The present invention refers to a method to make a core about which an electric winding mounted in an electric motor is placed. The present invention also refers to the core and the inductor thus obtained, and to the motor comprising the core. The invention refers preferably to an axial-flux electric motor, a case in which it has been demonstrated particularly effective.

Axial-flux electric motors have windings that each generate a magnetic flux with polar axis parallel to the rotor’s rotation axis. While US5861695 and US6255005 show examples of inductors, currently the state of the art for axial flux machine cores involves the use of sintered soft magnetic material (Soft Magnetic Composite) which, however, has performance limits in terms of magnetic permeability.

The magnetic properties (permeability and loss figure) of SMC are not optimal, but it is chosen anyway because of the need to make complex shaped cores.

A core of improved performance would then be needed.

The main object of the invention is to improve the present state of the art. This is achieved by a method according to the attached claims; other advantageous technical characteristics are defined in the dependent claims. A method is then presented to make a core around which an electric winding can be arranged to create a magnetic field generator, with the steps of making a central part of the core made with superimposed sheets of ferromagnetic material (e.g. iron...), surrounding the central part with powdered SMC material and then sintering it to obtain a solid composite core.

With the method it is possible to produce a composite core that combines the superior magnetic properties (permeability and loss figure) of a laminated ferromagnetic material and the design freedom in creating an overall core having complex geometries determined by the outer shell of SMC. The SMC material is shaped, for example in a mold. In particular, the method envisages placing the central part in the mould and then adding inside the mould the SMC material in powder form or granules. Then by sintering, i.e. a high-temperature heat treatment that, for example, can take place in said mould, the SMC material is transformed into an indivisible, solid material. The composite core also ensures electrical insulation and is easily shaped to optimize the coupling with the winding surrounding it.

Another advantage is that the sintered material can take a shape that maximizes the torque density of the entire machine; moreover, it makes no longer necessary the use of a polymeric body to safeguard the integrity of the conductors’ insulation.

In particular, the composite core has mechanical and above all magnetic characteristics that are overall superior to a core made only of SMC or laminated ferromagnetic material.

Another aspect of the invention concerns an inductor or coil to generate an electric field that comprises a core and an electric winding wound around the core, wherein the core is composite and formed by a central part of the core made of superimposed sheets of ferromagnetic material (e.g. iron...), and an outer shell surrounding the central part made of sintered SMC material.

Said central part may be composed by the superposition of sheets being all equal and/or with the same plan, or it may derive from the juxtaposition of a plurality of stacks of equal or different sheets. The outer shell may surround the central part completely, i.e. three-dimensionally, or it may surround it only partially, in which case preferably the shell is a ring that wraps the central part leaving two ends thereof uncovered.

As preferred examples of usable SMC material: sintered material consisting of iron powders coated with insulating resins, in different proportions and of different nature; allows the substantial reduction of electrical conductivity and the making of three- dimensional geometries and fluxes, e.g. Somaloy®.

As preferred examples of usable ferromagnetic material: soft ferromagnetic material bound to various elements (Si, etc.), to improve magnetic permeability and electrical resistivity properties, and/or laminated material to reduce the possible path of eddy currents. The different layers are glued to form a compact structure.

Another aspect of the invention concerns a stator of an axial flux electric motor comprising a constant angular pitch circular series of inductors or coils, made as defined above, to each generate a magnetic field. In an axial flux electric motor the stator is equipped with windings arranged in a circular series about a rotation axis of a rotor. Each winding acts to create a magnetic field, with a polar axis parallel to the rotation axis, through which a rotor is set into rotation thanks to the magnetic interaction between the generated magnetic fields and a corresponding circular series of magnetic elements of the rotor. This type of motor has a more complex structure than radial flux motors but is lighter and smaller the power being the same.

Further advantages will be clear from the following description, which refers to an example of a preferred core construction wherein:

- Figure 1 shows a three-dimensional view of a core,

- Figure 2 shows a three-dimensional view of a variant of core. The construction steps of a core for the stator of an axial flux electric motor will be now described. In order not to crowd the drawings multiple or repeated parts are not all indicated with numbers. Equal numbers indicate equal parts.

A core MC (fig. 1) is substantially wedge-shaped, to be arranged in a circular series. The core MC has a top face 50, a base face 52 and a side surface 54 (these relative spatial terms are used only for ease of description). The top face 50 and base face 52 are orthogonal to an imaginary axis Y, here indicated as the polar axis of the magnetic field that will pass through the core MC.

The volume of the core MC consists of a central part 10 and an outer shell 30 that incorporates the central part 10.

The central part 10 is formed by superimposed sheets 12 of ferromagnetic material, wherein the sheets 12 lie on a plane parallel to the Y axis. In the shown example there is a pack of sheets 12 all the same and stacked to form a parallelepiped of sheets 12.

The outer shell 30 is made instead of sintered SMC material, and surrounds the central part 10 forming a sort of ring coaxial to the Y axis. Short sides of the sheets 12 may remain visible on the top face 50 and the base face 52, but not necessarily.

An electrical winding (not shown) is wound around the side face 54 of the core MC, spiraling around the axis Y. By powering the winding, a magnetic field is created - as known. A second core MC2 (fig. 2) is substantially wedge-shaped, to be arranged in a circular series.

The core MC2 still has a top face 50, a base face 52 and a side surface 54 (these relative spatial terms are used only for ease of description).

The volume of the core MC2 is still composed of a central part 70 and an outer shell 80. This time the central part 70 is formed by two groups of superimposed sheets 14, 16 of ferromagnetic material, wherein the sheets 14, 16 lie on a plane parallel to the axis Y and the sheets 14, 16 of each group are equal to each other. In the example shown there is a pack of sheets 14 attached to a pack of sheets 16 to occupy or fill a larger internal volume of the shell 70 otherwise not achievable with a single pack. In the example shown, the sheets 14 are wider than the sheets 16.

The outer shell 70 is made like the shell 30.

Number and thickness of the sheets may vary from what is shown, the same applies for the number of attached groups of sheets of different sizes.