Note: Descriptions are shown in the official language in which they were submitted.
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POLYPHASE TRANSVERSE FLUX MOTOR
TECHNICAL FIELD
This invention relates to polyphase transverse flux do motors and in
particular,
but not solely, motors of the "inside out" type where the rotor rotates
externally of the
stator.
PRIOR ART
The use of term "polyphase" in relation to a do motor means a motor with a
plurality of windings, or a plurality of sets of windings, which when excited
sequentially from a do source to produce a rotating flux. Commutation of the
motor
phases is normally achieved using electronic switching devices in a bridge
arrangement
with the switching sequences controlled by a micro-processor.
The advantages of transverse flux machines are well known. A transverse flux
machine is capable of producing power densities several times greater than
conventional electrical machines. This arises from the geometry of transverse
flux
motors which enables a larger number of poles while maintaining the same
magnetomagnetic force (MMF) per pole as would be achieved in a conventional
machine design.
Transverse flux machines have in the past been difficult to implement because
standard core lamination techniques do not easily permit the three dimensional
magnetic flux flow required in transverse flux machines. This difficulty is
being
overcome by the use of sintered powdered iron cores. These may be formed by a
compression moulding technique.
Most transverse flux machine configurations disclosed hitherto are single
phase
machines. An example is disclosed in US patent 5,773,910 (Lange). Proposals
for
polyphase machines usually involve complex geometries which lead to
difficulties in
manufacture. For example US patents 5,117,142 (Von Zueygbergk), 5,633,551
(Weh)
and 5,854,521 (Nolle).
It is therefore an object of the present invention to provide a polyphase
transverse flux DC motor which is simple to manufacture.
DISCLOSURE OF THE INVENTION
Accordingly in one aspect the invention consists in a polyphase transverse
flux
do motor comprising:
a rotor having alternating magnetic pole polarities at the periphery; and
a stator mounted co-axially with said rotor so as to provide at least one air
gap
there between, said stator including:
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a first stator piece having a plurality of circumferentially disposed and
spaced
apart claw poles projecting in an axial direction,
a second stator piece complementary to said first stator piece and mounted co-
axially in facing relationship with said first stator piece such that there is
an axial
spacing there between and oriented about the common axis such that the claw
poles of
the second pole piece circumferentially alternate with the claw poles of the
first pole
piece;
a plurality of magnetically permeable bridging cores disposed about the stator
axis proximate to said claw poles and located between said first and second
stator
pieces to provide magnetic flux paths there between,
at least one said stator piece being provided with regions of magnetic high
reluctance between the sites of said bridging cores, and
stator windings disposed about each bridging core each of which when supplied
with an exciting current produce flux flow through those stator claw poles in
the first
and second pole pieces which are proximate to the corresponding bridging core
thereby
producing flux in said air gap adjacent such claw poles, each winding or a
selected set
of windings constituting the windings for one of a plurality of motor phases
which in
use are electronically commutated to produce a flux in said air gap which
rotates about
the stator axis.
In a further aspect the invention consists in a polyphase transverse flux do
motor
comprising:
a rotor having a plurality of permanent magnets circumferentially disposed and
separated by magnetically permeable material to provide alternating magnetic
pole
polarities at the periphery, said magnets being magnetised in the
circumferential
direction; and
a stator mounted co-axially with said rotor so as to provide at least one air
gap
therebetween, said stator including:
a first stator piece having a plurality of circumferentially disposed and
spaced
apart claw poles projecting in an axial direction,
a second stator piece complementary to said first stator piece and mounted co-
axially in facing relationship with said first stator piece such that there is
an axial
spacing therebetween and oriented about the common axis such that the claw
poles of
the second pole piece circumferentially alternate with the claw poles of the
first pole
piece,
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a plurality of magnetically permeable bridging cores disposed about the stator
axis proximate to said claw poles and located between said first and second
stator
pieces to provide magnetic flux paths therebetween,
at least one said stator piece being provided with regions of magnetic high
reluctance between the sites of said bridging cores, and
stator windings disposed about each bridging core each of which when supplied
with an exciting current produce flux flow through those stator claw poles in
the first
and second pole pieces which are proximate to the corresponding bridging core
thereby
producing flux in said at least one air gap adjacent to such claw poles, each
winding or
a selected set of windings constituting the windings for one of a plurality of
motor
phases which in use are electronically commutated to produce a flux in said
air gap
which rotates about the stator axis.
In a further aspect the invention consists in a method of making a stator for
a
polyphase transverse flux do motor, comprising the steps of:
forming a first stator piece having a plurality of circumferentially disposed
and
spaced apart claw poles projecting in an axial direction,
forming a second stator piece similar and complementary to said first stator
piece, providing a plurality of magnetically permeable bridging cores to be
symmetrically disposed about the stator axis proximate to said claw poles
between said
first and second stator pieces to provide magnetic flux paths therebetween,
providing regions of high magnetic reluctance between the sites of said
bridging
cores in either or both of the first or second stator pieces,
placing stator windings about each bridging core,
assembling the first and second stator pieces co-axially in facing
relationship
with each other and spaced axially apart by said bridging cores with said
second stator
piece oriented about the common axis such that the claw poles of said second
pole
piece circumferentially alternate with the claw poles of said first pole
piece;
each said winding or a selected set of said windings constituting the windings
for one of a plurality of motor phases such that in use when supplied with an
exciting
current produce flux flow through those stator claw poles in the first and
second pole
pieces which are proximate to the corresponding bridging core.
In yet a further aspect the invention consists in a rotor having a plurality
of
circumferentially disposed permanent magnets separated by segments of high
permeability material to form rotor poles,
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a stator mounted co-axially with said rotor so as to provide at least one air
gap
therebetween, said stator having a plurality of circumferentially disposed and
spaced
apart poles,
at least one stator winding per phase disposed such that when supplied with an
exciting current produce flux flow through stator poles which are proximate
thereto to
produce a flux in said air gap adjacent to said poles, said windings in use
being
electronically commutated to produce a flux in said air gap which rotates
about the
stator axis,
the improvement defined by the relationship wherein the number of motor
phases (P) is selected from the series 2, 3, ...,N, the number of windings per
phase (W)
is selected from the series 1, 2 ...M, the number of poles associated with
each winding
(PW) is selected from the series 2, 4, ...1, and the number of stator poles
(SP) is equal
to the product P*WP*PW and the number of rotor poles is SPEW.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 shows a diagrammatic diametrical cross-section through a motor
according to the present invention,
Figure 2 shows an exploded view of a stator for the motor in Figure 1 with
Figure 2A showing a first stator piece, Figure 2B showing a second
complementary
stator piece and Figure 2C showing four of six stator windings,
Figure 3 shows a partial pictorial view of the periphery of the stator
indicating
a representative flux path produced by a single winding,
Figure 4 shows a stator piece to which an electronics circuit board is
mounted,
Figure 5 shows a fragmentary view of the magnetic components of one preferred
form of rotor and the flux flow therethrough,
Figure 6 shows an alternative rotor configuration,
Figure 7 shows diagrammatically a three phase commutation circuit for the
motor,
Figure 8 shows one piece of a two piece mould for forming a stator piece of
the
motor, and
Figure 9 is a partial view of a stator piece showing a further pole
configuration.
BEST MODES FOR CARRYING OUT THE INVENTION
In one preferred form of the invention the rotor 2 is located externally of
the
stator 7 as indicated in Figure 1. As is mentioned later a variety of known
rotor
configurations may be used. Rotor 2 as shown comprises an annular ring of
axially
oriented magnetic material pieces 3A interspersed with similarly configured
permanent
magnets 3B (not shown in Figure 1). The permanent magnets 3B are magnetised in
the
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circumferential direction with successive magnets being magnetised with
opposite
polarities. The annular ring of magnetic components is supported by a
cylindrical non-
magnetic backing wall 4, preferably formed from a plastics material integrally
with a
base 5 and hub which carries the rotor shaft 6. The shaft is supported by
bearings
mounted either conventionally in a housing supporting the stator or within the
appliance which the motor is to power. An example of the latter type of
mounting in
a clothes washing machine is disclosed in US patent 5,150,589.
An alternative rotor configuration is shown in Figure 6. In this construction
the
rotor 200 comprises an annular ring of magnetisable and magnetically pemeable
material, rotating exteriorly of the stator. In one embodiment of this
configuration a
number of circumferentially orientated magnets 202 are disposed around the
internal
periphery of the rotor. The permanent magnets 202 are magnetised in the radial
direction, alternating in polarity, and abut an annular soft magnetic material
return path
204 to complete the magnetic circuit. The annular ring of magnetic components
is
supported by a cylindrical backing wall 206, preferably formed from a plastics
material
integrally with a base 208 and hub which carries the rotor shaft.
The motor stator 7 (see also Figure 2) is fabricated by two complementary
facing pieces 8 and 9 formed from material of high magnetic permeability
spaced
axially apart by bridging cores 10, also formed of a highly permeable
material. Each
stator piece 8 and 9 includes a number of spaced apart and axially directed
poles 12 and
13 respectively, located at the periphery. The stator poles are of the claw
pole type.
The bridging cores 10, of which there are six in the embodiment shown, are
symmetrically disposed about the axis of the stator and located in proximity
to the
stator poles. The purpose of these cores is to allow magnetic flux to flow
from one
stator piece to the other. Each bridging core also conveniently forms the core
for a
corresponding stator winding 11.
The stator illustrated is a three "phase", 60 pole stator with two windings
per
phase. The two primary stator pieces 8 and 9 are, in the embodiment
illustrated, similar
in form and are assembled together face to face with their respective axially
directed
poles 12 and 13 facing the opposing stator piece with the relative rotational
orientation
of each stator piece being such as to allow the poles 12 of the upper piece to
locate
within the interspacing of the poles 13 of the lower piece. In the preferred
form the
interpole spacing exceeds the width of each pole and the axial length of each
pole is
extended such that the oppositely directed poles of the two stator pieces
overlap. This
can be seen in Figure 3.
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Each stator piece can be visualised as a circular plate 15 carrying at its
periphery
spaced apart claw poles 12 and 13 respectively. A cavity 16 is provided
centrally in
each plate to conserve material and to allow the passage of the rotor shaft.
Each pole
is oriented axially and in the preferred embodiment has a circumferential
width less
than the interpole spacing. Each pole stands proud of "plate" 15 and the pole
tips are
rebated to form a reduced area tip 17 which has the effect of reducing leakage
flux
between adjacent pole tips and/or the other stator piece. Other pole
configurations can
be adopted to minimise flux leakage. For example, the claw poles can be
tapered in
one or more ways. In Figure 8 a pole is shown tapered in two directions. First
the side
faces 211 and 212 may taper from the root of the pole to its outer radial
face. Second
inner face 213 may taper from where it joins the stator piece "plate' 15 to
the tip 17.
Further, in the stepped pole embodiment shown in Figure 2 the step may be a
ramped
rebate instead of assuming the right angled rebate form shown.
To ensure at least the bulk of the flux produced by each winding flows through
the poles proximate to that winding and not through the plate material to
other windings
it is necessary to provide regions of high reluctance in at least one of the
plates 1 S
between the bridging cores. Magnetically these regions appear as "slots" and
in the
preferred embodiment suitable slots 30 are provided in the lower stator piece
plate as
shown in Figure 2B. In theory the slots 30 could be air gaps but to retain the
unitary
structure of each stator piece an engineering strength low permeability
material is used.
Preferably this is moulded into the stator piece and also forms the stator hub
as shown
in Figure 8.
The stator must be formed in two pieces to allow the several internal windings
to be put in place during manufacture. The two pieces must be magnetically
linked to
provide flux paths between the two and the bridging cores adopted to achieve
this are
formed by providing on the inner face of one or both stator plate 1 S raised
"islands" 10
which on assembly of the two pieces abut with their opposite number on the
facing
piece to provide a magnetic core about which a winding may be placed. The
bridging
cores may be formed integrally with one of the stator pieces. Alternatively
some
bridging cores can be formed integrally with one piece while the others are
formed
integrally with the second piece. As a further alternative "half height"
bridging cores
may be formed in each stator piece which during assembly of the stator are
physically
brought together in series to complete the magnetic circuit. This alternative
construction is that shown in Figure 2. In this embodiment the stator pieces
are similar
but not identical since the bridging cores 10 must align while at the same
time allowing
for misalignment of the poles of the respective pieces. In yet a further
alternative the
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bridging cores may be formed separately and located with the plates 15 during
assembly.
This stator geometry allows a single winding to produce flux through several
poles. Each winding is separately wound on single bobbins 14 (see Figure 2C)
according to conventional winding techniques. The bobbins 14 are preferably
formed
from a plastics material and are shaped so as to fit about each bridging core
10.
In the three phase stator described two diametrically opposite windings are
connected together in parallel or in series and on commutation are excited in
series with
the windings of another phase to clause flux to flow in the stator in the
vicinity of the
excited windings. One flux path so produced is shown in Figures 3 and 5. Each
winding in this embodiment causes flux to flow through five poles in each
stator piece.
For the path shown the flux passes through the bridging cores 10 (flux segment
a) into
the plate of the upper stator piece (flux segment b), then into a pole of the
upper piece
(segment c), leaves the pole and crosses the motor air gap (segment d)
radially into the
physically most proximate soft magnetic material piece 3A of rotor 2 (shown in
Figure
5), passes circumferentially into and through adjacent permanent magnet 3B and
into
the magnetic material piece 3A on the opposite side of this magnet (segment e,
visible
in Figure 5), passes through the soft magnetic material piece in an axial
direction and
leaves the soft magnetic material piece to cross the air gap radially (segment
f) to the
closest pole on the lower stator piece, travels through that pole (segment g)
to the lower
stator plate where it proceeds radially (segment h) to return to the bridging
core 10 to
complete the flux circuit.
In the case of the alternative rotor construction shown in Figure 6 the flux
path
through the rotor differs somewhat. The flux leaves the pole and crosses the
motor air
gap (segment d, Figure 3) radially into the physically most proximate
permanent
magnet (202, Figure 6) circumferentially through the soft magnetic material
return path
(204, Figure 6) and back radially through an adjacent permanent magnet to
cross the
air gap radially (segment f, Figure 3) to the closest pole on the lower stator
piece.
Only one flux path for two poles is shown for purposes of clarity. In reality
flux
flows three dimensionally through all five poles in the upper stator piece and
all five
poles of the lower stator piece which are excited by winding 11.
In a two phase firing embodiment (refer to Fig 7), after commutation of motor
current through the windings of phase A and phase B, motor current is then
commutated to flow through the phase A and phase C windings to cause the
radially
directed flux produced at the periphery of the stator to move around the
periphery of
the stator in the desired direction. The permanent magnets in rotor 2 which
are
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alternately located between the soft magnetic material pieces 3A are attracted
or
repelled by the stator flux to cause the rotor to move in synchronism with the
rotating
stator flux. The supply of the winding current and the commutation of the
windings
can be carned out in a known manner using two semi-conductor switching devices
per
phase ("totem pole") in a bridge configuration between do rails with the
devices being
switched under the control of a micro-processor (not shown) which stores
sequences
of switching patterns which are caused to execute in a cyclical manner to
produce a
flux which rotates about the stator in one direction or other as selected.
Such stator
winding commutation control is described in US patents 4,540,921 (Boyd),
4,857,814
(Duncan) and WO 98/35429 (Boyd et al), particularly with reference to figures
1
(which corresponds to present Figure 7) and 2 of the latter.
The present invention provides a polyphase transverse flux do motor having
simple geometry which is relatively easy to fabricate. As opposed to some
prior art
proposals the stator geometry allows for a motor having a single air gap.
1 S In a motor according to the present invention the following relationship
holds:-
If the number of phases P = 2, 3, ..., N;
the number of windings per phase W = 1, 2, ..., M;
and the number of poles per winding PW = 2, 4, ..., L;
then the number of stator poles SP is given by SP = P x W x PW; and
the number of rotor poles RP = SPEW per phase.
It is advantageous to make the number of windings per phase even to balance
the radial forces acting when the phase is excited and in some cases it is
desirable for
the number of poles per winding to be even when an opposed pair claw pole
geometry
is chosen. However, it is possible to have an odd number of poles per winding,
for
example 9.
When the number of windings per phase is two or more the windings may
connected in either series or in parallel. However a parallel connection may
have the
advantage in that it will reduce the radial force dissymmetry in the presence
of air gap
dissymmetries.
In the embodiment illustrated and described three phases have been chosen with
two windings per phase and 10 poles per winding. This results in a stator
having 60
poles and the rotor to use with the stator must have either 62 or 58 poles.
It is convenient for manufacturing purposes to locate the motor commutation
electronics in physical association with the motor. This is shown in Figure 4
where the
electronics are located on a printed circuit board 20 which is fixed to stator
piece 8.
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The stator pieces can be formed by pressing a soft magnetic material powder,
such as iron powder, into a mould 40 shaped for the purpose (see Figure 8).
For one
of the two stator pieces a former 41 made of low permeability material is
preferably
used to provide the high reluctance slots 30 in the stator pieces. The former
remains
as an integral part of the stator piece on removal of the piece from the
mould. This
former can also function as a bearing retainer. Pressing the soft magnetic
material
powder around the former allows very accurate concentricity between the
bearing and
the air gap. In the preferred embodiment the other stator piece does not
require slots
and no former is required.
