WO2018077788A1

WO2018077788A1 - An axial flux switched reluctance machine and an electric vehicle comprising the machine

Info

Publication number: WO2018077788A1
Application number: PCT/EP2017/076976
Authority: WO
Inventors: Pere ANDRADA GASCÓN; Eusebi MARTÏNEZ PIERA; Marcel TORRENT BURGUÉS; Balduí BLANQUÉ MOLINA; José Ignacio PERAT BENAVIDES; José Antonio SÁNCHEZ LÓPEZ
Original assignee: Universitat Politecnica De Catalunya
Priority date: 2016-10-26
Filing date: 2017-10-23
Publication date: 2018-05-03
Also published as: ES2666212A1; ES2666212B1

Abstract

The present invention relates to an axial flux switched reluctance machine, comprising: - a stator (S) comprising stator poles (p1) distributed along a first circumferential path on a stator plane; - electromagnet coils (L) wound on at least some of the stator poles (p1); and - a rotor (R) comprising rotor poles (p2) distributed along a second circumferential path on a rotor plane orthogonal to a rotation axis, parallel to said stator plane and separated therefrom by a gap along the rotation axis. At least the stator poles (p1) or the rotor poles (p2) are distributed non- equidistantly along the first and second circumferential paths, respectively. The invention also relates to an electric vehicle comprising an electric motor including the AFSRM of the invention.

Description

An axial flux switched reluctance machine and an electric vehicle comprising the machine

Technical field

The present invention generally concerns in general, in a first aspect, to an axial flux switched reluctance machine (AFSRM), and more particularly to an AFSRM with a specific distribution of stator or rotor poles which provides an improved efficiency to the machine.

A second aspect of the invention relates to an electric vehicle comprising an electric motor including the AFSRM of the first aspect of the invention.

State of the art

Nowadays, the switched reluctance machine (SRM) is the focus of increasing interest, because it is free of permanent magnets, its construction is simple and rugged, it has low manufacturing costs and high efficiency. From the point of view of electromechanical conversion, switched reluctance machine is a doubly salient pole rotational device with single excitation that usually works strongly saturated. The torque is produced by the tendency of its rotor to move to a position where the inductance of the excited phase winding is maximized, i.e. to reach the alignment of stator and rotor poles. Therefore, a power converter with solid state switches is needed to generate the right sequence of phase commutation for which it is required to known the position of the rotor. In variable speed applications the switched reluctance machine is operated in one of the three control modes: current mode, voltage mode and single pulse mode. Generally the switched reluctance machine is controlled in the low speed range by current control maintaining current within a given hysteresis band or by voltage control using PWM. At high speeds the conduction period and current waveforms adopts the natural shape according to the speed and torque requirements.

Usually the rotary switched reluctance machines are radial flux machines where the air gap flux is mainly in the radial direction relative to the axis of rotation. This type of SRM, usually have a cylindrical shape with a stator and a rotor that can be internal, the most common disposition, or outer. A less usual rotary switched reluctance machine is the axial flux SRM, in which the air gap flux is mostly parallel to the axis of rotation. The stator and rotor are parallel plates arranged perpendicular to the axis of rotation.

Recently, some studies carried out in axial flux switched reluctance motors, demonstrate that with this type of machine is possible to obtain higher torque density than in radial flux switched reluctance machines. These better features of the axial flux switched reluctance machine machine is due to the increase of volume including the air- gap area, which depend mainly on the diameter of the machine, whereas in the radial type machine the volume including the air-gap area depends mainly on the machine length.

The magnetic circuit of axial switched reluctance machines presents constructive drawbaks being difficult to built it using laminations.

Although a first axial variable reluctance motor was reported by Unnewher and Koch as early as 1973, recently, some authors have made important contributions to the development of axial flux switched reluctance machines (AFSRM). Some of said axial flux switched reluctance machines are disclosed in patent documents US5925965, US2002104909A1 , US20100295389A1 , US20140252913A1 , and also in the following papers:

- L. E. Unnewehr and W.H. Koch. "An Axial Air— Gap reluctance Motor for Variable Speed Applications." IEEE Transactions on Power Apparatus and Systems, Vol 93, issue 1 , January/February 1974

- H. Arihara and K. Akatsu. "Basic properties of an axial-type switched reluctance motor". IEEE Transactions on Industry Applications, Vol 49, No 1 , January/February 2013.

- A. Labak, N.C. Kar. "Designing and prototyping a novel five-phase pancake- shaped axial flux SRM for electric vehicle application through dynamic FEA incorporating flux-tube modeling". IEEE Transactions on Industry Applications, Vol. 49, No 3, May/June 2013.

- R. Madhavan, B.G. Fernandes. "Axial flux segmented SRM with a higher number of rotor segments for electric vehicles". IEEE Transactions on Energy Conversion, Vol. 28, No 1 , March 2013.

- T. Lambert, M. Biglarbegian, S. Mahmud. "A novel approach to the design of axial-flux switched reluctance motors". Machines 2015,3, 27-54; doi:10:10.3390/machines3010027.

- S. Murakami, H. Goto, O. Ichinokura. "A Study about Optimum Stator Pole Design of Axial-Gap Switched Reluctance Motor". 21th International Conference of Electrical Machines (ICEM), 2-5 September 2014, Berlin, Germany.

- T. Kellerer, O. Radler, T. Sattel, S. Purfurst. "Axial type switched reluctance motor of soft magnetic composite". Innovative Small Drives and Micro-Motor Systems, 19-20 September 2013, Nuremberg, Germany.

- J. Ma, R. Qu, J. Li. "Optimal design of an axial flux switched reluctance motor with grain oriented electrical steel". 18th International Conference on Electrical Machines and Systems (ICEMs), 25-28 October 2015, Pattaya City, Thailand. Arihara et al. have presented the basic design methodology for the axial counterpart of the classic rotary SRM. Murakami et al., have studied the optimization of an axial gap 18/12 SRM. Madahvan et al. have contributed to the development to the axial counterpart of rotor segmented SRM in a machine with two rotor and a stator with a toroidal type winding. Labak et al have proposed a novel multiphase pancake shaped SRM with a stator composed of a series of C-cores, each with an individual wound coil, perpendicularly disposed to a rotor made with aluminum in which a suitable number of cubes, the rotor poles, of high permeability material have been added. In this machine, the torque production is due to the tendency of any of these cubes to align with the two poles of an energized C-core. Some authors have exposed the manufacturing problems of these machines and proposed to use different materials for building its magnetic circuit as grain oriented electrical steel, Ma et al.; soft magnetic composite, Kellerer et al.; and sintered lamellar soft magnetic composite, Lambert et al.

Usually, the magnetic flux paths circulating through the poles of the the prior art AFSRM are long magnetic flux paths, as each of them generally passes through two diammetrically opposite stator poles.

However, some efforts have been made to reduce the length of the magnetic flux paths, and thus provide shorter magnetic flux paths, because the shortening of the magnetic length loops and the absence of flux reversal has as a consequence the reduction of the core losses. Thus the AFSRM proposed by A. Labak et al. has short magnetic flux paths but uses a cumbersome arrangement of stator C-cores and rotor cubes. T. Lambert et al., proposes combinations of stator and rotor that give as a result a short flux path but in the case to place a second rotor would result in two paths of magnetic flux. The arrangement of the toroidal winding, presented by R. Madhavan et al., generate two paths of magnetic flux each embracing the stator and one of the opposed rotors but returning both through the back iron.

Nevertheless, the AFSRMs disclosed in said papers are clearly improvable, both in terms of the length of the magnetic flux paths provided and also regarding the structure thereof and the distribution and arrangement of the poles of the rotor and/or the stator. Description of the invention

It is necessary to provide an alternative to the state of the art which covers the gaps found therein, providing an improved AFSRM, which allows shorter flux paths to pass through the poles thereof.

To that end, the present invention relates, in a first aspect, to an axial flux switched reluctance machine (AFSRM), comprising, in a known manner:

- a stator comprising stator poles distributed along a first circumferential path on a stator plane;

- electromagnet coils wound on at least some of said stator poles; and

- a rotor comprising rotor poles distributed along a second circumferential path on a rotor plane orthogonal to a rotation axis, parallel to said stator plane and separated therefrom by a gap along said rotation axis, such that for some rotation positions of said rotor about said rotation axis at least a portion of a rotor pole faces at least a portion of a stator pole.

Generally, said gap is an air gap, although a medium other than air can also be used to occupy said gap, such as other types of gases.

Contrary to the AFSRMs known in the state of the art, where the poles of both the stator and the rotor are distributed equidistantly along their respective circumferential paths, in the AFSRM proposed by the first aspect of the invention, in a characteristic manner, at least the stator poles or the rotor poles are distributed non-equidistantly along the above mentioned first and second circumferential paths, respectively.

Depending on the embodiment, the machine of the first aspect of the invention works as an electric motor or as an electric generator.

For a preferred embodiment, the said stator poles are distributed non- equidistantly along the first circumferential path and the rotor poles are distributed equidistantly along the second circumferential path.

According to an embodiment of the machine of the first aspect of the invention, the stator poles are spatially arranged in pairs along the first circumferential path, wherein the members of each pair of stator poles are angularly separated from each other an angle δ, ordered clockwise as first and second members, and wherein the first member of each of the pairs of stator poles is angularly separated with respect to the first member of each contiguous pair of stator poles an angle γ having a value which is greater than the value of said angle δ.

For an embodiment, the rotor poles are angularly separated from each other, along the second circumferential path, an angle a having a value which is equal or substantially equal to the result of subtracting the value of angle δ to the value of angle Y-

The machine of the first aspect of the invention comprises, for an embodiment, at least one electromagnet coil per stator pole, wherein the electromagnet coil wound on the stator pole of each pair of stator poles is electrically connected in series with the electromagnet coil wound on the adjacent stator pole of the contiguous pair of stator poles forming a phase winding.

For a basic variant of said embodiment, each phase winding is formed only by said two electrically connected electromagnet coils. That's the case when the machine includes three electrical phases and the stator has only three pairs of stator poles, each with a corresponding electromagnet coil wound thereon and interconnected as described in the above paragraph, so that three phase windings are formed, each by two respective electrically connected electromagnet coils.

For a more elaborated variant of said embodiment, at least two electromagnetic coils electrically connected in series and wound on two corresponding first adjacent stator poles of two contiguous pairs of stator poles form a phase winding for the same electrical phase than two electromagnetic coils electrically connected in series and wound on two corresponding second adjacent stator poles of two contiguous pairs of stator poles arranged in the first circumferential path diametrically opposite with respect to said first adjacent stator poles.

That's the case, for example, when the machine includes three electrical phases and the stator has six or more pairs of stator poles, each with a corresponding electromagnet coil wound thereon and interconnected according to said more elaborated variant of said embodiment, so that six or more windings are formed, each by two respective electrically connected electromagnet coils, wherein each winding is to be electrically connected to a diametrically opposite winding to form the same electrical phase.

Although for an embodiment, the machine of the first aspect of the invention comprises only one stator and only one rotor, configured as described above, for a preferred embodiment:

- the stator comprises further stator poles distributed along a third circumferential path on a further stator plane parallel and opposite to the above mentioned stator plane;

- and wherein the machine comprises a further rotor comprising rotor poles distributed along a fourth circumferential path on a further rotor plane which is parallel to said further stator plane and is separated therefrom by a gap along the rotation axis, such that for some rotation positions of said further rotor about the rotation axis at least a portion of a rotor pole of the further rotor faces at least a portion of one of said further stator poles.

Generally, said gap is also an air gap, although other medium other than air can be used to occupy said gap, such as other types of gases.

For other embodiments, the machine of the first aspect of the invention comprises more than one stator and more than two rotors, arranged like said stator and rotor and further rotor of the embodiment described in the above paragraph.

For a variant of said preferred embodiment, the further stator poles are also spatially arranged in pairs along the third circumferential path, and the machine comprises electromagnet coils wound on the further stator poles, at least one electromagnet coil per further stator pole, wherein the electromagnet coil wound on each further stator pole is electrically connected in series with the electromagnet coil wound on the respective opposite stator pole of the opposite stator plane, to form the same phase winding therewith, such that for each phase winding, when current is made circulate there through, a single magnetic flux loop is closed between two rotor poles of the rotor, two stator poles, two further stator poles and two further rotor poles of the further rotor.

Generally, the rotor and/or further rotor comprises a rotor support member from which the rotor poles or further rotor poles protrude towards the stator, wherein the rotor poles or further rotor poles are attached to or integral with said support member. Said support member is made of a ferromagnetic material which allows the above mentioned single magnetic flux to circulate there through between the corresponding two rotor poles or two further rotor poles.

For an embodiment, the stator also comprises a stator support member from which the stator poles protrude towards the rotor, or from which both the stator poles and the further stator poles protrude, from opposite faces of the stator support, substantially the same distance (although different distances are also embrace by the present invention, for less preferred embodiments), towards the rotor and towards the further rotor, respectively.

According to an embodiment, the rotor support member is an annular or circular ferromagnetic piece (or a plurality of individual straight or curved pieces interconnecting two or more rotor poles), the rotor poles and/or further rotor poles are made also of a ferromagnetic material, and the stator support member is an annular or circular piece (or a plurality of individual straight or curved pieces interconnecting two or more stator poles or further stator poles), the stator poles and/or further stator poles being made of a ferromagnetic material.

For the above mentioned embodiment for which the machine of the first aspect of the invention comprises only one stator and one rotor, the above mentioned stator support member is made of a ferromagnetic material, so that a magnetic flux path can circulate there through when going from a stator pole to the adjacent one.

In contrast, for those embodiments for which the machine of the first aspect of the invention comprises two or more rotors, and thus the magnetic flux paths don't circulate through the stator support but through the rotor supports, the above mentioned stator support member is made of a nonmagnetic material.

One of the more challenging aspects of the axial flux switched reluctance machine is the construction of its magnetic circuit, given the difficulty of making it using laminations. Therefore, for a preferred embodiment, the stator and the rotors of the machine of the first aspect of the present invention are built using sintered pieces made of SMC, Soft Magnetic Composites. SMC are iron powder particles separated with an electrically insulated layer. Basically SMC offer: unique combination of magnetic saturation and low eddy current losses, 3D-flux carrying capability and cost efficient production of 3D-net shaped component by the powder metallurgy process.

The machine of the first aspect of the invention can be used for many applications but is particular intended for an in-wheel direct traction motor.

According to an embodiment, the machine of the first aspect of the invention further comprises a shaft attached to the stator.

A second aspect of the present invention relates to an electric vehicle, comprising:

- an electrical motor including the machine of the first aspect of the invention;

- an electrical power source;

- an electronic control system fed by said electrical power source, and with output terminals connected to free terminals of said electromagnet coils to provide the latter with electric control signals to control the operation of the machine; and

- at least a wheel mechanically coupled to at least the rotor of the machine to rotate therewith under the control of said electronic control system.

Brief description of the drawings

The previous and other advantages and features will be more fully understood from the following detailed description of embodiments, with reference to the attached drawings, which must be considered in an illustrative and non-limiting manner, in which: Figure 1 is a schematic side view of the axial flux switched reluctance machine of the first aspect of the invention, for an embodiment for which the machine comprises one rotor and one stator;

Figure 2 is a front view of the stator of the AFSRM of the first aspect of the present invention, taken from the gap between the stator and the rotor, for an embodiment for which the stator comprises six poles distributed non-equidistantly along a circumference and with respective electromagnet coils electrically interconnected to form three phase windings;

Figure 3 is a front view of the rotor of the AFSRM of the first aspect of the present invention, taken from the gap between the stator and the rotor, for the same embodiment as Figure 2, for which the rotor comprises five poles distributed equidistantly along a circumference;

Figure 4 is a schematic side view of the AFSRM of the first aspect of the invention, for an embodiment for which the machine comprises two rotors and one stator with stator poles projecting towards the poles of both rotors;

Figure 5 is a schematic perspective and exploded view of the AFSRM of the first aspect of the invention, for the same embodiment as Figure 4, where one of the short magnetic flux paths passing through four stator poles and two poles of each of two rotors is depicted with a dashed arrow line;

Figure 6 schematically shows a portion of the rotors and stator of Figure 5, including the poles through which the depicted magnetic flux path passes, schematically showing the electrical connection of the coils wound on the stator poles (which has been omitted in Figure 5, for clarity sake) which constitute which has been denominated in the present document as double electromagnet though which the magnetic flux path passes;

Figure 7 is a perspective view of the AFSRM of the first aspect of the invention once mounted in a housing, for an embodiment;

Figure 8 is a schematic block diagram of a whole drive system including the AFSRM of the first aspect of the invention, an electronic power converter electrically connected to the coils thereof, a control unit and a position/speed transducer arranged to detect the position/speed of the rotation of the rotor(s) of the AFSRM;

Figure 9 is a diagram of the electronic power converter of the drive system of Figure 8, for an embodiment;

Figure 10 schematically shows a magnetic field line distribution for the resulting linear machine derived of the AFSRM of the first aspect of the invention, for the arrangement of Figure 5, calculated using 2D-FEA (Finite Element Analysis); Figure 1 1 is a diagram that shows the required wheel torque vs. speed and gradient the torque speed envelope that has to perform the motor drive system including the AFSRM of the first aspect of the invention that has been designed by the present inventors, for an embodiment;

Figure 12 shows magnetization curves of the AFSRM obtained using the 2D-FEA for the linear machine of Figure 9;

Figure 13 shows static torque curves of the AFSRM obtained using the 2D-FEA for the linear machine of Figure 9;

Figure 14 shows waveforms of phase voltage, phase current, bus current, phase torque and total torque (thin line curve of the bottom diagram) for an average torque of 122 Nm at 300 rpm, obtained from mathematical simulations with the AFSRM of the first aspect of the invention, for a designed drive system, performed with Matlab-Simulink using the results of the 2D-FEA;

Figure 15 shows the same kind of waveforms as in Figure 14, also obtained from mathematical simulations with the AFSRM of the first aspect of the invention, for the designed drive system, but for an average torque of 70 Nm at 600 rpm; and

Figure 16 is a graph showing a comparison between the expected values of torque-speed envelope with the simulated values (triangular marks) with the AFSRM of the first aspect of the invention, for the designed drive system.

Detailed description of several embodiments

Figure 1 shows a basic embodiment of the AFSRM of the first aspect of the invention, for which it comprises:

- one stator S comprising a stator support member Ms in the form of an annular ferromagnetic piece from a face of which stator poles p1 distributed along a first circumferential path on a stator plane protrude towards a rotor R;

- electromagnet coils L wound on each stator pole p1 ;

- one rotor R comprising a rotor support member Mr in the form of an annular ferromagnetic piece from a face of which rotor poles p2 protrude towards the stator S, and are distributed along a second circumferential path on a rotor plane which is orthogonal to a rotation axis, parallel to said stator plane and separated therefrom by a gap along said rotation axis, such that for some rotation positions of said rotor R about said rotation axis at least a portion of a rotor pole p2 faces at least a portion of a stator pole p1 ; and - a shaft E attached to the stator S, particularly to the stator support member Ms, and mounted to the rotor support member Mr at the central opening thereof by means of a bearing.

For the embodiment illustrated in Figure 2, the stator S comprises six poles p1 distributed non-equidistantly along the first circumferential path circumference, spatially arranged in pairs there along (the depicted dotted line oval encompasses one of said pairs), wherein the members of each pair of stator poles p1 are angularly separated from each other an angle δ, ordered clockwise as first and second members, and wherein the first member of each of said pairs of stator poles p1 is angularly separated with respect to the first member of each contiguous pair of stator poles p1 an angle γ having a value which is greater than the value of said angle δ.

The electromagnet coils L of the stator S of Figure 2 are electrically interconnected to form three phase windings W1 , W2 and W3, as shown in the Figure, i.e. the electromagnet coil L wound on the stator pole p1 of each pair of stator poles p1 is electrically connected in series with the electromagnet coil L wound on the adjacent stator pole p1 of the contiguous pair of stator poles p1 forming a phase winding. Each phase winding W1 , W2 and W3 has two free ends which constitute respective terminals through which control electrical signals will be applied when connected (connections not shown) to the electronic power converter (see Figure 8).

Figure 3 is a front view of the rotor of the AFSRM of the first aspect of the present invention, taken from the gap between the stator and the rotor, for the same embodiment as Figure 2, for which the rotor comprises five poles p2 distributed equidistantly along the second circumferential path, and angularly separated from each other an angle a having a value which is equal or substantially equal to the result of subtracting the value of said angle δ to the value of said angle γ.

A more elaborated embodiment of the AFSRM of the first aspect of the invention is shown in Figures 4 and 5, wherein:

- the stator S comprises further stator poles p3 distributed along a third circumferential path on a further stator plane parallel and opposite to said stator plane;

- and wherein the machine comprises a further rotor Rf comprising a further rotor support member Mrf also in the form of an annular ferromagnetic piece from a face of which further rotor poles p4 protrude towards the stator S, and are distributed along a fourth circumferential path on a further rotor plane which is parallel to said further stator plane and is separated therefrom by a gap along the rotation axis, such that for some rotation positions of the further rotor Rf about the rotation axis at least a portion of a further rotor pole p4 of the further rotor Rf faces at least a portion of one of said further stator poles p3.

For the embodiment of Figure 5, the shaft E traverses the further rotor Rf (particularly the further rotor support member Mrf), is attached to the stator S (to the support member Ms that is made of nonmagnetic material) and is mounted to both rotor support members Mr, Mrf at the central opening thereof by means of bearings.

Regarding the above mentioned angles δ, γ and a, they are also valid for the pole distribution of the embodiment of Figure 5.

Said angles and also other parameters of the AFSRM of the first aspect of the invention, which must be met for preferred embodiments, are further defined below.

For said preferred embodiments, the number of further stator poles p3 is equal to the number of stator poles p1 and together, with the electromagnetic coils L wound thereon, form Z double electromagnets, wherein the total number Ns of poles of the stator S, including the stator poles p1 and the further stator poles p3, is given according to the number of electrical phases of the machine, m, by the following relationships:

Z = k m

N_s = 2 Z = 2 k m

wherein k\s an integer denominated multiplicity.

For said preferred embodiments, the number of rotor poles p2 is equal to the number of further rotor poles p4 and equal to Ν_Ά defined by the formula:

N_R = k(2m - 1)

wherein:

and wherein:

360°·(Λ^ - {k - m))

δ = γ- = - k - m - N _R

The above relationships are also valid for the embodiment of Figure 2, with k =1 , being in this case the number of single electromagnets, Ns the total number of stator poles p1 , and N_R the number of rotor poles p2.

The preceding expressions relating to the angles for machines with 3 and 4 phases are shown in the table below: k m Z Ns N_R o(°) Y(°) δ(°)

1 3 3 6 5 72,00 120,00 48,00

2 3 6 12 10 36,00 60,00 24,00

3 3 9 18 15 24,00 40,00 16,00

4 3 12 24 20 18,00 30,00 12,00

1 4 4 8 7 51,43 90,00 38,57

2 4 8 16 14 25,71 45,00 19,29

3 4 12 24 21 17,14 30,00 12,86

4 4 16 32 28 12,86 22,50 9,64

In Figures 5 and 6 one of the short magnetic flux paths passing through four stator poles p1 and two poles p2, p4 of each of two rotors R, Rf is depicted, particularly when current is made circulate through one of the windings W1 , i.e. through the coils L of a double electromagnet. It can be seen that the flux path circulates also through the rotor support members Mr and Mrf.

The flux lines link the stator poles p1 , p3 of both sides of the stator S with the poles p2, p4 of the two rotors R, Rf forcing the alignment of these poles.

Figure 6 is a schematic view of a double electromagnet showing the connection of theirs coils L in the aligned position. In the case of k >1 the phases windings are obtained by connecting in a proper way the Z different double electromagnets of each phase.

Although not shown, for the embodiment of Figure 5, two electromagnetic coils L electrically connected in series and wound on two corresponding first adjacent stator poles p1 of two contiguous pairs of stator poles p1 form a phase winding W1 , W2, W3 for the same electrical phase than two electromagnetic coils L electrically connected in series and wound on two corresponding second adjacent stator poles p1 of two contiguous pairs of stator poles p1 arranged in the first circumferential path diametrically opposite with respect to said first adjacent stator poles p1 . In other words, the phase winding W1 shown in Figure 6 is electrically connected in series to a phase winding located diametrically opposite with respect thereto. The same will occur for the rest of phase windings W2, W3, thus constituting an AFSRM for three electrical phases, where each electrical phase will be connected to an electrical circuit formed by eight coils connected in series.

A possible final implementation of the AFSRM of the first aspect of the invention once mounted in a housing H, is shown in Figure 7. Preferably, the shaft E is a hollow shaft through which at least the electrical wires (not shown) connecting the phase windings W1 , W2, W3 with the electronic power converter pass.

Although the stator poles p1 , p3 and rotor poles p2, p4 shown in the Figures have a triangular cross section, other cross section shapes forms are embraced by the present invention, for other embodiments (not shown), such as round, square, rectangular or trapezoidal

In order to obtain a continuous torque, the different phases of the machine should be activated in a proper way. For this, as stated above, the AFSRM must be fed through an electronic power converter controlled by a switching sequence generator (control) based on the relative position between stator and rotor obtained by a position/speed transducer. Figure 8 shows a schematic block diagram of the whole drive system, including an electronic power converter to be electrically connected to the coils L, a control unit and a position/speed transducer arranged to detect the position/speed of the rotation of the rotor(s) of the AFSRM.

Figure 9 shows a schema of the electronic power converter for the case of a three phase machine. The electronic power converter has as many branches as phases, each branch is formed by two switches, in the case of Figure 9 IGBTs, and two diodes disposed as it is shown in Figure 9. When a phase is activated the two switches turn on and current flows into the machine phase from the power source being the phase voltage equal to the voltage of the power source. When the two switches turn off current continues flowing into the phase but through the diodes, being the voltage of opposite polarity of the voltage of the power source thereby enabling the demagnetization of the phase. Then the control should generate proper signals for the suitable activation of the switches according to the relative position between stator and rotor poles, determined by a position/speed transducer, and the requirements of the load.

The control generates proper signals for the suitable activation of the switches according to the relative position between stator and rotor poles, determined by a position/speed transducer, and the requirements of the load. In the low speed range the axial flux switched reluctance machine is controlled by current control maintaining current within a given hysteresis band or by voltage control using PWM, in both cases during the conduction period either of both of the switches may be chopped according to the control strategy. At high speeds both switches remain turned on along the conduction period and current waveforms adopts the natural shape according to the speed and torque requirements. When specific controls are needed, for instance if torque ripple should be minimized, then the turn on and turn off angles are carefully selected according to the control mode depending on the speed and torque required by the load.

A drive system including a motor implementing the AFSRM of the first aspect of the invention has been designed by the present inventors as will be described below.

MOTOR DRIVE SYSTEM REQUIREMENTS:

An e-scooter motor should be designed to provide the torque and speed requirements suitable with the size and driving conditions of the scooter that has to propel. In addition, motor performances should allow a reasonable autonomy for each battery charge. The determination of the torque-speed envelope is the first step in the design process of a traction motor. The motor must provide sufficient torque to overcome the rolling resistance, the aerodynamic drag and the scooter weight when climbing a gradient. In addition has to provide enough torque for acceleration. Therefore, the dynamic e uation of motion of the scooter a vehicle is iven by:

Where:

T, is the torque at the wheel (Nm)

m, is the gross (total) mass of the scooter (kg)

¾ is the angle of incline

R, is the radius of the tire (m)

μ_η is the rolling resistance coefficient

g, is the acceleration of gravity (m/s²)

p, is the air density (kg/m³)

A, is the frontal area (m²)

v, is the velocity of the scooter (m/s)

v₀. is the velocity of the wind (m/s)

C_D, is the aerodynamic drag coefficient

k_m, is the inertia coefficient

The angle of incline and the gradient in percentage (p) are related by: β = arctan— (²)

100

The present inventors have designed a direct motor drive system for the motorization of an e-scooter with the parameters listed in Table I. The requirements that have to verify the motor are summarized in Table II. -scooter:

TABLE II. Main requirements of the motor:

Requirements Values

Speed (max) 80 km/h (~ 900 rpm)

Voltage 48/72 V

Torque at 45 kiii/h 75 Nm

Gradient at 20 km h and

> 35 % ; 170 Nm

corresponding torque

Power 4 kW (20 < v < 80 km)

Efficiency > 88 % over a wide speed range Therefore, according with the values shown in the above tables and developing equations (1-2), the torque speed envelope that has to perform the motor drive system is shown in Fig. 1 1 . DESCRIPTION OF THE DESIGNED AFSRM:

As stated above, in order to verify that the proposed axial-flux switched reluctance motor meets the requirements of the Table II, a motor has been designed with the goal of reaching power densities similar to those of the permanent magnet synchronous motors with outer rotor currently used for the propulsion of e-scooters (250 W/kg). The exterior dimensions of the motor are limited to a diameter of 308 mm and an axial length of 1 16 mm to fit within a wheel of 13". The designed axial-flux switched reluctance motor has been designed according to the embodiment shown in Figure 5, i.e. with a stator S having twelve poles p1 , p2 per side, and two rotors R, Rf each having ten poles, and with the configuration given in Table III below.

TABLE III. Configurations of the motor designed:

As it is well known, performances of switched reluctance machines are very sensitive to the air-gap length, therefore the air-gap has been limited at 0.5 mm and constructive measures have been taken to assure this value and to avoid uneven airgaps on both sides of the stator.

ELECTROMAGNETIC ANALYSIS OF THE DESIGNED AFSRM:

The study of axial-flux machines involves a three-dimensional electromagnetic problem. So, the most accurate solution for modelling the machine is with three- dimensional finite element method, 3D-FEM. Nevertheless, this method is highly time consuming, and both the definition of the problem and the solving process are quite cumbersome.

A proved alternative is to perform simulations using 2D-FEM taking 2D planes of the machine geometry in several radiuses. That means to transform the axial-flux machine in a linear machine. Fig. 10 shows the field line distribution of the resulting linear machine for the average stator radius. The magnetization curves and the static torque curves obtained, using this methodology, are shown in Fig. 12 and Fig.13, respectively.

SIMULATION OF THE DESIGNED AFSRM DRIVE SYSTEM:

The drive system to be simulated includes the elements shown in Figure 8, i.e. the AFSRM, the power converter, an asymmetric converter (classic converter) with two switches and two diodes per phase (as shown in Figure 9), a control unit and a position/speed sensor. The control due to the limited speed range (0 to 900 rpm) is a hysteresis control with variable turn-on ( 9₀/ ) and turn-off angles and is implemented in Matlab-Simulink using the results of the finite element analysis of the axial-flux SRM. The waveforms of phase voltage, phase current, bus current and total torque are shown in Fig. 14 for an average torque of 122 Nm at 300 rpm with #_0/ν = -5° and 17°, and in Fig. 15 for an average torque of 70 Nm at 600 rpm with #_0/ν = - 2° and 14°. In Fig. 16, the expected torque-speed envelope of the e-scooter is compared with the results obtained from simulation.

The results of the above simulations show that the performances of the designed drive system match pretty well the requirements of the e-scooter. Nevertheless, due to the remarkable torque ripple, it would be advisable to change to direct torque control strategies.

A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims.

Claims

1. - An axial flux switched reluctance machine, comprising:

- a stator (S) comprising stator poles (p1 ) distributed along a first circumferential path on a stator plane;

- electromagnet coils (L) wound on at least some of said stator poles (p1 ); and

- a rotor (R) comprising rotor poles (p2) distributed along a second circumferential path on a rotor plane orthogonal to a rotation axis, parallel to said stator plane and separated therefrom by a gap along said rotation axis, such that for some rotation positions of said rotor (R) about said rotation axis at least a portion of a rotor pole (p2) faces at least a portion of a stator pole (p1 );

characterised in that at least said stator poles (p1 ) or said rotor poles (p2) are distributed non-equidistantly along said first and second circumferential paths, respectively.

2. - A machine according to claim 1 , wherein said stator poles (p1 ) are distributed non-equidistantly along said first circumferential path and said rotor poles (p2) are distributed equidistantly along said second circumferential path.

3. - A machine according to claim 1 or 2, wherein the stator poles (p1 ) are spatially arranged in pairs along the first circumferential path, wherein the members of each pair of stator poles (p1 ) are angularly separated from each other an angle δ, ordered clockwise as first and second members, and wherein the first member of each of said pairs of stator poles (p1 ) is angularly separated with respect to the first member of each contiguous pair of stator poles (p1 ) an angle γ having a value which is greater than the value of said angle δ.

4. - A machine according to claim 3, wherein the rotor poles (p2) are angularly separated from each other, along said second circumferential path, an angle a having a value which is equal or substantially equal to the result of subtracting the value of said angle δ to the value of said angle γ.

5. - A machine according to claim 3 or 4, comprising at least one electromagnet coil (L) per stator pole (p1 ), wherein the electromagnet coil (L) wound on the stator pole (p1 ) of each pair of stator poles (p1 ) is electrically connected in series with the electromagnet coil (L) wound on the adjacent stator pole (p1 ) of the contiguous pair of stator poles (p1 ) forming a phase winding (W1 , W2, W3).

6. - A machine according to claim 5, wherein two electromagnetic coils (L) electrically connected in series and wound on two corresponding first adjacent stator poles (p1 ) of two contiguous pairs of stator poles (p1 ) form a phase winding (W1 ,W2,W3) for the same electrical phase than two electromagnetic coils (L) electrically connected in series and wound on two corresponding second adjacent stator poles (p1 ) of two contiguous pairs of stator poles (p1 ) arranged in the first circumferential path diametrically opposite with respect to said first adjacent stator poles (p1 ).

7. - A machine according to any of the previous claims, wherein

- said stator (S) comprises further stator poles (p3) distributed along a third circumferential path on a further stator plane parallel and opposite to said stator plane;

- and wherein the machine comprises a further rotor (Rf) comprising further rotor poles (p4) distributed along a fourth circumferential path on a further rotor plane which is parallel to said further stator plane and is separated therefrom by a gap along the rotation axis, such that for some rotation positions of said further rotor (Rf) about the rotation axis at least a portion of a further rotor pole (p4) of the further rotor (Rf) faces at least a portion of one of said further stator poles (p3).

8. - A machine according to claim 7 when depending on claim 5 or 6, wherein the further stator poles (p3) are also spatially arranged in pairs along the third circumferential path, and wherein the machine comprises electromagnet coils (L) wound on the further stator poles (p3), at least one electromagnet coil (L) per further stator pole (p3), wherein the electromagnet coil (L) wound on each further stator pole (p3) is electrically connected in series with the electromagnet coil (L) wound on the respective opposite stator pole (p1 ) of the opposite stator plane, to form the same phase winding (W1 , W2, W3) therewith, such that for each phase winding (W1 , W2, W3), when current is made circulate there through, a single magnetic flux loop is closed between two rotor poles (p2) of the rotor (R), two stator poles (p1 ), two further stator poles (p3) and two further rotor poles (p4) of the further rotor (Rf).

9. - A machine according to claim 8, wherein the number of further stator poles (p3) is equal to the number of stator poles (p1 ) and together, with the electromagnetic coils (L) wound thereon, form double electromagnets, wherein the total number N_s of poles of the stator (S), including the stator poles (p1 ) and the further stator poles (p3), is given according to the number of electrical phases of the machine, m, by the following relationships:

Z = k m

N_s = 2 Z = 2 k m

wherein k\s an integer denominated multiplicity,

wherein the number of rotor poles (p2) is equal to the number of further rotor poles (p4) and equal to Ν_Ά defined by the formula:

N_R = k(2m - 1)

wherein: 360' and wherein:

360°{N_R -(k - m))

δ = γ- = — —

k^■ m^■ N_R

10. - A machine according to any of the previous claims, wherein said rotor (R) and/or further rotor (Rf) comprises a rotor support member (Mr, Mrf) from which the rotor poles (p2) or further rotor poles (p4) protrude towards the stator (S), wherein the rotor poles (p2) or further rotor poles (p4) are attached to or integral with said support member (Mr, Mrf).

1 1 . - A machine according to any of the previous claims, wherein said stator (S) comprises a stator support member (Ms) from which the stator poles (p1 ) protrude towards the rotor (R), or from which both the stator poles (p1 ) and the further stator poles (p3) protrude, from opposite faces of the stator support (Ms), substantially the same distance, towards the rotor (R) and towards the further rotor (Rf), respectively.

12. - A machine according to claim 1 1 when depending on claim 10, wherein said rotor support member (Mr, Mrf) is an annular or circular ferromagnetic piece, the rotor poles (p2) and/or further rotor poles (p4) being made also of a ferromagnetic material, and wherein said stator support member (Ms) is an annular or circular nonmagnetic piece, the stator poles (p1 ) and/or further stator poles (p3) being made of a ferromagnetic material.

13.- A machine according to any of claims 1 to 6, wherein said rotor (R) comprises a rotor support member (Mr) from which the rotor poles (p2) protrude towards the stator (S), wherein the rotor poles (p2) are attached to or integral with said support member (Mr).

14. - A machine according to any of claims 1 to 6, or according to claim 13, wherein said stator (S) comprises a stator support member (Ms) from which the stator poles (p1 ) protrude towards the rotor (R).

15. - A machine according to claim 14, comprising only one stator (S) and one rotor (R), wherein said rotor support member (Mr) is an annular or circular ferromagnetic piece, the rotor poles (p2) being made also of a ferromagnetic material, and wherein said stator support member (Ms) is an annular or circular ferromagnetic piece, the stator poles (p1 ) being made of a ferromagnetic material.

16. - A machine according to any of the previous claims, wherein at least said rotor poles (p2, p4) and stator poles (p1 , p3) are made of sintered pieces made of soft magnetic composites.

17.- An electric vehicle, comprising:

- an electrical motor including the machine of any of the previous claims;

- an electrical power source;