Note: Descriptions are shown in the official language in which they were submitted.
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Rotor for modulated pole machine
Field of the invention
The invention relates to a rotor for modulated pole machines, more particular
to a rotor for modulated pole machines that are easily manufacturable in
large quantities.
Background of the invention
Over the years, electric machine designs such as modulated pole machines,
claw pole machines, Lundell machines and transverse flux machines (TFM)
have become more and more interesting. Electric machines using the
principles of these machines were disclosed as early as about 1910 by
Alexandersson and Fessenden. One of the most important reasons for the
increasing interest is that the design enables a very high torque output in
relation to, for instance, induction machines, switched reluctance machines
and even permanent magnet brushless machines. Further, such machines
are advantageous in that the coil is often easy to manufacture. However, one
of the drawbacks of the design is that they are typically relatively expensive
to manufacture and that they experience a high leakage flux which causes a
low power factor and a need for more magnetic material. The low power
factor requires an up-sized power electronic circuit (or power supply when
the machine is used synchronously) that also increases the volume, weight
and cost of the total drive.
The modulated pole electric machine stator is basically characterised by the
use of a central single winding that will magnetically feed multiple teeth
formed by the soft magnetic core structure. The soft magnetic core is then
formed around the winding, while for other common electrical machine
structures the winding is formed around the tooth core section. Examples of
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the modulated pole machine topology are sometimes recognised as e.g.
Claw-pole-, Crow-feet-, Lundell- or TFM-machines. The modulated pole
machine with buried magnets is further characterised by an active rotor
structure including a plurality of permanent magnets being separated by rotor
pole sections. The active rotor structure is built up from an even number of
segments, whereas half the number of segments is made of soft magnetic
material and the other half number of segments is made from permanent
magnet material. The permanent magnets are arranged so that the
magnetization direction of the permanent magnets is substantially
circumferential, i.e. the north and south pole, respectively, is pointing in a
substantially circumferential direction.
Traditionally rotors are manufactured by producing a rather large number of
individual rotor segments, typically 10-50. The assembly process is however
complicated and time consuming, as a large number of components should
be brought together resulting in a well defined air-gap to preserve the
performance of the machine. The assembly process is further complicated by
the opposing polarisation direction of the permanent magnet segments that
will tend to repel the rotor pole sections from each other during the
assembly.
W02009116935 discloses a rotor and a method for manufacturing a rotor,
where the number of individual parts are reduced thereby reducing the time
needed to assembling the rotor. This approach however results in that the
complexity and cost of the individual parts are increased. Furthermore, it may
be difficult to reach good overall tolerances, since the components will show
large variation in cross-section areas that may lead to undesired deformation
like bending during heat-treatment. The thin integrated bridge sections may
also cause strength problems during assembly especially, if the structure
must be slightly deformed during assembly to fulfil demands on geometrical
tolerances
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It is generally desirable to provide a rotor for a modulated pole machine that
is relatively inexpensive in production and assembly. It is further desirable
to
provide such a rotor that has good performance parameters, such as high
structural stability, low magnetic reluctance, efficient flux path guidance
low
weight and inertia etc.
Summary
According to a first aspect, disclosed herein are embodiments of a rotor for a
modulated pole machine, the rotor being configured to generate a rotor
magnetic field for interaction with a stator magnetic field of a stator of the
modulated pole machine, wherein said rotor comprises:
- a tubular support structure defining a circumferential mounting
surface, the tubular support structure comprising a plurality of elongated
recesses in the mounting surface, the elongated recesses extending in an
axial direction of the tubular support structure;
- a plurality of permanent magnets magnetised in the
circumferential direction of said rotor so as to generate the rotor magnetic
field, the permanent magnets being separated from each other in the
circumferential direction of the rotor by axially extending rotor pole
sections
for directing the rotor magnetic field generated by said permanent magnets in
a radial direction,
wherein at least one permanent magnet or at least one rotor pole section
extends radially at least partly into one of the plurality of recesses. Hence
at
least one component chosen from a permanent magnet and a rotor pole
section extends at least partly into one of the plurality of recesses, such
that
a part of the component extends out of the recess.
Consequently, in embodiments of the rotor described herein, the permanent
magnets and rotor pole sections form a tubular rotor structure coaxial with
the tubular support structure. One of the circumferential surfaces of the
tubular rotor structure is connected to the circumferential mounting surface
of
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the tubular support structure. To this end, some or all of the permanent
magnets and/or some or all of the rotor pole sections project radially from
said one of the circumferential surfaces of the tubular rotor structure and
into
respective recesses of the mounting surface of the tubular support structure.
Embodiments of the rotor described herein provide an efficient and reliable
assembly process, where a well-defined air-gap is provided even with
relatively large tolerances on the individual components, and even when the
components to be assembled have limited strength and brittle behaviour.
In some embodiments, the plurality of recesses are adapted to allow the
position of the at least one permanent magnet or at least one rotor pole
section extending radially at least partly into one of the plurality of
recesses
to be adjusted radially, so as to allow the radial length of the part
extending
out of the recess to be adjusted.
A recess may be adapted to allow the position of said component to be
adjusted radially, by having a depth that is greater than the depth needed for
an average component. Thereby a component that is produced with a radial
length above average, as a result of production variance, can be inserted
deeper into the recess, allowing the radial length of the part extending out
of
the recess to be that of an average component. The reverse principle may be
used for components produced with a radial length below average.
In some embodiments of the invention the at least one permanent magnet or
at least one rotor pole section extending at least partly into one of the
plurality of recesses is in contact with the two side walls of said recess,
i.e. in
direct contact with or separated from the two side walls by an adhesive.
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The rotor may be any type of rotor such as an inner rotor, adapted to rotate
radially inside an outer stator, or an outer rotor adapted to rotate around an
inner stator.
5 The plurality of permanent magnets may be arranged so that every second
magnet around the circumference is reversed in magnetisation direction.
Thereby individual rotor pole sections may only interface with magnets
showing equal polarity.
The recesses may be positioned periodically along the mounting surface of
the tubular support structure. The walls of the recesses may extend in a
radial direction into the tubular support structure. The permanent magnet or
rotor pole section extending at least partly into one of the plurality of
recesses
may thus extend out of the recess in a radial direction.
In some embodiments the circumferential mounting surface is defined by an
inner surface of the tubular support structure. This design is beneficial for
outer rotors.
In some embodiments the circumferential mounting surface is defined by an
outer surface of the tubular support structure. This design is beneficial for
inner rotors.
The tubular support structure may comprise any number of recesses such as
between 2 and 200, between 5 and 60 or between 10 and 30. In some
embodiments of the invention all recesses are fitted with either a permanent
magnet or a rotor pole section. The tubular support structure may have any
axial length. In some embodiments of the invention the axial length of the
tubular support structure corresponds to the axial length of the permanent
magnets and/or the rotor pole sections. In some embodiments of the
invention the recesses extend along the entire axial length of the tubular
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support structure. In some embodiments of the invention the recesses
extends along a limited part of the axial length of the support structure. A
recess may be formed by a first and a second parallel side wall extending in
a radial direction into the tubular support structure connected by a third
wall.
In some embodiments of the invention the third wall is perpendicular to the
first and the second wall. In some embodiments of the invention the third wall
is curved, having a curve that approximately follows the curvature of the
tubular support structure. The rotor may have any size. The recesses of the
tubular support structure may be adapted to allow either the position of the
rotor pole sections or the permanent magnets to be adjusted radially so as to
allow the radial length of the part extending out of the recess to be
adjusted.
The rotor, e.g. the tubular support structure, may comprise means for
transferring the torque generated by the interaction between the rotor and the
stator. In some embodiments the tubular support structure is connected to a
shaft for transferring the generated torque. For example, the surface of the
tubular support structure opposite the mounting surface for mounting the
magnets and/or rotor pole sections may be used for mounting the rotor to a
hub, a shaft, etc.
The cost of manufacturing any product is closely related to the precision
requirements of the end product. High precision production requires either
complex and expensive production techniques or a relative large rejection
rate of the produced products, both approaches resulting in high production
cost. To secure an efficient interaction between the rotor and the stator of a
modulated pole machine, high precision requirements apply. This results in
corresponding high precision requirements for the components of the rotor,
e.g. the rotor pole sections and the permanent magnets. However, by
supplying the rotor with a support structure comprising a plurality of
recesses,
a rotor pole section or permanent magnet may be adjusted radially into a
recess of the tubular support structure, thereby allowing the length of the
part
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extending radially out of the recess to be adjusted. This will lower the
precision requirement of the rotor pole sections or permanent magnets,
thereby lowering the production cost correspondingly. In some embodiments
of the invention the gap between the component positioned in a recess and
the back of the recess is filled with a suitable material, such as a suitable
type of adhesive such as epoxy glue.
In some embodiments the support structure may comprise small recesses for
transporting glue axially during the radial adjustment of the rotor pole
pieces
or permanent magnets in the recesses. The small recesses may provide
respective channels for the glue to axially escape the areas under the pole
piece or permanent magnet and thereby enhance the tolerance adjustment
precision.
The tubular support structure may also serve to simplify the assembly
process of the rotor parts, by providing a frame wherein the rotor pole
section
and permanent magnets can be inserted. The tubular support structure will
additionally serve to provide a stiffer rotor, decreasing the risk of skewing
of
the rotor through use. As the tubular support structure can be produced with
great precision the resulting rotors will have reduced geometrical variation
increasing the overall quality of the product and reducing the risk of human
errors. Thereby fewer rotors will need to be discarded.
It is an advantage of the invention that by having recesses in the tubular
support structure, allowing the position of the permanent magnets or rotor
pole sections to be modified, higher tolerances of individual components can
be handled; this also includes the tolerance of the tubular support structure.
It
is a further advantage that the tubular support structure provides a frame for
assembling a rotor according to the invention easily and with good
concentricity.
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In some embodiments the tubular support structure may be a single
component or provided as a pluralit of segments or modules, e.g. sectioned
in axial and/or circumferential direction. Similarly, some or each of the
permanent magnets and/or pole-pieces may be modularized, e.g. sectioned
in the axial direction or otherwise split in several components.
In some embodiments of the invention the rotor pole sections are made from
a soft magnetic material such as soft magnetic powder. By making the rotor
pole sections from soft magnetic powder the manufacturing of the rotor may
be simplified and magnetic flux concentration, utilizing the advantage of
effective three-dimensional flux paths, may be more efficient.
In some embodiments of the invention the tubular support structure is made
of a non-magnetic material such as aluminium, plastic, e.g. extruded
aluminium, injection moulded plastics etc. and/or the like, and/or other
suitable non-magnetic materials. By producing the tubular support structure
of a non-magnetic material the magnetic properties of the rotor is
undisturbed.
According to a first aspect, the permanent magnets are fitted inside said
recesses of said tubular support structure, and a rotor pole section is placed
between two adjacent permanent magnets. By fitting the permanent magnets
inside the recesses of said support structure, the permanent magnets extend
radially beyond the rotor pole sections. This will allow a more efficient
utilization of the magnetic flux generated by the permanent magnets.
In some embodiments of the invention the rotor pole sections are fitted inside
said recesses of said support structure.
In some embodiments of the invention either the permanent magnets or the
rotor pole sections are fitted inside the recesses of the tubular support
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structure by a frictional fit formed by the side walls of said recess. By
using a
frictional fit an easy and reliable method of securing the permanent magnets
or rotor pole sections is provided. The frictional fit may be created by
designing the recesses to be slightly smaller than the permanent magnets or
rotor pole sections. An adjustment of the frictional forces may be facilitated
by a controlled deformation of the recess walls, e.g. by some integrated
design features like a lip of material that can be bent with desirable force
small enough to prevent damaging the pole section or magnet.
According to a second aspect, the invention relates to a rotor pole section
for
a rotor as disclosed above wherein the rotor pole section when fitted in said
recess extends radially from said recess defining a radial axis, where the
rotor pole section comprises,
- a first constant-width zone, forming a first end of said rotor pole
section, adapted to at least partly be fitted in a recess of said support
structure wherein said first constant-width zone has two parallel side walls
so
that the width of the rotor pole section in said first constant-width zone is
constant,
- a tapered zone starting at the point where the first constant-
width zone ends, wherein said tapered zone has two non-parallel side walls
such that the width of said rotor pole section in said tapered zone is non
constant.
Hence, the tapered zones of two adjacent rotor pole sections form a slot
opening with parallel walls for a permanent magnet, thereby facilitating a
simple, low cost geometry of the expensive permanent magnet.
In some embodiments of the invention the side walls of the first constant-
width zone are parallel with said radial axis.
In some embodiments of the invention the side walls of the tapered zone are
non-parallel with said radial axis.
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For the purpose of the present description, the length of a rotor pole section
is defined as the dimension extending along the radial axis of the tubular
support structure when the rotor pole section is fitted in the tubular support
5 structure, the height of the rotor pole section is defined as the dimension
extending along the axis of the tubular support structure, when the rotor pole
section is fitted in the tubular support structure, and the width of the rotor
pole
section is defined as the dimension being perpendicular to the length and
height of the rotor pole section.
The height of the rotor pole section may be constant through both the first
constant-width zone and the tapered zone. The length of the first constant-
width zone may approximately correspond to the depth of the recesses e.g.
the height (in radial direction) of the side walls of the recess. In some
embodiments of the invention the length of the first constant-width zone
corresponds to between 2 and 30 percent of the total length of the rotor pole
section. In some embodiments of the invention the length of the first
constant-width zone corresponds to between 5 and 20 percent of the total
length of the rotor pole section. In some embodiments of the invention the
length of the first constant-width zone corresponds to between 8 and 12
percent of the total length of the rotor pole section.
The tapered zone may have any length. In some embodiments of the
invention the length of the tapered zone corresponds to between 40 and 95
percent of the total length of the rotor pole section. In some embodiments of
the invention the length of the tapered zone corresponds to between 60 and
90 percent of the total length of the rotor pole section. The length of the
tapered zone may be determined by the radial length of the permanent
magnets. In some embodiments of the invention the two side walls of the
tapered zone are straight walls angled towards the centre radial axis such
that the width of the rotor pole section is monotonously decreasing along the
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radial axis with increasing distance to the first constant-width zone; this
design is advantageous when the rotor pole section is used in an outer rotor.
In some embodiments of the invention the two side walls of the tapered zone
are straight walls angled away from the centre radial axis such that the width
of the rotor pole section is monotonously increasing along the radial axis
with
increasing distance to the first constant-width zone; this design is
advantageous when the rotor pole sections is used in an inner rotor.
To secure a cylindrical shape of the rotor, in some embodiments of the
invention, the rotor pole sections preferably comprise a tapered zone. As
described above, the tapered zone secures that the width of the rotor pole
section is expanded for inner rotors and reduced for outer rotors. However,
by further having a first constant-width zone adapted to be positioned in a
recess of the tubular support structure the assembly of the rotors utilizing
the
rotor pole sections can be simplified as the rotor pole sections can be
inserted into the recesses in a movement along a radial axis. This has shown
to be superior over pushing the rotor pole sections into the recesses using an
axial movement as the height of rotor pole section typically is large, making
them unstable in beginning of the insertion process. Thereby the production
costs can be decreased. The first constant-width zone further serves to
secure a firmer fit when the rotor pole sections are used for inner rotors.
In some embodiments of the invention the rotor pole section further
comprises a second constant-width zone starting at the point where the
tapered zone ends, and forming a second end of said rotor pole section,
wherein the side walls of said second constant-width zone are parallel to
each other, so that the width of said rotor pole section is constant in said
second constant-width zone.
In some embodiment the two side walls of the second constant-width zone
are parallel with the radial axis.
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The second constant-width zone may have any length. In some
embodiments of the invention the second constant-width zone has a length
corresponding to between 2 and 20 percent of the total length of the rotor
pole section. In some embodiments of the invention the second constant-
width zone have a length corresponding to between 5 and 15 percent of the
total length of the rotor pole section.
By having a second constant-width zone, the width of the space formed by
two adjacent rotor pole sections can be decreased from the point where the
second constant-width zone starts. Thereby a magnet placed in the space
can be prevented from falling out of the pocket in a radial direction.
In some embodiments of the invention the height of the pole section is larger
than the length, and the length is larger than the width.
According to a third aspect, the invention relates to a method of
manufacturing a rotor pole section as disclosed above and in the following,
using powder compaction, comprising the steps of:
- obtaining a die having the inverse shape of a rotor pole section
as disclosed above and in the following, comprising a first constant-width
zone and a second constant-width zone;
- filling said die with magnetic powder such as e.g. iron or iron
based powder;
- compressing the deformable magnetic powder in the die, e.g.
using two or more punches, wherein at least one of the punches moves
against the other punch along the radial axis of the resulting rotor pole
section, partly entering at least one of the first constant-width zones or the
second constant-width zone of the die, such that the length of at least one of
the first constant-width zones or second constant-width zones of the resulting
rotor pole section is reduced during the compaction.
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The magnetic powder may e.g. be a soft magnetic Iron powder or powder
containing Co or Ni or alloys containing parts of the same. The soft magnetic
powder could be a substantially pure water atomised iron powder or a
sponge iron powder having irregular shaped particles which have been
coated with an electrical insulation. In this context the term "substantially
pure" means that the powder should be substantially free from inclusions and
that the amount of the impurities 0, C and N should be kept at a minimum.
The average particle sizes are generally below 300 pm and above 10 m.
However, any soft magnetic metal powder or metal alloy powder may be
used as long as the soft magnetic properties are sufficient and that the
powder is suitable for die compaction.
The electrical insulation of the powder particles may be made of an inorganic
material. Especially suitable are the type of insulation disclosed in US
6348265 (which is hereby incorporated by reference), which concerns
particles of a base powder consisting of essentially pure iron having an
insulating oxygen- and phosphorus-containing barrier. Powders having
insulated particles are available as Somaloy(R)500, Somaloy(R)550 or
Somaloy(R)700 available from Hoganas AB, Sweden.
Thereby the rotor pole sections are efficiently made in the same operation by
use of a powder forming method where the forming is made in a single
compaction tool set up.
By having the constant-width zones in the die, the punches may move a
variable degree into the zones without damaging the die. This allows a
greater tolerance in the compressibility of the iron powder, further lowering
the production costs.
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According to a fourth aspect, the invention relates to a method for
manufacturing a rotor for a modulated pole machine, said rotor comprising a
tubular support structure defining a circumferential mounting surface, the
tubular support structure comprising a plurality of elongated recesses
positioned periodically along the mounting surface of the support structure in
the mounting surface, the elongated recesses extending in an axial direction
of the tubular support structure, each recess having two side walls, the rotor
further comprising a plurality of permanent magnets separated in the
circumferential direction form each other by axially extending rotor pole
sections made from soft magnetic material, wherein the method comprises
the steps of:
- placing either a permanent magnet or a rotor pole section at least partly
inside each of the recesses, the permanent magnets or rotor pole sections
extending radially out of the recesses thereby forming a plurality of slots
between two adjacent recesses
- placing either a permanent magnet or a rotor pole section inside each of the
formed slots.
In some embodiments of the invention the method further comprises the step
of placing an air-gap fixture concentric with the support structure wherein a
rotor pole section or a permanent magnet is adjusted radially in a recess so
that the side of the permanent magnet or rotor pole section facing the air-
gap-fixture contacts said air-gap-fixture.
The air-gap fixture is preferably cylindrical when assembling an outer rotor
and tubular when assembling an inner rotor. The air-gap fixture may have
any axial length such as an axial length approximately equal to the axial
length of the support structure, an axial length lower than the support
structure or an axial length exceeding the axial length of the support
structure.
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By using an air-gap fixture a fast and easy way of assembling a rotor
according to the invention is provided, lowering the production cost. The air-
gap fixture may additionally be used in an automated production process
thereby further lowering the production costs. The air-gap fixture will as
well
5 serve to secure less variation in the end products.
In some embodiments of the invention the air-gap fixture further comprises a
magnetic device for strengthen the contact pressure between a rotor pole
section or a permanent magnet and the air-gap- fixture.
The magnetic device may be an arrangement of a magnetic flux circuit were
the pole pieces or the permanent magnets form a part of said magnetic
circuit so that the magnetic forces caused by said magnetic circuit can hold
the pole pieces and permanent magnets closely to a fixture that represent the
desired air-gap geometry of the application machine. The magnetic circuit
may contain a magnetic field source that could be an electromagnet using a
wire and coil holding controllable electric currents to generate the magnetic
field or by external permanent magnets. The external permanent magnets
may be the permanent magnets of the rotor. Additionally there may be radial,
axially extending recesses in the surface of the magnetic fixture surface to
further enhance the geometrical control of the rotor pole pieces and
permanent magnets during the assembly process.
By using an air-gap fixture comprising a magnetic device, magnetic energy
may be used to adjust the position of the rotor pole sections; this will
further
lower the production costs.
According to a fifth aspect the invention relates to an electrical, rotary
machine, said machine comprising: a first stator core section being
substantially circular and including a plurality of teeth, a second stator
core
section being substantially circular and including a plurality of teeth, a
coil
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arranged between the first and second circular stator core sections, and a
rotor as disclosed above and/or in the following, wherein the first stator
core
section, the second stator core section, the coil and the rotor are encircling
a
common geometric axis, and wherein the plurality of teeth of the first stator
core section and the second stator core section are arranged to protrude
towards the rotor; wherein the teeth of the second stator core section are
circumferentially displaced in relation to the teeth of the first stator core
section.
The different aspects of the present invention can be implemented in different
ways including the rotors and rotor pole sections described above and in the
following and further product means, each yielding one or more of the
benefits and advantages described in connection with at least one of the
aspects described above, and each having one or more preferred
embodiments corresponding to the preferred embodiments described in
connection with at least one of the aspects described above and/or disclosed
in the dependent claims. Furthermore, it will be appreciated that
embodiments described in connection with one of the aspects described
herein may equally be applied to the other aspects.
Brief description of the drawings
The above and/or additional objects, features and advantages of the present
invention, will be further elucidated by the following illustrative and non-
limiting detailed description of embodiments of the present invention, with
reference to the appended drawings, wherein:
Figure 1 a shows an exploded, perspective view of a prior art modulated pole
machine.
Figure 1 b shows a cross-sectional view of a prior art modulated pole
machine.
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Figure 2a shows a tubular support structure of an outer rotor according to
some embodiments of the invention.
Figure 2b shows a more detailed view of a recess of an outer rotor according
to some embodiments of the invention.
Figure 2c shows a tubular support structure comprising a plurality of
permanent magnets 203 of an outer rotor according to some embodiments of
the invention.
Figure 2d shows an outer rotor according to some embodiments of the
invention.
Figure 3 shows an inner rotor according to some embodiments of the
invention.
Figure 4 shows a rotor pole section 401 for an outer rotor according to some
embodiment of the invention.
Figure 5 shows a method of producing a rotor pole section 502 according to
some embodiments of the invention.
Figure 6a shows an outer rotor according to some embodiments of the
invention.
Figure 6b shows a more detailed view of a part of an outer rotor according to
some embodiments of the invention.
Figure 7a shows a rotor according to some embodiments of the invention.
Figure 7b shows a more detailed view of a rotor according to some
embodiments of the invention.
Figures 8a) and 8b) show an example of a magnetic air-gap fixture device.
Fig. 9 illustrates an example of a modulated pole machine. In particular, Fig.
9a shows a perspective view of the active parts of the machine including a
stator 10 and a rotor 30, while Fig. 9b shows an enlarged view of a part of
the
machine.
Fig. 10 illustrates an example of the stator 10 of the modulated pole machine
of fig. 9.
Fig. 11 illustrates an example of a 3-phase modulated pole machine. In
particular, Fig. 11 a illustrates the active parts of an example of a 3-phase
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modulated pole machine, while fig. 11 b shows an example of a stator of the
machine of fig. 11 a.
Detailed description
In the following description, reference is made to the accompanying figures,
which show by way of illustration how the invention may be practiced.
This invention is in the field of a modulated pole electric machine 100 of
which one example is shown in figure 1 a in a schematic, exploded,
perspective view. The modulated pole electric machine stator 10 is basically
characterised by the use of a central single winding 20 that will magnetically
feed multiple teeth 102 formed by the soft magnetic core structure. The stator
core is then formed around the winding 20 while for other common electrical
machine structures the winding is formed around the individual tooth core
section. Examples of the modulated pole machine topology are sometimes
recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM-machines. More
particularly the shown modulated pole electric machine 100 comprises two
stator core sections 14, 16 each including a plurality of teeth 102 and being
substantially circular, a coil 20 arranged between the first and second
circular
stator core sections, and a rotor 30 including a plurality of permanent
magnets 22. Further, the stator core sections 14, 16, the coil 20 and the
rotor
are encircling a common geometric axis 103, and the plurality of teeth of
the two stator core sections 14, 16 are arranged to protrude towards the rotor
30 for forming a closed circuit flux path. The machine in figure 1 is of the
25 radial type as the stator teeth protrudes in a radial direction towards the
rotor
in this case with the stator surrounding the rotor. However, the stator could
equally well be placed interiorly with respect to the rotor which type is also
illustrated in some of the following figures. The scope of invention as
presented in the following is not restricted to any specific type of modulated
30 pole electric machine and can equally well be applied to machines of both
the
axial and the radial type and for both interiorly and exteriorly placed
stators
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relative to the rotor. Similarly, the invention is not restricted to single
phase
machines but can equally well be applied to multi phase machines.
The active rotor structure 30 is built up from an even number of segments 22,
24 whereas half the numbers of segments also called rotor pole sections 24
are made of soft magnetic material and the other half of number of segments
of permanent magnet material 22. The state of art method is to produce
these segments as individual components. Often the number of segments
can be rather large typically of order 10 - 50 individual sections. The
permanent magnets 22 are arranged so that the magnetization directions of
the permanent magnets are substantially circumferential, i.e. the north and
the south pole, respectively, is facing in a substantially circumferential
direction. Further, every second permanent magnet 22, counted
circumferentially is arranged having its magnetization direction in the
opposite direction in relation to the other permanent magnets. The magnetic
functionality of the soft magnetic pole sections 24 in the desired machine
structure is fully three dimensional and it is required that the soft magnetic
pole section 24 is able to efficiently carry varying magnetic flux with high
magnetic permeability in all three space directions. A traditional design
using
laminated steel sheets will not show the required high permeability in the
direction perpendicular to the plane of the steel sheets and its here
beneficial
to use a soft magnetic structure and material that shows a higher magnetic
flux isotropy than a state of art laminated steel sheet structure.
Figure 1 b shows the same radial modulated pole electric machine as from fig
1 but in a cross-sectional view of the assembled machine showing more
clearly how the stator teeth 102 extend towards the rotor and how the stator
teeth of the two stator core sections 14, 16 are rotationally displaced in
relation to each other.
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In the following, examples of rotors will be described in greater detail that
may be used as a part of the modulated pole electric machine shown in
figures 1 a-b. It should be understood that the rotors described in this
application may be used together with stators of modulated pole machines of
5 different types than the one described above.
Figure 2a shows a tubular support structure 201 of an outer rotor according
to some embodiments of the invention. The tubular support structure 201 has
a radius and a height, where the height extends along an axial axis of the
10 tubular support structure 201. The tubular support structure 201 comprises
a
plurality of recesses 202 positioned periodically around the periphery of the
support structure 201 in a circumferential mounting surface, being the inner
surface of the tubular support structure 201. The tubular support structure
201 may be made of a non-permeable material e.g. a non-magnetic material
15 such as aluminium or plastic. The plurality of recesses 202 extend in an
axial
direction of the tubular support structure. Figure 2b shows a more detailed
view of a recess. The recess comprises two parallel side walls 205 and 206
extending in a radial direction into the tubular support structure. The two
parallel side walls 205 and 206 are connected by an end wall 207. The
20 recess extends through the entire height of the tubular support structure
201.
Figure 2c shows a tubular support structure comprising a plurality of
permanent magnets 203 of an outer rotor according to some embodiments of
the invention. Each of the plurality of recesses is fitted with a permanent
magnet 203. The permanent magnets 203 may be secured in the recesses
202 by a frictional fit and/or any kind of fastening means such as a suitable
type of glue.
Figure 2d shows an outer rotor according to some embodiments of the
invention. The outer rotor comprises a tubular support structure 201, a
plurality of permanent magnets 203 and a plurality of rotor pole section 204.
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The rotor pole sections 204 are fitted into the slots formed by the permanent
magnets fitted inside the recesses 202 of the support structure 201. The rotor
pole sections 204 may be fastened to the permanent magnet and/or support
structure by a frictional fit formed by the permanent magnets and/or any type
of fastening means e.g. a suitable type of glue. As the permanent magnets
203 are fitted into the recesses 202 of the support structure 201, they extend
radially further outwards than the rotor pole sections 204. Thereby a greater
portion of the magnetic field generated by the permanent magnets 203 can
be utilized by the polar sections to generate the rotor magnetic field. This
will
decrease the magnetic requirements of the permanent magnets, so that
smaller permanent magnets may be used, lowering the production costs.
Figure 3 shows an inner rotor corresponding to the outer rotor shown in
figure 2d.
Figure 4 shows a rotor pole section 401 for an outer rotor according to some
embodiments of the invention. The rotor pole section 401 has a width 407
and a length 406. The rotor pole section 401 comprises three zones: a first
constant-width zone 402, a tapered zone 403 and a second constant-width
zone 404. The first constant-width zone 402 is adapted to at least partly be
fitted in a recess of a support structure. The first constant-width zone 402
comprises two side walls being parallel to a radial axis of the rotor pole
section 401, thereby securing that the width of the rotor pole section 401 is
constant in the first constant-width zone 402. The length of the first
constant-
width zone may approximately correspond to the depth of the recesses e.g.
the extent of the two side walls of the recesses. The tapered zone 403
comprises two straight side walls having an equal but opposite angle in
regards to an radial axis of the rotor pole section 401, so that the width in
the
tapered zone is monotonically decreasing with increasing distance to the first
constant-width zone 402. However, in other embodiments the side walls of
the tapered zone is mirrored so that the width of the rotor pole section in
the
tapered zone is monotonically increasing with increasing distance to the first
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constant-width zone 402. The second constant-width zone 404 comprises
two side walls being parallel to a radial axis of the rotor pole section 401,
thereby securing that the width of the rotor pole section 401 is constant in
the
second constant-width zone. The second constant-width zone may further
comprise a concave end 405, when the rotor pole section is used for an outer
rotor and a convex end when the rotor pole section is used for an inner rotor.
In some embodiments of the invention the rotor pole section only comprises
a first constant-width zone 402 and a tapered zone 403.
Figure 5 illustrates a method of producing a rotor pole section 502 according
to some embodiments of the invention. The rotor pole section 502 is
manufactured by filling a die 501 with iron or iron-based powder, and by
compressing the iron powder with two punches 505 and 506. The die 501
has the inverse shape of the desired rotor pole section e.g. as shown in
figure 4, with the difference that the length of the first and second constant-
width zones 503 and 504 of the die 501 are extended. This enables the
punches 505 and 506 to move in an radial direction of the resulting rotor pole
section 502, partly entering the first and second constant-width zones 503
and 504 of the die 501, thereby compressing the iron powder in the die 501,
forming the rotor pole section 502.
Figure 6a shows an outer rotor according to some embodiments of the
invention. The outer rotor comprises a tubular support structure 601, a
plurality of permanent magnets 603 and a plurality of rotor pole sections 604
as shown in figure 4. The tubular support structure comprises a plurality of
recesses 602 positioned periodically around the periphery of the support
structure 601. The rotor pole sections 604 are fitted into the plurality of
recesses 602 of the tubular support structure 601 and the permanent
magnets 603 are fitted into the slots formed by two adjacent rotor pole
sections 604.
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Figure 6b shows a more detailed view of a part of the outer rotor shown in
figure 6a. Figure 6b shows how the shape of the rotor pole sections 604
fitted in the recesses 602 of the tubular support structure 601 influences the
space formed by two adjacent rotor pole sections 604. The tapered zone 607
of the rotor pole sections 604 secures that the width of the space formed
between two adjacent rotor pole sections 604 is constant along the tapered
zone 607 of the rotor pole sections 204. This enables a permanent magnet
605 having a constant width to be fitted in the space. By providing the rotor
pole sections 604 with a second constant-width zone 608, the width of the
space formed between two adjacent rotor pole sections 604, is decreased
along the second constant-width zone 608 of the rotor pole sections 604.
This secures the permanent magnets 603 enclosed in the space from sliding
in a radial direction out of the rotor.
Figure 7a shows a rotor according to some embodiments of the invention,
further comprising an air-gap fixture 605. The air-gap fixture may secure the
correct positioning of the rotor pole sections during manufacturing of the
rotor. The air-gap fixture 605 may have a cylindrical shape or alternatively a
cone shape when assembling an outer rotor, and a tubular shape when
assembling an inner rotor. The air-gap fixture 605 may be used to adjust the
rotor pole sections 604 radially in the recesses 602. The air-gap fixture may
comprise a magnetic device enabling magnetic energy to be used to adjust
the radial position of rotor pole sections 604 in the recesses 602. After
assembling the rotor, the air-gap fixture may be removed. Figure 7b shows a
more detailed view of figure 7a. By using an air-gap fixture a fast and easy
way of assembling a rotor according to the invention is provided, lowering the
production cost.
Figures 8a) and 8b) show an example of a magnetic air-gap fixture device.
The magnetic air-gap fixture 605 comprises a generally cylindrical body
having a circumferential recess 851 for accommodating a coil 852 for
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providing a controllable magnetic field for keeping the rotor pole sections
853
in place.
Fig. 9 illustrates an example of a modulated pole machine. In particular, fig.
9
shows the active parts of a single phase, e.g. a one-phase machine or a
phase of a multi-phase machine. Fig. 9a shows a perspective view of the
active parts of the machine including a stator 10 and a rotor 30. Fig. 9b
shows an enlarged view of a part of the machine.
Fig. 10 illustrates an example of the stator 10 of the modulated pole machine
of fig. 9. In particular, fig. 10 shows a cut-view of the stator 10.
The machine comprises a stator 10 which comprises a central single winding
that magnetically feeds multiple teeth 102 formed by a soft magnetic core
15 structure. The stator core is formed around the winding 20 while for other
common electrical machine structures the winding is formed around the
individual tooth core section. More particularly the modulated pole electric
machine of figs. 9 and 10 comprises two stator core sections 14, 16 each
including a plurality of teeth 102 and being substantially annular, a winding
20 20 arranged between the first and second annular stator core sections, and
a
rotor 30 including a plurality of permanent magnets 22. Further, the stator
core sections 14, 16, the coil 20 and the rotor 30 are encircling a common
geometric axis, and the plurality of teeth 102 of the two stator core sections
14, 16 are arranged to protrude towards the rotor 30 for forming a closed
circuit flux path. The stator teeth of the two stator core sections 14, 16 are
circumferentially displaced in relation to each other.
Each stator section comprises an annular core back portion 261 providing a
circumferential flux path between neighbouring teeth. The stator further
comprises a flux bridge or yoke component 18 providing at least an axial flux
path between the two stator core sections. In the machine in figures 9 and 10
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the stator teeth protrude in a radial direction towards the rotor, in this
case
with the rotor surrounding the stator. However, the stator could equally well
be placed exteriorly with respect to the rotor. Embodiments of the rotor
described herein may be used in single and/or in multi-phase machines.
5
The active rotor structure 30 is built up from an even number of segments 22,
24 wherein half of the number of segments - also called rotor pole sections
24 - is made of soft magnetic material and the other half of the number of
segments is made of permanent magnetic material 22. These segments may
10 be produced as individual components. For illustration purposes, only the
magnetically active parts of the rotor are shown in figs. 9-10. The tubular
support structure described herein is not explicitly shown in figs. 9-10.
The permanent magnets 22 are arranged so that the magnetization
15 directions of the permanent magnets are substantially circumferential, i.e.
the
north and the south poles, respectively, face in a substantially
circumferential
direction. Further, every second permanent magnet 22, counted
circumferentially is arranged having its magnetization direction in the
opposite direction in relation to its neighbouring permanent magnets. The
20 magnetic functionality of the soft magnetic pole sections 24 in the desired
machine structure is fully three dimensional and each soft magnetic pole
section 24 is able to efficiently carry varying magnetic flux with high
magnetic
permeability in all three space directions.
25 This design of the rotor 30 and the stator 10 has the advantage of enabling
flux concentration from the permanent magnets 22 so that the surface of the
rotor 30 facing a tooth of the stator 10 may present the total magnetic flux
from both of the neighboring permanent magnets 22 to the surface of the
facing tooth. The flux concentration may be seen as a function of the area of
the permanent magnets 22 facing each pole section 24 divided with the area
facing a tooth. In particular, due to the circumferential displacement of the
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teeth, a tooth facing a pole section results in an active air gap that only
extends partly across the axial extent of the pole section. Nevertheless, the
magnetic flux from the entire axial extent of the permanent magnets is axially
and radially directed in the pole section towards the active air gap. These
flux
concentration properties of each pole section 24 make it possible to use
weak low cost permanent magnets as permanent magnets 22 in the rotor
and makes it possible to achieve very high air gap flux densities. The flux
concentration may be facilitated by the pole section being made from
magnetic powder enabling effective three dimensional flux paths. Further, the
design also makes it possible to make more efficient use of the magnets than
in corresponding types of machines.
Still referring to figs. 9 and 10, the single phase stator 10 may be used as a
stator of a single-phase machine as illustrated in figs. 9 and 10, and/or as a
stator phase of a multi-phase machine, e.g. one of the stator phases 10a-c of
the machine of fig. 11. The stator 10 comprises two identical stator core
sections 14, 16, each comprising a number of teeth 102. Each stator core
section is made of soft magnetic powder, compacted to shape in a press tool.
When the stator core sections have identical shapes, they may be pressed in
the same tool. The two stator core sections are then joined in a second
operation, and together form the stator core with radially extending stator
core teeth, where the teeth of one stator core section are axially and
circumferentially displaced relative to the teeth of the other stator core
section.
Each of the stator core sections 14, 16 may be compacted in one piece.
Each stator core section 14, 16 may be formed as an annular disc having a
central, substantially circular opening defined by a radially inner edge 551
of
an annular core back portion 261. The teeth 102 protrude radially outward
from a radially outer edge of the annular disc-shaped core back. The annular
part between the inner edge 551 and the teeth 102 provides a radial and
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circumferential flux path and a side wall of a circumferential cavity
accommodating the coil 20. Each stator core section comprises a
circumferential flange 18 at or near the inner edge 551. In the assembled
stator the circumferential flange 18 is arranged on the inner side of the
stator
core section, i.e. the side facing the coil 20 and the other stator core
section.
In the embodiment shown in figs 9 and 10, the stator core sections 14, 16 are
formed as identical components. In particular both stator core sections
comprise a flange 18 protruding towards the respective other stator core
section. In the assembled stator, the flanges 18 abut each other and form an
axial flux bridge allowing the provision of an axial magnetic flux path
between
the stator core sections. In the assembled stator for an outer rotor machine
the coil thus encircles the stator core back formed by flanges 18.
Each of the teeth 102 has an interface surface 262 facing the air gap. During
operation of the machine, the magnetic flux is communicated through the
interface surface 262 via the air gap and through a corresponding interface
surface of a pole piece of the rotor.
Fig. 11 a illustrates the active parts of an example of a 3-phase modulated
pole machine, while fig. 11 b shows an example of a stator of the machine of
fig. 11 a. The machine comprises a stator 10 and a rotor 30. The stator 10
contains 3 stator phase sections 10a, b, c each as described in connection
with figs. 9 and 10. In particular each stator phase section comprises a
respective stator component pair 14a, 16a; 14b, 16b; and 14c, 16c,
respectively, each holding one circumferential winding 20a-c, respectively.
Hence, as in the example of figs. 9 and 10, each electric modulated pole
machine stator phase section 10a-c of fig. 11 comprises a central coil 20a-c,
e.g. a single winding, that magnetically feeds multiple teeth 102 formed by
the soft magnetic core structure. More particularly, each stator phase 10a-c
of the shown electric modulated pole machine 100 comprises two stator core
sections 14, each including a plurality of teeth 102 and being substantially
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annular, a coil 20 arranged between the first and second circular stator core
sections. Further, the stator core sections 14 and the coil 20 of each stator
phase encircle a common axis, and the plurality of teeth 102 of the stator
core sections 14 are arranged to protrude radially outward. In the example of
fig. 11 the rotor 30 is arranged coaxially with the stator 10 and encircling
the
stator so as to form an air gap between the teeth 102 of the stator and the
rotor. The rotor may be provided as alternating permanent magnets 22 and
pole pieces 24 as described in connection with figs. 9 and 10, but axially
extending across all stator phase sections.
Although some embodiments have been described and shown in detail, the
invention is not restricted to them, but may also be embodied in other ways
within the scope of the subject matter defined in the following claims. In
particular, it is to be understood that other embodiments may be utilised and
structural and functional modifications may be made without departing from
the scope of the present invention.
Embodiments of the invention disclosed herein may be used for a direct
wheel drive motor for an electric-bicycle or other electrically driven
vehicle, in
particular a light-weight vehicle. Such applications may impose demands on
high torque, relatively low speed and low cost. These demands may be
fulfilled by a motor with a relatively high pole number in a compact geometry
using a small volume of permanent magnets and wire coils to fit and to meet
cost demands by the enhanced rotor assembly routine.
In device claims enumerating several means, several of these means can be
embodied by one and the same item of hardware. The mere fact that certain
measures are recited in mutually different dependent claims or described in
different embodiments does not indicate that a combination of these
measures cannot be used to advantage.
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It should be emphasized that the term "comprises/comprising" when used in
this specification is taken to specify the presence of stated features,
integers,
steps or components but does not preclude the presence or addition of one
or more other features, integers, steps, components or groups thereof.
