Variable air-gap electric motor

Abstract

An axial flux electric motor is described comprising: • a rotor which is rotatable about a rotation axis and provided with magnetic members, • a stator with windings for creating a magnetic field which has a polar axis parallel to the rotation axis and can interact with the magnetic members of the rotor to make the latter rotate, • an output shaft integral with the rotor to communicate torque outside the motor, wherein the rotor is mounted translatable along the rotation axis so as to be translated relative to the stator. Said rotor described in this invention, which allows varying the width of an air gap at any time between the rotor and the stator, allows a reduction in losses and an expansion of the speed range compared to a conventional motor.

Description

[0001] VARIABLE AIR-GAP ELECTRIC MOTOR

[0002] The present invention relates to an axial-flow electric motor applicable to pedal- assisted electric bicycles, or electric vehicles in general (e.g. a car, truck, train, or ship) aboard which the motor is installed to propel the vehicle.

[0003] Normally an electric motor is designed according to parameters that take into account its specific use. These parameters are many, the most important of which are: maximum power up to a certain rpm and maximum torque.

[0004] For example, when pedaling without the assistance of the electric motor on an electric bicycle, the muscle force must move the bicycle and also drag the motor rotor. In this case, the motor functions as a dynamo, and is subject to the magnetic dissipative phenomenon (eddy currents) resulting from the interaction between the magnetic parts of the rotor and the stator. The dissipated power due to eddy currents is greater the faster you pedal.

[0005] Laws for E-Bike certification require that the motor must not assist pedaling beyond a certain speed of the bicycle, a condition that certainly triggers undesirable parasitic resistance to pedaling by human force alone.

[0006] In addition, normally, if the electric motor is operated at a speed for which a higher induced voltage is generated than the supply voltage, the motor needs a high deflussing current for its operation, in order to still generate useful power, with detriment of the vehicle's autonomy.

[0007] The object of the invention is to solve or at least mitigate the above problems, and is achieved by what is stated in the appended claims; advantageous technical features are defined in the dependent claims.

[0008] An axial-flow electric motor is proposed comprising: a rotor that is rotatable about a rotation axis and provided with magnetic members (e.g. arranged in a circular array), a stator with windings (e.g. arranged in a circular series) - and preferably associated ferromagnetic cores - to create a magnetic field that has a polar axis parallel to the rotation axis and can interact with the magnetic members of the rotor to bring the latter into rotation, an output shaft integral with the rotor to communicate torque outside the motor, wherein the rotor is mounted translatable along the rotation axis so that it can be translated relative to the stator and thereby vary the width of the air gap between the windings (and ferromagnetic cores) and the magnetic members. By allowing the air gap between the rotor and stator to be varied at any time, the motor achieves a reduction in losses and an expansion of the speed range in which the motor can work compared to a conventional motor. The aforementioned electric motor can vary its operating range as needed, as if it were a mechanical gearbox, without additional waste of electrical energy.

[0009] Moving the rotor away from the stator has the advantageous effect of reducing the concatenated magnetic flux, particularly at high speed, thereby reducing both losses in the stator iron and load losses on the output shaft (and e.g. on the pedals) due to the dissipative magnetic effect described above.

[0010] Advantageously, it is preferred that the motor forms a complete module and also comprises means for translating the rotor relative to the stator along the rotation axis thereby varying the width of said air gap.

[0011] In a preferred variant, the output shaft comprises a surface toothing that is meshed on a complementary internal toothed crown provided on the rotor. Thus the rotor can slide linearly on the output shaft transferring mechanical power to it.

[0012] In a preferred variant for motor efficiency and compactness, to translate the rotor relative to the stator there is provided an actuator member (e.g. a disc) that is mounted axially between the stator and the rotor (i.e. the stator and rotor respectively extend mainly along two parallel planes and the actuator member extends mainly along a plane parallel to, and placed between, the said two planes), is translatable along the axis, and has relative distance along the axis which is adjustable with respect to the stator.

[0013] For compactness of the motor, preferably the actuator member is coupled to the rotor by a bearing coaxial to said axis.

[0014] In a compact preferred variant, the actuator member comprises a first thread and the stator comprises a second thread complementary to - and engaged on - the first thread, the motor comprising means for rotating the actuator member relative to the stator to advance the first thread on the second thread thereby varying the relative axial position between the stator and the actuator disc (and thus the relative axial position between the stator and the rotor). Specifically, the first thread and the second thread are respectively arranged on two circumferences lying in a plane orthogonal to the rotation axis and centered on such axis. The threads provide stability and positioning accuracy.

[0015] In a preferred variant, the means for rotating comprise a rotary motor or a linear, e.g. electric or pneumatic, motor. In particular, the means for rotating and the actuator member are coupled to each other by means of a gear; e.g. the means for rotating comprise a pinion gear meshed on a toothed crown of the actuator member, or vice versa.

[0016] In a preferred variant, the motor comprises an elastic member, e.g. a coil spring, configured to exert a thrust against the actuator member to move it away from the stator. In particular, the elastic member is mounted in or on the stator. The elastic member facilitates the action of the means for rotating and allows the means for rotating to be depowered, making them smaller and easily to be installed in the rotor.

[0017] In a preferred variant, the motor comprises two equal rotors, mounted on opposite sides of the stator.

[0018] In a preferred variation, said output shaft is a shaft that connects two pedals of an electric bicycle. In a more preferred variant, said output shaft is permanently connected to a wheel of the bicycle (so that if the wheel turns, so does the motor). E.g. said output shaft comprises a sprocket on which a chain is engaged that transfers mechanical power to the wheel. Thus one can fit the motor into a two-chain bicycle structure, see e.g. W02020230038.

[0019] In preferred variants: the shaft passes coaxially through the stator, by which it is supported via bearings; and / or the shaft at one end thereof mounts a gear for the connection with a bicycle chain; and e.g. said gear is mounted downstream of a planetary gear driven by the rotor and coaxial to the shaft.

[0020] The motor preferably comprises an electronic unit to control the means for moving in order to adjust the distance between the driving member and the stator, thus adjusting the magnetic air gap between the windings and the magnetic members.

[0021] To facilitate the positional control of the actuator member, preferably the motor comprises a linear position sensor capable of sensing the stroke or position along said axis of the actuator member. Preferably the sensor is connected to said electronic unit, particularly to feedback control the means for translating.

[0022] Another aspect of the invention concerns a method, which shares the variants described for the motor, for controlling the operation of an aforementioned axial-flow electric motor, wherein the rotor is translated along the rotation axis to vary the width of the aforementioned air gap.

[0023] In a preferred variant, the rotor is translated relative to the stator by means of an actuator disc or member mounted axially between the stator and rotor.

[0024] In a compact preferred variant, the actuator member is screwed into the stator to advance the actuator member along said axis.

[0025] In a preferred variant, the actuator member is electronically controlled to adjust its distance from the stator.

[0026] In a preferred variant, the stroke or position of the actuator member along said axis is detected via a sensor, and more preferably the position of the actuator member is controlled through a feedback loop.

[0027] Further advantages will become clear from the following description, which refers to an example of a preferred embodiment of a motor in which:

[0028] - Figure 1 shows an exploded view of an electric motor;

[0029] - Figure 2 shows a cross-sectional view of the motor in Fig. 1 ;

[0030] Figures 3 and 4 show enlarged views of components of Fig. 1 .

[0031] Equal numbers in the figures indicate equal or substantially equal parts. To avoid crowding the drawings, equal members are not all numbered.

[0032] Fig. 1 illustrates an electric motor MC comprising a stator 30, an output shaft 10, and a rotor 20. The shaft 10 is rotationally integral with the rotor 20, both rotatable about an X axis. The shaft 10 passes coaxially through the stator 30, by which it is supported through bearings, and mounts at one end thereof a gear 88 for connection with, for example, a bicycle chain. Alternatively, the gear 88 is mounted downstream of a planetary gear driven by the rotor 20 and coaxial to the shaft 10.

[0033] The motor MC may also have only one rotor 20, but preferably, to drive the shaft 10, it has two equal rotors 20 each mounted on an opposite side of the stator 30, which on such opposite sides has a mirror-like structure.

[0034] For simplicity we will limit the description to one side of the stator 30, the other side functioning in the same way.

[0035] The stator 30 comprises a cylindrical casing 32 in which, around the X axis, a circular array of known electrical windings 34 is installed, containing a ferromagnetic core, which in use generate magnetic fields with a polar axis parallel to the X axis to bring the rotor 20 into rotation. The rotor 20 carries a circular array of magnets 22 interacting with the windings 34. An optional bell 90 encloses the rotor 20 in the casing 32.

[0036] The output shaft 10 has a surface toothing 14 on which an inner gear 24 of the rotor 20 meshes, so that the rotor 20 can transfer driving torque to the shaft 10 while being able to translate on the shaft 10 along the X axis. The rotor 20 abuts against an actuator disc 50 mounted coaxially between the stator 30 and the rotor 20. A bearing 98 between the actuator disc 50 and the rotor 20 rotationally decouples them about the X axis.

[0037] The actuator disc 50 acts to move the rotor 20 along the X axis, and is in turn translatable along the X axis relative to the stator 30.

[0038] The means for translating the actuator disc 50 may vary, and a preferred embodiment thereof is shown in the figures.

[0039] Said means for translating preferably comprise a first thread 52 obtained on the actuator disc 50, a second thread 38 that is made on the stator 30 and is complementary to - and engaged on - the first thread 52, and means for rotating the actuator disc 50 relative to the stator 30 to advance the first thread 52 on the second thread 38.

[0040] The threads 52, 38 are arranged respectively on two circumferences lying on a plane orthogonal to the rotation axis X and with center on such X axis.

[0041] The relative screwing between the threads 52, 38 results in the variation of the relative axial position between the stator 30 and the actuator disc 50, which can move the rotor 20 away from the stator 20. Thus the actuator disc 50 can vary the relative axial position between the stator 30 and the rotor 20, and consequently the size of the air gap between the magnets 22 and the windings 34.

[0042] The means for rotating may also have various embodiments. In the example shown, they comprise a rotary motor 80 that can drive a pinion 82 into rotation. For example, the motor 80 is mounted on the stator 30 via a bracket 96. The pinion 82 meshes with a sprocket 56 of the actuator disc 50, so that the rotation of the pinion 82 brings the actuator disc 50 into rotation.

[0043] By properly controlling the motor 80, the distance between the actuator disc 50 and the stator 30 can be adjusted, thereby adjusting the magnetic air gap between the circular array of windings 34 and the circular array of magnets 22.

[0044] To facilitate positional control of the actuator disc 50, preferably the motor MC comprises a linear position sensor 94 capable of detecting the stroke or position along the X-axis of the actuator disc 50. By means of the sensor 94, the motor 80 can be feedback controlled thereby increasing the accuracy of the adjustment of said air gap. The sensor 94 is e.g. a potentiometer, e.g. a linear potentiometer, connected to the actuator disc 50; or a Hall-effect or inductive sensor.

[0045] Preferably, the sensor 94 is connected to an electronic unit 100 adapted to control the motor 80, e.g. with a positional feedback loop.

[0046] In operation, the rotor 20 is attracted by the stator 30 by magnetic force, which can counteract the action of the motor 80. In a preferred variant then the motor MC comprises an elastic member 92, e.g. a coil spring, mounted in the stator 30 and configured to exert a thrust against the actuator disc 50 in order to move it away from the stator 30. Such thrust opposes the magnetic attraction between the rotor 20 and the stator 30, and decreases the load on the motor 80.

Claims

CLAIMS1 . Axial flux electric motor comprising:• a rotor which is rotatable about a rotation axis and provided with magnetic members,• a stator with windings for creating a magnetic field which has a polar axis parallel to the rotation axis and can interact with the magnetic members of the rotor to make the latter rotate,• an output shaft integral with the rotor to communicate torque outside the motor, wherein the rotor is mounted translatable along the rotation axis so as to be translated relative to the stator and thereby vary the width of an air gap between the windings and the magnetic members.

2. Motor according to claim 1 , comprising means for translating the rotor relative to the stator along the rotation axis, thus varying the width of said air gap.

3. Motor according to claim 2, wherein said means for translating comprises an actuator member which is mounted axially between the stator and the rotor, is translatable along said axis, and has an adjustable relative distance along the axis with respect to the stator.

4. Motor according to claim 3, wherein the actuator member comprises a first thread and the stator comprises a second thread complementary to - and engaged on - the first thread, the motor comprising means for rotating the actuator member relative to the stator to advance the first thread on the second thread thereby changing the relative axial position between the stator and the actuator member.

5. Motor according to claim 4, wherein the first thread and the second thread are arranged respectively on two circumferences lying on a plane orthogonal to the rotation axis and having center on such axis.

6. Motor according to claim 4 or 5, wherein the means for rotating comprise a rotary motor or a linear motor.

7. Motor according to claim 6, wherein the means for rotating comprise a pinion gear meshed with a toothed crown of the actuator member, or vice versa.

8. Motor according to any preceding claim 3 to 7, comprising an elastic member configured to exert a thrust against the actuator member to move it away from the stator.

9. Motor according to claim 8, wherein the elastic member is mounted in or on the stator.

10. Motor according to any preceding claim, wherein said output shaft is a shaft joining two pedals of an electric bicycle.5

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