U.S. patent application number 15/684490 was filed with the patent office on 2017-12-07 for high efficiency permanent magnet machine.
The applicant listed for this patent is Louis Chow, Martin Epstein, Jon Harms, Yang Hu, Hanzhou Liu, Wei Wu, Xinzhang Wu. Invention is credited to Louis Chow, Martin Epstein, Jon Harms, Yang Hu, Hanzhou Liu, Wei Wu, Xinzhang Wu.
Application Number | 20170353062 15/684490 |
Document ID | / |
Family ID | 51060454 |
Filed Date | 2017-12-07 |
United States Patent
Application |
20170353062 |
Kind Code |
A1 |
Wu; Xinzhang ; et
al. |
December 7, 2017 |
HIGH EFFICIENCY PERMANENT MAGNET MACHINE
Abstract
The present invention is a high efficiency permanent magnet
machine capable of maintaining high power density. The machine is
operable over a wide range of power output. The improved efficiency
is due in part to copper wires with a current density lower than
traditional designs and larger permanent magnets coupled with a
large air gap. In a certain embodiment, wide stator teeth are used
to provide additional improved efficiency through significantly
reducing magnetic saturation resulting in lower current. The
machine also has a much smaller torque angle than that in
traditional design at rated load and thus has a higher overload
handling capability and improved efficiency. In addition, when the
machine is used as a motor, an adaptive phase lag compensation
scheme helps the sensorless field oriented control (FOC) scheme to
perform more accurately.
Inventors: |
Wu; Xinzhang; (Oviedo,
FL) ; Liu; Hanzhou; (Oviedo, FL) ; Hu;
Yang; (Oviedo, FL) ; Chow; Louis; (Orlando,
FL) ; Harms; Jon; (Seminole, FL) ; Epstein;
Martin; (Seminole, FL) ; Wu; Wei; (Orlando,
FL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wu; Xinzhang
Liu; Hanzhou
Hu; Yang
Chow; Louis
Harms; Jon
Epstein; Martin
Wu; Wei |
Oviedo
Oviedo
Oviedo
Orlando
Seminole
Seminole
Orlando |
FL
FL
FL
FL
FL
FL
FL |
US
US
US
US
US
US
US |
|
|
Family ID: |
51060454 |
Appl. No.: |
15/684490 |
Filed: |
August 23, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14449208 |
Aug 1, 2014 |
9780608 |
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15684490 |
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14146059 |
Jan 2, 2014 |
8829742 |
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14449208 |
|
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61803993 |
Mar 21, 2013 |
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61748998 |
Jan 4, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K 21/14 20130101;
H02K 3/28 20130101; H02K 21/16 20130101; H02P 6/153 20160201; H02P
6/182 20130101; H02K 2213/03 20130101; H02K 3/12 20130101; H02K
1/274 20130101; H02K 1/16 20130101 |
International
Class: |
H02K 1/16 20060101
H02K001/16; H02K 21/14 20060101 H02K021/14; H02K 1/27 20060101
H02K001/27; H02P 6/15 20060101 H02P006/15; H02K 3/28 20060101
H02K003/28; H02K 3/12 20060101 H02K003/12; H02P 6/182 20060101
H02P006/182; H02K 21/16 20060101 H02K021/16 |
Claims
1. An electric machine, comprising: a rotor, the rotor further
including: a cylindrical shape and a central axis; an axial bore
adapted to receive a shaft; an outer circumference adapted to
receive a plurality of permanent magnets comprising of permanent
magnet material; a stator, the stator further including: a
cylindrical shape and a longitudinal axis; an outer circumference;
a plurality of teeth disposed in an equidistant angular
relationship, each tooth projecting inwardly toward the
longitudinal axis, the teeth creating a discontinuous inner
circumference configured to accept the rotor within the stator such
that the longitudinal axis of stator is aligned with the central
axis of the rotor and the rotor is rotatable within the inner
circumference of the stator; a stator slot of predetermined shape
and size disposed between each tooth and adapted to receive a
plurality of strands of conducting material to form an electrical
winding around at least one tooth; each stator tooth having a width
between 60 and 80 percent of a distance between corresponding
points in adjacent stator slots; and an air gap between the outer
circumference of the rotor and the inner circumference of the
stator when the rotor is disposed within the stator.
2. The electric machine according to claim 1, further comprising a
current density ranging between 3 and 8 Amp/mm2.
3. The electric machine according to claim 1, further comprising a
torque angle between 2 and 10 degrees.
4. The electric machine according to claim 1, further comprising a
three-phase electrical winding scheme where each phase has two
groups and each group includes two coils connected in series.
5. The electric machine according to claim 1, further comprising
the rotor having a plurality of rotor sheets affixed to one another
by a thermal insulating adhesive disposed between adjacent rotor
sheets.
6. The electric machine according to claim 1, further comprising
the stator having a plurality of stator sheets affixed to one
another by a thermal insulating adhesive layer disposed between the
adjacent stator sheets.
7. The electric machine according to claim 1, further comprising a
sensorless field oriented control, where a rotor angle is estimated
by a sliding mode observer, the sliding mode observer module
containing a first order low-pass filter for back electromotive
force estimation.
8. The electric machine according to claim 1, further comprising a
control module configured to automatically calculate a phase lag
compensation value based on rotational speed of the rotor and is
applied to feedback increasing accuracy of the field oriented
control.
9. The electric machine according to claim 1, further comprising a
housing enclosing the stator and rotor, wherein the housing
includes a passive cooling mechanism.
10. The electric machine according to claim 1, further comprising
the rotor having between 8 and 16 poles when the intended
rotational speed of the rotor is less than 3000 rpm.
11. The electric machine according to claim 1, further comprising
the rotor having between 2 and 12 poles when the intended
rotational speed of the rotor is greater than 6000 rpm.
12. An electric machine, comprising: a rotor, the rotor further
including: a cylindrical shape and a central axis; an axial bore
adapted to receive a shaft; an outer circumference adapted to
receive a plurality of permanent magnets comprising of permanent
magnet material; a stator, the stator further including: a
cylindrical shape and a longitudinal axis; an outer circumference;
a plurality of teeth disposed in an equidistant angular
relationship, each tooth projecting inwardly toward the
longitudinal axis, the teeth creating a discontinuous inner
circumference configured to accept the rotor within the stator such
that the longitudinal axis of stator is aligned with the central
axis of the rotor and the rotor is rotatable within the inner
circumference of the stator; a stator slot of predetermined shape
and size disposed between each tooth and adapted to receive a
predetermined number of strands of conducting material to form an
electrical winding around at least one tooth; the conducting
material having a diameter that results in a current density
ranging between 3 and 8 Amp/mm2; and an air gap between the outer
circumference of the rotor and the inner circumference of the
stator when the rotor is disposed within the stator.
13. The electric machine according to claim 12, wherein each
permanent magnet has a magnetic flux density between 70 and 95
percent of a residual magnetic flux density of the permanent magnet
material.
14. The electric machine according to claim 12, further comprising
a torque angle between 2 and 10 degrees.
15. The electric machine according to claim 12, further comprising
a three-phase electrical winding scheme where each phase has two
groups and each group includes two coils connected in series.
16. The electric machine according to claim 12, further comprising
a sensorless field oriented control, where a rotor angle is
estimated by a sliding mode observer, the sliding mode observer
module containing a first order low-pass filter for back
electromotive force estimation.
17. The electric machine according to claim 12, further comprising
a control module configured to automatically calculate a phase lag
compensation value based on rotational speed of the rotor and is
applied to feedback increasing accuracy of the field oriented
control.
18. The electric machine according to claim 12, further comprising
the stator teeth each having a generally uniform width, wherein the
magnitude of the width is between 60 and 80 percent of the
magnitude of a slot pitch.
19. An electric machine, comprising: a rotor, the rotor further
including: having a cylindrical shape and a central axis; an axial
bore adapted to receive a shaft; an outer circumference adapted to
receive a plurality of permanent magnets comprising of permanent
magnet material; a stator, the stator further including: having a
cylindrical shape and a longitudinal axis; an outer circumference;
a plurality of teeth disposed in an equidistant angular
relationship, each tooth projecting inwardly toward the
longitudinal axis, the teeth creating a discontinuous inner
circumference configured to accept the rotor within the stator such
that the longitudinal axis of stator is aligned with the central
axis of the rotor and the rotor is rotatable within the inner
circumference of the stator; a stator slot of predetermined shape
and size disposed between each tooth and adapted to receive strands
of conducting material to form an electrical winding around at
least one tooth; an angle between the induced voltage and terminal
voltage with a magnitude between 2 and 10 degrees; and an air gap
between the outer circumference of the rotor and the inner
circumference of the stator when the rotor is disposed within the
stator.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This nonprovisional application is a continuation of and
claims priority to nonprovisional application Ser. No. 14/449,208
entitled "HIGH EFFICIENCY PERMANENT MAGNET MACHINE," filed Aug. 1,
2014, which is a continuation of and claims priority to
nonprovisional application Ser. No. 14/146,059 entitled "HIGH
EFFICIENCY PERMANENT MAGNET MACHINE," filed Jan. 2, 2014 and
patented on Sep. 9, 2014 as U.S. Pat. No. 8,829,742, which in turn
claim priority to provisional application No. 61/803,993 entitled
"HIGH EFFICIENCY PERMANENT MAGNET MACHINE," filed Mar. 21, 2013,
and to provisional application No. 61/748,998, entitled "HIGH
EFFICIENCY LOW SPEED PERMANENT MAGNET MACHINE," filed Jan. 4,
2013.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] This invention relates, generally, to electrical machines.
More specifically, it relates to a high efficiency electrical motor
or generator.
2. Brief Description of the Prior Art
[0003] Electrical motors and generators are machines capable of
converting electrical energy into mechanical energy and converting
mechanical energy into electrical energy, respectively. These
electrical machines have many similarities and can often be
operated as either an electrical generator or motor.
[0004] Known electrical machines include a rotor, a stator, a
plurality of electrical windings, and a plurality of permanent
magnets. The rotor is a generally cylindrical shape with an outer
circumference and an axial bore creating an inner circumference.
The inner circumference is adapted to receive a shaft such that the
two may rotate as one. The outer circumference of the rotor
contains a plurality of permanent magnets (PMs) disposed
longitudinally parallel to the axis of rotation or central axis of
the rotor in a preferably generally uniform and concentric
manner.
[0005] Current stator designs have a generally cylindrical shape
with an outer circumference and a predetermined number of teeth
projecting, a predetermined distance, inwardly towards the
longitudinal axis of the stator. The teeth create a discontinuous
inner circumference allowing the rotor and PM assembly to be
disposed within the stator such that the rotor assembly is freely
rotatable within the inner circumference of stator. Between each
tooth is a stator slot having predetermined shape and size adapted
to receive electrical windings. Electrical windings are typically
strands of conductive materials, such as copper or aluminum, which
are arranged into coil groups around the stator teeth. The
electrical windings interact with the PMs to produce either
mechanical or electrical energy. When configured as a motor, the
electrical machine uses current flowing through the electrical
windings to generate rotating magnetic fields which interact with
the PMs attached to the rotor and cause the rotor and shaft to
rotate. When configured as a generator, the PMs, and their
respective magnetic fields, are rotated and interact with the
electrical windings to produce electricity.
[0006] In recent years, the push towards green energy has increased
the demand associated with developing efficient electric machine
technology. The U.S. Department of Energy estimates that electric
motors in the U.S. consume more than half of all electrical energy
in the states. Therefore, improving the efficiency of these
electric machines will greatly decrease the United States' carbon
footprint.
[0007] Currently, some commercial off-the-shelf electric motors are
designed for high efficiency, but have power densities on the order
of only 0.1 horse power per pound (HP/lb.). Electric motors
developed for use in aircraft propulsion (small unmanned aerial
vehicles), on the other hand, have power densities between 1 and 2
HP/lb., while direct-drive electric motors can attain power
densities greater than 5 HP/lb. only through increased operating
speeds.
[0008] Traditionally in electric machine applications, the higher
the electric machine's efficiency, the less energy wasted and the
easier the thermal management system; however, the efficiency
generally comes at the cost of increased size and weight. Copper
loss is the term often given to heat produced by electrical
currents in the conductors of transformer windings, or other
electrical devices. Copper losses are an undesirable transfer of
energy, as are core losses, which result from induced currents in
adjacent components. Copper loss is the most significant in all the
losses in electric machines, so reducing the copper loss is the key
to building highly efficient machines. It is known that copper loss
is inversely proportional to the wire's cross-sectional area.
Therefore, copper wires having a greater cross-sectional area
(large diameter) and lower current density will also require a
larger slot area. The larger slot area requires the stator size to
increase or tooth size to decrease. If the stator size increases,
the machine becomes larger and the power density decreases. If the
tooth size decreases, the magnetic saturation increases, and so the
current must increase resulting in decreased efficiency.
[0009] Accordingly, what is needed is a highly efficient scalable
permanent magnet machine having relatively high power density while
being capable of operating at a wide range of power outputs.
However, in view of the art considered as a whole at the time the
present invention was made, it was not obvious to those of ordinary
skill in the field of this invention how the shortcomings of the
prior art could be overcome.
[0010] All referenced publications are incorporated herein by
reference in their entirety. Furthermore, where a definition or use
of a term in a reference, which is incorporated by reference
herein, is inconsistent or contrary to the definition of that term
provided herein, the definition of that term provided herein
applies and the definition of that term in the reference does not
apply.
[0011] While certain aspects of conventional technologies have been
discussed to facilitate disclosure of the invention, Applicants in
no way disclaim these technical aspects, and it is contemplated
that the claimed invention may encompass one or more of the
conventional technical aspects discussed herein.
[0012] The present invention may address one or more of the
problems and deficiencies of the prior art discussed above.
However, it is contemplated that the invention may prove useful in
addressing other problems and deficiencies in a number of technical
areas. Therefore, the claimed invention should not necessarily be
construed as limited to addressing any of the particular problems
or deficiencies discussed herein.
[0013] In this specification, where a document, act or item of
knowledge is referred to or discussed, this reference or discussion
is not an admission that the document, act or item of knowledge or
any combination thereof was at the priority date, publicly
available, known to the public, part of common general knowledge,
or otherwise constitutes prior art under the applicable statutory
provisions; or is known to be relevant to an attempt to solve any
problem with which this specification is concerned.
BRIEF SUMMARY OF THE INVENTION
[0014] The long-standing but heretofore unfulfilled need for a more
efficient permanent magnet electric machine is now met by a new,
useful, and nonobvious invention.
[0015] The novel structure includes a laminated cylindrical rotor
and a laminated cylindrical stator enclosed in a housing. The rotor
has an axial bore adapted to receive a shaft and an outer
circumference adapted to receive a plurality of permanent magnets.
Each permanent magnet has a magnetic flux density between 70 and 95
percent of a residual magnetic flux density of the rotor's
permanent magnet material.
[0016] The stator includes an outer circumference, a slot pitch, a
longitudinal axis, and plurality of teeth projecting a
predetermined distance inwardly towards the longitudinal axis of
the stator. The teeth create a discontinuous inner circumference
allowing the rotor to be disposed within the stator such that a
central axis of the rotor aligns with the stator's longitudinal
axis. The rotor is freely rotatable within the inner circumference
of stator. The stator also includes stator slots of predetermined
shape and size disposed between each tooth and adapted to receive a
predetermined number of strands of conducting material to form an
electrical winding around each tooth.
[0017] An air gap exists between the outer circumference of the
rotor and the inner circumference of the stator when the rotor is
disposed within the stator, such that the air gap is inversely
proportional to the torque angle of the machine. The torque angle
is between about 2 and about 10 degrees, which is significantly
less than the standard, and is proportional to the thickness of the
permanent magnets.
[0018] Additionally, the electrical machine includes a
predetermined number of poles, where the number of poles is
directly proportional to the electrical frequency of the electric
machine and inversely proportional to a number of required coil
windings. The present invention utilizes a three-phase electrical
winding scheme where each phase has two groups and each group
includes two coils connected in series. The windings are comprised
of strands of conducting material having a cross-sectional area
that is related to current density with the preferred current
density ranging between about 3 and about 8 Amp/mm.sup.2.
[0019] In an embodiment, the stator teeth each have a generally
uniform width, wherein the magnitude of the width is between about
60 and about 80 percent of the magnitude of the slot pitch. This
design feature provides decreased magnetic saturation and therefore
increased efficiency.
[0020] In an embodiment, the present invention may utilize a
sensorless field oriented control, where a rotor angle is estimated
by a sliding mode observer and the sliding mode observer module
contains a first order low-pass filter for back electromotive force
estimation. In such an embodiment, a phase lag compensation value,
that is automatically calculated based on rotational speed of the
rotor, is applied to feedback resulting in increased accuracy of
the field oriented control.
[0021] These and other important objects, advantages, and features
of the invention will become clear as this disclosure proceeds.
[0022] The invention accordingly comprises the features of
construction, combination of elements, and arrangement of parts
that will be exemplified in the disclosure set forth hereinafter
and the scope of the invention will be indicated in the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a fuller understanding of the invention, reference
should be made to the following detailed description, taken in
connection with the accompanying drawings, in which:
[0024] FIG. 1 depicts an exemplary B-H curve.
[0025] FIG. 2a depicts a phasor diagram illustrating torque angle
between induced voltage E.sub.A and terminal voltage V.sub..phi.
and between net flux .lamda..sub.net and rotor flux
.lamda..sub.f.
[0026] FIG. 2b depicts a phasor diagram illustrating the
relationship between torque angle and magnetic flux density.
[0027] FIG. 3 is a perspective view of a first embodiment of the
present invention.
[0028] FIG. 4 is an exploded view of the first embodiment in FIG.
3.
[0029] FIG. 5 is a perspective view of a second embodiment of the
present invention.
[0030] FIG. 6 is an exploded view of the second embodiment in FIG.
3.
[0031] FIG. 7 is a top view the first embodiment illustrating the
rotor displaced inside the stator.
[0032] FIG. 8 is a close up view of the second embodiment
illustrating the air gap between the stator teeth and the permanent
magnets on the rotor.
[0033] FIG. 9 is a close up view of the first embodiment
illustrating the air gap between the stator teeth and the permanent
magnets on the rotor.
[0034] FIG. 10 is a top view the second embodiment illustrating the
rotor displaced inside the stator.
[0035] FIG. 11 depicts a winding scheme diagram of the second
embodiment with the stator slots being identified by numerical
indicators 1-36.
[0036] FIG. 12 emphasizes Phase A of the winding scheme diagram
shown in FIG. 11.
[0037] FIG. 13 emphasizes Phase B of the winding scheme diagram
shown in FIG. 11.
[0038] FIG. 14 emphasizes Phase C of the winding scheme diagram
shown in FIG. 11.
DETAILED DESCRIPTION OF THE INVENTION
[0039] In the following detailed description of the preferred
embodiments, reference is made to the accompanying drawings, which
form a part thereof, and within which are shown by way of
illustration specific embodiments by which the invention may be
practiced. It is to be understood that other embodiments may be
utilized and structural changes may be made without departing from
the scope of the invention.
[0040] The present invention is a high efficiency electrical
machine operable as a motor or generator. The electrical machine
includes a rotor, a stator, a plurality of electrical windings, and
a plurality of permanent magnets all contained in a housing. The
rotor has a generally cylindrical shape with an outer circumference
and an axial bore creating an inner circumference. The inner
circumference is adapted to receive a shaft such that the two may
rotate as one. The outer circumference of the rotor contains a
plurality of permanent magnets (PMs) disposed longitudinally
parallel to the axis of rotation or central axis of the rotor in a
preferably generally uniform and concentric manner. In a certain
embodiment, the PM's are arranged in a manner such that the outward
radially facing magnetic poles are alternating in orientation
between each PM.
[0041] The stator has a generally cylindrical shape with an outer
circumference and a predetermined number of teeth projecting, a
predetermined distance, inwardly towards the longitudinal axis of
the stator. The teeth create a discontinuous inner circumference
allowing the rotor and PM assembly to be disposed within the inner
circumference of the stator such that the rotor assembly is freely
rotatable within the inner circumference of stator. Between each
tooth is a stator slot of predetermined shape and size adapted to
receive a predetermined number of, preferably copper, conducting
strands or wires to form a winding around a predetermined number of
teeth.
[0042] The present invention utilizes a design providing decreased
copper loss and decreased windage loss to improve the overall
efficiency of the electrical machine. In order to decrease copper
loss, the present invention uses copper wires having a larger
diameter than traditional designs. The wires have a diameter
related to current density as shown in the equation below:
J = 4 I .pi. d 2 ( 1 ) ##EQU00001##
[0043] As shown in equation 1, low current density will result in
large wire diameter which will reduce the copper loss since the
copper loss is inversely proportional to the wire's cross-sectional
area. The preferred value of current density is in the range of 3
to 8 Amp/mm.sup.2. In electrical machine design, winding current
density is an important factor. Due to the heat dissipation
limitation, current density must not exceed a certain value,
otherwise the motor will overheat. For air cooling, the current
density is usually lower than 8 A/mm.sup.2 and the current density
has such a relationship with the wire diameter as seen in equation
(1). So, under the same current, larger wire diameter could result
in a lower current density which provides the motor with more
overload capability (capable of operating with a higher current),
and less heat stress. Additionally, to reduce the eddy current
effects and proximity effects resulting in increased efficiency, a
certain embodiment may utilize a known stranded Litz wire instead
of solid wire.
[0044] Although a thicker wire requires a larger stator slot area,
a highly efficient machine generates much less heat and therefore
does not need to use any active cooling methodologies (such as fan,
liquid cooling or spray cooling); and hence the overall power
density of the whole system (including cooling devices) may not
decrease much. A passive cooling mechanism, such as fins on the
machine's outside housing, is sufficient when the efficiency is
high. In a certain embodiment, the housing may contain a passive
cooling mechanism or an active cooling mechanism, depending on the
application of the electrical machine.
[0045] In order to counteract the need to increase the slot area to
account for larger wires, the number of turns in each coil, or in
other words the amount of wire in the slot area, is decreased.
However, decreasing the number of turns per coil requires higher
electrical frequency. The electrical frequency depends on the
number of rotor pole pairs (PP) and mechanical frequency (MF) of
the rotor as shown in the equation below:
EF=PP.times.MF (2)
[0046] The electrical frequency is not exactly a constant when the
motor runs. Instead a motor is typically driven using a motor drive
system, which provides electrical frequency specified by a
controller. For example, in an application for truck APU, the motor
speed is almost a constant. If using a large number of pole pairs,
the electric frequency will increase. The number of turns per coil
for the stator winding is inversely proportional to the electric
frequency as shown below in equation 3.
N c = 1.1 V .phi. rated 2 2 .pi. f e nk w B m Dl ( 3 )
##EQU00002##
[0047] Where N.sub.c is the effective turns per coil, V.sub. rated
is phase voltage, f.sub.e is electrical frequency, n is number of
group windings per pole, k.sub.w is the back EMF due to stator
pitch, B.sub.m is a sum of residual magnetic flux density, D is the
diameter of the motor, and l is the length of the motor.
[0048] Thus, with higher electric frequency, a lower number of coil
turns suffices, therefore leading to smaller slot areas.
Ultimately, an increase in the number of pole pairs, resulting in
increased electrical frequency, allows the present invention to
utilize a decreased number of turns per coil, thus resulting in a
smaller required slot area and preventing the need to increase the
overall size of the machine.
[0049] The preferred embodiment of the present invention uses 8 to
16 rotor poles when the intended mechanical speed is less than
3,000 revolutions per minute. Although the present invention may
seem to use a comparatively large slot area for a low speed
machine, when the electrical frequency is increased, the number of
turns decreases, and the slot area required can be comparable to
that of a 2 to 6 pole design utilizing a smaller diameter copper
wire.
[0050] Moreover, the design utilizes a smaller torque angle, a
large air gap and large PM's to increase efficiency. Traditionally,
the high cost of PM's has led innovators to develop electric
machines with small PM's ultimately forcing the design to contain a
small air gap (0.5 mm-10 mm). However, a small air gap leads to a
higher windage loss (shear forces on the air between the rotor and
stator), which results in less efficient electric machines. To
avoid demagnetization the present invention utilizes a B.sub.mR
(the magnetic flux density from the rotor PM's) between about 70
and 95 percent of Br (the residual magnetic flux density of the
permanent magnet material on the rotor). Then, using the material's
B-H curve as shown in FIG. 1, H.sub.mR (the magnetic field
intensity in the PM's) is determined. The following equation
represents the relationship between the size of the air gap and the
size of the PM's when the total magnetomotive force (MMF) in the
loop is zero.
g.sub.effB.sub.mR+.mu..sub.0H.sub.mRl.sub.m=0 (4)
[0051] Where g.sub.eff is effective air gap, which is proportional
to the real air gap, and l.sub.m is the PM thickness. Because
B.sub.mR and H.sub.mR have a certain relationship and can be
considered constant for a particular design, and H.sub.mR is
negative (See FIG. 1), it is apparent that the air gap and PM
thickness are proportional.
[0052] Torque angle is also related to air gap and therefore
permanent magnet size. Traditional electric machines utilize a
torque angle between about 15 and 30 degrees at the machine's rated
power and speed. The present invention uses a much lower torque
angle of 2 to 10 degrees at rated power and speed. Torque angle is
.delta. as shown in FIG. 2a, and can be represented as the angle
between the induced voltage E.sub.A and terminal voltage
V.sub..phi. or the angle between net flux .lamda..sub.net direction
(total flux, which is the result of interaction between flux from
winding excitation and rotor flux) and the rotor flux .lamda..sub.f
direction. The torque angle can also be seen as the angle that the
rotor poles lags behind the rotating field.
[0053] When torque angle is 0 degrees, there is no torque, but at
90 degrees, the torque is at its maximum. However, if the torque
angle goes over 90 degrees, the motor will lose synchronization and
stop. When the motor is running at a small torque angle at rated
load, it has more room and potential to provide more torque if
needed (when the load torque increases, the motor will have a
larger torque angle for compensation and try to produce more torque
output). However, torque angle needs to be kept at a certain range
because if it increases too much there is danger of losing
synchronization. Therefore, having the motor running at a small
torque angle for rated power, means it has better overload power
handling capability and efficiency.
[0054] As shown in FIG. 2b, the magnetic flux density from the
rotor (B.sub.R) and the magnetic flux density in the stator
armature (B.sub.S) have the following relationship:
B.sub.S=B.sub.R sin .delta. (5)
[0055] Where B.sub.mR (which is B.sub.R in this equation) is
determined when the working point is chosen and can be considered
as a constant in a particular design. Therefore, a small torque
angle .delta. will result in a smaller B.sub.S. Additionally, the
magnetic flux density generated from the armature windings has a
relationship with air gap as shown below.
g ^ total = 4 .pi. .mu. 0 B a , p k N ^ a P 1.5 2 I A , rated ( 6 )
##EQU00003##
[0056] Where .sub.total is the effective total airgap, {circumflex
over (N)}.sub.a is effective number of series turns per phase of
armature winding, P is the number of rotor poles, I.sub.A,rated is
the rated phase current, .mu..sub.0 is a vacuum permeability
constant, and B.sub.a,pk is the B.sub.S described above. Equation
(6) proves that a smaller B.sub.a,pk will result in a larger air
gap. Therefore, smaller torque angle will lead to a smaller
B.sub.S, which results in a larger air gap.
[0057] The present invention's use of a small torque angle between
about 2 degrees and about 10 degrees, a large air gap based on the
torque angle from equations (5-6), and large PM's based on the air
gap from equation (4). The large air gap, as accentuated in FIGS.
7-10, helps to reduce the windage loss and noise level, while
increased PM thickness helps to avoid demagnetization.
[0058] In a certain embodiment, the present invention utilizes a
sensorless field oriented control (FOC), where the rotor angle is
estimated by a sliding mode observer (SMO). The SMO module contains
a first order low-pass filter for back EMF estimation. However,
this low pass filter will cause a delay in the estimated angle.
Rotor angle is a critical parameter in FOC control: to accurately
achieve FOC control, compensation is made to the estimated angle.
The phase lag differs at different speeds, and therefore, the
compensation must be adaptive. The angle delay may be calibrated at
different speeds utilizing a function to interpolate the delay
angle vs. speed curve. The appropriate phase lag compensation value
is automatically calculated based on the speed of the motor and
applied to the feedback. This adaptive phase lag compensation
increases the accuracy of the control.
[0059] In an embodiment of the invention intended for use in
electrical machines operating at mechanical speeds of greater than
3,000 rpm, a lower number of rotor poles than specified above may
be preferable. For an electrical machine that operates at speeds
greater than 6,000 rpm, 2-12 poles are sufficient. Alternative
embodiments may involve more poles for electrical machines intended
to operate at lower speeds. When the mechanical speed is high, less
poles are preferred because of the mechanical strength of the
rotor.
[0060] A certain embodiment may employ a laminated stator and/or a
laminated rotor, such that both are made of laminated sheets held
together by an adhesive having thermal insulation properties. Such
a design reduces the need to have additional internal cooling
devices to prevent the electric machine from overheating. The
laminated design also provides the additional known benefit of
reducing eddy currents.
Example 1
[0061] As shown in FIG. 3, a certain embodiment, generally denoted
as reference numeral 100, of the present invention is a high
efficiency electrical motor having shaft 114, laminated stator 102,
and laminated rotor 106 all contained within housing 112. Laminated
stator 102 contains a plurality of windings 104, and laminated
rotor 106 contains a plurality of permanent magnets 108. Rotor 106
is disposed within the inner circumference of stator 102 such that
the longitudinal axis of stator 102 is aligned with the central
axis of rotor 106 and rotor 106 is freely rotatable within stator
102. Stator 102 has a plurality of stator teeth 110 and a plurality
of stator slots wherein each stator slot is disposed between two
stator teeth 110. Windings 104 are positioned around each tooth 110
passing through stator slots on either side of each respective
tooth 110. Windings 104 are illustrated in the figures as having a
rectangular shape for clarity purposes. The windings, being
composed of multiple strands of conducting material would be
difficult to illustrate in the figures as they actually exist with
proper clarity.
[0062] As shown in FIGS. 3, 4, 7, and 9, stator 102 has been
designed to further reduce copper loss by using wider teeth 110.
Since the copper loss is proportional to the current squared,
reducing the current is an effective way to reduce copper loss and
improve efficiency. The current is reduced by decreasing magnetic
saturation and magnetic saturation is inversely related to tooth
width. So, the width of the stator teeth was increased to decrease
magnetic saturation, in turn decreasing current, and ultimately
decreasing copper loss. Teeth 110 have a width preferably in the
range of 60% to 80% of the stator's slot pitch. Slot pitch is a
circumferential distance along the inner circumference of the
stator from the center line of one tooth or slot to the center line
of the adjacent tooth or slot. Traditional electric machine designs
utilize stator teeth having widths of about 40% to 60% of the slot
pitch resulting in greater magnetic saturation and less efficiency
than the present invention.
[0063] A certain embodiment of the motor may implement a winding
scheme having 3 phases (A, B, C) with each phase having two groups
and each group including two coils connected in series. For
example, a stator having 12 stator slots, 3 phases, and two groups
per phase would have coils arranged as shown in Table 1 below.
TABLE-US-00001 TABLE 1 Winding table illustrating a concentric
winding scheme for a 12-slot stator. Slot Number Phase A Phase B
Phase C 1 In Out 2 Out & Out 3 In Out 4 In & In 5 Out In 6
Out & Out 7 Out In 8 In & In 9 Out In 10 Out & Out 11
In Out 12 In & In
[0064] In an alternate embodiment, the groups may be connected in
parallel as is known by a person having ordinary skill in the
art.
Example 2
[0065] As shown in FIG. 5, a certain embodiment, generally denoted
as reference numeral 200, is a high efficiency electrical
generator. Similar to the motor, the generator design includes
shaft 214, stator 202, rotor 206, electrical windings 204, and PMs
208 all contained in housing 212. The rotor has four PMs 208 for
use at a desired operational speed of 6,000 rpm to prevent
excessive electrical frequency associated with too many rotor
poles. Stator 202 is comprised of thirty-six stator teeth 210 and
therefore thirty-six stator slots. Similar to windings 104,
windings 204 are illustrated in the figures as having a rectangular
shape for clarity purposes. As shown in FIG. 5, a winding
encompasses two stator teeth. This illustration is simply for
clarity. Embodiment 200 actually contains 2 coils per slot. FIGS.
11-14 illustrate the three-phase winding diagram, in which there is
1 turn per coil, 2 coils per slot, and all the groups are connected
in series. Each coil contains about 120 strands of conducting
material. In an alternate embodiment, the groups may be connected
in parallel as is known by a person having ordinary skill in the
art.
Glossary of Claim Terms
[0066] Back Electromotive Force Estimation: is the estimation of
the voltage induced in electric motors where there is relative
motion between the armature of the motor and the magnetic
field.
[0067] Current Density: is the electrical current per unit area of
cross section.
[0068] Electrical Frequency: is the number of cycles of electricity
per unit of time.
[0069] Electrical Winding: is a number of strands of conducting
material wound around stator teeth.
[0070] Low-Pass Filter: is a filter that passes low-frequency
signals and attenuates signals with frequencies higher than the
cutoff frequency.
[0071] Magnetic Flux Density: is the amount of magnetic flux per
unit area taken perpendicular to the direction of the magnetic
flux.
[0072] Passive Cooling Mechanism: is heat dissipation without the
aid of a pump or fan.
[0073] Permanent Magnet: is a magnet that retains its magnetic
properties in the absence of an inducing field or current.
[0074] Phase Lag Compensation Value: is the angle that calculated
based on rotor speed and feedback loop delay. It is applied to the
estimated rotor angle to increase accuracy.
[0075] Pole: point where electric or magnetic force appears to be
concentrated.
[0076] Rotor: is a rotary part of a machine.
[0077] Rotor Sheet: is a thin layer of material adapted to be
laminated to additional rotor sheets to form the final rotor
dimensions.
[0078] Sensorless Field Oriented Control: is a sensorless variable
frequency drive control method.
[0079] Sliding Mode Observer: is a non-linear high-gain
observer.
[0080] Slot Pitch: is a distance between corresponding points in
adjacent stator slots. It can also be expressed as an angle.
[0081] Stator Sheet: is a thin layer of material adapted to be
laminated to additional stator sheets to form the final stator
dimensions.
[0082] Stator Slot: is an opening between two stator teeth.
[0083] Stator Tooth Width: is a measurement of the minimum width of
the tooth.
[0084] Stator: is a mechanical device consisting of the stationary
part of a motor or generator in or around which the rotor
revolves.
[0085] Thermal Insulating Adhesive: is a substance having a
tendency to stick and the capability to reduce heat transfer
between objects in thermal contact or in range of radiative
influence.
[0086] Torque Angle: is the angle between the induced voltage
E.sub.A and terminal voltage V.sub..phi. or the angle between net
flux .lamda..sub.net direction (total flux, which is the result of
interaction between flux from winding excitation and rotor flux)
and the rotor flux .lamda..sub.f direction. The torque angle can
also be seen as the angle that the rotor poles lags behind the
rotating field.
[0087] The advantages set forth above, and those made apparent from
the foregoing description, are efficiently attained. Since certain
changes may be made in the above construction without departing
from the scope of the invention, it is intended that all matters
contained in the foregoing description or shown in the accompanying
drawings shall be interpreted as illustrative and not in a limiting
sense.
[0088] It is also to be understood that the following claims are
intended to cover all of the generic and specific features of the
invention herein described, and all statements of the scope of the
invention that, as a matter of language, might be said to fall
therebetween.
* * * * *