U.S. patent application number 11/012870 was filed with the patent office on 2006-01-19 for axial field electric machine.
Invention is credited to Yuval Shenkal, Stephen H. Smith.
Application Number | 20060012263 11/012870 |
Document ID | / |
Family ID | 22252111 |
Filed Date | 2006-01-19 |
United States Patent
Application |
20060012263 |
Kind Code |
A1 |
Smith; Stephen H. ; et
al. |
January 19, 2006 |
Axial field electric machine
Abstract
An axial field electric machine having an improved efficiency
includes a number of magnetic elements (e.g., as a rotor) as
annular disks magnetized to provide multiple sector-shaped poles.
Each sector has a polarity opposite that of an adjacent sector, and
each sector is polarized through the thickness of the disk. The
poles of each disk are aligned with opposite poles of each adjacent
magnet. Metal members adjacent the outermost disks contain the
flux; The axial field electric machine also includes one or more
conductor elements (e.g., as a stator) which include a number of
conductor phases that traverse the flux emanating between poles of
axially adjacent magnetic elements. The design of the axial field
electric machine including the gap spacing between adjacent
magnetic elements, the transition width between adjacent poles on
each magnetic element, the number of poles, the number and width or
conductor phases in the conductor element is based on the physical
characteristics of the magnetic elements to increase
efficiency.
Inventors: |
Smith; Stephen H.;
(Leucadia, CA) ; Shenkal; Yuval; (Cardiff,
CA) |
Correspondence
Address: |
KENYON & KENYON
1500 K STREET NW
SUITE 700
WASHINGTON
DC
20005
US
|
Family ID: |
22252111 |
Appl. No.: |
11/012870 |
Filed: |
December 14, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10745702 |
Dec 29, 2003 |
|
|
|
11012870 |
Dec 14, 2004 |
|
|
|
10175733 |
Jun 19, 2002 |
|
|
|
10745702 |
Dec 29, 2003 |
|
|
|
09095455 |
Jun 10, 1998 |
6411002 |
|
|
10175733 |
Jun 19, 2002 |
|
|
|
08763824 |
Dec 11, 1996 |
5982074 |
|
|
09095455 |
Jun 10, 1998 |
|
|
|
Current U.S.
Class: |
310/268 |
Current CPC
Class: |
H02K 16/00 20130101;
H02K 1/02 20130101; H02K 15/165 20130101; H02K 15/03 20130101; H02K
2203/03 20130101; H02K 3/26 20130101; H02K 1/182 20130101; H02K
3/47 20130101; H02K 3/28 20130101; H02K 1/2793 20130101; H02K 29/08
20130101; H02K 1/27 20130101; H02K 3/04 20130101; H02K 21/24
20130101; H02K 2203/09 20130101 |
Class at
Publication: |
310/268 |
International
Class: |
H02K 1/22 20060101
H02K001/22 |
Claims
1-5. (canceled)
6. An axial field electric machine comprising: a shaft; a plurality
of conductor elements, each conductor element including a number of
conductors and at least one pin for selectively coupling a
conductor from one of said conductor elements to another of said
conductor elements; and a plurality of magnetic elements capable of
being mounted to said shaft, each of said magnetic elements being
mounted adjacent to at least one of said conductor elements; said
conductor elements and said magnetic elements are capable of being
selectively added to and subtracted from said axial field electric
machine.
7-19. (canceled)
Description
RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of U.S.
application Ser. No. 08/763,824, the disclosure of which is hereby
incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to electric machines
or motor/generators and, more specifically, to permanent magnet,
axial field electric machines.
[0004] 2. Description of the Related Art
[0005] An electric motor/generator, referred to in the art as an
electric machine, is a device that converts electrical energy to
mechanical energy and/or mechanical energy to electrical energy.
Since electric machines appear more commonly as motors the ensuing
discussion often assumes that electric energy is being converted to
mechanical energy. However, those knowledgeable in the art
recognize that the description below applies equally well to both
motors and generators.
[0006] Electric machines generally operate based on Faraday's law,
which can be written as e=BLv, and the Lorentz force equation,
which is often written as F=BLi. In electric machines that utilize
rotational motion, these equations can be written as
e=k.sub.1BL.omega. and T=k.sub.2BLi respectively. Faraday's law
describes the speed voltage or back EMF (electromotive force), e,
that appears across motor conductors due to the geometrically
orthogonal interaction of a magnetic field having flux density B
with conductors of length L traveling at a rotational speed
.omega.. The Lorentz force equation describes the torque T
generated by the geometrically orthogonal interaction of a magnetic
field having flux density, B, with conductors of length L carrying
current i. The coefficients k.sub.1 and k.sub.2 are constants that
are a function of motor geometry, material properties, and design
parameters.
[0007] A variety of electric machine types exist in the art based
on how they generate the magnetic field and on how they control the
flow of electrical energy in the conductors exposed to the magnetic
field. The present invention pertains to electric machines where
the magnetic field B is primarily generated by permanent magnets
affixed to the rotating assembly, or rotor of the machine; whereas
the conductors are affixed to the stationary assembly, or stator of
the machine and electronic circuitry is used to control the flow of
electrical energy. In the art this type of machine is commonly
called a brushless DC motor or a brushless permanent magnet
machine. In addition, such electric machines can be modified to use
induction to generate the magnetic field. In this case the machine
is commonly called an induction motor.
[0008] Electric machines that produce rotational motion are
classified as either radial field or axial field. Radial field
machines have a radially directed magnetic field interacting with
axially directed conductors, leading to rotational motion. On the
other hand, axial field machines have an axially directed magnetic
field interacting with radially directed conductors, leading to
rotational motion. Of these two machine topologies, the axial field
machine appears much less often. In the art, axial field machines
are most often found in applications where: (i) there is
insufficient axial length to accommodate a radial field machine,
(ii) relatively little torque is needed, and (iii) motor energy
conversion efficiency is not a primary concern. The reasons why
axial field machines generally appear less often than radial field
machines include: (a) more familiarity with radial field machines,
(b) the desire to minimize cost by reusing existing radial field
machine tooling, and (c) the lack of market incentive to address
manufacturing issues unique to axial field machines.
[0009] In terms of quantity produced, the spindle motor in computer
floppy disk drives is the most commonly appearing axial field
electric machine. In this application minimizing cost is the most
critical design goal. As a result, this motor does not utilize
materials, design steps, or construction techniques that lead to
high efficiency over a broad range of speeds, high motor constant,
or high power density. The floppy disk spindle motor uses an axial
field topology solely because there is insufficient axial space
available inside the floppy disk housing to use a radial field
motor. This motor is typically manufactured with one rotor element
and one stator element, with the stator element being constructed
from a steel-backed printed circuit board upon which the stator
windings and motor electric drive circuitry are connected.
[0010] The present invention discloses design aspects for axial
field machines that offer greater performance than common axial
field machines and performance that meets, exceeds, or is
competitive with radial field machines. Performance in this case
includes the measures of: (i) energy conversion efficiency, (ii)
motor constant, (iii) gravimetric power density, (iv) volumetric
power density, (v) manufacturing cost, and (vi) construction
flexibility due to modular construction.
[0011] Energy conversion efficiency describes how well an electric
machine converts energy. For a motor, efficiency can be written as
.eta.=(Power Out)/(Power
In)=(T.omega.)/(T.omega.+P.sub.r+P.sub.c+P.sub.m) (Eq. 1) where T
is torque, .omega. is rotational speed, P.sub.r is resistive loss
i.e., the so called I.sup.2R loss, which represents power converted
to heat by the resistance of the current carrying conductors in the
motor, P.sub.c is the core loss, which represents power converted
to heat due to hysteresis and eddy current losses in the conductive
and magnetic materials used in the motor, and P.sub.m is the
mechanical loss, which includes bearing loss, windage, etc. Core
and mechanical losses generally increase with the square of speed,
so efficiency typically increases from zero at zero speed, to some
peak value at some rated speed, then decreases beyond that rated
speed. For constant speed applications, achieving high peak
efficiency at a constant rated speed is all that is important. For
variable speed applications, however, it is important to maximize
the range of speeds over which maximum efficiency can be achieved.
As defined in Eq. 1, efficiency is unitless and is often expressed
as a percentage, where 100% efficiency reflects the ideal electric
machine.
[0012] Referring to FIG. 30, a graph is presented showing the
efficiency of a typical electric machine known in the art at
various speeds and torque. The operation of the electric machine is
bounded by a peak speed, a peak torque, and a maximum power output.
In this example, the electric machine has a peak efficiency of 90%
at a particular operating point (i.e., at a particular rated speed
and torque). At other operating points, however, the efficiency
drops off precipitously as indicated by the contours of constant
efficiency. In a traction application, for example, when the
electric machine is operated at different operating points on the
graph, the average efficiency will be much lower than peak
efficiency.
[0013] In servomotor applications where a motor does not turn
continuously but rather starts and stops frequently, efficiency is
not a good measure of motor performance because efficiency is zero
at zero speed, i.e., .omega.=0. Under these conditions, the ability
to produce torque with minimum losses is important. In the art the
term motor constant describes the motor characteristic. Motor
constant can be written and simplified as K m = T P r = K T .times.
I I 2 .times. R = K T R ( Eq . .times. 2 ) ##EQU1## where K.sub.T
is the motor torque constant, I is the net motor current, and R is
the net motor resistance. Core loss and mechanical loss are not
included in the motor constant because these losses are zero at
zero speed. The square root of P.sub.r is used in Eq. 2 because it
makes the motor constant independent of current, which makes it
independent of any motor load and makes it easier to compare the
performance of different motors. Based on Eq. 1 and Eq. 2, it is
clear that a motor exhibiting high efficiency will generally
exhibit a high motor constant. Likewise, if a motor exhibits
minimal core loss and mechanical loss, a motor having a high motor
constant will also exhibit high efficiency.
[0014] Gravimetric and volumetric power density are defined as the
ratio of output power, e.g., T.omega. for a motor, to the mass and
volume of the machine, respectively. As such, gravimetric power
density is often specified in terms of watts per pound, horsepower
per pound, or kilowatts per kilogram. Likewise, volumetric power
density is often specified in terms of watts per cubic inch or
kilowatts per cubic meter. In most cases, there is a high degree of
correlation between these two measures of power density. That is,
given that electric machines are generally constructed from the
same types of materials, their mass is directly proportional to
their volume, thus a motor having a high gravimetric power density,
will also exhibit a high volumetric power density. Given this
correlation, it is common to use the term power density to mean
either gravimetric or volumetric power density or both. In any
case, since output power is the product of torque and speed, power
density increases linearly with speed to the point where it is no
longer possible to maintain torque production, at which point power
density decreases. In addition, given that torque is generally
proportional to current as shown in Eq. 2, the ability to produce
torque is only limited by the ability to remove the heat created by
the resulting I.sup.2R loss P.sub.r and the speed dependent losses
P.sub.c and P.sub.m, which decrease efficiency. As a result, power
density is generally proportional to efficiency because more power
can be safely produced in a more efficient motor. For example, a
highly efficient motor generates less heat for a given torque
output than a less efficient motor, which in turn implies that the
more highly efficient motor can generate more torque and therefore
have higher power density, while generating the same amount of heat
as the less efficient motor.
[0015] In the art, electric machines of varying outputs generally
require significant unique tooling for each voltage and torque
level. For a given diameter it is typical to specify a number of
rotor and stator lengths, with similar but different parts and
tooling required for each rotor and stator. For example, in a
brushless DC motor each stator may be made from the same stator
laminations stacked to various lengths, but the windings are unique
for every length as well as for every voltage level at any fixed
length. As a result, additional cost is incurred in traditional
motors due to the additional capital expense and inventory required
to support a family of motors at a given diameter.
[0016] In view of the above, there is a need for an improved axial
field-electric machine that provides a high efficiency over a wide
variety of speeds and torque and a high gravimetric and volumetric
power density over a wide range of speeds and torque. There is also
a need for an improved axial field electric machine that allows for
easy modification of the rotor and/or stator to increase or
decrease the power output of the electric machine.
SUMMARY OF THE INVENTION
[0017] These and other needs are satisfied by the axial field
electric machine of the present invention. Based on the above
discussion, the present invention discloses design aspects for an
axial field electric machine that maximize efficiency, motor
constant, power density, as well as offer the benefits of modular
construction, and the potential for reduced cost. Efficiency and
motor constant are maximized by maximizing the production of torque
while incurring minimal losses. In particular, one aspect of the
invention eliminates all ferromagnetic material that incurs core
loss, thereby essentially eliminating P.sub.c from Eq. 1, above
(although eddy current losses in the conductors must be
considered). Doing so increases peak efficiency, broadens the range
of speeds over which efficiency is high, and increases power
density by eliminating the high mass associated with the added
stationary ferromagnetic material. In addition, other aspects of
the invention minimizes P.sub.r, which maximize motor constant and
maximizes the peak efficiency. Power density is maximized further
according to an embodiment of the present invention by optimum
selection of the amount of permanent magnet material relative to
stator volume. Modular construction allows a whole family of motors
at varying power levels to be constructed by stacking sets of
identical rotor components and stator components axially within the
same motor. Since each rotor and stator is identical, no
duplication of capital cost is incurred to produce a whole family
of motors. In addition, other aspects of the invention make it
possible to select a variety of voltage levels by simply changing
the way individual stators are connected, thereby minimizing the
inventory required to support a whole family of motors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a pictorial view of an exemplary axial field
electric machine of the present invention;
[0019] FIG. 2 is an enlarged sectional view taken on line 2-2 of
FIG. 1;
[0020] FIG. 3 is a sectional view taken on line 3-3 of FIG. 2;
[0021] FIG. 4 is a face view of a magnetic element of the axial
field electric machine, showing the polarization of the magnet;
[0022] FIG. 5 is a side elevation view of a magnetic element;
[0023] FIG. 6 is a graphical illustration of the magnetic flux
emanating from a magnetic element;
[0024] FIG. 7 is a plot showing the demagnetization characteristics
of permanent magnets and the operating point of a magnet when used
in an electric machine constructed according to the present
invention;
[0025] FIG. 8 is a block diagram showing an air conditioner unit
including an axial field electric machine constructed according to
an embodiment of the present invention.
[0026] FIG. 9 is a pictorial view of a shaft for use in the axial
field electric machine of the present invention;
[0027] FIG. 10 is a pictorial view of a hub that can be mounted to
the shaft of FIG. 9;
[0028] FIG. 11 is a pictorial view of a conductor element of the
axial field electric machine;
[0029] FIG. 12 is a cross-sectional view of an axial field electric
machine constructed according to an embodiment of the present
invention;
[0030] FIG. 13 is a schematic diagram of the conductor element
winding arrangement of FIG. 11;
[0031] FIG. 14 is a pictorial view of an alternative conductor
element winding arrangement having single-turn, rectangular
cross-section conductors;
[0032] FIG. 15 is a flux diagram for a plurality of magnetic
elements;
[0033] FIGS. 16a-f are views of a plurality of subassemblies in an
alternative conductor element;
[0034] FIG. 17 is a top plan view of another subassembly in an
alternative conductor element illustrating both sides of the
subassembly;
[0035] FIG. 18 is a sectional view taken along line 18-18 of FIG.
17, showing multiple subassemblies;
[0036] FIG. 19 is a sectional view taken along line 19-19 of FIG.
17;
[0037] FIG. 20 is a partial top plan view similar to FIG. 17, but
showing the portion of the conductor element winding arrangement
relating to 12 phases of windings of one of the subassemblies;
[0038] FIG. 21 is a block diagram of a motor controller;
[0039] FIG. 22 is a timing diagram of the motor signals generated
by the motor controller of FIG. 21;
[0040] FIG. 23 is a schematic diagram of the conductor elements
connected to one another in a configuration selected to operate the
axial field electric machine at a first voltage;
[0041] FIG. 24 is a schematic diagram of the conductor elements
connected to one another in a configuration selected to operate the
axial field electric machine at a second voltage;
[0042] FIG. 25 is a schematic diagram of the conductor elements
connected in a configuration selected to operate the axial field
electric machine at a third voltage;
[0043] FIG. 26 is, in part, a front elevation view of a vehicle
having the axial field electric machine disposed within a wheel
and, in part, a cross-sectional detail view of an alternative
embodiment of the axial, field electric machine suitable for
installation within the wheel.
[0044] FIG. 27 is a plan view of a frame used for a conductor
element in an axial field electric machine constructed according to
an embodiment of the present invention.
[0045] FIG. 28 is a view of a connector support element for an
axial field electric machine constructed according to an embodiment
of the present invention.
[0046] FIG. 29 is a view of a partially completed axial field
electric machine constructed according to an embodiment of the
present invention.
[0047] FIG. 30 is a graph depicting contours of constant efficiency
for a typical electric machine known in the art.
[0048] FIG. 31 is a graph depicting contours of constant efficiency
for an electric machine constructed according to an embodiment of
the present invention.
DETAILED DESCRIPTION
[0049] With reference to the drawing figures, a number of
embodiments of the present invention are shown that maximize peak
efficiency, efficiency over a broad range of speeds, motor
constant, and power density. Also with reference to the drawing
figures, an axial field electric machine according to embodiments
of the present invention will be described that has a modular
design that allows for cost effective creation of an entire family
of machines for varying applications.
Conductor Element
[0050] As used herein, the term conductor element refers to an
element of the axial field electric machine that provides
conductors that traverse the magnetic flux generated by an adjacent
magnet or magnetic element. In a motor application, the conductors
in the conductor element carry electric current in response to a
motor controller, and in a generator application, electric voltage
is induced across the conductors by the magnet or magnetic element.
In the examples given below, the stator of the axial field electric
machine includes one or more conductor elements. One skilled in the
art will recognize that in an alternative embodiment, the conductor
elements could also from the rotor of the axial field electric
machine, in which case magnetic elements form the stator of the
electric machine.
[0051] To achieve a high power density, each conductor element is
designed so as to maximize the amount of conductive material (e.g.,
as conductor phases) that traverses the magnetic flux from adjacent
magnets or magnetic elements. Conductive material that does not
traverse this magnetic flux contributes to the mass and losses of
the machine and thereby reduces the efficiency and power density of
the machine. To achieve a modular design, it is advantageous if
each conductor element is similar in construction.
[0052] A first embodiment of a conductor element is shown in FIG.
14. In this embodiment, the conductor element 121 includes four
phase windings 122, 124, 126, and 128, each having a dielectric
coating or the like to electrically insulate one phase winding from
the other. Each conductor in the phase winding has a generally
rectangular cross-sectional shape with a generally constant axial
thickness and a width that tapers linearly with the radius of the
conductor element. As shown in FIG. 14, each phase winding starts
at a first terminal point (e.g., first terminal point 126a of phase
winding 126) at the outer periphery of the conductor element 121,
extends in a radial direction as a radial conductor section 126c
towards the center of the conductor element, extends in an arcuate
path at the center (not visible) and extends away from the center
as a radial conductor section 126d to the outer periphery of the
conductor element to form a loop. Each phase winding may include a
number of loops around the conductor element. In this example,
phase winding 126 includes four such loops and eight radial
conductor sections 126c-126j between first terminal point 126a and
a second terminal point 126b distributed uniformly around the
conductor element. The radial conductor sections are connected at
the center and periphery by arcuate sections such as outer arcuate
section 126k and inner arcuate section 126m.
[0053] The current path in conductor element 121 of FIG. 14 is
shown in FIG. 13. Phase winding 126, for example is shown as
extending between a point labeled .phi..sub.3- and .phi..sub.3+ and
crosses between the periphery of conductor element 121 and an inner
portion of the element eight times.
[0054] Returning to FIG. 14, in this example, each radial section
of a conductor winding is offset from another radial section in the
same phase winding by radial sections of each of the other phase
windings. In other words, radial section 126c and radial section
126d are offset from each other by a radial section from each of
phase windings 122, 124, and 128. The phase windings define a
generally planar or wheel-like structure, with a total of 32 radial
sections arranged in a spoke-like manner. In this example,
conductive sockets 42 are provided at the terminal points for
electrically coupling one conductor element to another.
[0055] As will be described in further detail below, conductor
element 12.1 is adapted to be placed axially adjacent to a magnetic
element such as a magnetic disk with sector shaped poles. The
relationship of one of these poles to conductor element 121 is
shown in FIG. 13 in a dashed outline form as element 100. To
maximize the amount of conductive material in the conductor phases
adjacent to the magnetic poles, each radial section is tapered or
wedge-shaped, i.e., their widths decrease in a radially inward
direction, thereby allowing them to be packed closely together in
the spoke-like arrangement. Phase windings 122, 124, 126 and 128
are made of metal, preferably cast or otherwise formed into the
illustrated winding shape, but it may also be suitable to form
dielectric coated rectangular tapered metal wire into the
illustrated winding shape to reduce eddy currents in the
conductors. Packing conductors 122, 124, 126 and 128 closely
together maximizes the amount of their conductive material that
passes through the flux. The ratio between the volume of conductive
material that passes through the magnetic flux and the volume of
the entire conductor element that passes through the flux is known
as the "fill factor." The fill factor for the stator shown in FIG.
14 is generally greater than 80 percent and is typically between
60% and 90%. Increasing the fill factor maximizes the efficiency
and motor constant as given in Eqs. 1 and 2 above by minimizing the
resistance R of the conductive material. Power density is also
improved by maximizing the fill factor even though the conductive
material adds mass to the machine because the added conductive
material promotes torque production.
[0056] In an alternative embodiment of the stator, each conductor
element comprises one or more subassemblies, each formed, for
example, of printed circuit material that has been suitably etched
to form the conductor pattern and electrical interconnections
between subassemblies described below. The printed circuit material
and etching process may be any such material and process known in
the art that is commonly used to manufacture printed circuit boards
or flexible printed circuits in the electronics industry. The
subassemblies can be bonded together or otherwise attached to one
another. The resulting multiple-layer printed circuit conductor
element functions in the same manner as conductor element 121 in
FIGS. 13 and 14, described above. In that regard, this alternative
conductor element may have any suitable number of conductor
windings and conductor phases. The alternative stator assembly may
have a thickness as small as about 0.1 inches, thereby facilitating
the construction of smaller axial field electric machines.
Nevertheless, a typical alternative conductor element for a small
electric machine (e.g., one producing 7.5 HP) may have a thickness
of about 0.25 inches. Larger motors may be constructed using an
alternative conductor element having a thickness as great as about
two inches.
[0057] This alternative conductor element includes one or more
subassemblies such as subassembly 129 in FIGS. 16a-b. Subassembly
129 includes a substrate 129a having first and second sides, which
is made of a suitable dielectric or insulating material. Multiple
conductive traces 131 are formed on substrate 129a to provide
conductor windings in subassembly 129. For example, substrate 129a
can be made of a common substrate material such as FR4 or other
thin sheet-like plastic material. In this embodiment, subassembly
129 includes a composite material sheet, commonly referred to as
"flex PC," where substrate 129a is a thin sheet-like plastic which
is bonded to copper. For example, substrate 129a can have a
thickness less than about 0.010 --inches (10 mils) thick and is
preferably 1 to 3 mils thick. The flex PC material includes a
dielectric substrate and a 3 mil thick layer of copper on each of
the first and second sides of the substrate. Conductive traces 131
are formed on substrate 129a by etching away copper between
adjacent traces. To increase the amount of conductive material in
each subassembly 129, the thickness of the conductive traces 131 is
then increased to six mils on the first and second sides of the
substrate. This can be achieved using a well-known mask and
sputtering technique. The space between adjacent conductive traces
131 is filled with a dielectric resin. In this embodiment, the
dielectric material for substrate 129a and for separating adjacent
traces 131 is rated to 2000 volts. The spacing between adjacent
conductive traces in this example is on the order of ten mils and
is preferably about four mils.
[0058] In this example, each conductive trace has a thickness of
six mils, but can be increased to 15 mils. As shown in FIG. 16a,
each conductive trace 131 includes an outer section 131b, a radial
section 131c that extends in a generally radial direction from an
outer diameter to an inner diameter of the conductor element, and
an inner section 131d that extends from the radial section 131c
towards a center of the conductor element. As with the conductor
element 121 of FIG. 14, subassembly 129 is designed to maximize the
amount of conductive material adjacent to the magnetic poles of an
axially adjacent magnetic element (described below). In other
words, subassembly 129 is designed to maximize the amount of
conductive material in the radial sections 131c of each conductive
trace 131. Doing so maximizes the fill factor which in turn
contributes to maximizing efficiency, motor constant, and power
density.
[0059] As illustrated in FIGS. 17 and 18, each subassembly can have
conductive traces on both sides of substrate 129a, in the manner
associated with what is commonly known as a two-sided printed
circuit board. In FIG. 17, conductive traces 131 on the first side
are shown in a solid line, and conductive traces 137 on the second
side are shown in a dashed line. Conductive traces 131 and 137 are
essentially identical, mirroring one another in size and position.
Each end of a conductor 131 is electrically connected to an end of
a conductor 137 via an inter-side through-hole 139. Each inter-side
through-hole 139 is plated on its interior to provide a conductive
path in a manner well-known in multi-layer printed circuit board
manufacture.
[0060] A first terminal through-hole 141 is disposed at one end of
one of conductive traces 131 (i.e., coupled to a terminal portion
131a of conductive trace 131), and a second terminal through-hole
143 is disposed at one end of another of conductive traces 131.
Terminal through-holes 141 and 143 are plated through-holes similar
to inter-side through-holes 139, but they do not connect conductive
trace 131 to conductive trace 137. Rather, terminal through-holes
141 and 143 form the terminals of an electrical circuit. The
conductor path of the circuit, a portion of which is indicated by
arrows 145 in FIG. 17, begins at terminal 141, follows one of
conductors 131 on the first side of substrate 129a changes sides
via one of inter-side through-holes 139, and continues through one
of conductors 137 on the second side of the subassembly. The
portion of the conductor path indicated by arrows 145 defines a
winding. (In this example, the winding has only a single turn of
conductor, in a manner similar to the embodiment described above
with respect to FIG. 14.) The circuit then follows a second winding
by again changing sides via another of inter-side through-holes
139, and continues through another of conductive traces 131. The
connections continue in such a manner (e.g., in a clockwise manner)
until bridge portion 145a. The connections proceed in an opposite
direction (e.g., in a counter-clockwise manner) to terminal 143.
The circuit shown in FIG. 17 includes twelve windings between the
two sides of the subassembly.
[0061] As shown in FIGS. 16 and 17, and as stated above, each
conductive trace 131 includes an outer section 131b, a radial
section 131c, and an inner section 131d. The inner section 131d and
the inner section 137d of a conductive trace on the other side of
substrate 129a are coupled via an inter-side through-hole 139. In
this embodiment, the inner sections 131d and 137d form
substantially 45.degree. angles with a line 138 tangential to an
inner radius of subassembly 129. Connecting the inner connector
portions 131b and 137b in such a manner minimizes the resistive or
I.sup.2R loss P.sub.r in the electric machine. Likewise, in this
embodiment, the outer connector portions 131b and 137b, coupled
together by an inter-side through-hole 139, form substantially 45
angles with a line 140 tangential to an outer radius of subassembly
129 for the same purpose.
[0062] Although a conductor element may include only the windings
of a single subassembly 129, such as that shown in FIG. 17, a
conductor element can include windings of multiple subassemblies
electrically connected in series or parallel. As illustrated in
FIG. 18, subassemblies 129 are bonded together to form a conductor
element. A plastic sheet 147 (e.g., of a dielectric or insulating
material such as the commonly known Prepreg material) between
layers 129 bonds the laminations together when heated and subjected
to pressure, and also electrically insulates conductive traces 137
of one subassembly from conductive traces 131 of an adjacent
subassembly. As illustrated in FIG. 19, terminal through-holes 141
of all subassemblies are electrically connected together, and
terminal through-holes 143 of all subassemblies are electrically
connected together, thereby electrically connecting the windings in
parallel to form a conductor element.
[0063] Referring to FIGS. 16a-f, the different subassemblies are
connected in serial. One skilled in the art will appreciate that
individual-subassemblies can be joined in series and/or parallel,
as desired, in a conductor element. FIGS. 16a-b depict the first
and second sides of a topmost subassembly, FIGS. 16c-d depict the
first and second sides of a second subassembly (i.e., under the
subassembly of FIGS. 16a-b), and FIGS. 16e-f depict the first and
second sides of the bottom subassembly. In this example, one of the
conductor windings begins at a terminal portion 150a and extends in
a generally radial direction towards the center of the conductor
element 129. In FIGS. 16a-f, arrows are used to show a relative
direction of current in radial portions of this conductor winding.
Conductor 150a is coupled to conductor 150b on the opposite side of
the subassembly as shown in FIG. 16b via an inter-side through
hole. Conductor 150b is coupled to conductor 150c (FIG. 16a) via
another inter-side through hole. Accordingly, in FIGS. 16a-b,
conductors 150a-1 are coupled together. Referring to FIG. 16b, a
bridge portion 151 couples conductor 1501 to 150m. Conductors
150m-x are coupled together in a manner similar to conductors
150a-1. In summary, the conductor winding in the subassembly shown
in FIGS. 16a-b starts at terminal portion 150a and continues
through conductors 150b-1, bridge portion 151, conductors 150m-x
and terminal section 152.
[0064] In this embodiment, terminal section 152 on an upper side of
the subassembly shown in FIGS. 16a-b is coupled to terminal section
153a on a bottom side of the subassembly shown in FIGS. 16c-d via a
terminal through hole. In a manner similar- to what is described
above, the conductor winding in the subassembly shown in FIGS.
16c-d starts at terminal portion 153a and continues through
conductors 153b-1, bridge portion 154a, conductors 153m-x and
terminal section 154. Terminal section 154 is coupled to another
terminal section in the next subassembly not shown via a terminal
through hole 155a (FIG. 16a). Terminal through holes 155b-k
likewise couple terminal sections of conductor windings in adjacent
subassemblies in the conductor element (as do the other unlabelled
terminal through holes). In the conductor assembly of FIGS. 16a-f,
thirteen subassemblies are coupled together in such a manner.
Terminal through hole 155k couples a terminal section in the
twelfth subassembly (not shown) to the terminal section 156a of the
thirteenth subassembly shown in FIGS. 6e-f. The conductor winding
starts at terminal section 156a and continues through conductors
156b-1, bridge section 157, conductors 156m-x and terminal section
158. Terminal section 158 is coupled to the uppermost side of the
first subassembly via a terminal through hole 1551. Accordingly, a
serial connection of the conductor windings begins at the terminal
section 150a and ends at terminal through hole 1551 in FIG.
16a.
[0065] In the example of FIG. 20, the conductor element includes
twelve conductor phase windings. In the 12-phase conductor element
of FIG. 20, conductive traces 131 and 137 are arranged at an
angular spacing of 2.5 degrees and two conductive traces 131 from
one phase are separated by eleven conductive traces 131 from the
other phase windings. For purposes of clarity, only a portion of
the conductor element is shown in FIG. 20, illustrating the pair of
terminals for phase-1, labeled ".phi..sub.1.sup.+" and
".phi..sub.1.sup.-" and the pair of terminals for phase-2, labeled
".phi..sub.2.sup.+" and ".phi..sub.2.sup.+" Nevertheless, the
completed conductor element would have 12 pairs of terminals for
phases 1-12. In the example of FIGS. 16a-f, the conductor element
includes eight conductor phase windings. In this example, adjacent
conductive traces from one phase are separated by one conductive
trace from each of the other seven phases.
[0066] In the embodiments of the conductor element described above
with respect to FIGS. 16-20, a fill factor for the conductor
element can be between 60 and 90% and is typically between 80% and
84%.
[0067] In view of the embodiments illustrated in FIGS. 14, and
16-20, persons of skill in the art will understand that in other
embodiments the conductors may have any suitable size, shape, and
number of windings and turns. For example, in an embodiment similar
to that illustrated in FIG. 14, each winding may have two turns of
rectangular wire having wedge-shaped elongated portions.
Magnetic Element
[0068] In the axial field electric machine of the present
invention, one or more magnetic elements are provided that interact
with the conductor elements discussed above with respect to FIGS.
14 and 16-20. For example, the rotor of the axial field electric
machine can include one or more magnetic elements such as the rotor
disk 14 shown in FIG. 4. Again, to achieve a high efficiency, motor
constant, and power density for the axial field electric machine,
it is advantageous if the magnetic elements have a low density and
a high energy product (as discussed further below).
[0069] As illustrated in FIG. 4, each rotor disk 14 may include an
annular magnet 54 mounted on a hub 56. Hub 56 can have hub
ventilation openings 58 with angled, vane-like walls for impelling
cooling air through housing 10. Each magnet 54 may be made from a
suitable ferroceramic material, such as M-V through M-VIII,
oriented barium ferrite (BaO--6Fe.sub.6--O), strontium ferrite
(SrO--6Fe.sub.6--O.sub.2), or lead ferrite
(PbO--6Fe.sub.6--O.sub.2). Alternatively, magnets 54 may be made
from a bonded or sintered neodymium-iron-boron (NdFeB) material.
Both ferroceramic magnets and NdFeB magnets are known in the art
and commercially available. As illustrated in FIG. 4, magnet 54 is
polarized to provide multiple magnetic poles or sectors 57
uniformly distributed angularly around magnet 54. Alternatively,
each magnetic element or rotor disk can include a plurality of
individual sector-shaped magnets that are joined together into an
annular shape with an appropriate adhesive or support
structure.
[0070] As illustrated in FIG. 5, each sector is polarized through
the thickness of magnet 54. Thus, each sector has opposite poles on
opposite faces 60 and 62 of the magnet 54. In addition, the poles
of sectors 57 on face 60 alternate with those of adjacent sectors
57 on face 60, and the poles of sectors 57 on face 62 alternate
with those of adjacent sectors on face 62. In this embodiment, each
rotor disk 14 is to be mounted on a shaft with the poles of its
magnet 54 axially aligned with opposite poles of any adjacent
magnets 54 (i.e., a North pole on face 62 of a first rotor magnet
54 will be axially aligned with a South pole on face 60 of a second
axially adjacent rotor magnet 54). Magnetic flux therefore travels
axially between such axially aligned poles.
[0071] As discussed above in this example, magnet 54 is mounted to
a hub 56 which in turn is mounted to a shaft. Referring to FIG. 9,
an example of a shaft 16a is shown. Shaft 16a is splined and
provides a mating surface for the central portion of the hub 56a as
shown in FIG. 10. It is preferable if magnet 54 is mounted to hub
56a before being magnetized to ensure proper orientation between
adjacent magnets when the hub 56a is placed onto shaft 16a.
[0072] As illustrated in FIG. 15, annular disks or endplates 64 and
66, made of a suitable high-permeability material such as steel,
are mounted to outer faces 60 of the magnet 54 of the endmost two
rotor disks 14. Endplates 64 and 66 contain the magnetic flux
between adjacent poles of the rotor magnet 54 adjacent to endplate
64 or 66. By mounting high permeability endplates to the endmost
two rotor disks, the endplates rotate with the rotor magnet thereby
eliminating the core loss associated with the high permeability
material in the flux path of the magnets. As a result, the
efficiency of the electric machine is maximized.
[0073] As illustrated in FIG. 15, conceptually, the magnetic flux
only "flows" from a sector 57 of a first one of the two endmost
rotor disks 14, through axially aligned sectors 57 of adjacent
magnets 54 until reaching the second one of the two endmost rotor
disks 14, where one of endplates 64 and 66 directs the flux to an
angularly adjacent sector 57. The flux then returns axially through
aligned sectors 57 of adjacent magnets 54 until again reaching the
first endmost rotor disk 14, where the other of endplates 64 and 66
directs the flux to an angularly adjacent sector 57. The magnets 54
other than the two endmost magnets 54 may be referred to herein for
convenience as inner rotor disks or magnets 54. The flux thus
follows a serpentine pattern, weaving axially back and forth
through aligned sectors 57 of magnets 54.
[0074] Magnet 54 has at least one South and one North pole on each
side 60 and 62. The minimum number of magnet poles distributed
around each face 60 or 62 of magnet 54 is a function of the
demagnetization characteristics of the magnet material used. If the
demagnetization characteristic has a "knee" in the second quadrant
of its B-H curve at room temperature, the number of magnet poles
must be sufficiently large to keep the magnet poles from being
irreversibly demagnetized before magnet 54 is assembled into the
electric machine.
[0075] FIG. 6 illustrates the axially-directed flux density B
profile emanating from magnet pole faces assembled into the
electric machine. The flux density is generally positive over North
poles and negative over South poles. Between North and South poles
the flux density passes through zero flux density at the midpoint.
70 between poles of magnet 54. When magnet 54 is formed by a single
piece of annular magnet material, the interpolar region 72 between
magnet poles represents permanent magnet material that is
nonuniformly magnetized due to limitations inherent in the
magnetizing process. When magnet 54 is formed from a plurality of
sector shaped magnets, the interpolar region 72 represents the
unmagnetized adhesive or support structure holding the magnets
together. The transition width d shown in FIG. 6 is the width
generally over the midpoint 70 where the axial flux density is
significantly diminished with respect to its peak value. As
explained in further detail below, this transition width d is used
as part of a design algorithm for the electric machine.
Electric Machine Design
[0076] As described in further detail below, two embodiments of an
electric machine designed according to embodiments of the present
invention will be shown. The first uses the conductor element
design shown in FIG. 14, the second uses the conductor element
design shown in FIGS. 16-20. According to an embodiment of the
present invention, the magnetic and conductor elements are designed
and ihe electric machine is designed so as to maximize the
efficiency, motor constant, and power density of the electric
machine. The embodiments described below have a modular design
allowing a user to select the number of conductor elements and
magnetic elements that are needed for a particular application.
First Embodiment
[0077] As illustrated in FIGS. 1-3, a first embodiment of the axial
field electric machine designed according to an embodiment of the
present invention is shown. The axial field electric machine
includes a housing 10 (the center section of which is shown
removed), multiple stator assemblies 12 (e.g., each including a
conductor element similar to the one shown in FIG. 14) connected to
one another and disposed within housing 10, and magnetic elements
14 (e.g., similar to the one shown in FIG. 4) connected to a shaft
16 that extends axially through housing 10. In this example, the
conductor elements make up the stator of the electric machine and
the magnetic elements make up the rotor. One skilled in the art
will appreciate that in an alternative embodiment, conductor
elements can serve as the rotor and the magnetic elements can serve
as the stator in the electric machine.
[0078] Housing 10 includes two endpieces 18 and 20, each having
multiple housing ventilation openings 22. Housing 10 may also
include at least one removable midsection piece between endpieces
18 and 20 that is indicated as a phantom line in FIGS. 1-3 but not
shown for purposes of clarity. Endpieces 18 and 20 and the
removable midsection pieces can be made of a light-weight plastic
or metal (e.g., aluminum). Bolts 24 extend from endpiece 18 axially
through housing 10 through each stator assembly 12 and are secured
by nuts 26 at endpiece 20. At one end of housing 20, ball bearings
28 retained between a first bearing race 30 connected to shaft 16
and a second bearing race 32' connected to endpiece 18 facilitate
rotation of shaft 16 with respect to housing 10. A similar bearing
arrangement having ball bearings 34 retained between a first
bearing race 36 connected to shaft 16 and a second bearing race 38
connected to endpiece 20 facilitate rotation of shaft 16 at the
other end of housing 10.
[0079] In this embodiment, magnetic elements 14 are interleaved
with stator assemblies 12 in the axial field electric machine. As
shown in FIG. 14, conductor element 121 may include sockets 42
allowing any number of the stator assemblies 12 to be assembled
into the electric machine. Conversely, the stator assemblies can be
removed from the electric machine as desired. Removable pins 40
plug into sockets 42 to electrically connect each stator assembly
12 to an axially adjacent stator assembly 12. Accordingly,
depending on the desired application (e.g., power output
requirements), a selected number of stator assemblies 12 and
magnetic elements 14 can be added to or subtracted from the
electric machine as necessary.
[0080] An example of a stator assembly is shown in FIG. 11. In this
embodiment, conductor element 121 is embedded, molded or similarly
encased in a substantially annular stator casing 104 made of a
suitable dielectric or insulative material. Stator assembly 12 has
bores 106 through which bolts 24 may be extended to physically
interconnect them, as described above with respect to FIGS. 1 and
2. As similarly described above, stator assembly 12 has sockets 42
that may be electrically interconnected by removable pins 40.
Stator casing 104 has a central opening 108 through which shaft 16
extends when the electric machine is assembled, as illustrated in
FIG. 2. The diameter of shaft 16 is less than that of central
opening 108 to facilitate airflow through the axial field electric
machine.
[0081] The modular construction of the electric machine facilitates
selection of an operating voltage. Operating voltage is
proportional to the total conductor length for each phase. Thus, an
operating voltage may be selected by adjusting the total conductor
length for each phase. Each stator assembly 12 has conductors 110,
112, 114 and 116, each defining one of the four phases. (See, e.g.,
FIG. 13.) By connecting, for example, conductor 110 in each stator
assembly 12 in parallel with conductor 110 in all other stator
assemblies 12, the total conductor length for phase-1 is minimized.
Conversely, by connecting, for example, conductor 110 in each
stator assembly 12 in series with conductor 110 in all other stator
assemblies 12, the total conductor length for phase-1 is maximized.
The modular construction facilitates selectively connecting the
conductors of adjacent stator assemblies in either series or
parallel.
[0082] One skilled in the art will appreciate that the magnetic
element in the electric machine described herein can be replaced
with an suitably constructed aluminum disk to operate the electric
machine as an induction machine.
[0083] As illustrated in FIG. 1, each stator assembly 12 has
indicia 158, 160 and 162, such as adhesive labels, each indicating
one of the voltages that may be selected. An operating voltage can
be selected by connecting each stator assembly 12 in an angular
orientation in which the indicia indicating a certain voltage are
aligned. Indicia 158 are labeled "120" to indicate 120 volts;
indicia 160 are labeled "480" to indicate 480 volts; and indicia
162 are labeled "960" to indicate 960 volts. In the exemplary
embodiment and the relative angular orientation of stator
assemblies 12 shown in FIG. 1, indicia 158 are aligned to select an
operating voltage of 120 volts. To change the operating voltage,
one need only uncouple one or more stator assemblies 12 and rotate
them to realign indicia 158 such that they align to indicate a
different operating voltage.
[0084] As illustrated schematically in FIG. 23, stator assemblies
12 are interconnected to select a first operating voltage, such as
120 volts. Broken lines indicate an electrical connection. With
respect to phase-1, each end of conductor 110 in each stator
assembly 12 is connected by a removable pin 40 to the corresponding
end of conductor 110 in another stator assembly 12. Thus, all
conductors 110 are connected in parallel. Similarly, with respect
to phase-2, each end of conductor 112 in each stator assembly 12 is
connected by a removable pin 40 to the corresponding end of
conductor 112 in another stator assembly 12. Thus, all conductors
112 are connected in parallel. All conductors 114 and 116 are
similarly connected in parallel. Pins 40 at one of the endmost
stator assemblies 12 may be connected to electrical power leads 44
(FIG. 1). It should be noted that all indicia 158 are aligned, but
indicia 160 and indicia 162 are not aligned.
[0085] As illustrated schematically in FIG. 24, stator assemblies
12 are interconnected to select a second operating voltage, such as
960 volts. As in FIG. 24, broken lines indicate an electrical
connection. With respect to phase-1, with the exception of the two
endmost stator assemblies 12, a first end of conductor 110 in each
stator assembly 12 is connected by a removable pin 40 to a second
end of conductor 110 in another stator assembly 12. Thus, all
conductors 110 are connected in series. Similarly, with respect to
phase-2, with the exception of the two endmost stator assemblies
12, a first end of conductor 112 in each stator assembly 12 is
connected by a removable pin 40 to a second end of conductor 112 in
another stator assembly 12. Thus, all conductors 112 are connected
in series. All conductors 114 and 116 are similarly connected in
series. Pins 40 at the endmost stator assemblies 12 may be
connected to electrical power leads 44 (FIG. 1). It should be noted
that all indicia 162 are aligned, but indicia 158 and indicia 160
are not aligned.
[0086] As illustrated schematically in FIG. 25, stator assemblies
12 are interconnected to select a third operating voltage, such as
480 volts. In the same manner as in FIGS. 23 and 24, broken lines
indicate an electrical connection. With respect to phase-1, with
the exception of the two endmost stator assemblies 12, the
corresponding first and second ends of conductors 110 in two
adjacent stator assemblies 12 are connected to each other by a
removable pin 40; a first end of conductor 110 in one of those
stator assemblies 12 is connected by a removable pin 40 to a second
end of conductor 110 in a third stator assembly 12; and the
corresponding first and second ends of conductors 110 in the third
stator assembly 12 and an adjacent fourth stator assembly 12 are
connected to each other by a removable pin 40. Thus, two conductors
110 are connected in parallel form a group, and then these groups
are connected in series. Similarly, with respect to phase-2, with
the exception of the two endmost stator assemblies 12, the
corresponding first and second ends of conductors 112 in two
adjacent stator assemblies 12 are connected to each other by a
removable pin 40; a first end of conductor 112 in one of those
stator assemblies 12 is connected by a removable pin 40 to a second
end of conductor 112 in a third stator assembly 12; and the
corresponding first and second ends of conductors 112 in the third
stator assembly 12 and an adjacent fourth stator assembly 12 are
connected to each other by a removable pin 40. Thus, groups of two
conductors 112 are connected in parallel, and then these groups are
connected in series. All conductors 114 and 116 are similarly
connected in parallel groups of two that are connected in series.
Pins 40 at the endmost stator assemblies 12 may be connected to
electrical power leads 44 (FIG. 1). It should be noted that all
indicia 160 are aligned, but indicia 158 and indicia 162 are not
aligned.
[0087] Those skilled in the art will appreciate that the conductors
may be interconnected in various combinations of series and
parallel groups to provide more than three selectable voltages.
Moreover, the illustrated set of voltages is exemplary only; in
view of the teachings herein, persons of skill in the art will
readily be capable of constructing a electric machine operable at
other voltages.
[0088] Electrical power leads 44 extend into housing 10 and have
plugs 46 that connect to sockets 42 in one of the two endmost
stator assemblies 12. Although FIG. 3 illustrates a power lead 44
connected to the endmost stator assembly 12 adjacent endpiece 20,
it could alternatively be connected to the endmost stator assembly
12 adjacent endpiece 18 or an intermediate stator assembly 12. As
illustrated in FIGS. 1 and 3, openings or ports 48 and 50 in
endpieces 18 and 20, respectively, admit plugs 46 into housing-10.
A sensor 52, such as a Hall-effect sensor, is mounted to endpiece
20. Sensor 52 is adjacent the endmost magnetic element 14 for
sensing pole transitions, as described below with respect to the
operation of the electric machine. One skilled in the art will
appreciate that other devices can be used to sense pole transitions
in a magnetic element 14. For example, an optical grating may be
placed around the periphery of an magnetic element and an
opticoupler can be used to sense reflected light from the grating
using a stationary light source to indicate the position of the
magnetic poles relative to the stator assemblies.
Second Embodiment
[0089] A second embodiment of the electric machine of the present
invention is shown in FIGS. 12, 27-29 using the conductor element
of FIGS. 16-20. Referring to FIG. 12, a cross section of this axial
field electric machine is shown. The axial field electric machine
200 is similar in construction to the electric machine of FIGS. 1
and 3. Electric machine 200 includes a plurality of magnetic
elements 201, such as rotor disks, attached to a shaft 205. In this
example, shaft 205 has a configuration similar to that which is
shown in FIG. 9. Hubs of axially adjacent magnetic elements are
separated by a ring separator 209. Electric machine 200 includes a
plurality of conductor elements 202 and connector support elements
203, the construction of which is described in further detail
below. As with the electric machine design of FIGS. 1 and 3,
electric machine 200 has a modular design in that any number of
conductor elements 202 (and connector support elements 203) and
magnetic elements 201 may be added to or subtracted from the
electric machine as desired.
[0090] In this embodiment, each conductor element includes a frame,
such as frame 210 shown in FIG. 27. In the front view of FIG. 27,
frame 210 includes mounting holes 212, for insertion of a bolt or
the like to secure one frame to one or more such frames in the
electric machine. Frame 210 also includes apertures 211 to allow
air flow into and out of the electric machine.
[0091] Referring to FIG. 28, a front view of the connector support
element 203 is shown. The connector support element 203 also
includes mounting holes 212 (as in FIG. 27) for mounting to an
adjacent frame 210. Connector pin assemblies 217 are provided to
electrically connect selected conductor phases of one conductor
element to selected conductor phases of an axially adjacent
connector assembly. In this embodiment, the connector pin assembly
includes a number of pins 220 coupled to a number of sockets 221.
Accordingly, pins 220 of one connector support element 203 mate
with sockets 221 of an axially adjacent connector support element
203. A Hall sensor 216 can be provided for sensing pole transitions
in a magnetic element rotating within an opening of the connector
support element. Also, high voltage switches 218 can be provided to
switch power on and off to the conductor phases of the conductor
element (see FIG. 29).
[0092] Referring to FIG. 29, a partially completed axial field
electric machine is shown with a conductor element of FIGS. 16-20,
the connector support element 203 of FIG. 28, and the magnet of
FIG. 3. The high voltage switches 218 and connector pin assemblies
217 are selectively coupled to conductor phases of the conductor
element. In this example, the conductor element is shown in FIG. 16
and includes mounting holes for mounting it to adjacent conductor
elements.
Controller
[0093] As illustrated in FIG. 21, the electric machine may be
configured as a motor by connecting a brushless motor controller
130 of an essentially conventional design. In this example,
brushless motor controller 130 receives a pole sense signal 132
from sensor 52 (FIG. 3) and generates signals 134 (.phi..sub.1-),
136 (.phi..sub.1+), 138 (.phi..sub.2-), 140 (.phi..sub.2+), 142
(.phi..sub.3-), 144 (.phi..sub.3+), 146 (.phi..sub.4-) and 148
(.phi..sub.4+) for the conductor phases in conductor element 121
in. FIG. 14. Signals 134, 136, 138, 140, 142, 144, 146 and 148 are
coupled to electrical leads 44, as described above with respect to
FIG. 2.
[0094] As shown in the timing diagram of FIG. 22, brushless motor
controller 130 attempts to drive current in each phase while that
phase is subjected to flux from a pole sector in magnet 54. As
described in further detail herein, it is preferable if the width
of the radial portion of the conductor phases that pass through the
flux of the magnet 54 have a width that does not exceed the
transition width d between adjacent poles as shown in FIG. 6.
Accordingly, as a phase conductor travels across one magnet pole
face, current is being driven into each phase conductor 75% of the
time.
[0095] In the timing diagram of FIG. 22, the voltage amplitude
signals for each of the phases are shown. In this example, the
voltage amplitude for each phase fluctuates between +350V, 0V, and
-350V D.C. The brushless motor controller 130 includes a chopping
or pulse-width modulating (PWM) circuit, as is known in the art,
which converts the D.C. voltage signal into a square wave signal
having a duty cycle between 0 and 100%. In this example, the
frequency of the pulse-width modulation is 20 KHz. Looking at the
voltage signal for .phi..sub.1, the signal is at 0V when the
.phi..sub.1 phase conductor is completely within a transition width
between magnet poles. In FIG. 22, the pole sense signal is
generated when a transition width is passing the pole sensor. As
the phase conductor passes from the transition width to the next
pole sector, the voltage amplitude jumps to .+-.350V (depending on
direction of rotation) and the duty cycle is set to a low value
(e.g., 5%). The duty cycle can be raised as the phase conductor
moves into the pole sector, and the duty cycle is at a maximum when
the phase conductor is completely within a pole sector. The
selection of a maximum duty cycle depends on the desired current in
each conductor phase (e.g., based on torque, speed, and/or power
requirements). The duty cycle is again lowered when the phase
conductor once again begins to move within the next transition
width. The duty cycle is lowered to zero when the phase conductor
is completely within the transition width. As the phase conductor
moves into the next pole, the duty cycle is increased, but the
voltage level is inverted (i.e., from positive to negative or
negative to positive). In FIG. 22, one pole sense signal is
generated which is related to the presence of the .phi..sub.1
conductors in the transition width. Pole sense signals relative to
the phase conductors .phi..sub.2-.phi..sub.4 can be generated based
on the pole sense signal for .phi..sub.1. Alternatively, pole sense
signals can be generated for all poles (e.g., using an optical
grating pattern around the periphery of a magnet).
[0096] The motor controller can be easily modified to provide the
same voltage signals for any number of phases, such as the eight
phases shown in FIG. 16 and the twelve phases shown in FIG. 20. In
the case of eight phases, current will be conducted in each phase
87.5% of the time. In the case of twelve phases, current WIll be
conducted in each phase 91.67% of the time.
[0097] As shown in these embodiments, only the phase conductor
within the transition width d closest to the midpoint (e.g., 70 in
FIG. 6) between magnet poles is nonconducting at any rotor
position. Therefore at any given rotor position a motor having N
phase windings will have 100(N-1)/N percent of its phase conductors
conducting current and producing torque. As a result, the electric
machine maximizes its conductor utilization, which maximizes
efficiency, motor constant, and power density.
Design Considerations
[0098] With the structure of the axial field electric machine given
above for the first embodiment, the specific design of the
conductor elements 121 and the magnetic elements 14 to achieve high
efficiency, high motor constant, and high power density is given
below. With this design algorithm, the axial field electric machine
of this embodiment minimizes the I.sup.2R loss denoted P.sub.r
earlier, minimizes the core loss P.sub.c, minimizes eddy current
losses, and maximizes the production of torque. As a result, the
electric machine will achieve and maintain high efficiency over a
wide range of speeds, will exhibit a high motor constant, and
achieve high power density because torque production is
optimized.
[0099] Referring to FIG. 31, a graph is shown of the efficiency of
an electric machine constructed according to the present invention.
In comparison to FIG. 30, the electric machine obtains a higher
efficiency over a broader range of operating points. Accordingly,
in a traction application requiring operation of the electric
machine at several operating points, the average efficiency will be
far in excess of a typical electric machine.
[0100] A first design objective is to select an axial spacing
between adjacent magnetic elements in the axial field electric
machine. As discussed above, the stator assemblies 12 are disposed
between adjacent magnetic elements. The permanent magnet flux, as
described by its flux density B, that passes from one magnetic
element axially through a stator assembly, then through the
adjacent magnetic element determines the torque and back EMF (i.e.,
the performance) of the axial field electric machine. As such, it
defines the operating point of the motor. This operating point is
commonly characterized in the art as the intersection between the
magnetic circuit load line and the demagnetization curve of the
permanent magnet material used in the magnetic elements. Here the
magnetic circuit is a mathematical characterization of the physical
path taken by the magnetic field and its interaction with the
materials in that path. Two example demagnetization curves of a
magnet are shown in FIG. 7. As is known in the art, curve 71 is the
demagnetization curve of a magnet that does not have a knee,
whereas demagnetization curve 72 has a knee where the
characteristic bends toward the horizontal axis when the curve
nears the axis. The presence of a knee, the slope of the curves,
and the intersection of the curves with the two axes is a function
of the magnet material type as well as temperature, with higher
performance and generally more expensive magnet material having
higher points of intersection and no knee at room temperature.
[0101] Also shown in FIG. 7 are three example magnetic circuit load
lines, 81, 82, and 83 each having a different slope. The absolute
value of the load line slope is known in the art as the permeance
coefficient, PC, which is illustrated in FIG. 7. In its simplest
form, the permeance coefficient is approximated by
PC=L.sub.m/L.sub.g (Eq. 3) where L.sub.m is the magnet length in
the direction of magnetization (i.e., the axial direction in this
invention) and L.sub.g is the net magnetic flux path length in air
(including that through stator assemblies disposed between adjacent
magnetic elements). Based on this approximation and with reference
to FIG. 7, for a fixed magnet length L.sub.m, the electric machine
operating flux density B.sub.m is inversely proportional to
L.sub.g. See for example, B.sub.m marking flux density at the
intersection of magnet demagnetization curve 71 and load line 82.
As L.sub.g increases, the flux density operating point B.sub.m
decreases and as L.sub.g decreases, B.sub.m increases.
[0102] With this understanding of the inverse relationship between
the electric machine flux density operating point B.sub.m and the
net magnetic flux path length in air L.sub.g, the optimum spacing
between magnetic elements is based on the ideas (a) if L.sub.g is
zero, B.sub.m is maximized giving the potential for high torque
since torque is proportional to flux density. However, if L.sub.g
is zero there is no room between axially adjacent magnetic elements
for stator assemblies containing conductor elements through which
torque can be created. Therefore L.sub.g=0 is not feasible. (b) On
the other hand, if L.sub.g is made very large, the conductor
elements can be made very thick in the axial direction, which
minimizes the I.sup.2R losses. However, making L.sub.g large forces
the flux density operating point B.sub.m to such a small value that
little torque can be generated. Therefore making L.sub.g large is
not feasible. (c) The product of field intensity H (i.e., the
horizontal axis in FIG. 7) and flux density B (i.e., the vertical
axis in FIG. 7) is energy density. As such, it is known in the art
that operating a permanent magnet where the absolute value of the
product of the flux density operating point B.sub.m and the field
intensity point H.sub.m is greatest, maximizes the usable energy
available from the magnet material. In other words, operating at
the maximum energy density point provides the maximum flux density
for the least magnet volume or mass. For an electric machine
seeking to maximize power density, this is an optimum operating
point. For most commonly available permanent magnet materials, the
maximum energy density point occurs at or near a permeance
coefficient of one. FIG. 7 illustrates this point at the
intersection of demagnetization curve 71 and load line 82. Using
this value, a permeance coefficient of one as dictated by Eq. 3
implies that the optimum spacing between adjacent magnetic elements
("S" in FIG. 15) is equal to the axial length of the magnet
(L.sub.m in FIG. 6).
[0103] A second design objective is to determine the optimum size
of the transition width ("d" in FIG. 6) between adjacent poles of a
magnet 54. In the transition width area, flux emanating from one
magnet pole flows in approximately a semicircular path to an
adjacent pole on the same magnet 54, rather than traversing axially
to an adjacent magnet. Under the assumption that the transition
between axial flow to an adjacent magnet versus semicircular flow
to an adacent magnet occurs when the flux paths are equal in
length, the transition width is given by d=2L.sub.g/.pi. (Eq. 4)
where L.sub.g is the spacing 77 in FIG. 15. Therefore, once the
spacing 77 is determined by the maximum energy density point of the
magnet, Eq. 4 gives the transition width.
[0104] A third design objective is to determine the maximum width
of each conductor phase, i.e., the section that extends radially in
the conductor element through which torque producing magnetic flux
flows, e.g., 131c in FIG. 16. According to an embodiment of the
present invention, the maximum width of each conductor phase is
selected to be no wider than the transition width d as given be Eq.
4. This choice maximizes motor efficiency as well as motor constant
and power density for two reasons. First, it minimizes losses due
to eddy currents induced in each conductor phase due to motion of
the magnet 54. By limiting the width of the conductor phase to the
transition width, at no time does any conductor phase
simultaneously experience significant magnetic flux in both the
North and South directions. As a result, there are no instants
where significant eddy currents are induced in any conductor phase,
which in turn increases motor efficiency and indirectly power
density. Second, by limiting the width of conductor phases, more
conductor phases can be placed radially around the circumference of
the conductor element, thereby increasing the number of phase
windings and the percentage of the conductor phases conducting
current and producing torque simultaneously. As stated earlier,
this maximizes torque production while minimizing losses. For
example, the exemplary conductor element in FIG. 14 has thirty-two
radial sections and four phase windings or motor phases. The number
of motor phases is generally given by length of the outer periphery
of sector 57 divided by the transition width. The number of motor
phases is in effect the number of transition widths that fit within
sector 57. If R is the outer radius of a sector 57, N.sub.s is the
number of sectors needed to form a complete annulus, and d is the
transition width, the number of motor phases N.sub.p is given by
N.sub.p(2.pi.R)/(dN.sub.s) (Eq. 5) Those skilled in the art will
recognize that some dimensional variations in R and d are typically
required to make the number of motor phases given by Eq. 5 closely
approximate an integer. Uses
[0105] The axial field electric machine may be used to power any
suitable type of device, machine or vehicle. For example, it may be
used in domestic appliances such as refrigerators and washing
machines. It may also be used to power vehicles such as
automobiles, trains and boats. One such use as a power plant in a
vehicle is illustrated in FIG. 26. In the embodiment illustrated in
FIG. 26, the axial field electric machine is mounted in a casing
164 that functions as the hub for a traction device such as the
rubber tire 166 of an automotive vehicle 168. The shaft 170 is
fixedly, i.e., non-rotatably, connected to the body of vehicle 168.
The rotor disks 172, which are of substantially the same
construction as described above with respect to other embodiments,
are fixedly connected to casing 164 and thus rotate with tire 166.
The stator assemblies 174 are fixedly connected to shaft 170 but
are otherwise constructed as described above with respect to other
embodiments. In operation, the rotation of rotor disks 172 propels
the vehicle while the shaft remains stationary with respect to the
ground.
[0106] In another application shown in FIG. 8, the axial field
electric machine of the present invention can be used to reduce
operating costs for an air conditioner unit. In FIG. 8, an axial
field electric machine 230 operating as a motor and constructed
according to an embodiment of the present invention is coupled to a
compressor 231 in the air conditioner unit 232. Due to its small
size (i.e., relative to other motors used in these units) and high
efficiency, the axial field electric machine 230 can be sealed
within the compressor 231 in the air conditioner unit 232. Because
of the high efficiency of electric machine 230, the operating costs
for the air conditioner unit 232 can be substantially reduced.
[0107] The axial field electric machine of the present invention
can be used in a variety of other applications. While this electric
machine can be used in virtually any electric machine application,
its high efficiency, motor constant, and power density make it
attractive for applications where these traits have significant
value to the end user or product. For example, the electric machine
of the present invention is attractive for many battery driven
applications such as electric vehicles, including wheel chairs,
scooters for elderly people, golf carts, and undersea vehicles. In
these applications, the low mass and high efficiency of the present
invention increases the vehicle range before battery recharging.
The electric machine of the present invention is also valuable in
other portable applications such as portable generators for
commercial and military use. In these applications, the low mass of
the present invention makes it easier to transport the end product
and also saves fuel due to the increased energy conversion
efficiency of the generator. Yet another area where the electric
machine of the present invention will be useful is in applications
requiring tight integration of the electric machine with the end
product. Examples in this area include robotics, semiconductor
processing equipment, embedded pumps and compressors, and a variety
of other high throughput automatic tasks. As it stands, the
electric machine of the present invention is superior or
competitive in almost all applications. The degree to which it
makes inroads in any application is dependent upon the degree with
which high efficiency, motor constant, and power density impact the
end product in which the electric machine appear. For example, it
is unlikely that the electric machine of the present invention will
become popular in hand-held consumer hair dryers, residential
vacuum cleaners, and consumer appliances. Since high efficiency,
motor constant, and power density are not as important as cost in
these applications, the present invention will appear in these
applications only if the materials and manufacturing cost of the
present invention become competitive with the electric machines
currently used in these applications.
[0108] Other embodiments and modifications of the present invention
will occur readily to those of ordinary skill in the art in view of
these teachings. Therefore, this invention is to be limited only by
the following claims, which include all such other embodiments and
modifications when viewed in conjunction with the above
specification and accompanying drawings.
* * * * *