U.S. patent application number 12/317730 was filed with the patent office on 2010-06-10 for induction motor with improved torque density.
This patent application is currently assigned to Tesla Motors, Inc.. Invention is credited to Yifan Tang.
Application Number | 20100141080 12/317730 |
Document ID | / |
Family ID | 41600455 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100141080 |
Kind Code |
A1 |
Tang; Yifan |
June 10, 2010 |
INDUCTION MOTOR WITH IMPROVED TORQUE DENSITY
Abstract
An induction motor embodiment includes a stator defining a
stator bore, the stator including a stator yoke having a stator
yoke thickness and a plurality of stator teeth, the teeth having a
common length, with each of the stator teeth including a stator
tooth center portion that extends from a stator tooth bottom
portion proximal the yoke to a stator tooth tip portion, with
adjacent stator teeth defining a stator slot between them, each
stator slot having a stator slot bottom that extends along a stator
slot bottom length. In the embodiment, the center portion has a
stator tooth width that is less than or equal to one half the
stator slot bottom length. In the embodiment, the stator tooth
width is smaller than a stator slot opening width distance. In the
embodiment, a ratio of stator yoke thickness to stator tooth width
is at least 5:1. A rotor is rotably mounted in the stator.
Inventors: |
Tang; Yifan; (Los Altos,
CA) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG & WOESSNER, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Tesla Motors, Inc.
San Carlos
CA
|
Family ID: |
41600455 |
Appl. No.: |
12/317730 |
Filed: |
December 29, 2008 |
Current U.S.
Class: |
310/216.111 |
Current CPC
Class: |
H02K 17/20 20130101;
H02K 2213/03 20130101; H02K 1/265 20130101; H02K 17/12 20130101;
H02K 1/165 20130101 |
Class at
Publication: |
310/216.111 |
International
Class: |
H02K 1/16 20060101
H02K001/16 |
Claims
1. Apparatus, comprising: a stator defining a stator bore, the
stator comprising a stator yoke having a stator yoke thickness, and
a plurality of stator teeth, the teeth having a common length, with
each of the stator teeth including a stator tooth center portion
that extends from a stator tooth bottom portion proximal the yoke
to a stator tooth tip portion, with adjacent stator teeth defining
a stator slot between them, each stator slot having a stator slot
bottom that extends along a stator slot bottom length, wherein the
center portion has a stator tooth width that is less than or equal
to one half the stator slot bottom length, the stator tooth width
also being smaller than a stator slot opening width distance
between adjacent stator tooth center portions, wherein a ratio of
stator yoke thickness to stator tooth width is at least 5:1; and a
rotor mounted in the bore to rotate.
2. The apparatus of claim 1, wherein a ratio of rotor yoke
thickness to rotor tooth width is at least 5:1.
3. The apparatus of claim 1, wherein a sum of the stator yoke
thickness and the stator tooth length approximately equals a sum of
the rotor yoke thickness and a rotor tooth length that extends
radially away from the rotor yoke.
4. The apparatus of claim 1, wherein the rotor and stator have
plate means for driving most of the stator teeth of a pole and most
of the rotor teeth of the pole into deep magnetic saturation
simultaneously.
5. The apparatus of claim 1, wherein the rotor and stator plates
are adapted to simultaneously put most of the teeth of the rotor
and most of the teeth of the stator into approximately equal levels
of deep magnetic saturation, with the rotor and the stator yokes
being in at most a light magnetic saturation that is less than a
full magnetic saturation.
6. The apparatus of claim 1, wherein the rotor has a plurality of
rotor teeth, with each of the rotor teeth having a rotor tooth
center portion that extends from a rotor tooth bottom portion
proximal the rotor yoke to a rotor tooth tip portion, the rotor
center portion having a rotor tooth width, and wherein a ratio
between the rotor tooth width and the stator tooth width is between
3:4 and 4:3.
7. The apparatus of claim 6, wherein a ratio between the rotor
tooth width and the stator tooth width is approximately 1:1.
8. The apparatus of claim 1, wherein the stator is generally
circular with an inner diameter greater than an axial stack length
of the stator, and wherein the air gap distance between the
circular stator and the circular rotor is at least 0.5 mm.
9. Apparatus, comprising: a stator defining a stator bore, the
stator comprising a stator yoke having a stator yoke thickness, and
stator teeth, the teeth having a common stator tooth width, wherein
the ratio of stator yoke thickness to stator tooth width is at
least 5:1; and a rotor mounted in the bore to rotate, the rotor
comprising a rotor yoke having a rotor yoke thickness, and rotor
teeth, the rotor teeth each having a rotor tooth width, wherein the
ratio of rotor yoke thickness to rotor tooth width is at least
5:1.
10. The apparatus of claim 9, wherein a sum of the stator yoke
thickness and the stator tooth length approximately equals a sum of
the rotor yoke thickness and the rotor tooth length.
11. The apparatus of claim 9, wherein the stator has at least four
teeth per pole per phase.
12. The apparatus of claim 11, wherein the stator has 60 teeth in
total and the rotor has 74 teeth in total.
13. The apparatus of claim 11, wherein the rotor and stator have
plates, the plates of the rotor and the stator being adapted to
drive at least two stator teeth into a deep magnetic saturation
simultaneously and is adapted to drive at least two rotor teeth
into the deep magnetic saturation simultaneously.
14. The apparatus of claim 9, wherein the stator has an axial stack
length and an outside diameter perpendicular to the length, and the
ratio of the outside diameter to the axial stack length is
approximately 2:1.
15. Apparatus, comprising: a stator defining a stator bore, the
stator comprising a stator yoke having a stator yoke thickness, and
a plurality of stator teeth, the stator teeth having a stator tooth
length, wherein the stator yoke thickness is greater than the
stator tooth length; and a rotor mounted in the bore to rotate, the
rotor comprising a rotor yoke having a rotor yoke thickness, and
rotor teeth coupled to the yoke, the rotor teeth having a rotor
tooth length, wherein the rotor yoke thickness is greater than the
rotor tooth length, wherein a sum of the stator yoke thickness and
the stator tooth length approximately equals a sum of the rotor
yoke thickness and the rotor tooth length.
16. The apparatus of claim 15, wherein the rotor and stator have
plates, the plates of the rotor and the stator being adapted to
drive most of the stator teeth of a pole and most of the rotor
teeth of the pole into approximately equal levels of deep magnetic
saturation simultaneously.
17. The apparatus of claim 15, wherein each of the stator teeth
includes a center portion that extends from a bottom portion
proximal the yoke to a tip portion, with adjacent teeth defining a
stator slot between them, each stator slot having a stator slot
bottom extending along a stator slot bottom length, and wherein the
center portion has a tooth width that is less than or equal to one
half the stator slot bottom length, the tooth width also being
smaller than the distance between adjacent stator tooth center
portions.
18. The apparatus of claim 15, wherein a stator winding coupled to
the stator is adapted to generate a substantially non-sinusoidal
magneto-motive-force curve.
19. The apparatus of claim 18, wherein a stator winding coupled to
the stator is adapted to generate an approximately trapezoidal
magneto-motive-force curve.
20. The apparatus of claim 15, wherein there are at least two
layers of coils in a single stator slot.
Description
BACKGROUND
[0001] Electric motors used in applications such as electric road
vehicles should be able to provide varying torque, and at times
very high torque peaks. High torque peaks enable drivers to
experience quick acceleration or to climb a steep hill, for
example. Many preexisting induction motors are unable to
accommodate the widely varying torque levels drivers desire. These
motors become magnetically saturated in too many real-world
conditions. Those that do meet more torque demands often are not as
efficient as is desired. Motors that resist saturation often suffer
from other drawbacks, such as high cost, poor reliability,
undesirable mass and undesired-field weakening difficulty at high
speeds. The undesired field weakening demonstrated by these motors
decreases the torque available for drivers. An improved motor is
desired that accommodates high peak torque demands while avoiding
these drawbacks.
BRIEF DESCRIPTION OF THE DRAWINGS
[0002] FIG. 1A shows an axial view of an improved induction motor,
according to various embodiments.
[0003] FIG. 1B shows a side view of the improved induction motor
100 of FIG. 1A.
[0004] FIG. 1C illustrates an example magnetic field distribution
for an improved induction motor.
[0005] FIG. 2 shows a portion of a plate of a stator taken at line
2-2 in FIG. 1A, including a schematic illustration of windings.
[0006] FIG. 3A shows a portion of a plate of a stator taken at line
3A-3A in FIG. 1A, including a schematic illustration of
windings.
[0007] FIG. 3B shows an optional closed rotor slot motor for an
improved induction motor, including a schematic illustration of
windings.
[0008] FIG. 4 is a diagram showing an increased air gap flux
linkage of an example of an improved induction motor for high
torque density.
[0009] FIG. 5 is a diagram showing a higher stator flux linkage for
one example of an improved induction motor.
[0010] FIG. 6 is a diagram of a saturation curve shown as motor
terminal line-to-line voltage as a function of phase current,
according to some embodiments.
[0011] FIG. 7 is a diagram of a saturation curve of an improved
induction motor shown as saturated magnetizing inductance as a
function of phase current, according to some embodiments.
[0012] FIG. 8 is a diagram of a saturation curve of an improved
induction motor shown as saturated magnetizing inductance as a
function of air gap flux linkage, according to some
embodiments.
[0013] FIG. 9 is a diagram showing flux density distribution in the
air gap versus electrical angle between a rotor and a stator of an
idealized conventional induction motor, at a given instant of time
under a balanced three-phase excitation.
[0014] FIG. 10 is a diagram of two different idealized conventional
motors showing, in an upper portion, stator yoke flux density and
flux density for the stator teeth for a stator having 72 slots, and
in a lower portion, rotor yoke flux density and flux density for
the rotor teeth for a rotor having 82 slots.
[0015] FIG. 11 is a diagram showing air gap flux density over 360
degrees for an improved motor, such as the one illustrated in FIG.
1A.
[0016] FIG. 12 shows stator yoke and tooth flux densities over a
stator circumfluent of 60 slots, and rotor yoke and tooth flux
densities over rotor circumfluent of 74 slots, for the motor
illustrated in FIG. 11.
[0017] FIG. 13 shows flux densities for an improved induction motor
embodiment, such as the embodiment illustrated in FIG. 20.
[0018] FIG. 14 is a diagram showing stator yoke and teeth flux
densities over stator circumfluent of 48 slots and rotor yoke and
teeth flux densities over rotor circumfluent of 68 slots, for the
motor illustrated in FIG. 13.
[0019] FIG. 15 is a diagram showing the total magneto-motive-force
("MMF") of a single phase of an example 4-pole configuration with
short-pitch winding placement.
[0020] FIG. 16 is a diagram showing the total MMF of a phase of an
example 4-pole design with full-pitch winding placement.
[0021] FIG. 17A shows a flux density along the middle of the
air-gap, according to some embodiments.
[0022] FIG. 17B shows flux density across the stator teeth and flux
density across the stator yoke, according to some embodiments.
[0023] FIG. 17C shows flux density across the rotor teeth and flux
density across the rotor yoke, according to some embodiments.
[0024] FIG. 18A shows peak torque over a speed range, according to
some embodiments.
[0025] FIG. 18B shows voltage over a speed range, according to some
embodiments.
[0026] FIG. 18C shows current over a speed range, according to some
embodiments.
[0027] FIG. 18D shows stator flux linkage over a speed range,
according to some embodiments.
[0028] FIG. 19A is a diagram including several peak torque curves
at different operating frequencies.
[0029] FIG. 19B is a diagram including several peak stator flux
linkage curves at different operating frequencies.
[0030] FIG. 20 shows an axial view of an additional embodiment of
an improved induction motor, according to various embodiments.
DETAILED DESCRIPTION
[0031] In the following description, reference is made to the
accompanying drawings that form a part hereof, and in which is
shown by way of illustration specific embodiments which may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the invention, and it
is to be understood that other embodiments may be utilized and that
structural, logical and electrical changes may be made without
departing from the scope of the present invention. The following
description of example embodiments is, therefore, not to be taken
in a limited sense, and the scope of the present invention is
defined by the appended claims.
[0032] The induction motors disclosed here address the needs of
vehicles such as automobiles that wish to use induction motors for
propulsion. Such vehicles include, but are not limited to, roadway
capable battery powered electric vehicles ("EVs") and hybrid
electric vehicles. The present subject provides examples of
improved induction motors that perform better than a comparable
existing or conventional induction motor in EV applications. A
comparable motor is one having a similar external size and one that
has access to a similar power supply.
[0033] Road-going EVs that are mass produced should be cost
effective and should deliver torque near or at the levels of their
fuel-burning counterparts. Unfortunately, conventional induction
motors do not work well and do not achieve the torque levels that
drivers desire. Conventional induction motor designs for modern
high performance applications are described in the following
references: (1) J. Kim, etc., "Optimal Stator Slot Design of
Inverter-Fed Induction Motor in Consideration of Harmonic Losses,"
IEEE Transactions on Magnetics, Vol. 41, No. 5, May 2005, pp.
2012-2015; (2) S. Park, etc., "Stator Slot Shape Design of
Induction Motors for Iron Loss Reduction," IEEE Transactions on
Magnetics, Vol. 31, No. 3, May 1995, pp. 2004-2007; (3) J. L.
Kirtley, Jr., "Designing Squirrel Cage Rotor Slots with High
Conductivity," Proceedings of International Conference on Electric
Machines, Krakow, Poland, Sep. 5-8, 2004, the subject matter of
each of which is incorporated herein by reference in its entirety.
They can work well in industrial applications, but these
applications generally operate at a steady state with limited
dynamic requirements. In addition, operating efficiency is often
not a top priority in the motor design for an induction motor
linked to a power line such as a municipal power grid. Industrial
operating conditions differ from EV operating conditions that
include, but are not limited to, long driving ranges, fast
acceleration and deceleration and fast control dynamic response.
For these and other reasons, others have relied on permanent magnet
("PM") motors for EVs.
[0034] However, PM motors suffer from many shortcomings. For
example, PM motors suffer from field weakening at higher speeds.
This limits the load the motor can be exposed to at higher speeds,
which limits the torque it can produce. Limiting the speed range of
the motor to avoid this problem is problematic, as vehicle
designers seek availability of variable motor speeds so they can
simplify or eliminate multi-speed transmissions or gearboxes. PM
motors also suffer from demagnetization under certain environmental
conditions and/or excitation conditions. Additionally, PM traction
motors can be expensive to manufacture, as they require large,
delicate and specialized magnets. Some designs also use more parts,
which can add cost and decrease reliability.
[0035] To add high speed torque capability in PM motors, previous
efforts have relied on size or power increases. Size increases are
undesirable in automotive applications because weight affects
range, acceleration and cost. Further attempts have relied on
higher power. Higher power can increase operating temperatures,
which can damage other parts of the car. Higher powered
applications additionally require more expensive power
converters.
[0036] Further attempts to increase high speed torque have resulted
in hybrid PM-reluctance motor designs that add reluctance torque to
the idealized conventional PM torque. Such hybrid motors have
increased the complexity and cost of the mechanical structures.
They also use control systems which have increased complexity and
cost.
[0037] When induction motors have been used in EVs, they have
suffered from performance problems, as is set out below in a number
of charts that compare the performance of idealized conventional
motors to that of the improved induction motors disclosed
herein.
[0038] For electric car applications in general, induction motors
produce from about 50 kilowatts to about 300 kilowatts of peak
power, depending on particular vehicle design specifications. The
present induction motors disclosed here provide high torque
capability over a wide speed range. They provide this improvement
without undesired increases in size and weight. Some of the motors
disclosed here are able to produce short duration peak torque at 5
or more times that of continuous running torque. The level of the
continuous running torque is generally limited by the ability of
the motor to cool, as well as the efficiency of the motor. In some
examples, the level of the peak torque is generally limited by
electromagnetic considerations. In additional examples, the level
of the peak torque is limited by external motor drive current.
Providing a high ratio of peak torque to continuous running torque
provides a number of benefits, including, but not limited to, fast
acceleration, improved hill climbing, and a reduced or eliminated
need for multi-speed transmissions.
[0039] The present induction motors may use improved magnetic steel
sheet plate shapes (i.e., steel laminations) and associated
waveforms, among other things, to produce improved acceleration and
deceleration and to provide high torque for their size in a wide
speed range, when compared to known existing conventional motors.
In some embodiments, these improved induction motors reduce the
cost of a power inverter because they reduce the stress on the
inverter by lowering the required current and achieving a better
power factor. Such motors are easier to package, as they are
smaller. In various embodiments, these motors reduce switching
losses, switching device ratings, and inverter and energy source
power capacity ratings. In some embodiments, variable-voltage
variable-frequency inverters are used (e.g., the curves of FIGS.
19A-B are generated with variable-voltage variable-frequency
inverters).
[0040] Changes in plate shape versus a conventional design are
discussed in FIGS. 1-3. Curves comparing features, characteristics
and performance between an idealized conventional motor and an
improved motor are set out in FIGS. 4-8. Waveforms for conventional
motors and improved motors are discussed in relation to FIGS. 9-14.
FIGS. 15 and 16 show MMF for two different winding configurations.
FIGS. 17-19 provide performance metrics for an improved induction
motor. FIG. 20 illustrates a further embodiment of an improved
induction motor.
[0041] FIG. 1A shows an axial view of an improved induction motor
100, according to various embodiments. The diagram is simplified
for the sake of explanation, as windings and other components are
omitted. The general characteristics of the geometry when compared
to a conventional induction motor include, but are not limited to,
one or more of a radial air gap increased over the air gap size of
conventional motors, a shorter axial stack length of the plate
stack, wider stator slots, narrower stator teeth, narrower and
shorter rotor teeth, a stator yoke that is thicker than the
stator's tooth length and much thicker than the tooth width, a
rotor yoke that is thicker than the rotor's tooth length and much
thicker than the rotor's tooth width, an increased number of stator
slots per pole per phase, and an increased number of rotor
slots.
[0042] A rotor 102 is encircled by a stator 104. There is an air
gap 106 between them. The air gap 106 between the stator 104 and
rotor 102 is sized to obtain desirable levels of the magnetizing
inductance and the leakage inductances, as disclosed here (e.g.,
FIGS. 4-8 show the performance of a conventional design compared to
the performance of an improved design). The air gap 106
additionally has an effect on the saturation levels and harmonic
levels of the magnetic flux proximal the air gap. In various
embodiments, the air gap is at least 0.5 mm. A shaft 108 is coupled
to the rotor 102. The shaft 108 can be coupled to downstream
devices, including but not limited to an axle, a gearbox and the
like.
[0043] The rotor 102 includes a plurality of rotor teeth 110.
Between each tooth and the tooth next to it (i.e., between each
tooth pair), is a rotor slot 112. The teeth define the slots
between them. The rotor also has a rotor yoke 114. The stator 104
includes a stator yoke 1 16, a plurality of stator teeth 118, and
stator slots 120 that are defined by the stator teeth 118. In
various embodiments, the teeth and the yoke of the rotor are
continuous blocks of material, i.e. monolithic. For example, in
some embodiments, the rotor is comprised of a stack of plates
electrically insulated from one another, and each of the plates is
stamped or otherwise excised from a single piece.
[0044] FIG. 1B shows a side view of the improved induction motor
100 of FIG. 1A. The illustration shows the stator 104, the rotor
102, the air gap 106 and the shaft 108. The illustration also shows
end rings 122. Various embodiments include rotor bars, stator
windings or coils including an electrical insulation system, a
frame, end caps, bearings, and other components. Some embodiments
include speed and temperature sensors.
[0045] A plurality of windings is disposed around each of the
stator teeth. In various embodiments the windings are copper, but
other materials are possible. An example of a winding is shown in
each of FIGS. 2-3. In various embodiments, the plurality of
windings provides a plurality of poles. Some of the induction motor
improved designs disclosed here are illustrated using three-phase
induction motor examples, although other numbers of phases are
possible, and the disclosed improvements are applicable to other
numbers of phases. In various embodiments, the windings are
configured to provide a total of twelve poles spread across three
phases (i.e., 4-poles/phase, commonly referred to as a 4-pole
motor, or a 2-pole pair motor).
[0046] In various embodiments, the stator is generally circular
with a stator diameter D.sub.S (also seen in FIG. 1A) greater than
an axial stator length L.sub.A. Various embodiments have a stator
diameter D.sub.S to axial stator length L.sub.A ratio of at least
2:1. In various embodiments, a plurality of plates is stacked along
the stator length L.sub.A and defines the stator. In various
embodiments, the teeth and the yoke of the stator are continuous
blocks of material, i.e. monolithic. For example, in some
embodiments the stator is comprised of a stack of plates
electrically insulated from one another, and each of the plates is
stamped or otherwise excised from a single billet.
[0047] Magnetic saturation occurs when an increasing magnetic field
excitation level (also know as magnetic field intensity, or "H")
fails to increase the resulting magnetic flux density (or "B") in a
linear manner. In unsaturated states, the magnetic flux density is
linearly proportional to the magnetic field intensity. Accordingly,
as magnetic saturation increases, the responsiveness of magnetic
flux density to changes in magnetic field intensity decreases.
[0048] FIG. 1C illustrates an example magnetic field distribution
for an improved induction motor. The figure shows saturation levels
and flux paths for a balanced three-phase excitation for a
three-phase motor. The motor includes a first pole 150, a second
pole 152, a third pole 154, and a fourth pole 156. A balanced
three-phase excitation results in a symmetrical magnetic field
distribution, in various embodiments.
[0049] The flux lines 152 represent circular and closed magnetic
flux paths. The magnetic flux density level for the motor 150 is
represented by a color distribution 158 that also includes flux
density values ("B") expressed in Tesla units ("T"). Magnetic flux
density levels near the top 160 of the scale represent deep
saturation for laminations formed of magnetic steel. In various
examples, a magnetic flux density level of at least 1.5 Tesla, with
most at 2.0 Tesla and above, represents a deep magnetic saturation
of the lamination at the particular location. In the figure, such a
flux density is generally red in appearance.
[0050] In this example, most of the stator teeth 118 and most of
the rotor teeth 110 carry flux lines 152 and are in deep
saturation. The remainders of the teeth have flux density levels of
lower than 2.0 Tesla. In some embodiments, the saturation level may
be lower than 1.5 Tesla. The example further illustrates similarly
saturated flux density in the stator teeth 118 and rotor teeth 110,
and similarly unsaturated flux density in the stator yoke 116 and
rotor yoke 114.
[0051] FIG. 2 shows a portion of a plate of a stator taken at line
2-2 in FIG. 1A. FIG. 3A shows a section of a rotor taken at line
3A-3A in FIG. 1A. FIG. 3B shows an optional closed rotor slot motor
for an improved induction motor, such as the motor illustrated in
FIG. 20. In these figures, a portion of a plate is illustrated. In
various embodiments, induction motors using these improved plate
shapes are able to provide improved torque density versus that of a
conventional motor.
[0052] Various embodiments include a stator 104 defining a stator
bore 124. In various embodiments, a rotor 102 is mounted in the
stator bore 124 to rotate in the stator bore 124. Bearings and/or
other mounting apparatus may be used. The stator yoke 116 has a
stator yoke thickness T.sub.SY. A plurality of stator teeth (118 is
typical of the plurality) are coupled to the stator yoke 116. In
various embodiments, the stator teeth extend radially inward toward
an axis of the stator bore. A solid conductor is illustrated having
a first layer 125 and a second layer 127. A stator winding can
include a bundle of wires, or it can be formed of a solid conductor
(i.e., a rotor bar) as illustrated. The two coils in the stator
slot define a two-layer winding. The coils may belong to the same
phase or not. In additional embodiments, coils having a number of
strands or windings are used. The present subject matter extends to
embodiments having another number of layers as well. An adhesive or
similar mounting structure can optionally occupy interstices to fix
the position of the rotor bars.
[0053] The stator teeth have a common length L.sub.ST. Each of the
stator teeth includes a stator tooth center portion 126 that
extends from a stator tooth bottom portion 128 proximal the yoke
116 to a stator tooth tip portion 130, with adjacent stator teeth
defining a stator slot 120 between them. Each stator slot has a
stator slot bottom portion 132 that extends along a stator slot
bottom length L.sub.SSB. In various embodiments, the stator slot
bottom portion 132 is generally rounded. In various embodiments,
the slot bottom length L.sub.SSB includes a flat bottom and
radiused corners. In additional embodiments, the slot bottom is
entirely arcuate. In various embodiments, the stator teeth have an
approximately uniform width W.sub.ST, not including the lip 131. In
various embodiments, the tooth center portion 126 has a stator
tooth width W.sub.ST that is less than or equal to one half the
stator slot bottom length L.sub.SSB. In some embodiments, the
stator tooth width W.sub.ST is smaller than the stator slot opening
width D.sub.AST between adjacent stator tooth center portions. In
various embodiments, the distance D.sub.AST does not include the
lip 131.
[0054] FIG. 3A illustrates an example portion of a rotor of an
improved induction motor. The rotor 102 includes a rotor yoke 114.
The rotor yoke 114 has a rotor yoke thickness T.sub.RY. Coupled to
the rotor are a plurality of rotor teeth (tooth I 10 is typical of
the plurality) that extend radially away from a center axis of the
rotor. In various embodiments, each of the teeth includes a rotor
tooth center portion 134 and a rotor tooth tip portion 136. Between
adjacent rotor teeth is a rotor slot 112 having a bottom portion
138. The slot bottom portion 138 has a slot bottom length
L.sub.RSB. The rotor teeth have a common length L.sub.RT. The
distance between the tips of the rotor teeth in this embodiment is
D.sub.ART. A single layer winding 137 comprising a solid conductor
(i.e., a rotor bar) is shown disposed in the slot 112. The slot 112
may be filled with a coil that can be solid or that can include
windings. In various embodiments, the windings extend to the top of
the slot 112, proximal the tip portion 136. The coil material, in
various embodiments, is formed of one or more materials including,
but not limited to, copper and aluminum.
[0055] Stators of the improved induction motors described here have
a higher than conventional thickness T.sub.SY to stator tooth width
W.sub.ST ratio, as is illustrated in FIG. 2. Rotors of these motors
have a high rotor yoke thickness T.sub.RY to rotor tooth width
ratio W.sub.RT. These ratios provide a difference in the saturation
levels of the yoke and the teeth for the rotor and the stator (i.e.
the difference in the flux density level) that is improved when
compared to conventional induction motors. For example, in
4-pole/phase and higher pole/phase count induction motors the ratio
of the flux density in the yoke to the tooth for one or both of the
stator and the rotor may be at least 5:1 or more. In a 2-pole/phase
induction motor, the ratio may be at least 5:1.
[0056] In various embodiments, stator yoke thickness T.sub.SY, as
is illustrated in FIG. 2, is larger than the stator tooth length
L.sub.ST, and the rotor yoke thickness T.sub.RY is larger than
rotor tooth length L.sub.RT. In various embodiments, a sum of the
stator yoke thickness T.sub.SY and the stator tooth length L.sub.ST
approximately equals a sum of the rotor yoke thickness T.sub.RY and
a rotor tooth length L.sub.RT.
[0057] In various embodiments, the plates of the rotor and of the
stator are shaped so that a maximum inverter drive current for peak
torque load will drive at least two or more or most of the stator
and rotor teeth of each pole of each phase into deep magnetic
saturation. Some examples simultaneously saturate all of the stator
and rotor teeth of the induction motor. In these examples, most of
the teeth are in deep saturation, as described above in association
with FIG. 1C, while the stator and rotor yokes are in light
magnetic saturation, or are not saturated at all. Light magnetic
saturation is a saturation level less than 2.0 Tesla. In various
embodiments, the saturation levels of the rotor teeth and the
stator teeth across an air gap between the rotor and stator are at
approximately equal levels.
[0058] Various embodiments use a high stator and rotor tooth number
combination. In some examples, there are more rotor teeth than
stator teeth. The example of FIG. 1A includes 60 stator teeth and
74 rotor teeth. Various embodiments use around a 1:1 ratio of
stator tooth width W.sub.ST to rotor tooth width W.sub.RT. In some
embodiments, the ratio between the two tooth widths is less than or
equal to one-fourth of each other. Various embodiments have at
least 4 stator teeth per pole per phase, although other numbers are
possible. For example, in some embodiments, each of the rotor teeth
has a rotor tooth center portion 134 that extends from a rotor
tooth bottom portion 138 proximal the rotor yoke 102 to a rotor
tooth tip portion 136, the rotor center portion 138 having a rotor
tooth width W.sub.RT, wherein a ratio between the rotor tooth width
W.sub.RT and the stator tooth width W.sub.ST is between 3:4 and
4:3.
[0059] FIG. 3B shows an optional closed rotor slot motor for an
improved induction motor. A closed rotor slot motor has improved
aerodynamics, including reduced windage losses. Further, the design
can be easier to manufacture. For example, it is easier to
manufacture embodiments in which the rotor bars are made from a low
cost die-cast copper or aluminum injection process. A winding 141
is shown in the form of a rotor bar. In various embodiments, the
rotor teeth of FIGS. 3A and 3B are similarly shaped to those in
FIG. 3B, but for the tooth tip closure portions (139 is typical)
that bridge tooth tips. The tip closure portion improves the mutual
coupling of the stator and rotor electromagnetic fields in various
embodiments. Under high torque load conditions, the tip closure
portions 139 are driven into deep magnetic saturation so that the
tip closure portions 139 approximate function of open air in an
open slot motor. In these examples, the magnetic flux distribution
of the closed rotor slot motor resembles the open rotor slot motor
under high torque conditions.
[0060] A number of comparison curves show characteristics and
performance of example embodiments. To increase the peak torque
capability of-the motor without increasing the motor size and drive
voltage and current, the air gap flux linkage is increased by
increasing the flux density level in the air gap. In various
examples, this is achieved with magnetic steel material.
[0061] FIGS. 4-17 demonstrate characteristics and performance of a
conventional motor and several improved induction motors. For these
curves, the same power inverter is used to drive the conventional
motors and the improved motors, and the conventional motors and the
improved motors have similar external sizes, such as having the
same stator outside diameters. The curves illustrate that the
improved induction motors provide higher magnetizing inductance,
lower leakage inductances, higher stator flux linkage, and higher
air-gap flux linkage. One or more of these characteristics, alone
or in combination, provide a higher torque capability with the same
drive current.
[0062] FIG. 4 is a diagram showing an increased air gap flux
linkage of an example of an improved induction motor for high
torque density, such as the motor illustrated in FIG. 1A. The
improved embodiment is represented by a solid line 402, and the
conventional motor is shown as a dotted line 404. The curve
represents air-gap flux linkage as a function of phase current. The
air-gap flux linkage illustrated in FIG. 4 and the stator flux
linkage illustrated in FIG. 5 represent the fluxes passing through
areas associated with current carrying stator windings and rotor
bars. More specifically, the air-gap flux linkage of a single phase
refers to the flux linked by both the stator windings and the rotor
bars of the phase, whereas the stator flux linkage of a phase
refers to the total flux linked by the stator windings of the
phase.
[0063] "Per unit" refers to the per-unit measurement system for
power electronics, in which an actual value of a physical variable
is divided by a certain fixed base value of the same variable, the
base value often being the nominal value or the maximum rating
value. For a given plate constructed from a material such as
magnetic steel, saturation level is represented by a B-H curve,
where as noted earlier B represents magnetic flux density and H
represents magnetic field intensity.
[0064] The magnetic material B-H curve resembles the shape of the
curves shown in FIG. 4, as the air-gap flux linkage is proportional
to B and the phase current is proportional to H. The improved
induction motor operates in saturation mode starting at less than
0.1 p.u. (per unit) drive current, and goes into deeper saturation
when the phase current is increased to 1.0 p.u. One p.u. in this
example is the power inverter maximum drive RMS current rating.
FIG. 4 illustrates some of the characteristics of the improved
design based on configurations of the present subject matter.
[0065] FIG. 5 is a diagram showing a higher stator flux linkage for
one example of an improved induction motor, such as the motor
illustrated in FIG. 1A. The improved embodiment is represented by a
solid line 502, and the conventional motor is shown as a dotted
line 504. The difference between the air-gap flux linkage of FIG. 4
(taken at a phase current) and the stator flux linkage of FIG. 5 is
the stator leakage flux. The stator leakage flux represents the
flux produced by excited stator windings that are not linking with
rotor bars.
[0066] For a given drive current, the stator leakage inductance is
reduced for the improved induction motor, thus reducing the stator
leakage flux. According to certain embodiments of the present
subject matter, both the stator and rotor leakage inductances are
reduced in the improved induction motor. In various embodiments,
increasing the air gap in the radial direction reduces leakage flux
by increasing the leakage flux path reluctance through the air.
Shortening the axial stack or rotor length also reduces the total
leakage flux for the same reason. Narrower stator teeth and
narrower rotor teeth facilitate the deeper magnetic saturation of
the stator teeth and the rotor teeth. This also results in the
saturation of the stator and rotor leakage inductances. The reduced
leakage inductances increase the breakdown torque (i.e., pull-out
torque) especially at high speeds, resulting in an improved peak
torque capability.
[0067] FIG. 6 is a diagram of a saturation curve shown as motor
terminal line-to-line voltage as a function of phase current, for
an improved induction motor such as the motor illustrated in FIG.
1A. An improved motor embodiment is shown as a solid line 602,
whereas a conventional motor is shown as a dotted line 604. In
saturation, for the same current, the improved induction motor
operates at a higher terminal voltage. This is an indication that
more electrical power, which is proportional to a product of
voltage and current, and which is transmitted to the motor to
produce a higher torque, despite construction of the motors from
the same magnetic material with the same material saturation
limitation according to the material B-H curve. FIG. 6 illustrates
a desirable characteristic of an improved induction motor according
to certain embodiments of the present subject matter.
[0068] FIG. 7 is a diagram of a saturation curve of an improved
induction motor shown as saturated magnetizing inductance as a
function of phase current, for an improved induction motor such as
the motor illustrated in FIG. 1A. Again, the improved embodiment is
represented by a solid line 702, and the conventional motor is
shown as a dotted line 704. An increase in the magnetizing
inductance can be noted in a saturated operating condition, which
generally occurs when the phase current is higher than 0.1 per
unit.
[0069] FIG. 8 is a diagram of a saturation curve of an improved
induction motor shown as saturated magnetizing inductance as a
function of air-gap flux linkage, for an improved induction motor
such as the motor illustrated in FIG. 1A. The improved embodiment
is represented by a solid line 802, and the conventional motor is
shown as a dotted line 804. FIGS. 4 through 8 disclose
characteristics of the improved induction motors according to
certain embodiments of the present subject matter, illustrated via
the relationships among important motor parameters including air
gap flux linkage, stator flux linkage, line to line voltage,
magnetizing inductance and phase current.
[0070] FIG. 9 shows flux density distribution in the air gap for an
idealized conventional induction motor, versus electrical angle
between a rotor and a stator, at a given instant of time under a
balanced three-phase excitation, according to some embodiments. The
electrical angle from 0 to 360 degrees is a span that covers a pair
of opposing poles. The shape of a waveform depends on the number of
motor poles. Different waveforms are generated if the physical
(i.e., mechanical) angle of the rotor with respect to the stator
changes. One differentiates electrical angle from mechanical angle
because the poles can rotate around the stator, even if the rotor
is not moving with respect to the stator.
[0071] High-frequency sinusoidal ripples 902 are shown to be
superimposed over a sinusoidal fundamental curve 904. The
fundamental curve is defined by a series of values that represent
the instantaneous value of the flux density minus slot ripples
having a higher frequency than that of the fundamental waveform.
The fundamental value has a first fundamental sinusoidal component.
A substantially square, trapezoidal shaped waveform can be
decomposed into a first fundamental sinusoidal component as well as
multiple higher order sinusoidal components using Fourier
transforms. These multiple higher-order sinusoidal components are
also commonly referred to as harmonics. The ripples 902 are due to
the slotting effects of the stator and rotor slots.
[0072] FIG. 10 is a diagram showing, in an upper portion, stator
yoke flux density 1004 and flux density for the stator teeth 1002
for an idealized conventional stator having 72 slots. In a bottom
portion, stator yoke flux density 1008 and flux density for the
stator teeth 1006 for a rotor having 82 slots is illustrated. The
prefixes Bg, Bsy, Bst, Bry and Brt represent flux densities of air
gap, stator yoke, stator tooth, rotor yoke and rotor tooth,
respectively. The additional suffix "nl" represents no-load, which
is an operating condition, established in the induction motor arts,
to characterize magnetic saturation capability of the motor.
[0073] FIGS. 11 and 12 show flux densities for an improved
induction motor embodiment. In FIG. 11, ripples 1102 are
superimposed over a fundamental curve 1104. FIG. 12 shows, in an
upper portion, flux density for a stator yoke 1202 and in the
stator teeth 1204, and in a lower portion, flux density for a rotor
yoke 1206 and for rotor teeth 1208. In FIGS. 11-12, the improved
induction-motor embodiment uses a stator having 60 slots and a
rotor having 74 slots (e.g., the embodiment of FIG. 1A), although
other slot number combinations are possible.
[0074] FIG. 13 and FIG. 14 show flux densities for a further
improved induction motor embodiment. FIG. 13 shows flux density
ripples 1302 superimposed over a fundamental curve 1304 for an air
gap. FIG. 14, shows, in an upper portion, flux density for a stator
yoke 1402 and in the stator teeth 1404, and in a lower portion,
flux density for a rotor yoke 1406 and for rotor teeth 1408. In
FIGS. 13 and 14, the improved induction motor embodiment uses a
stator having 48 slots and a rotor having 68 slots (e.g., the
embodiment of FIG. 20).
[0075] For a given voltage and amperage, the torque capability of
conventional motors is unfavorably limited by a lower pull-out
torque and decreased air-gap flux linkage. Improved air-gap flux
linkage is described above in associate with FIGS. 4-5. Pull-out
torque is also known as breakdown torque, which is the maximum
torque that can be obtained at a speed point and under the
excitation limitations, i.e., the voltage and current limitations.
The curve 1902 for the improved motor embodiment in FIG. 19A has a
maximum in curve 1902 which represents the pull-out torque for the
motor, for example.
[0076] FIG. 11 is a diagram showing air gap flux density over 360
electrical degrees for an improved motor. This motor performs
better in EV applications than the conventional induction motor of
FIGS. 9 and 10. The improved motor produces increased stator and
air-gap flux linkages and increased air-gap flux density. This
provides increased torque. The waveforms are not sinusoidal,
instead taking a substantially square, trapezoidal shape. Across
the air gap, both the peak and the RMS of the fundamental flux
density are increased when compared to the curve in FIG. 9, showing
a performance improvement of the improved motor of FIG. 11 when
compared to the conventional design of FIG. 9.
[0077] FIG. 12 shows stator yoke and tooth flux densities over a
stator circumfluent of 60 slots, and rotor yoke and tooth flux
densities over rotor circumfluent of 74 slots. In FIG. 12, for the
stator and rotor teeth, both the peak and the RMS of the flux
density are increased. The substantially square trapezoidal MMF
pattern, produced in some examples by using a full-pitch stator
winding placement discussed in association with FIG. 16, generates
a substantially square or trapezoidal flux density distribution.
Some embodiments have two or more layers of coils in a single slot.
An example of a coil that has two layers is illustrated in FIG.
2.
[0078] FIGS. 9-14 refer to embodiments in which a motor has four
poles per phase. For the improved motor embodiments disclosed in
FIGS. 11-14, most of the teeth in each pole are driven to nearly
deep saturation, i.e., increasing the MMF produced by the current
through the windings surrounding the teeth would not increase the
flux density appreciably. The saturation level of the several
central teeth of a particular pole is similarly high, unlike the
conventional motor which has a more sinusoidal gradient. The
saturation level of most or all the teeth of a particular pole is
deep, and the saturation level of the outside teeth is less than
that of the central teeth. For example, each of the teeth 1101-1116
and 1181-1190 (see FIG. 1C) are at or above approximately 1.5
Tesla. Those teeth are in full saturation, in various embodiments.
In some embodiments, the saturation level is above 2.0 Tesla. In
one example, simultaneous saturation of all the stator and rotor
teeth of the entire motor occurs, in which where most of the teeth
are in deep saturation while the stator and rotor yokes are only
lightly saturated or not saturated.
[0079] FIG. 12 illustrates flux densities for an improved motor
having 60 stator slots and 74 rotor slots. The flux densities
illustrated are a snapshot of what is measured as a point of
reference travels 360 degrees around the stator measuring flux
density. As is illustrated in FIG. 1C, many of the stator teeth and
rotor teeth are in deep saturation. In the illustration, they have
a flux density of about 2.0 Tesla. The no-load stator tooth
flux-density is shaped like a substantially square wave, while the
no-load stator yoke flux density is shaped like a saw tooth. The
stator yoke and the rotor yoke have a flux density ranging from not
saturated to lightly saturated to a very limited region of deep
saturation.
[0080] The improved induction motor embodiments disclosed here are
able to show characteristics of a permanent-magnet brushless DC
("BLDC") motor with salient stator and rotor poles and with
substantially square-wave current excitations. For example, the
improved motors disclosed here can mimic BLDC motors at high-torque
operating points. The saliency of the substantially square,
trapezoidal flux density is obtained with distributed stator
windings, instead of with the concentrated stator windings of a
BLDC. In contrast to BLDC motors, the embodiments disclosed here
produce torque from the interaction between the first fundamental
sinusoidal components of the traveling air-gap flux distribution
and the first fundamental sinusoidal components of the traveling
rotor MMF provided by the induced rotor bar currents.
[0081] The forward traveling air-gap flux distribution is the
combined result of the forward traveling stator MMF and the forward
traveling rotor MMF (the latter lags the former by the slip speed),
while the forward traveling stator MMF is the combined result of
the varying stator drive current and spatially distributed stator
windings of multiple phases. Compared to the conventional motor,
the non-sinusoidal wave flux density distribution results in higher
fundamental sinusoidal components than a conventional design.
Additionally, the non-sinusoidal wave MMF distribution can result
in higher fundamental sinusoidal components than a conventional
design.
[0082] Compared to a conventional motor fitted with the same number
of teeth, the improved induction motor embodiments disclosed here
carry flux across the air gap using more stator teeth and more
rotor teeth than a conventional design does. The deep tooth
saturation for the main magnetizing flux paths reduces both stator
and rotor leakage inductances while allowing higher air gap flux
density. This improves pull-out torque and constant-power range as
is discussed here.
[0083] Due to the deep saturation level of the stator and rotor
teeth, some embodiments maintain the saturation levels of the bulk
of the stator and the rotor yokes at a low level to reduce the need
for high magnetization current under varying load conditions. This
tends to keep the motor power factor and efficiency at a high
level. Some embodiments match the saturation levels and the above
saliency effect on both the stator and the rotor to provide a
smooth torque production and minimal low order torque
pulsations.
[0084] The improved saturation level management is possible through
the improvement of the plate geometries, winding patterns and
excitation control. These characteristics are discussed in
association with FIGS. 1-3. Wide stator slots and narrow stator
teeth allow for distributed and wide-span deep tooth saturation.
They also provide increased winding areas so that a higher number
of turns can be used. This increases the MMF which increases the
flux density through the stator teeth to the air gap. This increase
generally takes place at speeds below those where field weakening
begins. Short stator and rotor teeth (as compared to yoke
thickness), most of which are driven to deep saturation in peak
torque condition, require reduced magnetization current. The need
for magnetization current is further reduced by using thicker
stator and rotor yokes that are less saturated. Short and narrow
stator and rotor teeth also reduce stator and rotor leakage fluxes
which increase pull-out torque (discussed, e.g., in association
with FIG. 5).
[0085] A high number of stator teeth allows for the distributed
wide-span deep saturation in the stator teeth. This creates a
substantially square-wave like stator-tooth and air-gap flux
density distribution with high fundamental levels as shown above. A
higher number of rotor teeth allows for the matching of deep
saturation of the stator and rotor teeth, as well as allowing a
similar distributed wide-span deep saturation in the rotor teeth to
create the substantially square-wave like rotor teeth flux density
distribution with high fundamental levels. A large stator outer
diameter to axial stack length ratio, such as a ratio of 2:1,
provides more room to increase stator slot area, to increase MMF
and to increase stator and rotor yoke thickness. This also allows
for high stator and rotor tooth and slot numbers without adding
manufacturing difficulty. This ratio provides for a shorter plate
axial stack length and allows for a limited motor volume and weight
while increasing motor outer diameter. This can lower the stator
and rotor resistances and leakage inductances. Stator and rotor
tooth widths that are sized similarly to each other allow for
improved air-gap flux density levels and provide the
torque-producing effect of a high air-gap flux density. These tooth
sizes also reduce rotor-bar leakage fluxes.
[0086] One example includes 60 stator slots and 74 rotor slots. In
this example, the torque ripple is reduced. Radial force and stray
load losses due to slot harmonics and winding harmonics are reduced
largely due to the ability to match the rotor tooth width to the
stator tooth width and to match the saturation levels of the rotor
teeth and the stator teeth. The motor retains other general
benefits of high tooth numbers, such as lower resistances and
leakage inductances. As described earlier, a high number of stator
and rotor teeth allow the distributed multiple teeth saturation and
a high fundamental flux density distribution.
[0087] A stator outer diameter (OD) to stack length ratio of at
least 2:1 is desirable, as shown in FIG. 1. Compared to longer
stack motors the benefits are reduced stator and rotor resistances
and leakage inductances, as they are all proportional to the stack
length. End-turn and end-ring effects are insignificant until the
stator OD to stack length ratio is substantially increased. The
shorter stack's compromise in air-gap flux linkage is compensated
for by an increase in flux linkage due to one or more of the
features depicted herein as increasing flux linkage. Its compromise
in thermal contact area is compensated by larger stator and rotor
OD. The increased stator slot area reduces stator current density
and increases the winding-to-iron contact area. This can reduce
thermal resistance and provide an improved thermal performance
(i.e., reduce heating in operation). Alternatively, short-stack
motors include motors with a stator inner diameter (ID) to stack
length ratio of greater than 1. Other aspects of the improved
embodiments may be varied to provide further benefits. For example,
the size of the slot openings of the stator and rotor slots can be
adjusted to reduce torque pulsations and leakage fluxes.
[0088] As set out above, a reduced rotor outer diameter and
increased stator inner diameter provides a larger stator slot area.
The stator MMF can be increased when compared to a conventional
motor with the same winding pattern as the conventional motor. This
further increases the air gap flux density level, as well as
reducing the slot ripple harmonic magnitude and rotor inertia.
[0089] FIG. 13 shows an embodiment which has a fundamental flux
density peak that is increased, when compared to FIG. 11, from 0.90
Tesla to 1.05 Tesla. The flux density ripple maximum magnitude has
reduced from 1.10 Tesla to 0.80 Tesla, as compared to the
configuration set out in FIG. 11.
[0090] FIG. 14 is a diagram showing the above embodiment with
stator yoke and teeth flux densities over a stator circumfluent of
48 slots and rotor yoke and teeth flux densities over a rotor
circumfluent of 68 slots. The figure shows a motor with improved,
deep, identical-level saturation of the stator and rotor teeth, and
with improved, shallower, identical-level saturation of the stator
and rotor yokes.
[0091] FIG. 15 is a diagram showing the total MMF 1502 of a phase
of an example 4-pole configuration with short-pitch winding
placement, with accumulated MMF of all coils and turns of the phase
over the 60 stator slots. The short-pitch winding configuration is
disclosed in table 1, as follows:
TABLE-US-00001 Slot Number Top Bottom 1 A C- 2 A C- 3 A C- 4 A C- 5
A C- 6 C- B 7 C- B 8 C- B 9 C- B 10 C- B 11 B A- 12 B A- 13 B A- 14
B A- 15 B A- 16 A- C 17 A- C 18 A- C 19 A- C 20 A- C 21 C B- 22 C
B- 23 C B- 24 C B- 25 C B- 26 B- A 27 B- A 28 B- A 29 B- A 30 B- A
31 A C- 32 A C- 33 A C- 34 A C- 35 A C- 36 C- B 37 C- B 38 C- B 39
C- B 40 C- B 41 B A- 42 B A- 43 B A- 44 B A- 45 B A- 46 A- C 47 A-
C 48 A- C 49 A- C 50 A- C 51 C B- 52 C B- 53 C B- 54 C B- 55 C B-
56 B- A 57 B- A 58 B- A 59 B- A 60 B- A
[0092] In table 1 and table 2 (below), a minus sign indicates
current in a slot in a coil extending in an opposite direction to
coils without a minus sign. A complete coil travels through two
slots with opposite signs and completes a current loop. Coils can
be connected in serial or parallel or combination. Table 1 shows a
4-pole motor with 4 groups of coils and four current loops for each
phase, resulting in 4 maximum (2 positive, 2 negative) peaks of the
3-phase combined MMF in FIG. 15.
[0093] The left of the table includes the slot numbers for a 60
slot stator. A two-layer winding pattern is shown, including top
layer (occupying top half of a slot) and bottom layer (occupying
bottom half of a slot). This embodiment has 60 stator slots. There
are 4 poles per phase. There are 5 coils per pole. The coils each
span 10 teeth. Coil 1 spans slots 1 to 11, and coil 5 spans slots 5
to 15. The short-pitch winding configuration provides a near
sinusoidal MMF which is an efficient method to excite the induction
motor magnetic paths to produce a near sinusoidal flux density
distribution such as the one illustrated in FIG. 9.
[0094] FIG. 16 is a diagram showing the total MMF 1602 of a phase
of an example 4-pole design with full-pitch winding placement, with
accumulated MMF of all coils and turns of the phase over the 60
stator slots. The full-pitch winding configuration is disclosed in
table 2, as follows:
TABLE-US-00002 Slot Number Top Bottom 1 A A 2 A A 3 A A 4 A A 5 A A
6 C- C- 7 C- C- 8 C- C- 9 C- C- 10 C- C- 11 B B 12 B B 13 B B 14 B
B 15 B B 16 A- A- 17 A- A- 18 A- A- 19 A- A- 20 A- A- 21 C C 22 C C
23 C C 24 C C 25 C C 26 B- B- 27 B- B- 28 B- B- 29 B- B- 30 B- B-
31 A A 32 A A 33 A A 34 A A 35 A A 36 C- C- 37 C- C- 38 C- C- 39 C-
C- 40 C- C- 41 B B 42 B B 43 B B 44 B B 45 B B 46 A- A- 47 A- A- 48
A- A- 49 A- A- 50 A- A- 51 C C 52 C C 53 C C 54 C C 55 C C 56 B- B-
57 B- B- 58 B- B- 59 B- B- 60 B- B-
[0095] Table 2 shows a 4-pole motor with 4 groups of coils and four
current loops for each phase, resulting in 4 maximum (2 positive, 2
negative) peaks of the 3-phase combined MMF in and FIG. 16. This
embodiment has 60 stator slots. There are 4 poles per phase. There
are 5 coils per pole. The coils each span 15 teeth. Coil 1 spans
slots 1 to 16, and coil 5 spans slots 5 to 20, for example. The
full-pitch winding configuration provides a non-sinusoidal or
substantially square, trapezoidal MMF that contains lower order
harmonics which is an efficient method to excite the induction
motor magnetic paths to produce a substantially square, trapezoidal
flux density distribution, such as the ones illustrated in FIGS.
11-14. Alternative winding configurations are also contemplated,
including those having coils spanning between 10 and 15 slots.
[0096] Winding patterns allowing more substantially square-wave
trapezoidal shaped MMF provide increased torque. For example, the
winding pattern of FIG. 16 provides more torque than that of FIG.
15. These are more efficient in producing a substantially
square-wave or trapezoidal shaped flux density distribution
analogous to that of BLDC motors. For a more smooth torque
production at lower torque operating points, a more sinusoidal MMF
is used. This also provides a more sinusoidal flux density
distribution. In some examples, a substantially square-wave like or
trapezoidal shaped MMF is provided by using at full-pitch 2 or more
layers of same-phase coils per slot and by using a high number of
turns per coil. Higher numbers of poles can also allow for a
substantially square-wave MMF.
[0097] FIGS. 17A-17C show a finite-element analysis model of flux
density magnitude at peak torque, the flux density indicated across
180 mechanical degrees or 1/2 of the induction motor, according to
embodiments of the present subject matter. The charts represent 1
pole pair of a 4-pole motor. FIG. 17A shows a flux density 1702
along the middle of the air-gap. FIG. 17B shows flux density across
the stator teeth 1704 and flux density across the stator yoke 1706.
FIG. 17C shows flux density across the rotor teeth 1710 and flux
density across the rotor yoke 1708.
[0098] The leakage flux densities 1707, 1710 through the stator and
rotor slots is shown to be under 0.3 Tesla in this example, as
indicated by the minimum pulses of the non-smooth pulsed curves of
FIG. 17B and FIG. 17C, for the stator slots and the rotor slots,
respectively. The leakage flux density level is lower than in a
conventional motor due to the higher and more prevalent saturation
of the stator and rotor teeth which are also part of the leakage
flux paths. The stator and rotor leakage flux can saturate in the
high torque operating points, resulting in saturated and reduced
stator and rotor leakage inductances. In the improved motor, by
distributing the leakage fluxes as well as reducing the stack
length, the total leakage flux linkages and the leakage inductances
of the stator and rotor are further reduced, resulting in an
increased pull-out torque.
[0099] FIGS. 18A-18D show peak torque 1802, voltage 1804, current
1806 and stator flux linkage 1808 over a speed range, for
embodiments of the present subject matter. FIGS. 19A-19B show
variable-frequency control and constant flux 1902 range up to the
base speed, with a field weakening range, for embodiments of the
present subject matter. The base speed is shown to be at 1 p.u.
(per unit). The base speed is the speed around which the peak
torque 1902 starts to decrease (as can be seen in FIG. 19A) and the
peak stator flux linkage 1904 starts to decrease (as can be seen in
FIG. 19B). The improved induction motors may provide a 6:1 or
greater peak/continuous torque ratio. They also provide a wide
constant-peak power range that is extended from the base speed to
more than twice the base speed. Further, they provide a wide
field-weakening range that is extended from the base speed to more
than twice the base speed. These effects are provided under various
constraints, such as limited available maximum current and voltage.
The improved induction motors have an improved power factor and
improved efficiency when compared to a conventional motor. Further,
they perform without unwanted increases in motor weight and
size.
[0100] Below the base speed, the peak flux linkage level is kept
under a limit in deep saturation. This is to obtain an improved
torque/current ratio. This is possible with the adjustment of the
drive voltage as shown in the example voltage profile in FIGS.
18A-D. At less than peak torque operating points below the base
speed, the flux linkage level can be adjusted by adjusting the
drive voltage and current, achieving the demanded torque while
meeting other performance criteria such as desired efficiency or
reduced drive current. The adjustment of the flux linkage allows
for the operation of the motor in substantially reduced saturation,
and the flux density distribution will now become more sinusoidal
if the winding pattern is sinusoidal (which is not necessary as is
discussed above).
[0101] FIGS. 19A-B illustrate a variable frequency control. At
speeds above the base speed of 1 p.u., the drive voltage is limited
and the flux linkage is weakened. The pull-out torque, which is the
maximum torque in each of the single frequency torque-speed curves,
becomes lower as the speed increases. It is beneficial to provide a
high pull-out torque, as this increases the vehicle operator's
perception that torque is available for acceleration. This can be
done through reducing both the stator and rotor leakage fluxes, and
hence leakage inductances. The short and narrow stator and rotor
teeth of the improved induction motor embodiments provide this
advantage by providing less steel area for the leakage flux paths.
In addition, the deep tooth saturation further reduces both stator
and rotor leakage inductances while allowing higher air gap flux
density. This improves pull-out torque. Increasing the pull-out
torque also allows the full utilization of the peak inverter power
rating by providing a wide constant-power range.
[0102] FIG. 20 shows an axial view of an improved induction motor
2000, according to various embodiments. The diagram is simplified
for the sake of explanation, as windings and other components are
omitted. The general characteristics of the geometry when compared
to a conventional induction motor include, but are not limited to,
one or more of a radial air gap increased over the air gap size of
conventional motors, a shorter axial stack length, wider stator
slots, narrower stator teeth, narrower and shorter rotor teeth, a
stator yoke that is thicker than the stator's tooth length and much
thicker than the tooth width, a rotor yoke that is thicker than the
rotor's tooth length and much thicker than the rotor's tooth width,
an increased number of stator slots per pole per phase, and an
increased number of rotor slots.
[0103] A rotor 2002 is encircled by a stator 2004. There is an air
gap 2006 between them. The air gap 2006 between the stator 2004 and
rotor 2002 is larger than those used in conventional designs. In
some embodiments, it is at least 0.5 mm, although the present
subject matter is not so limited. In various embodiments, the air
gap size is selected to obtain certain levels of the magnetizing
inductance and the leakage inductances, as disclosed herein (e.g.,
FIGS. 4-8 show the embodiments of FIG. 1A compared to a
conventional design). The air gap 2006 additionally has an effect
on the saturation levels and harmonic levels of the magnetic flux
proximal the air gap. A shaft 2008 is coupled to the rotor 2002.
The shaft 2008 can be coupled to downstream devices, including but
not limited to an axle, a gearbox and the like.
[0104] The rotor 2002 includes a plurality of rotor teeth 2010.
Between each tooth and the tooth next to it (i.e., between each
tooth pair), is a rotor slot 2012. The teeth define the slots
between them. In this embodiment, the rotor slots are closed. The
rotor also has a rotor yoke 2014. The stator 2004 includes a stator
yoke 2016, a plurality of stator teeth 2018, and stator slots 2020
that are defined by the stator teeth 2018. In various embodiments,
the teeth and the yoke of the rotor are continuous blocks of
material, i.e. monolithic. For example, in some embodiments the
rotor is comprised of a stack of plates electrically insulated from
one another, and each of the plates is stamped or otherwise excised
from a single billet.
[0105] The Abstract is provided to comply with 37 C.F.R.
.sctn.1.72(b) to allow the reader to quickly ascertain the nature
and gist of the technical disclosure. The Abstract is submitted
with the understanding that it will not be used to interpret or
limit the scope or meaning of the claims.
* * * * *