JP2000083387A - Control method for three-phase voltage inverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence difficulties in high frequency current leakage in a three-phase AC equipment by suppressing a common mode voltage fluctuation itself at a load by controlling an inverter by two-arm-on switching. SOLUTION: Three-phase AC is generated by alternately repeating a switching control (two-arm-on switching) in which an upper arm of a certain phase is turned on, the lower arm of the other phase is turned on, and both the upper arm and lower arm are turned off for a remaining phase among each phase of U, V, W of a three-phase voltage type inverter 10. By doing this, a voltage of E/2 is applied to one of the phases U, V, W, and a voltage of -E/2 to one of the others, thereby resulting in uniform application of voltage as a whole so that the common mode voltage as the voltage at the virtual neutral point of delta connection load 30 is always at zero potential. Because of this, no high frequency current is generated which leaks to the outside of the load.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a three-phase voltage-source inverter (power conversion device) for generating a three-phase alternating current device such as an induction motor by generating an alternating voltage from a direct current voltage.

[0002]

2. Description of the Related Art With the development of semiconductor devices, AC motors using three-phase voltage type inverters having advantages such as ease of maintenance and control have been widely used. The control carrier frequency of the three-phase voltage-source inverter has also been increased in order to prevent wave current and speed up the control response.

FIG. 8 is a connection diagram showing such a conventional three-phase voltage source inverter for driving a load such as an induction motor in a star connection. Each phase U of the star connection load 20,
Each of V and W is connected to each of the output terminals 11, 12 and 13 of the three-phase voltage source inverter 10, and each of the output terminals 11, 12 and 13 is positively connected via each of the switching elements 11P, 12P and 13P. Input DC voltage + E / 2, and switching elements 11N, 1
It is connected to the negative input DC voltage -E / 2 via 2N and 13N. Hereinafter, the switching element connected to the positive input DC voltage + E / 2 is referred to as an upper arm, and the negative input DC voltage −
The switching element connected to E / 2 is called a lower arm.

Conventionally, when driving a three-phase AC device using such a three-phase voltage source inverter, the upper arm of a certain phase is turned on and the lower arm of the other two phases is turned on, or a certain phase is turned on. A three-phase alternating current is generated by alternately repeating switching control (hereinafter referred to as three-arm on-switching) in which the lower arm is turned on and the remaining two-phase upper arms are turned on. FIG. 9 is a diagram for explaining such a three-arm on-switching. The switching state of each of the U, V, and W phases is set such that the upper arm on (lower arm off) is 1, the lower arm on (upper arm off) is -1, For example, when the upper arm of the U phase is turned on and the lower arms of the remaining V and W phases are turned off, the expression is (1, -1, -1).

When the switching state is (1, -1, -1), (+ E / 2, -E / 2, -E / 2) are applied to the U, V, W phase terminals of the star connection load 20, respectively. ) Is applied. Therefore, the current flowing in the U phase is I, and the voltage at the connection point 21 of each phase (hereinafter, referred to as a common mode voltage) is V.
_Assuming that _c , a current of −I / 2 flows in the V and W phases, and V _c = −
E / 6, and the U, V, W phases is the applied voltage, respectively (2E / 3, -E / 3 , -E / 3) next to the U-phase direction as shown in the voltage vector V ₁ column of FIG. 9 A heading voltage vector is obtained. Therefore, the voltage vector rotates counterclockwise is generated by changing the sequential switching state as shown in V ₆ column from V ₁ column of FIG. 9, for example. Note that the magnitude of the two-phase converted voltage vector and current vector in the case of this star connection load is (2/3)
^1/2 E and (3/2) ^1/2 I.

However, in such three-arm on-switching, the common mode voltage alternately takes a value of ± E / 6 depending on the switching state as shown in FIG. For example, taking the inverter for driving a motor as an example, the following problems have become apparent. That is, high-frequency leakage current due to fluctuations in the common mode voltage caused by increasing the carrier frequency flows between the motor and the inverter through the grounding wire, etc., via the stray capacitance between the motor winding and the frame, and the conductivity and radiation Cause electromagnetic interference (EMI),
Electric corrosion of motor bearings has occurred, and control circuits of inverters have been malfunctioning.

[0007] Therefore, in order to suppress the influence of the fluctuation of the common mode voltage, a passive suppression method of inserting a common mode choke or an LC filter between the inverter and the load, or a negative feedback by detecting the common mode voltage. Various methods such as active cancellation have been proposed.

[0008] For example, FIG. 10 shows a paper D of the Institute of Electrical Engineers of Japan, Vol. 117, No. 5 (1997), pages 565-5, by Ogasawara et al.
"Active cancellation of common-mode voltage generated by voltage-source PWM inverters" published on page 71
Described, detects a common mode voltage V _c via a capacitor C ₁ -C ₃ connected to the output line of the three-phase voltage source inverter 10 in connection diagram showing a common mode noise canceller, the complementary transistor T _r1 this, amplified by the emitter-follower constituting a T _r2, the common mode transformer 40 the output voltage, by negative feedback to the output line, it is canceled AC-variation of the common mode voltage.

[0009]

However, such a conventional suppression method is not sufficient in the passive suppression method using a common mode choke or an LC filter. In this case, there is a problem of resonance, and in the case of cancellation by negative feedback, there is a problem of control delay due to the time constant of the feedback circuit, and in any method, additional components such as filters, chokes, and common mode transformers Need, price side,
There were problems in terms of size, weight and power loss.

The present invention has been made in order to solve such a problem.
By removing the occurrence of the fluctuation of the common mode voltage itself, it is possible to drive the three-phase AC device without causing the problem of high-frequency leakage current and without requiring any additional components as in the conventional example. It is an object of the present invention to provide a control method of a three-phase voltage source inverter which can be performed.

[0011]

In order to solve the above-mentioned problems, a control method of a three-phase voltage source inverter according to the present invention comprises the steps of: setting first, second and third output terminals to the positive side, respectively; If controlled, connect to the positive input terminal,
It has first, second and third switching means which are connected to the negative input terminal when controlled to the negative side and open from the positive and negative input terminals when controlled to neutral, By controlling the first, second, and third switching means, the DC power supply voltage supplied to the positive and negative input terminals is switched to output a three-phase output from the first, second, and third output terminals. A method for controlling a three-phase voltage source inverter that outputs a voltage, wherein the first switching means is set to a positive side, the second switching means is set to neutral, and the third switching means is set to a negative side; Controlling the first switching means to neutral, the second switching means to the positive side, and the third switching means to the negative side;
Controlling the first switching means to the negative side, the second switching means to the positive side, the third switching means to the neutral side, and the first switching means to the negative side;
Controlling the second switching means to be neutral and the third switching means to be positive; and setting the first switching means to neutral, the second switching means to be negative,
Controlling the switching means to the positive side;
The second switching means on the positive side, the second switching means on the negative side, and the third switching means on the neutral side, or the first, second and third switching steps. The method is characterized in that the method includes a step of sequentially repeating the whole arm-off step of controlling all of the means to neutral.

In this case, it is preferable that when switching between the two arm on stages, the all arm off stage be transited transiently.

Alternatively, the first, second and third switching means may connect the respective output terminals of the first, second and third output terminals to the positive / negative when each of them is controlled to be neutral. It is preferable to have a function of connecting to the neutral point of the DC power supply voltage input to the positive and negative input terminals by opening the input terminal of the DC power supply.

The PWM of the three-phase voltage source inverter
When performing (pulse width modulation) control, a step of generating first, second and third variable amplitude signal waves having phases different from each other by 120 ° from the carrier wave; The first, second, and third signals each have a logic 1 during a period in which the potential of each AC component of the variable amplitude signal wave No. 3 is equal to or higher than the potential of the AC component of the carrier wave, and have a logic -1 during the other periods. Generating an intermediate control signal;
A first control signal having the same logic as the first intermediate control signal during a period in which the logics of the first and second intermediate control signals are equal, and a second control signal having the same logic as the first intermediate control signal in other periods; 3
During the period when the logics of the intermediate control signals are equal to 0,
In other periods, the second control signal having the same logic as the second intermediate control signal and the logic of the third and first intermediate control signals are equal to logic 0 during the same period, and in other periods, Generating a third control signal having the same logic as the third intermediate control signal; and wherein the first, second, and third control signals each have a logic 1, and Each of the second and third switching means is controlled to be positive, and the first, second, and third switching means are controlled during a period in which each of the first, second, and third control signals is logic 0. Each of the first, second, and third switching means is controlled to be negative while each of the first, second, and third control signals is at logic-1. And a step of controlling.

In this case, the first, second, and third control signals are transiently set to logic 0 even when the respective logics of the third, first, and second intermediate control signals change. Preferably, it is controlled.

Alternatively, the first, second and third switching means may connect the respective output terminals of the first, second and third output terminals to the positive / negative when each of them is neutrally controlled. It is preferable to have a function of connecting to the neutral point of the DC power supply voltage input to the positive and negative input terminals by opening the input terminal of the DC power supply.

In any case, it is preferable that a delta connection load is connected to the first, second, and third output terminals.

Still further, the first, second and third output terminals are connected to a neutral point of a DC power supply voltage input to the positive and negative input terminals via capacitors having the same capacitance. preferable.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. As described above, the three-phase AC device driving apparatus and the driving method according to the present invention are more preferably delta-connected loads. First, an embodiment will be described using a delta-connected load as an example.

FIG. 2 is a wiring diagram showing a three-phase voltage source inverter driving a delta connection load.
8 is the same as FIG. 8 except that 0 is replaced by the delta connection load 30, and the duplicated description is omitted.

In the present invention, instead of the three-arm on-switching described above, the upper arm of one of the U, V, and W phases is turned on, the lower arm of the other phase is turned on, and the remaining arm is turned on. For a phase, a three-phase alternating current is generated and generated by alternately repeating switching control (hereinafter, referred to as two-arm on-switching) for turning off both the upper arm and the lower arm.

FIG. 1 is a diagram showing an example of such two-arm on-switching. For example, the switching state (1, 0, -1) in the column of the voltage vector V ₁ ′ corresponds to the U-phase of the three-phase voltage source inverter 10. Turn on upper arm 11P, lower arm 1
1N off, V-phase upper and lower arms 12P and 12N off,
This indicates that the upper arm 13P of the W phase is turned off and the lower arm 13N is turned on. In this case, the WU connection of the delta connection load 30 is a phase, the UV connection is b phase, and the VW connection is c.
Assuming the phases, a voltage of -E is applied to the a phase, and a voltage of E / 2 is applied to the b and c phases. Therefore, assuming that the current flowing through the U-phase of the three-phase voltage source inverter 10 is I, a,
Currents of -2I / 3, I / 3, and I / 3 flow in the respective phases b and c, and a voltage vector in the -WU direction, that is, the direction from the W phase to the U phase can be obtained as shown in FIG. it can.

Therefore, for example, by changing the switching state sequentially as shown in the columns V ₁ ′ to V ₆ ′ in FIG. 1, the voltage vector rotating counterclockwise in the case of the star connection load 20 as in FIG. Can be generated. The magnitudes of the two-phase converted voltage vector and current vector in the case of this delta connection load are (3/2)
^1/2 E and (2/3) ^1/2 I.

That is, if the impedance of each phase of the load is the same, in the case of the delta connection compared to the star connection load, the voltage of the DC power supply is set to 2/3 and the current capacity of the three-phase voltage source inverter 10 is set to 3 By setting / 2, the same output can be obtained. Conversely, if the output voltage and current capacity of the three-phase voltage source inverter 10 are the same, for example, the conductor cross-sectional area of each phase winding of the motor is 2/3 and the number of turns is 3/2.
The same output can be obtained by setting the load impedance of each phase to 9/4 of the star connection load.

Further, in such a two-arm on-switching, one of the U, V, and W phases has E / 2 and the other has one.
Since the voltage of -E / 2 is uniformly applied as a whole, the common mode voltage which is the voltage at the virtual neutral point of the delta connection load 30 in the steady state of each switching state is always constant as shown in FIG. Since the potential becomes zero, no high-frequency current leaking out of the load is generated.

When the delta-connection load is an inductive load such as a motor, a transient current flows via flywheel diodes provided in each switching element of the three-phase voltage source inverter 10 when switching the switching state. Figure 3 is a schematic diagram showing a transient current flowing through the load phase when switching 'from the switching state of the field V _2' V ₁ in FIG. 1 to the switching state of the field, V ₁ shown in FIG. 3 (a) 'column Is in the switching state, the U-phase upper arm 11P is turned off, so that the load current flows through the flywheel diode of the switching element of the U-phase lower arm 11N, and the U-phase is connected to the negative power supply. Becomes Therefore, the potential of each phase of the load immediately after the switching is V-phase as shown in FIG.
/ 2, form U-phase and W-phase are both connected to the negative power source potential -E / 2, i.e. the common mode voltage of -E / 6 becomes switching state equivalent to V ₃ column in FIG 9 is generated. As described above, when the switching state is directly switched between the columns V ₁ ′ to V ₆ ′, the common mode voltage fluctuates due to the same transient current.

However, two-arm on-switching
In, for example, the voltage vector V ₁’-V₆’Between
V when switching₀’Column (all arms off)
The common mode caused by this transient current
It is also possible to prevent the fluctuation of the load voltage. FIG. 4 shows FIG.
V₁’Column from the switching state₀’Column (All Arms
F) flows through each phase of the load when switching to the switching state
FIG. 4A is a schematic diagram showing a transient current, and FIG.₁’Column
When the load current in the switching state is
1P and 13N switching elements are turned off
Switches of arms 11N and 13P in a complementary relationship
Flowing through the flywheel diode of the switching element
However, the potential of each phase of the load immediately after switching is as shown in FIG.
The UW2 phase is connected to the opposite potential, that is, in FIG.
V_Four”Column and the common mode
The voltage is also maintained at zero potential. V_Two’Column to V_Five'Switch
V from the ching state₀’Column switching status (all arms
The same applies to the switching to off).

Therefore, when driving an inductive load such as a motor in the two-arm on-switching, all the arms are temporarily turned off, that is, each of the V ₁ ′ to V ₆ ′ columns is switched via the switching state in the V ₀ ′ column of FIG. It is desirable that the switching state be controlled so as to prevent the fluctuation of the common mode voltage due to the transient current.

Further, as the three-phase voltage source inverter 10,
When the upper and lower arms of each phase are in the off state, for example, by using a neutral point clamp voltage type inverter, they are connected to neutral point potential nodes connected to the respective positive and negative power supplies via a sufficiently large capacitance element. By controlling, the three-phase voltage source inverter 10 is controlled such that two of the phases are always connected to the positive / negative power supply and the other phase is connected to the neutral potential node, including the transient state, and the common mode potential is controlled. It may be maintained at zero potential.

As described above, by controlling the three-phase voltage inverter by the two-arm on-switching of the present invention, the specifications of the inverter or the load are only slightly changed, and the filters, chokes, and common elements as in the conventional example are changed. It is possible to eliminate the trouble of the high-frequency leakage current caused by the fluctuation of the common mode voltage without adding a mode transformer or the like and without causing power loss.

Further, in the three-arm ON switching shown in FIG. 9, for example, in order to change the voltage vector from V ₁ to V ₂ , all the switches of each switch are switched so that the V phase is switched from the lower arm ON to the upper arm ON. At
It is necessary to switch on and off the upper and lower arms at the same time. For example, a short-circuit prevention period is required to prevent both the upper and lower arms from being turned on due to a variation in control signal delay. However, in the two-arm ON switching shown in FIG. 1, since all switches are controlled to be in the upper arm ON or the lower arm ON via the OFF state, such on-delay compensation is required. With the advantages that do not require.

When PWM (pulse width modulation) control of a load is performed using a three-phase voltage source inverter, it is necessary to generate a zero voltage vector without driving the load. It switched had turned on the arm or the lower arm on the Zenso as of the voltage vector V ₇ or V ₀ column in FIG. 9, for example. In the two-arm on-switching, all the arms are turned off as shown in the voltage vector V ₀ ′ column of FIG. In this case, if a three-phase voltage-type inverter whose arm is connected to the neutral point voltage is not used as in the case of the neutral point-clamped voltage-type inverter described above, all phases of the load are cut off from the power supply and Although the mode voltage may be unstable, a neutral point is created by combining impedance elements C _{1 to} C ₃ on the output side of the three-phase voltage source inverter 10 as shown in FIG. , The load potential of all arms off can be stabilized.

Although the embodiment in which the two-arm on-switching of the present invention is applied to a delta-connected load has been described above, the present invention is not limited to the delta-connected load. There are differences such as the point that the voltage waveform applied to each phase becomes a simple rectangular wave by applying 2-arm on-switching, but the high-frequency leakage current due to the fluctuation of the common mode voltage is the same as in the case of the delta connection load. Inverter control that does not occur can be performed.

FIG. 6 shows the switching state (1,
FIG. 7 is a conceptual diagram showing a voltage vector at the time of a star connection load corresponding to (0, -1). As shown in FIG. 6, by performing the same switching as in FIG.
As V, a rotation voltage vector different from the delta connection load by 30 ° can be obtained. In this case, the magnitudes of the two-phase converted voltage vector and the current vector are respectively (1/1 /
2) ^1/2 E and 2 ^1/2 I

Hereinafter, a specific example of the PWM control by the three-phase voltage source inverter using the two-arm on-switching of the above embodiment will be described with reference to the waveform diagram of FIG. In this waveform diagram, the expression of the zero voltage vector V ₀ ′ as a measure against the transient current is omitted. One of the commonly used methods of generating a PWM switching signal of a three-phase voltage-source inverter by three-arm on-switching has a triangular carrier and a cycle three times the triangular carrier shown in phases a to c in FIG. There is a method using three sinusoidal signal waves having phases different from each other by 120 °.

For example, if the a-phase sine wave signal ≧ triangular carrier, then a _s = 1 if the a-phase sine wave signal <triangular carrier, then the control signal a for each phase is set such that a _s = −1. _s , b _s , and c _s are generated, and using these control signals a _s , b _s , and c _s , when each is 1, the upper arm of each of the U, V, and W phases is turned on. By turning on the lower arm of each of the U, V, and W phases in the case of 1, as shown in FIG.
And outputs a ₀ ~V _7. By controlling the amplitude of the sine wave signal wave in this manner, the pulse widths of the zero voltage vectors V ₀ and V ₇ are modulated to control the output of the three-phase voltage source inverter.

When PWM control is performed by the two-arm on-switching of the embodiment shown in FIG.
_s, v _s, the w _s, for example, as shown in FIG. 7, if a _s = b _s, if _{u s = 0 a s ≠ b} s, generated for each phase such that u _s = a _s, By using the control signals u _s , v _s , and w _s of each phase, U, V,
W Turn on the upper arm of each phase, and
By turning on the lower arm of each phase of U, V, W in the case of 1, and turning off the upper and lower arms of each phase of U, V, W in the case of each 0, as shown in FIG. The power vectors V ₀ ′ to V ₆ ′ of the two-arm ON switching shown in FIG.

The generation of a PWM control signal by two-arm on-switching using a variable amplitude sine wave signal wave and a triangular wave carrier having a frequency three times that of the variable amplitude sine wave signal wave has been described above. , Or when switching with a carrier having a higher frequency ratio, a PWM control signal by two-arm on-switching can be generated in exactly the same manner.

[0039]

As described above, by controlling the three-phase voltage source inverter by the two-arm on-switching according to the present invention, the fluctuation itself of the common mode voltage of the load can be suppressed. In addition, there is no need for additional components used for suppressing high-frequency leakage current in the case of conventional three-arm on-switching, such as a common mode transformer, and there is no accompanying power loss. It is possible to prevent a current fault from occurring.

Further, according to the two-arm on-switching of the present invention, it is not necessary to set the short-circuit prevention period of the upper and lower arms in the case of the conventional three-arm on-switching, and to eliminate the on-delay compensation for compensating for the output distortion. In particular, with respect to the delta connection load, PWM control with a simpler and more stable and high quality waveform can be performed as compared with the conventional three-arm ON switching of the star connection load.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a voltage vector and a switching state of two-arm ON switching according to the present invention.

FIG. 2 is a connection diagram showing a three-phase voltage source inverter driving a delta connection load.

FIG. 3 is a schematic diagram showing a transient current flowing through each phase of a load when switching from the switching state in the V ₁ ′ column to the switching state in the V ₂ ′ column in FIG. 1;

FIG. 4 is a schematic diagram showing a transient current flowing through each phase of a load when switching from the switching state in the V ₁ ′ column to the switching state in the V ₀ ′ column in FIG. 1;

FIG. 5 is a connection diagram illustrating an example of a load potential stabilization circuit.

FIG. 6 is a conceptual diagram showing a voltage vector at the time of a star connection load corresponding to the switching state (1, 0, −1) of FIG. 1;

FIG. 7 is a waveform diagram illustrating an embodiment of PWM control by a three-phase voltage source inverter using two-arm ON switching.

FIG. 8 is a connection diagram showing a three-phase voltage source inverter driving a star connection load.

FIG. 9 is a diagram illustrating a relationship between a voltage vector of three-arm ON switching and a switching state.

FIG. 10 is a connection diagram showing a common mode noise canceller according to the prior art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Three-phase voltage type inverter 11, 12, 13 Output terminal 11P, 11N, 12P, 12N, 13P, 13N Switching element 20 Star connection load 21 Connection point 30 Delta connection load 40 Common mode transformer

Claims (8)

[Claims] Each of the first, second, and third output terminals is connected to a positive input terminal when controlled to a positive side, and connected to a negative input when controlled to a negative side. A method of controlling a three-phase voltage source inverter having first, second and third switching means connected to an input terminal and opened from the positive and negative input terminals when controlled neutrally, wherein the first switching Controlling the means to the positive side, setting the second switching means to neutral, and setting the third switching means to negative; and setting the first switching means to neutral, the second switching means to the positive side, Controlling the third switching means to the negative side, controlling the first switching means to the negative side, the second switching means to the positive side, and the third switching means to the neutral side; Switching Controlling the stage to be negative, the second switching means to be neutral, and the third switching means to be positive; the first switching means being neutral, the second switching means being negative, A two-arm on state comprising the steps of: controlling the third switching means to the positive side; controlling the first switching means to the positive side, the second switching means to the negative side, and the third switching means to the neutral side. A method of controlling a three-phase voltage-source inverter, comprising a step of repeating the steps sequentially or sequentially with an all-arms-off step of controlling all of the first, second and third switching means to neutral. . 2. The method for controlling a three-phase voltage source inverter according to claim 1, wherein the switching of each of the two-arm on stages is performed by transiently passing through the all-arms off stage. 3. The first, second, and third switching means, when each of them is controlled to be neutral, sets the respective one of the first, second, and third output terminals to the positive or negative. 3. The control method for a three-phase voltage source inverter according to claim 1, further comprising a function of opening the input terminal of the inverter and connecting to a neutral point of the DC power supply voltage input to the positive and negative input terminals. 4. Each of the first, second and third output terminals is connected to a positive input terminal when controlled to the positive side, and connected to a negative input when controlled to the negative side. In a PWM (pulse width modulation) control method for a three-phase voltage source inverter having first, second and third switching means connected to an input terminal and opened from the positive and negative input terminals when controlled neutrally. , The first and second carriers, each having a phase difference of 120 °
Generating a third variable amplitude signal wave and a third variable amplitude signal wave, wherein the potential of the AC component of each of the first, second and third variable amplitude signal waves is higher than the potential of the AC component of the carrier wave. Generating first, second, and third intermediate control signals each having a logic -1 during the other period, and logic 0 during a period when the logic of the first and second intermediate control signals is equal. In the other periods, the first control signal having the same logic as the first intermediate control signal, and the second and third control signals
During the period when the logics of the intermediate control signals are equal to 0,
In other periods, the second control signal having the same logic as the second intermediate control signal and the logic of the third and first intermediate control signals are equal to logic 0 during the same period. Generating a third control signal having the same logic as the third intermediate control signal; and wherein each of the first, second, and third control signals has a logic 1, Each of the second and third switching means is controlled to be positive, and the first, second, and third switching means are controlled during a period in which each of the first, second, and third control signals is logic 0. Each of the first, second, and third switching means is set to a negative side while each of the first, second, and third control signals is at logic-1. Controlling the three-phase voltage source. How to control the barter. 5. The control so that the first, second, and third control signals transiently become logic 0 even when the respective logics of the third, first, and second intermediate control signals change. 5. The method according to claim 4, wherein the control is performed. 6. The first, second, and third switching means, when each of them is controlled to be neutral, sets the respective one of the first, second, and third output terminals to the positive or negative. 5. The control method for a three-phase voltage source inverter according to claim 4, further comprising a function of opening the input terminal of the inverter and connecting to a neutral point of the DC power supply voltage input to the positive and negative input terminals. 7. The control method for a three-phase voltage source inverter according to claim 1, wherein a delta connection load is connected to the first, second, and third output terminals. . 8. The first, second, and third output terminals are connected to a neutral point of a DC power supply voltage input to the positive and negative input terminals via capacitors having the same capacitance. The method for controlling a three-phase voltage source inverter according to claim 2 or 5, wherein