U.S. patent application number 12/995798 was filed with the patent office on 2011-06-02 for magnetic gear.
This patent application is currently assigned to MAGNOMATICS LIMITED. Invention is credited to Kais Atallah, Jan Jozef Rens.
Application Number | 20110127869 12/995798 |
Document ID | / |
Family ID | 39638066 |
Filed Date | 2011-06-02 |
United States Patent
Application |
20110127869 |
Kind Code |
A1 |
Atallah; Kais ; et
al. |
June 2, 2011 |
MAGNETIC GEAR
Abstract
Embodiments of the present invention relate to magnetic gears
comprising first and second moveable members arranged to interact
in a magnetically geared manner via a first electrical winding
arrangement arranged to generate, at least in part, a first
magnetic flux having a first number of pole-pairs, and one or more
pole-pieces arranged to modulate the first magnetic flux to
interact with a second magnetic flux having a second number of
pole-pairs, wherein the first number of pole-pairs is less than the
second number of pole-pairs.
Inventors: |
Atallah; Kais; (Sheffield,
GB) ; Rens; Jan Jozef; (Sheffield, GB) |
Assignee: |
MAGNOMATICS LIMITED
Sheffield, South Yorkshire
GB
|
Family ID: |
39638066 |
Appl. No.: |
12/995798 |
Filed: |
May 29, 2009 |
PCT Filed: |
May 29, 2009 |
PCT NO: |
PCT/GB2009/001365 |
371 Date: |
February 7, 2011 |
Current U.S.
Class: |
310/94 ;
310/103 |
Current CPC
Class: |
H02K 7/1838 20130101;
H02K 49/06 20130101; Y02E 10/72 20130101; Y02E 10/725 20130101;
H02K 7/11 20130101; H02K 49/102 20130101 |
Class at
Publication: |
310/94 ;
310/103 |
International
Class: |
H02K 49/06 20060101
H02K049/06; H02P 15/00 20060101 H02P015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 3, 2008 |
GB |
0810097.6 |
Claims
1. A magnetic gear comprising input and output moveable members
arranged to interact in a magnetically geared manner and transmit
torque therebetween via a first electrical winding arrangement
arranged to generate, at least in part, a first magnetic flux
having a first number of pole-pairs, and one or more pole-pieces
arranged to modulate the first magnetic flux to interact with a
second magnetic flux having a second number of pole-pairs, such
that asynchronous harmonics of the magnetic field that is produced
by each flux-producing member are created which have the same
number of poles as the other flux-producing member and so interact,
wherein the first number of pole-pairs is less than the second
number of pole-pairs.
2. The magnetic gear as claimed in claim 1, wherein the second
magnetic flux is generated by a plurality of permanent magnets.
3. The magnetic gear according to claim 1, wherein the first
electrical winding arrangement is associated with the input
member.
4. The magnetic gear according to claim 1, wherein the plurality of
permanent magnets is associated with the second member.
5. The magnetic gear according to claim 1, wherein the plurality of
permanent magnets is associated with a stator.
6. The magnetic gear as claimed in claim 1, wherein the second
magnetic flux is generated, at least in part, by a second
electrical winding arrangement.
7. The magnetic gear according to claim 6, wherein the first
electrical winding arrangement is associated with the input member
and the second electrical winding arrangement is associated with a
stator.
8. The magnetic gear according to claim 6, wherein the first
electrical winding arrangement is associated with a stator and the
second electrical winding arrangement is associated with one of the
input or output members.
9. The magnetic gear as claimed in claim 6, wherein the first and
second electrical winding arrangements are associated with the
input and output members, respectively.
10. The magnetic gear as claimed in claim 1, wherein the first, or
at least one, electrical winding arrangement is arranged to be
provided with an electrical current controlled in response to a
torque level transmitted by the magnetic gear.
11. The magnetic gear as claimed in claim 10, wherein the
electrical current is controlled to maintain a load-angle in a
predetermined range.
12. (canceled)
13. The magnetic gear as claimed in claim 10, wherein the
electrical current is DC.
14. The magnetic gear as claimed in claim 10, wherein the
electrical current is AC.
15. (canceled)
16. The magnetic gear as claimed in claim 1, wherein the first
magnetic flux is partly generated by one or more permanent
magnets.
17. The magnetic gear as claimed in claim 16, wherein the one or
more permanent magnets are associated with a moveable member
carrying the first electrical winding arrangement.
18. The magnetic gear as claimed in claim 1, wherein the first
electrical winding arrangement is formed by a plurality of
separately controllable groups of electrical windings, wherein the
groups of electrical windings are selectively controlled to vary a
gear ratio between the input and output members.
19. The magnetic gear as claimed in claim 1, wherein a first
electrical winding arrangement is arranged to form salient
poles.
20. The magnetic gear as claimed in claim 1 wherein an electrical
winding arrangement is arranged to form a distributed winding.
21. (canceled)
22. The magnetic gear as claimed in claim 1, wherein the
pole-pieces are associated with a stator.
23. The magnetic gear as claimed in claim 1, wherein the
pole-pieces are associated with a moveable member.
24. (canceled)
25. The magnetic gear as claimed in claim 1, wherein the input and
output members are interposed by a stator having the pole-pieces
associated therewith.
26. The magnetic gear as claimed in claim 1, wherein the first
electrical winding arrangement is associated with an inner moveable
element.
27. The magnetic gear as claimed in claim 1, wherein the plurality
of electromagnets is associated with an outer moveable element.
28. The magnetic gear as claimed in claim 1 in which the input and
output members are rotatable members.
29. The magnetic gear as claimed in claim 1 in which the input and
output members comprise translatable members.
30. A magnetic gear system comprising a magnetic gear according to
claim 1 and a controller arranged to provide an electrical current
to, at least, the first electrical winding arrangement, wherein the
controller is arranged to control the electrical current in
response to a torque level transmitted by the magnetic gear.
31. The magnetic gear system of claim 30, wherein the controller is
arranged to control the electrical current to maintain a load-angle
in a predetermined range.
32. (canceled)
33. The magnetic gear system of claim 30, wherein the controller is
arranged to provide electrical current to first and second groups
of electrical windings associated, 5 such that a gear ratio between
the moveable members is changed.
34. A magnetic gear comprising first and second rotatable members
arranged to interact in a magnetically geared manner via a
plurality of electromagnets associated with the first rotatable
member, the plurality of electromagnets being arranged to generate
a first magnetic flux having a first number of pole-pairs, and one
or more pole-pieces arranged to modulate the first magnetic flux to
interact with a second magnetic flux having a second number of
pole-pairs generated by a plurality of permanent magnets associated
with the second rotatable member, wherein the first number of
pole-pairs is less than the second number of pole-pairs.
35. A magnetic gear comprising first and second rotatable members
arranged to interact in a magnetically geared manner via a
plurality of electromagnets associated with a stator, the plurality
of electromagnets being arranged to generate a first magnetic flux,
having a first number of pole-pairs, and one or more pole-pieces
associated with the first rotatable member being arranged to
modulate the first magnetic flux to interact with a second magnetic
flux having a second number of pole-pairs generated by a plurality
of permanent magnets associated with the second rotatable member,
wherein the first number of pole-pairs is less than the second
number of pole-pairs.
36. A magnetic gear comprising first and second rotatable members
arranged to interact in a magnetically geared manner via a
plurality of electromagnets and one or more permanent magnets
associated with the first rotatable member, the one or more
permanent magnets and, selectively, the plurality of electromagnets
being arranged to generate a first magnetic flux, having a first
number of pole-pairs, one or more pole pieces being arranged to
modulate the first magnetic flux to interact with a second magnetic
flux, having a second number of pole-pairs, generated by a second
plurality of electromagnets associated with a stator, wherein the
first number of pole-pairs is less than the second number of
pole-pairs.
37. A magnetic gear comprising first and second rotatable members
arranged to interact in a magnetically geared manner via a first
plurality of electromagnets associated with the first rotatable
member, the plurality of electromagnets being arranged to generate
a first magnetic flux having a first number of pole-pairs, and one
or more pole-pieces associated with the second rotatable member and
arranged to modulate the first magnetic flux to interact with a
second magnetic flux having a second number of pole-pairs generated
by a plurality of permanent magnets or a second plurality of
electromagnets associated with a stator, wherein the first number
of pole-pairs is less than the second number of pole-pairs.
38. A turbine comprising a magnetic gear or magnetic gear system
according to claim 1.
39. A wind turbine comprising a magnetic gear or magnetic gear
system according to claim 1.
40. A method of controlling a magnetic gear, comprising energising
a plurality of electromagnets to generate, at least in part, a
first magnetic flux having a first number of pole-pairs, and
modulating the first magnetic flux to cause a geared interaction
with a second magnetic flux having a second number of pole-pairs,
wherein the first number of pole-pairs is less than the second
number of pole-pairs.
41. The method as claimed in claim 40, comprising determining the
first magnetic flux level according to a level of torque
transmitted by the magnetic gear.
42. The method as claimed in claim 41, wherein the magnetic flux
level is determined to maintain a load-angle in a predetermined
range.
43. (canceled)
44. The method according to claim 40, wherein the plurality of
electromagnets is energised when a level of torque transmitted by
the magnetic gears exceeds a predetermined value.
45. The method as claimed in claim 40, wherein one or more
electromagnets comprise a spatially distributed multi-phase winding
and energizing one or more of the electromagnets by multiphase ac
currents that are temporally displaced.
46. The method as claimed in claim 40, comprising selectively
energizing one or more electromagnets of the plurality of
electromagnets so as to vary a gear ratio between the first and
second moveable members.
47. (canceled)
48. (canceled)
Description
FIELD OF THE INVENTION
[0001] Embodiments of the present invention relate to magnetic
gears.
BACKGROUND TO THE INVENTION
[0002] Mechanical gearboxes are extensively used to match the
operating speed of prime-movers to the requirements of their loads
for both increasing rotational speed such as, for example, in a
wind-powered generator or reducing rotational speed such as, for
example, in an electric-ship propulsion arrangement. It is usually
more cost and weight effective to employ a high-speed electrical
machine in conjunction with a mechanical gearbox to achieve
requisite speed and torque characteristics. However, while such a
high-speed electrical machine in conjunction with a mechanical
gearbox allows high system torque densities to be realised, such
mechanical gearboxes usually require lubrication and cooling.
Furthermore, reliability can also be a significant issue.
Consequently, direct drive electrical machines are employed in
applications where a mechanical gearbox cannot be used.
[0003] Several techniques of achieving magnetic gearing, using
permanent magnets, are known within the art. For example, FIG. 1
shows the most commonly used topology for magnetic gears. It can be
appreciated that FIG. 1 shows a magnetic gear 100 comprising a
first, high-speed, rotor 102 bearing a plurality of permanent
magnets 104 that is magnetically coupled, in a geared manner, to a
second, low speed, rotor 106 comprising a number of permanent
magnets 108. A significant disadvantage of the magnetic gear 100
shown in FIG. 1 is that the topology suffers from a very poor
utilisation of the permanent magnets since very few of the
permanent magnets simultaneously contribute to torque transmission
at any given time. The very poor torque transmission capability has
limited the use of magnetic gearing.
[0004] The problem associated with the magnetic gear 100 of FIG. 1
is solved by the magnetic gear 200 shown in FIG. 2. FIG. 2 shows a
rotary magnetic gear 200 comprising a first or inner rotor 202, a
second or outer rotor 204 and a number of pole pieces 206,
otherwise known as an interference or an 5 interference means. The
first rotor 202 comprises a support 208 bearing a respective first
number of permanent magnets 210. In the illustrated magnetic gear,
the first rotor 202 comprises 8 permanent magnets or 4 pole-pairs
arranged to produce a spatially varying magnetic field. The second
rotor 204 comprises a support 212 bearing a respective second 10
number of permanent magnets 214. The second rotor 204 comprises 46
permanent magnets or 23 pole-pairs arranged to produce a spatially
varying field. The first and second numbers of permanent magnets
are different. Accordingly, there will be little or no useful
direct magnetic coupling or interaction between the permanents
magnets 210 and 214 such that rotation of one rotor will not cause
rotation of the other rotor.
[0005] Pole pieces 206 are used to allow the fields of the
permanent magnets 210 and 214 to interact in a geared manner. The
pole pieces 206 modulate the magnetic fields of the permanent
magnets 210 and 214 so they interact to the extent that rotation of
one rotor will induce rotation of the other rotor in a geared
manner. Rotation of the first rotor 202 at a speed .omega..sub.1
will induce rotation of the second rotor 204 at a speed
.omega..sub.2 where .omega..sub.1.noteq..omega..sub.2 The gear
ratio is directly related to the ratio of the number of pole-pairs
on the outer rotor to the number of pole-pairs on the inner rotor.
In the given example, which has 23 pole-pairs on the outer rotor
and 4 pole-pairs on the inner rotor, the 25 gear ratio is 5.75:1.
Therefore, in a magnetic gear, the rotors always have a different
number of pole pairs.
[0006] However, the magnetic gear topology shown in FIG. 2 has the
disadvantages that it is large in size for given operating
conditions. That is, the magnetic gear must be designed to transmit
a peak torque level to be encountered in operation, even though
that peak torque may not be encountered more than momentarily.
Hence the magnetic gear is large in size and expensive to produce.
Further, iron losses are significant, for the magnetic flux which
is provided by the permanent magnets must be sufficient to couple
the rotors when subjected to the peak torque, even though this peak
torque rarely occurs.
[0007] It is an object of embodiments of the present invention to
at least mitigate one or more of the above problems of the prior
art.
SUMMARY OF EMBODIMENTS OF THE INVENTION
[0008] Accordingly, a first aspect of embodiment of the present
invention provides a magnetic gear comprising first and second
moveable members arranged to interact in a magnetically geared
manner via a first electrical winding arrangement arranged to
generate, at least in part, a first magnetic flux having a first
number of pole-pairs, and one or more pole-pieces arranged to
modulate the first magnetic flux to interact with a second magnetic
flux having a second number of pole-pairs, wherein the first number
of pole-pairs is less than the second number of pole-pairs.
[0009] Further aspects of the invention are defined in the appended
claims.
[0010] Magnetic gears according to embodiments of the present
invention exhibit significant advantages in terms of simplicity,
size and cost.
[0011] The winding arrangement may be known as an electromagnet.
Preferably, the first magnetic flux generated by the electromagnets
is modulated by the pole pieces such that asynchronous harmonics
are created which have the same number of poles as the second
magnetic flux. Preferably the pole pieces are ferromagnetic
members.
[0012] The electromagnets may be energised to provide the first
magnetic flux. The first magnetic flux is preferably modulated to
cause coupling of moveable members.
[0013] The electromagnets may be arranged within a chamber interior
to a moveable member. The electromagnets may be arranged around
teeth of a moveable member. Alternatively, the electromagnets may
be arranged within open slots of one or more flux-producing
members. In this case, the electromagnets may be arranged in pairs
in stacked relation. The electromagnets are preferably formed by
windings or coils. In some embodiments, the plurality of
electromagnets are associated with a moveable member and arranged
in one relation, whilst a second plurality of electromagnets are
associated with a second moveable member. The second plurality of
electromagnets may be arranged in a different relation. Both first
and second magnetic fluxes may be produced by electromagnets.
[0014] The electromagnets may be supplemented with permanent
magnets. The permanent magnets may interpose one or a plurality of
electromagnets.
[0015] The electromagnets are preferably supplied with a
current/which varies according to a torque level transmitted
through the magnetic gear. The current may be supplied by a
controller. The current may be AC or DC. The controller may only
supply current when the torque level exceeds a nominal torque
level. The controller may disengage the magnetic gears by reducing
the current and hence magnetic flux of the electromagnets. In this
way, the magnetic gears may act like a clutch. The controller
preferably attempts to maintain a load-load angle .delta. which is
as close to 90 degrees as is feasible in the given application for
varying torque levels.
[0016] Magnetic gears according to embodiments of the present
invention preferably couple an input shaft to an output shaft and
transmit torque there-between. Preferably, and particularly
suitably for the present invention, the torque level varies over
time. The torque level may vary between a nominal torque and a peak
torque. In some embodiments, the electromagnets are only energised
when the torque exceeds the nominal torque or a predetermined
torque value. Embodiments of the present invention may be applied
to turbines which generate torque at varying power levels, such as
wind turbines.
[0017] Other embodiments are described below and claimed in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Embodiments of the present invention will now be described,
by way of example only, with reference to the accompanying drawings
in which:
[0019] FIG. 1 shows a conventional magnetic gear;
[0020] FIG. 2 shows a further conventional magnetic gear;
[0021] FIG. 3 shows a magnetic gear according to a first
embodiment;
[0022] FIG. 4 shows a graph of load-angle against torque for a
prior art magnetic gear;
[0023] FIG. 5 illustrates a graph of load-angle against torque for
a magnetic gear according to embodiments of the present
invention;
[0024] FIG. 6 depicts a magnetic gear according to a further
embodiment;
[0025] FIG. 7 shows another embodiment of a magnetic gear; and
[0026] FIG. 8 illustrates yet another embodiment of a magnetic
gear;
[0027] FIG. 9 illustrates yet another embodiment of a magnetic
gear;
[0028] FIG. 10 illustrates yet another embodiment of a magnetic
gear; and
[0029] FIG. 11 illustrates yet another embodiment of a magnetic
gear.
[0030] Detailed description of preferred embodiments FIG. 3 shows a
magnetic gear 300 according to a first embodiment. The magnetic
gear 300 comprises an inner rotor 302, an outer rotor 304 and a
stator 306 interposing the inner 302 and outer 304 rotors. The
inner 302 and outer 304 rotors are rotatable, as will be explained,
and the stator 306 fixedly or statically arranged.
[0031] The inner rotor 302 comprises a plurality of salient teeth
302a around which windings 303 have been fitted, or wound, to form
electromagnets. Due to the inner rotor 302 being rotatable, the
plurality of windings 303 carried upon the inner rotor 302 are
provided with an electrical current/from a controller (not shown)
via one or more of a slip ring, rotating connection part, rotating
supply or transformer. In the present embodiment, there are 4
windings arranged to form 2 pole pairs, although it will be
realised that other numbers and arrangements of windings may be
provided to provide other numbers of pole-pairs. In the first
embodiment, the windings 303 are concentrated around the teeth 302a
to provide salient poles.
[0032] The outer rotor 304 is formed by, for example, a back iron
or other like substrate and comprises a number of permanent magnets
305 mounted on an inwardly facing surface thereof. In the shown
embodiment there are 50 permanent magnets 305 arranged to form 25
pole-pairs, although it will be realised that other numbers
permanent magnets may be provided to provide other numbers of
pole-pairs. The number of poles carried by the outer rotor 304 is
greater than the number of poles formed by the windings 303 carried
on the inner rotor 302.
[0033] The stator 306 comprises a plurality of ferro-magnetic pole
pieces 307. The pole pieces 307 are magnetically coupled to the
magnetic field from the inner 302 and outer 304 rotors to produce a
geared rotation between the inner 302 and outer 304 rotors using
the above described principles, that is, to modulate the magnetic
fields of, and couple in a geared manner, the permanent magnets 305
of the outer rotor 304 and the windings 303, when energised, of the
inner rotor 302. When modulated, the magnetic fields interact to
the extent that rotation of one rotor will induce rotation of the
other rotor in a geared manner. That is, the pole-pieces 307
modulate the magnetic fields of the electromagnets 303 and
permanents magnets 305 such that asynchronous harmonics of the
magnetic field that is produced by each flux-producing member are
created which have the same number of poles as the other
flux-producing member and so interact. The asynchronous harmonics
rotate at a different rate than the fundamental harmonic and create
a gear ratio between the rotors.
[0034] The first embodiment 300 comprises a stator 306 which
contains a plurality of ferro-magnetic pole-pieces 307 and two
rotors 302, 304 consisting of flux-producing members 303, 305.
However, it will be understood by those skilled in the art that the
principle of the invention does not depend on which member is taken
as the stator. Any member of the embodiment may form the stator,
with the other two members forming the input and output rotors of
the magnetic gear. It is further possible to allow all members to
rotate, such that two rotating members constitute the input and
output rotors, whilst the third member is rotated by an external
electric machine to affect the gear ratio between the input and
output rotors.
[0035] Advantages of the above-described arrangement will now be
described. Firstly, the use of electromagnets to replace permanent
magnets upon one or more rotating elements of a magnetic gear
allows a physically smaller and consequently cheaper magnetic gear
to be produced, due to the use of less materials. Typically, peak
torque transmitted by a magnetic gear is only encountered for a
small duration of an operating lifetime. However, a prior art
magnetic gear arrangement must be designed to transmit the peak
torque, and is consequently large in size. However, a magnetic gear
according to embodiments of the present invention may be designed
to transmit a nominal torque level which is typically encountered
for a majority of the operating lifetime. When it is desired to
transmit a torque level greater than the nominal torque, such as
the peak torque level, a magnetic flux of the electromagnets
carried upon one or more of the flux-producing members may be
increased accordingly by the controller supplying an increased
electrical current/to the windings. As a result, a physically
smaller and cheaper to produce mechanical gear arrangement may be
used. Such an advantage is particularly prevalent in applications
in which a transmitted torque level is time variant. Further, the
material which is typically used to construct a permanent magnet,
NdFeB for example, is typically many times more expensive than the
material which is used to build an electromagnet which can produce
an equivalent magnetic field, such that a magnetic gear which uses
electromagnets is generally cheaper than one which uses permanent
magnets.
[0036] Secondly, during a majority of an operating lifetime when
operating at the nominal torque level, the magnetic flux produced
by the windings is reduced, compared to that generated by the
permanent magnets of the prior art which is designed to transmit
the peak torque. This results in reduced iron losses in the
magnetic gear.
[0037] Thirdly, magnetic gears according to embodiments of the
present invention may be used as a clutch, negating the need for a
separate mechanically actuated clutch to be provided. In order to
operate as a clutch, the magnetic flux of the windings 303 is
reduced, or switched-off, by reducing the current/applied to the
windings 303 such that the flux-producing members are magnetically
decoupled.
[0038] Another advantage of the presented gear is its increased
ease of manufacture. Handling permanent magnets and securing them
in place is a complicated and highly skilled task. Because
electromagnets produce no magnetic flux without a supply current,
the assembly of the invention is much simplified compared to the
prior art gear.
[0039] It is well understood by those skilled in the art that the
advantages of the use electromagnets over the use of permanent
magnets to provide flux in an electrical machine are more
pronounced for machines of a larger size. This is due to the
increase of the physical area of each magnetic pole with an
increased machine size. Hence, it is envisaged that the advantages
of the invention over the prior art are more pronounced for large
gears, and for those gears where the flux produced by the
flux-producing member with the lowest number of poles, and hence
the largest pole-area, is provided by electromagnets.
[0040] Reference will now be made to FIGS. 4 and 5 which compare
load-angle d against transmitted torque for magnetic gears of the
prior art and the present invention.
[0041] Referring firstly to FIG. 4, load-angle against torque for a
prior art magnetic gear is shown. Load-angle .delta. is an
electrical angle between a magnetic field produced by a magnet
array and a magnetic field with the same pole-number produced by
the other magnetic array. The torque which is transmitted through
the gear is a sinusoidal function of the load-angle, i.e. the load
angle will automatically re-adjust when the torque through the gear
varies. As is well understood from the theory of synchronous
machines, the magnetic gear operates at a stable operating point
when the load angle is smaller than 90.degree., shown on the
left-hand side of the graph, whilst load angles larger than
90.degree., shown on the right-hand side of the graph in a dashed
line, result in unstable operating points. At maximum torque
transmission T.sub.max, the gear will operate with a load-angle
d.sub.max of 90.degree. If a torque greater than T.sub.max is
applied, then the gear will slip. At nominal load, the input torque
is smaller, and the gear will automatically operate at a load-angle
d.sub.nominal which is less than 90.degree.. Because the flux-level
in the prior-art magnetic gear is fixed, the torque carrying
capacity of a prior art permanent magnet magnetic gear is fixed,
and the gear must therefore be designed to be capable of
transmitting a maximum operating torque T.sub.max expected in
operation, even if this maximum torque is only expected to be
encountered for a small fraction of the operating duration, or
infrequently.
[0042] FIG. 5 shows the load-angle against torque for embodiments
of the present invention for different values of the current which
is supplied to the electromagnets. Because the flux in the
embodiments of the invention can be varied through varying the
current which flows through the windings of the electromagnets, the
peak torque which can be transmitted by the magnetic gear is a
function of the current in the windings. A controller controls the
current/which is supplied to the electromagnets in order to
maintain the load angle within the stable operating region but as
close to 90.degree. as possible. Advantageously, this allows the
magnetic gear to be designed to transmit the nominal torque
T.sub.nominal and to be consequently smaller and cheaper to produce
than a prior art magnetic gear, which must be designed for the peak
torque T.sub.max. Peak torque is accommodated in the invention by
increasing the current/flowing in the windings to a maximum
value/.sub.max. The maximum torque that can be transmitted through
the gear is a function of the maximum current of the electromagnets
and the duration for which the peak torque is encountered and this
current must be maintained, since over-heating of the windings may
become an issue. At part load, current in the windings is reduced
in order to maintain the load-angle close to 90.degree.. This
consequently reduces copper losses. Iron losses are also reduced by
the consequential reduction in magnetic flux at part loads.
[0043] FIG. 6 shows a second preferred embodiment of the present
invention. The embodiment shown replaces the salient poles with
concentrated windings on the inner flux-producing member of the
first embodiment with a plurality of distributed windings which are
fitted in a number of slots distributed around an inner rotor.
[0044] The second preferred embodiment 400 comprises an outer rotor
404 carrying a plurality of permanent magnets 405 and a stator 406
comprising a plurality of pole pieces 407 as in the first preferred
embodiment.
[0045] The second embodiment 400 further comprises an inner rotor
402 carrying a plurality of windings 403 which form electromagnets.
The windings 403 are arranged in open slots, or blind-apertures,
distributed around the outer circumference or periphery of the
inner rotor 402. The windings 403 are arranged in layers within the
open slots. The windings which are shown in FIG. 6 are configured
to result in a magnetic field which has 4 poles. The current in the
windings 403 may be an AC or a DC current. When a DC current is
applied to the windings 403, the magnetic field is stationary
compared to the inner rotor 402, and hence moves at the same speed
as the mechanical speed of the inner rotor 402 compared to an
external reference frame. In case an AC current is applied to the
windings 403, the magnetic field that is produced by the windings
moves at a different speed than the mechanical speed of the inner
rotor 402. As explained previously, the outer rotor 404 couples
with an asynchronous harmonic of the magnetic flux that is produced
by the inner rotor 402, and hence the mechanical speed of the outer
rotor 404 is a function of the speed of the magnetic field that is
produced by the inner rotor 402, which is not equal to the
mechanical speed of the inner rotor 402 when an AC current is
applied. Therefore, the application of an AC current allows for a
gear ratio between the input and output rotors 402, 404 of the
magnetic gear which is variable and which is a function of the
frequency of the AC current. More particularly in this embodiment,
a spatially distributed multi-phase winding is supplied by
multiphase ac currents that are temporally displaced. The generated
field of the rotor hosting the winding then rotates relative to the
rotor (in either direction) allowing a gear ratio to be varied.
[0046] The second embodiment operates in a like-manner to the above
described first embodiment. However, the second embodiment is
particularly suited to high speed applications in which the inner
rotor 402, in particular, rotates at least during a part of the
operating period at high-speed. The high-speed suitability of the
second embodiment is provided by the inner rotor 402 having less
aerodynamic drag compared with the first embodiment.
[0047] FIG. 7 shows a third preferred embodiment of the present
invention. In this embodiment, an outer rotor magnetic field is
produced by electromagnets.
[0048] The third embodiment 500 comprises an inner rotor 502
carrying a plurality of permanent magnets 503 on an outer periphery
thereof and a stator 506 carrying a plurality of pole pieces 507,
as in the prior art. However, an outer rotor 504 comprises a
plurality of open slots distributed around the inner circumference
or periphery of an inner surface of the outer rotor 504. A
plurality of winding 505 are arranged in layers within the slots to
provide 4 pole-pairs, however the winding can be configured to
generate a magnetic field with another number of pole pairs as will
be appreciated. It will also be realised that the outer rotor 504
could comprise concentrated windings and salient magnetic poles as
in the first embodiment.
[0049] Although as described with the inner rotor 502 and outer
rotor 504 being rotatable around and within, respectively, the
fixed pole pieces 507 it will also be realised that the outer rotor
504 may be fixed, hence becoming a stator, and the pole-pieces 507
being mounted upon a rotor to rotate in cooperation with the inner
rotor 502. Such a structure has the advantage that all windings to
be provided with an electrical current are mounted in a fixed
position and slip rings and the like are not required.
[0050] FIG. 8 shows a fourth preferred embodiment of the present
invention. In this embodiment, both inner rotor and outer rotor
magnetic fields are produced by electromagnets.
[0051] The fourth embodiment 600 comprises an inner rotor 602
having an identical arrangement to the first embodiment 300,
comprising a plurality of concentrated windings 603 which are wound
on salient poles. As shown, the fourth embodiment 600 comprises two
pole-pairs on the inner rotor 602, although other numbers of
pole-pairs can be envisaged. The fourth embodiment 600 further
comprises a stator 606 carrying a plurality of ferro-magnet pole
pieces 607 and an outer rotor 604 comprising a plurality of
windings 605 forming electromagnets which are distributed around an
inner periphery of the outer rotor 604, as in the third
embodiment.
[0052] The fourth embodiment 600 has two primary advantages.
Firstly, due to replacement of all permanent magnets with windings
603, 605, the magnetic gear 600 is cheaper to produce and easier to
manufacture. Secondly, it would be possible to change the gear
ratio of the magnetic gear by only selectively energising
pluralities of windings from the plurality of windings carried by
both rotors 604, 602 or by re-arranging the configuration of the
windings such that they produce a magnetic field at a different
number of poles. It will be recalled that the torque of the
magnetic gear is established by modulation by the pole pieces 607
of the magnetic flux that is generated by each flux-producing
member, such that asynchronous harmonics are created which have the
same number of magnetic poles as the other flux-producing member.
Therefore, the variation of the number of magnetic poles on one
flux-producing member only, would result in asynchronous harmonics
with a number of pole-pairs which is different than the number of
pole pairs on the other flux-producing member, such that no torque
would be transmitted by the gear. Therefore, for torque
transmission, the number of poles on each flux-producing member
must be changed simultaneously. This would lead to a corresponding
change in the gear ratio between rotors 602, 604.
[0053] FIG. 9 shows a fifth embodiment of the present invention. In
the fifth embodiment 700, a combination of permanent magnets and
windings are carried by one or more flux-producing members.
[0054] The fifth embodiment 700 comprises an outer rotor 704
carrying a plurality of permanent magnets 705 and a stator 706
carrying a plurality of ferro-magnetic pole-pieces 707, as in the
first embodiment 300. An inner rotor 702 carries a plurality of
permanent magnets 708 in combination with a plurality of windings
709 forming electromagnets, such that the flux from the inner
magnet is produced by both permanent magnets and electromagnets. In
the shown embodiment, the magnets and the windings are equally
spaced and radially interpose each other, but it will be realised
that other arrangements of permanent magnets and windings 708, 709
may be envisaged. Further, the outer rotor 704, or both the inner
and outer rotors 702, 704, may carry a combination of permanent
magnets 708 and windings 709.
[0055] An advantage of the use of one or more rotors carrying a
combination of permanent magnets and windings will now be
described. A magnetic gear may be designed to carry a nominal
torque level for a majority of an operating period. A determined
number of permanent magnets may be arranged about one or more
rotors, such that the nominal torque level, or the nominal torque
level and a safety margin, may be transmitted by the magnetic gear.
If, in a conventional magnetic gear, it was then attempted to
transmit a torque level in excess of the nominal torque level, or
the nominal torque level and safety margin, slippage of the
magnetic gear would then occur. However, in the fifth embodiment
700, the controller is arranged to energise one or more windings
709 when a greater torque level is desired to be transmitted. That
is, the controller would energise at least some windings 709 to
allow a peak torque level to be transmitted. Energising the
windings 709, it will be realised, does not have to be a binary
operation, but may be gradually energised by increasing current/to
carry increasing torque if desired.
[0056] Embodiments of the present invention are particularly suited
to applications in which torque load varies over time. For example,
embodiments of the present invention are particularly suited to use
in power generation equipment, such as turbines, wind turbines,
wave-power turbines etc. Further, embodiments of the present
application are suited to propulsion applications, such as
electric-drive propulsion apparatus, and also to variable-load
industrial applications such as pumps.
[0057] Also, although the above embodiments have been described
with reference to radial field rotors and rotation, embodiments can
equally well be realised using axial field rotors and rotation as
well as translators and translation, that is, the principles of
embodiments of the present invention can be realised in the context
of linear gears.
[0058] FIG. 10 illustrates a sixth embodiment of the invention. An
axial-field magnetic gear 800 is axisymmetric around axis 811 and
comprises a low-speed rotor 806 which is connected with two discs
811a and 811b carrying a plurality of pole-pieces 807a and 807b. An
inner rotor 802 carries a plurality of windings 803 forming
electromagnets. The embodiment further comprises stators 804 to
which two discs 809a and 809b are attached, which contain
pluralities of permanent magnets 80Sa and 805b. Although the
embodiment is described in relation to rotors 802/806 and a stator
804, it is not essential to the principle of the invention which
component is configured to be stationary, and which component
functions as the input rotor or output rotor.
[0059] The sixth embodiment 800 is shown to be symmetric around a
symmetry plane which cuts through the centre of disc 802 and is
perpendicular to the symmetry axis 811. The components which are
symmetric to one another have been referred to in FIG. 10 with
identical numbers but with different indices a) and b). Such a
symmetric embodiment has the advantage that the axial forces on
each component cancel, such that no net axial force is exerted on
any component of the shown embodiment, and inexpensive bearing
system can be used. However, it will be understood that the shown
embodiment can equally operate without this symmetry, as can be
achieved by, for example, omitting all parts in FIG. 10 with index
b), although a more robust and expensive bearing system will
result.
[0060] FIG. 11 shows an embodiment of the present invention as
realised in a linear magnetic gear 900, which could be implemented
as a planar device or a tubular device. The magnetic gear 900
comprises an outer stator 904 to which a plurality of permanent
magnets 905 have been connected. The embodiment further comprises a
low-speed translator 906 which contains a plurality of
ferro-magnetic pole-pieces 907. The gear 900 also comprises an
inner translator 902, to which a plurality of windings 903 has been
attached. It will be understood by those skilled in the art that
the shown embodiment operates in an essentially similar manner to
previously described embodiments, and that it is not essential
which part of the gear is configured as a stator and which parts
have been configured to operate as input/output translator.
[0061] The above embodiments have been described with reference to
the inner rotor driving the outer rotors. However, it will be
appreciated that embodiments can be realised in which an outer
rotor drives an inner rotor 30 thereby reversing the gear ratio.
Further, all previous embodiments have been described with
reference to an inner and outer rotor and a stator which is
positioned between the rotors. However, it will be understood that
any of the previous embodiments could operate with a moving middle
pole-piece structure, with either the inner or outer flux-producing
member configured to be static. It is further possible to rotate
all three members, such that two of the moving members are
configured as the input and output rotors, whilst the third moving
member is rotated to change the speed-relationship between the
input and output rotors, as is known in the prior-art magnetic
gears.
[0062] Further, it will be understood that combinations of aspects
of the various embodiments which have been described are part if
the invention. To this respect, the illustrated embodiments serve
as examples of the introduction of electro-magnets within a
magnetic gear. It will be clear, however, that other winding
arrangements are possible, which are well-known in the art of
electrical machine design.
* * * * *