restrict the flow in an electric machine of the type shown in Fig. 1 with 24 pole pieces 16. Fig. 19 shows the improvement in the uniformity of the temperature distribution without the heat-exchange portions 60 (original stator flow geometry) and with the heat-exchange portions 60 (improved stator flow geometry), as determined by a simulation for a fixed inlet flow rate, inlet temperature and pole piece heat load.

Further discussion of the manner in which the heat-conduction members 54 lower the temperature of the inner layers in the concentrated windings 52 is as follows. Concentrated windings 52 suffer from a temperature gradient across the turns of the conductor 51.

Considering a winding 52 with three concentrated winding layers (inner, middle and outer) as an example, when a constant heat load is applied, the temperature of each winding layer and iron increases with time as shown in Fig. 20 which plots measured and simulated results. It can be seen from Fig. 20 how the inner layer, which is further apart from the coolant, has the highest thermal resistances and suffer from the highest temperature.

The hot spot temperature within the winding layers of each winding 52 can be reduced by introducing the heat-conduction member 54 in the winding 52. This shortens the thermal resistance of the inner winding layers and reduces the temperature. A comparison of the winding temperatures is shown in Fig. 21 which shows a temperature map of some windings 52 derived using a CFD model. The upper winding is a

conventional construction without a heat-conduction member 54. The middle winding is a winding 52 with a heat-conduction member 54 between the core 50 and the inner winding layer. The lower winding is a winding 52 with a heat-conduction member 54 between the first and second winding layers.

As will be appreciated, the detailed description above is not limitative and the teachings may be applied to other forms of winding construction, including but not limited to the following examples.

Although in the illustrated embodiments, the entire heat-conduction member 54 is illustrated as being sheet material, this is not essential. For example, just the first portion 56 could be sheet material to enable it to fit within the winding 52, but the heat-exchange portion could be much thicker or in some other form, such as a heat-sink, if desired.

Although in the description and drawings, the winding has generally been described as formed of conventional (drawn, cylindrical) wire, it is envisaged that other electrical conductors could be used, such as in the form of strips or other shapes. An example of a winding construction using a strip electrical conductor that can be used with embodiments of the present invention is disclosed in an International Application (J A Kemp ref:

N404688WO) being filed by the same applicant on the same date as this application, which is incorporated herein by reference.

Reference is made herein to the winding comprising layers. Typically it is envisaged that each layer is a mono-layer of turns of the relevant conductor; however, that is not essential, and a 'layer' can comprise a partial-layer or multi-layer of turns of the conductor.

Embodiments of the winding construction described herein could be applied to every winding 52 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 inner 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 with a passive heat-conduction member. 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 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; and

a heat-conduction member, having a first portion formed of a sheet-like material, said first portion being inside the winding.

Claims

1. A winding construction, for an electric machine, comprising:

a winding comprising a plurality of turns of an electrical conductor; and a heat-conduction member, having a first portion formed of a sheet material, said first portion being inside the winding.

2. A winding construction according to claim 1, wherein the heat-conduction member comprises a heat-exchange portion that extends into a coolant flow path.

3. A winding construction according to claim 2, wherein the heat-exchange portion is shaped to direct the flow of the coolant.

4. A winding construction according to claim 2 or 3, wherein said heat-exchange portion is also formed of sheet material and includes multiple folds.

5. A winding construction according to any one of the preceding claims, wherein the turns of electrical conductor are wound around a winding axis, and wherein the heat- conduction member comprises at least one radial portion extending in a direction substantially perpendicular to said winding axis of the winding.

6. A winding construction according to any one of the preceding claims, further comprising a core around which the winding is provided, wherein the first portion of the heat-conduction member is provided between the winding and the core.

7. A winding construction according to any one of claims 1 to 5, further comprising a core around which the winding is provided, wherein the winding comprises a plurality of layers of turns of electrical conductor, and wherein the first portion of the heat-conduction member is interleaved within the winding such that at least one layer of the winding is provided between the first portion of the heat-conduction member and the core.

8. A winding construction according to claim 7, wherein a single layer of the winding is provided between the first portion of the heat-conduction member and the core.

9. A winding construction according to any one of the preceding claims, wherein the heat-conduction member is in the form of a strip or ribbon.

10. A winding construction according to any one of the preceding claims, wherein the heat-conduction member extends circumferentially around substantially one turn in the same sense as the winding, and wherein there is a gap such that heat-conduction member does not complete one turn.

11. A winding construction according to any one of the preceding claims, wherein the heat-conduction member is formed of a material with a conductivity greater than 10 W/mK.

12. An electric machine comprising plural winding constructions each according to any one of the preceding claims.

13. An electric machine according to claim 12, further comprising a housing that houses the plural winding constructions, the housing defining a coolant flow path around the plural winding constructions.

14. An electric machine according to claim 12 or 13, wherein the heat-exchange portion of the winding construction is shaped to direct the flow of a coolant in the coolant flow path in a manner that the flow is distributed more uniformly around the winding constructions than if the heat-exchange portion were not present.

15. An electric machine according to any one of claims 12 to 14, wherein the heat- exchange portion of the winding construction is shaped to direct the flow of a coolant in the coolant flow path in a manner that the temperature distribution of the windings is more uniform than if the heat-exchange portion were not present.

16. An electric machine according to any one of claims 11 to 15, being a rotary electric machine.

17. A electric machine according to 16, wherein the winding axis is in the axial direction of the rotary electric machine.