U.S. patent application number 16/805924 was filed with the patent office on 2021-09-02 for current ripple reduction for a direct current source powering an alternating current load.
This patent application is currently assigned to The Boeing Company. The applicant listed for this patent is The Boeing Company. Invention is credited to Kamiar J. Karimi, Frederic Lacaux, Shengyi Liu, Eugene V. Solodovnik.
Application Number | 20210273554 16/805924 |
Document ID | / |
Family ID | 1000004718338 |
Filed Date | 2021-09-02 |
United States Patent
Application |
20210273554 |
Kind Code |
A1 |
Liu; Shengyi ; et
al. |
September 2, 2021 |
Current Ripple Reduction for a Direct Current Source Powering an
Alternating Current Load
Abstract
Systems and methods for current ripple reduction for a direct
current (DC) source powering an alternating current (AC) load. In
accordance with one embodiment, the system and method involve
interleaved operation of a 3.times.3-phase AC motor having multiple
groups of windings. In accordance with another embodiment, the
system and method involve interleaved operation of multiple
co-shafted 3-phase AC motors. In accordance with a further
embodiment, the system and method involve interleaved operation
multiple 3-phase AC motors (not co-shafted) of the same level of
power. The interleaved operation entails interleaved switching
inside a set of inverters which are connected in parallel between a
DC bus and the windings of the AC motor (motors).
Inventors: |
Liu; Shengyi; (Sammamish,
WA) ; Solodovnik; Eugene V.; (Lake Stevens, WA)
; Lacaux; Frederic; (Woodinville, WA) ; Karimi;
Kamiar J.; (Kirkland, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Boeing Company |
Chicago |
IL |
US |
|
|
Assignee: |
The Boeing Company
Chicago
IL
|
Family ID: |
1000004718338 |
Appl. No.: |
16/805924 |
Filed: |
March 2, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 1/15 20130101; H02M
7/537 20130101; H02P 27/06 20130101 |
International
Class: |
H02M 1/15 20060101
H02M001/15; H02M 7/537 20060101 H02M007/537; H02P 27/06 20060101
H02P027/06 |
Claims
1. A system comprising a DC source, a DC bus connected to the DC
source, a DC-to-AC converter connected to the DC bus, and an AC
motor connected to the DC-to-AC converter, wherein: the AC motor
comprises a stator, a first winding group at a first angular
position on the stator, a second winding group at a second angular
position on the stator different than first angular position, and a
third winding group at a third angular position on the stator
different than first and second angular positions; the DC-to-AC
converter comprises first and second inverters connected in
parallel to receive direct current from the DC bus; three phases of
the first inverter are connected in sequence to a first winding of
the first winding group, to a first winding of the second winding
group, and to a first winding of the third winding group; and three
phases of the second inverter are connected in sequence to a second
winding of the first winding group, to a second winding of the
second winding group, and to a second winding of the third winding
group.
2. The system as recited in claim 1, further comprising: a first
inverter controller configured to control a switch configuration of
the first inverter; and a second inverter controller configured to
control a switch configuration of the second inverter.
3. The system as recited in claim 2, wherein: the first inverter is
configured to output first, second, and third AC power signals
having respective first, second, and third phases to the first,
second, and third winding groups respectively; the second inverter
is configured to output fourth, fifth, and sixth AC power signals
having respective fourth, fifth, and sixth phases to the first,
second, and third winding groups respectively; the first through
sixth phases are different; and the first through sixth AC power
signals are interleaved.
4. The system as recited in claim 3, further comprising a system
controller communicatively coupled to the first and second inverter
controllers and configured to supervise and coordinate the first
and second inverter controllers so that the first through sixth AC
power signals are interleaved.
5. The system as recited in claim 1, wherein: the DC-to-AC
converter further comprises a third inverter connected in parallel
with the first and second inverters to receive direct current from
the DC bus; and three phases of the third inverter are connected in
sequence to a third winding of the first winding group, to a third
winding of the second winding group, and to a third winding of the
third winding group.
6. The system as recited in claim 5, further comprising: a first
inverter controller configured to control a switch configuration of
the first inverter; a second inverter controller configured to
control a switch configuration of the second inverter; and a third
inverter controller configured to control a switch configuration of
the third inverter.
7. The system as recited in claim 6, wherein: the first inverter is
configured to output first, second, and third AC power signals
having respective first, second, and third phases to the first,
second, and third winding groups respectively; the second inverter
is configured to output fourth, fifth, and sixth AC power signals
having respective fourth, fifth, and sixth phases to the first,
second, and third winding groups respectively; the third inverter
is configured to output seventh, eighth, and ninth AC power signals
having respective seventh, eighth, and ninth phases to the first,
second, and third winding groups respectively; the first through
ninth phases are different; and the first through ninth AC power
signals are interleaved.
8. The system as recited in claim 7, further comprising a system
controller communicatively coupled to the first, second, and third
inverter controllers and configured to supervise and coordinate the
first, second, and third inverter controllers so that the first
through ninth AC power signals are interleaved.
9. The system as recited in claim 8, wherein the AC motor further
comprises a shaft and a rotor mounted to the shaft, and the rotor
is driven to rotate by electromagnetic inductive coupling with the
stator in response to receipt of the first through ninth AC power
signals.
10. A system comprising a DC source, a DC bus connected to the DC
source, a DC-to-AC converter connected to the DC bus, and first and
second AC motors connected to the DC-to-AC converter, wherein: the
first AC motor comprises a first stator and first, second, and
third windings disposed at first, second, and third angular
positions respectively on the first stator; the second AC motor
comprises a second stator and first, second, and third windings
disposed at first, second, and third angular positions respectively
on the second stator; the DC-to-AC converter comprises first and
second inverters connected in parallel to receive direct current
from the DC bus; the first inverter is connected to the first,
second, and third windings on the first stator; and the second
inverter is connected to the first, second, and third windings on
the second stator.
11. The system as recited in claim 10, further comprising: a first
inverter controller configured to control a switch configuration of
the first inverter; and a second inverter controller configured to
control a switch configuration of the second inverter.
12. The system as recited in claim 11, wherein: the first inverter
is configured to output first, second, and third AC power signals
having respective first, second, and third phases to the first,
second, and third windings respectively on the first stator; the
second inverter is configured to output fourth, fifth, and sixth AC
power signals having respective fourth, fifth, and sixth phases to
the first, second, and third windings respectively on the second
stator; the first through sixth phases are different; and the first
through sixth AC power signals are interleaved.
13. The system as recited in claim 12, further comprising a system
controller communicatively coupled to the first and second inverter
controllers and configured to supervise and coordinate the first
and second inverter controllers so that the first through sixth AC
power signals are interleaved.
14. The system as recited in claim 10, further comprising a third
AC motor, wherein: the third AC motor comprises a third stator and
first, second, and third windings disposed at first, second, and
third angular positions respectively on the third stator; the
DC-to-AC converter further comprises a third inverter connected in
parallel with the first and second inverters to receive direct
current from the DC bus; and the third inverter is connected to the
first, second, and third windings on the third stator.
15. The system as recited in claim 14, further comprising: a first
inverter controller configured to control a switch configuration of
the first inverter; a second inverter controller configured to
control a switch configuration of the second inverter; and a third
inverter controller configured to control a switch configuration of
the third inverter.
16. The system as recited in claim 15, wherein: the first inverter
is configured to output first, second, and third AC power signals
having respective first, second, and third phases to the first,
second, and third windings respectively on the first stator; the
second inverter is configured to output fourth, fifth, and sixth AC
power signals having respective fourth, fifth, and sixth phases to
the first, second, and third windings respectively on the second
stator; the third inverter is configured to output seventh, eighth,
and ninth AC power signals having respective seventh, eighth, and
ninth phases to the first, second, and third windings respectively
on the third stator; the first through ninth phases are different;
and the first through ninth AC power signals are interleaved.
17. The system as recited in claim 16, further comprising a system
controller communicatively coupled to the first, second, and third
inverter controllers and configured to supervise and coordinate the
first, second, and third inverter controllers so that the first
through ninth AC power signals are interleaved.
18. The system as recited in claim 17, wherein: the first, second,
and third AC motors further comprise a common shaft; the first AC
motor further comprises a first rotor mounted to the common shaft;
the second AC motor further comprises a second rotor mounted to the
common shaft; the third AC motor further comprises a third rotor
mounted to the common shaft; and the common shaft is driven to
rotate by electromagnetic inductive coupling of the first, second,
and third rotors with the first, second, and third stators
respectively in response to receipt of the first through third AC
power signals, fourth through AC power signals, and sixth through
ninth AC power signals respectively.
19. The system as recited in claim 17, wherein: the first AC motor
further comprises a first shaft and a first rotor mounted to the
first shaft, and the first rotor is driven to rotate by
electromagnetic inductive coupling with the first stator in
response to receipt of the first through third AC power signals;
the second AC motor further comprises a second shaft and a second
rotor mounted to the second shaft, and the second rotor is driven
to rotate by electromagnetic inductive coupling with the second
stator in response to receipt of the fourth through sixth AC power
signals; and the third AC motor further comprises a third shaft and
a third rotor mounted to the third shaft, and the third rotor is
driven to rotate by electromagnetic inductive coupling with the
third stator in response to receipt of the seventh through ninth AC
power signals.
20. A method for providing AC power to motors, the method
comprising: connecting a DC source to a DC bus; connecting first
through third inverters in parallel to the DC bus and to windings
of first through third 3-phase AC motors respectively; controlling
switches inside the first inverter to convert a DC power signal
into first through third AC power signals having a same amplitude
and shifted in phase by 120 degrees with respect to each other;
supplying the first through third AC power signals to first, second
and third windings respectively of the first 3-phase AC motor;
controlling switches inside the second inverter to convert the DC
power signal into fourth through sixth AC power signals having the
same amplitude and shifted in phase by 120 degrees with respect to
each other and by 40 degrees with respect to the first through
third AC power signals; supplying the fourth through sixth AC power
signals to first, second and third windings respectively of the
second 3-phase AC motor; controlling switches inside the third
inverter to convert the DC power signal into seventh through ninth
AC power signals having the same amplitude and shifted in phase by
120 degrees with respect to each other, by 80 degrees with respect
to the first through third AC power signals, and by 40 degrees with
respect to the fourth through sixth AC power signals; and supplying
the seventh through ninth AC power signals to first, second and
third windings respectively of the third 3-phase AC motor.
21. The method as recited in claim 20, further comprising mounting
respective rotors of the first through third 3-phase AC motors to a
common shaft.
22. A method for providing AC power to a motor, the method
comprising: connecting a DC source to a DC bus; connecting first
through third inverters in parallel to the DC bus and to windings
of first through third winding groups disposed at first, second and
third angular positions respectively on a stator of a 3-phase AC
motor disposed; controlling switches inside the first inverter to
convert a DC power signal into first through third AC power signals
having a same amplitude and shifted in phase by 120 degrees with
respect to each other; supplying the first through third AC power
signals to first, second and third windings respectively of the
first winding group; controlling switches inside the second
inverter to convert the DC power signal into fourth through sixth
AC power signals having the same amplitude and shifted in phase by
120 degrees with respect to each other and by 40 degrees with
respect to the first through third AC power signals; supplying the
fourth through sixth AC power signals to first, second and third
windings respectively of the second winding group; controlling
switches inside the third inverter to convert the DC power signal
into seventh through ninth AC power signals having the same
amplitude and shifted in phase by 120 degrees with respect to each
other, by 80 degrees with respect to the first through third AC
power signals, and by 40 degrees with respect to the fourth through
sixth AC power signals; and supplying the seventh through ninth AC
power signals to first, second and third windings respectively of
the third winding group.
Description
BACKGROUND
[0001] The present disclosure generally relates to electrical power
conversion systems and, in particular, to power conversion systems
for converting direct current into alternating current. In
particular, the present disclosure relates to a method and an
apparatus for reducing the total number of components in order to
reduce the overall weight, cost, and size of the conversion
system.
[0002] One type of an electrical power conversion system
(hereinafter "power conversion system") is a system of one or more
devices used to convert direct current (DC) into alternating
current (AC). In certain systems, a centralized power conversion
system may be used to interface DC power sources with various DC
and AC distribution buses. For example, aircraft power generation
and distribution systems may use a centralized power conversion
system to interface low-voltage DC power sources with various DC
and AC distribution buses. A low-voltage DC power source may be,
for example, a fuel cell, a battery pack, a solar panel, or some
other type of power source.
[0003] A power conversion system may include, for example, a
converter for increasing, or stepping-up, the voltage level of a
low-voltage DC power source to form a high-voltage DC (HVDC) power
source. As used herein, a converter is an electrical or
electromechanical device used to change the voltage level of the DC
current power source. As used in the aerospace industry and herein,
the term "high voltage" in the context of direct current means any
DC voltage higher than 500 V.sub.DC.
[0004] The high-voltage DC current power source formed by the
converter may then be fed to an inverter of the power conversion
system to form a high-voltage AC power source. An inverter, as used
herein, is an electrical or electromechanical device used to
convert direct current into alternating current. Inverters may take
various forms, including, but not limited to, single-phase
inverters and three-phase inverters. Three-phase inverters
(hereinafter "3-phase inverters) are used for variable-frequency
drive applications and/or for high-power applications such as AC
power transmission. A basic 3-phase inverter consists of three
single-phase inverters each of which consists of two switches in
series with the center point connected to one of the three load
terminals. For the most basic control scheme, the operation of the
six switches of the three phase legs is coordinated so that one
switch operates at each 60 degree point of the fundamental output
waveform. This creates a line-to-line output waveform that has six
steps. The six-step waveform has a zero-voltage step between the
positive and negative sections of the square wave such that the
harmonics that are multiples of three are eliminated as described
above. When carrier-based PWM techniques are applied to six-step
waveforms, the basic overall shape, or envelope, of the waveform is
retained so that the third harmonic and its multiples are
cancelled. To construct inverters with higher power ratings, two
six-step 3-phase inverters can be connected in parallel for a
higher current rating or in series for a higher voltage rating. In
either case, the output waveforms are phase shifted to obtain a
12-step waveform. If additional inverters are combined, an 18-step
inverter is obtained with three inverters etc. Although inverters
are usually combined for the purpose of achieving increased voltage
or current ratings, the quality of the waveform is improved as
well.
[0005] In all-electric or hybrid electric systems, an AC load, for
example, a synchronous electric motor (hereinafter "motor") for
either a propulsion or non-propulsion purpose, is commonly driven
by an inverter which draws a DC current from a DC bus supplied and
stabilized by one or more DC sources (typically non-ideal).
Oftentimes, the current drawn by the inverter contains ripples
(hereinafter "current ripples") which are detrimental to the DC
source due to heat production and other adverse effects. A current
ripple is a periodic non-sinusoidal waveform derived from an AC
power source and characterized by high-amplitude narrow-bandwidth
pulses. The pulses coincide with peak or near-peak amplitude of an
accompanying sinusoidal voltage waveform. Depending upon switching
schemes and system operation conditions, the magnitude of the
current ripple varies. For a given system setup and inverter
modulation method, the current ripple typically has a fixed
pattern.
[0006] There are many ways to reduce the magnitude of a current
ripple, but each way has a limitation. For example, a front filter
(either passive or active) may be added, but the size of a passive
filter size can be large for low-frequency current ripple
filtering, whereas using an active filter complicates the system
design and operation and adds weight and reliability issues. An
alternative solution is to select an appropriate inverter switching
modulation, such as pulse width modulation, space vector
modulation, or phase shift modulation, but none of these modulation
techniques can eliminate or achieve satisfactory reduction of
current ripples.
[0007] A system and method for current ripple reduction of a DC
source powering an AC load which improves upon the state of the art
would be beneficial.
SUMMARY
[0008] The subject matter disclosed in some detail below is
directed to systems and methods for current ripple reduction for a
DC source during DC-to-AC power conversion for supplying AC power
to an electric motor (hereinafter "AC motor") that is driven by
alternating current. The systems and methods disclosed herein focus
on the setup and operation of the AC load. In accordance with
various embodiments, the system and method involve interleaved
operation of: (1) a 3.times.3-phase AC motor having multiple groups
of windings (hereinafter "embodiment"); (2) multiple co-shafted
3-phase AC motors (hereinafter "second embodiment"); or (3)
multiple 3-phase AC motors (not co-shafted) of the same level of
power (hereinafter "third embodiment"). The interleaved operation
entails interleaved switching inside a set of inverters which are
connected in parallel between a DC bus and the windings of the AC
motor (motors). Depending upon the number of groups of windings in
the 3.times.3-phase AC motor of the first embodiment or the number
of 3-phase AC motors in the second and third embodiments, the
interleaved operation method may vary.
[0009] As used herein, the term "interleaved operation" means
interleaved switching inside inverters that produces AC power
signals having interleaved waveforms. As used herein, the term
"interleaved waveforms" means separately generated waveforms having
the same shape and period, but differing in phase, which may be
combined into a waveform in which, for example, the peaks of the
separately generated waveforms alternate in time. In the context of
this disclosure, interleaved waveforms are separately generated by
inverters and then separately applied to different sets of windings
(a.k.a. "coils"), not combined and applied to one set of
windings.
[0010] Because of the independence of the interleaved operational
method disclosed herein, the method generally applies to all
inverter switching modulation schemes, as long as the same
modulation scheme is used for all three phases of a motor or for
all 3-phase AC motors. The method proposed herein does not rely on
the setup of the front end or the condition of the DC sources.
Therefore the method has a broad application for any DC source
driving an AC load.
[0011] The benefits of the innovative technology disclosed herein
include: (1) current ripple reduction or minimization for a DC
source (for example, a battery, thereby extending the life of the
battery); and (2) reduction of the front-end filter size, thereby
reducing system weight, size, and complexity.
[0012] Although various embodiments of systems and methods for
current ripple reduction for a DC source powering an AC load will
be described in some detail below, one or more of those embodiments
may be characterized by one or more of the following aspects.
[0013] One aspect of the subject matter disclosed in detail below
is a system comprising a DC source, a DC bus connected to the DC
source, a DC-to-AC converter connected to the DC bus, and an AC
motor connected to the DC-to-AC converter, wherein: the AC motor
comprises a stator, a first winding group at a first angular
position on the stator, a second winding group at a second angular
position on the stator different than first angular position, and a
third winding group at a third angular position on the stator
different than first and second angular positions; the DC-to-AC
converter comprises first, second and third inverters connected to
receive direct current from the DC bus; the three phases A, B, and
C of the first inverter are connected, sequentially, to a first
winding of the first winding group, to a first winding of the
second winding group, and to a first winding of the third winding
group; the three phases A, B and C of the second inverter are
connected, sequentially, to a second winding of the first winding
group, to a second winding of the second winding group, and to a
second winding of the third winding group; and the three phases A,
B, and C of the third inverter are connected, sequentially, to a
third winding of the first winding group, to a third winding of the
second winding group, and to a third winding of the third winding
group. The inverters are configured by respective inverter
controllers to output AC power signals having respective phases
which are interleaved to reduce current ripple that may adversely
impact the DC source.
[0014] Another aspect of the subject matter disclosed in detail
below is a system comprising a DC source, a DC bus connected to the
DC source, a DC-to-AC converter connected to the DC bus, and first,
second, and third AC motors connected to the DC-to-AC converter,
wherein: the first AC motor comprises a first stator and first,
second, and third windings disposed at first, second, and third
angular positions respectively on the first stator; the second AC
motor comprises a second stator and first, second, and third
windings disposed at first, second, and third angular positions
respectively on the second stator; the third AC motor comprises a
third stator and first, second, and third windings disposed at
first, second, and third angular positions respectively on the
third stator; the DC-to-AC converter comprises first, second, and
third inverters connected to receive direct current from the DC
bus; the first inverter is connected to the first, second, and
third windings on the first stator; the second inverter is
connected to the first, second, and third windings on the second
stator; and the third inverter is connected to the first, second,
and third windings on the third stator. The inverters are
configured by respective inverter controllers to output AC power
signals having respective phases which are interleaved to reduce
current ripple that may adversely impact the DC source.
[0015] In accordance with one embodiment of the system described in
the immediately preceding paragraph, the first, second, and third
AC motors further comprise a common shaft; the first AC motor
further comprises a first rotor mounted to the common shaft; the
second AC motor further comprises a second rotor mounted to the
common shaft; the third AC motor further comprises a third rotor
mounted to the common shaft; and the common shaft is driven to
rotate by electromagnetic inductive coupling of the first, second,
and third rotors with the first, second, and third stators
respectively.
[0016] In accordance with an alternative embodiment, the first AC
motor further comprises a first shaft and a first rotor mounted to
the first shaft, and the first rotor is driven to rotate by
electromagnetic inductive coupling with the first stator; the
second AC motor further comprises a second shaft and a second rotor
mounted to the second shaft, and the second rotor is driven to
rotate by electromagnetic inductive coupling with the second
stator; and the third AC motor further comprises a third shaft and
a third rotor mounted to the third shaft, and the third rotor is
driven to rotate by electromagnetic inductive coupling with the
third stator.
[0017] A further aspect of the subject matter disclosed in detail
below is a method for providing AC power to a motor, the method
comprising: connecting a DC source to a DC bus; connecting first
through third inverters in parallel to the DC bus and to windings
of first through third winding groups disposed at first, second and
third angular positions respectively on a stator of a 3-phase AC
motor disposed; controlling switches inside the first inverter to
convert a DC power signal into first through third AC power signals
having a same amplitude and shifted in phase by 120 degrees with
respect to each other; supplying the first through third AC power
signals to first, second and third windings respectively of the
first winding group; controlling switches inside the second
inverter to convert the DC power signal into fourth through sixth
AC power signals having the same amplitude and shifted in phase by
120 degrees with respect to each other and by 40 degrees with
respect to the first through third AC power signals; supplying the
fourth through sixth AC power signals to first, second and third
windings respectively of the second winding group; controlling
switches inside the third inverter to convert the DC power signal
into seventh through ninth AC power signals having the same
amplitude and shifted in phase by 120 degrees with respect to each
other, by 80 degrees with respect to the first through third AC
power signals, and by 40 degrees with respect to the fourth through
sixth AC power signals; and supplying the seventh through ninth AC
power signals to first, second and third windings respectively of
the third winding group.
[0018] Yet another aspect of the subject matter disclosed in detail
below is a method for providing AC power to motors, the method
comprising: connecting a DC source to a DC bus; connecting first
through third inverters in parallel to the DC bus and to windings
of first through third 3-phase AC motors respectively; controlling
switches inside the first inverter to convert a DC power signal
into first through third AC power signals having a same amplitude
and shifted in phase by 120 degrees with respect to each other;
supplying the first through third AC power signals to first, second
and third windings respectively of the first 3-phase AC motor;
controlling switches inside the second inverter to convert the DC
power signal into fourth through sixth AC power signals having the
same amplitude and shifted in phase by 120 degrees with respect to
each other and by 40 degrees with respect to the first through
third AC power signals; supplying the fourth through sixth AC power
signals to first, second and third windings respectively of the
second 3-phase AC motor; controlling switches inside the third
inverter to convert the DC power signal into seventh through ninth
AC power signals having the same amplitude and shifted in phase by
120 degrees with respect to each other, by 80 degrees with respect
to the first through third AC power signals, and by 40 degrees with
respect to the fourth through sixth AC power signals; and supplying
the seventh through ninth AC power signals to first, second and
third windings respectively of the third 3-phase AC motor.
Optionally, the method further comprises mounting respective rotors
of the first through third 3-phase AC motors to a common shaft.
[0019] Other aspects of systems and methods for current ripple
reduction of a DC source powering an AC load are disclosed
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The features, functions and advantages discussed in the
preceding section may be achieved independently in various
embodiments or may be combined in yet other embodiments. Various
embodiments will be hereinafter described with reference to
drawings for the purpose of illustrating the above-described and
other aspects.
[0021] FIGS. 1A and 1B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter
for powering a 3.times.3-phase AC motor using interleaved switching
of three parallel 3-phase inverters in accordance with the first
embodiment.
[0022] FIG. 2 is a diagram representing a pair of sensors for
measuring the voltage and current of the AC power signals output by
the inverters.
[0023] FIGS. 2A and 2B are diagrams respectively representing
voltage and current sensors measuring the voltage and current of
the AC power signals output by the inverters.
[0024] FIG. 3 is a graph showing current (vertical axis) versus
time (horizontal axis) for current ripples produced at a DC bus by
single 3-phase resistive loads (waveform WF1) and by three 3-phase
resistive loads interleaved with a 769 phase angle (waveform
WF2).
[0025] FIGS. 4A and 4B are high-frequency spectrum graphs showing
that some harmonics are eliminated (indicated by dashed arrows in
FIG. 4A) or the magnitudes of some harmonics are reduced (indicated
by dashed arrows in FIG. 4B) by using interleaved switching of
parallel 3-phase inverters.
[0026] FIGS. 5A and 5B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter
for powering three co-shafted 3-phase AC motors using interleaved
switching of three parallel 3-phase inverters in accordance with
the second embodiment.
[0027] FIGS. 6A and 6B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter
for powering three separate 3-phase AC motors using interleaved
switching of three parallel 3-phase inverters in accordance with
the third embodiment.
[0028] Reference will hereinafter be made to the drawings in which
similar elements in different drawings bear the same reference
numerals.
DETAILED DESCRIPTION
[0029] Illustrative embodiments of systems and methods for current
ripple reduction for a DC source powering an AC load are described
in some detail below. However, not all features of an actual
implementation are described in this specification. A person
skilled in the art will appreciate that in the development of any
such embodiment, numerous implementation-specific decisions must be
made to achieve the developer's specific goals, such as compliance
with system-related and business-related constraints, which will
vary from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
[0030] FIGS. 1A and 1B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter 50
(seen in FIG. 1A) for powering a 3.times.3-phase AC motor 20 (seen
in FIG. 1B) using a method of current ripple reduction in
accordance with the first embodiment. The DC-to-AC converter 50 is
connected to a DC bus 38. A DC power source 18 (e.g., a battery or
other non-ideal DC power source) is also connected to the DC bus
38.
[0031] As seen in FIG. 1A, the DC-to-AC converter 50 includes a
system controller 10 and three inverter controllers 12a-12c which
are communicatively coupled to receive control signals from the
system controller 10 and send feedback signals to the system
controller 10. The system controller 10 performs a role of
supervision and coordination for all inverter controllers 12a-12c.
It also interfaces with a higher level controller. The operation of
the DC source 18 may be controlled and managed by a control and
management system and may interact with the system controller 10
(neither of which feature is shown in FIGS. 1A and 1B).
[0032] The DC-to-AC converter 50 further includes three front-end
signal conditioning circuits 14a-14c (hereinafter "front-end
circuits 14a-14c") which receive DC power signals from DC bus 38
via respective DC power lines 4. The DC-to-AC converter 50 further
includes three 3-phase inverters 16a-16c (hereinafter "inverters
16a-16c") which receive conditioned DC power signals from the
respective front-end circuits. The front-end circuits 14a-14c and
the inverters 16a-16c are connected in parallel to the
3.times.3-phase AC motor 20. The operation of inverters 16a-16c is
controlled by inverter controllers 12a-12c respectively, which send
switch control signals to and receive switch state signals from the
inverters 16a-16c via switch signal lines 7.
[0033] An inverter is a power electronic device or circuit that
changes direct current to alternating current. In one simple
inverter circuit, DC power is connected to a transformer through
the center tap of the primary winding. A switch is rapidly switched
back and forth to allow current to flow back to the DC source
following two alternate paths through one end of the primary
winding and then the other end. The alternation of the direction of
current in the primary winding of the transformer produces
alternating current in the secondary circuit. Transistors and other
types of semiconductor switches may be incorporated into the
inverter circuit design.
[0034] More specifically, the inverters 16a-16c may include a
switch system, set of inductors, a set of capacitors, and an
electromagnetic interference filter. The switch system may include
different numbers of switches, depending on the type of inverter.
Each of the switches may be implemented using, for example, without
limitation, a bipolar transistor device, a metal-oxide
semiconductor field-effect transistor (MOSFET) device, an
insulated-gate bipolar transistor device, or some other type of
semiconductor device or switching device.
[0035] In the embodiment depicted in FIGS. 1A and 1B, the inverter
controllers 12a-12c control the operation (switching) of inverters
16a-16c so that the switching inside the inverters is interleaved.
Inverters 16a-16c output respective sets of three AC power signals
that have respective phase angles which differ by 120 degrees,
which sets in turn have phase angles which differ by 40 degrees.
The result is the production of nine AC power signals with
respective phase angles that differ by 40 degrees. For example,
inverter 16a may produce AC power signals that have respective
phase angles of 0, 120 and 240 degrees, while inverter 16b produces
AC power signals that have respective phase angles of 40, 160 and
280 degrees, and inverter 16c produces AC power signals that have
respective phase angles of 80, 200 and 320 degrees. The modifier
"interleaved" as used herein is referring to the fact that the
inverters 16a-16c are switched in an alternating sequence to
produce interleaved AC power signals in the following sequence:
1A.fwdarw.2A.fwdarw.3A.fwdarw.1B.fwdarw.2B.fwdarw.3B.fwdarw.1C.fwdarw.2C.-
fwdarw.3C (where the numbers 1, 2, and 3 respectively designate the
three inverters 16a-16c and the letters A, B, and C respectively
designate respective phases of the designated inverter).
[0036] As seen in FIG. 1B, the 3.times.3-phase AC motor 20 receives
AC power from the inverters 16a-16c via AC power lines 6. The
3.times.3-phase AC motor 20 includes a rotor 30 mounted to a shaft
32 and a stator 36 separated from the rotor 30 by an air gap 28.
The stator has an outer diameter 24 and an inner diameter 26. The
stator 36 has a multiplicity of windings 22. More specifically, the
windings 22 include three groups of windings 22 (hereinafter
"winding groups 21A-21C"), which winding groups are arranged at
respective angular positions on the stator 36. The rotor 30 has a
multiplicity of windings (not shown in the drawings), or a
permanent magnet array, which windings or permanent magnet array
interact with the magnetic field produced by the stator windings to
generate the forces that turn the shaft 32. The 3.times.3-phase AC
motor 20 further includes a speed and position sensor 34 which
detects the speed of rotation and position of the rotor 30 and
sends speed and position signals 40 to inverter controllers
12a-12c.
[0037] More specifically, each of winding groups 21A-21C include
windings which are respectively referred to herein as "first,
second, and third windings". Thus, each of the winding groups
21A-21C includes first, second, and third windings which receive
respective AC power signals having different phases from respective
inverters 16a-16c. For the avoidance of indefiniteness, the first,
second, and third windings of winding group 21A receive AC power
signals having phases which are different than the phases of the AC
power signals received by the first, second, and third windings of
winding group 21B. Likewise, the first, second, and third windings
of winding group 21B receive AC power signals having phases which
are different than the phases of the AC power signals received by
the first, second, and third windings of winding group 21C; and the
first, second, and third windings of winding group 21A receive AC
power signals having phases which are different than the phases of
the AC power signals received by the first, second, and third
windings of winding group 21C.
[0038] As seen in FIGS. 1A and 1B, the inverter 16a outputs 3-phase
AC power signals 1A, 1B, and 1C which are respectively supplied to
respective windings 22 of winding group 21A via respective AC power
lines of a first subset of AC power lines 6; the inverter 16b
outputs 3-phase AC power signals 2A, 2B, and 2C which are
respectively supplied to respective windings 22 of winding group
21B via respective AC power lines of a second subset of AC power
lines 6; and the inverter 16c outputs 3-phase AC power signals 3A,
3B, and 3C which are respectively supplied to respective windings
22 of winding group 21C via respective AC power lines of a third
subset of AC power lines 6. In other words, three phases A, B, and
C of the first inverter 16a are connected in sequence to respective
first windings of the first, second, and third winding groups
respectively; three phases A, B, and C of the second inverter 16a
are connected in sequence to respective second windings of the
first, second, and third winding groups respectively; and three
phases A, B, and C of the third inverter 16c are connected in
sequence to respective third windings of the first, second, and
third winding groups respectively.
[0039] Referring again to FIG. 1A, the DC-to-AC converter 50
further includes a multiplicity of pairs of sensors 5 which measure
the voltages and currents of the 3-phase AC power signals 1A-1C,
2A-2C, and 3A-3C respectively output by the inverters 16a-16c. Each
pair of sensors 5 is represented by an open circle on a magnified
scale in FIG. 2. The pair of sensors 5 includes a voltage sensor 5a
for measuring the voltage of an AC power signal (as seen FIG. 2A)
and a current sensor 5b for measuring the voltage of an AC power
signal (as seen FIG. 2B). Typical voltage sensors may include: Hall
effect sensors, resistive or capacitive voltage dividers,
electronic sensors, etc.; typical current sensors include: Hall
effect sensors, transformer type, resistor current sensors,
electronic sensors, etc. The 3-phase voltage and current signals
representing the measured voltage and current of the AC power
signals output by inverter 16a are fed back to inverter controller
12a; the 3-phase voltage and current signals representing the
measured voltage and current of the AC power signals output by
inverter 16b are fed back to inverter controller 12b; and the
3-phase voltage and current signals representing the measured
voltage and current of the AC power signals output by inverter 16c
are fed back to inverter controller 12c. Thus, the feedback signals
for each inverter controller consist of three voltages and three
currents.
[0040] The inverter controllers 12a-12c are configured to control
switching inside the inverters 16a-16c in accordance with an
interleaved switching scheme that reduces current ripple. The
method of current ripple reduction proposed herein is applicable to
permanent magnet motors, induction motors, and synchronous motors.
Therefore, no detail regarding rotor design is shown in the
drawings since such details are not relevant here.
[0041] In addition, FIGS. 1A and 1B show how the stator windings
interact with the control system to implement interleaved
operation. The technology disclosed herein is not intended for use
with any specific stator winding design. Each of the motor windings
may represents n pairs of windings that are appropriately
allocated, orientated, and dispersed on the stator, where
n.gtoreq.1. The motor stator windings can be concentrated or
distributed. As a convention, each pair of 3-phase stator windings
are Y-connected.
[0042] The 3.times.3-phase AC motor 20 shown in FIG. 1B is
effectively a 9-phase AC motor (nine AC phases having the same
amplitude and sequentially shifted by 40 degrees). The first group
of windings 21A receives modulated AC power signals 1A, 1B, and 1C,
consecutively separated by 120 degrees, similar to a typical single
3-phase AC motor. Note that distributed windings may be applied to
the method disclosed herein, but FIG. 1B shows only concentrated
windings for a matter of convenience. The second group of windings
21B receives modulated AC power signals 2A, 2B, and 2C; and the
third group of windings 21C receives modulated AC power signals 3A,
3B, and 3C in a similar fashion. There is no physical phase shift
(difference in angular position) among the windings in any one of
winding groups 21A-21C.
[0043] The inverter topology does not affect the current ripple
reduction method disclosed here. So the detailed inverter topology
is ignored here. The current ripple reduction method requires that
the three inverters 16a-16c possess the same types of devices and
topology.
[0044] Similarly, all three inverter controllers 12a-12c are of the
same type. The detailed controller construction is not important
here. The controller functions as usual to control and regulate the
voltages, currents, motor speed and torque, and other functions.
These functional controls are independent from the interleaved
operation of the motor winding groups or motors, which is also
performed by the controllers.
[0045] The front-end circuits 14a-14c may comprise any one of or a
combination of two or more of the following types of devices: a
step-up or step-down converter, a filter network, a protective
circuit, or a contactor. The essential component nature and circuit
topology of the front-end circuits 14a-14c does not affect the
current ripple reduction method disclosed herein.
[0046] The DC source or DC sources are typically non-ideal. The
voltage and impedance of the DC source is usually subject to change
during a mission cycle. A change of the terminal voltage or
impedance may affect the ripple size for a given load condition,
but such change does not impact the current ripple reduction effect
produced by the technology proposed herein.
[0047] The principle of operation of the DC-to-AC converter 50 will
now be described with reference to FIG. 3, which is a graph showing
current (vertical axis) versus time (horizontal axis) for current
ripples produced at a DC bus by single 3-phase resistive loads and
by three 3-phase resistive loads interleaved with a .pi./9 phase
angle. Under an ideal condition, a single 3-phase resistive load
would draw a current at the DC bus 38 with six current ripple
pulses within a 2.pi. mechanical angle (assume a two-pole machine)
as shown by waveform WF1 in FIG. 3. When three 3-phase loads draw
an equivalent power, and if three groups of loads draw the current
at an interleaved angle of .pi./9, the current at the DC bus 38
would have an 18-pulse ripple uniformly distributed across an angle
of 2.pi., but with much smaller size, as shown by waveform WF2 in
FIG. 3.
[0048] A non-ideal DC voltage waveform can be viewed as a composite
of a constant DC component (offset) with an alternating (AC)
voltage--the ripple voltage--overlaid. It can be shown that the
ripple factor of a 6-pulse operation is 0.04 and the ripple factor
of an 18-pulse operation is 0.005--a reduction by a factor of 10.
The ripple factor RF is defined as
R .times. F = v r .times. m .times. s 2 - v a .times. v .times. g 2
v a .times. v .times. g ##EQU00001##
where .nu..sub.rms is the root mean square voltage and .nu..sub.avg
is the average voltage.
[0049] In practice, neither the source nor the load are ideal. In
the case depicted in FIGS. 1A and 1B, the load is a motor and
inductive, whereas the DC source may be a battery. Furthermore, the
inverter switching may result in significant current ripple due to
switching transients. However, for given source and load condition,
one can expect that the ripple current will be reduced
significantly if a motor of multiple 3-phase windings, as shown in
FIG. 1B, operates in an interleaved fashion as compared to a single
3-phase AC motor operated at the same power level.
[0050] In general, for a motor having n 3-phase windings, the
interleaved angle .theta..sub.int between each consecutive phase
will be
.theta. i .times. n .times. t = .pi. 3 .times. n ##EQU00002##
[0051] This would produce a total of 23n ripple pulses uniformly
distributed across a period of 2.pi..
[0052] The above discussion is for the current ripple in an
interleaved time scale which is in a low frequency range (motor
operating frequency range). The current ripples at the DC bus 38
due to different phase currents can be effectively reduced using
interleaved operation of multiple 3-phase currents.
[0053] Current ripples are also generated due to switching actions,
for example, due to the pulse width modulation process, which
switching is in a much higher frequency range. The high-frequency
current ripples are superimposed on the DC side, which is also
detrimental to DC sources.
[0054] In an interleaved operation of a motor of three 3-phase
windings, all three inverter controllers and inverters are operated
in accordance with the same switching modulation scheme, regardless
of the specific modulation technique used. In this way, any
switching actions that generate a voltage pulse in phase 1A will be
followed by a second voltage pulse in phase 2A but inter-leaved by
.pi./9 degrees, and also followed by a third voltage pulse
interleaved with the first pulse by 2.pi./9 degree.
[0055] The effect of the interleaved switching sequence among three
inverters described above is that some high-frequency harmonics can
be eliminated or reduced. Additionally, the interleaved switching
can be designed for eliminating or reducing some specific
harmonics. FIGS. 4A and 4B are high-frequency spectrum graphs
showing that some harmonics are eliminated (see dashed arrows in
FIG. 4A) or the magnitudes of some harmonics are reduced (see
dashed arrows in FIG. 4B) by using interleaved switching
(interleaved modulation) of parallel 3-phase inverters.
[0056] FIGS. 5A and 5B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter 50
for powering three co-shafted 3-phase AC motors 20a-20c using
interleaved switching of parallel 3-phase inverters 16a-16c in
accordance with the second embodiment. The components of DC-to-AC
converter 50 and their interconnections are the same as those of
converter 2 seen in FIG. 1A.
[0057] As seen in FIG. 5B, the system setup includes three 3-phase
AC motors 20a-20c installed on a common shaft 32*. The 3-phase AC
motors 20a-20c receive AC power from the inverters 16a-16c
respectively via AC power lines 6. Each of the 3-phase AC motors
20a-20c includes a respective rotor 30 mounted to common shaft 32*
and a respective stator 36 separated from the rotor 30 by an air
gap 28. Each stator 36 has respective sets of windings 22a-22c
(only one winding is shown for each set), which sets of windings
are arranged at respective angular positions on the stators 36. The
windings 22a-22c of each motor 20a-20c receive respective AC power
signals having different phases from respective inverters 16a-16c.
Each of the 3-phase AC motors 20a-20c further includes a respective
speed and position sensor 34 which detects the speed of rotation
and position of rotor 30 and sends respective speed and position
signals 40a-40c to inverter controllers 12a-12c respectively.
[0058] As seen in FIGS. 5A and 5B, the inverter 16a outputs 3-phase
AC power signals 1A, 1B, and 1C which are respectively supplied to
respective windings 22a-22c of 3-phase AC motor 20a via respective
AC power lines of a first subset of AC power lines 6; the inverter
16b outputs 3-phase AC power signals 2A, 2B, and 2C which are
respectively supplied to respective windings 22a-22c of 3-phase AC
motor 20b via respective AC power lines of a second subset of AC
power lines 6; and the inverter 16c outputs 3-phase AC power
signals 3A, 3B, and 3C which are respectively supplied to
respective windings 22a-22c of 3-phase AC motor 20c via respective
AC power lines of a third subset of AC power lines 6.
[0059] The principle of interleaved operation for reducing current
ripple, previously described with reference to FIGS. 1A and 1B, is
equally applicable to the co-shafted-motor system shown in FIGS. 5A
and 5B, assuming all three motors 20a-20c are drawing the same
level of power and producing the same torque. The co-shafted-motor
system configuration shown in FIGS. 5A and 5B is common for various
applications such as propulsion systems and actuation systems. The
same benefits (such as the reduced current ripple seen in FIG. 3
and the eliminated or reduced harmonics shown in FIGS. 4A and 4B)
would occur in the co-shafted-motor system depicted in FIGS. 5A and
5B.
[0060] FIGS. 6A and 6B are respective interconnected parts of a
diagram representing a system that includes a DC-to-AC converter 50
for powering three separate 3-phase AC motors 20a-20c using
interleaved switching of parallel 3-phase inverters 16a-16c in
accordance with the third embodiment. The components of DC-to-AC
converter 50 and their interconnections are the same as those of
converter 2 seen in FIGS. 1A and 5A.
[0061] As seen in FIG. 6B, the system setup includes three separate
3-phase AC motors 20a-20c. The 3-phase AC motors 20a-20c receive AC
power from the inverters 16a-16c respectively via AC power lines 6.
Each of the 3-phase AC motors 20a-20c includes a respective rotor
30 mounted to a respective shaft 32 and a respective stator 36
separated from the rotor 30 by an air gap 28. Each stator 36 has
respective sets of windings 22a-22c (only one winding is shown for
each set), which sets of windings are arranged at respective
angular positions on the stators 36. The windings 22a-22c of each
motor 20a-20c receive respective AC power signals having different
phases from respective inverters 16a-16c. Each of the 3-phase AC
motors 20a-20c further includes a respective speed and position
sensor 34 which detects the speed of rotation and position of rotor
30 and sends respective speed and position signals 40a-40c to
inverter controllers 12a-12c respectively.
[0062] As seen in FIGS. 6A and 6B, the inverter 16a outputs 3-phase
AC power signals 1A, 1B, and 1C which are respectively supplied to
respective windings 22a-22c of 3-phase AC motor 20a via respective
AC power lines of a first subset of AC power lines 6; the inverter
16b outputs 3-phase AC power signals 2A, 2B, and 2C which are
respectively supplied to respective windings 22a-22c of 3-phase AC
motor 20b via respective AC power lines of a second subset of AC
power lines 6; and the inverter 16c outputs 3-phase AC power
signals 3A, 3B, and 3C which are respectively supplied to
respective windings 22a-22c of 3-phase AC motor 20c via respective
AC power lines of a third subset of AC power lines 6.
[0063] The principle of interleaved operation for reducing current
ripple, previously described with reference to FIGS. 1A and 1B, is
equally applicable to the three-motor system shown in FIGS. 6A and
6B, assuming all three motors 20a-20c are drawing the same level of
power and producing the same torque. The three-motor system
configuration shown in FIGS. 6A and 6B is useful in a distributed
propulsion system where multiple motors are mounted on an aircraft
wing for the purpose of propulsion and are generally operated at
the same power level. The same benefits (such as the reduced
current ripple seen in FIG. 3 and the eliminated or reduced
harmonics shown in FIGS. 4A and 4B) would occur in the three-motor
system depicted in FIGS. 6A and 6B.
[0064] The motor drawings in FIGS. 1B, 5B, and 6B are intended for
convenience of illustrating an interleaved operation scheme in
conjunction with the inverter systems shown in FIGS. 1A, 5A, and
6A, and should not be construed as a representation of an actual
design. Specifically, each stator winding depicted in the figure
represents a pair or multiple pairs, depending upon a specific
motor design, of electromagnetic poles which are typically
distributed along the peripheral inner surface of the stator. For
example, the phase 1A winding of winding group 21A in FIG. 1B may
represent a pair of electromagnetic poles with north and south
poles separated by 180 degree along the peripheral inner surface of
the stator; similarly, the phase 1B winding of winding group 21B,
which is shifted by 120 degree with respect to winding group 21A,
may represent a pair of electromagnetic poles with its north and
south poles separated by 180 degree along the peripheral inner
surface of the stator; and so forth for the phase 1C winding of
winding group 21C etc.
[0065] The motors 20 and 20a-20c may be of the types used on
aircraft to drive actuators for flight control surfaces, for
landing gear, or for performing any other appropriate functions or
combinations of functions on the aircraft. In other applications,
the motor drive a pump, a vehicle drive train, an actuator for
performing another function, or any other appropriate load or
combination of loads.
[0066] The inverter controllers 12a-12c may be implemented using
hardware or hardware in combination with software. For example,
inverter controllers 12a-12c may be implemented using configurable
hardware, a programmable device, or both. Configurable hardware may
comprise hardware that is configurable to perform one or more
functions of the inverter controller. A programmable device may
comprise any device that is programmable to implement one or more
functions of the inverter controller. For example, without
limitation, the programmable device may comprise a programmable
microcontroller or a digital signal processor. The programmable
device may be configured to run software or firmware in the form of
program instructions to implement one or more functions of the
inverter controller. Program instructions may be stored in any
appropriate non-transitory tangible computer-readable storage
medium for use by, or transfer to, the programmable device.
[0067] While systems and methods for current ripple reduction of a
DC source powering an AC load have been described with reference to
various embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the teachings herein. In addition, many modifications may be
made to adapt the teachings herein to a particular situation
without departing from the scope thereof. Therefore it is intended
that the claims not be limited to the particular embodiments
disclosed herein.
[0068] As used in the claims, the term "controller" should be
construed broadly to encompass a system having at least one
computer or processor, and which may have multiple computers or
processors. As used in the preceding sentence, the terms "computer"
and "processor" both refer to devices having a processing unit
(e.g., a central processing unit) and some form of memory (i.e.,
computer-readable medium) for storing a program which is readable
by the processing unit. For example, the term "controller"
includes, but is not limited to, a small computer on an integrated
circuit containing a processor core, memory and programmable
input/output peripherals.
* * * * *