Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
1999-12-16
2001-11-20
Patel, Rajnikant (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S065000
Reexamination Certificate
active
06320767
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an inverter apparatus.
As a control apparatus for an AC motor such as an induction motor subjected to variable-speed control, a voltage-fed multiple inverter apparatus of a PWM control scheme in
FIG. 1
is known.
The apparatus shown in
FIG. 1
is an inverter in which two single-phase inverters
12
are connected in series to form one phase, and three sets of such single-phase inverters are star-connected. A motor
13
is driven by using output terminals U, V, and W. In this case, a transformer
11
supplies insulated three-phase power to the AC input terminals of the respective single-phase inverters. Each single-phase inverter
12
will be described in detail with reference to FIG.
2
. Referring to
FIG. 2
, three-phase AC power is converted into DC power by a three-phase bridge rectifier
14
, and the current is smoothed by a smoothing capacitor
15
. This DC power is converted into an AC power by a single-phase bridge inverter
16
. In this conversion, the DC power is converted into an AC power of a desired voltage and frequency by so-called PWM control.
According to the arrangement shown in
FIG. 1
, an output three times higher than the voltage output from each single-phase inverter can be obtained, and hence a large-capacity inverter can be formed. By shifting the PWM control timings of two single-phase inverters constituting one phase from each other, the PWM frequency appearing at the output terminal is doubled. As a consequence, each voltage step is reduced to ½ from the viewpoint of the overall phase, and an output similar to a sine wave can be obtained.
In the above multiple inverter, a PWM signal can be obtained by shifting the phases of carrier signals.
FIG. 3
shows examples of output voltage waveforms. By alternately switching output voltages U
1
and U
2
from two single-phase inverters, a waveform more similar to a sine wave can be obtained as a whole. In addition, by shifting the phase of the secondary winding of the transformer
11
, input harmonics can be reduced. Consider two sets of single-phase inverters on the upper and lower stages. Since the single-phase inverters on each stage output three-phase power, the power is smoothed from the viewpoint of each stage, and a so-called “12-pulse arrangement” with the phases of the windings of the transformer being shifted by 30° is formed. This arrangement can reduce input harmonics.
FIG. 1
shows an example of two stages. Obviously, however, an improved result can be obtained with three or more stages.
However, single-phase inverters require a larger number of parts than a three-phase inverter, and have power pulsations having a frequency twice the output frequency at the DC portion. Since the peaks of power pulsations must be considered, the use efficiency of the rectifier of each single-phase inverter is lower than that of the rectifier of the three-phase inverter. In addition, large current ripples are produced in the smoothing capacitor of each single-phase inverter. Furthermore, since the current ripples in the smoothing capacitor greatly affect the service life of the capacitor, a sufficient design margin must be ensured. This greatly increases the cost of the inverter apparatus.
In order to process regeneration power from a load, DC power in all the single-phase inverters must be consumed by using resistors, or a special power converting circuit is required, resulting in problems in terms of both cost and size. Furthermore, as the number of stages of single-phase inverters decreases, the input harmonics of currents increase.
The present invention is therefore made in consideration of the above problems, and has as its object to provide an inverter apparatus which can attain increases in the capacity and voltage of the inverter by combining a three-phase inverter and single-phase inverters, improve reliability and cost performance by reducing the number of parts, and realizing a reduction in input harmonics and a regeneration function at a low cost.
BRIEF SUMMARY OF THE INVENTION
In order to achieve the above object, according to first aspect of the present invention, there is provided an inverter apparatus comprising:
a three-phase inverter section having a three-phase bridge rectifier for converting three-phase AC power into DC power and a three-phase bridge inverter for converting the DC power converted by the three-phase bridge rectifier into three-phase AC power;
3×n (n is a natural number) single-phase inverters, of which n single-phase inverters are connected in series with phase outputs of the three-phase inverter section to convert the three-phase AC power into DC power, convert the converted DC power into single-phase AC power; and
a transformer for supplying insulated AC powers to the three-phase inverter and the 3n single-phase inverters, respectively.
According to the second aspect of the present invention, there is provided an inverter apparatus of the first aspect,
wherein the single-phase inverter comprises a rectifying section for converting the AC power supplied from the transformer into DC power and an inverter section for converting the DC power rectified by the rectifying section into the single-phase AC power.
According to the third aspect of the present invention, there is provided an inverter apparatus of the second aspect,
wherein the AC power supplied from the transformer is supplied to the three-phase bridge rectifier of the three-phase inverter section and the rectifying section of the single-phase inverter.
According to the present invention, the single-phase inverters and the three-phase inverter are connected in series with each other. Therefore, the arrangement formed by combining single-phase inverters and a three-phase inverter can decrease the number of parts as compared with the arrangement constituted by only single-phase inverters. In addition, since three-phase outputs balance at the DC portion, restrictions on a smoothing capacitor and rectifying circuit can be reduced. This improves reliability and cost performance.
According to the fourth aspect of the present invention, there is provided an inverter apparatus of the first aspect,
further comprising:
a first control circuit provided for each phase to generate a first control signal for the three-phase bridge inverter from a reference sine signal of a corresponding phase; and
a second control circuit provided for each phase to generate a second control signal for controlling the single-phase inverter so as to reduce a difference between the first control signal, generated by the first control circuit, and the reference sine signal of the corresponding phase.
According to the present invention, each single-phase inverter supplies the difference between a sine waveform and an output waveform from the three-phase inverter, and a plurality of such single-phase inverters are connected in series, a low-voltage, high-speed switch device can be used. In addition, since each single-phase inverter supplies the difference between the sine waveform and the output waveform from the three-phase inverter, a sine output waveform can be obtained as a whole.
According to the fifth aspect of the present invention, there is provided an inverter apparatus of the first aspect,
wherein the single-phase inverter is used in a bypassed state when an operation speed of the inverter apparatus is not higher than a predetermined speed.
According to the present invention, only the three-phase inverter is PWM-controlled in low-speed operation, and the remaining single-phase inverters are bypassed. This can prevent power pulsations having a frequency twice the output frequency in low-frequency output operation, which poses a problem in a single-phase inverter, reduces restrictions on the parts used, and improves reliability.
According to the sixth aspect of the present invention, there is provided an inverter apparatus of the first aspect,
wherein the three-phase inverter comprises a resistor for consuming power generated by regeneration, the inverter apparatus further co
Oka Toshiaki
Shimoura Takuji
Banner & Witcoff , Ltd.
Kabushiki Kaisha Toshiba
Patel Rajnikant
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