The invention relates - generally - to a method for producing a cooled stator for an electric motor, e.g. mounted on electric vehicles, and to a stator so produced. The stator may be advantageously applied especially to high-power electric motors for vehicles, taken herein as an example.

High-power vehicles have electric motors with power ratings of hundreds of kW, hence the primary need to cool them. Two modes most frequently used in the cooling systems are the oil bath and the peripheral water circuit. In the oil bath, borrowed from transformer technology, the motor coils are immersed in insulating oil, which, through a primary circuit consisting of a pump and an oil/water exchanger (intercooler), transfers heat to a separate water circuit. The peripheral water circuit dissipates heat through a circuit consisting of a second pump and a water/air radiator.

The weaknesses of the oil bath system are the lower heat transfer capability of oil compared to water and the presence of two pumps and the intercooler.

The major defect of the peripheral water circuit is the reduced heat exchange surface between the heat source (the windings) and the circuit of water, which circulates isolated, on specific channels, with respect to the windings to be cooled.

The main object of the invention is to improve this state of the art.

Another object is to make a stator with more efficient cooling.

Such objects are achieved by a stator and/or method according to the appended claims, wherein the dependent ones define advantageous variants.

A system for cooling the windings is integrated into the stator. For this purpose the the stator comprises an outer ring, an inner ring concentric to the outer ring, segments extending radially from the inner ring toward the outer ring, wherein the rings and segments are internally hollow bodies and join to form one or more continuous channels inside them capable of carrying a cooling fluid along a path going from one ring to the other.

The rings and segments are arranged to form or delimit pass-through openings able to accommodate and surround each individual winding.

The stator allows fluid to circulate around the windings, increasing the heat pickup area. In addition, the continuous channel provides a watertight seal with respect to the electrical circuit of the windings, so water can be used as a fluid to take advantage of its high heat transfer capability.

The method described may be used for radial motors and for axial motors. The following description and drawings show the application to axial flux motors. An axial flux motor has a stator with a circular array of windings, arranged around the rotation axis of a rotor, which generate a magnetic flux with a polar axis parallel to the rotation axis of the rotor. This type of motor has more complex structure than radial flux motors but is lighter and smaller for the same power.

The rings and segments are hollow internally and joined to form a continuous channel within them capable of carrying a cooling fluid along a path that begins on one ring, passes into the other ring, and returns to the starting ring.

For simplicity of construction, the outer ring and inner ring describe a circumference.

Preferably, for simplicity of construction, the segments are linear segments.

Preferably, for simplicity of construction, the outer ring and the inner ring lie substantially on one plane and are substantially coplanar.

The outer ring, the inner ring, and the segments are hollow shells.

Preferably, in order to give robustness to the stator and to increase the heat transmission generated by the windings, the outer ring, the inner ring and the segments are made of technopolymers being electrically insulating but having high thermal conductivity, typically made by mixing a polymer with a mineral filler (Boron Nitride, Alumina, Zinc Oxide or other) and heat transfer capacity of several Wm/K.

Preferably, the continuous channel forms a path that makes a complete turn around the center of the rings, so as to draw heat extensively from the windings.

Preferably, the segments extend radially along an axis passing through the center of the rings, and in particular with polar symmetry with respect to said center. As a result, the segments form for the rings a kind of sunburst-pattern that advantageously skims the sides of the windings from which it removes heat. We will call these segments spokes.

Preferably there is more than one continuous channel within the component, particularly two or more ducts. Multiple channels allow for faster heat dissipation and balancing of the dissipation, e.g. preventing the fluid at the end of a channel from being too hot to remove heat effectively.

Preferably, to maximize heat dissipation, said pass-through openings have a contour complementary to the perimeter of the surrounding windings. In particular, the surface of the inner edge of the outer ring and the surface of the outer edge of the inner ring comprise cusps with tips directed radially and facing the cusps of the opposite edge.

According to an embodiment, the stator is produced in one piece, and comprises an outer ring, an inner ring concentric to the outer ring, a plurality of spokes, e.g. linear spokes, extending radially from the inner ring toward the outer ring, wherein the rings and spokes are hollow internally to have internal channels, which channels join to form a continuous channel within them capable of carrying a cooling fluid along a path from one ring to the other, the rings and spokes being arranged to form or delimit pass-through openings capable of accommodating and surrounding the windings.

Each pass-through opening is configured to accommodate and surround a single winding. Thus, each winding is surrounded on two opposite sides by two spokes which cool it. Even more preferably, each pass-through opening has an annular shape, without cusps; in particular it consists of two equal radial walls joined by two curved walls, wherein one curved wall is a surface of the inner ring with a first radius of curvature and the other curved wall is a surface of the outer ring with a second radius of curvature, wherein the first radius of curvature is smaller than the second radius of curvature.

Preferably, each spoke comprises inside it two or more radial channels, fluidically isolated from each other and preferably parallel to each other, putting into communication a circular channel present inside the outer ring with a circular channel present inside the inner ring. Thus, the volume of the spoke is traversed internally by more channels for the benefit of a better and more uniform withdrawal of heat from the windings.

The cooling fluid can be fed into the outer and/or inner ring.

A method according to the invention relates to a method of constructing a stator as defined above.

It is known to first make a stator by injection molding or 3D printing, then installing the windings thereon, and finally embedding the molded stator into a resin to fill the voids around the windings.

During the process the resin is sucked into the molded or printed stator by depression, and inevitably bubbles or rarefied areas may form between the plastic stator and the windings, where the thermal conductivity is very low. Then the thermal dissipation in the final stator may be insufficient for high power applications, also due to the fact that resins developed for filling electrical machines (motors or transformers) have modest thermal conduction coefficients, typically 1 Wm' ¹ K' ¹ .

Instead, a method according to the invention envisages arranging the copper windings constituting the stator portion of the motor within a mold, and over-injecting on the windings plastic material, e.g. the technopolymer previously mentioned, in order to obtain, after the solidification of the plastic material (and extraction from the mold), a monolithic stator consisting of a single body carrying the electrical conductors (windings) and the hydraulic cooling circuit without interruption between plastic material (or polymer) and copper.

An advantage of the method is that the inner ring, the outer ring, the spokes or the linear segments and the windings end up as one-piece where the molded plastic material formed the inner ring, the outer ring, the spokes and the linear segments; and the windings end up embedded as one-piece in the high thermal conductivity plastic material. Over-injection ensures maximum interpenetration between the windings and the plastic material of the stator, which in turn ensures no empty gaps in the stator or under-filled areas. The method increases the thermal conductivity between the windings and the surrounding plastic material, and improves the overall thermal dissipation of the stator and the motor that mounts it.

Another advantage is that the thermal conductivity of a plastic material is on average higher than that of the resin, currently at least 7 Wm’ ¹ K' ¹ , so the method allows distributing around the windings material which is more thermally conductive.

Preferably, the stator injection molding is done so that a cavity, e.g. a pass-through cavity, remains in the molded stator at the center of one or each winding or coil. The cavity is useful for aeration and weight control (lightening) for the stator. Thus, in effect, the molding makes a conduit around one or each winding.

Another aspect of the invention relates to an electric vehicle equipped with the stator as defined above in one or each of the variants.

The advantages of the invention will be clearer from the following description of a preferred embodiment of a fluid-cooled stator and a method for producing it, reference being made to the attached drawing wherein

• Fig. 1 shows a three-dimensional view of a known stator,

• Fig. shows a cross-sectional view of the stator of Fig. 1 according to plane ll-ll;

• Fig. 3 shows a three-dimensional view of a stator according to the invention; • Fig. 4 shows a cross-sectional view of the stator of Fig. 3 according to the plane IV-IV;

• Fig. 5 shows a cross-sectional view of the stator of Fig. 3 according to the plane V-V;

• Figs. 6a, 6b, and 7 show the results of a simulation.

In order not to crowd the drawings, some equal elements are not all marked with a number.

Fig. 1 shows a stator of an axial-flux electric motor, where there are visible windings or coils 10 arranged circularly around an X-axis and surrounded by a cooling component 20. The component 20 comprises an outer circular ring 30, an inner circular ring 40 concentric to the outer ring 30, and straight segments or spokes 50 radially joining the two rings 30, 40.

The outer ring 30 and the inner ring 40 have centers on the X-axis.

Two adjacent segments 50 and the arcs of ring 30, 40 comprised by them delimit pass-through cavities 36 having perimeter complementary to the windings 12. The number of segments or spokes 50 may vary, thus varying the number of side-by-side windings 12 provided between two adjacent segments 50.

The rings 30, 40 and the segments 50 are hollow shells and collectively form a continuous channel within them for conveying a cooling fluid, which enters the component 20 from an inlet and exits from an outlet.

Fluid circulation within the component 20 occurs along a path involving at least once the two rings 30, 40 and at least two segments 50. That is, fluid circulates within the component 20 by passing from the ring 30 to the ring 40 via a segment 50 and then passing from the ring 40 to the ring 30 via a different segment 50. As it flows, the fluid skims the windings and draws heat away from them.

The number of channels for the cooling fluid within the component can vary, particularly the number of independent channels. Two or more separate channels can better remove heat from the windings12, providing a more uniform operating temperature for the motor.

The pass-through cavities 36 have perimeters complementary to a single winding 12.

After the windings are mounted in the component 20, they are coated with resin 56 that is distributed around them (Fig. 2) to form the final stator 10.

As noted above, the resin 56 offers modest thermal dissipation performance, which can be improved with the stator 60 of Fig. 3, produced by a method according to the invention.

The internal and geometric structure of the stator 60 is the same as in Fig. 1 .

The stator 60 is for an axial-flux electric motor, and there are visible windings or coils 12 arranged circularly around an X axis.

The stator 60 consists of an outer circular ring 72, an inner circular ring 74 concentric to the outer ring 72, and straight segments or spokes 76 radially joining the two rings 72, 74.

The outer ring 72 and the inner ring 74 have centers on the X-axis.

The number of segments or spokes 76 may vary, thus varying the number of windings 12 too.

Again, the rings 72, 74 and the segments 76 are hollow shells and altogether form a continuous channel within them for conveying a cooling fluid, which for example enters the stator 60 from an inlet and exits from an outlet. The inlets of the radial channels 80 from the ring 72 to the ring 74 are visible, for example, on the side of the stator 60 in Fig. 3 or Fig. 5.

In particular, the stator 60 is formed by the superposition (or stacking) along X of a replicated structure, that consisting of an outer circular ring 72, an inner circular ring 74 and associated hollow segments or spokes 76. The channels within each ring 72, 74 and associated hollow segments or spokes 76 may be totally isolated from, or communicating with, those of an adjacent structure to form an overall fluid path.

The circulation of the fluid inside the stator 60 occurs as already explained for fig. 1 .

The steps to produce the stator 60 are as follows.

The windings 12 are arranged inside a mold (not shown) in the position they assume in the final stator, and the mold is closed.

Plastic material is injected into the mold. The material embeds the windings 12 and simultaneously forms one or more axially overlapping structures consisting of an outer circular ring 72, an inner circular ring 74 and associated hollow segments or spokes 76.

When extracted from the mold, the molded part is formed by the intimate and integral union between the windings 12 and the plastic forming one or each outer circular ring 72, one or each inner circular ring 74 and associated segments or hollow spokes 76. In particular, the plastic material is in direct contact with the windings 12. See in this regard the cross-sections of Fig. 4 and Fig. 5, which illustrate the internal structure of the stator 60. The plastic material completely surrounds the windings 12 but leaves a pass- through cavity 78 in the center of each winding 12. The cavity 78 is subsequently used to accommodate the ferromagnetic core that completes the coils (windings) of the motor and is made, for example, from shaped and overlapping ferromagnetic laminations or from sintered SMC (Soft Magnetic Composite) material.

The method according to the invention, based on simulation and experimental tests, provides significantly better thermal performance than the variant with resin 56: about 15- 20 % higher heat transfer (dissipation).

Figure 6b shows the results of a simulation for the stator 10, where the shades of gray indicate the temperature at a winding 12 in steady state. Figure 6a shows the results of a simulation for the stator 60, where the shades of gray indicate the temperature in correspondence of a winding 12 in steady state. It is evident that with the same circulating fluid the winding 12 of the stator 60 is on average everywhere colder.

Figure 7 shows in a comparative graph, the graph 110 and 160 respectively, the results of a simulation for the stator 10 and 60. The graphs show the average temperature at a winding in steady-state after the motor has been started and given time to bring itself up to operating temperature. Again, it is evident from the temperature/time trend how a winding 12 of the stator 60 remains cooler for the same amount of power dissipated by the motor.