FIELD OF THE INVENTION
5 The present invention relates to a method of controlling an axial flux machine, a
controller for an axial flux machine, and an axial flux machine. In particular, to a
method, controller and machine having a reduced mechanical resonance.
10
BACKGROUND OF THE INVENTION
All structures have natural frequencies of vibration at which exciting forces cause
amplification. This amplification occurs when the excitation frequency coincides with
the natural frequency in a condition called resonance.
15 Rotating machines are particularly prone to resonance induced mechanical vibration
which occurs when a natural frequency is at or close to an exciting frequency, such as
rotor speed. For machinery-such as pumps, turbines, electric motors and generators
-resonance can amplify small vibratory forces from machine operation, and severe
sometimes destructive vibration levels can result.
20
With variable-speed drives, exciting frequencies change with motor speed and result in
resonance each time exciting harmonic frequencies cross a natural resonance
frequency. Strength of resonance varies with excitation amplitude and proximity to the
natural frequency, as well as with, damping, mass, and stiffness of subject machine
25 components associated with the natural frequency.
Natural resonance frequencies of rotating machines are a consideration in a machine's
design and may be adjusted i.e. minimised and perhaps shifted in frequency according
to material properties and design of key components, typically stators, rotors, and
30 housings.
Nevertheless despite efforts to shift and minimise resonance through materials choice
and mechanical design, its consequences of noise, vibration and harshness (NVH) are
often a challenge to overcome in finished machines.
wo 2021/239849 PCT /EP2021/064120
2
Once a machine's design is committed and thereby fundamental resonances fixed,
there remains opportunity to further reduce impact of resonance by modifying excitation
frequencies and strengths.
5 For variable speed axial flux machines it is usually rotors that present greatest risk of
troublesome resonances because of their planar, disc-like shape which present several
common modes of vibration.
It is the fundamental mode (0, 1 ), tO, f01 and sometimes called the zeroth mode that is
10 of prime interest in the present invention because this mode is easily excited. Though
the invention may equally be applied to higher order natural resonant modes, the
following descriptions and focus will be on affecting the fundamental mode of
resonance. It will be understood these same techniques may be applied to higher order
modes.
15
20
The fundamental mode of resonance and vibration has additional importance to axial
flux machines because such machines are often power, and torque optimised by using
stator to rotor airgaps of approximately 1 mm. Even at lower amplitudes noise from
resonance induced vibration may become intrusive.
The principle of resonant vibration I noise suppression via application of countering
currents is well known and has been applied to permanent magnet machines to reduce
vibration and noise produced by interaction of permanent magnet rotors on opposing
stator poles wherein rotor positions alternate between high and low energy states.
25 Some examples include AU2015396604, which relates to flux-weakening to reduce
cogging forces and thereby reduce excitation of resonance. US2005/0231143 teaches
vibration reduction in stators of radial machines. US2008315818 teaches applying pairs
of harmonic currents whose order differ by two to the sinusoidal fundamental wave.
US2015108938 teaches applying a compensation current to counteract oscillations
30 caused by asymmetries in the machine.
35
We have appreciated the need for improved suppression of planar resonances in axial
flux motor rotors.
wo 2021/239849 PCT /EP2021/064120
3
SUMMARY OF THE INVENTION
The present invention therefore provides a method of controlling an axial flux machine,
a controller for an axial flux machine, and an axial flux machine in accordance with the
5 independent claims appended hereto. Further advantageous embodiments are also
provided by the dependent claims, also appended hereto.
In particular, we will describe a method of controlling an axial flux machine, the axial
flux machine comprising a stator comprising a stator housing enclosing a plurality of
1 0 stator pole pieces disposed circumferentially at intervals around an axis of the
machine, each of the stator pole pieces having a set of coils wound therearound for
generating a magnetic field; and a rotor comprising a set of permanent magnets and
mounted for rotation about the axis of the machine, the rotor being spaced apart from
the stator along the axis of the machine to define a gap between the stator and rotor
15 and in which magnetic flux in the machine is generally in an axial direction, the method
comprising: controlling an alternating current supplied to the plurality of coils to inject a
compensation current for reducing a mechanical resonant component of the rotor, the
compensation current being injected when the rotor is rotating over one or more ranges
of rotational speeds, each of the one or more ranges of rotational speeds of the rotor
20 including a respective determined rotational speed of the rotor, wherein the alternating
current through each coil is represented as vectored direct current components
comprising a Direct current (ld) component and a Quadrature current (lq) component
that are orthogonal to one another, and wherein the compensation current comprises a
modulated current component added to at least one of the Quadrature Current (lq) and
25 the Direct Current (ld) components, the modulated current component having an
electrical frequency that varies over a range of frequencies between a first frequency
and a second frequency depending on the rotational speed of the rotor, the range of
frequencies including a frequency that is substantially the same as a fundamental
mechanical resonant frequency of the rotor; and a phase that is out of phase with the
30 fundamental mechanical resonant frequency of the rotor.
By applying the above method, a reduction in the resonance in the rotor is observed.
The one or more respective determined rotational speeds of the rotor may be
35 dependent on one or more respective mechanical resonant excitation orders of the
wo 2021/239849 PCT /EP2021/064120
4
rotor, for example one or more of the 121h, 181h, 361h and ?2nd excitation orders. These
orders are of interest for certain topologies of machine, for example an 18/12 topology.
Other topologies may have other excitation orders that are of interest, for example for a
12/8 machine, the excitation orders may be 81h, 121h, 241h and 481h orders. For a 24/16
5 topology the 161h , 241h, 481h and 961h orders may be interest.
10
The one or more respective determined rotational speeds of the rotor may be defined
by the relationship:
rotor fundamental resonant frequency
determined rotational speed = 60 * - - d -
- - excitation_or er
Each of the ranges of rotational speed of the rotor may be based on a percentage
change of the rotor fundamental mechanical resonant frequency for a given mechanical
resonant excitation order of the rotor. The percentage change of the rotor fundamental
mechanical resonant frequency may be ±1 %, ±5%, ±1 0%, ±15% or ±20% of the rotor
15 fundamental mechanical resonant frequency.
In each of the one or more ranges of rotational speeds of the rotor, an amplitude of the
compensation current may be ramped between a lower amplitude and a peak
amplitude over at least a portion of the range of rotational speeds of the rotor, and
20 wherein the peak amplitude of the compensation current substantially coincides with
the respective determined rotational speed of the rotor.
The method may also comprise: receiving vibration data from a vibration sensor, the
vibration sensor detecting mechanical vibrations in the rotor; identifying a mechanical
25 resonant component of the rotor from the vibration data; and injecting the
compensation current in response to an identified mechanical resonant component of
the rotor. Such a method is a closed loop method where the compensation current is
injected in response to data from the vibration sensor indicating that there is a
resonance.
30
35
In this closed loop method, the compensation current may only be injected when an
amplitude of the identified mechanical resonant component is above a threshold value.
An amplitude of the compensation current may be proportional to an amplitude of the
identified mechanical resonant component.
wo 2021/239849 PCT /EP2021/064120
5
In the closed loop method, the vibration sensor may be an accelerometer.
In any of the above methods, in each of the one or more ranges of rotational speeds of
the rotor, the modulated current component may have a frequency at the first
5 frequency when the rotor is rotating at a rotational speed corresponding with a lowest
rotational speed within the respective range of rotational speeds of the rotor; and
wherein the modulated current component may have a frequency that is at the second
frequency when the rotor is rotating at rotational speed corresponding with the highest
rotational speed within the respective range of rotational speeds of the rotor.
10
15
In each of the one or more ranges of rotational speeds of the rotor, the modulated
current component may have a frequency substantially the same as the fundamental
mechanical resonant frequency of the rotor at a rotational speed of the rotor
corresponding with the respective determined rotational speed.
The first frequency may be lower than the second frequency. Furthermore, the range of
frequencies of the modulated current component between the first frequency and
second frequency may be based on a percentage change of the rotor fundamental
mechanical resonant frequency, and wherein the percentage change of the rotor
20 fundamental mechanical resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the
rotor fundamental mechanical resonant frequency.
The alternating current supplied to the plurality of coils may be a three-phase
alternating current, and wherein ld and lq represent vectored direct current components
25 of the combination of all three-phases.
The axial flux machine may be a motor or a generator.
The present invention also provides a controller for controlling an axial flux machine,
30 the axial flux machine comprising a stator comprising a stator housing enclosing a
plurality of stator pole pieces disposed circumferentially at intervals around an axis of
the machine, each of the stator pole pieces having a set of coils wound therearound for
generating a magnetic field; and a rotor comprising a set of permanent magnets and
mounted for rotation about the axis of the machine, the rotor being spaced apart from
35 the stator along the axis of the machine to define a gap between the stator and rotor
wo 2021/239849 PCT /EP2021/064120
6
and in which magnetic flux in the machine is generally in an axial direction, the
controller comprising: one or more electrical inputs for receiving one or more electrical
currents; one or more electrical outputs for supplying one or more alternating currents
to the axial flux machine coils, wherein the controller is configured to: control an
5 alternating current supplied to the plurality of coils to inject a compensation current for
reducing a mechanical resonant component of the rotor , the compensation current
being injected when the rotor is rotating over one or more ranges of rotational speeds,
each of the one or more ranges of rotational speeds of the rotor including a respective
determined rotational speed of the rotor, wherein the alternating current through each
1 0 coil is represented as vectored direct current components comprising a Direct current
(ld) component and a Quadrature current (lq) component that are orthogonal to one
another, and wherein the compensation current comprises an alternating current
component added to at least one of the Quadrature Current (lq) and the Direct Current
(ld) components, the modulated current component having an electrical frequency that
15 varies over a range of frequencies between a first frequency and a second frequency
depending on the rotational speed of the rotor, the range of frequencies including a
frequency that is substantially the same as a fundamental mechanical resonant
frequency of the rotor, and a phase that is out of phase with the fundamental
mechanical resonant frequency of the rotor.
20
The one or more respective determined rotational speeds of the rotor may be
dependent on one or more respective mechanical resonant excitation orders of the
rotor, for example one or more of the 121h, 181h, 361h and ?2nd excitation orders. These
orders are of interest for certain topologies of machine, for example an 18/12 topology.
25 Other topologies may have other excitation orders that are of interest, for example for a
12/8 machine, the excitation orders may be 81h, 121h, 241h and 481h orders. For a 24/16
topology the 161h , 241h, 481h and 961h orders may be interest.
The one or more respective determined rotational speeds of the rotor is defined by the
30 relationship:
rotor fundamental resonant frequency
determined rotational speed = 60 * - - d -
- - excitation_or er
Each of the ranges of rotational speed of the rotor may be based on a percentage
change of the rotor fundamental mechanical resonant frequency for a given resonant
35 excitation order of the rotor, for example the percentage change of the rotor
wo 2021/239849 PCT /EP2021/064120
7
fundamental mechanical resonant frequency may be ±1 %, ±5%, ±1 0%, ±15% or ±20%
of the rotor fundamental mechanical resonant frequency.
The controller may be configured to ramp an amplitude of the compensation current
5 between a lower amplitude and a peak amplitude over at least a portion of the range of
rotational speeds of the rotor, and wherein the peak amplitude of the compensation
current substantially coincides with the respective determined rotational speed of the
rotor.
1 0 The controller may also comprise: a vibration sensor input for receiving vibration data
from a vibration sensor, the vibration sensor detecting mechanical vibrations in the
rotor, wherein the controller is configured to: identify a mechanical resonant component
of the rotor from the vibration data; and inject the compensation current in response to
an identified mechanical resonant component of the rotor.
15
20
The controller may be configured only to inject the compensation current when an
amplitude of the identified mechanical resonant component is above a threshold value.
An amplitude of the compensation current may be proportional to an amplitude of the
identified mechanical resonant component.
The vibration sensor may be an accelerometer.
In each of the one or more ranges of rotational speeds of the rotor, the controller may
control the modulated current component to have a frequency at the first frequency
25 when the rotor is rotating at a rotational speed corresponding with a lowest rotational
speed within the respective range of rotational speeds of the rotor; and wherein the
controller may control the modulated current component to have a frequency that is at
the second frequency when the rotor is rotating at rotational speed corresponding with
the highest rotational speed within the respective range of rotational speeds of the
30 rotor.
In each of the one or more ranges of rotational speeds of the rotor, the controller may
control the modulated current component to have a frequency substantially the same
as the fundamental mechanical resonant frequency of the rotor at a rotational speed of
35 the rotor corresponding with the respective determined rotational speed.
wo 2021/239849 PCT /EP2021/064120
8
The first frequency may be lower than the second frequency. The range of frequencies
of the modulated current component between the first frequency and second frequency
may be based on a percentage change of the rotor fundamental mechanical resonant
5 frequency, and wherein the percentage change of the rotor fundamental mechanical
resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the rotor fundamental
mechanical resonant frequency.
The one or more alternating currents supplied to the plurality of coils may be a three-
1 0 phase alternating current, and wherein ld and lq represent vectored direct current
components of the combination of all three-phases.
The axial flux machine may be a motor or a generator.
15 The present invention also provides an axial flux machine, compns1ng: a stator
comprising a stator housing enclosing a plurality of stator pole pieces disposed
circumferentially at intervals around an axis of the machine, each of the stator pole
pieces having a set of coils wound therearound for generating a magnetic field; and a
rotor comprising a set of permanent magnets and mounted for rotation about the axis
20 of the machine, the rotor being spaced apart from the stator along the axis of the
machine to define a gap between the stator and rotor and in which magnetic flux in the
machine is generally in an axial direction, wherein the axial flux machine is coupled to a
controller as described above, the controller supplying alternating currents to the
plurality of coils.
25
The axial flux machine may comprise a vibration sensor mounted to the machine for
sensing vibrations in the rotor. The vibration sensor may be an accelerometer.
The stator housing may have an annular shape forming a hollow region about the axis
30 of the machine, and wherein the rotor may be formed of an annulus and having a
hollow central region about the axis of the machine.
The axial flux machine may comprise a second rotor disposed on an opposite side of
the stator to the first rotor, the second rotor comprising a set of permanent magnets on
35 a first side of the second rotor facing the stator, the second rotor being mounted for
5
10
wo 2021/239849 PCT /EP2021/064120
9
rotation about the axis of the machine and relative to the stator, the second rotor being
spaced apart from the stator along the axis of the machine to define an axial gap
between the stator and second rotor and in which magnetic flux in the machine is
generally in an axial direction.

CLAIMS:
1. A method of controlling an axial flux machine, the axial flux machine comprising
a stator comprising a stator housing enclosing a plurality of stator pole pieces disposed
5 circumferentially at intervals around an axis of the machine, each of the stator pole
pieces having a set of coils wound therearound for generating a magnetic field; and a
rotor comprising a set of permanent magnets and mounted for rotation about the axis
of the machine, the rotor being spaced apart from the stator along the axis of the
machine to define a gap between the stator and rotor and in which magnetic flux in the
10 machine is generally in an axial direction, the method comprising:
controlling an alternating current supplied to the plurality of coils to inject a
compensation current for reducing a mechanical resonant component of the rotor, the
compensation current being injected when the rotor is rotating over one or more ranges
of rotational speeds, each of the one or more ranges of rotational speeds of the rotor
15 including a respective determined rotational speed of the rotor,
wherein the alternating current through each coil is represented as vectored direct
current components comprising a Direct current (ld) component and a Quadrature
current (lq) component that are orthogonal to one another, and
wherein the compensation current comprises a modulated current component added to
20 at least one of the Quadrature Current (lq) and the Direct Current (ld) components, the
modulated current component having an electrical frequency that varies over a range
of frequencies between a first frequency and a second frequency depending on the
rotational speed of the rotor, the range of frequencies including a frequency that is
substantially the same as a fundamental mechanical resonant frequency of the rotor;
25 and a phase that is out of phase with the fundamental mechanical resonant frequency
of the rotor.
2. A method according to claim 1, wherein the one or more respective determined
rotational speeds of the rotor is dependent on one or more respective mechanical
30 resonant excitation orders of the rotor.
35
3. A method according to claim 2, wherein the one or more respective mechanical
resonant excitation order is one or more of the 121h, 181h, 361h and ?2nd excitation
orders.
5
10
wo 2021/239849 PCT /EP2021/064120
25
4. A method according to claim 2 or 3, wherein the one or more respective
determined rotational speeds of the rotor is defined by the relationship:
rotor fundamental resonant frequency
determined rotational speed = 60 * - - d -
- - excitation_or er
5. A method according to any preceding claim, wherein each of the ranges of
rotational speed of the rotor is based on a percentage change of the rotor fundamental
mechanical resonant frequency for a given mechanical resonant excitation order of the
rotor.
6. A method according to claim 5, wherein the percentage change of the rotor
fundamental mechanical resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the
rotor fundamental mechanical resonant frequency.
7. A method according to any preceding claim, wherein, in each of the one or
15 more ranges of rotational speeds of the rotor, an amplitude of the compensation
current is ramped between a lower amplitude and a peak amplitude over at least a
portion of the range of rotational speeds of the rotor, and wherein the peak amplitude of
the compensation current substantially coincides with the respective determined
rotational speed of the rotor.
20
8. A method according to any preceding claim, comprising:
receiving vibration data from a vibration sensor, the vibration sensor detecting
mechanical vibrations in the rotor;
identifying a mechanical resonant component of the rotor from the vibration
25 data; and
injecting the compensation current in response to an identified mechanical
resonant component of the rotor.
9. A method according to claim 8, wherein the compensation current is only
30 injected when an amplitude of the identified mechanical resonant component is above
a threshold value.
1 0. A method according to claim 8 or 9, wherein an amplitude of the compensation
current is proportional to an amplitude of the identified mechanical resonant
35 component.
5
wo 2021/239849 PCT /EP2021/064120
26
11 . A method according to any one of claims 8 to 1 0, wherein the vibration sensor
is an accelerometer.
12. A method according to any preceding claim, wherein, in each of the one or
more ranges of rotational speeds of the rotor, the modulated current component has a
frequency at the first frequency when the rotor is rotating at a rotational speed
corresponding with a lowest rotational speed within the respective range of rotational
speeds of the rotor; and
1 0 wherein the modulated current component has a frequency that is at the second
frequency when the rotor is rotating at rotational speed corresponding with the highest
rotational speed within the respective range of rotational speeds of the rotor.
13. A method according to any preceding claim, wherein, in each of the one or
15 more ranges of rotational speeds of the rotor, the modulated current component has a
frequency substantially the same as the fundamental mechanical resonant frequency of
the rotor at a rotational speed of the rotor corresponding with the respective determined
rotational speed.
20 14. A method according to claim 12 or 13, wherein the first frequency is lower than
the second frequency.
15. A method according to claim 12, 13 or 14, wherein the range of frequencies of
the modulated current component between the first frequency and second frequency is
25 based on a percentage change of the rotor fundamental mechanical resonant
frequency, and wherein the percentage change of the rotor fundamental mechanical
resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the rotor fundamental
mechanical resonant frequency.
30 16. A method according to any preceding claim, wherein the alternating current
supplied to the plurality of coils is a three-phase alternating current, and wherein ld and
lq represent vectored direct current components of the combination of all three-phases.
17. A method according to any preceding claim, wherein the axial flux machine is a
35 motor or a generator.
wo 2021/239849 PCT /EP2021/064120
27
18. A controller for controlling an axial flux machine, the axial flux machine
comprising a stator comprising a stator housing enclosing a plurality of stator pole
pieces disposed circumferentially at intervals around an axis of the machine, each of
5 the stator pole pieces having a set of coils wound therearound for generating a
magnetic field; and a rotor comprising a set of permanent magnets and mounted for
rotation about the axis of the machine, the rotor being spaced apart from the stator
along the axis of the machine to define a gap between the stator and rotor and in which
magnetic flux in the machine is generally in an axial direction, the controller comprising:
10 one or more electrical inputs for receiving one or more electrical currents;
one or more electrical outputs for supplying one or more alternating currents to
the axial flux machine coils,
wherein the controller is configured to:
control an alternating current supplied to the plurality of coils to inject a
15 compensation current for reducing a mechanical resonant component of the rotor , the
compensation current being injected when the rotor is rotating over one or more ranges
of rotational speeds, each of the one or more ranges of rotational speeds of the rotor
including a respective determined rotational speed of the rotor,
wherein the alternating current through each coil is represented as vectored direct
20 current components comprising a Direct current (ld) component and a Quadrature
current (lq) component that are orthogonal to one another, and
wherein the compensation current comprises an alternating current component added
to at least one of the Quadrature Current (lq) and the Direct Current (ld) components,
the modulated current component having an electrical frequency that varies over a
25 range of frequencies between a first frequency and a second frequency depending on
the rotational speed of the rotor, the range of frequencies including a frequency that is
substantially the same as a fundamental mechanical resonant frequency of the rotor,
and a phase that is out of phase with the fundamental mechanical resonant frequency
of the rotor.
30
19. A controller according to claim 18, wherein the one or more respective
determined rotational speeds of the rotor is dependent on one or more respective
mechanical resonant excitation orders of the rotor.
wo 2021/239849 PCT /EP2021/064120
28
20. A controller according to claim 19, wherein the one or more respective
mechanical resonant excitation order is one or more of the 121h, 181h, 361h and ?2nd
excitation orders.
5 21. A controller according to claim 19 or 20, wherein the one or more respective
determined rotational speeds of the rotor is defined by the relationship:
rotor fundamental resonant frequency
determined rotational speed = 60 * - - d -
- - excitation_or er
22. A controller according to claim any one of claims 18 to 21, wherein each of the
1 0 ranges of rotational speed of the rotor is based on a percentage change of the rotor
fundamental mechanical resonant frequency for a given resonant excitation order of
the rotor.
23. A controller according to claim 22, wherein the percentage change of the rotor
15 fundamental mechanical resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the
rotor fundamental mechanical resonant frequency.
24. A controller according to any one of claims 18 to 23, wherein the controller is
configured to ramp an amplitude of the compensation current between a lower
20 amplitude and a peak amplitude over at least a portion of the range of rotational
speeds of the rotor, and wherein the peak amplitude of the compensation current
substantially coincides with the respective determined rotational speed of the rotor.
25
30
25. A controller according to any one of claims 18 to 24, comprising:
a vibration sensor input for receiving vibration data from a vibration sensor, the
vibration sensor detecting mechanical vibrations in the rotor,
wherein the controller is configured to:
identify a mechanical resonant component of the rotor from the vibration data;
and
inject the compensation current in response to an identified mechanical
resonant component of the rotor.
26. A controller according to claim 25, wherein the controller is configured only to
inject the compensation current when an amplitude of the identified mechanical
35 resonant component is above a threshold value.
5
wo 2021/239849 PCT /EP2021/064120
29
27. A controller according to claim 25 or 26, wherein an amplitude of the
compensation current is proportional to an amplitude of the identified mechanical
resonant component.
28. A controller according to any one of claims 25 to 27, wherein the vibration
sensor is an accelerometer.
29. A controller according to any one of claims 18 to 28, wherein, in each of the one
1 0 or more ranges of rotational speeds of the rotor, the controller controls the modulated
current component to have a frequency at the first frequency when the rotor is rotating
at a rotational speed corresponding with a lowest rotational speed within the respective
range of rotational speeds of the rotor; and
wherein the controller controls the modulated current component to have a frequency
15 that is at the second frequency when the rotor is rotating at rotational speed
corresponding with the highest rotational speed within the respective range of rotational
speeds of the rotor.
30. A controller according to any one of claims 18 to 29, wherein, in each of the one
20 or more ranges of rotational speeds of the rotor, the controller controls the modulated
current component to have a frequency substantially the same as the fundamental
mechanical resonant frequency of the rotor at a rotational speed of the rotor
corresponding with the respective determined rotational speed.
25 31 . A controller according to claim 29 or 30, wherein the first frequency is lower
than the second frequency.
32. A controller according to claim 29, 30 or 31, wherein the range of frequencies of
the modulated current component between the first frequency and second frequency is
30 based on a percentage change of the rotor fundamental mechanical resonant
frequency, and wherein the percentage change of the rotor fundamental mechanical
resonant frequency is ±1 %, ±5%, ±1 0%, ±15% or ±20% of the rotor fundamental
mechanical resonant frequency.
5
10
wo 2021/239849 PCT /EP2021/064120
30
33. A controller according to any one of claims 18 to 32, wherein the one or more
alternating currents supplied to the plurality of coils are a three-phase alternating
current, and wherein ld and lq represent vectored direct current components of the
combination of all three-phases.
34. A controller according to any preceding claim, wherein the axial flux machine is
a motor or a generator.
35. An axial flux machine, comprising:
a stator comprising a stator housing enclosing a plurality of stator pole pieces
disposed circumferentially at intervals around an axis of the machine, each of the stator
pole pieces having a set of coils wound therearound for generating a magnetic field;
and
a rotor comprising a set of permanent magnets and mounted for rotation about
15 the axis of the machine, the rotor being spaced apart from the stator along the axis of
the machine to define a gap between the stator and rotor and in which magnetic flux in
the machine is generally in an axial direction,
20
wherein the axial flux machine is coupled to a controller according to any one of
claims 18 to 33, the controller supplying alternating currents to the plurality of coils.
36. An axial flux machine according to claim 35, comprising a vibration sensor
mounted to the machine for sensing vibrations in the rotor.
37. An axial flux machine according to claim 36, wherein the vibration sensor is an
25 accelerometer.
38. An axial flux machine according to claim 35, 36 or 37, wherein the stator
housing has an annular shape forming a hollow region about the axis of the machine,
and wherein the rotor is formed of an annulus and having a hollow central region about
30 the axis of the machine.
39. An axial flux machine according to any one of claim 35 to 38, comprising a
second rotor disposed on an opposite side of the stator to the first rotor, the second
rotor comprising a set of permanent magnets on a first side of the second rotor facing
35 the stator, the second rotor being mounted for rotation about the axis of the machine
5
wo 2021/239849 PCT /EP2021/064120
31
and relative to the stator, the second rotor being spaced apart from the stator along the
axis of the machine to define an axial gap between the stator and second rotor and in
which magnetic flux in the machine is generally in an axial direction.
40. An axial flux machine according to any one of claims 35 to 39, wherein the
machine is a motor or a generator.