U.S. patent application number 09/286966 was filed with the patent office on 2002-06-06 for axial flux machine and method of fabrication.
Invention is credited to JANSEN, PATRICK LEE, KLIMAN, GERALD BURT, STEPHENS, CHARLES MICHAEL.
Application Number | 20020067091 09/286966 |
Document ID | / |
Family ID | 23100906 |
Filed Date | 2002-06-06 |
United States Patent
Application |
20020067091 |
Kind Code |
A1 |
KLIMAN, GERALD BURT ; et
al. |
June 6, 2002 |
AXIAL FLUX MACHINE AND METHOD OF FABRICATION
Abstract
An axial flux machine includes a rotatable shaft; at least one
rotor disk coupled to the rotatable shaft; at least one permanent
magnet supported by the at least one rotor disk; at least one
stator extension positioned in parallel with the at least one rotor
disk; at least two molded iron pole elements attached to the at
least one stator extension and facing the at least one permanent
magnet; and at least two electrical coils, each wrapped around a
respective one of the at least two molded iron pole elements.
Inventors: |
KLIMAN, GERALD BURT;
(NISKAYUNA, NY) ; STEPHENS, CHARLES MICHAEL;
(PATTERSONVILLE, NY) ; JANSEN, PATRICK LEE;
(ALPLAUS, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY
CRD PATENT DOCKET RM 4A59
P O BOX 8, BUILDING K-1-SALAMONE
SCHENECTADY
NY
12301
|
Family ID: |
23100906 |
Appl. No.: |
09/286966 |
Filed: |
April 6, 1999 |
Current U.S.
Class: |
310/156.01 |
Current CPC
Class: |
H02K 21/24 20130101;
H02K 1/14 20130101; H02K 29/03 20130101 |
Class at
Publication: |
310/156.01 |
International
Class: |
H02K 015/12 |
Claims
1. An axial flux machine comprising: a rotatable shaft; at least
one rotor disk coupled to the rotatable shaft; at least one
permanent magnet supported by the at least one rotor disk; at least
one stator extension positioned in parallel with the at least one
rotor disk; at least two molded iron pole elements attached to the
at least one stator extension and facing the at least one permanent
magnet; at least two electrical coils, each wrapped around a
respective one of the at least two molded iron pole elements.
2. The machine of claim 1 wherein each of the at least two molded
iron pole elements comprises a base portion around which a
respective one of the at least two electrical coils is wrapped and
an shield portion extending over at least part of the respective
one of the at least two electrical coils.
3. The machine of claim 1 wherein each of the at least two
electrical coils is wrapped in a substantially round
configuration.
4. The machine of claim 1 wherein the at least one stator extension
includes at least two openings and wherein the at least two molded
iron pole elements are situated within the at least two
openings.
5. The machine of claim 1 wherein the at least one rotor disk
includes at least two rotor disks; wherein the at least one
permanent magnet supported by the at least one rotor disk includes
at least two permanent magnets, each permanent magnet supported by
a respective one of the at least two rotor disks and facing the
other permanent magnet; wherein the at least one stator extension
is positioned in parallel with and between the at least two rotor
disks; wherein the at least two molded iron pole elements attached
to the at least one stator extension comprise at least four molded
iron pole elements with at least two of the at least four molded
iron pole elements being attached to an opposite side of the stator
extension than at least two others of the at least four molded iron
pole elements; and wherein the at least two electrical coils
comprise at least four electrical coils, each wrapped around a
respective one of the at least four molded iron pole elements.
6. The machine of claim 5 wherein at least two of the at least four
molded iron pole elements are positioned in directly opposing
positions.
7. The machine of claim 6 wherein the at least two of the at least
four molded iron pole elements positioned in directly opposing
positions are coupled.
8. The machine of claim 1 wherein the at least one stator extension
comprises at least two stator extensions, each positioned on an
opposite side of the at least one rotor disk; wherein the at least
two molded iron pole elements attached to the at least one stator
extension comprise at least four molded iron pole elements with at
least two being attached to a respective stator extension and
facing the at least one permanent magnet; and wherein the at least
two electrical coils comprise at least four electrical coils, each
wrapped around a respective one of the at least four molded iron
pole elements.
9. The machine of claim 8 wherein the at least one permanent magnet
extends through the rotor disk.
10. The machine of claim 8 wherein the at least one permanent
magnet comprises at least two permanent magnets, each facing a
respective one of the at least two stator extensions.
11. The machine of claim 10 further including at least two
retainers each retaining a respective one of the at least two
permanent magnets.
12. The machine of claim 2 wherein the shield portions of the at
least two molded iron pole elements are shaped to provide an uneven
air gap between the at least two molded iron pole elements and at
least one permanent magnet.
13. The machine of claim 2 wherein the shield portions of the at
least two molded iron pole elements are bifurcated.
14. The machine of claim 2 wherein the shield portions of the at
least two molded iron pole elements are asymmetrically
bifurcated.
15. The machine of claim 2 wherein the shield portions of the at
least two molded iron pole elements each include at least one
notch.
16. The machine of claim 2 wherein the shield portions of the at
least two molded iron pole elements each include a plurality of
radially extending notches.
17. The machine of claim 16 wherein adjacent molded iron pole
elements form slots between adjacent shield portions and wherein
the slots and radially extending notches are spaced at
substantially uniform intervals.
18. The machine of claim 17 wherein the at least one permanent
magnet comprises at least one permanent magnet with a magnetization
skew trajectory.
19. The machine of claim 18 wherein the magnetization skew
trajectory of the at least one permanent magnet is represented by
the following equation: 4 r ( ) = r 0 2 + r i 2 ( 1 - ) for 0 .
wherein .alpha. represents a pitch angle between the substantially
uniform intervals; .theta. represents a trajectory coordinate;
r.sub.i represents a radius between a center point of the at least
one stator extension and an inner surface of the pole element, and
r.sub.o represents a radius between the center point of the at
least one stator extension and an outer surface of the pole
element.
20. The machine of claim 15 wherein the at least one notch
comprises a radially extending notch, wherein adjacent molded iron
pole elements form slots between adjacent shield portions, and
wherein the slots and radially extending notches are spaced at
substantially uniform intervals.
21. The machine of claim 20 wherein the notches and the slots are
geometrically skewed.
22. The machine of claim 21 wherein geometrical skews of the
notches and slots are represented by the following equation: 5 r (
) = r 0 2 + r i 2 ( 1 - ) for 0 . wherein .alpha. represents a
pitch angle between the substantially uniform intervals; .theta.
represents a trajectory coordinate; r.sub.i represents a radius
between a center point of the at least one stator extension and an
inner surface of the pole element, and r.sub.o represents a radius
between the center point of the at least one stator extension and
an outer surface of the pole element.
23. An axial flux machine comprising: a rotatable shaft; at least
one rotor disk coupled to the rotatable shaft; at least one
permanent magnet supported by the at least one rotor disk; at least
one stator extension positioned in parallel with the at least one
rotor disk; at least two molded iron pole elements attached to the
at least one stator extension and facing the at least one permanent
magnet; at least two electrical coils, each wrapped around a
respective one of the at least two molded iron pole elements, each
of the at least two molded iron pole elements comprises a base
portion around which a respective one of the at least two
electrical coils is wrapped and an shield portion extending over at
least part of the respective one of the at least two electrical
coils.
24. The machine of claim 23 wherein each of the at least two
electrical coils is wrapped in a substantially round configuration
and wherein the at least one stator extension includes at least two
openings and wherein the at least two molded iron pole elements are
situated within the at least two openings.
25. The machine of claim 23 wherein the shield portion of the at
least one molded iron pole element is shaped to provide an uneven
air gap between the at least one molded iron pole element and at
least one permanent magnet.
26. A method for fabricating an axial flux machine comprising:
coupling at least one rotor disk supporting least one permanent
magnet to a rotatable shaft; attaching at least two molded iron
pole elements to the at least one stator extension; positioning
each of at least two electrical coils around respective ones of the
at least two molded iron pole elements; positioning at least one
stator extension in parallel with the at least one rotor disk.
27. The method of clam 26 wherein positioning the at least two
electrical coils occurs prior to attaching the at least two molded
iron pole elements to the at least one stator extension.
28. The method of claim 27 wherein positioning the at least two
electrical coils includes pre-winding the at least two electrical
coils.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to axial flux
machines.
[0002] Axial flux machines, sometimes referred to as disk,
axial-gap, or pancake motors, are presently used in appliances that
have low to modest power requirements such as video cassette
recorders and blenders, for example. Theoretically, high pole
numbers can be useful for motor drive cost reduction, but prior
motor designs such as those used in laminated claw pole motors and
radial flux electronically commutated motors have been complex and
expensive.
[0003] Thus, there is a particular need for a simple fabrication
process for providing an axial flux motor having increased power
density and efficiency.
BRIEF SUMMARY OF THE INVENTION
[0004] Briefly, in accordance with one embodiment of the present
invention, an axial flux machine includes a rotatable shaft; at
least one rotor disk coupled to the rotatable shaft; at least one
permanent magnet supported by the at least one rotor disk; at least
one stator extension positioned in parallel with the at least one
rotor disk; at least two molded iron pole elements attached to the
at least one stator extension and facing the at least one permanent
magnet; and at least two electrical coils, each wrapped around a
respective one of the at least two molded iron pole elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The features of the invention believed to be novel are set
forth with particularity in the appended claims. The invention
itself, however, both as to organization and method of operation,
together with further objects and advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings, where like numerals
represent like components, in which:
[0006] FIGS. 1 and 2 are a side view of a prior art machine and a
front view of a prior art stator extension including electrical
coils.
[0007] FIGS. 3 and 4 are side and front views of a stator extension
including molded iron pole elements and electrical coils 10
according to one embodiment of the present invention.
[0008] FIG. 5 is a side view of a molded iron pole element
according to another embodiment of the present invention.
[0009] FIGS. 6-9 are side views of areas, flux paths, and forces of
the molded iron pole elements.
[0010] FIG. 10 is a side view of a machine according to another
embodiment of the present invention.
[0011] FIG. 11 is a side view of two molded iron pole elements
arranged in a configuration useful for the embodiment of FIG.
10.
[0012] FIG. 12 is a side view of a machine according to another
embodiment of the present invention.
[0013] FIGS. 13 and 14 are side views of a machine according to
another embodiment of the present invention.
[0014] FIGS. 15 and 16 are perspective views of molded iron pole
elements according to several embodiments of the present
invention.
[0015] FIG. 17 is a front view of a molded iron pole element
according to another embodiment of the present invention.
[0016] FIG. 18 is a front view of a molded iron pole element
according to another element of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] FIGS. 1 and 2 are a side view of a prior art, commercially
available and a front view of a prior art stator extension 18
including electrical coils 24. The machine includes disks 12 and 14
coupled to a rotatable shaft 10. Rotor disk 12 supports at least
one permanent magnet 16. Stator extension 18 is positioned in
parallel with the rotor disk and supports a plurality of electrical
coils 24 and 25. One such design, for example, is described in J.
R. Hendershot Jr. et al., Design of Brushless Permanent-Magnet
Motors, pp. 2-10 through 2-12 (Magna Physics Publishing and Oxford
University Press 1994).
[0018] The at least one permanent magnet 16 comprises either (a) a
plurality of separate, axially magnetized, thin trapezoids, or (b)
a continuous ring with a multipole pattern impressed thereon. Rotor
disk 12 typically comprises a solid iron disk which serves as a
magnetic flux path and provides mechanical support.
[0019] Stator extension 18 typically comprises a printed circuit
board coupled to a stator mount 20. Electrical coils 24 and 25 have
trapezoidal shapes formed of straight radial segments 30 and 32,
where electromagnetic force is developed, and large end windings 34
and 36 to complete the circuit at inner and outer radii. The radial
extent of permanent magnet 16 is typically the length of radial
segments 30 and 32.
[0020] Electrical coils 24 and 25 are coupled by electrical
connections 26 which can be soldered to the printed circuit board
with solder 28, for example. The electrical coils in the
embodiments of FIGS. 1 and 2 are precision wound around pins or
temporary support pieces (not shown) to minimize space requirements
and to avoid lengthening the magnetic gap between disks 12 and 14.
To prevent or reduce eddy and circulating current losses, the
electrical coils are finely stranded and transposed. Disk 14 serves
as a solid steel rotating back iron (yoke) which provides an air
gap flux path (from disk 12 to disk 14) with low core losses
because the air gap flux appears stationary. Permanent magnet 16
must be sufficiently thick to drive adequate magnetic flux density
through the electrical coil thicknesses, the stator extension
thicknesses, and the mechanical clearances around the stator
extension.
[0021] Thus the prior art axial flux machine technology has a
number of limitations: large magnet volume, winding losses from end
windings and eddy currents, low power density from poor utilization
of volume, a complex stator fabrication process, and
inapplicability to high power applications.
[0022] FIGS. 3 and 4 are side and front views of a stator extension
118 including molded iron pole elements 122 and 123 and electrical
coils 124 and 125 wound around the molded iron pole elements
according to one embodiment of the present invention. At least two
molded iron pole elements 122 and 123 (and, more preferably, at
least four molded iron pole elements) are attached to the at least
one stator extension and face permanent magnet 16 (shown in FIG.
1). The molded iron pole elements may comprise bonded iron powder
or pressed flakes, for example, and preferably are of low
electrical conductivity.
[0023] A molded iron pole element is useful for reducing the
effective gap (that is, the distance between the permanent magnet
and the nearest magnetically conductive material) because the
molded iron pole element provides a good flux path and thus reduces
the amount of permanent magnet material required to drive adequate
magnetic flux density. Furthermore, molded iron pole elements can
be used to provide proper division of the poles. Such proper
division would be difficult to achieve using laminations. If
cylindrical molded iron pole elements are used, molding is
simplified.
[0024] In the embodiment of FIG. 3, each molded iron pole element
122 comprises a base portion 38 around which a respective one of
the electrical coils 124 is wrapped and a shield portion 40
extending over at least part of the respective electrical coils.
The shield portion serves to extend the magnetic flux collection
area of the molded iron pole element and shields the electrical
coil from the air gap flux. When the air gap flux does not flow
through the electrical coils, the electrical coils do not need to
be transposed and can be random-would instead of precision-wound.
Precision winding is still useful for minimizing the dimensions and
minimizing winding losses. In the embodiment of FIG. 3, the molded
iron pole elements can be glued to the stator extension.
[0025] If a high permeability pole element is used, the electrical
coils no longer need to be trapezoidal. In one embodiment, the base
portion of the pole element and the electrical coil are both
circular in cross section (a feature that simplifies the coil
winding process). The high permeability pole elements, by the
action of the large area shield portion 40 in communication with
the small area base portion 38, concentrate the magnetic flux as
passed to coils 124 and allow the developed length of the coils to
be shorter, thereby reducing losses. The entire coil can be used
for torque production (rather than just the straight radial
segments of FIG. 2) because torque is developed on the pole
elements instead of the coils.
[0026] In one embodiment, fabrication of the machine includes the
following steps: coupling at least one rotor disk 12 supporting at
least one permanent magnet 16 to a rotatable shaft 10; attaching at
least two molded iron pole elements 122 and 123 to the at least one
stator extension; positioning each of at least two electrical coils
124 and 125 around respective ones of the at least two molded iron
pole elements; and positioning at least one stator extension 118 in
parallel with the at least one rotor disk. For ground insulation,
the molded iron pole elements can be dipped or fitted with
insulation pre-forms (not shown).
[0027] The above fabrication embodiment is particularly useful
because the electrical coils can be pre-wound prior to being
positioned around the molded iron pole elements. Further, such
windings are not limited to the conventional trapezoidal shapes and
may be substantially round, for example. In one embodiment, after
the windings are positioned around the molded iron pole elements,
the molded iron pole elements can conveniently be attached to the
stator extension by gluing or force-fitting for example.
[0028] In the present invention, it is not necessary to wind
individual electrical coils and connect the coils with electrical
connections on a printed circuit board. If round electrical coils
are used in the present invention, the electrical coils can be
bobbin wound in a continuous string in the proper polarity and then
slid over the molded iron pole elements. The only connections that
need to be made are to an external circuit, and suitable guides for
these connections (not shown) can be molded into the stator
extension.
[0029] FIG. 5 is a side view of a molded iron pole element 222
according to another embodiment of the present invention. In this
embodiment, openings 19, extend completely or partially through
stator extension 118, and the molded iron pole elements 222 are
force fit or otherwise situated within the respective openings.
[0030] One process for assembling a stator extension 118 with
openings 19 (FIG. 5), molded iron pole elements 122 (FIG. 3), and
electrical coils 124 begins by punching and interlocking standard
lamination sheet stock to produce a compact stack of laminations to
form a stator extension 118. The punching process includes punching
holes 19 to receive molded iron pole elements 122 and shaft 10. The
stator extension can be annealed in a conventional manner.
[0031] The molded iron pole elements 122 can be molded in a small
press in their final form. A cylindrical base portion 38, upon
which the coil will be mounted and which will be inserted into the
holes provided in the stator extension, and a trapezoidal-like
shield portion 40 can be formed as an integral unit. A plurality of
such pole elements can be molded simultaneously in a single molding
step if desired. If more convenient, the cylindrical and
trapezoidal-like portions may be molded separately and then joined
together. In such embodiments, it is preferred to mold the
trapezoidal-like shield portion with a circular hole to receive the
cylindrical base portion.
[0032] The molded iron pole elements can then be positioned and
aligned in a jig (not shown). In one embodiment, the jig includes a
plate into which cavities have been formed to match the
trapezoidal-like shape of the shield portions. Thus the pole
elements are held in the proper orientation and spacing. Prior to
being placed in the jig the pole elements can be dipped in
insulating varnish (not shown) or coated by other conventional
means for ground insulation. Insulation pre-forms (not shown) can
be placed over the cylindrical base portions as an alternative or
additional ground insulation.
[0033] The electrical coils can be wound on a mandrel (not shown)
in bunches, containing the proper number of turns for each pole
element, in a continuous fashion with enough wire between them to
reach from pole to pole. The electrical coils are then slid off the
mandrel and onto the pole elements one at a time. Every other
electrical coil is flipped over before being slid onto the pole
element to form pole pairs. Depending on whether the machine is to
be single phase, two phase, or three phase, one or more pole
elements may be skipped to be subsequently wound with a different
phase. In another embodiment, adjacent pole elements may be wound
with the same polarity to create a longer pole pitch. An
alternative technique is to spin the wire directly onto the pole
elements using conventional apparatus.
[0034] Next the stator extension is positioned over the parts of
the cylindrical base portions of the molded iron pole elements that
are not covered by electrical coils. This step may be performed
with a small amount of clearance and an adhesive or the step can be
performed by force fitting. The completed assembly can be
impregnated with varnish and baked if desired.
[0035] FIGS. 6-9 are side views of areas A.sub.1 and A.sub.2, flux
paths B.sub.1 and B.sub.2, and forces F.sub.1 and F.sub.2 of the
molded iron pole elements. If, as shown in FIGS. 6 and 8, no
leakage flux (B.sub.L) exists, a net axial force on a molded iron
pole element will be zero (that is, F.sub.1 balances F.sub.2).
Typically, however, the force equilibrium will be unstable.
Mechanical stabilization can be provided by stator extension 118
(shown in FIGS. 3 and 5). Conventional stator extensions comprise
printed circuit boards. With the present invention, a simpler
construction can be used. In one embodiment, a material of
composition such as fiberglass can be used without patterned
circuit interconnections. Because magnetic flux is carried through
the molded iron pole elements, the thickness of the stator
extension is not critical.
[0036] In the embodiment of FIG. 8, when no leakage flux B.sub.L is
present, flux B.sub.2 is equal to flux B.sub.1 multiplied by the
ratio of the area A.sub.1 of the surface including shield portions
40 over the area A.sub.2 of the surface of the base portion 38.
F.sub.1 and F.sub.2 can be approximated by the following
equations:
F.sub.1.congruent.(.mu..sub.o/2)*B.sub.1.sup.2*A.sub.1,
F.sub.2.congruent.(.mu..sub.o/2)*B.sub.2.sup.2*A.sub.2,
.congruent.(.mu..sub.o/2)*(A.sub.1/A.sub.2).sup.2*B.sub.1.sup.2*A.sub.2,
and
F.sub.2.congruent.(A.sub.1/A.sub.2)*F.sub.1,
[0037] wherein .mu..sub.o represents permeability of free space
(that is .mu..sub.o=4.pi.10.sup.-7 Henries per meter).
[0038] When leakage flux B.sub.L is present, the forces F.sub.1 and
F.sub.2 on the surfaces of the molded iron pole elements 122, 222
becomes unbalanced and can result in a mechanical instability or
noise problem. In the embodiment of FIG. 9, shield portions 40 can
be used for balancing the forces (at least under no load
conditions) because, as discussed above, the magnetic force is
proportional to the square of the flux density in the respective
area ratios A1 and A2. Thus the area ratios can be adjusted to
balance the expected forces. Although armature reaction will tend
to unbalance the forces by distortion of the fields, due to the
large effective gap of the magnets, any effect is minimal.
[0039] FIG. 10 is a side view of a machine according to another
embodiment of the present invention wherein a double sided geometry
is used for stator extension 118. The conventional geometry of FIG.
1 may not be appropriate for high power machines. In the embodiment
of FIG. 10, at least two permanent magnets 16 and 116 are situated
on two rotor disks 112 and 114 and facing each other. Stator
extension 118 is positioned in parallel with and between the at
least two rotor disks. Two molded iron pole elements 122 and 123
(with electrical coils 124 and 125) are attached to an opposite
side of stator extension 118 than two other molded iron pole
elements 322 and 323 (with electrical coils 324 and 325). As
further shown in FIG. 10 by molded iron pole elements 122 and 322
and molded iron pole elements 123 and 323, the molded iron pole
elements can be positioned back-to-back in directly opposing
positions.
[0040] FIG. 11 is a side view of two molded iron pole elements 122
and 322 arranged in a configuration useful for the embodiment of
FIG. 10. In this embodiment, the molded iron pole elements are
positioned in directly opposing positions and coupled in any
appropriate manner such as gluing or a snap configuration of
portions 42 and 44 with protrusions 46 and 48, for example.
[0041] FIG. 12 is a side view of a machine according to a double
stator embodiment of the present invention. In this embodiment each
of two stator extensions 418 and 518 positioned on an opposite side
of the a rotor disk 13. Molded iron pole elements (shown as 422,
423, 522, and 523) are attached to the stator extensions and face
permanent magnet 216. In the embodiment of FIG. 12, at least one
permanent magnet 216 extends through the rotor disk. The rotor disk
in this embodiment comprises a non-magnetic material suitable for
high speed operation such as aluminum.
[0042] FIGS. 13 and 14 are side views of a machine according to
another embodiment of the present invention which is similar to
that of FIG. 12 except that instead of at least one permanent
magnet which extends through the rotor disk, permanent magnets 316
and 416 are mounted on opposite sides of a rotor disk 113. In one
embodiment the rotor disk comprises steel. If the magnets are
mounted on a central rotor disk 113 and if the magnet polarities
are in sequence, magnetic flux will travel directly across the
rotor disk. Thus the thickness of the rotor disk is a function only
of mechanical needs.
[0043] FIG. 13 additionally illustrates optional retainer rings 50
and 51 for retaining the permanent magnets. The retainer rings are
useful at higher machine speeds. Appropriate retainer materials
include aluminum or stainless steel, for example.
[0044] If the stator extensions of FIG. 14 comprise a material such
as silicon steel or molded iron, then they are more robust than
composition board and have better heat transfer. Because the stator
extension flux is ac and in the plane of the rotor disk, the stator
extensions can be made of laminations 918. Molded iron pole
elements are still preferred due to varying flux directions and
useful complex shapes. At high speeds, high frequency losses in
molded iron are lower than in laminations, and molding iron is a
lower cost process than forming elements from thin laminations.
FIG. 14 further shows a stator frame 62, end shields 66 and 68, and
bearings 60.
[0045] The electrical coils which form the stator windings in any
of the above embodiments of the present invention may be of any
phase number including single, two (also referred to as
single-phase bifilar-wound) and three phase windings, for example.
Conventional control systems (not shown) can be used to control the
machines. For example, conventional Hall sensors (not shown) can be
positioned on and/or between the molded iron pole elements to
control commutation. Hall sensors can be positioned directly in the
active area of a magnet or near the radial ends of the magnet where
leakage flux could be detected.
[0046] FIGS. 15 and 16 are perspective views of molded iron pole
elements according to several embodiments of the present invention.
Techniques to reduce reluctance torque (cogging) in radial flux
motors were disclosed with respect to electronically commutated
motors in commonly assigned Harms et al., U.S. Pat. No. 4,933,584.
In U.S. Pat. No. 4, 933,584, stator teeth include notches to mimic
the magnetic reluctance of the space between the teeth and a
helical rotor magnet imprint which forms a skew angle with the axis
of rotation. The notches in the stator teeth and a skewed magnetic
field formed by the magnetic elements reduce cogging between the
rotor and the stator during rotation.
[0047] In the present invention, to avoid deadpoints common with
single and two-phase machines, the molded iron pole elements 122 or
222 (FIGS. 3 and 5, respectively) can be shaped or notched to
provide an uneven air gap between the molded iron pole elements and
the permanent magnets as shown by molded iron pole element 822 in
the embodiment of FIG. 15. When the stator is unenergized, the
rotor disk will rest in a position enabling starting (particularly
in single phase machines). Direction of rotation can also be
controlled by the orientation of the uneven air gap. The embodiment
of FIG. 15 does not reduce cogging.
[0048] In the embodiment of FIG. 16, the shield portion of the at
least one molded iron pole element 922 includes two notches 930 and
932 to reduce cogging. An asymmetry is created, for example, by
recessing one of the faces 934 adjacent notch 932 to avoid
deadpoints common with single-phase machines. In another
embodiment, a single notch can be asymmetrically positioned to
avoid deadpoints.
[0049] In the embodiment of FIG. 17, molded iron pole element 1022
includes a plurality (shown as 2 for purposes of example) of
notches 1058 and 1060. Preferably in this embodiment, there is a
skew of the imprint on the magnetic element (not shown in FIG. 17)
with a trajectory, for example, represented by trajectory 1062. In
an alternative embodiment, as shown in FIG. 18, instead of being
present on the magnetic imprint, skews are present in one or more
notches 1260 of a molded iron pole element 1122 as well in slots
1264 between the molded iron pole elements.
[0050] Notches 1058 and 1060 in one embodiment are intended to
duplicate the magnetic reluctance of a slot 1064 between molded
iron pole elements and the slots and notches are positioned at
regular, uniform intervals about the periphery of the machine. In
this embodiment, the pitch angle between the slots or notches is
defined as .alpha..
[0051] The magnetization skew trajectory spans one pitch for a
single phase machine or one-half pitch for a three-phase machine,
for example. The trajectory of the skew in one embodiment follows a
linear relationship between the trajectory angle .theta. and the
incremental magnetic energy in the airgap.
[0052] For an axial flux machine, the skew trajectory can be
determined by assuming that the pole element edges and the notch
edges follow lines of constant angle. In this embodiment, the
proportion of the skew pitch area enclosed by .theta. can be
expressed in relation to .alpha. as: 1 A = 2 ( ( r ( ) ) 2 - r i 2
) ;
[0053] wherein r.sub.i represents the radius between a center point
1070 of the stator extension and an inner surface 1072 of the pole
element and A .sub..alpha. represents the skew pitch area as: 2 A =
2 ( r 0 2 - r i 2 ) ,
[0054] wherein r.sub.o represents the radius between the center
point and an outer surface 1072 of the pole element.
[0055] By combining these relationships, the radial trajectory
coordinate r corresponding to the peripheral trajectory coordinate
.theta. can be determined as follows: 3 r ( ) = r 0 2 + r i 2 ( 1 -
) for 0 .
[0056] Asymmetry in the magnetic pole configuration is useful for a
single phase machine for achieving resting positions that will
readily permit machine starting.
[0057] Although the above discussion relates to a magnetization
skew, the analysis and resulting formula is identical for the
geometrical skew calculations of the notches and slots of FIG. 18.
In some embodiments, a magnetization skew may be present in
combination with a geometrical skew with a net skew given by the
above equation.
[0058] While only certain preferred features of the invention have
been illustrated and described herein, many modifications and
changes will occur to those skilled in the art. It is, therefore,
to be understood that the appended claims are intended to cover all
such modifications and changes as fall within the true spirit of
the invention.
* * * * *