ELECTRIC MACHINE

Description

Disclosure domain

[0001] The invention belongs to the technical field of electrical machines, capable of forming an electric motor or generator. State of the art

[0002] The current rapid expansion of electric vehicles necessitates the design of electric motors that combine compactness with high power. The recent use of electric motors to power the new generation of land and air vehicles requires the development of more efficient and higher-performing electric motor series. Increasing the power density of electric motors can be limited by electrical and thermal constraints. The temperature increase in electric machines results from electromagnetic and mechanical losses, generating heat in the motor components.

[0003] An electric motor typically consists of a stator, a rotor, and a housing. The stator is generally made of laminations and a winding, which is a coil of conductive wire, most of which is embedded in slots within the stator. An electric motor operates by passing an electric current through the winding, which generates a magnetic field. This field creates a magnetic moment, causing the rotor to rotate.

[0004] The conversion of this electrical energy into mechanical energy by the motor does not occur without voltage losses. For electric motors, depending on the type of motor, between 5 and 15% of the input electrical power is lost in this way.

[0005] These losses can be classified into several categories: mechanical losses, iron losses, losses related to parasitic loads and Joule losses.

[0006] Mechanical losses are mainly due to friction inside the bearings and aerodynamic heating resulting from a very high velocity gradient in the air gap between the rotor and the stator.

[0007] Iron losses and Joule losses are vectors of thermal losses distributed respectively in the stator laminations and the winding. Iron losses are produced by magnetic effects (hysteresis and eddy currents), and Joule losses in the windings are due to the resistivity of the copper conductors.

[0008] Therefore, effective thermal management of the electric motor is essential to achieving high power density. Efficient cooling allows for a combination of high power and compact motor design, reducing the overall size of the vehicle's components. Furthermore, a lower operating temperature prevents the degradation of the thermal and electrical insulation surrounding each conductor in the winding. The lifespan of this insulation decreases as the temperature rises.

[0009] For a permanent magnet motor, one of the most widely used electric motors today, the magnetic properties of the permanent magnets are highly dependent on the operating temperature. A significant temperature increase can lead to their irreversible demagnetization. Furthermore, an increase in motor temperature results in increased Joule heating losses. It should be noted that the electrical resistance of the conductors increases linearly with increasing motor temperature.

[0010] To limit the temperature, a heat transfer fluid is commonly used to dissipate the heat generated by the motor. A conventional cooling method involves passing a flow of water or air over the casing surrounding the stator. However, such cooling offers limited performance because the heat generated by the coils must pass through areas and interfaces where thermal conductivity is low.

[0011] It is also known to insert a sleeve between the stator and the casing of the electric motor, said sleeve delimiting channels allowing the circulation of a cooling fluid.

[0012] Documents FR 3 036 880 A1 and WO 2021 / 236565 disclose an electrical machine comprising a stator and a rotor capable of pivoting about an axis, and a cooling element extending around at least a portion of the stator, between the stator and the housing. The cooling element is in the form of an annular sleeve with circulation channels allowing the flow of a cooling fluid, said sleeve being in contact both with the housing, radially externally, and with the stator, radially internally.

[0013] The invention aims to provide an electrical machine that can adapt to different phases of operation of said machine. Statement of Disclosure

[0014] To meet such a need, the invention proposes an electrical machine comprising a stator and a rotor capable of pivoting about an axis, an annular cooling element extending around at least a part of the stator, said cooling element having at least one circulation channel capable of allowing the circulation of a cooling fluid, an annular housing extending around said axis and at least partially surrounding the cooling element, characterized in that the stator has a first coefficient of thermal expansion, the cooling element having a second coefficient of thermal expansion, greater than the first coefficient of thermal expansion, said cooling element being pre-stressed around the stator, the housing having a third coefficient of thermal expansion, greater than or equal to the first coefficient of thermal expansion and less than the second thermal coefficient, said housing at least partially surrounding the cooling element,A radial annular gap is formed between the cooling element and the housing at a first temperature, said gap being able to be filled by differential expansion between the cooling element and the housing at a second temperature.

[0015] The terms axial, radial and circumferential are defined with respect to the axis of rotation of the rotor.

[0016] The pre-stressed mounting, or tight mounting, means that the diameter of the cooling element at rest, i.e. not mounted around the stator, is less than the diameter of the cooling element in its mounted state around the stator.

[0017] Thus, during operation, as the temperature of the cooling element and the stator increases, the cooling element expands faster than the stator. In other words, the radially external diameter of the cooling element increases more rapidly than the external diameter of the stator. This partially reduces the preload on the cooling element. However, this preload is determined so that, at a normal operating temperature, for example, between 90°C and 120°C, it is not zero. In other words, regardless of the operating temperature, the contact between the cooling element and the stator is maintained.

[0018] Such contact makes it possible, for example, to ensure a seal between the cooling element and the stator and / or to dissipate heat, by contact between said cooling element and the stator.

[0019] Furthermore, the contact between the cooling element and the casing beyond the second temperature allows the latter to be used as a heat sink.

[0020] Such contact also allows for better vibration damping during operation, particularly when the cooling element is made of elastomer material.

[0021] Thus, in a first case of operation, the temperature of the fluid circulating in the channel of the cooling element is at a low temperature, for example at the first temperature.

[0022] In such a case, it is generally necessary to increase the temperature of this fluid to ensure optimal operation of the electrical machine. Indeed, if the temperature is too low, the fluid may have excessive viscosity, which can lead to overconsumption of fluid and / or energy required for its circulation. This is achieved in this document by limiting the heat exchange surfaces, that is, by preventing contact between the cooling element and the housing, thanks to the radial clearance between them.

[0023] Conversely, once the optimal temperature (for example, the second temperature) is reached, it is generally necessary to maximize heat dissipation. In this document, such dissipation is achieved through contact between the cooling element and the housing.

[0024] Differential expansion phenomena are thus used to form a thermal switch that is beneficial to the operation of the system.

[0025] The stator and the cooling element can each extend around the X-axis.

[0026] The stator can consist of an annular body and a winding. The body can be metallic. In particular, the body can be formed from a stack of metal laminations. The body can be made of steel.

[0027] The first coefficient of thermal expansion can be between 10 x 10 -6< K -1< and 12 x 10 -6< K -1< .

[0028] The second coefficient of thermal expansion can be between 40 x 10 -6< K -1< and 140 x 10 -6< K -1< .

[0029] The third coefficient of thermal expansion can be between 20 x 10 -6< K -1< and 30 x 10 -6< K -1< .

[0030] The casing can be metallic. The casing can be made of aluminum.

[0031] The radial clearance between the cooling element and the housing can be between 1 and 10 mm.

[0032] The second temperature can be between 70 and 110°C.

[0033] The matrix of the cooling element can be, at least in part, made of elastomeric material, for example rubber.

[0034] The elastomeric material can be formulated to exhibit a thermal conductivity λ > 1 Wm-1.K-1.

[0035] The said elastomeric material is for example suitable for resisting oils and / or high temperatures, for example of the order of 150°C, over long periods of time or continuously.

[0036] The said cooling element is for example made of an elastomer chosen from the following list: polyacrylics ACM, ethylene acrylate copolymers AEM, fluorocarbon polymers FKM, fluorosilicones FVMQ, hydrogenated nitrile HNBR, or is made of a mixture of several elastomers from this list.

[0037] The said elastomer may contain fillers, such as for example carbon fillers (e.g. graphite, carbon nanotubes, carbon black or graphene), metallic fillers (e.g. Ag, Cu, Al, TiO2, AIN) and / or ceramic fillers (BN, Si).

[0038] Such loads facilitate heat exchange and the removal of heat, radially towards the outside.

[0039] The mass loading rate is, for example, between 30 and 50%.

[0040] The cooling element may include elongated reinforcement elements embedded and attached to a matrix of synthetic material of the cooling element, the reinforcement elements being regularly distributed around the circumference, at least part of the reinforcement elements extending each along a spiral trajectory around the jacket, the projection of the spiral trajectory of each reinforcement element onto a plane parallel to the axis forming an angle with said axis, the angle being between 46 and 54°.

[0041] The presence and orientation of the reinforcing elements help to limit the axial expansion of the cooling element. It should be noted that, during such expansion, the reinforcing elements will tend to reorient themselves at a so-called neutral angle, the value of which is 54.75°.

[0042] The radially external surface of the cooling element may include at least one textured area comprising recessed and / or protruding parts, intended to bear against the housing.

[0043] The aforementioned recessed and / or protruding parts are designed to gradually bear against the housing as the cooling element expands due to temperature. In other words, the contact area between the cooling element and the housing can gradually increase with temperature.

[0044] Reinforcing elements can be wires.

[0045] The threads may be, at least in part, textile threads, for example polyamide or polyester. The threads may also be, at least in part, metallic threads, for example stainless steel or copper.

[0046] The diameter of each wire can be between 0.25 and 2 mm, for example around 0.5 mm.

[0047] The cooling fluid can be a heat transfer fluid or a refrigerant.

[0048] Said channel may open, at least partially, at the level of the radially internal surface of the cooling element, said channel being delimited partly by said cooling element and partly by the external surface of the stator.

[0049] In such a case, the pre-stressing of the cooling element on the stator also ensures the sealing of said channel.

[0050] At least one axial end of the cooling element may be located axially recessed relative to the corresponding end of the stator, at the first temperature.

[0051] The term "recessed" means that the said end of the cooling element does not axially protrude from the stator.

[0052] Said end of the cooling element may be located axially at or beyond said corresponding end of the stator, at a second temperature, higher than the first temperature.

[0053] The axial distance between said end of the cooling element and said corresponding end of the stator, at said second temperature, can be limited, for example less than 1 mm. This ensures that the axial expansion of the cooling element beyond the stator remains controlled, so as to avoid, for example, any unwanted interaction with surrounding elements.

[0054] The opposite axial end of the cooling element can be adapted to bear against a flange axially fixed to the stator and / or the housing, so as to prevent the displacement of said opposite axial end beyond the corresponding end of the stator. Such a feature makes it possible to limit the axial expansion of the cooling element in only one direction.

[0055] The coolant can be oil or a non-electrically conductive fluid.

[0056] The cooling element may have several channels arranged in parallel, extending circumferentially.

[0057] Each channel may have a first end forming a coolant inlet and a second end forming a coolant outlet.

[0058] The first ends of the channels can be connected together to form a common input. The second ends of the channels can be connected together to form a common output.

[0059] The inlet of each channel can be located at the top of the cooling element. The outlet of each channel can be located at the bottom of the cooling element.

[0060] The cooling element may have at least two opposing channels, each extending from an upper to a lower part of the cooling element.

[0061] The inlets of said two opposing channels may form a common inlet. The outlets of said two channels may be axially opposed and terminate at two opposite axial ends of the cooling element.

[0062] The stator may comprise an annular body with axis X and a winding extending axially beyond the body at an axial end zone of the winding, the channel comprising an outlet opening at an axial end of the element, opposite a corresponding axial end of the body and opposite the corresponding axial end zone of the winding.

[0063] The channel outlet can be located in the lower part of the cooling element.

[0064] The cooling element may also include an auxiliary channel opening axially in the upper part of the cooling element, opposite a corresponding axial end of the body and opposite the corresponding axial end area of ​​the winding.

[0065] The winding can extend axially on either side of the body. In such a case, the element can include two auxiliary channels opening axially opposite each of the end zones of the winding.

[0066] The electric machine may include a supply tank capable of providing each channel with cooling fluid and a collection tank capable of collecting the cooling fluid from each channel. A heat exchanger may be combined with the collection tank to maintain a constant temperature of the cooling fluid.

[0067] The fuel supply reservoir can be located opposite the upper part of the stator. The collection reservoir can be located opposite the lower part of the stator.

[0068] The electric machine may include a pump to convey cooling fluid from the collection tank to the feed tank.

[0069] This document also proposes a method of operating an electrical machine of the aforementioned type, in which at the first temperature, a radial annular gap is formed between the cooling element and the housing, and in which, at the second temperature, said gap is filled by differential expansion between the cooling element and the housing. Brief description of the figures

[0070] Other features and benefits of this disclosure will appear in the following detailed description, referring to the attached drawings in which: [ Fig. 1 ] is an exploded, perspective view of an electrical machine according to one embodiment of the invention, [ Fig. 2 ] is a perspective view of part of the electrical machine, with the cooling element represented "in negative", so as to show only the channels of the cooling element, [ Fig. 3] is a perspective view of part of the electrical machine, [ Fig. 4 ] is a perspective view of the cooling element, on which the reinforcing elements are schematically represented in dotted lines, [ Fig. 5 ] is a view of a portion of the cooling element developed or projected onto a plane parallel to the axis of said cooling element, the reinforcing elements being represented by dashed lines, [ Fig. 6 ] is a radial cross-sectional view illustrating the sections of the stator, the cooling element and the housing, at the first operating temperature, [ Fig. 7 ] is a view corresponding to the figure 6 , at the second operating temperature, [ Fig. 8 ] illustrates different forms of realization of protruding and / or recessed areas of the radially external surface of the cooling element. Detailed description of the disclosure

[0071] THE figures 1 to 7illustrate an electrical machine 1 according to a first embodiment of the invention.

[0072] This includes a stator 2 which may have an annular body 4 with axis X and a winding 5 which may extend axially beyond the body 4, on either side of the body 4, at the level of axial end zones 6 of the winding 5.

[0073] The body 4 can be formed from a stack of metal sheets, for example steel. Furthermore, the body 4 can include an annular part 7 and comprise, for example, three fixing lobes 8 ( figure 1 ), extending radially outwards from the annular part 7. Each lobe 8 can extend axially and has a hole 9 for the passage of a fixing screw 10 ( figure 2 ) suitable for securing the body 4 to a housing 3. Of course, the annular part may be without lobes 9. The housing 3 may be made of aluminium.

[0074] The winding 5 may be located radially inside the body 4.

[0075] The electrical machine 1 includes an annular cooling element 11, extending around the body 4.

[0076] The cooling element 11 may be in the form of a jacket or layer of elastomeric material conforming to the general shape of the radially external surface of the body 4. The cooling element 11 may thus include complementary areas 12 to the lobes 8 of the body 4.

[0077] The cooling element 11 may include a first cooling circuit 13 and a second cooling circuit 14.

[0078] The first cooling circuit 13 may have two parts that are generally symmetrical with respect to a vertical plane. Each part may have several parallel channels 15, for example three channels 15, extending circumferentially between a cooling fluid inlet 16 and a fluid outlet 17.

[0079] The fluid inlet 16 can be located in the upper part of the cooling element 11 and can be common to the three channels 15 of the first part and the three channels 15 of the second part of the first cooling circuit 13. In particular, the fluid inlet 16 can be connected to the upper ends of the channels 15 of each part by an upper connecting zone 18. Each part of the first cooling circuit can further include a fluid outlet 17 common to the corresponding channels 15, and opening at an axial end 11a, 11b of the cooling element 11.

[0080] The fluid outlet 17 of the first part of the first circuit 13 can open at a first axial end 11a of the cooling element 11 and the fluid outlet 17 of the second part of the first circuit 13 can open at a second axial end 11b of said element 11, opposite to the first end 11a.

[0081] The second cooling circuit 14 ( figure 2 ), optional, may comprise a first part 14a and a second part 14b, each located in the upper part of the cooling element 11.

[0082] Each part 14a, 14b of the second circuit 14 may have a common fluid inlet 19 located in the upper part and several channels 20 extending from said inlet and opening at an axial end 11a, 11b of the cooling element 11. Each channel 20 of the second circuit 14 may have an outlet 20a separate from the other channels 20 or may open at a common outlet.

[0083] The outputs of the channels 20 of the first part 14a of the second circuit 14 can open at the first axial end 11a of the cooling element 11 and the outputs of the channels 20 of the second part 14b can open at the second axial end 11b of the cooling element 11.

[0084] The inlet 16 of the first circuit 13 and the two inlets 19 of the second circuit 14 can be supplied with cooling fluid from a supply tank 21 ( figure 3) located in the upper part of the electrical machine 1.

[0085] A collection tank 22 located in the lower part can be suitable for collecting the cooling fluid from the different circuits 13, 14. A heat exchanger can be combined with the collection tank in order to keep the temperature of the cooling fluid constant.

[0086] A pump 23 and pipes 24 can be used to convey the cooling fluid from the collection tank 22 to the supply tank 21.

[0087] The coolant is, for example, oil.

[0088] Each channel 15, 20 of each circuit 13, 14 may have a rectangular section and may include a radially internal surface and a radially external surface in the shape of a portion of a cylinder, connected by radial flanks.

[0089] The radially internal surface can be formed by the radially external surface 4a of the body 4 of the stator 2. In other words, the channels 15, 20 can be opening radially inwards and closed by the body 4.

[0090] The cooling element 11 may include a matrix of elastomeric material capable of withstanding oils and / or high temperatures, for example on the order of 150°C, over long periods of time or continuously.

[0091] The matrix of the cooling element 11 is for example made of an elastomer chosen from the following list: polyacrylics ACM, ethylene acrylate copolymers AEM, fluorocarbon polymers FKM, fluorosilicones FVMQ, hydrogenated nitrile HNBR, or is made of a mixture of several elastomers from this list.

[0092] Furthermore, said elastomer may contain fillers, such as carbon fillers (e.g. graphite, carbon nanotubes, carbon black or graphene), metallic fillers (e.g. Ag, Cu, Al, TiO2, AIN) and / or ceramic fillers (BN, Si).

[0093] The mass loading rate is, for example, between 30 and 50%.

[0094] The elastomeric material can be formulated to exhibit a thermal conductivity λ > 1 Wm-1.K-1.

[0095] As illustrated in Figures 4 and 5 , the cooling element 11 may also include elongated reinforcing elements 25, for example wires, which are embedded and integral with the elastomer matrix.

[0096] The threads may be, at least in part, textile threads, for example polyamide or polyester. The threads may also be, at least in part, metallic threads, for example stainless steel or copper.

[0097] The reinforcing elements 25 can be regularly distributed around the circumference, at least some of the reinforcing elements 25 extending each along a spiral trajectory around the jacket, the projection of the spiral trajectory of each reinforcing element onto a plane parallel to the axis ( figure 5 ) forming an angle α with said axis, the angle α being between 46 and 54°.

[0098] The coefficient of thermal expansion of the body 4 is lower than the coefficient of thermal expansion of the cooling element 11. In order to ensure contact and sealing between the body 4 and the cooling element 11, the latter is mounted pre-stressed or clamped onto the body 4.

[0099] As previously stated, the pre-stressed mounting, or tight mounting, means that the diameter of the cooling element 11 at rest, i.e. not mounted around the body 4, is less than the diameter of the cooling element 11 in its state mounted around the body 4.

[0100] Furthermore, the annular housing 3 may have a coefficient of thermal expansion which is greater than or equal to the coefficient of thermal expansion of the body 4 and which is less than the coefficient of thermal expansion of the cooling element 11.

[0101] A radial annular set j ( figure 6 A gap (for example, between 1 and 10 mm) can be formed between the cooling element 11 and the housing 3 at a first temperature, for example 20°C. This gap j can be filled by differential expansion between the cooling element 11 and the housing 3 from a second temperature, for example 90°C.

[0102] Of course, it is possible to adapt the electric machine 1 so that the second temperature is different from 90°C, for example between 70 and 110°C.

[0103] Furthermore, the cooling element 11 has a first axial end 26 and a second axial end 27.

[0104] As illustrated in the figure 6 , the first axial end 26 can be located axially recessed relative to the corresponding end of the stator 2, at the first temperature.

[0105] The term "recessed" means that the end 26 of the cooling element 11 does not extend axially beyond the stator. Such a recess r is, for example, between 0 and 3 mm.

[0106] The second axial end 27 of the cooling element 1 may be able to bear against a flange 28 axially attached to the stator 2 and / or the housing 3, so as to prevent the displacement of said opposite axial end 27 beyond the corresponding end of the stator 2.

[0107] The radially external surface 29 of the cooling element 11 may include at least one textured area comprising recessed and / or protruding parts, intended to bear against the housing 3.

[0108] The projecting or recessed parts may include grooves 30, with a rectangular or triangular cross-section for example, oriented axially or at an angle to the X-axis (see embodiments 2 and 4 in the figure 8 ), or present different grooves 30 with different orientations (see embodiments 1 and 3 in the figure 8). Of course, any other type of protruding and / or recessed area can be used. Thus, other types of straight or curved bosses, illustrated in embodiments 5 and 6 of the figure 8 for example, can be used.

[0109] During the operation of the electric machine 1, cooling fluid is brought by the pump 23 into the supply tank 21 and circulates through the channels 15, 20 of the first and second cooling circuits 13, 14 before exiting at the upper and lower parts of the axial ends 11a, 11b of the cooling element 11, through the outlets 17, 20a opposite the end areas 6 of the windings 5 ​​in order to cool them.

[0110] At the start of operation, the cooling fluid and more generally the various elements of the electrical machine, in particular the stator 2, the cooling element 11 and the casing 3, are at a low temperature (or first temperature), for example 0°C or 20°C depending on the external environmental conditions for example.

[0111] At this temperature, the cooling element 11 is in contact only with the stator 2, a clearance j existing between the radially external surface 29 of the cooling element 11 and the housing 3. Furthermore, the end 26 is recessed relative to the corresponding end of the stator 2.

[0112] During the operation of the electric machine 1, the cooling fluid's temperature rises to reach an optimal operating temperature (or second temperature), for example 90°C. During this heating, the stator 2, the cooling element 11, and the housing 3 expand differently. During such heating, the expansion of the cooling element 11, in particular, tends to catch up with the clearance j, until the external surface 29 of the cooling element 11 comes into contact with the housing 3. Furthermore, the reinforcing elements 25 limit axial expansion so that, even if the cooling element 11 can extend slightly beyond the stator 2 at the second temperature, such axial extension e remains limited ( figure 7 ).

[0113] The cooling element 11 can then dissipate heat through the casing 3, which helps to regulate the temperature of the cooling fluid.

[0114] It should be noted that the recessed and / or protruding parts 30, 31 present on the external surface 29 of the cooling element 11 are capable of progressively bearing against the housing 3 during the expansion of the cooling element 11, due to the increase in temperature. In other words, the contact area between said cooling element 11 and the housing 3 can gradually increase with temperature.

[0115] Of course, the disclosure is in no way limited to the embodiment(s) described by way of illustration, not limitation.

Claims

1. An electric machine (1) comprising a stator (2) and a rotor which is capable of pivoting about an axis (X), an annular cooling element (11) extending around at least a portion of the stator (2), said cooling element (11) comprising at least one circulation channel (15) capable of allowing the circulation of a coolant, an annular casing (3) extending around said axis (X) and at least partly surrounding the cooling element (11), characterized in that the stator (2) has a first coefficient of thermal expansion, the cooling element (11) having a second coefficient of thermal expansion which is greater than the first coefficient of thermal expansion, said cooling element (11) being mounted around the stator (2) in a prestressed manner, the casing having a third coefficient of thermal expansion which is greater than or equal to the first coefficient of thermal expansion and is less than the second thermal coefficient, said casing (3) surrounding the cooling element (11) at least in part, a radial annular clearance (j) being formed between the cooling element (11) and the casing (3), at a first temperature, said clearance (j) being capable of being filled in by the differential expansion between the cooling element (11) and the casing (3), at a second temperature2. Electric machine (1) according to the preceding claim, wherein the matrix of the cooling element (11) is made, at least in part, of elastomer material, for example rubber.

3. Electric machine (1) according to one of the preceding claims, said cooling element (11) comprising elongate reinforcement elements (25) embedded in and integral with a matrix of synthetic material of the cooling element (11), the reinforcement elements (25) being regularly distributed around the circumference, at least a portion of the reinforcement elements (25) each extending along a spiral path around the jacket, the projection of the spiral path of each reinforcement element onto a plane parallel to the axis (X) forming an angle with said axis (X), the angle being between 46 and 54°.

4. Electric machine (1) according to the preceding claim, wherein the reinforcement elements (25) are wires or threads.

5. Electric machine (1) according to one of the preceding claims, wherein the radially outer surface (29) of the cooling element (11) comprises at least one textured area comprising recessed portions and / or protruding portions (30, 31), intended to bear against the casing (3).

6. Electric machine (1) according to one of the preceding claims, wherein the cooling element (11) comprises at least one circulation channel (15) for circulating a coolant.

7. Electric machine (1) according to the preceding claim, wherein said channel (15) leads, at least in part, to the radially inner surface of the cooling element (11), said channel (15) being partly delimited by said cooling element (11) and partly by the outer surface of the stator (2).

8. Electric machine (1) according to one of the preceding claims, wherein at least one axial end (26) of the cooling element (11) is axially recessed (r) relative to the corresponding end of the stator (2), at the first temperature.

9. A method of operating an electric machine (1) according to one of the preceding claims, wherein, at the first temperature, a radial annular clearance (j) is formed between the cooling element (11) and the casing (3), and wherein, at the second temperature, said clearance (j) is filled in by the differential expansion between the cooling element (11) and the casing (3).