U.S. patent application number 11/246800 was filed with the patent office on 2006-02-09 for multifunction hybrid intelligent universal transformer.
Invention is credited to Frank Goodman, Jih-Sheng Lai, Arindam Maitra, Arshad Mansoor.
Application Number | 20060028848 11/246800 |
Document ID | / |
Family ID | 34592321 |
Filed Date | 2006-02-09 |
United States Patent
Application |
20060028848 |
Kind Code |
A1 |
Lai; Jih-Sheng ; et
al. |
February 9, 2006 |
Multifunction hybrid intelligent universal transformer
Abstract
A multifunction hybrid intelligent universal transformer
includes a conventional transformer coupled with power electronics
on the secondary side to enhance the functionality of power
conversion. The universal transformer includes features for
overcoming the deficiencies associated with conventional
transformers, including voltage sag compensation, instantaneous
voltage regulation, outage compensation, capacitor switching
protection, harmonic compensation, single-phasing protection, DC
output, and variable frequency output.
Inventors: |
Lai; Jih-Sheng; (Blacksburg,
VA) ; Mansoor; Arshad; (Oak Ridge, TN) ;
Maitra; Arindam; (Knoxville, TN) ; Goodman;
Frank; (Palo Alto, CA) |
Correspondence
Address: |
MORGAN, LEWIS & BOCKIUS, LLP.
2 PALO ALTO SQUARE
3000 EL CAMINO REAL
PALO ALTO
CA
94306
US
|
Family ID: |
34592321 |
Appl. No.: |
11/246800 |
Filed: |
October 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10723620 |
Nov 25, 2003 |
6954366 |
|
|
11246800 |
Oct 7, 2005 |
|
|
|
Current U.S.
Class: |
363/39 |
Current CPC
Class: |
Y02E 40/40 20130101;
H02J 3/01 20130101; H02J 3/32 20130101; H02M 5/293 20130101 |
Class at
Publication: |
363/039 |
International
Class: |
H02M 1/12 20060101
H02M001/12 |
Claims
1. A power conversion device, comprising: a transformer having a
number of primary windings adapted to be coupled to a three-phase
power source and a number of secondary .DELTA.-.DELTA. and
.DELTA.-Y connections; a first switched inverter coupled to the
.DELTA.-.DELTA. secondary connection; and a second switched
inverter coupled to the .DELTA.-Y secondary connection and to the
first switched inverter for providing a harmonically compensated
waveform.
2. The device of claim 1, further comprising: AC/DC converter
blocks coupled to the secondary .DELTA.-.DELTA. and .DELTA.-Y
connections for converting Alternating Current (AC) waveforms into
Direct Current (DC) waveforms.
3. The device of claim 2, wherein the AC/DC converter block is a
diode bridge.
4. The device of claim 1, further comprising: an energy storage
device coupled to the input of at least one of the first and second
switched inverters.
5. The device of claim 1, further comprising: a filter coupled to
the output of at least one of the first and second switched
inverters.
6. A power conversion device, comprising: a transformer adapted to
be coupled to a power source; power electronics coupled to the
transformer for transforming the power source into a desired
waveform; and an energy storage device coupled to the transformer
and the power electronics for mitigating interruptions in the power
source.
7. The device of claim 6, further comprising: a converter coupled
to the transformer for converting the power source from Alternating
Current (AC) waveforms to Direct Current (DC) waveforms.
8. The device of claim 6, wherein the power electronics include an
actively switched inverter for providing a desired output
waveform.
9. The device of claim 8, wherein the switched inverter is
controlled by pulse width modulation.
10. The device of claim 6, wherein the converter is a switched
full-bridge converter.
11. The device of claim 6, further comprising: a filter coupled to
the power electronics for smoothing the output waveform.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of pending U.S. patent
application Ser. No. 10/723,620, filed Nov. 25, 2003. U.S. patent
application Ser. No. 10/723,620 is incorporated herein by reference
in its entirety.
TECHNICAL FIELD
[0002] The present invention relates generally to power conversion
technology, and in particular to a universal transformer for
enhancing the functionality of power conversion in electrical
distribution systems.
BACKGROUND
[0003] Transformers make up a large portion of power delivery
systems throughout the world and are the backbone of electric power
conversion systems. The positive attributes of conventional
distribution transformers have been well documented for years and
include low cost, high reliability, and high efficiency. Were it
not for these highly reliable devices, the distance separating
generators from consumers would have been significantly greater.
Indeed, to distribute power over such distances would require many
households and industries to operate their own substations,
resulting in electricity being a much less practical form of
energy.
[0004] Like other devices in modern electrical distribution
systems, the conventional transformer has some drawbacks. The
drawbacks include voltage drop under load, inability to mitigate
"flicker," sensitivity to harmonics, environmental impacts when
mineral oil leaks occur, limited performance under DC-offset load
unbalances, inability to convert single-phase service to
three-phase for powering certain types of equipment and no
energy-storage capacity. One consequence of not having energy
storage capacity is that the output can be easily interrupted
because of a disturbance at the input. Also, when the output load
current generates harmonics and reactive power, the conventional
transformer reflects them back to the input side.
[0005] Power-line disturbances, such as voltage sags and momentary
interruptions, cost electric utility customers billions of dollars
every year. With today's increased complexity of process
automation, even if only a small segment of a process is
vulnerable, power disruptions may interrupt the entire automated
process. In response to this perceived market need, a broad range
of distribution-class, power-conditioning devices, commonly known
as "custom power" devices have been introduced in the market. These
custom power devices include reactive power and harmonic
compensation devices, such as the Adaptive Var Compensator (AVC)
and voltage sag and momentary interruption protection devices, such
as a Dynamic Voltage Restorer (DVR). Unfortunately, the customer
response to these new lines of products has not been receptive,
mainly because of their cost.
SUMMARY
[0006] A multifunction hybrid intelligent universal transformer
includes a conventional transformer coupled with power electronics
on the secondary side to enhance the functionality of power
conversion.
[0007] In some embodiments of the present invention, a power
conversion device comprises a transformer having a primary input
winding and multiple secondary output windings. A switched inverter
circuit is coupled to the secondary output windings and
configurable to couple a first output winding with a parallel path
inverter and configurable to couple a second output winding with a
series path inverter.
[0008] In some embodiments, an energy storage device (e.g.,
ultra-capacitor, battery) can be coupled to the parallel path
inverter for mitigating voltage disturbances.
[0009] In some embodiments, an active or diode-bridge AC/DC
converter block (e.g., full-bridge rectifier) can be coupled to the
parallel path inverter for converting Alternating Current (AC)
waveforms into Direct Current (DC) waveforms to maintain clean
input current with unity power factor.
[0010] In some embodiments, a filter can be coupled to the output
of the switched inverter for smoothing output waveforms.
[0011] A significant advantage of the present invention is the
combining of a conventional distribution transformer with the
functionalities of one or more custom power devices into a single,
tightly integrated, electrical customer interface, rather than the
costly conventional solution of adding separate custom power
devices to the power distribution system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a circuit diagram of a three-phase universal
transformer, in accordance with some embodiments of the present
invention.
[0013] FIG. 2 is a circuit diagram of an active AC/DC converter
block, in accordance with some embodiments of the present
invention.
[0014] FIG. 3 is a graph illustrating harmonic compensation, in
accordance with some embodiments of the present invention.
[0015] FIG. 4 is a graph illustrating voltage sag compensation,
instantaneous voltage regulation and outage compensation, in
accordance with some embodiments of the present invention.
[0016] FIG. 5 is circuit diagram of an alternative version of the
universal transformer having a reduced number of switches and
transformer windings, in accordance with some embodiments of the
present invention.
[0017] FIG. 6 is a circuit diagram of a cascaded inverter based
universal transformer, in accordance with some embodiments of the
present invention.
[0018] FIG. 7 is a graph illustrating individual universal
transformer input currents, in accordance with some embodiments of
the present invention.
[0019] FIG. 8 is a graph illustrating an inverter output voltage,
in accordance with some embodiments of the present invention.
[0020] FIG. 9 is a block diagram of a converter/inverter control
system, in accordance with some embodiments of the present
invention.
DETAILED DESCRIPTION
[0021] As pressures increase on electric service providers to
provide a higher quality and reliable product on demand when
customers need it and at a price point that is acceptable to
customers, there is desire to increase utilization of conventional
transformers. One possible improvement is to integrate the
functionalities of custom power devices into an existing
distribution transformer. Such a "hybrid" transformer would
significantly alter the way electric utilities serve their
respective customers and expand the capabilities of a conventional
distribution transformer from primarily a voltage transformation
device to an integrated, electrical customer interface. The hybrid
transformer would enable service providers to broaden their
traditional service offerings, satisfy a myriad of customer
requirements for power quality, and at the same time provide
advanced distribution automation functionalities.
[0022] A multifunction hybrid intelligent universal transformer
would ideally combine a conventional distribution transformer with
the functionalities of one or more custom power devices to provide
an integrated electrical customer interface. The universal
transformer should include features for overcoming the deficiencies
associated with conventional transformers, including voltage sag
compensation, instantaneous voltage regulation, outage
compensation, capacitor switching protection, harmonic
compensation, single-phasing protection, DC output, and variable
frequency output (e.g., 50 Hz, 60 Hz, 400 Hz, etc.).
Overview of Universal Transformer
[0023] FIG. 1 is a circuit diagram of a three-phase universal
transformer 100, in accordance with some embodiments of the present
invention. For each phase-leg of the universal transformer 100
there is a power conversion circuit 102-1 through 102-3. For
simplification, only the power conversion circuit 102-1 (phase-leg
a) will be described, since the power conversion circuits 102-2 and
102-3 (phase-legs b and c) operate in a similar manner.
[0024] The power conversion circuit 102-1 includes a conventional
transformer 104 having a primary input winding 106 and secondary
output windings 108-1 and 108-2, a DC bus capacitor circuit 110 and
an actively switched inverter circuit 112. The switched inverter
circuit 112 includes semiconductor switches 114-1 through 114-6
that can be rapidly switched (approximately at 30,000 to 40,000 Hz)
to convert the DC voltage stored at the DC bus capacitor circuit
110 to an AC waveform. In some embodiments, the number of switches
114 and their rate of switching may be different from the exemplary
values of the embodiment shown in FIG. 1.
[0025] The switches 114 can be configured to provide a parallel
path inverter coupled to the secondary winding 108-1 or a series
path inverter coupled to the secondary output winding 108-2. The
switched inverter 112 can have many different inverter circuit
topology options. For example, the parallel path inverter can be a
half bridge-based inverter, which relies on capacitor-split sources
and phase-leg switches to produce PWM (pulse width modulated)
output. The DC bus capacitor 110 can be any DC voltage source
(e.g., capacitor bank, battery) capable of maintaining voltage for
a sufficient period of time to compensate for a disturbance or
interruption.
[0026] In some embodiments, the conventional transformer 104 steps
down a high voltage level (e.g., 12000 Volts) received from utility
lines to a low voltage level (e.g., 120 Volts) suitable for
consumer applications at a variable frequency (e.g., 50 Hz, 60 Hz,
or 400 Hz). In addition to transforming voltage, the transformer
104 isolates the input voltage and current from the secondary or
load side of the transformer 104. Thus, transients generated by a
power factor correction capacitor switching event will not
propagate to the secondary or load side of the transformer 104.
[0027] The secondary output winding 108-1 of the transformer 104 is
coupled across the switched inverter 112. The secondary output
winding 108-2 has a first terminal coupled to switches 114-1 and
114-4 (node a') and a second terminal coupled to ground. The node
between switches 114-2 and 114-5 (node a''') is also coupled to
circuit ground. The switches 114 can include Gate-Turn-Off (GTO)
Thyristors, Integrated Gate Bipolar Transistors (IGBTs), MOS
Turn-off Thyristors (MTOs), Integrated-Gate Commutated Thyristors
(IGCTs), Silicon Controlled Rectifiers (SCRs) or any other
semiconductor devices that have a turn-off capability.
[0028] In some embodiments, a filter circuit 116 is coupled to the
output of the inverter 112 (node a'') for smoothing the output AC
waveform. By adding the filter circuit 116 to the output, the AC
output waveform is sinusoidal with substantially reduced ripple. In
some embodiments, the filter circuit 116 includes an inductive
element L.sub.a coupled to a shunt capacitance C.sub.a to form a
low pass filter. In other embodiments, the inductive element
L.sub.a can be coupled in series with the capacitance C.sub.a to
form a low pass filter. Note that other combinations of passive
and/or active devices can be coupled to the switched inverter 112
for smoothing the output waveform using well-known filter design
techniques.
[0029] If the transformer 100 is used in an application or system
that requires outage compensation or short-term interruption
protection, an energy storage device 118 can be coupled across the
inputs of the parallel path inverter to ride-through these
disturbances. When the input source voltage drops for a short
period of time, the energy storage device 118 compensates for the
deficit and maintains constant output voltage. The total period of
compensation as a function of the amount of energy storage can be
adapted as desired. The energy storage device 118 can include
capacitor banks, ultra-capacitors, flywheels, batteries, or any
other suitable storage media (or any combination thereof). In some
embodiments, the energy storage device 118 can be switched into the
power conversion circuit 102-1 upon detection of a voltage sag
and/or to provide outage compensation.
[0030] In some embodiments, the secondary output winding 108-1 is
coupled to an AC/DC converter block 120 to obtain a DC source, and
the secondary output winding 108-2 is coupled in series with
switches 114-4, 114-6 and 114-2, which can be configured as a
full-bridge inverter, enabling the output voltage to be actively
compensated when there is an interruption at the primary side of
the transformer 100. In some embodiments, depending upon the power
flow direction requirement of the AC/DC converter block 120, the
AC/DC converter block 120 can be a diode bridge or an active
full-bridge inverter. If the AC/DC converter block 120 output is a
unidirectional power flow that allows power transmission only from
the primary side to the secondary side of the transformer 100, then
a diode bridge can be used as a low cost alternative. If the AC/DC
converter block 120 output needs to allow power flow back to the
primary side of the transformer 100, then the AC/DC converter block
120 can be replaced with a full-bridge converter to avoid high
harmonic contributions at the primary side of the transformer
100.
[0031] FIG. 2 is a circuit diagram of an AC/DC converter block 120,
in accordance with some embodiments of the present invention. In
some embodiments, the AC/DC converter block 120 is a full-bridge
converter that can be used as an active rectifier block. The input
to the converter block 120 is AC and the output is DC. With
switching control (not shown), the input current can be controlled
such that the input is a clean sinusoidal waveform and in phase
with the input voltage, regardless of the type of output load
(e.g., linear or nonlinear output loads). Note that each switch 114
includes an IGBT and an anti-paralleled diode. If all the IGBTs are
removed, then the current becomes a diode-bridge that only allows
current or power to flow in one direction (i.e., from AC input to
DC output).
[0032] FIG. 3 is a graph illustrating harmonic compensation, in
accordance with some embodiments of the present invention. The
waveforms shown in FIG. 3 were generated by a simulation of the
universal transformer 100, with the universal transformer 100
output connected to a nonlinear load and the output currents,
i.sub.La, i.sub.Lb, and i.sub.Lc harmonic distorted. As can be
observed from in FIG. 3, with an actively switched converter block
120 for rectification, the input current i.sub.SA becomes
sinusoidal and in phase with the input source voltage, v.sub.AN.
Note that FIG. 3 also shows the input line-to-line voltage,
v.sub.AB, which is 30 degrees leading the input source voltage,
v.sub.AN.
Operation of Universal Transformer
[0033] The operation of the universal transformer 100 can be
described by examining the operation of the power conversion
circuit 102-1 for various switch 114 configurations. Note that the
power conversion circuits 102-2 and 102-3 (phase-legs b and c) of
the transformer 100 operate in a similar manner.
[0034] In some embodiments, the switches 114-1 (S.sub.a1), 114-3
(S.sub.a3), 114-4 (S.sub.a4) and 114-6 (S.sub.a6) form an inverter
that has an AC output in series with the transformer 104 output.
The basic operation is to switch S.sub.a1-S.sub.a6 and
S.sub.a4-S.sub.a3 pairs in an alternating fashion so that the
inverter 112 output voltage is an alternating chopped DC voltage.
The filter 116 smoothes the chopped DC voltage into a clean,
sinusoidal waveform.
[0035] In some embodiments, the switches 114-2 (S.sub.a2), 114-3
(S.sub.a3), 114-5 (S.sub.a5) and 114-6 (S.sub.a6) form an inverter
that can produce AC output independent from the input voltage,
v.sub.AB. The basic operation is to switch the S.sub.a3-S.sub.a2
and S.sub.a5-S.sub.a6 pairs in an alternating fashion so that the
inverter 112 output voltage is an alternating chopped DC voltage
independent from the input voltage, v.sub.AB. The chopped DC
voltage is then smoothed into a clean, sinusoidal waveform by the
filter circuit 116.
[0036] The switches 114 can be controlled by an external control
means using either analog or digital control signals in a manner
commonly known to one of ordinary skill in the art. For example,
the states of switches 114 can be controlled using Pulse-width
modulation (PWM) techniques. In PWM, the width of pulses in a pulse
train are modified in direct proportion to a small control voltage.
By using a sinusoid of a desired frequency as the control voltage,
it is possible to produce a waveform whose average voltage varies
sinusoidally in a manner suitable for driving the switches 114. An
embodiment of a pulse-width modulation inverter control circuit is
described below with respect to FIG. 9.
[0037] Table I below shows the two basic configurations for
switches 114-1 through 114-6 and the corresponding inverter output.
Note that an "X" in Table I indicates that the switch is used to
perform the output function. TABLE-US-00001 TABLE I Switch
Configurations S.sub.a1 S.sub.a2 S.sub.a3 S.sub.a4 S.sub.a5
S.sub.a6 Output Function X X X X AC output in series with
transformer output X X X X AC output independent of transformer
input X DC output
[0038] In some embodiments, the universal transformer 100 can be
configured to provide single-phase protection. For example, if the
input power source has a missing phase or is running under a
single-phase condition, the switched inverter 112 can be configured
to turn off one or more phase-legs to prevent the universal
transformer 100 from operating under an abnormal source
condition.
[0039] In some embodiments, the universal transformer 100 can be
configured to provide DC output. For example, the switched inverter
112 can be configured to provide an interleaved three-leg DC/DC
converter to provide DC output with only S.sub.a3, S.sub.b3 and
S.sub.c3 conducting.
[0040] FIG. 4 is a graph illustrating voltage sag compensation,
instantaneous voltage regulation and outage compensation, in
accordance with some embodiments of the present invention. The
waveforms shown in FIG. 4 were generated from a simulation in which
the first two cycles show normal operation and the third cycle
shows a 75% voltage sag (shown as V.sub.a'n). The voltage sag may
last a few cycles, but the output voltage V.sub.an can be
compensated by the switching inverter 112 to maintain a
full-voltage output. With the series inverter path, the universal
transformer 100 is capable of supplying full voltage continuously
without the need of the energy storage device 120 even if the input
source voltage drops to 50% of its normal voltage.
[0041] FIG. 5 is circuit diagram of a universal transformer 500
having a reduced number of switches and transformer windings, in
accordance with some embodiments of the present invention with
omission of the series connecting path. The transformer 500
includes a power conversion circuit 502-1 through 502-3 for each
phase-leg. Each power conversion circuit 502 includes a
conventional transformer 504 having a primary winding 506 and a
secondary output winding 508, a AC/DC converter block 520, an
energy storage device 518, a DC bus capacitor circuit 510 and an
actively switched inverter circuit 514. Each switched inverter 514
includes semiconductor switches 516-1 through 516-4, which can be
controlled (e.g., using PWM control) to convert DC voltage stored
at the DC bus capacitor circuit 510 to an AC waveform. A low pass
filter 518 is coupled to an output of the inverter 514 (node a').
The node between switches 516-3 and 516-4 (node a'') is coupled to
circuit ground.
[0042] The operation of the universal transformer 500 relies on the
switched inverter 514, which produces output voltages that are
independent of the source voltage. The transformer 500 performs all
the functions of the transformer 100 shown in FIG. 1, except that
it does not include a series path inverter. Thus, it relies on the
energy storage devices 518 for voltage sag and under-voltage
compensation.
[0043] FIG. 6 is a circuit diagram of a cascaded inverter-based
universal transformer 600, in accordance with some embodiments of
the present invention. For simplification, FIG. 6 shows a
three-phase input and only a single-phase output. In some
embodiments, however, the transformer 600 is reconfigured to
provide three-phase output.
[0044] The input side of the transformer 600 includes a
.DELTA.-.DELTA. connection 606 and a .DELTA.-Y connection 608,
resulting in output waveforms that are separated by a phase angle
(e.g., 30 degrees). The connection 606 is coupled in series with an
AC/DC converter block 610 and a switched inverter 602. The
connection 608 is coupled in series with an AC/DC converter block
612 and a switched inverter 604. The inverters 602, 604 are coupled
together at nodes x and y to form a cascade inverter. In some
embodiments, the AC/DC converter blocks 610 and 612 can be actively
switched full-bridge rectifiers (FIG. 2). In other embodiments, the
AC/DC converter blocks 610 and 612 can be simple diode bridges, as
shown in FIG. 6.
[0045] The inverters 602 and 604 include semiconductor switches 614
and 616, respectively, which can be controlled to convert the DC
voltage stored at a DC blocking capacitors (C.sub.d) 620 and 622
with PWM (pulse width modulation) or any other suitable waveforms,
such as a staircase type waveform. A filter circuit 618 can be
coupled to the output of the inverter 602 (node z) to smooth the
output waveforms.
[0046] While the individual rectifier currents i.sub.d.DELTA. and
i.sub.dY are harmonic distorted, the sum of the transformer
currents at the input/source side (i.sub..DELTA..DELTA.,
i.sub..DELTA.y) provide a significantly improved waveform that has
the 5.sup.th and 7.sup.th harmonics cancelled. This type of
phase-shift transformer and diode bridge connection provides
"12-pulse rectification." FIG. 7 shows the individual transformer
input currents, i.sub..DELTA..DELTA. and i.sub..DELTA.Y, and output
currents, i.sub..DELTA.2 and i.sub.Y2, and the sum of the two
transformer input currents, i.sub.SA. The operation of cascaded
inverters is further described in U.S. Pat. No. 5,642,275, which is
incorporated by reference herein in its entirety.
[0047] FIG. 8 shows the inverter output voltage, v.sub.an, of
transformer 600 with one set of inverters switching with
high-frequency PWM, and the other set of inverters switching only
once per fundamental cycle. By optimizing the switching angle, the
output waveform is less distorted and the filter size can be
reduced. Other variations of the transformer 600 are also possible,
such as adding more levels of cascaded inverters.
[0048] FIG. 9 is a block diagram of a converter/inverter control
system 900 for controlling a converter or inverter 902, in
accordance with some embodiments of the present invention. The
feedback control system 900 includes a processor 906 (e.g.,
microcomputer, digital signal processor), a scaling factor circuit
908, a set of gate drivers 910 and a command interface 912. The
processor further includes a pulse width modulator 914, a
controller 916 and memory 918 (e.g., DRAM, CD-ROM). The scaling
factor circuit 908 and the gate drivers 910 isolate control signals
from the power.
[0049] In operation, the processor 906 compares a command voltage
V.sub.ref and a scaled feedback output signal V.sub.sense to
determine an error signal V.sub.error. The feedback signal,
V.sub.sense, is taken from the output of the converter/inverter
902. The error signal V.sub.error is received by the controller
916, which generally applies a proportional (P),
proportional-integral (PI), or proportional-integral-differentiator
(PID) gain to the error signal. The output of the controller is a
smooth duty cycle signal, d(t). Note that in a typical application
either a load (e.g., adjustable speed drive) or another
converter/inverter 904 is coupled to the output of the
converter/inverter 902.
[0050] The duty cycle of each switch is computed by the processor
906 based on one or more computer programs or gate pattern logic
stored in memory 918. The resulting duty cycle signal, d(t), is
then sent to the pulse width modulator 915 (PWM), which generally
includes a set of voltage comparators. In some embodiments, one
comparator is used for each pair of switches. For example, the
switch pair S.sub.1-S.sub.4 in the actively switched inverter 112
(FIG. 1) can be controlled by a first comparator and the switch
pair S.sub.3-S.sub.6 can be controlled by a second comparator. The
PWM signals are then fed into the gate drivers 910 to turn the
switches in the converter/inverter 902 on or off. The number of
switches in the converter/inverter 902 depends on how many voltage
levels and phases are to be controlled.
[0051] The control voltages d(t) (and therefore the output pulse
width) can be varied to achieve different frequencies and voltage
levels in any desired manner. For example, the processor 906 can
implement various acceleration and deceleration ramps, current
limits, and voltage-versus-frequency curves by changing variables
(e.g., via the command interface 912) in control programs or gate
pattern logic in processor 906.
[0052] If the duty cycle d(t) is greater than the voltage level of
a reference waveform (e.g., a triangular waveform) at any given
time t, then the PWM circuit 914 will turn on the upper switches
(e.g., switches S.sub.a1 and S.sub.a4) of inverter 112 and turn off
the lower switches (e.g., switches S.sub.a3 and S.sub.a6) of the
inverter 112. For a three-phase PWM inverter embodiment (e.g., the
embodiment shown in FIGS. 1, 5 and 7), three single-phase control
circuits can be used with control voltages comprising sinusoidal
waveforms shifted by 120 degrees between phases using techniques
well-known in the art.
[0053] In some embodiments, the control system 900 includes a
detection circuit configured to detect when the input poser source
has a missing phase or is running under a single-phase condition
and to generate control signals to be used by the command interface
912 to shut off the switches in one or more phase-legs of the
universal transformer.
[0054] The foregoing description, for purpose of explanation, has
been described with reference to specific embodiments. However, the
illustrative discussions above are not intended to be exhaustive or
to limit the invention to the precise forms disclosed. Many
modifications and variations are possible in view of the above
teachings. The embodiments were chosen and described in order to
best explain the principles of the invention and its practical
applications, to thereby enable others skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated.
* * * * *