JP2000308368A - Power conversion circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a system whose efficiency is high and which is small by a method, wherein an inverter output voltage is increased without increasing a DC link voltage, a motor current is reduced and a copper loss and the generation of heat are suppressed. SOLUTION: This power conversion circuit is provided with a power-supply side leg LG, in which two semiconductor switch parts are connected in series. The power conversion circuit is provided with a smoothing capacitor Cdc which is connected to both ends of the leg. The power conversion circuit is provided with a polyphase inverter INV, whose DC input side is connected to both ends of the capacitor. The power conversion circuit is provided with a single-phase AC power supply AC connected to the neutral point of a polyphase motor M, one end of which is connected to the middle point of the power-supply-side leg LG and the other end of which is star-connected to the AC output side of an inverter INV. In the power sonversion circuit, the zero-volt vector of the inverter INV is controlled, the neutral-point potential of the motor M is controlled, the semiconductor switch parts in the power-supply side leg LG are operated, the intermediate-point potential of the power-supply side leg LG is operated, and the input current of the inverter INV is controlled. Then, a zero-phase voltage is superposed on the phase output voltage command value of the inverter INV, and the maximum output voltage of the inverter INV is increased.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power conversion circuit for converting a single-phase AC power supply to a multi-phase AC power supply, the output voltage of which is increased and the DC link voltage utilization factor is improved. Here, the voltage utilization rate means the peak value of the maximum output line voltage fundamental wave of the power conversion circuit (inverter) / DC link voltage.

[0002]

2. Description of the Related Art The prior art will be described below by exemplifying a single-phase to three-phase power conversion circuit. FIG. 3 shows a prior art of a single-phase to three-phase power conversion circuit for converting a single-phase AC into a three-phase AC with a small number of switching elements and a simple configuration, and a neutral point of a winding of a three-phase AC motor as a load. It is a thing using. This technique is disclosed in Japanese Patent Application Laid-Open No. 10-3370 by the present applicant.
Although it is widely known in FIG. 2 of JP-A-47-47 and “The National Institute of Electrical Engineers of Japan in 1997,” the circuit configuration and operation are briefly described as follows.

In FIG. 3, S1 and S2 are semiconductor switch sections in which two anti-parallel circuits of a switching element and a diode are connected in series.
G is configured. Also, S11 to S16 are similar semiconductor switch units, and these constitute a three-phase voltage source inverter INV. C _dc is a smoothing capacitor. Each phase output terminal of the inverter INV is connected to one end of a winding of an AC motor M such as a three-phase induction motor, and its neutral point is connected via a single-phase AC power supply AC to a semiconductor switch section S1, It is connected to the middle point (virtual neutral point) of the series circuit of S2. In the motor M, the stator windings are connected in a star shape, and the induced voltage of each winding is shown by the symbol of the AC power supply.

In the configuration shown in FIG. 3, since the neutral point potential v ₀ of the motor M can be controlled by controlling the zero voltage vector of the inverter INV, the inverter IV is controlled by the zero voltage vector of the inverter INV and the power supply side leg LG.
Controls the input current of NV. As a result, the input current can be controlled as in the case of using the conventional single-phase full-bridge AC / DC converter. Since the voltage applied to the motor M is the line voltage of the inverter INV, the zero-phase component voltage does not appear on the motor M side and does not affect the motor driving. Therefore, the zero phase voltage of the inverter INV has a degree of freedom. Therefore, in the circuit of FIG. 3, the input current of the inverter INV is controlled using the zero-phase component, and the motor current is controlled using the positive-phase component.

A control block diagram of this circuit is shown in FIG.
It is configured as follows. 4, 1 is an automatic voltage regulator (AVR), 2, 6, 7, 8, 9 are comparators, 3 is a PLL (phase locked loop) circuit, and 4, 1
Reference numerals 0, 11, and 12 denote sine tables, 5 denotes an automatic current regulator (ACR), and 20, 21, 22, and 23 denote adders.

In the configuration shown in FIG. 4, the deviation between the DC link voltage command value V _dc ^* and the detected value V _dc is input to AVR1. The output of the comparator 2 to the power supply voltage V _s is input is input to the sin table 4 through the PLL circuit 3,
Sine wave data synchronized with the power supply voltage is read. The sine wave data is multiplied by the output of AVR1, the source current command value i _s ^*. The deviation between the command value i _s ^* and the detection value i _s is input to the ACR5 sought by the adder 23. The output of the ACR 5 is input to the comparator 6 as a voltage command value v _x ^{* at} the middle point of the power supply leg LG, and is used for comparison with a carrier (triangular wave). The output of the comparator 6 and its inverted signal are connected to the semiconductor switch units S1 and S1 of the power side leg LG.
It becomes a PWM pulse for driving the switching element of S2.

On the other hand, the rotational speed command value (angular frequency command value) ω
^* Is input to the sine tables 10, 11, and 12
Three-phase sine wave data having a phase difference of π / 3 [rad] is output. These sine wave data are multiplied by the amplitude command value a ^* , and the result is input to the comparators 7, 8, and 9.
These comparators 7, 8, and 9 compare the sine wave data with the carrier and determine the semiconductor switch S of the inverter INV.
PWM pulses for the switching elements 11 to S16 are generated.

In this control block, the DC link voltage V
_{When dc} has a relationship of 2√2 times or more of the power supply voltage V _s (effective value), the zero-phase voltage of the voltage command value on the inverter INV side is set to zero, and the neutral point potential v ₀ of the motor M is connected to the DC link. The voltage _Vdc was regarded as the midpoint potential, and the AC / DC conversion operation was controlled in the same manner as the conventional half-bridge AC / DC converter.

[0009]

[SUMMARY OF THE INVENTION] Figure 3, the control method of FIG. 4, the superimposing the supply current i _s as zero-phase current to the electric motor M, the motor current increases. As a result, copper loss increases and heat generation of the electric motor M also increases. In order to reduce the copper loss of the motor M, it is necessary to reduce the positive-phase current flowing through the motor M. In this case, even if the positive-phase current is reduced, the same motor output can be obtained by increasing the rated input voltage of the motor M (that is, the output voltage of the inverter INV). Therefore, in order to obtain a desired output while reducing the loss and heat generation in the motor M, it is desired to increase the output voltage of the inverter INV while reducing the motor current.

In the conventional control method, the inverter IN
In the control circuit of V, the PWM pulse is obtained by comparing the sine wave and the triangular wave, so that the peak value of the phase voltage fundamental wave of the inverter INV as viewed from the midpoint potential of the DC link voltage is (1/1).
2) V _dc is maximum. Since the line voltage is √3 times the phase voltage, the peak value of the line voltage fundamental wave is (√3 / 2)
V _dc is the limit. On the other hand, in order to increase the output voltage of the inverter INV, it is conceivable to simply increase the DC link voltage _Vdc. However, this method is not preferable because the withstand voltage of the element to be used increases.

Therefore, the present invention improves the DC link voltage utilization rate and increases the output voltage of the inverter without depending on the method of increasing the DC link voltage, and suppresses heat generation by reducing the motor current. An object is to provide a highly efficient and compact power conversion circuit.

[0012]

In order to solve the above-mentioned problems, according to the present invention, the zero-phase voltage of the voltage command value on the inverter side is not zero, the line voltage of the inverter becomes a desired value, and the PWM inverter The zero-phase voltage is superimposed so that the modulation rate of the signal does not exceed 1 (so as not to be larger than the carrier to be compared). Accordingly, the zero-phase voltage is superimposed on the voltage command value of the power supply side leg.

That is, according to the first aspect of the present invention, there are provided a power supply side leg in which two semiconductor switches are connected in series, a smoothing capacitor connected to both ends of the power supply side leg, and a DC input to both ends of the smoothing capacitor. Side, and one end is connected to the midpoint of the power supply side leg, and the other end is connected to the neutral point of a polyphase load that is star-connected to the AC output side of the inverter. A single-phase AC power supply, and controls the zero-voltage vector of the inverter to control the neutral point potential of the multi-phase load, and operates the semiconductor switch of the power-side leg to operate the midpoint of the power-side leg. In a power conversion circuit configured to control an input current of the inverter by controlling a potential, a zero-phase voltage is superimposed on an output voltage command value of each phase of the inverter, and a maximum output voltage of the inverter is obtained. It is intended to be pressurized.

According to a second aspect of the present invention, in the power conversion circuit according to the first aspect, the zero-phase voltage is superimposed on a voltage command value for controlling a midpoint potential of the power supply side leg.

According to a third aspect of the present invention, the zero-phase voltage is a multi-harmonic voltage (for example, a third harmonic voltage) equal to the number of inverter phases with the output frequency of the inverter as a fundamental frequency. good.

As described above, in order to reduce the copper loss of the motor, it is desirable to reduce the positive-phase current of the motor. Since the power is proportional to the product of the voltage and the current, increasing the voltage can reduce the current under the same power. Therefore, it is necessary to increase the output voltage of the inverter as much as possible.

Since the power supply side leg LG shown in FIG. 3 is constituted by a semiconductor switch section (switching element and diode), even if the neutral point potential v ₀ of the load fluctuates, the power supply leg LG is taken into consideration. Midpoint potential v of side leg LG
_{If x} is controlled by the operation of the semiconductor switch unit, the input current of the inverter INV can be controlled. In general, as a method of improving the voltage utilization rate of a three-phase inverter, there is a method of superimposing the third harmonic of the output voltage of the inverter on each phase voltage command value of the inverter. At this time, the voltage command value of the inverter is given by Equation 1.

[0018]

## EQU1 ## v _u ^* = (V _dc / 2) ・ a ｛sin ωt + (1 /
^{_{6) · sin3ωt} v v *}} = (V dc / 2) · a · {sin (ωt-2π / 3) +
_{(1/6) · sin3ωt} v w} * = (V dc / 2) · a · {sin (ωt-4π / 3) +
(1/6) ・ sin3ωt｝

That is, in the present invention, the third harmonic is superimposed on each phase voltage command value as a zero-phase component. In order to obtain the output voltage of the inverter without distortion, the maximum value of each phase voltage command value of the inverter must be _Vdc / 2 or less. For this reason, the modulation ratio a of the output voltage command value of the inverter was 1 at the maximum in the past (when there is no second term on the right side of Equation 1), but in the present invention, (V _dc /
2) Since the maximum value in parentheses excluding a is about 0.866, the modulation ratio a can be increased to 1.15. This means that the output voltage can be increased by 15% as compared with the conventional case. The above corresponds to the first and third aspects of the present invention.

At this time, from Equation 1, the neutral point potential v ₀ of the motor M is represented by Equation 2.

[0021]

## EQU2 ## v ₀ = (1/12) · V _dc · a · sin3ωt

On the other hand, the AC / DC control of the power supply side leg LG satisfies the relationship of Expression 3.

[0023]

## EQU3 ## v ₀ = v _x −v _s

Therefore, the midpoint potential v _x of the power supply side leg LG
Is obtained by Expression 4.

[0025]

## EQU4 ## v _x = (1/12) · V _dc · a · sin3ωt + √2
V _s · sin ω _s t

In the present invention, when the third harmonic is superimposed on each phase voltage command value of the inverter INV, the control of the power supply side leg LG follows Expression 4. Specifically, since the second term on the right side of Equation 4 is obtained from the output of the ACR, the first term is added to the output of the ACR. As a result, AC
R may have a response equivalent to that of the conventional power supply current control, and the power supply current control can be performed without giving a distortion due to the third harmonic. The above corresponds to claims 2 and 3.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a control block diagram showing an embodiment of the invention described in claims 1 and 3, and is a control circuit for the circuit of FIG.

The control circuit of FIG. 1 is different from that of FIG. 4 in that the third harmonic (sine wave data having a frequency three times the output voltage of the inverter INV) in the right side {} of equation 1 is stored. The table 15 is provided, and the multiplication result of the third harmonic and the amplitude command value a ^* is superimposed on each phase voltage command value in the adders 20, 21, 22 so that the final phase voltage command value v _u ^{* is obtained.} , V _v ^* , v _w ^* .
From the sin tables 10, 11, and 12, three-phase sine wave data having a phase difference of 2π / 3 [rad] are output based on the rotational speed command value ω ^* as in FIG.
^* Is multiplied.

In the present embodiment, the control of the power supply leg LG is performed as follows. As in the prior art, the deviation between the DC voltage command value V _dc ^* and the detected value V _dc is input to AVR1. Meanwhile, the power supply voltage detection value V _s inputted to the comparator 2, a sine wave synchronized to the power supply voltage via the PLL circuit 3, sin table 4 is generated. Multiplication result between the sine wave synchronized with the output power of AVR1 becomes the source current command value i _s ^*. The deviation between the power source current command value i _s ^* and the detection value i _s is input to the ACR5 sought by the adder 23. By comparing the midpoint potential command value v _x ^* of the power supply leg LG, which is the output of the ACR 5, with the carrier by the comparator 6, a PWM pulse for the semiconductor switch section of the power supply leg LG can be obtained as in the related art. .

As described above, in the present embodiment, a sine table 15 in which the third harmonic is stored and adders 20, 21, and 22 are added to the prior art, and a zero-phase component is added to each phase voltage command value. By superimposing the third harmonic, it is possible to make the peak value of the maximum output line voltage fundamental wave of the inverter INV equal to the DC link voltage, and to increase the output voltage of the inverter INV by about 15% as compared with the conventional art by the formula 1. it can.

Next, FIG. 2 shows an embodiment of the present invention described in claims 2 and 3. The difference from FIG. 1 is that, in the control of the power supply leg LG, the output of the ACR 5 is controlled so that the third harmonic as the zero-phase component superimposed on the inverter INV cancels the influence on the control of the power supply leg LG. This is to superimpose the same third harmonic as that on the inverter INV side.
That is, the third harmonic similar to that on the inverter INV side is superimposed by the adder 24 provided on the output side of the ACR 5 in FIG.

Since the third harmonic superimposed on the inverter INV has a frequency three times as high as the inverter output voltage, the frequency becomes very high during high-speed operation of the motor. As a result, if the ACR 5 attempts to follow this, an ACR with a very high-speed response is required, and the cost of the control circuit increases. Therefore, in the present embodiment, the configuration of the ACR 5 is the same as that of the prior art, and the AC power is obtained by adding the third harmonic corresponding to the first term of Equation 4 to the output of the ACR 5.
The input current can be controlled using an ACR having a speed equivalent to that of a conventional AC / DC converter without reducing the load on R and applying distortion due to the third harmonic.

The present invention can be applied not only to the three-phase output of the above embodiment, but also to a multi-phase output power conversion circuit other than the three-phase output.

[0034]

As described above, according to the present invention, the DC link voltage utilization factor can be improved by making the peak value of the maximum line output voltage fundamental wave of the inverter equal to the DC link voltage value. That is, conventionally, the DC link voltage of 86
%, Which is the limit, the present invention makes it possible to increase the output voltage by about 15%. As a result,
Without increasing the DC link voltage of the inverter, the voltage applied to the motor can be increased more than before.
Reduced motor current suppresses copper loss increase and heat generation,
A compact and highly efficient system can be realized.

[Brief description of the drawings]

FIG. 1 is a control block diagram showing a first embodiment of the present invention.

FIG. 2 is a control block diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a conventional technique.

FIG. 4 is a control block diagram of FIG. 3;

[Explanation of symbols]

 Reference Signs List 1 AVR (automatic voltage regulator) 2, 6, 7, 8, 9 comparator 3 PLL circuit 4, 10, 11, 12, 15 sin table 5 ACR (automatic current regulator) 20, 21, 22, 23, 24 Adder

Claims (3)

[Claims] 1. A power supply leg having two semiconductor switches connected in series, a smoothing capacitor connected to both ends of the power supply leg, and a polyphase inverter having a DC input connected to both ends of the smoothing capacitor. A single-phase AC power supply, one end of which is connected to the midpoint of the power supply side leg, and the other end of which is connected to the neutral point of a polyphase load which is star-connected to the AC output side of the inverter. Controlling the neutral voltage of the polyphase load by controlling the zero voltage vector of the inverter, and controlling the midpoint potential of the power supply leg by operating the semiconductor switch section of the power supply leg. In a power conversion circuit configured to control an input current, a power output characterized by increasing a maximum output voltage of the inverter by superimposing a zero-phase voltage on an output voltage command value of each phase of the inverter. Circuit. 2. The power conversion circuit according to claim 1, wherein the zero-phase voltage is superimposed on a voltage command value for controlling a midpoint potential of the power supply side leg. 3. The power conversion circuit according to claim 1, wherein the zero-sequence voltage is a multi-harmonic voltage equal to the number of inverter phases whose basic frequency is the output frequency of the inverter. Conversion circuit.