JPH03178564A - Inverter unit - Google Patents

Abstract

PURPOSE:To obtain an economical inverter unit having high performance by providing two DC voltage sources, inverters and output transformers, operating them in a direct coupling inverter in a low frequency range, and operating them as multiplex inverters in a high frequency range. CONSTITUTION:DC voltage sources Vd1, Vd2 connected in series are connected to a load unit LOAD through an inverter unit having inverters INV-1, INV-2, switches SW1-SW5 and output transformers TR1, TR2. If an output frequency is low, the switches SW1-SW4 are opened, and the switch SW5 is closed. That is, power is supplied directly from the inverter to the unit LOAD. If the output frequency becomes a predetermined value or higher, the switch SW5 is opened, and the switches SW1-SW4 are closed. That is, the power is supplied to the unit LOAD through the transformers TR1, TR2.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a large-capacity inverter device that supplies variable voltage and variable frequency power to a load such as an AC motor.

(Prior Art) In recent years, large-capacity self-extinguishing elements (eg, gate turn-off thyristors, etc.) have been actively developed and are being used in power conversion devices such as inverters. especially,
2. Description of the Related Art Pulse width modulation controlled inverters (referred to as PWM inverters) have been widely used as driving power sources for AC motors because they can provide a sine wave output with variable voltage and variable frequency.

Furthermore, as the capacity of AC motors increases, inverters with high voltage and large current are required, and high-capacity inverters with series-parallel connections of elements or large-capacity inverters with multiple connections using output transformers have been proposed.

FIG. 10 shows the configuration of a conventional inverter device, in which multiple connections are made using output transformers.

In the figure, Vd is a DC voltage source, INV-1 and NV-2 are the first and second inverters with output transformers, and TRU□
.

TRU2 is an output transformer, INV-3 is a direct inverter without an output transformer, LUI RIJ? VCU indicates the inductance, resistance and back emf of the load, respectively.

The figure shows one output phase (U phase), and the U phase of the first inverter INV-1 includes self-extinguishing elements (such as GTO) 81□~SL4 and freewheel diodes D1□~D. It is composed of □. The (2) phase and the W phase are similarly constructed. The third inverter (direct-coupled inverter) INV-3 is connected in a three-phase bridge, and the U phase component is composed of self-extinguishing elements S3□, S32 and a freewheel diode 0311032.

A neutral point N of the load is connected to a midpoint Nd of the DC voltage Vd. That is, Vdx = Vdz = Vd/2
It becomes.

If the load is a variable speed motor, variable voltage, variable frequency power must be output from the converter.

If the output frequency is zero, no voltage can be obtained from an inverter with an output transformer. Therefore, a voltage is output from the direct-coupled inverter INV-3 to supply the voltage drop due to the load resistance.

When the output frequency fo becomes larger than a certain value fmin, the inverters with output transformers INV-1 and INV-2
A voltage Vs>r VCl2 is generated from. This voltage V
υ1r VU2 is increased or decreased approximately in proportion to the output frequency fO to prevent the iron core of the output transformer from being saturated.

The capacity of the above conventional inverter device can be increased by increasing the number of inverters with output transformers, and by controlling the inverters with multiplexed pulse width modulation, a sine wave output with few harmonics can be obtained. There is a characteristic that

(Problems to be Solved by the Invention) However, this conventional inverter device has the following problems. In other words, inverter INV- with output transformer
1, INV-2 has a single-phase component and is connected in a full bridge with four elements, whereas the direct-coupled inverter INV-3 has a single-phase component and has two elements in a half-bridge connection. Therefore, the output voltage that can be generated from the direct-coupled inverter INV-3 is
) is half of the voltage generated from

In addition, the output voltage of an inverter with an output transformer can be freely selected by providing multiple stages, but the output voltage of a direct-coupled inverter is uniquely determined by the voltage of the DC power supply.

This direct-coupled inverter needs to generate at least a sum voltage of the resistance drop of the AC load, the speed electromotive force of the motor up to the lowest frequency at which the output transformer can operate, and the inductance drop of the load.

Therefore, when the above-mentioned minimum frequency increases or when the resistance drop of the load increases, the output voltage borne by the direct-coupled inverter also increases, and the DC voltage must be increased.

On the contrary, the DC voltage of a multiplex inverter having output transformers becomes lower as the number of multiplexed inverters increases.

As a result, it becomes difficult to share a DC power supply, and the DC power supply for the direct-coupled inverter and the DC power supply for the multiplex inverters must be separated. Therefore, the main circuit configuration becomes complicated, resulting in an uneconomical system.

In addition, transformer-equipped inverters can increase the control frequency of the output voltage by performing multiplexed PWM control, whereas half-bridge-connected direct-coupled inverters generate multiple voltages that are different from those of transformer-equipped inverters. The control frequency of its output voltage matches the carrier frequency of PWM control and remains low. Therefore, the voltage applied to the load remains a pulsating portion of the output voltage of the direct-coupled inverter, causing an increase in load current ripple.

In order to eliminate this drawback, there is a method of cutting off the direct-coupled inverter when the output frequency becomes high and performing multiplex operation using only the inverter with a transformer, but in that case, the direct-coupled inverter is operated only in the low frequency range and can handle more voltage. This means that the required high frequency range will not be effectively utilized.

The present invention has been made in view of the above problems, and can operate as a direct-coupled inverter that can obtain a higher output voltage in the low frequency region, and as a multiplex inverter that can obtain an output voltage with less pulsation in the high frequency region. The purpose is to provide a high-performance and economical inverter device.

[Structure of the invention]

(Means for Solving the Problems) In order to achieve the above object, the present invention comprises two DC voltage sources connected in series, and a first and second single-phase full bridge connected in series on the DC side. a wired voltage type inverter circuit, a first switch that opens and closes an intermediate point between the DC voltage source and an intermediate point on the DC side of the two inverter circuits;
a second output transformer connected to the output terminal of the inverter circuit via a second switch; and a second output transformer connected to the output terminal of the second inverter circuit via a third switch. An output transformer, an alternating current load, and an output frequency of the first and second output transformers with the first to third switches closed in an operating range where the output frequency for the alternating current load is higher than a certain set value. a fourth switch that is turned on so that the sum of the output voltages is applied; and a fourth switch that is turned on so that the sum of the output voltages is applied;
a fifth switch that is turned on so that an output voltage between the midpoint of the DC voltage source and the DC midpoint of the two inverter circuits is applied when the switch is opened. ing.

(Function) A DC power source is generated by an AC/DC converter (for example, a PWM control converter) and a DC smoothing capacitor connected to its DC side terminal. Two of these DC power supplies are prepared, and each DC power supply is connected to the first and second single-phase full bridge connected PWM with output voltage transformers via the first switch.
Connect the inverter.

In an operating region where the output frequency is low, one half of the two inverters is used, the first to fourth switches are opened, and the inverter operates as a half-bridge inverter connected to the entire DC power supply. The half-bridge inverter is directly connected to a load device via a fifth switch. Therefore, the output voltage that can be generated from this direct-coupled inverter is twice that of the conventional device, making it possible to increase the lowest frequency at which switching to the inverter with a transformer is required, and reducing the number of output transformers.

In an operating region where the output frequency is high, the fifth switch is released and the first to fourth switches are closed to operate as two full-bridge connected inverters with transformers. The secondary windings of the two output transformers are connected in series, and the sum voltage is applied to the load device via the fourth switch. At this time, the two inverters are controlled to be multiplexed by shifting the phase of the PWM-controlled carrier wave signal, and the voltage applied to the load becomes a voltage with few high frequency components.

The present invention further includes a first freewheeling diode connected between one side output side terminal of the first inverter circuit and an intermediate point of the DC voltage source, and a first freewheeling diode connected between one side output side terminal of the first inverter circuit and a midpoint of the DC voltage source; A second freewheeling diode connected between an output side terminal and an intermediate point of the DC voltage source is provided, and when the half-bridge inverter is operated as a direct-coupled inverter, the half-bridge inverter is operated as a neutral point clamp type inverter. I also suggest. That is, when the output frequency is low, the first to fourth switches are released, the fifth switch is closed, and of the four elements that make up the half bridge, the upper two, the middle two, and the lower PWM control is performed in three modes on the two sides to generate three levels of output voltage: positive voltage, zero voltage, and negative voltage. This reduces the pulsation of the output voltage of the direct-coupled inverter by half, and has the effect of reducing load current ripple during low frequency operation. The above two diodes are
When in zero voltage mode, it functions to circulate the load current.

In this way, it is possible to provide a high-performance and economical inverter device that can operate as a direct-coupled inverter that provides a higher output voltage in the low frequency range and as a multiplex inverter that provides an output voltage with less pulsation in the high frequency range.

(Embodiment) FIG. 1 is a configuration diagram showing an embodiment of an inverter device of the present invention. In the figure, Vdz is a DC voltage source, I
NV-4 and INV-2 are first and second inverter circuits, SW to SW are switches, TR is an output transformer, and LOAD is a load device. Inverter circuit I
NV-1 has a single-phase full bridge connection, and is equipped with a self-extinguishing element (e.g., gate turn-off thyristor; GT○
)31□~S work, and freewheeling diode D work□
- Consists of DL4. Inverter circuit INV-2
The two inverters INV-1 and INV-2 are connected in series on the DC side, and the intermediate point d is connected to the intermediate point 0 of the DC power supply through the first switch 51110. connected.

The output terminal of the first inverter INV-1 is connected to the first output transformer via the second switch Sv2.

Further, the output terminal of the second inverter INV-2 is connected to the third inverter INV-2.
The output transformer is connected to the second output transformer via the switch S. The secondary windings of the first and second output transformers TR1, TR are connected in series and connected to the load LOAD via a fourth switch.

On the other hand, the intermediate point d on the DC side of the two inverters INV-1 and INV-2 and the intermediate point C of the DC power supply are connected to the switch Sv
, are connected to both ends of the load LOAD.

When operating this inverter device with VVVF (variable voltage variable frequency), when the output frequency is low, the transformer TR
1. It is difficult to obtain voltage from TR, so switch S
W□~SW is released, the switch sw is closed, and power is directly supplied from the inverter to the load LOAD.

When the output frequency exceeds a certain frequency, the switch Sv5 is released, and the switches SW, ~5IA1. is applied to the load LOA via the output transformers TR1 and TR2.
Supplying Power to D6 First, the operation of directly supplying power to the load LOAD without an output transformer will be explained.

The switches Sv1 to Sv4 are opened and only the switch Sli is closed. In addition, inverter circuits INV-1, INV-
2 half elements 5i31S14 and 5231524
is gated off, and the other half of the elements S1□, S
12 and S21°S2□ are operated to directly supply power to the load LOAD via the switch SW.

FIG. 2A shows a main circuit connection diagram during low frequency operation, and FIG. 2B shows a control circuit block diagram. In the figure, CT.

is a current detector, C is a comparator, GL(S) is a current control compensation circuit, and PWMD is a pulse width modulation control circuit. Other symbols follow the construction drawings, and switches are omitted.

FIG. 3 shows a time chart for explaining the PWM control operation of the circuit of FIG. 2. The PWM control carrier wave signal xD and the input signal eD are compared to generate a gate signal go.

That is, when eD≧X, go=1, and the element sti+S
12 is on, element S2□+S, □ is off, when eo<Xo, gD=O, and element 5ill s
xz is off, and element S2□1322 is on. At this time, the output voltage vL of the inverter is VL=+Vd when the elements Si□ and S2 are on, VL=+Vd and the element sx
When x+szz is off, VL=Vda.

As a result, the voltage vL applied to the load LOAD is
It will look like the bottom row of the figure. The average value vL is proportional to the input signal eD.

The load current ■ is controlled as follows.

That is, load current 1. is detected by a current detector CT and inputted to a comparator C. In the comparator CL, the current command value ■L* and the current detected value 1. Compare and get the deviation ε=
1. ” -IL is determined. The deviation eL is amplified by the next current control compensation circuit GL(S) and is used as the input signal el) of the PWM control circuit P needle.

■If the machining process becomes rough, the deviation ε will be a positive value,
By increasing the input signal eD of the PWM control circuit PwMD, that is, the output voltage vL of the inverter, the load current 1. is controlled so that IL*=I. On the contrary, IL*
<IL, the deviation εL becomes a negative value, and the PWM
The input signal e[) of the control circuit puMo, that is, the output voltage vL of the inverter, is reduced to reduce the load current. After all, it settles down to IL””IL.

In this way, when the output frequency is low, the inverter device shown in FIG. 1 can perform half-bridge connected PWM operation overnight by utilizing the full voltage of the DC power source. As a result, the output voltage can be increased to twice that of conventional devices, and operation up to a higher frequency can be achieved as a direct-coupled inverter.

Next, the operation of the apparatus shown in FIG. 1 when the output frequency becomes high and is operated as an inverter with a transformer will be explained.

Figure 4A shows the device shown in Figure 1 releasing the switch SWs.
A main circuit connection diagram is shown when the switches SW1 to Sv4 are turned on, and FIG. 2B is a block diagram of the control circuit. However, the switch has been saved.

In the figure, CT, CT, 0 is the current detector, C is the current detector, C2 is the comparator, Got (S) r Goa
(S) tGL(S) is a current control compensation circuit, A1.
A2 is an adder, PIIM, and P2H4 is a pulse width modulation control circuit.

The ↓-th inverter INV-1 uses the DC power source Va as a voltage source and outputs variable voltage variable frequency AC power via the transformer TR. Similarly, the second inverter IN
V-2 uses the DC power supply Vdz as a voltage source and outputs variable voltage variable frequency AC power via the transformer TR2.

FIG. 5 shows a time chart diagram for explaining the PWM control operation of the circuit of FIG. 4.

The PWM control circuit PWM□ receives its input signal e□ and the carrier wave signal X1. Compare Y(inverted value of x4) and generate gate signal g(2g) for inverter INV-1. That is, when e1≧X, g=↓, element S1□: on, S
Work 2: Off e□〈When xl, g□=0, element S work□
: Off, S1 is turned on. Further, when e1≧Y4, g%=1 and element S1. : Off, S
14: On When e1<Yl, g□'=o, element S,
, : on, S14: off.

The output voltage V□ of inverter INV-1 is the transformer TRI
When the primary and 72nd turns ratio of is 1, When elements S and S are on, ■Yo = 10vd, When elements SLZ and SXa are on, VL = + Vdt In other modes , V,=O. The average value Q1 (indicated by a broken line) is a value proportional to the aforementioned control input signal e.

Similarly, in the PWM control circuit PWM2, its input signal e2
is compared with carrier wave signal X, (phase shifted by 90 degrees with respect to X□), and y2 (inverted value of x2) to generate gate signal fh+gz' for inverter INV-2. That is, when e2≧×2, ga”1, element S, □: on, S
22: Off e2(X, when 1g2=Q, element S8,
: Off, S2 is turned on. Also, when e2≧Y2, gz'=1, element 523=off,
S24: On When e2<Y2, gz'=o, element S
23: on, 524=off.

The output voltage v2 of the inverter INV-1 is the transformer TR2
When the primary/secondary turns ratio of elements Ski and S2. When is on, V2=+Vd2 When elements S22 and 323 are on, Vz=Vdz and other modes, V2=O. The average value V2 (indicated by a broken line) is a value proportional to the aforementioned control input signal e2.

The sum of the output voltages of the transformer TR and TR2 is applied to the load LOAD, and the voltage at the bottom of FIG. 5 becomes vL=v□+v2.

In FIG. 4, the load current is controlled as follows.

First, the load current is detected by the current detector CT,
Input to comparator C. Comparator CL compares the load current command value IL East and the above detected value IL, and calculates the deviation εL =
IL' Find IL. The deviation eL is amplified by the next load current control compensation circuit GL (S) and sent to each adder.

It is input to the PWM control circuits PWM and PWMz via A2.

When IL*>IL, the deviation ε becomes a positive value,
Input signal e□ of PWM control circuit PIIM and PWM
Increase le2. Therefore, inverter INV-1,
INV-2 output voltage V0. V2 increases, causing the load current to increase.

Therefore, finally 1. It is controlled so that *=IL.

Conversely, when I*<I, the deviation ε becomes a positive value, and the PWM control circuit PIIM1. Decrease the input signal eIJ02 of Pli'M2. As a result, the output voltage of the inverter, V, = V, 10 V, decreases, and the load current, E, decreases. Finally, IL*=1. I yell and calm down.

In addition, Fig. 4 shows the inverter INV-1, INV
-2 controls the excitation current of each output transformer TR. That is, the excitation current of TRi ■. , is controlled as follows. However, to simplify the explanation, the primary/secondary turns ratio of the transformers TR□ and TR is assumed to be 1:1.

The current detector CT detects the primary current of the transformer TR, and calculates the difference between the detected value of the load 'r'Il current and the detected value of the exciting current. Let x=I, IL. The excitation current command value ■ is determined by the comparator C□. 1* and the above detection value. 1 and the deviation ε. □=Eng. --I. Find □.

The deviation ε. , is the following excitation current control compensation circuit G. , (S
) and input to the PWM control circuit P111M via the adder A□.

■. ,′〉■. If it becomes 1, the deviation ε. , becomes a positive value and increases the input signal e of PWM. Therefore, the output voltage of inverter INV-1 increases and transformer TR
□ Exciting electrician. , increase ■. ,=I. □It is controlled so that it becomes K.

On the contrary, ■. ,'< 工. , then the deviation ε. □ takes a negative value and reduces the input signal e of the PWM. Therefore, the output voltage of the inverter INV-1 decreases, and the exciting current of the transformer TR decreases. , and again ■. ,=
Engineering. , controlled to become a tree.

Excitation current engineer for transformer TR. Similarly, □ also has its command value 1
,,' is controlled to match.

At this time, if the ratings of the two transformers TR□ and TR2 are the same, the command value of the excitation current. 1*.

I02* is the setting value of the voltage to be applied to the load LOAD, L* is the transformer TR, the mutual inductance of TR is M, and the output angular frequency is ω. When closed, it is given as the following equation.

■. 1*=Eng. I=vL*/(jω.M) In this way, the exciting current I of the output transformers TR1, TR. i+■
○ is the corresponding command value ■. 1*, ■. It is controlled to always match 2*. Therefore, even if a DC bias voltage is applied to the transformer TR 2 for some reason, the bias voltage will be automatically corrected and no DC bias will occur.

FIG. 6 shows a configuration of another embodiment of the device of the present invention, and compared with FIG. 1, a freewheeling diode Dt+D2 is added.

When the output frequency increases and power is supplied to the load via the output transformers TR1 and TR2, the operation is the same as that of the device shown in the upper figure.

When the output frequency is low, the switches S~51N4 are released and the switches S~51N4 are closed. In addition, the inverter INV-
1.

Element S work on one half of INV-2 31 Si2 and 52
3T S24 is gated off.

FIG. 7A shows a main circuit connection diagram during low frequency operation, and FIG. 7B shows a control circuit block diagram. In this circuit, four self-extinguishing elements S11. S-engineer 2. S2□ and S22 are divided into three modes, with two pairs each.

That is, Mode 1: When S□ and SiZ are on (S21. S2□ are off), the output voltage vL = 10vd□ Mode 2: When S12 and S2□ are on (S1t and Szz are off), the output Voltage V, = O Mode 3: When S21 and S22 are on (S, , S, , are off), the output voltage is VL'' Vdz. Voltage output can now be obtained, and the load current can be precisely controlled.

FIG. 8 shows a time chart for explaining the PWM control operation of the circuit shown in FIG.

Input signal 8N of PWM control circuit PWMN and carrier wave signal X
N, YN (XN(7) inverted value), and
-5at gate signal gN□ and element S□2. Create a gate signal gNi for S2□. That is, when eN >
S1□: ON, S2□=ON Also, (when IN<XN and eN<YN, gN2
= 1, S□2: Off, S2□: On 8N > XN or eN > YN, gN
When z=o, S1□: on, S2□: off, and the output voltage ■L has the waveform shown in the bottom row of FIG.

Its average value V, (indicated by a broken line) is proportional to the input signal eN.

In this way, according to the circuit shown in FIG. 7, the control frequency of the output voltage V□ is the same as the carrier frequency of PWM control, but
When the input signal eN is positive, 1L is PWM-controlled with either -Vd□ or O, and when the input signal eN is negative, 2L is controlled with either -Vdz or O. As a result, harmonic components of the output voltage are reduced, and load current ripple is also reduced.

Note that the operation of load current control is the same as that shown in FIG. 2, so a description thereof will be omitted.

FIG. 9 shows a configuration diagram of yet another embodiment of the device of the present invention, in which four single-phase full bridge inverters are prepared for the single-phase load LOAD.

In the figure, Vdxt and Vdz are DC voltage sources, INV-1,
INV-2゜INV-3, INV-4 are single-phase full bridge connection inverter circuits, ~TR4 is an output transformer,
D engineer? 02 is a freewheeling diode, 51111~SV5
is a switch, and LOAD is a load device.

Inverter INV-1 and inverter INV-2 are connected in series on the DC side and supply power to negative 1LOAD via an output transformer. Further, inverter INV-3 and inverter INV-4 are constructed in the same manner as shown in FIG. 6, and generate three-level voltage outputs during low frequency operation, and operate as a multiplex inverter with one lance during high frequency operation.

That is, in the region where the output frequency is low, the switches SW, ~
Release SW4 and turn on S. At this time, the gates of inverters INV-1 and INV~2 are blocked.

Therefore, it becomes the same as the main circuit diagram shown in FIG. 7, and operates as a PWM inverter that generates three levels of output voltage.

Moreover, when the output frequency becomes high to a certain extent, the switch SWs is released and SW, to SW are turned on to utilize all four inverters and operate as a multiplex inverter with a transformer. The sum of the output voltages of each output transformer is applied to the load LOAD. That is, Vshi=V□+V, +V, +V4. At this time, four full bridge inverters IN
By performing PWM control by shifting the phases of the carrier wave signals V-1 to INV-4 by 45'', an output voltage with few harmonic components can be obtained.

In this way, by increasing the number of stages of transformer-equipped inverters, the capacity of the converter can be easily increased.

Note that the above embodiment describes a single-phase load.

However, it goes without saying that the same method can be applied to a three-phase load or a multi-phase load.

〔Effect of the invention〕

As described above, according to the device of the present invention, when operated as a direct-coupled inverter during low frequency operation, an output voltage twice as high as that of the conventional device can be obtained, and output voltage harmonics during high frequency operation are significantly reduced. It becomes possible to do so. Furthermore, by adding a freewheeling diode during operation of the direct-coupled inverter and generating three levels of voltage output, the harmonic components of the output voltage can be reduced by half, making it possible to suppress the pulsation of the load current.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing one embodiment of the inverter device of the present invention, Fig. 2A is a main circuit connection diagram for explaining the operation of the device in Fig. 1 during low frequency operation, and Fig. 2B is a Control circuit block diagram, FIG. 3 is a time chart diagram for explaining the PWM control operation of the circuit in FIG. 2, and FIG. 4A is a main circuit for explaining the operation of the device in FIG. 1 during high frequency operation. In the connection diagram, FIG. 4B is a control circuit block diagram, FIG. 5 is a time chart diagram for explaining the PWM control operation of the circuit in FIG. 4, and FIG. 6 is a configuration showing another embodiment of the device of the present invention. Figure, Figure 7A
is a main circuit connection diagram for explaining the operation of the device in Fig. 6 during low frequency operation, Fig. 7B is a control circuit block diagram, and Fig. 8 is for explaining the PWM control operation of the circuit in Fig. 7. 9 is a block diagram showing still another embodiment of the device of the present invention, and FIG. 10 is a block diagram showing a conventional inverter device. v, i + Vdz'...DC voltage source INV-1...1st inverter INV-2...2nd inverter TR, TR2...output transformer S, ~SW,...opening/closing Device LOAD...Load device

Claims (2)

[Claims] (1) Two DC voltage sources connected in series, first and second single-phase full-bridge connected voltage inverter circuits connected in series on the DC side, and an intermediate point between the DC voltage sources and the two a first switch that opens and closes an intermediate point on the DC side of the inverter circuit; a first output transformer connected to the output terminal of the first inverter circuit via a second switch;
A second output transformer connected to the output side terminal of the second inverter circuit via a third switch, an AC load, and an output frequency for the AC load in an operating region exceeding a certain set value. a fourth switch that is closed so that the sum of the output voltages of the first and second output transformers is applied with the first to third switches closed, and to the AC load; In an operating region where the output frequency is lower than the set frequency, the output voltage between the midpoint of the DC voltage source and the DC midpoint of the two inverter circuits is 5th input to be applied
An inverter device comprising a switch. (2) two DC voltage sources connected in series; first and second single-phase full-bridge connected voltage-type inverter circuits connected in series on the DC side; a first switch that opens and closes an intermediate point on the DC side of the inverter circuit; a first freewheeling diode connected between one output side terminal of the first inverter circuit and an intermediate point of the DC voltage source; , a second freewheeling diode connected between one output side terminal of the second inverter circuit and the midpoint of the DC voltage source; and a second switching diode connected to the output side terminal of the first inverter circuit. a first output transformer connected to the output side terminal of the second inverter circuit via a third switch, an AC load, and the AC load. In an operating range where the output frequency exceeds a certain set value, the first to third switches are closed so that the sum of the output voltages of the first and second output transformers is applied. and a fourth switch that connects the DC voltage source to the intermediate point of the DC voltage source in a state where the first to fourth switches are released in an operating region where the output frequency for the AC load is lower than the set frequency. an inverter device comprising: a fifth switch that is turned on so that an output voltage between the inverter circuit and the DC intermediate point of the inverter circuit is applied;