Motor windings with improved cooling

Abstract

An assembly comprising a winding (20) and a cooler in contact with the conductors of the winding, the cooler comprising a container (26) forming a heat dissipator and comprising a phase change material (28) having the ability to absorb excess heat when the conductors of the winding are subjected to an increase in their current density, and the heat dissipator being formed by at least two hollow parts (26A, 26B) separated from each other and nested for containing the phase change material.

Description

Technical Field

[0001] The present invention relates to the field of cooling of electric machines with high power density and more particularly to windings with improved cooling and any assembly comprising such windings. Background Art

[0002] The development of "more electric" aircraft and the subsequent need for high-power electric machines (of the order of one hundred kilowatts to several megawatts) requires that heat transfer be considered from the design stage of the electric machine, since the current density can reach very high values.

[0003] In this context, the windings are often the main source of losses in the motor. However, the maximum temperature of the windings (determined by the maximum temperature of the conductor's insulation) limits the current density that the machine can withstand. In addition, the conductor resistivity of these windings and the subsequent Joule losses increase with temperature, which reduces the efficiency of the machine.

[0004] There are three main solutions commonly used to cool electric motors: natural convection, air forced convection and liquid forced convection. Natural convection is the simplest solution, with most of the heat being removed by the machine casing. Air forced convection generally includes a fan, which increases the overall heat exchange coefficient, but has disadvantages in terms of reliability and reducing overall weight. "Water jacket" type liquid forced convection allows good heat extraction, but the fluid circulation system may require the addition of pumps and heat exchangers, which can complicate the cooling system, among other things. However, in most of the cooling systems described previously, the heat generated in the windings must first pass through the yoke and then through the machine's casing before it can be removed to the outside.

[0005] Therefore, the solution for improving heat transfer is to provide direct cooling of the coil heads of the windings, which, due to their positioning at the periphery of the machine, usually form the hot spots of the machine. Various known methods allow such direct cooling of the coil heads. Nozzles can be used to cool the axial or outer periphery of the coil heads (if it is a so-called jet cooling method). As an alternative, a cooling fluid can circulate in the rotor of the machine, usually oil or fuel, and be sprayed directly onto the coil heads using the centrifugal effect caused by the rotation of the rotor. In this case, the fluid contacts the inner periphery of the coil heads. A disadvantage of these methods is that the oil can penetrate into the air gap, which can cause additional friction losses. Another disadvantage is that if the injection speed is too high, potential damage to the insulator is caused by erosion phenomena.

[0006] Figure 3is a simplified exploded view of an electric machine 40 having a longitudinal rotating shaft 42 and comprising a laminated rotor 44 in whose slots permanent magnets are mounted and an equally laminated stator 46, the rotor-stator assembly being mounted in a housing 48. The stator comprises a plurality of windings (conductors 50) which surround respective portions of the stator (core, yoke or poles 52) and whose portions 54 are exposed at the outer periphery. This exposed portion of each conductor is in contact with a finned dissipator type cooler 56 to discharge the heat generated in the coil heads towards the periphery of the machine. The dissipation is accomplished by conduction between the coil heads and the exchange surfaces of the dissipator, and then by convection with the ambient air through the fins of the dissipator, which increase the convection surface and therefore the heat dissipation.

[0007] It is known to resort to phase change materials (PCM) as a simple and lightweight alternative for cooling electric machines and transformers, in particular the use of heat pipes or incorporation into the housing or electronic components of the electric machine to be cooled. It is also provided to use these phase change materials directly in the impregnation material (usually epoxy resin) of the windings to improve their mechanical and thermal resistance and to increase the electrical insulation between the conductors.

[0008] However, in a limited environment (rack or hot machine with poor ventilation), these solutions do not allow sufficient cooling of the windings to absorb the losses associated with the current density required for high powers. This is explained by the distance between the phase change material and the area of ​​maximum thermal stress of the winding (coil head), which does not allow the heat to be absorbed quickly enough, thus causing an increase in the winding temperature.

[0009] Existing solutions incorporate PCMs into the active part of the machine, i.e. inside the slots. However, this space is usually filled with conductors, and the Joule losses in a conductor are inversely proportional to its cross-sectional area. Incorporating PCMs into the slots is not always the best solution, as it reduces the space available for the conductors, thus potentially increasing the Joule effect losses within the windings. A compromise must be found between the minimization of the generated losses and the installation of a cooling system. Summary of the invention

[0010] The present invention thus provides a new structure for cooling the windings which improves the heat transfer characteristics of the machine (maximizing heat exchange) by convection as well as by conduction. It is also an object of the present invention to reduce the Joule effect losses as much as possible.

[0011] To this end, an assembly is disclosed, comprising a winding and a cooler in contact with the conductors of the winding, the cooler comprising a container forming a heat dissipator and a phase change material, and the phase change material has the ability to absorb excess heat when the conductors of the winding are subjected to an increase in their current density, characterized in that the heat dissipator comprises at least two hollow parts separated from each other and nested for containing the phase change material.

[0012] Thus, by providing new cooling structures in areas not normally used within the machine, and by resorting to phase change materials to cool, for example, the conductors of the windings of the motor, and in particular the coil heads (stator coil heads in induction motors, or rotor coil heads), it is possible to absorb a large amount of heat while limiting the maximum temperature reached. Furthermore, cooling is improved (and Joule effect losses are reduced) without increasing the generation of losses within the conductors, and by placing these materials close to the windings, the heat generated by the windings can be quickly extracted.

[0013] In particular, the fact that the container is nestable allows simplified pre-filling with PCM and rapid incorporation into the coil head. In particular, the dissipator can be inserted in an already completed machine or during the winding process, depending on the case.

[0014] Preferably, the phase change material has a phase change temperature between 100°C and 300°C and is advantageously a nitrate or hydroxide, preferably filled with graphite.

[0015] According to a specific embodiment, the heat dissipator is preferably made of an electrically insulating and thermally conductive material having a thermal conductivity greater than 10 W / (mK).

[0016] Preferably, when the heat dissipator is made of a conductive material, an electrically insulating and thermally conductive material is disposed between the conductor and the heat dissipator. For example, the electrically insulating and thermally conductive material is aluminum oxide or nylon mixed aluminum powder (Alumide).

[0017] Advantageously, the heat dissipater is in direct contact with the housing to allow heat to be transferred to the exterior by conduction before or after the phase change of the phase change material.

[0018] According to another embodiment, the heat dissipater may include external fins to provide insulation between the conductors, or internal fins to better transfer heat within the phase change material, and the container may also form a mold covering the conductors of the winding and embedding the conductors in the phase change material.

[0019] The invention also relates to a component of a motor, a transformer or an inductor, or a motor, a transformer or an inductor comprising a winding as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other features and advantages of the present invention will become apparent from the detailed description given below, made with reference, without any limitation, to the following drawings, in which:

[0021] Figure 1A : Figure 1A An external view of an electric machine comprising windings with improved cooling according to the invention is shown in a simplified manner.

[0022] Figure 1B : Figure 1BAn external view of several coil heads and their associated coolers of a distributed winding electric machine according to the invention is shown.

[0023] Figure 1C : Figure 1C An external view of several coil heads and circular conductors of a distributed winding machine and its associated cooler is shown.

[0024] Figure 2 : Figure 2 is a curve showing the temperature variation of the winding for different thermal contact resistances, and

[0025] Figure 3 : Figure 3 An example of a prior art electric machine is shown. DETAILED DESCRIPTION

[0026] Figure 1A The motor is shown very schematically and comprises an outer rotor and an inner stator, the outer rotor comprising a rotor yoke 12, on the surface of which permanent magnets 14 are arranged, and the inner stator comprising a stator yoke 16 concentric with the rotor. Around each tooth 18 of the stator is wound a winding 20 formed by a plurality of conductors and having a coil body 22 and two coil heads 24 (a second one opposite to the first one is not shown) exposed at the periphery 18A of the body. Each coil head 20 comprises an inner peripheral surface 20A, an inner radial peripheral surface 20B and an outer radial peripheral surface 20C.

[0027] According to the invention, each coil head 24 is mounted on a container forming a heat dissipater 26, in order to quickly extract the heat generated by these conductors and increase the exchange surface between the conductors and the environment. Advantageously, the heat dissipater, formed by two separate hollow parts 26A, 26B nested in each other, is interposed between the conductors and the body of the coil head and comprises an inner cavity 26C for receiving a liquid phase change material (PCM 28). This phase change material has the ability to absorb excess heat when the conductors of the windings are subjected to an increase in their current density, without increasing the maximum temperature reached driven by the inherent limitations of the insulation used.

[0028] Various geometries can be considered to improve the thermal contact between the coil and the dissipator, which can also include external fins (not shown) to provide insulation between the conductors, or include internal fins for better heat transfer within the phase change material. More precisely, the inner peripheral surface 20A of the coil head covers the part of the heat dissipator including the phase change material 28, and the inner radial peripheral surface 20B and the outer radial peripheral surface 20C of the coil head are in contact with the wall of the heat dissipator, thus ensuring the retention of the conductors. The positioning of the container in the space usually left free for such a concentric winding between the coil head and the teeth can limit the volume of the machine, since its total length is not increased, as would be the case if the container were set at the outer periphery of the coil head. Note that this solution is not limited to concentric windings and can be used in the case of distributed windings described below.

[0029] In order to be able to incorporate phase change materials into the windings of electric machines, these materials must have a phase change temperature of about 100° C. to 300° C. They can be nitrates or hydroxides (LiNO3, NaNO3, Li2CO3, etc.), preferably filled with graphite, and the phase change must have characteristics that are as consistent as possible to guarantee very significant liquid-solid cycles.

[0030] The dissipator can be made of an electrically insulating and thermally conductive material with good thermal conductivity (greater than 10 or 20 W / (mK)), and the phase change material has a low thermal conductivity of about 0.15 W / (mK). On the other hand, if the dissipator is made of a conductive material, an electrically insulating and thermally conductive material must be provided between the conductor and the heat dissipator. Suitable materials such as aluminum oxide or nylon mixed aluminum powder (Alumide) have been identified as good thermal conductors and electrical insulators.

[0031] Another solution that is not limited to the previous concentric winding is Figure 1B , and relates to a distributed winding of a motor stator with distributed windings, wherein the coil heads of the motor bear the reference numeral 30. These coil heads, for example, can house a container forming a heat dissipator 32 which can be inserted at the periphery of the windings to cool them. As in the previous example, this heat dissipator 32, intended to be filled with a liquid-solid phase change material, can be made of several hollow parts which are then nested in one another. The positioning of the container in the space usually left free for such a distributed winding inside the coil head makes it possible to limit the volume of the machine, since its overall length is not increased, as would be the case if the container were arranged at the outer periphery of the coil head.

[0032] Whatever the scenario envisioned, the heat dissipater may be in direct contact with the housing or the slot wedge to allow heat to be transferred to the exterior by conduction either before or after the phase change of the phase change material.

[0033] Figure 1CYet another solution shown for machines with distributed windings and circular conductors consists in making the (electrically insulated) conductors in direct contact with the phase-change material and then placing around the coil heads a container forming a sealed sheath 34 (e.g. "ring-shaped") to contain the dispersion of the material while it is in the liquid state. It should be noted that in this case, the phase-change material is in direct contact with the conductors and the sealed sheath (the mold that covers the conductors and embeds them in the PCM) does not need to be made of a material with good thermal conductivity, since the heat does not have to pass through it before being absorbed by the phase-change material. It should be noted that in this solution, the coil heads are not impregnated.

[0034] However, when the phase change material is in direct contact with the windings, it must not be chemically unstable or must have neutral (non-corrosive) properties to avoid damaging or corroding the insulator or copper.

[0035] Figure 2 The first results of computer simulations incorporating phase-change materials near the windings are presented, Figure 2 The variation of the winding temperature for several contact thermal resistances is shown. In well-defined cycles, it can be seen that in the absence of such a material (reference case - first straight line) the temperatures reached can be higher than 200 °C, which would, for example, lead to consequences such as damage to the winding insulation. On the other hand, by adding the phase change material and taking into account the optimized thermal resistance between the material and its backing, the maximum temperature reached by the sensitive element is kept below its limit temperature (lower curve).

[0036] Thus, with the present invention, an increase in electromagnetic performance is obtained by increasing the current density passing through the windings, significantly greater than that of the prior art machines. As an alternative, a reduction in the wire section of the conductors is obtained, thereby reducing the weight of the machine. Finally, the cooling system is simplified (no fans or pumps, and therefore no risk of failures) and is lightweight. Furthermore, the fact of placing the dissipator in a space that is usually left free makes it possible to minimize the volume occupied by the electric machine.

[0037] It should be noted that the field of the invention is not limited to the cooling of electric machine windings, but also includes the cooling of any system comprising a winding element, such as an inductor or a transformer.

Claims

1. An assembly comprising a stator, wherein: The stator comprises a plurality of teeth protruding from the stator, so that each tooth of the plurality of teeth forms a cooler in contact with a conductor of a winding, the winding being formed by a plurality of conductors and having a coil body and a coil head, the cooler comprising a container forming a heat dissipator and comprising a phase change material having the ability to absorb excess heat when the conductors of the winding are subjected to an increase in their current density, wherein the heat dissipator comprises at least two hollow parts separated and nested from each other for accommodating the phase change material, wherein the heat dissipator is interposed between the coil body and the coil head.

2. The assembly of claim 1, wherein: The phase change temperature of the phase change material is between 100°C and 300°C.

3. The assembly of claim 1, wherein: The phase change material is a nitrate or hydroxide filled with graphite.

4. The assembly of claim 1, wherein: The heat dissipator is made of an electrically insulating and thermally conductive material having a thermal conductivity greater than 10 W / (mK).

5. The assembly according to claim 1, wherein: When the heat dissipator is made of an electrically conductive material, an electrically insulating and thermally conductive material is provided between the conductor and the heat dissipator.

6. The assembly of claim 4, wherein: The electrical insulating and thermally conductive material is alumina or nylon mixed aluminum powder.

7. The assembly of claim 1, wherein: The heat dissipater is in direct contact with the housing to allow heat to be transferred to the outside by conduction before or after the phase change of the phase change material.

8. The assembly of claim 1, wherein: The heat dissipater may include external fins to provide insulation between the conductors, or internal fins to better transfer heat within the phase change material.

9. The assembly of claim 1, wherein: The container forms a mold covering the conductors of the winding and embedding them in the phase change material.

10. A component of a motor, a transformer or an inductor comprising the assembly according to claim 1, or a motor, a transformer or an inductor.

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