EP4200960A1

EP4200960A1 - Aircraft electrical machine with improved heat transfer by means of a phase change material and associated method

Info

Publication number: EP4200960A1
Application number: EP21769765.5A
Authority: EP
Inventors: Camel SERGHINE; Thomas Klonowski; Pierre Gabriel Marie QUINIO; Sabrina Siham AYAT; Eric ROULAND; Sébastien YON; Albert MUTABAZI
Original assignee: Safran Helicopter Engines SAS; Safran SA
Current assignee: Safran Helicopter Engines SAS; Safran SA
Priority date: 2020-08-20
Filing date: 2021-08-13
Publication date: 2023-06-28
Also published as: JP2023538567A; FR3113546B1; CN116195173A; WO2022038326A1; US20230318416A1; FR3113546A1

Abstract

Disclosed is a method for protecting windings from excessive heating in an aircraft electrical machine comprising a stator (12) and a rotor (14) configured to be driven in rotation with respect to one another, the stator comprising a plurality of notches (120) receiving the same or different plurality of windings, the method comprising the following successive steps: inserting an electrical insulator (16) into the notches or on the teeth of the stator, positioning the windings (18) in the notches or on the teeth of the stator, casting a phase-change material (20) into the notches or onto the teeth provided with the windings, the electrical insulator forming a casting mould.

Description

ELECTRIC AIRCRAFT MACHINE WITH IMPROVED THERMAL TRANSFER BY MEANS OF A PHASE CHANGE MATERIAL AND ASSOCIATED METHOD

Technical area

The field of the invention is that of direct or alternating current electric machines integrated in aircraft engines, in particular those of helicopters or VTOL (for "Vertical Take Off and Landing"), allowing the generation and/or or the motorization of certain electrical parts of the aircraft, including the electric propulsion.

Prior technique

With the aim of reducing the overall mass of a helicopter or VTOL propulsion unit, one of the preferred ways is to reduce the mass of the electric generation and/or starting, assistance or even electric propulsion systems for the VTOLs. Indeed, the weight of these systems can reach several tens of kilograms for powers not exceeding a few tens of kilowatts, the power/mass ratio of current electric machines rarely exceeding 3 kW/kg.

However, the electrical components to be controlled often operate under a network voltage of 28Vdc, while the power demands are several kW or kVA, which gives high intensity currents that can reach several hundred amperes, requiring an increase in significantly the cross-section of the copper wires within the electrical machine, the latter (as well as its structure and its sizing in general) being a direct function of the amplitude of these currents. More generally, increasing the mass power densities of these electrical machines requires an increase in current densities while optimizing the onboard copper mass.

This is why, to keep the assembly at a temperature not exceeding the melting temperatures of the insulators making up the wires of the winding of the electric machine or to avoid slower degradation of the properties of the insulator which would lead to risks of partial discharges, it is known to use different cooling systems, such as natural convection by means of a finned dissipator, forced convection by means of a fan, forced cooling by liquid or heat exchanger or cooling by thermoelectric module (Peltier effect type).

However, in aeronautical applications, i.e. in the context of an embedded system requiring strong constraints in terms of compactness, mass and reliability, these solutions are not without drawbacks. Natural convection is cumbersome, massive and requires an air flow around the periphery of the electric machine, forced convection is also bulky and moreover harms the reliability of the electric machine, forced liquid cooling is similarly bulky, massive and intrusive towards the electrical machine and also requires frequent shutdowns for maintenance, and cooling by thermoelectric module can only concern very localized areas and requires a stabilized power supply allowing the thermoelectric power supply.

Also, finding that the electrical power demand of an electrical system can be very high but over times that do not exceed a few tens of seconds or even a minute, which means significant but cyclical or transient heat dissipation, the applicant in its application FR3012698 proposed the use of phase change materials (PCM) to allow better management of heat transfers as close as possible to the critical elements that are the windings of the electric machine.

As shown in FIG. 6, in order to clip the temperature within the winding and to protect the electrical conductors from alternating current losses during high-speed operation (therefore at high electrical frequency), these phase-change materials 2 are placed in the stator 4 of the electric machine at the bottom of the slots 6 receiving the windings 8 of the stator windings. Thus, the electromagnetic performance is increased by allowing an increase in the intensity of the electric current passing through the windings or, for an identical electric current intensity value, the wire section of the stator winding is reduced and therefore a reduction in the mass of the machine.

This solution is generally satisfactory. However, it can still be improved and in particular it is possible to improve the heat transfer properties while reducing the mass and the size of the electric machine and this independently of the voltage of the electric network. Disclosure of Invention

The present invention therefore proposes to improve the management of heat transfers within an aircraft electrical machine comprising a stator and a rotor configured to be driven in rotation with respect to each other, the stator comprising a plurality of slots receiving the same plurality or not of windings, method characterized in that it comprises the following successive steps:

- insertion of an electrical insulator in the notches or on the teeth of the stator,

- installation of the windings in the notches or on the teeth of the stator,

- casting of a phase change material in the notches or on the teeth fitted with the windings, the electrical insulation forming a casting mould.

Thus, by carrying out an encapsulation of the phase change material in a container formed by the electrical insulation ensuring the protection of the stator winding, the current intensity that can pass through the windings is significantly increased without significantly modifying the manufacturing process and the configuration of the electric machine, but giving it increased robustness in the face of external attacks (heat, dust, water in particular). Encapsulation also ensures good mechanical strength of the winding, particularly in highly vibrating environments.

When the notches are open and the insulation thus sectorized, the step of casting the phase change material is preceded by a step of inserting a wedge or a hoop to close the notches and thus prevent the casting. phase change material outside the notches.

Preferably, the electrical insulator, sectorized or not, comes from a first casting of a standard resin conventionally used to encapsulate the windings of electrical machines. According to an advantageous embodiment, the phase change material is mixed beforehand with a resin of the epoxy, polyurethane or silicone type.

The invention also relates to an aircraft electrical machine comprising a stator and a rotor configured to be driven in rotation relative to each other, the stator comprising a plurality of notches or teeth receiving a same plurality or not of windings, characterized in that, to protect the windings from excessive heating, it comprises an electrical insulator inserted in the notches or on the teeth to successively receive the said windings and a phase change material of which it forms a casting mold .

Preferably, the electrical insulation and the winding encapsulated in the phase change material can advantageously form an independent module that can be directly inserted on each of the stator teeth.

According to the embodiment envisaged, said electrical insulator may be formed of a plurality of elements corresponding to the plurality of notches or teeth, each of the elements thus sectorized being configured to be inserted individually into each of the notches or on each of the teeth or a single element adapted to the geometry of the stator and configured to be inserted together in all the slots.

Said electrical insulator can be one of the following insulators: paper, mica, polyethylene terephthalate, polyester, fiberglass, or be obtained by an additive manufacturing process from a plastic material, such as PEEK ( PolyEtherEtherKetone) or Polyamide 66 (PA66), with suitable electrical insulation and thermal resistance characteristics.

Preferably, the phase change material is a nitrate or a hydroxide preferably filled with graphite having a phase change temperature between 150°C and 300°C.

The invention also relates to an independent module consisting of an electrical insulator and windings encapsulated in a phase change material and insertable directly on each of the teeth of a stator of an electric machine thus mentioned.

The invention finally relates to an aircraft engine of the VTOL type or not comprising at least one electric machine as mentioned above. Brief description of the drawings

Other characteristics and advantages of the present invention will emerge from the detailed description given below, with reference to the following figures, which are not of any limiting nature and in which:

[Fig. 1] Figure 1 is a first example of an electric machine according to the present invention,

[Fig. 2] Figure 2 is a radial sectional view illustrating the encapsulation of the windings at the level of the coil heads in accordance with the present invention,

[Fig. 3] Figure 3 is a second example of an electric machine according to the present invention,

[Fig. 4] Figure 4 represents the evolution of temperatures according to the percentage of phase change material filling the notches of the stator of figure 1,

[Fig. 5] Figure 5 shows part of an internal stator of an electric machine according to the present invention, and

[Fig. 6] Figure 6 shows an electric machine stator of the prior art.

Description of embodiments

A first example of an aircraft electrical machine 10, comprising a stator 12 having a plurality of slots 120 and a rotor 14, for example with permanent magnets 140, the stator and rotor being configured to be driven in rotation one relative to each other, is shown in Figure 1. In the example shown, the stator typically made of ferromagnetic laminations is with concentric winding (also called dental) supplied with alternating current by a single-phase or polyphase system, typically three-phase (not shown). This winding is called "Double Layer" because the copper wires are directly wound on each of the teeth of the stator but a so-called "Single Layer" winding where certain teeth are not surrounded by copper wires is of course also possible.

In accordance with the invention, an electrical insulator which, in the example illustrated, is formed of a single element 16, is inserted together in one go into the notches 120 of the stator 12 to successively receive the windings of copper wires 18 and a phase change material 20.

This electrical insulator 16 thus forms a casting mold for the phase change material 20 which is prevented from spreading out of the notches 120 by a hoop 22 which radially closes off these notches and therefore closes the mold during casting. In order to limit the amount of air inside the phase change material, making it possible to increase the amount of material molded and therefore the amount of heat that can be extracted, the impregnation of the windings by the liquid PCM will advantageously be carried out in a vacuum chamber. The hoop is removed once the solidification has taken place to allow the positioning of the rotor, but it can also be preserved in certain specific applications.

In order to have an optimized shape, adapted to the geometric constraints of the stator, the electrical insulation will preferably be produced by a known additive manufacturing process (SLA for "Stereo Lithograph Apparatus" or PIM for "Plastic Injection Molding" for example), from a plastic material with characteristics of good electrical insulation and good thermal resistance, such as PEEK (PolyEtherEtherKetone) or Polyamide 66 (PA66).

In fact, and as the section of FIG. 2 illustrates, the winding 18 generally comprises at its two ends located outside the notches of the zones 180 called "coil head" or "bun" which should also be cover with electrical insulation 16 forming a mold to ensure complete encapsulation of the winding by the phase change material 20 (shown in the solid phase after the liquid casting phase).

The phase change material is preferably a nitrate or a hydroxide (LiNO3, NaNO3, U2CO3, etc.) preferably filled with graphite, both chemically neutral and an excellent electrical and thermal conductor, and typically has a temperature of phase change between 150°C and 300°C. It must not be chemically unstable and be of a neutral character so as not to degrade or corrode the copper wires. In order to guarantee a very significant liquid-solid phase change, the phase change material must have the property of being as congruent as possible and have a very low expansion coefficient. A second example of an aircraft electrical machine 30, also comprising a stator 32 with a plurality of notches 320 and a rotor 34, for example with permanent magnets 340, is illustrated in FIG. 3. In the example shown, however, the stator has a distributed winding and it comprises lateral expansions 322 on the lower part of the teeth (known as “Tooth Tips”). It will be noted that, in this configuration with distributed winding, the number of slots is typically greater than in the preceding configuration with concentric winding. In another configuration (not shown), the stator may include completely closed notches over its entire periphery.

Unlike the first embodiment, the electrical insulator is no longer formed of a single element but of a plurality of elements 36 corresponding to the plurality of notches, and each inserted individually in a different notch 320 of the stator 32 to successively receive, as before, the windings of copper wires 38 then a phase change material 40.

Each of the elements 36 forming the electrical insulator thus divided into sectors constitutes a casting mold for the phase change material 40 which is prevented from spreading out of the notches 320 by a wedge 42, supported by the lateral expansions 322, coming to block these radially notches and thus close the mold during casting. The assembly will then advantageously be placed in a vacuum chamber to facilitate the step of impregnating the phase change material in the liquid state before it solidifies during cooling. The wedge 42 can be removed once this solidification has taken place or left in place when the nature of its material allows it, for example PEEK (PolyEtherEtherKetone) or Polyamide 66 (PA66).

It will be noted that in this embodiment, the insulation of the slots normally present in the stators of an electrical machine is obtained by one of the following insulators: paper, mica, polyethylene terephthalate, polyester, fiberglass , can advantageously constitute the mold inside which the phase change material will be cast.

However, this electrical insulator, sectored or not, covering the walls of the slots can also advantageously come from a first casting of a standard resin conventionally used to encapsulate the windings of machines electrical and adapted to the application (epoxy, silicone, polyurethane, or any other usual resin), which will spare the windings and will contain the internal cavity to accommodate the phase change material (PCM). This will of course require a first rigid mold which will be removed once this resin has solidified.

If it was previously only a question of the casting of a phase change material (PCM), it should be noted that it is also possible to produce a premix of a resin / PCM assembly, in the form liquid or in solid form in mass proportions determined beforehand by experience and/or by thermal calculations within the reach of any person skilled in the art, and to pour this mixture into the electrical insulation instead of the PCM alone. The resin used for this mixture will typically be of the epoxy, polyurethane or silicone type. In this case, it may be useful to place the assembly in an oven to trigger or accelerate the solidification of the resin.

Figure 4 shows the evolution of the temperature as a function of the rate of filling of the notches with phase change material, in this case an Erythritol compound. It can be seen that for a fixed temperature limit value (here 150 °C), the use of a mixture based on 100% (curve A) or 40% (curve B) of a phase change material (the other 60% being made up of a simple epoxy resin) makes it possible to maintain the thermal stress for several tens of additional seconds compared to filling the notches with a simple epoxy resin (curve C - 0% of MCP) or without any filling (curve D).

Thus, compared to a standard electric machine, the invention allows a minimization of the volume and the mass of the winding of about 10% of the mass of the electric machine, i.e. about 1.2 kg for a machine of the order of 10kg. It avoids any addition of additional cooling systems degrading the mass balance, the size and the reliability rate of the electrical machine. It can be integrated in transiently very hot environments (> 150°C).

Of course, the encapsulation of the phase change material in the notches of the stator is also valid for topologies of electrical machines with a stator placed inside and a rotor placed outside as shown in FIG. an inner stator part 50 provided with a tooth stator 52 on which is inserted the electrical insulator 54 receiving the winding 56 and serving as a mold for the phase change material 58 (shown in solid phase after the casting phase). It will be noted that the electrical insulator 54 and the winding 56, dental or concentric, encapsulated in the phase change material 58, can advantageously form an independent module directly insertable on each of the stator teeth 52.

It will be noted that the invention also finds application in induction/asynchronous machines or variable reluctance machines, as in cylindrical machines with axial flux and discoid machines with linear flux.

Claims

[Claim 1] Method for protecting the windings from excessive heating in an aircraft electrical machine comprising a stator (12, 50) and a rotor (14) configured to be driven in rotation relative to each other , the stator comprising a plurality of notches (120) or teeth (52) receiving the same plurality or not of windings (18, 56), method characterized in that it comprises the following successive steps:

- insertion of an electrical insulator (16, 54) in the notches or on the teeth of the stator,

- installation of the windings in the notches or on the teeth of the stator,

- Casting of a phase change material (20, 58) in the notches or on the teeth provided with the windings, the electrical insulation forming a casting mould.

[Claim 2] Method according to claim 1, characterized in that, when the notches are open and the electrical insulation thus sectorized, the step of casting the phase change material is preceded by a step of inserting a wedge (42) or a hoop (22) pou6r close the notches and thus prevent the casting of the phase change material outside the notches.

[Claim 3] Process according to claim 1 or claim 2, characterized in that the electrical insulation, sectorized or not, is derived from a first casting of a resin for encapsulating the windings of electrical machines.

[Claim 4] Process according to Claim 1 or Claim 2, characterized in that the phase change material is mixed beforehand with a resin of the epoxy, polyurethane or silicone type.

[Claim 5] Aircraft electrical machine comprising a stator (12, 50) and a rotor (14) configured to be driven in rotation relative to each other, the stator comprising a plurality of notches (120) or teeth (52) receiving the same plurality or not of windings (18, 56), characterized in that, to protect the windings from excessive heating, it comprises an electrical insulator (16, 54) inserted in the notches or on the teeth to successively receive said windings and a phase change material (20, 58) of which it forms a casting mould.

[Claim 6] Electrical machine according to Claim 5, characterized in that the electrical insulation (54) and the winding (56) encapsulated in the phase-change material (58) can advantageously form an independent module which can be inserted directly on each of the stator teeth (52).

[Claim 7] Electrical machine according to claim 5, characterized in that said electrical insulator is formed of a plurality of elements (36) corresponding to the plurality of notches (320) or teeth (52), each of the elements thus sectorized being configured to be inserted individually in each of the notches or on each of the teeth.

[Claim 8] Electrical machine according to Claim 7, characterized in that the said electrical insulator is one of the following insulators: paper, mica, polyethylene terephthalate, polyester, fiberglass.

[Claim 9] Electrical machine according to Claim 5, characterized in that the said electrical insulator is formed of a single element (16) adapted to the geometry of the stator and configured to be inserted together in all the slots (120).

[Claim 10] Electrical machine according to Claim 7 or Claim 9, characterized in that the said electrical insulator is obtained by an additive manufacturing process from a plastic material, such as PEEK (PolyEtherEtherKetone) or Polyamide 66 (PA66 ), with appropriate electrical insulation and thermal resistance characteristics.

[Claim 11] Electrical machine according to any one of Claims 5 to 10, characterized in that the phase change material is a nitrate or a hydroxide, preferably filled with graphite, having a phase change temperature of between 150°C and 300°C. [Claim 12] Independent module consisting of an electrical insulator (54) and windings (56) encapsulated in a phase change material (58) and insertable directly on each of the teeth (52) of a stator (50) d an electric machine according to any one of claims 5 to 11. [Claim 13] Aircraft engine comprising at least one electric machine according to any one of claims 5 to 11.