U.S. patent application number 09/541373 was filed with the patent office on 2002-05-30 for electromagnetic apparatus employing high permeability low conductivity element.
Invention is credited to Scott, Harold C..
Application Number | 20020063492 09/541373 |
Document ID | / |
Family ID | 24159311 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020063492 |
Kind Code |
A1 |
Scott, Harold C. |
May 30, 2002 |
ELECTROMAGNETIC APPARATUS EMPLOYING HIGH PERMEABILITY LOW
CONDUCTIVITY ELEMENT
Abstract
A pole for use in a magnetic circuit includes a shaft for
passing magnetic flux and a cap. The shaft has a face through which
at least some of the magnetic flux passes and to which the cap is
fixed. The cap is made of a permeable material and is formed to
impede eddy currents in the cap. The cap may have a top and bottom
surface joined by a shoulder to provide a distribution of magnetic
flux through the shoulder that is less concentrated than the flux
distribution through a 90-degree corner. The pole may be
incorporated into a generator, a motor, a linear motor, or a
portable engine-driven generator to increase the efficiency
thereof.
Inventors: |
Scott, Harold C.;
(Lafayette, CO) |
Correspondence
Address: |
Squire Sanders & Dempsey LLP
Two Renaissance Square
40 North Central Avenue Suite 2700
Phoenix
AZ
85004-4498
US
|
Family ID: |
24159311 |
Appl. No.: |
09/541373 |
Filed: |
March 31, 2000 |
Current U.S.
Class: |
310/261.1 |
Current CPC
Class: |
H02K 1/02 20130101; H02K
1/2786 20130101; H02K 21/227 20130101 |
Class at
Publication: |
310/261 |
International
Class: |
H02K 001/22 |
Claims
What is claimed is:
1. A pole for use in a magnetic circuit, the pole comprising: a. a
shaft for passing magnetic flux, the shaft comprising a face
through which at least a first portion of the magnetic flux passes;
and b. a cap fixed to the face, the cap comprising a permeable
material for passing at least a second portion of the magnetic
flux, the cap formed to impede eddy currents in the cap.
2. A generator that produces an output current, the generator
comprising: a. a stator having a plurality of stator poles and a
winding wound to produce the output current in accordance with
changes in magnetic flux in a pole of the plurality; and b. a rotor
to be driven by a provided engine in rotation with respect to the
stator, the rotor comprising: (1) a surface that faces the
plurality of stator poles; and (2) a multiplicity of rotor poles in
radial spaced apart relation affixed to the surface, at least one
of the rotor poles comprising the magnetic pole of claim 1 wherein:
(a) the shaft comprises a permanent magnet; and (b) the cap is
mounted to the magnet so as to form an air gap between the cap and
at least one stator pole, the cap comprising a particulate
permeable material disposed in a binder for reduced
interparticulate electrical conductivity.
3. A motor comprising: a. a stator having a plurality of stator
poles and a winding wound to accept an input current to effect
changes in magnetic flux in a pole of the plurality; and b. a rotor
that rotates with respect to the stator in response to the changes
in magnetic flux, the rotor comprising: (1) a surface that faces
the plurality of stator poles; and (2) a multiplicity of rotor
poles in radial spaced apart relation affixed to the surface, at
least one of the rotor poles comprising the magnetic pole of claim
1 wherein: (a) the shaft comprises a permanent magnet; and (b) the
cap is mounted to the magnet so as to form an air gap between the
cap and at least one stator pole, the cap comprising a particulate
permeable material disposed in a binder for reduced
interparticulate electrical conductivity.
4. A linear motor comprising: a. a stator having a plurality of
stator poles and a winding wound to accept an input current to
effect changes in magnetic flux in a pole of the plurality; and b.
a rotor that rotates with respect to the stator in response to the
changes in magnetic flux, the rotor comprising: (1) a surface that
faces the plurality of stator poles; and (2) a multiplicity of
rotor poles in radial spaced apart relation affixed to the surface,
at least one of the rotor poles comprising the magnetic pole of
claim 1 wherein: (a) the shaft comprises a permanent magnet; and
(b) the cap is mounted to the magnet so as to form an air gap
between the cap and at least one stator pole, the cap comprising a
particulate permeable material disposed in a binder for reduced
interparticulate electrical conductivity.
5. A portable engine-driven generator that produces an output
current, the engine-driven generator comprising an engine and a
generator, the generator comprising: a. a stator having a plurality
of stator poles and a winding wound to produce the output current
in accordance with changes in magnetic flux in a pole of the
plurality; and b. a rotor, in mechanical communication with the
engine so as to be driven by the engine in rotation with respect to
the stator, the rotor comprising: (1) a surface that faces the
plurality of stator poles; and (2) a multiplicity of rotor poles in
radial spaced apart relation affixed to the surface, at least one
of the rotor poles comprising the magnetic pole of claim 1 wherein:
(a) the shaft comprises a permanent magnet; and (b) the cap is
mounted to the magnet so as to form an air gap between the cap and
at least one stator pole, the cap comprising a particulate
permeable material disposed in a binder for reduced
interparticulate electrical conductivity.
6. The cap of claim 1 further comprising: a. a first surface facing
the shaft; and b. a second surface opposite to the first surface,
the second surface being joined to the first surface to form a
shoulder having a magnetic flux distribution less spatially
concentrated than a 90 degree corner.
7. The cap of claim 6 wherein the shoulder is shaped in accordance
with a radius.
8. The cap of claim 7 wherein the radius is less than or equal to a
thickness of the cap.
9. The cap of claim 1 further comprising a plurality of laminations
arranged to impede the flow of eddy current.
10. The cap of claim 1 wherein the permeable material comprises
iron.
11. The cap of claim 1 wherein the permeable material comprises
nickel.
12. The cap of claim 1 wherein the permeable material comprises
soft iron.
13. The cap of claim 12 wherein a majority by weight of the soft
iron particles have a size in the range from about 60 microns to
about 220 microns.
14. The cap of claim 12 further comprising a binder having lower
electrical conductivity than the particles so as to impede eddy
currents in the cap.
15. The cap of claim 12 wherein the particles are disposed in a
binder comprising a thermoplastic material.
16. The cap of claim 6 wherein the second surface is formed in
accordance with a radius for providing a more uniform air gap
measured between the cap and a provided second magnetic pole.
17. A method of forming the cap of claim 1 comprising: providing a
powder comprising particles having magnetically soft permeability
each coated with a binder, the binder having lower conductivity
than soft iron; molding the powder in the shape of the cap; and
heating the powder so that the powder forms a matrix of soft iron
particles disposed in a solid formed from the binder.
18. The method of claim 17 wherein the particles comprise iron.
19. The method of claim 17 wherein the particles comprise soft
iron.
20. The method of claim 17 wherein the particles comprise nickel.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to electromagnetic apparatus
and to elements in the magnetic circuits of such apparatus for
reducing eddy currents.
[0002] In general, electromagnetic apparatus including machines for
converting between the mechanical and electrical energy are well
known, e.g., motors and generators. Such conversion machines
typically comprise a rotor and stator arranged for relative motion,
e.g., rotation or linear displacement. In an electric motor,
varying magnetic fields are typically generated by the stator.
These fields interact with a magnetic field associated with the
rotor, imparting motion to the rotor, e.g., causing the rotor to
turn. Conversely, in a generator, the rotor is driven by a
mechanical energy source (e.g., a motor or engine) and generates a
magnetic field (using either permanent magnets or windings), which
interacts with windings formed on the stator. As the magnetic field
of the rotor intercepts the windings on the stator, an electrical
current is induced in the stator windings. The induced current is
typically applied to a bridge rectifier, regulated, and provided as
a DC output power signal from the generator. The output of the
bridge rectifier may be applied to an inverter to provide an AC
output power signal. Electromagnetic apparatus includes machines
that operate with direct current, single phase alternating current,
or polyphase alternating current. Such apparatus may operate with
relatively continuous motion (e.g., a turbine driven generator) or
intermittent motion (e.g., a stepper motor or a linear positioning
motor).
[0003] Examples of portable engine driven generators (gensets) are
described in U.S. Pat. Nos. 5,929,611 issued to Scott et al. on
Jul. 27 1999; 5,625,276 issued to Scott et al. on Apr. 29, 1997;
5,705,917 issued to Scott et al. on Jan. 6, 1998; 5,780,998 issued
to Scott et al. on Jan. 14, 1998; 5,886,504 issued to Scott et al.
on Mar. 23 1999; 5,900,722 issued to Scott et al. on May 4, 1999;
and 5,929,611 issued to Scott et al. on Jan. 27, 1999; all commonly
assigned with the present invention. Portable power conversion
systems find particular utility as power sources and vehicular
battery charger/jump start units. Power may be used for lights,
small appliances, or in connection with recreational vehicles.
These uses typically arise at construction or camping sites.
[0004] Generators that use permanent magnets to develop the
requisite magnetic field tend to be lighter and smaller than
generators that use field windings for that purpose. However, the
power supplied by a permanent magnet generator has historically
been difficult to regulate or control. The voltage provided by a
generator may vary significantly according to the rotational speed
of the rotor (e.g., measured in RPM). In addition, this output
voltage tends to vary inversely with the current delivered so that,
as the current delivered to a given load increases, the output
voltage provided by the generator across the load decreases.
[0005] Rotors employing high energy product magnets and consequence
poles are also known. Such rotors are described in, for example,
the aforementioned U.S. Pat. No. 5,705,917. That rotor employs a
body of soft-magnetic material carrying a plurality of permanent,
high energy product magnets. These magnets may have a flux density
of at least on the order of five kilogauss and may be suitably
formed of a rare earth alloy such as neodymium iron boron, or
samarium cobalt. These magnets are disposed on a peripheral surface
of the body of the rotor, mounted in recesses (also called insets)
formed in the rotor surface. The portions of the soft magnetic
rotor body between the recesses form respective consequence
poles.
[0006] Rare earth materials tend to be extremely expensive.
Accordingly, it is desirable to minimize the amount of these
materials used. However, at the same time, it is desirable to
generate relatively high flux densities. In the preferred
embodiment of the aforementioned U.S. Pat. No. 5,705,917, the
magnets employed are relatively thin (e.g. about {fraction (1/10)}
of an inch thick) and present a relatively large area (e.g., about
3/4 of an inch by about one inch) to minimize the amount of high
energy product magnet used. By maximizing the area of the permanent
magnet relative to the area of the consequence pole, a desirable
total flux is achieved with a smaller diameter core. Less weight
and less magnetic material are typically used with a smaller
diameter core.
[0007] In addition, it is also known that permanent magnets are
subject to demagnetization when exposed to high temperatures and/or
high magnetic flux. Heating permanent magnets to near their Curie
temperature can cause demagnetization and result in loss of other
performance characteristics. Permanent magnets installed in
conventional rotors are susceptible to physical damage and/or
demagnetization from eddy current heating and heat associated with
mechanical vibrations induced from air gap harmonics. Such heat may
be particularly evident at the permanent magnet surface (e.g.,
during exposure to flux changes across the air gap). In addition,
permanent magnets on rotors are subject to the electromagnetic
fields generated by current induced in the stator windings. Shorts
in simple turn-to-turn or phase-to-phase stator windings may
produce dramatic heating of magnets installed in conventional
rotors and thus lead to demagnetization.
[0008] Attempts have been made to a avoid demagnetization of rotor
magnets due to electromagnetic fields associated with current flow
in the stator windings. For example, U.S. Pat. No. 5,298,827 issued
to Sugiyama on Mar. 29, 1994 describes a permanent magnet
dynamo-electric machine rotor in which a ferromagnetic material
(e.g., high magnetic permeability soft iron) is attached to the
outer magnetic pole face of each of the permanent magnets. The
magnetic flux generated by the stator windings passes through the
ferromagnetic material so that demagnetization of the permanent
magnets caused by flux generated by the stator windings can be
reduced.
[0009] The flux path or magnetic circuit formed in a conventional
electromagnetic machine is generally supported by high permeability
materials formed as stacks of laminations electrically insulated
from each other. Electrical insulation interrupts eddy current
flow, reducing energy losses and avoiding localized resistive
heating in the regions of the eddy currents. The flow of flux
between laminations is inhibited. Because conventional laminations
are generally planar, electromagnetic machine design has been
limited to implementing flux paths in two dimensions through
laminated members.
SUMMARY OF THE INVENTION
[0010] A pole for use in an electromagnetic apparatus in an
implementation according to various aspects of the present
invention has a magnetic circuit that includes a shaft for passing
magnetic flux and a cap. The shaft has a face through which at
least some of the magnetic flux passes and to which the cap is
fixed. The cap is made of a permeable material and is formed to
impede eddy currents in the cap.
[0011] By forming the cap of material having relatively low
conductivity (e.g., an insulator or substantially nonconductive
material), energy losses in eddy currents may be reduced or
avoided. The heat generated by such eddy currents may be reduced or
avoided. The detrimental consequences of local heating (e.g.,
tempering, expansion, convection, infrared radiation, etc.) due to
such eddy currents is consequently reduced or avoided. In one
implementation, the cap is formed from a particulate permeable
material disposed in a binder. A method of forming the cap may
include the steps of: (a) providing a powder comprising particles
having magnetically soft permeability each coated with a binder,
the binder having lower conductivity than soft iron; (b) molding
the powder to form the cap; and (c) heating the powder so that the
powder forms a matrix of soft iron particles disposed in a solid
formed from the binder.
[0012] The cap may have a top and bottom surface joined by a
shoulder to provide a distribution of magnetic flux through the
shoulder that is less concentrated than the flux distribution
through a 90-degree corner. When concentrations of flux are
distributed, eddy current losses may be lower and heating due to
eddy current losses may be reduced or avoided.
[0013] The pole may be incorporated into any electromagnetic
apparatus to increase an operating efficiency, for example, a
motor, a linear motor, an alternator, a generator, or a portable
engine-driven generator.
[0014] An electromagnetic apparatus according to various aspects of
the present invention has a flux circuit supported at least in part
by a permeable material that is formed to impede eddy currents. In
an implementation for rotary motion, the apparatus may include a
first pair of permanent magnet poles spaced apart on a
circumference centered on the axis of rotation. Each pole may
include a permanent magnet and a cap as discussed above. The
implementation may further include a second pair of poles
implemented to face the first pair to complete a flux circuit. The
second pair of poles may be provided in magnetic communication with
a support and spaced apart on a second circumference centered on
the axis of rotation. The support may be formed at least in part by
a permeable material that is formed to impede eddy currents. When a
winding is provided between the second poles to pass through the
flux circuit, rotation of the first poles with respect to the
second poles may give rise to a current in the winding. When the
second poles are spaced apart in separate parallel planes, the
winding may be formed in a plane generally between the second poles
and encircling the support within a relatively small range of radii
from the axis of rotation.
[0015] By forming the support of a permeable material that impedes
eddy currents, the electromagnetic apparatus operates at higher
energy efficiency. Localized heating with consequential dimensional
changes can be avoided.
BRIEF DESCRIPTION OF THE DRAWING
[0016] The preferred exemplary embodiment of the present invention
will hereinafter be described in conjunction with the appended
drawing, where like designations denote like elements, and:
[0017] FIG. 1 is a perspective cross section of a three-phase
alternator according to various aspects of the present
invention;
[0018] FIG. 2 is a cross section of the alternator of FIG. 1;
[0019] FIG. 3 is a cross section of a cap used in the alternator of
FIGS. 1 and 2;
[0020] FIG. 4 is a cross section of the alternator of FIG. 1 taken
as indicated at plane A-A in an implementation having a cap formed
with laminations;
[0021] FIG. 5 is a cross section of a stator for use in an
alternate electromagnetic apparatus according to various aspects of
the present invention;
[0022] FIG. 6 is a perspective cross section view of a single-phase
alternator according to various aspects of the present invention;
and
[0023] FIG. 7 is a cross section of a portion of the alternator of
FIG. 6.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0024] Electromagnetic apparatus generally includes apparatus for
converting motion to and from electrical energy. In operation, an
electric current may produce a physical force (or vice versa) in
accordance with one or more magnetic circuits, each typically
including an electromagnet through which the electric current
passes. The physical force generally acts to move one or more
components of a magnetic circuit (as in a motor) or is externally
supplied to convert the force to a resulting current that passes
through a coil or other portion of a magnetic circuit (as in a
generator). The magnetic circuit typically includes components of
high magnetic permeability arranged in a closed path for passage of
magnetic flux. As implied above, a conventional motor or generator
may have numerous magnetic circuits each contributing to the
overall purpose of the apparatus. Components formed of high
permeability material provide a low impedance path for passage of
magnetic flux and so provide a mechanism for producing regions
(e.g., gaps) where magnetic flux density is high for efficient
interaction with other members of the electromagnetic
apparatus.
[0025] A magnetic pole (or herein simply a pole) includes any
structure used in a magnetic circuit for the passage of magnetic
flux (or herein simply flux). A pole may operate, for example, as a
central member of an electromagnet, a portion of a stator, or a
portion of a rotor, and may provide a face defining a gap as may be
necessary to allow motion among members of one or more magnetic
circuits as discussed above. A pole typically includes a portion
(herein called a shaft) that passes flux over some necessary or
desirable distance which may be linear or any convenient nonlinear
form. The shaft may (though not necessarily) include one or more
permanent magnets, electromagnets, or a combination of permanent
magnets and electromagnets. Any number of poles may be formed as a
physical unit or joined together to cooperate for passing flux and
providing any number of defined regions of high flux density,
perhaps arranged in numerous spacial orientations as desired.
[0026] A pole structure (herein also briefly called a pole)
operative as one or more magnetic poles, according to various
aspects of the present invention, suitably includes physical
characteristics for reducing eddy currents which may otherwise flow
in the structure. Eddy current flow may be reduced according to
various aspects of the present invention: (a) by including a cap in
a region of varying magnetic flux density and/or direction, the cap
formed of material having relatively low electrical conductivity;
(b) by including a cap as above formed of laminated material
arranged to impede eddy currents; (c) by providing a shoulder in a
region of the pole where high flux densities are expected, the cap
for lowering the spacial concentration of flux; or (d) by a
suitable combination of the above techniques.
[0027] In a three-phase AC rotary alternator implementation of an
exemplary electromagnetic apparatus according to various aspects of
the present invention, (FIGS. 1 through 4) poles of a stator and
opposing poles of a rotor may be suitably arranged along first and
second concentric arcs (typically extending 360 degrees). The first
arc may have a smaller radius than the second (e.g., a stationary
internal stator with external coaxial rotor) or vice versa. For
example, referring to FIG. 1, an exemplary alternator 100 includes
stator 110 and rotor 140 that rotates about axis 108. Stator 110
includes conventional windings 130 (shown in partial cross section)
about each pole (e.g., 24 poles) of stator 110 that generate
current according to each phase (i.e., a conventional three-phase
winding). Rotor 140 includes eight poles in alternating spaced
arrangement about an interior face and circumference of rotor 140
represented by active pole 142 and consequence pole 152. As shown,
rotor 140 is driven by a prime mover (not shown) to generate
current in windings 130 of stator 110.
[0028] Referring to FIG. 2, an exemplary pie-shaped segment of a
cross section of alternator 100 is represented by an arc 106 of
about 45 degrees between radial lines 102 and 104 which extend from
the center of rotation 108. Substantially identical segments are
suitably repeated about center 108 to complete a circular cross
section. Alternator 100 includes stator 110, windings 130 (cross
hatched), rotor 140, and gap 156 (enlarged for clarity of
presentation). Gap 156 is typically maintained by conventional
bearings and may be relatively small (e.g., in the range of, for
example, 0.02 to 0.06 inch, and preferably about 0.03 inch).
[0029] Stator 110 suitably includes a plurality of poles 112, 114,
116, 118, 120 and 122 and winding 130, conventionally arranged to
accomplish the purpose of the electromagnetic apparatus. The
structure of each pole may be uniform or may vary from one pole to
the next and may be of conventional construction or constructed in
accordance with principles of the present invention. For example,
all poles of stator 110 are of a conventional laminated structure,
the laminations being parallel to the plane of rotation. Winding
130 may be formed of multiple conductors in any conventional
manner. For example, enameled magnet wire may be wound
automatically on the shaft portions of poles 112 through 122. In a
conventional three phase manner, poles 112 and 118 may be phase A,
poles 114 and 120 may be phase B, and poles 116 and 122 may be
phase C. Each pole includes a shaft and a face. For example, pole
112 includes face 111 having a corner 113; and, pole 118 includes
face 119 having corner 121. Faces 111 and 119 may be flat or
slightly rounded for less variation in the operative gap 156 as the
face moves across a region of a rotor pole. Stator 110 may be
electrically and magnetically configured in a manner suitable for
alternator 100 by applying the principles of stator construction
described in aforementioned U.S. Pat. No. 5,886,504.
[0030] Rotor 140 suitably includes an arcuate body 139 for
supporting a number of poles arranged to accomplish generating an
alternating current in winding 130. Body 139 serves also as a
casing and may be formed of soft iron, steel, or nickel alloy. The
number of rotor poles may be similar or different from the number
of stator poles, e.g., poles 112 through 122. Rotor poles may be of
uniform structure or may vary in structure from one pole to the
next. Variation may be according to conventional construction or
construction in accordance with principles of the present
invention. For example, rotor 140 includes two types of poles
represented by active pole 142 and consequence pole 150 that are
arranged in alternating sequence about center 108.
[0031] Any member of a magnetic circuit for directing flux to a
desired gap may constitute a consequence pole. Typically, a
consequence pole is a passive member of a magnetic circuit in as
much as it does not include a source of flux. For example,
consequence pole 150 includes radial shaft 154 and generally
transverse face 153. Face 153 meets a side of shaft 154 at comer
152. Corner 152 may include an angle of about 90 degrees between
the side of shaft 154 and face 153 in the region near corner
152.
[0032] Pole 142 likewise suitably includes a radial shaft and a
generally transverse face. The shaft includes a base 144, a magnet
146, and a cap 148. Conventional cements (preferably flexible to
accommodate different thermal expansion coefficients) may be used
to affix base 144 to inner surface 141 of rotor 140, to affix
magnet 146 to base 144, and to affix cap 148 to magnet 146. Base
144 and cap 148 are designed, inter alia, to reduce the cost of
magnet 146. To reduce cost, magnet 146 suitably provides
substantially flat mounting surfaces and is as thin as practical
while still meeting operating goals of alternator 100. Such a
magnet may be formed in a flat strip and economically cut to
appropriate dimensions.
[0033] Base 144 provides a flat surface for mounting magnet 146,
and manifests suitable permeability to serve its role in the
magnetic circuit comprising portions of rotor 140 and stator 110.
Base 144 also manifests suitable thermal characteristics (e.g.,
dimensional stability, bonding characteristics, and capable of
expected thermal cycles). Base 144 may be formed in the same manner
discussed below with reference to cap 148, or formed of
magnetically soft material (e.g., iron, soft iron, steel, or nickel
alloy) in any conventional manner. Base 144 may be integral to body
139 or affixed to body 139 in any conventional manner (e.g.,
suitable fasteners or adhesives). Base 144 may be omitted and
surface 141 may be formed directly on body 139 (e.g., by casting,
extrusion, or milling) to provide a flat surface (e.g., an inset,
flat, or plateau) on which to seat magnet 146. Surfaces 141 and the
opposing surfaces of base 144 (if used) may be prepared in any
conventional manner to facilitate location, alignment, seating, or
attachment of magnet 146, including for example, slotted or notched
surface configurations.
[0034] A magnet provides a source of magnetic flux. For example,
magnet 146 may be a relatively light weight high energy product
permanent magnet (e.g. flux density of at least on the order of 5
kilogauss) comprising a rare earth alloy (e.g., neodymium iron
boron, or samarium cobalt). Such rare earth materials tend to be
extremely expensive, and, accordingly, it is desirable to minimize
the amount of material used. However, at the same time, it is
desirable to generate relatively high flux densities. In the
preferred embodiment, magnet 146 is relatively thin (e.g., on the
order of 0.1 inch thick measured radially from center 108) and has
a relatively large cross sectional area (e.g., about 0.75 sq. inch,
3/4" by 1") to minimize the amount of high energy product magnet
used. A magnet having an energy product greater than 40,000,
preferably about 48,000 may be used as provided, for example, by
Grades 44 or 48 material marketed by UGIMax Inc. These materials
are suitable for alternator maximum operating temperatures of about
80 degrees Celsius.
[0035] In accordance with one aspect of the present invention, the
overall size of the electromagnetic apparatus and amount (e.g.,
total volume) of high energy product magnetic material used are
minimized for a given total flux in each magnetic circuit.
Specifically, the cross sectional area of magnet 146 may be greater
than the area of face 153 of consequence pole 150 by approximately
the ratio of the flux density produced by magnet 144 to the allowed
flux density of consequence pole 150. By maximizing the cross
sectional area of magnet 144 relative to consequence pole 150,
alternator 100 may have a smaller overall diameter for a desired
total flux. A smaller overall diameter results in less weight and
less magnetic material being required for a desired total flux.
[0036] Consequence pole 150 is spaced apart from pole 142 to an
extent significantly larger than air gap 156 between rotor and
stator (e.g., at least five times greater) to ensure that the
majority of magnetic energy is directed into the stator rather than
between shaft portions of the respective poles.
[0037] In an alternate implementation of alternator 100, poles 142
and 150 may be part of a hub and oriented to face away from center
108; and, poles 112 through 122 may be part of the inside of a
hollow cylinder coaxial with center 108. Either the hub or the
cylinder (or both) may move with respect to each other. Pole 142
(and poles of similar structure located about center 108) may be
wrapped in a non-metallic material (e.g., fiberglass tape) to
secure its components (144, 146, 148) against centrifugal forces
generated by rotation. Further, alternator 100 is generally
representative of electromagnetic apparatus including generators
and motors, as discussed above. Principles of construction and
operation discussed with reference to alternator 100 apply
analogously to these other types of electromagnetic apparatus.
[0038] Magnetic circuit efficiency may be improved by reducing eddy
current through use of a high permeability, low conductivity (HPLC)
element. Such an element may also provide structure for support or
mounting of other components of the electromagnetic apparatus,
provide routing of flux in unconventional paths, or provide a
measure of insulation from environmental effects including thermal,
vibration, or corrosion resistance. An HPLC element, according to
various aspects of the present invention, includes any member of a
magnetic circuit that passes a varying flux yet impedes the flow of
eddy current that would otherwise flow in the element due to
variation in the flux. Flux may vary in magnitude, in spacial
direction, or in both magnitude and direction.
[0039] HPLC elements may be formed using a particulate having a
predetermined magnetic permeability or range of permeability. The
particulate may be introduced and/or mixed with a suitable
conventional material (generally herein called a binder) for
forming an element of suitable stable dimensions (e.g., rigid,
flexible, malleable, or elastic). The binder may comprise a
conventional adhesive, plastic, molding compound, or potting
compound. The binder may be in fluid, paste, or powder form to
facilitate mixing with the particulate. The particles may be coated
with the binder and then treated to effect interparticle bonding.
Preferably, the binder is applied to coat each particle to
facilitate handling. Suitable coatings include inorganic compounds
(e.g., phosphide preparations) or organic compounds (e.g., resins,
thermoplastics, or Bakelite marketed by E. I. Dupont). Treatment to
effect bonding may include any conventional combination of pressure
and/or heat. Heat may be applied to cure the coating on the
particles and/or to activate adhesion among the particles (e.g.,
when molded in a desired shape). Other activated coatings may be
used including multi-part adhesives and plastics activated by
mixing, and conventional materials activated by ultraviolet
light.
[0040] Particles may include any mix including high permeability
material (e.g., iron, soft iron) and may include non-magnetic and
paramagnetic material (e.g., alloys of iron and/or alloys of
nickel) where desired to achieve suitable structural or insulative
properties perhaps at the expense of some permeability. Particles
may be coated to avoid corrosion, facilitate storage, or facilitate
formation of HPLC elements, as discussed above. Particles may be of
any size or mix of sizes (e.g., from about 45 to about 600 microns,
preferably from about 75 to about 150 microns) suitable for storage
and formation of HPLC elements. For example, ATOMET EM1 marketed by
Quebec Metal Powders, Inc. may be used.
[0041] An HPLC element may be formed in accordance with a method
comprising: (a) providing a powder comprising particles having
magnetically soft permeability each coated with a binder, the
binder having lower conductivity than soft iron; (b) molding the
powder in the shape of the HPLC element; and (c) heating the powder
so that the powder forms a matrix of soft iron particles disposed
in a solid formed from the binder. The powder suitably comprises
soft iron particles varying in size primarily (by weight) in the
range from about 60 to about 220 microns, preferably from about 75
to about 150 microns in diameter. The coating may comprise
Bakelite. Powder may be poured, blown, or pressed into a
conventional mold for example of a type typically used for plastic
parts manufacturing. The interior of the mold may be treated with a
conventional release agent. Curing sufficient to form a rigid
matrix of particles that are electrically isolated from each other
may be accomplished by one or more cycles of heating followed by
cooling to avoid cracking, chafing, or other surface irregularities
using conventional techniques. Vibration of the mold may promote
isolation of particles to good effect. Vibration of the mold may be
used to promote obtaining a particular size particle in a desired
position or region of the mold.
[0042] Composition and manufacture of HPLC elements of the type
described herein may be selected from exemplary materials and
processes described in the following publications. Each publication
and its references are incorporated herein. Lubricated Iron Powder
Mixes for Low Frequency Soft Magnetic Applications by L. -P.
Lefebvre, et al.; Effect of Curing Conditions on Properties of
Iron-Resin Materials for Low Frequency AC Magnetic Applications by
C. Gelinas, et al.; Effect of Thermal Cycling on Properties of Soft
Magnetic Iron-Resin Composites by S. Pelletier, et al.; Iron-resin
Composite Materials for AC Magnetic Applications by C. Gelinas and
J. M. Battison; and Imminent Changes in Soft Magnetic Devices by J.
Mark Battison and Claude Gelinas; all published on the World Wide
Web by Quebec Metal Powders Limited at www.qmppowders.com.
[0043] Particles of predetermined geometry, e.g., substantially
spherical particles, that are separated from each other by a
generally uniform distance suitable for providing a desired
breakdown voltage may be used to limit (e.g., reduce, minimize, or
practically eliminate) eddy currents in regions of wide dynamic
range of magnetic flux density and/or direction. Substantially
uniform permeability in all directions may be so obtained. Where
permeability is to be higher in one or more directions with
reference to a reference direction, suitable particle shape, size,
and orientation may be used. To assure a desired orientation of
particles, curing (e.g., activating the binder to facilitate
interparticle binding) may be accomplished under the influence of a
stable magnetic field. To assure random alignment of particles,
curing may be accomplished under the influence of a demagnetizing
field (e.g., fields from moving permanent magnets, moving
electromagnets, an alternating current electromagnet or an RF
field).
[0044] An HPLC element may be used adjacent to a gap or
discontinuity of materials in any electromagnetic apparatus
discussed above. An entire pole may constitute an HPLC element
(e.g., consequence pole 150 may be entirely homogeneous HPLC
material); or, one or more portions of a pole (e.g., where eddy
current losses are to be limited) may constitute HPLC elements. For
example, base 144 and/or cap of pole 142 may be formed as HPLC
elements.
[0045] Cap 148 suitably provides the transverse face of active pole
142 adjacent to gap 156 in the magnetic circuits comprising
permanent magnet 146. In accordance with one aspect of the present
invention, cap 148 limits eddy current flow with consequential
benefits in reduced heat generation and higher efficiency of
electromagnetic apparatus operation. For example, cap 148 passes
flux between stator poles 112-122 and permanent magnet 146. When a
pole (e.g., pole 112) is a strong source or sink of flux, the
direction of flux passing through cap 148 varies considerably as
the pole (e.g., 112) moves into, through, and out of the active
range of pole 142. Because cap 146, when formed as an HPLC element,
provides high permeability for all directions of flux and because
cap 146 may serve as a bulk electrical insulator, eddy currents are
limited which may otherwise have been of substantial magnitude.
[0046] Referring now to FIG. 3, cap 148 suitably includes a
mounting surface 302, a face 149, and shoulders 147 and 304 that
join surface 302 and face 149. Preferably, the height of a cap
(e.g., height H of cap 148) is determined in accordance with the
following trade-off: (a) increasing cap thickness decreases
operative motive force because reduced flux magnitude in the
operative magnetic circuit results from the series effect of
imperfect permeability of the cap; yet, (b) increasing cap
thickness reduces the variation in flux direction and magnitude
passed by magnet 146, consequently reducing eddy current energy
losses and eddy current induced heating of magnet 146. When magnet
146 is about 0.1 to about 0.2 inches thick, cap 148 height H is
preferably about 0.1 to 0.2 inches thick also. Cap height may be
from about 50% to about 75% of magnet thickness, preferably from
about 60% to about 70%.
[0047] The above trade-off may be restated in terms of the cost
(i.e., amount of material) of magnet 146. The amount of material
needed in magnet 146 is determined to meet design requirements of
alternator 100 and to avoid premature demagnetization of magnet 146
(e.g., torque, speed, mechanical power, voltage, current, or
electrical power, and operative life). Demagnetization may occur as
a consequence of exposure to heat (e.g., produced in part by eddy
currents) and flux passing through the permanent magnetic that is
not aligned with the flux provided by the permanent magnet. Due to
the effect of increased cap thickness as in (a) above, more
material may be needed for magnet 146. Due to the effect of
increased cap thickness as in (b) above, magnet 146 may be less
likely to become demagnetized and less magnet material may be
needed. In addition, higher energy product magnetic materials may
be used with a suitable cap because the magnet is then not exposed
to eddy current or hysteresis induced heat. A suitable cap also
reduces hysteresis induced losses in the magnet (e.g., heat) by
redirecting flux as discussed above.
[0048] Shoulders 147 and 304 may comprise any surface configuration
that avoids the spacial concentration of flux near the face of the
cap that would otherwise occur if the face of the cap terminated in
a corner of 90 degrees or less. For example, referring again to
FIG. 2, flux is more highly concentrated in the immediate vicinity
of line 151 passing through corner 152 of consequence pole 150 than
concentrated in the immediate vicinity of line 141 passing through
shoulder 147 of cap 148. In addition to providing lower flux
concentration (and consequently less associated eddy currents and
heat), the material in the immediate vicinity of shoulder 147 is
not exposed to the greater range of flux direction experienced in
the immediate vicinity of corner 152. More efficient operation of
alternator 100 results at least in part (directly or indirectly)
from so limiting the range of flux direction variation.
[0049] A shoulder in cross section may comprise a series of
straight segments to avoid angles of 90 degrees or less; with one
or a series of nonlinear segments; or with any combination of
straight and curved segments. For example, as shown in FIG. 3,
shoulder 147 preferably exhibits a cross section of a circular arc
formed with radius 308. Radius 308 may be determined with reference
to height H, for example in the range 89% H<R<111% H or
approximately equal to height H as shown. The adverse affects on
magnet 146 that may occur as a result of H being too thin, as
discussed above, may be accounted for by increasing cap length L
(and magnet material) when a substantial portion of length L does
not exhibit the full height H.
[0050] A cap may provide a face that facilitates a uniform gap
dimension. For example, by forming face 149 with a radius 310
measured from center 108, a gap dimension measured radially from
any pole toward face 149 is maintained relatively uniform.
[0051] An alternate cap may provide a shoulder only on the leading
edge of a pole with respect to the relative motion seen by the
pole.
[0052] Another alternate cap, according to various aspects of the
present invention, is formed as a laminated structure instead of an
HPLC element, as discussed above. For example, pole 442 of FIG. 4
is shown in cross section as if installed in rotor 140. (As an HPLC
element, cap 148 in such a cross section view would appear as a
homogeneous solid.) Pole 442 suitably comprises an HPLC element
formed as base 444, a permanent magnet 446, and a cap 448. A gap
456 separates cap 448 from stator pole 413. Laminations used to
form cap 448 and stator pole 413 may consist of thin sheets of
magnetically soft material (e.g., iron, soft iron, steel, or nickel
alloy). The laminations used to form cap 448 may have the
dimensions and features similar to cap 148 as discussed above with
reference to FIG. 3.
[0053] A series of HPLC elements may be included in several
independent magnetic circuits, according to various aspects of the
present invention. For example, referring to FIG. 5, an
electromagnetic apparatus 500 includes a stator 510, encircled by a
band 504. Stator 510 includes winding 530 and poles 512 through 522
corresponding generally in function to similarly numbered elements
of alternator 100. Poles 512 through 522 include caps 513 through
523 formed as a unit in band 504 and spaced by material 526 between
caps. Each cap may have corners or shoulders 528 as desired to
provide performance as discussed above. Cap regions of band 504 may
be formed in situ. Alternatively, band 504 may be formed as a tape
(or in arcs) and later wrapped about or attached to stator 510.
Alignment of caps on poles may be facilitated by energizing one or
more poles of stator 510 during assembly of band 504 on stator
510.
[0054] Though a laminated structure may conduct flux equally well
in the 2 major dimensions of each lamination, an HPLC element may
be used to conduct flux in any direction. When, in an
electromagnetic apparatus, flux through a solid can follow paths
each wholly contained in one plane, that solid may be constructed
of laminated material to reduce eddy current losses. On the other
hand, where flux is desired to flow through a solid along paths
such that each path cannot wholly be contained in a plane, that
solid may advantageously be formed consisting of or comprising in
relevant part an HPLC element.
[0055] In a rotary alternator implementation of an exemplary
electromagnetic apparatus according to various aspects of the
present invention, poles of a stator and opposing poles of a rotor
may be suitably arranged along first and second concentric arcs
(e.g., circles) and spaced apart along the circumference and along
the axis of rotation to define magnetic circuits having non-planar
flux paths. The first arc may be smaller in radius than the second,
providing a stationary internal stator with an external coaxial
rotor. For example, referring to FIG. 6, an exemplary single-phase
alternator 600 includes a stator 602 and a rotor 604 that rotates
about axis 603. Dimensions for an exemplary configuration are shown
in FIG. 7. Alternate designs may employ dimensions and dimensional
ratios similar to the configuration of FIG. 7. Dimensional ratios
may range plus or minus 20% from ratios based on the values
shown.
[0056] Rotor 604 includes body 606 of soft iron or steel and active
poles 607 through 611 (poles 607 and 611 shown in cross section).
Active poles 607 through 611 are affixed to body 606 in any
conventional manner, as discussed above. Each active pole 607
through 611 may include a ferrite magnet or a high energy product
permanent magnet comprising a rare earth material as discussed
above (e.g., 613, 615, 617, 642, and 646) and a cap (e.g., 614,
616, 618, 644, and 648) comprising an HPLC element as discussed
above with reference to cap 148. Poles 607 through 611 are affixed
in spaced arrangement about an interior face and circumference of
body 606 with magnetic poles in alternating north-south
sequence.
[0057] Stator 602 includes a first four-pole plate 634, a hub 630,
a winding 638, and a second four-pole plate 636. Stator 602 may be
assembled by aligning and press-fitting plate 634 onto hub 630
until it rests against face 633, placing winding 638 over hub 630,
and finally aligning and press-fitting plate 636 onto hub 630 until
it rests against face 631. Plates 634 and 636 may serve to retain
winding 638 in a predetermined position along the z coordinate
(i.e., along axis 603). Plates 634 and 636 may be formed of
laminations (as shown) or as HPLC elements. Hub 630 is preferably
formed as an HPLC element. Plate 634 and hub 630 may be formed as a
unit. Hub 630 may be formed in sections to permit thermal
expansion. A hub section may include stator poles used in place of
plates 634 and 636.
[0058] Positions about hub 630 may be established for plates 634
and 636 and for winding 638 using a key way (e.g., 632) or in any
conventional manner (e.g., adhesives, noncircular mating surfaces,
or fasteners). Plates 634 and 636 may be identical in structure and
placed on hub 604 at any coordinate angle .theta. (i.e., measured
about axis 603) relative to each other (e.g., 45 degrees as shown
for an 8 pole stator). Every other pole of stator 604 is thus
aligned at an index +45 degrees from the preceding pole and is
located in a plane different from the preceding pole. Alternately,
poles may be grouped to form a magnetic flux circuit (e.g., a
single or multiple phase circuit), each member of the group being
indexed in both z and 0 coordinates relative to another member of
the group. Plate 634 is also indexed an incremental amount along
the z coordinate (e.g., along the length of axis 603 of rotation).
The spacing along the z coordinate is sufficient for locating
winding 638 between plates 634 and 636.
[0059] Winding 638 may be a single-phase winding formed for example
by a predetermined number of clockwise turns about axis 603.
Winding 638 may be made as a unit from conventional enameled magnet
wire or insulated conductive tape and may be potted to simplify
assembly and reduce vibration. Winding 638 may be wound directly on
hub 630 or on a bobbin by conventional techniques. The inner radius
R1 of winding 638 may be less than or equal to the radius R2 of
plate 636 and/or 638.
[0060] In operation, rotor 602 is driven by a prime mover (not
shown) to generate current in winding 638 of stator 604. Winding
638 passes through a magnetic circuit that includes two poles of
rotor 602 and two poles of stator 604. Because adjacent stator
poles are indexed in both z and .theta., it is desirable to
facilitate flux in hub 630 to flow in nonplanar paths. For example,
representative flux paths include: path a-b-c-d-e and path
p-q-s-t-u-v. Other paths (not shown) may include path segments in
cap 644 analogous to segments b and q in cap 648. These other paths
may connect with path segment r. Note that some path segments in
caps 644 and 648 as well as all path segments in hub 630 are
advantageously nonplanar to more evenly distribute flux density and
to employ the full capability of each of poles 635, 610, 611, and
634.
[0061] Particular advantages are provided by hub 630 when
completely formed of HPLC material: (a) flux paths in all
directions are equally facilitated; and (b) eddy currents in all
directions are limited by the nonconductive property of the binder
of the HPLC element. Though HPLC materials reach saturation flux
density at lower flux density than soft iron, these materials are
generally lighter weight that soft iron and more HPLC material may
generally be used in comparison to soft iron to reduce flux density
without exceeding a total weight allowance for the stator. Further,
by providing a shoulder on caps 614, 616, 618, 644, and 648, flux
from magnets 613, 615, 617, 642, and 646 may be directed away from
adjacent opposite polarity poles and into an adjacent stator pole
for higher energy efficiency. Consequence poles may be used in
alternator 600 in a manner similar to consequence poles discussed
above. Affixing a cap on each consequence pole is preferred for
reducing flux density in the consequence pole.
[0062] The principles of construction and operation discussed above
with reference to electromagnetic apparatus for generating
electricity may be applied to converting electricity into motion.
For example, a motor formed as described with reference to
alternator 600 may be operated by driving winding 638 with an
alternating current (e.g., comprising a sine, square, triangle, or
pulsed waveform). Motor rotation may be started with a suitable
mechanism such as an auxiliary electromagnetic element, a further
set of stator poles suitably indexed and driven from a second
winding or coil, an external manually input rotation, or an
external starting prime mover. Once in operation, the rotational
speed (e.g., RPM) of the motor may be controlled by adjusting the
frequency of AC voltage applied to the winding. An auxiliary
electromagnetic element may include a tapped toroidally wound
winding having portions energized in sequence to urge rotor
movement in a desired direction. A self-starting three-phase motor
may include a winding similar to winding 638 for each phase. One or
two pole plates for each phase may sandwich the appropriate phase
winding. The pole plates and winding for each phase may be spaced
along the axis of rotation to decouple magnetic interaction or may
be integrated (e.g., one pole plate participating in the magnetic
circuits of more than one phase).
[0063] The foregoing is a description of preferred exemplary
embodiments and best mode of the invention known to the applicant
at the time of filing the application. The invention is not limited
to the specific forms shown. For example, modifications may be made
in the design and arrangement of the elements within the scope of
the invention, as expressed in the appended claims.
* * * * *
References