U.S. patent application number 14/969872 was filed with the patent office on 2017-06-15 for system and method for operating a three-phase load from a single phase source.
The applicant listed for this patent is Eaton Corporation, Zhejiang University. Invention is credited to Huaqiang Li, Zhengyu Lu, Wenxi Yao.
Application Number | 20170170753 14/969872 |
Document ID | / |
Family ID | 58738032 |
Filed Date | 2017-06-15 |
United States Patent
Application |
20170170753 |
Kind Code |
A1 |
Li; Huaqiang ; et
al. |
June 15, 2017 |
SYSTEM AND METHOD FOR OPERATING A THREE-PHASE LOAD FROM A SINGLE
PHASE SOURCE
Abstract
A power converter for operating a three-phase AC electrical
machine from a single phase AC power source includes an ASD having
a rectifier and inverter, and an add-on power conversion module
external to the ASD that is electrically connected to each of the
power source and the ASD. The power conversion module includes one
or more input inductors configured to store and filter single-phase
AC power received from the power source and a pair of switching
devices connected to each of the input inductors at an output end
thereof. A controller operably connected to the power conversion
module selectively controls switching of the pair of switching
devices associated with each of the one or more input inductors so
as to cause a sinusoidal single phase AC power to be output from
the add-on power conversion module for rectification by the
rectifier.
Inventors: |
Li; Huaqiang; (Menomonee
Falls, WI) ; Yao; Wenxi; (Hangzhou, CN) ; Lu;
Zhengyu; (Hangzhou, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Zhejiang University
Eaton Corporation |
Hangzhou
Cleveland |
OH |
CN
US |
|
|
Family ID: |
58738032 |
Appl. No.: |
14/969872 |
Filed: |
December 15, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 5/00 20130101; Y02P
80/10 20151101; H02P 2201/15 20130101; H02P 27/06 20130101; H02P
1/265 20130101 |
International
Class: |
H02P 1/26 20060101
H02P001/26; H02P 27/06 20060101 H02P027/06 |
Claims
1. A power converter for operating a three-phase AC electrical
machine from a single phase AC power source, the power converter
comprising: an adjustable speed drive (ASD) including: a rectifier
configured to convert a single-phase AC power to DC power on a DC
bus, the DC bus having first and second rails to provide a
relatively fixed DC voltage, and a DC bus capacitor across the
first and second rails to smooth voltage ripple; and an inverter
for receiving DC power from the DC bus and converting the DC power
to a three-phase AC power, the inverter comprising a plurality of
switches that are selectively controllable to control current flow
and terminal voltages in the three-phase AC electrical machine; and
an add-on power conversion module external to the ASD and
electrically connected to each of the single phase AC power source
and the ASD, the add-on power conversion module including: one or
more input inductors each comprising an input end and an output
end, with each of the one or more input inductors configured to
store and filter single-phase AC power received from the single
phase AC power source; and a pair of switching devices connected to
each of the one or more input inductors at the output end thereof;
and a controller operably connected to the add-on power conversion
module to selectively control switching of the pair of switching
devices associated with each of the one or more input inductors so
as to cause a sinusoidal single phase AC power to be output from
the add-on power conversion module for rectification by the
rectifier.
2. The power converter of claim 1 wherein the rectifier comprises a
three phase rectifier bridge that includes three bridge legs each
having an upper diode and a lower diode positioned thereon.
3. The power converter of claim 2 wherein the ASD further comprises
three input terminals for providing AC power to the rectifier
bridge, wherein the input terminals are tied together and connected
to midpoints of one or more of the three bridge legs.
4. The power converter of claim 1 wherein the upper diodes of the
three phase rectifier bridge and the lower diodes of the three
phase rectifier bridge are paralleled so as to share AC power input
received by the rectifier, with the paralleling of the upper diodes
and the lower diodes equally dividing a full load current of the AC
power input to the rectifier.
5. The power converter of claim 1 wherein the one or more input
inductors comprises a single input inductor, and wherein a single
pair of switching devices is connected to the single input inductor
at the output end thereof.
6. The power converter of claim 1 wherein the one or more input
inductors comprises three input inductors, and wherein a pair of
switching devices is connected to each of the three input inductors
at the output end thereof, such that six switching devices are
included in the add-on power conversion module.
7. The power converter of claim 6 wherein the controller is
programmed to switch the six switching devices in an interleaved
fashion to increase the effective switching frequency of the add-on
power conversion module.
8. The power converter of claim 1 wherein the controller is
programmed to selectively control switching of the pair of
switching devices associated with each of the one or more input
inductors according to a pulse width modulation (PWM) scheme to
cause the sinusoidal single phase AC power to be output from the
add-on power conversion module.
9. The power converter of claim 1 wherein the ASD further comprises
an outer housing configured to house the rectifier and the inverter
therein, and wherein the add-on power conversion module positioned
external to the outer housing of the ASD.
10. The power converter of claim 1 wherein the ASD is free of DC
chokes on the DC bus, with functions of the DC chokes on the DC bus
being performed by the one or more input inductors.
11. The power converter of claim 1 wherein a total harmonic
distortion of the sinusoidal single phase AC power output from the
add-on power conversion module is less than 5%.
12. A method for operating a three-phase AC electrical machine from
a single phase power source, the method comprising: providing a
single phase AC power to an add-on power conversion module, the
add-on power conversion module comprising: one or more input
inductors each comprising an input end and an output end; and a
pair of switching devices connected to each of the one or more
input inductors at the output end thereof; and selectively
switching each of the pair of switching devices associated with
each of the one or more input inductors via an operably connected
controller so as to cause a sinusoidal single phase AC power to be
output from the add-on power conversion module; providing the
sinusoidal single phase AC power to an adjustable speed drive (ASD)
separate from and operably connected to the add-on power conversion
module; and generating a three-phase AC power via the ASD for
operating the three-phase AC electrical machine, with the ASD
rectifying the sinusoidal single phase AC power to generate a DC
power and inverting the DC power to generate the three-phase AC
power.
13. The method of claim 12 wherein providing the sinusoidal single
phase AC power to the ASD comprises providing the sinusoidal single
phase AC power in parallel to upper and lower diodes on each of
three bridge legs of a rectifier bridge of the ASD, with a single
tied input of the ASD providing the sinusoidal single phase AC
power in parallel to the upper and lower diodes.
14. The method of claim 13 wherein the input of the ASD is
connected to a midpoint of each of the three bridge legs of the
rectifier bridge to provide the sinusoidal single phase AC power in
parallel to the upper or lower diodes on the three bridge legs, or
the input of the ASD is connected to a midpoint of two of the three
bridge legs of the rectifier bridge to provide the sinusoidal
single phase AC power in parallel to two of the upper or lower
diodes on the three bridge legs.
15. The method of claim 13 wherein the input of the ASD is
connected to a midpoint of each of the three bridge legs of the
rectifier bridge to provide the sinusoidal single phase AC power in
parallel to the upper or lower diodes on the three bridge legs, or
the input of the ASD is connected to a midpoint of one of the three
bridge legs of the rectifier bridge to provide the sinusoidal
single phase AC power to the same leg of the upper or lower diode
on the three bridge legs, while the midpoints of the other two legs
are disconnected.
16. The method of claim 12 wherein the one or more input inductors
comprises a single input inductor, and wherein a single pair of
switching devices is connected to the single input inductor at the
output end
17. The method of claim 12 wherein the one or more input inductors
comprises three input inductors, and wherein a pair of switching
devices is connected to each of the three input inductors at the
output end thereof, such that six switching devices are included in
the add-on power conversion module; and wherein selectively
switching each of the pair of switching devices via the operably
connected controller comprises switching the six switching devices
in an interleaved fashion to increase the effective switching
frequency of the add-on power conversion module.
18. The method of claim 12 wherein, in, generating a three-phase AC
power via the ASD for operating the three-phase AC electrical
machine, the ASD is de-rated between 0% to less than 50%
19. A power converter for operating a three-phase AC electrical
machine from a single phase AC power source, the power converter
comprising: an adjustable speed drive (ASD) having an input
connectable to an AC power source and a three phase output
connectable to an input terminal of a load, the ASD comprising a
housing enclosing a rectifier and inverter operable to provide a
three-phase AC output to the load; and an add-on power conversion
module positioned external to the housing of the ASD and being
electrically connected to each of the single phase AC power source
and the ASD, the add-on power conversion module including: one or
more input inductors positioned at an input of the add-on power
conversion module connected to the single phase AC power source;
and a pair of switching devices connected to each of the one or
more input inductors at an output end thereof; and a controller
operably connected to the add-on power conversion module to
selectively control switching of the pair of switching devices
associated with each of the one or more input inductors; wherein
the controller is programmed to control switching of the pair of
switching devices so as to cause a sinusoidal single phase AC power
to be output from the add-on power conversion module, the
sinusoidal single phase AC power output having a total harmonic
distortion of less than 5%.
20. The power converter of claim 19 wherein the rectifier of the
ASD comprises a three phase rectifier bridge that includes three
bridge legs each having an upper diode and a lower diode positioned
thereon; and wherein the upper diodes of the three phase rectifier
bridge and the lower diodes of the three phase rectifier bridge are
paralleled so as to share AC power input received by the rectifier
through a tied input terminal, with the paralleling of the upper
diodes and the lower diodes equally dividing a full load current of
AC power input to the rectifier.
21. The power converter of claim 19 wherein the one or more input
inductors and the pair of switching devices connected to each of
the one or more input inductors comprises: a single input inductor
and a single pair of switching devices connected to the single
input inductor at the output end thereof; or three input inductors
and a pair of switching devices connected to each of the three
input inductors at the output end thereof, such that a pair of
switching devices is provided on each of three bridge legs in the
power conversion module.
Description
BACKGROUND OF THE INVENTION
[0001] Embodiments of the invention relate generally to motor drive
systems for operating a three-phase electric motor and, more
particularly, to an apparatus and method for operating a
three-phase motor from a single phase power source.
[0002] One type of system commonly used in industry that performs
power conversion is an adjustable or variable speed drive (ASD)
circuit, which is an industrial control device that provides for
variable frequency, variable voltage operation of a driven system,
such as a three-phase AC induction motor. ASDs have an AC to DC
rectifier unit with a large DC capacitor to smooth the voltage
ripple. In all ASDs, the DC bus voltage is inverted to three-phase
variable voltage, variable frequency output to control the speed
and torque of three-phase AC motors. For many loads, it is
customary and often required to power up the AC to DC rectifier
section from a three-phase AC source. However, the input AC to DC
rectifier can be powered up from a single-phase AC source,
especially in locations where three-phase AC power is unavailable
due to logistics and other reasons. In such cases, some utilities
allow three-phase ASDs to be powered from a single-phase AC source
provided the peak current flowing into the AC to DC rectifier
system is within the rating of the single-phase AC source. Many ASD
manufacturers impose restrictions on the rating of the ASD when
they are subject to a single phase AC source.
[0003] There exist important concerns while operating a three-phase
ASD from a single-phase AC source--with such concerns tied to high
peak current and zero current conditions associated with a single
phase AC power, as can be seen in the single phase AC waveform
shown in FIG. 1 by high peak current 10 and zero current 12. With
respect to the input AC current provided from the single phase AC
source to the ASD, it is recognized that the input diodes of the
ASD have to handle the higher demand current for a given load (with
the RMS value of the input AC current being significantly higher
than that when a three-phase supply is used for a given load), and
thus de-rating of the inverter is often undertaken to address this
concern--with the de-rating being--50% in some cases. The higher
input current affects the input AC power terminal blocks and, while
in many cases the rectifier diodes of the ASD may be able to handle
the higher values of RMS current, the terminal blocks and the
rectifier diodes of the ASD may not be rated to handle the peak
current on a continuous basis. An additional concern is that input
harmonic distortion is high when single-phase input is used as an
AC source for a three-phase inverter. Poor harmonics are associated
with lower input power factor, which affects the efficiency of
power conversion. Single-phase AC supply results in higher ripple
voltage across the DC bus. Higher ripple voltage translates to
higher ripple current through the DC bus capacitors and more
heating of these capacitors. The inverter is typically de-rated to
handle the higher ripple current. The current drawn from the
single-phase AC source feeding a three-phase ASD is discontinuous.
When the pulsed current flows from the AC source, it creates
voltage drop that mimics the pulsed current waveform to some
extent. The resulting voltage drop can affect other loads connected
to the same AC source.
[0004] There are many known techniques that are employed to improve
the current waveform and reduce the overall current
harmonics--including both passive and active techniques. One known
passive approach creates a resonant circuit across the DC bus, with
energy being stored in the resonant components and released
naturally at the appropriate time to support the sagging DC bus
voltage and thereby reduce the ripple across the bulk capacitors of
the DC bus. However, while the resonant circuit provides benefits
of extending the diode conduction period during the charging cycle
to reduce the input harmonics and improve the input power factor
and reducing the DC bus capacitor ripple, it is recognized that the
passive resonant circuit has disadvantages as well. These
disadvantages include that the resonant components are bulky and
expensive, the peak diode current is reduced but the improvement is
not conspicuous, and the average DC bus voltage is still low and
the ASDs need to be de-rated, though the level of de-rating is
smaller than that without the DC bus resonant circuit.
[0005] One active solution commonly used in single-phase AC to DC
power supplies uses a boost converter that boosts the input voltage
to a desired DC bus voltage level under all load conditions. Use of
the boost converter beneficially reduces the overall DC bus voltage
ripple and makes the input current continuous, which reduces the
input current harmonic distortion (thereby eliminating the peak
current stress in the input diodes) and resulting in lower thermal
loss in the AC system. However, it is recognized that the boost
converter (and operation thereof) has disadvantages associated
therewith of being expensive due to the boost switch having to be
rated to carry peak of input current and to handle the boosted
voltage (i.e., a large stress across the switch of the boost
converter) and of requiring an input EMI filter to limit switching
noise observed in the input AC voltage waveform from propagating
into the AC source.
[0006] One particular active solution used to improve the current
waveform and reduce the overall current harmonics is disclosed in
U.S. application Ser. No. 14/672,967. FIG. 2 illustrates a single
phase front end circuit 14 for use with a ASD 16, where the front
end circuit 14 includes a bidirectional switch 18 positioned
between an input to the rectifier 20 and a midpoint 22 (formed by a
pair of capacitors) positioned across the rails of the DC bus 24,
so as to form a single-phase partial boost converter. The
bidirectional switch 18 is controlled to inject current into the
midpoint of the DC bus 24 even during the time when the input AC
voltage is lower than the DC bus voltage, with such current being
typically only about 50% of the rated current of the AC to DC
rectifier. The current flow is limited by an external inductor 26,
which behaves like a boost inductor boosting the main DC bus
voltage when the switch is forced to turn off at sometime near to
the peak of input AC voltage.
[0007] However, while the front end circuit 14 operates to
successfully reduce the ripple across the DC bus capacitor 28 and
reduce the peak current flowing through the diodes of rectifier 20,
the front end circuit 14 does not provide an optimum solution for
conditioning the single phase current waveform and reducing the
overall current harmonics, including such concerns tied to a high
peak current and zero current conditions. As seen in FIG. 3, which
illustrates a current waveform 30 output from the front end
rectifier circuit 14 resulting from operation thereof (i.e., from
operation of bidirectional switch 18), the current provided by the
front end rectifier circuit 14 is non-sinusoidal--such that a large
amount of total harmonic distortion is still present in the
waveform. Additionally, periods/intervals are present where zero
current (indicated at 32) is output from the front end circuit 14,
which is problematic for operation ASD 16 in an optimum
fashion.
[0008] It would therefore be desirable to have a system and method
for operating a three-phase motor from a single phase power source
that overcomes the aforementioned drawbacks. It would also be
desirable for such a system and method to utilize a standard ASD
with an add-on kit, to provide a non-intrusive solution that
minimizes the cost, weight and size of the system in single phase
applications, minimizing the need of large derating. It would still
further be desirable for such a system and method to provide an
input current harmonics reduction that makes it possible to meet
industry regulatory power quality mandatory requirements.
BRIEF DESCRIPTION OF THE INVENTION
[0009] Embodiments of the invention are directed to a power
converter and method of operating thereof for providing power to a
three-phase motor from a single phase power source.
[0010] According to an aspect of the invention, a power converter
for operating a three-phase AC electrical machine from a single
phase AC power source comprises an adjustable speed drive (ASD)
that includes a rectifier configured to convert a single-phase AC
power to DC power on a DC bus, the DC bus having first and second
rails to provide a relatively fixed DC voltage, and a DC bus
capacitor across the first and second rails to smooth voltage
ripple. The ASD also includes an inverter for receiving DC power
from the DC bus and converting the DC power to a three-phase AC
power, the inverter comprising a plurality of switches that are
selectively controllable to control current flow and terminal
voltages in the three-phase AC electrical machine. The power
converter also includes an add-on power conversion module external
to the ASD and electrically connected to each of the single phase
AC power source and the ASD, with the add-on power conversion
module having one or more input inductors configured to store and
filter single-phase AC power received from the single phase AC
power source and a pair of switching devices connected to each of
the one or more input inductors at an output end thereof. The
add-on power conversion module also includes a controller operably
connected to the add-on power conversion module to selectively
control switching of the pair of switching devices associated with
each of the one or more input inductors so as to cause a sinusoidal
single phase AC power to be output from the add-on power conversion
module for rectification by the rectifier.
[0011] According to another aspect of the invention, a method for
operating a three-phase AC electrical machine from a single phase
power source includes providing a single phase AC power to an
add-on power conversion module that includes one or more input
inductors each comprising an input end and an output end and a pair
of switching devices connected to each of the one or more input
inductors at the output end thereof. The method also includes
selectively switching each of the pair of switching devices
associated with each of the one or more input inductors via an
operably connected controller so as to cause a sinusoidal single
phase AC power to be output from the add-on power conversion
module, providing the sinusoidal single phase AC power to an
adjustable speed drive (ASD) separate from and operably connected
to the add-on power conversion module, and generating a three-phase
AC power via the ASD for operating the three-phase AC electrical
machine, with the ASD rectifying the sinusoidal single phase AC
power to generate a DC power and inverting the DC power to generate
the three-phase AC power.
[0012] According to yet another aspect of the invention, a power
converter for operating a three-phase AC electrical machine from a
single phase AC power source includes an adjustable speed drive
(ASD) having an input connectable to an AC power source and a three
phase output connectable to an input terminal of a load, the ASD
comprising a housing enclosing a rectifier and inverter operable to
provide a three-phase AC output to the load. The power converter
also includes an add-on power conversion module positioned external
to the housing of the ASD and being electrically connected to each
of the single phase AC power source and the ASD, with the add-on
power conversion module having one or more input inductors
positioned at an input of the add-on power conversion module
connected to the single phase AC power source and a pair of
switching devices connected to each of the one or more input
inductors at an output end thereof. The power converter further
includes a controller operably connected to the add-on power
conversion module to selectively control switching of the pair of
switching devices associated with each of the one or more input
inductors, wherein the controller is programmed to control
switching of the pair of switching devices so as to cause a
sinusoidal single phase AC power to be output from the add-on power
conversion module, the sinusoidal single phase AC power output
having a total harmonic distortion of less than 5%.
[0013] Various other features and advantages of the present
invention will be made apparent from the following detailed
description and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The drawings illustrate preferred embodiments presently
contemplated for carrying out the invention.
[0015] In the drawings:
[0016] FIG. 1 illustrates a current waveform of a conventional
single phase AC power source of an ASD.
[0017] FIG. 2 is a schematic diagram of an ASD including a single
phase active front end circuit, as known in the prior art.
[0018] FIG. 3 illustrates a current waveform output from the single
phase active front end circuit of FIG. 2.
[0019] FIG. 4 is a schematic diagram of a power converter including
an ASD and an add-on power conversion module, according to an
embodiment of the invention.
[0020] FIG. 5 illustrates a current waveform output from the add-on
power conversion module of FIG. 4.
[0021] FIGS. 6A-6D illustrate current flows through the power
converter of FIG. 4 corresponding to different switching patterns
of switches in the add-on power conversion module.
[0022] FIG. 7 is a schematic diagram of a power converter including
an ASD and an add-on power conversion module, according to an
embodiment of the invention.
DETAILED DESCRIPTION
[0023] Embodiments of the invention are directed to a power
converter and method of operating thereof for providing power to a
three-phase motor from a single phase power source. The power
converter includes an adjustable speed drive (ASD) and an add-on
module connected to the ASD, with the add-on module configured to
condition a single phase AC input received thereby for providing to
the ASD, so as to reduce a total harmonic distortion of an input
current and minimize the need to derate the ASD.
[0024] Referring to FIG. 4, a power converter 40 is illustrated
according to an embodiment of the invention. The power converter 40
is composed generally of an ASD 42 and an add-on power conversion
module 44 that operates in conjunction with the ASD 42 to receive a
single phase AC power input from a single phase AC power source 46,
condition the single phase AC power input, rectify the conditioned
AC input, and perform a DC/AC conversion of the rectified segment
into a three-phase alternating voltage of variable frequency and
amplitude that is supplied to a load 48.
[0025] As shown in FIG. 4, the ASD 42 is constructed as a
conventional motor drive having a standard construction, such that
the ASD 42 includes a three-phase rectifier bridge 50 and a
inverter circuit 52 for performing rectification and inversion of a
received AC input. The three-phase rectifier bridge 50 includes
three bridge legs 54--each of which includes an upper diode 56 and
a lower diode 58 thereon. An AC input may be fed to the three-phase
rectifier bridge 50, with the rectifier bridge 50 converting the AC
power input to a DC power such that a DC voltage is present on DC
link 60 between the rectifier bridge 14 and the inverter 52. The
link voltage is smoothed by a DC link capacitor bank 62. The
inverter 52 may be comprised of a series of insulated gate bipolar
transistor switches (IGBTs) and anti-parallel diodes (not shown),
such as an arrangement of six IGBTs and diodes, that collectively
form inverter 52--with the inverter 52 being controlled via a pulse
width modulation (PWM) technique, for example, to synthesize AC
voltage waveforms with a fixed frequency and amplitude for delivery
to the load 48, which is in the form of a three-phase electrical
machine such as an induction motor, for example. While the inverter
52 is described above as including IGBTs, it is recognized that
other embodiments of the invention contemplate other power
switching devices as known in the art, such as MOSFETs, for
example.
[0026] Control of the ASD 42 and operation of the inverter 52 is
via a controller 64, which may further be comprised of a plurality
of controllers that perform high speed operations such as
volts-per-hertz or vector control algorithms, space-vector
modulation, DC link voltage decoupling, and protection, for
example. The controller system 64 interfaces to the inverter 52 via
gate drive signals and sensing of the DC link voltage and pole
currents (by way a voltage sensor and current sensors, for example)
such that changes in DC link voltage can be sensed. These voltage
changes can be interpreted as transient load conditions and are
used to control switching of the switches of inverter 52 such that
near steady-state load conditions are maintained.
[0027] The add-on power conversion module 44 is positioned external
to the ASD 42 (i.e., outside of a housing 66 enclosing ASD 42) and
is electrically connected thereto. Accordingly, the add-on power
conversion module 44 may be implemented with an existing ASD 42 and
electrically tied thereto to "retrofit" an existing ASD 42.
According to the embodiment of FIG. 4, the add-on power conversion
module 44 is composed of an input inductor 68 and a pair of
switching devices 70a, 70b that operate to condition the single
phase AC power provided from single phase source. The input
inductor 68 is positioned on an input terminal 72 of the power
conversion module 44 to receive a single phase AC power from source
46, with the input inductor 68 providing energy storage and
filtering during operation of power converter 40 and load 48. While
power conversion module 44 is shown in FIG. 4 as including only a
single input inductor 68 positioned at input terminal 72, it is
recognized that, in another embodiment, the input inductor 68 could
be split onto two input terminals of the power conversion module
44, so as to provide common mode rejection capability.
[0028] In an exemplary embodiment, each of the pair of switching
devices 70a, 70b included in power conversion module 44 are
provided as an IGBT 73 and anti-parallel diode 75. While the
switching devices 70a, 70b are shown as including IGBTs, it is
recognized that other embodiments of the invention contemplate
other power switching devices as known in the art, such as MOSFETs,
for example. The switching devices 70a, 70b are arranged to provide
a half-bridge circuit topology, with the switches being controlled
according to a PWM scheme to condition the AC power received from
the power source 46 to an AC waveform. Switching devices 70a, 70b
are operated in an alternating manner in which one switch is an on
state while the other switch is in the off state. Controlling which
switching device 70a, 70b is in the on state via the PWM scheme
causes the AC waveform output therefrom to be generated according
to a desired frequency. Control of the switching devices 70a, 70b
may be provided via a dedicated controller 74 associated with power
conversion module 44. Alternatively, control of the switching
devices 70a, 70b may be provided via controller 64, such that
controller 64 controls operation of both the ASD 42 and the power
conversion module 44.
[0029] While ASD 42 is described above as comprising a conventional
motor drive having a standard construction, it is seen in FIG. 4
that the ASD 42 is modified in several respects. First, it is seen
in FIG. 4 that DC link chokes that are typically positioned on the
positive and negative rails of the DC link 60 (for providing energy
storage and filtering on the DC link) are not present--i.e., they
may be removed. That is, the inclusion of the input inductor 68 in
the power conversion module 44 allows for removal of such DC link
chokes, as functions of the DC link chokes are performed by the
input inductor 68. Additionally, it is seen in FIG. 4 that the AC
inputs 76 to the three-phase rectifier bridge 50--which are
typically provided via a plurality of separate input terminals to
the legs 54 of the rectifier bridge 50--are tied together in the
ASD 42 and are connected to respective midpoints 77 of the legs 54,
such that the upper and lower diodes 56, 58 of the three-phase
rectifier bridge 50 share the full load current in the ASD 42
during operation, as will be explained in further detail below.
[0030] In operation of power converter 40, power is provided from
the single phase AC source 46 to the add-on power conversion module
44. Upon receiving the single phase AC power, the state of
switching devices 70a, 70b is controlled by controller 74 to
selectively provide for a flow of power therethrough, with
switching of the switching devices 70a, 70b being performed with a
selected variable duty cycle to provide for conditioning of the AC
power. Power is selectively stored and filtered in/by the input
inductor 68 during operation of the switching devices 70a, 70b so
as to provide a conditioned AC power output from the power
conversion module 44 having a desired waveform that induces less
stress across the diodes 56, 58 of the rectifier 50 and reduces the
ripple current into the DC link capacitor bank 62. As shown in FIG.
5, based on the controlling of switching devices 70a, 70b, a
sinusoidal AC power 78 is output from the power conversion module
44 having a reduced peak current. The purely sinusoidal AC current
waveform 78 output from the power conversion module 44 has a lower
peak current value than the single phase AC power provided to the
power conversion module 44 from the AC source 46, with the presence
of any intermittent zero current condition being removed--in
contrast to existing front end circuits and associated drives that
operate a three-phase motor from a single phase power source.
[0031] Referring now to FIGS. 6A-6D, operation of the power
conversion module 44 in four different operating modes and the
current flows through the power converter 40 resulting therefrom
are illustrated in greater detail. Referring first to FIG. 6A,
operation of the power conversion module 44 in a first mode is
illustrated--in which a positive current flow is output from single
phase AC power source 46 and a charging of the input inductor 68 is
performed. In the first mode of operation, IGBT 73 of switching
device 70a is in a conducting state such that current is allowed to
flow therethrough in a modulated fashion. Specifically, current
flows in a first/positive direction from power source 46, through
input inductor and through IGBT 73 of switching device 70a, so as
to be output from power conversion module 44 to ASD 42. The
sinusoidal single phase AC power output from the power conversion
module 44 (through IGBT 73 of switching device 70a) is provided to
the ASD 42 and is fed to the three-phase rectifier bridge 50. In
receiving the single phase AC power at the three-phase rectifier
bridge 50, the connection of the tied inputs 76 to the midpoints 77
of the rectifier bridge legs 54 causes current to flow through the
three lower diodes 58 in parallel--with the paralleling of the
three lower diodes 58 serving to equally divide the full load
current of AC power input to the three-phase rectifier bridge 50,
so as to reduce the current flow through each diode and thereby
drastically reduce or eliminate the need to de-rate the ASD 42
(i.e., de-rate from 0% to less than 50%). As can be seen in FIG.
6A, the flow of current continues out through tied inputs 76 and
back into power source 46, with current flow along the illustrated
flow path continuing in the first mode until a build-up/charging of
energy in the input inductor 68 has reached a desired level.
[0032] Upon a build-up of energy in input inductor 68 reaching a
desired level, the power conversion module 44 then changes to a
second operating mode, as illustrated in FIG. 6B. In the second
operating mode, with a positive current flow being output from
single phase AC power source 46, a releasing of the energy stored
in the input inductor 68 is performed. In the second mode of
operation, IGBT 73 of switching device 70a is in a non-conducting
state such that current is caused to flow through the diode 75 of
switching device 70b. Accordingly, current flows from power source
46, through input inductor and through the diode 75 of switching
device 70b, so as to be output from power conversion module 44 to
ASD 42. The power output from the power conversion module 44 during
the second mode of operation is provided to the ASD 42 and is fed
to the DC link 60 so as to provide power to the inverter 52 for
inversion/conditioning and output to the load 48. As further shown
in FIG. 6B, power output from the power conversion module 44 during
the second mode of operation is also routed through the three lower
diodes 58 of the rectifier bridge 50 in parallel (as previously
described with respect to FIG. 6A) and out through tied inputs 76
back to power source 46.
[0033] Referring now to FIG. 6C, operation of the power conversion
module 44 in a third mode is illustrated--in which a negative
current flow is output from single phase AC power source 46 and a
charging of the input inductor 68 is performed. In the third mode
of operation, IGBT 73 of switching device 70b is in a conducting
state such that current is allowed to flow therethrough in a
modulated fashion. Specifically, current flows in a second/negative
direction from power source 46, through upper diodes 56 of
rectifier bridge 50, through IGBT 73 of switching device 70b, and
to input inductor 68 to provide for a build-up or charging of
energy therein. As shown in FIG. 6B, negative current output from
power source 46 is fed to the tied inputs 76 and provided to the
three-phase rectifier bridge 50. In receiving the negative current
at the three-phase rectifier bridge 50, the connection of the tied
inputs 76 to the midpoints 77 of the rectifier bridge legs 54
causes current to flow through the three upper diodes 56 in
parallel--with the paralleling of the three upper diodes 56 serving
to equally divide the full load current of AC power input to the
three-phase rectifier bridge 50, so as to reduce the current flow
through each diode. Upon flowing through the three upper diodes 56,
the flow of current is provided to the power conversion module 44
such that it is caused to flow through IGBT 73 of switching device
70b in a modulated fashion and then to/through the input inductor
68. As can be seen in FIG. 6C, the flow of current continues in the
second direction back into power source 46, with current flow along
the illustrated flow path continuing in the third mode until a
build-up/charging of power in the input inductor 68 has reached a
desired level.
[0034] Upon a build-up of energy in input inductor 68 reaching a
desired level, the power conversion module 44 then changes to a
fourth operating mode, as illustrated in FIG. 6D. In the fourth
operating mode, with a negative current flow being output from
single phase AC power source 46, a releasing of the energy stored
in the input inductor 68 is performed. In the fourth mode of
operation, IGBT 73 of switching device 70b is in a non-conducting
state such that current is caused to flow through the diode 75 of
switching device 70a. Accordingly, power output from the power
conversion module 44 during the fourth mode of operation (i.e.,
released from input inductor 68) flows through upper diodes 56 of
rectifier bridge 50 and is fed to the DC link 60 so as to provide
power to the inverter 52 for inversion/conditioning and output to
the load 48. As further shown in FIG. 6D, current that flows
through three upper diodes 56 (and to inverter 52) is also caused
to flow through the diode 75 of switching device 70a and out from
power conversion module 44 back to power source 46.
[0035] While the power converter 40 illustrated in FIG. 4 and FIGS.
6A-6D is shown as having the inputs 76 to the ASD 42 being tied to
the midpoint 77 on each of the three bridge legs 54 of rectifier
50, it is recognized that not all inputs 76 need be tied to the
midpoints 77. That is, according to additional embodiments of the
invention, the inputs 76 may only be tied to one or two of the
bridge leg midpoints 77. In such embodiments, current would only
flow through the diodes 56, 58 on the bridge legs 54 to which the
inputs 76 are tied to the midpoints 77--such that current could
flow through two diodes in parallel or only through a single
diode.
[0036] Addition of the add-on power conversion module 44 to an
existing ASD 42 thus provides for input of a pure sinusoidal AC
power output 78 to be supplied to the ASD 42 that has a lower peak
current value than a current provided directly from a single phase
AC power source. The sinusoidal current waveform 78 output from the
add-on power conversion module 44 has a reduced total harmonic
distortion as compared to the current waveform output directly from
the single phase AC power source 46--with total harmonic distortion
of the sinusoidal current waveform 78 being less than 5%.
Additionally, the tying of inputs 76 to the midpoints of the
rectifier legs 54 and the resulting paralleling of the three upper
diodes 56 and the three lower diodes 58 provides for the equal
division of the full load current of AC power input to the
three-phase rectifier bridge 50, so as to reduce the current flow
through each diode and thereby reduce/eliminate the need to de-rate
the ASD 42.
[0037] Referring now to FIG. 7, a power converter 80 is illustrated
according to another embodiment of the invention. The power
converter 80 is composed generally of an ASD 42 and an add-on power
conversion module 82 that operates in conjunction with the ASD 42
to receive a single phase AC power input, condition the single
phase AC power input, rectify the conditioned AC input, and perform
a DC/AC conversion of the rectified segment into a three-phase
alternating voltage of variable frequency and amplitude that is
supplied to a load 48. The construction of ASD 42 is identical to
the ASD 42 included in the power converter 40 of FIG. 4, and thus
ASD 42 and the components thereof in the power converter 80 of FIG.
7 are numbered likewise to the ASD 42 and the components thereof in
the power converter of FIG. 4.
[0038] As shown in FIG. 7, the add-on power conversion module 82 of
power converter 80 is positioned external to the ASD 42 (i.e.,
outside of housing 66 enclosing ASD 42) and is electrically
connected thereto. Accordingly, the add-on power conversion module
82 may be implemented with an existing ASD 42 and electrically tied
thereto to "retrofit" an existing ASD 42. According to the
embodiment of FIG. 7, the add-on power conversion module 82 is
composed of three single phase AC input inductors 84 and six
switching devices 86a, 86b, 86c, 86d, 86e, 86f (each including an
IGBT 73 (or MOSFET) and anti-parallel diode 75) that operate to
condition the single phase AC power provided from single phase
source 46. The input inductors 84 are positioned/arranged on an
input terminal 72 of the power conversion module 82 to receive a
single phase AC power from source, with the input inductors 84
providing energy storage and filtering during operation of power
converter 80 and load 48. While power conversion module 82 is shown
in FIG. 7 as including only the three input inductors 84 positioned
at input terminal 72, it is recognized that, in another embodiment,
the input inductors could be split onto two input terminals, so as
to provide common mode rejection capability.
[0039] As shown in FIG. 7, the switching devices 86a, 86b, 86c,
86d, 86e, 86f are arranged such that a pair or group of switching
devices is provided on each of three bridge legs 88, 90, 92--with
switching devices 86a, 86b on leg 88, switching devices 86c, 86d on
leg 90, and switching devices 86e, 86f on leg 92. The pair/group of
switching devices on each leg--i.e., switching devices 86a, 86b on
leg 88, switching devices 86c, 86d on leg 90, and switching devices
86e, 86f on leg 92--may be switched in a certain time sequence from
interleaving (i.e., interleaved switching) to provide an output
current with a desired waveform, with the switching being performed
according to a PWM scheme to condition the AC power received from
the single phase power source 46 to a desired sinusoidal AC
waveform on the output. Beneficially, the interleaved switching of
the switching devices 86a, 86b, 86c, 86d, 86e, 86f increases the
effective switching frequency of the power conversion module 82 (as
compared to the effective switching frequency of the power
conversion module 44 of FIG. 4), as switching of each pair/group of
switching devices 86a and 86b, 86c and 86d, 86e and 86f at 5 kHz
would result in an overall effective switching frequency of 15 kHz
for the power conversion module 82, for example. Of further
benefit, the inclusion of a pair of switching devices on each of
the three bridge legs 88, 90, 92 in a skewed arrangement--along
with the selective and independent switching of the IGBT 73 in each
switching device--enables the switching devices 86a, 86b, 86c, 86d,
86e, 86f on each leg to carry only one third of the load current
provided from the single phase AC power source 46. Control of the
switching devices 86 may be provided via a dedicated controller 74
associated with the power conversion module 82. Alternatively,
control of the switching devices 86 may be provided via controller
64, such that controller 64 controls operation of both the ASD 42
and the power conversion module 82.
[0040] Similar to the operation of the power conversion module 44
(FIG. 4 and FIGS. 6A-6D), it is recognized that power conversion
module 82 is operable in four different operating modes based on
the conductive state of each of the switching devices 86a, 86b,
86c, 86d, 86e, 86f. First and second operating modes may be
implemented during periods of a positive current being output from
single phase AC power source 46--with the first and second
operating modes providing for an energy build-up in input inductors
84 and energy release from input inductors 84, respectively. Third
and fourth operating modes may be implemented during periods of a
negative current being output from single phase AC power source
46--with the third and fourth operating modes providing for an
energy build-up in input inductors 84 and energy release from input
inductors 84, respectively. The flow of current through power
converter 80 and ASD 42 is dictated by the selective and
independent switching of the IGBT 73 in each switching device 86a,
86b, 86c, 86d, 86e, 86f, with current flow paths in the four
operating modes being essentially similar to those illustrated in
FIGS. 6A-6D.
[0041] Beneficially, inclusion of the arrangement of six switching
devices 86 and three input inductors 84 in the power conversion
module 82--and the switching of the switching devices 86 at a high
frequency--allows for the size of the input inductors 84 to be
reduced. That is, because each phase input inductor 84 only carries
one third of the load current and the equivalent switching
frequency is three times that of each individual bridge leg 88, the
overall inductor size can be scaled down significantly. The
size/rating of each switching device 86 (i.e., the IGBT) can be
reduced, with the rating of each switching device 86 being one
third of the load current rating. It is also recognized that
inclusion of the arrangement of six switching devices 86 and three
input inductors 84 in the power conversion module 82 allows for an
even further reduction in the total harmonic distortion present in
the sinusoidal current waveform output from the power conversion
module 82--with total harmonic distortion of the sinusoidal current
waveform being approximately 3%.
[0042] As set forth in the above descriptions of the power
converters 40, 80, embodiments of the invention thus provide an
add-on power conversion module that may be utilized with a
standard, existing ASD to operate a three-phase motor from a single
phase power source. The add-on power conversion module provides a
non-intrusive solution that minimizes the cost, weight and size of
a power converter system in single phase applications, such as by
negating the need for DC link chokes in the ASD, without the need
of large derating of the ASD. The add-on power conversion module
reduces input current harmonics so as to make it possible to meet
industry regulatory power quality mandatory requirements.
[0043] Therefore, according to an embodiment of the invention, a
power converter for operating a three-phase AC electrical machine
from a single phase AC power source comprises an adjustable speed
drive (ASD) that includes a rectifier configured to convert a
single-phase AC power to DC power on a DC bus, the DC bus having
first and second rails to provide a relatively fixed DC voltage,
and a DC bus capacitor across the first and second rails to smooth
voltage ripple. The ASD also includes an inverter for receiving DC
power from the DC bus and converting the DC power to a three-phase
AC power, the inverter comprising a plurality of switches that are
selectively controllable to control current flow and terminal
voltages in the three-phase AC electrical machine. The power
converter also includes an add-on power conversion module external
to the ASD and electrically connected to each of the single phase
AC power source and the ASD, with the add-on power conversion
module having one or more input inductors configured to store and
filter single-phase AC power received from the single phase AC
power source and a pair of switching devices connected to each of
the one or more input inductors at an output end thereof. The
add-on power conversion module also includes a controller operably
connected to the add-on power conversion module to selectively
control switching of the pair of switching devices associated with
each of the one or more input inductors so as to cause a sinusoidal
single phase AC power to be output from the add-on power conversion
module for rectification by the rectifier.
[0044] According to another embodiment of the invention, a method
for operating a three-phase AC electrical machine from a single
phase power source includes providing a single phase AC power to an
add-on power conversion module that includes one or more input
inductors each comprising an input end and an output end and a pair
of switching devices connected to each of the one or more input
inductors at the output end thereof. The method also includes
selectively switching each of the pair of switching devices
associated with each of the one or more input inductors via an
operably connected controller so as to cause a sinusoidal single
phase AC power to be output from the add-on power conversion
module, providing the sinusoidal single phase AC power to an
adjustable speed drive (ASD) separate from and operably connected
to the add-on power conversion module, and generating a three-phase
AC power via the ASD for operating the three-phase AC electrical
machine, with the ASD rectifying the sinusoidal single phase AC
power to generate a DC power and inverting the DC power to generate
the three-phase AC power.
[0045] According to yet another embodiment of the invention, a
power converter for operating a three-phase AC electrical machine
from a single phase AC power source includes an adjustable speed
drive (ASD) having an input connectable to an AC power source and a
three phase output connectable to an input terminal of a load, the
ASD comprising a housing enclosing a rectifier and inverter
operable to provide a three-phase AC output to the load. The power
converter also includes an add-on power conversion module
positioned external to the housing of the ASD and being
electrically connected to each of the single phase AC power source
and the ASD, with the add-on power conversion module having one or
more input inductors positioned at an input of the add-on power
conversion module connected to the single phase AC power source and
a pair of switching devices connected to each of the one or more
input inductors at an output end thereof. The power converter
further includes a controller operably connected to the add-on
power conversion module to selectively control switching of the
pair of switching devices associated with each of the one or more
input inductors, wherein the controller is programmed to control
switching of the pair of switching devices so as to cause a
sinusoidal single phase AC power to be output from the add-on power
conversion module, the sinusoidal single phase AC power output
having a total harmonic distortion of less than 5%.
[0046] Embodiments of the present invention have been described in
terms of the preferred embodiment, and it is recognized that
equivalents, alternatives, and modifications, aside from those
expressly stated, are possible and within the scope of the
appending claims.
* * * * *