JPH11146649A - Pwm cycloconverter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a PWM cycloconverter which can be prevented from destruction due to overvoltage even if switching means become defective, by suppressing terminal voltages of both output terminals of it. SOLUTION: A cycloconverter has the phases of an AC power supply directly connected to respective phases at the output side by means of two-way switches 112-117 having self-distinguishing capabilities and outputs AC and DC voltage of any value by controlling the open/close of the two-way switches 112-117 by PWM control according to an output voltage command. In the case that the two-way switches 112-117 become faulty, the faults are detected by a fault detecting circuit 110 and then a fault signal Sf is outputted. Based on the faulty signal Sf, the voltages at output terminals A, B of the PWM cycloconverter are suppressed by an overvoltage protection circuit 109.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a PWM cycloconverter as a power conversion device for converting an AC voltage into a DC or an AC having a different frequency and amplitude, and more particularly to a PWM cycloconverter even when a failure of switching means occurs. P which can suppress the voltage across the output terminal of the cycloconverter and prevent the destruction of the PWM cycloconverter due to overvoltage
It relates to a WM cyclo converter.

[0002]

2. Description of the Related Art Here, a PWM inverter having a failure detection and protection function will be exemplified, and the case where the protection method in the PWM inverter is applied to a PWM cycloconverter will be discussed. As a conventional PWM inverter protection method, generally, when a failure detection circuit detects a failure of a PWM inverter, a control signal to a switching element (power transistor) constituting the inverter, that is, a gate signal is all turned off. It is a target. A protection method in a conventional PWM inverter will be described with reference to the drawings. First, FIG. 9 is a configuration diagram of an AC motor drive control device to which a PWM inverter according to a first conventional example is applied. In the figure, 801 is a three-phase AC power supply, 835 is an AC motor to be controlled, 832 is a diode bridge that forms a converter, 833 is a smoothing capacitor, 834 is a transistor module that forms an inverter, and 836 is a gate of a transistor module 834. A gate driver that generates a control signal to the electrode, 810 is a failure detection circuit that detects a failure of the transistor from the state of the gate signal and the voltage across each transistor of the transistor module 834, and 806 is a PWM command according to an output voltage command. 837 is a failure detection circuit 810
Gate driver 83 of PWM command based on the detection result of
6 is a gate block circuit that controls supply to the gate block 6. In the PWM inverter of the first conventional example having such a configuration, first, the controller 806 performs PWM control in accordance with an output voltage command.
When the command is created, the PWM command is supplied to the gate driver 836 via the gate block circuit 837, and the gate driver 836 drives each transistor of the transistor module 834 by isolating the gate signal based on the PWM command. If a failure occurs in the transistor module 834 constituting the inverter, the failure detection circuit 810 outputs a failure signal to the gate block circuit 837 and completely supplies the PWM command to the gate driver 836 via the gate block circuit 837. Turn off the phase. Therefore, the gate signal to the transistor module 834 is turned off in all phases, the current flowing in the AC motor 835 flows into the smoothing capacitor 833 through the free-foil diode in the transistor module 834, and the terminal voltage of the AC motor 835 sharply increases. It will be suppressed.

Next, as a second conventional example, a single-phase output PWM
FIG. 10 shows a multiplexed PWM inverter configured by connecting a plurality of inverters. FIG. 10 shows a multiplex P according to the second conventional example.
1 is a configuration diagram of an AC motor drive control device to which a WM inverter is applied. In the figure, 940 is a single-phase output PWM inverter, 906 is a controller that outputs a PWM command according to an output voltage command, and 935 is an AC motor to be drive-controlled. FIG. 11 is a configuration diagram of the single-phase output PWM inverter 940. In the figure, 1001 is a three-phase AC power supply, 1032 is a diode bridge that forms a converter, 1033 is a smoothing capacitor, 1034 is a single-phase output transistor module that forms an inverter, 103
6, a gate driver for generating a control signal to the gate electrode of the single-phase output transistor module 1034;
Is a failure detection circuit that detects a failure of the transistor from the state of the gate signal and the voltage across each transistor of the single-phase output transistor module 1034, and 1037 sends a PWM command to the gate driver 836 based on the detection result of the failure detection circuit 1010. Is a gate block circuit that controls the supply of data. The protection method in the multiplex PWM inverter of this conventional example also operates in the same manner as the first conventional example. When the single-phase output transistor module 1041 in one of the single-phase output PWM inverters 940 constituting the multiple PWM inverter fails, the failure detection circuit 1010 sends a failure signal to the controller 906 and the gate block circuit 10.
37. At this time, first, the gate block circuit 1
Single-phase output inverter 1 where 037 operates and a fault occurs
Turn off the PWM command to 041. Next, the controller 906, which has received the failure signal, sets all the single-phase outputs PW
Turn off the PWM command of the M inverter. That is, although there is a time delay between the operation of the gate block circuit 1037 of the failed single-phase output PWM inverter 940 and the operation of the gate block circuit of the other single-phase output PWM inverter, Output PWM inverter 940
Due to the operation of the smoothing capacitor 1033 inside, rapid voltage rise can be avoided, and concentration of the residual voltage of the AC motor 935 on the failed single-phase output PWM inverter 940 can be avoided.

[0004]

However, when the above-described protection methods in the first and second conventional PWM inverters are applied to a PWM cycloconverter, an energy absorbing element such as a smoothing capacitor or a free-foil diode is not used. Therefore, there is a problem that an overvoltage is generated at the output terminal of the PWM cycloconverter and the PWM cycloconverter cannot be protected.
In particular, when an AC motor is used as a load in a multiplex system, the induced voltage of the AC motor is concentrated on the output terminal of the failed PWM cycloconverter, causing a fatal destruction. The present invention has been made in view of the above-described conventional problems, and suppresses the voltage across the output terminal of a PWM cycloconverter even when a failure of a switching unit occurs, thereby preventing the destruction of the PWM cycloconverter due to overvoltage. It is an object to provide a PWM cycloconverter that can block.

[0005]

To solve the above-mentioned problems, a PWM cycloconverter according to a first aspect of the present invention has a self-extinguishing capability for each phase of an AC power supply and each phase on an output side. A PWM which is directly connected by a switching means, and performs an open / close control of the switching means by a PWM control according to an output voltage command to output arbitrary AC and DC voltages.
In the cycloconverter, a failure detection means for detecting a failure of the switching means and outputting a failure signal, and connected to both ends of an output terminal of the PWM cycloconverter, for suppressing a voltage of the output terminal based on the failure signal And an overvoltage protection means. The PWM cycloconverter according to claim 2 is the PWM cycloconverter according to claim 1, wherein the overvoltage protection unit includes a second switching unit that is opened and closed based on the failure signal. . The PWM cycloconverter according to a third aspect is the PWM cycloconverter according to the second aspect, wherein the overvoltage protection unit includes a second switching unit that is controlled to open and close based on the failure signal; And a resistor connected in series to the switching means. Also,
The PWM cycloconverter according to claim 4 is the third embodiment.
Wherein the overvoltage protection means rectifies the voltage between the output terminals of the PWM cycloconverter, and connects the series-connected resistor and the second
And a diode bridge applied to both ends of the switching means. The PWM according to claim 5
The cycloconverter is a PWM cycloconverter according to claim 4, wherein the overvoltage protection means includes the resistor connected in series and the capacitor connected in parallel with the second switching means. further,
In the PWM cycloconverter according to the sixth aspect, each phase of the AC power supply and each phase on the output side are directly connected by switching means having a self-extinguishing ability, and the PWM corresponding to the output voltage command.
In a PWM cycloconverter for performing an open / close control of the switching means by M control and outputting an arbitrary AC and DC voltage, a failure detection means for detecting a failure of the switching means and outputting a failure signal is provided. That is, the switching means of the non-existing phase is closed and the output terminal voltage of the PWM cycloconverter is forcibly reduced to zero.

In the PWM cycloconverter according to the present invention, the failure of the switching means, which is controlled to be opened and closed by the PWM control, is detected by the failure detection means and a failure signal is output, and based on the failure signal, the PWM is provided by the overvoltage protection means. The voltage across the output terminal of the cycloconverter is suppressed. Thus, even when a failure occurs in the switching means, the voltage between both ends of the output terminal of the PWM cycloconverter can be suppressed, and the destruction of the PWM cycloconverter due to the overvoltage can be prevented. In particular, the configuration of the overvoltage protection means includes a second switching means that is controlled to open and close based on a failure signal, a configuration in which a resistor connected in series to the configuration is added, and a series connection to the additional configuration. A configuration in which a resistor and a diode bridge applied to both ends of the second switching means are added, or a resistor connected in series and a capacitor connected in parallel with the second switching means are further added to the additional configuration. It is desirable to adopt a configuration in which: In either configuration, it is possible to drive the second switching means by the failure signal to lower the voltage across the output terminal of the PWM cycloconverter. Further, in the PWM cyclo-converter of the present invention, the failure of the switching means, which is controlled to be opened and closed by the PWM control, is detected by the failure detection means and a failure signal is output, and based on the failure signal,
The switching means of the non-failed phase is closed-controlled so that the voltage across the output terminal of the PWM cycloconverter is forcibly set to zero. As a result, even when a failure occurs in the switching means, the voltage between both ends of the output terminal of the PWM cycloconverter is suppressed, and P
Destruction of the WM cycloconverter can be prevented.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a first embodiment of a PWM cycloconverter according to the present invention will be described.
The second embodiment will be described in detail with reference to the drawings. [First Embodiment] FIG. 1 is a configuration diagram of an AC motor drive control device to which a PWM cycloconverter according to a first embodiment of the present invention is applied. In FIG. 1, a PWM cycloconverter according to the present embodiment includes an AC power supply 101, an AC line filter 102, a power supply voltage detection circuit 103, a gate driver 104, a commutation sequence switching circuit 10
5, a controller 106, a current direction detection circuit 107, an AC motor 108 as a load, an overvoltage protection circuit 109,
The configuration includes a failure detection circuit 110 and a bidirectional switch group 111. The bidirectional switch group 111 has a configuration including bidirectional switches 112 to 117 as switching means.
Reference numerals 2 to 117 denote two IGBTs (Insulated Gate Bipols) having both low-voltage characteristics in the on state of the bipolar transistor and high-speed switching characteristics of the power MOSFET.
ar Transistor) in which one emitter and the other collector are connected in series. The output of the AC power supply 101 is supplied via the AC line filter 102 to the bidirectional switches 112 to 114 connected to the output terminal A side and the bidirectional switches 115 to 117 connected to the output terminal B side. I have. The opposite side of the bidirectional switches 112 to 114 is connected to the AC motor 108 via an A terminal, and the opposite side of the bidirectional switches 115 to 117 is connected to the other terminal of the AC motor 108 via a B terminal. . The voltage detection circuit 103 takes in the output voltage of the AC power supply 101, insulates it, and steps down the voltage, and then passes the power supply voltage detection value to the controller 106. Further, the controller 106 determines and outputs a PWM command based on the output voltage command and the power supply voltage detected value by the voltage detection circuit 103. The current direction detection circuit 107 detects and outputs each switch current direction from the difference between the voltages across the bidirectional switches 112 to 117.
The commutation sequence switching circuit 105 is provided with a PWM command from the controller 106 and a current direction detection circuit 107.
Commutation order is determined based on the current direction detection value by
The WM command is converted into a gate signal and output. The gate driver 104 insulates the gate signal and controls the bidirectional switch 1
12 to 117 are output. Bidirectional switch 1
12 to 117 receive the drive signal, and
08. Further, the failure detector 110 detects a failure from the voltages of both ends of the bidirectional switches 112 to 117 and the state of the gate signal, and outputs a failure signal as a gate block signal to the gate driver 104 and a drive signal to the overvoltage protection circuit 109. Output. Further, upon receiving the failure signal (drive signal), the overvoltage protection circuit 109 starts an overvoltage protection operation.

Next, FIG. 2 is a detailed circuit configuration diagram of the gate drive circuit 104 and the failure detection circuit 110 in the present embodiment. Note that FIG.
Switching element (IGBT) Tr
The circuit configuration per ig is shown. In the figure, 219 is an insulated photocoupler for the gate signal Sg, and 220 is a failure signal Sf
R1 to R9 are resistors, B1 and B2 are bias power supplies, Tr1 to Tr4 are transistors, D1 is a diode, D2 is a Zener diode, and C1 and C2 are capacitors. In FIG. 2, first, the gate signal Sg
Is off, the output transistor of the gate signal isolation photocoupler 219 is turned off, and the transistor Tr1 is turned on. At this time, the transistor Tr4 is turned on, the gate signal of the switching element Trig is negatively biased, and the switching element Trig is opened. At this time, the cathode voltage of the Zener diode D2 changes to the failure signal Sf because the transistor Tr1 is turned on.
Does not become a voltage sufficient to drive the transistor Tr3 driving the transistor Tr3, and therefore the failure signal Sf is not output. When the gate signal Sg is on, the output transistor of the gate signal insulating photocoupler 219 turns on, and the transistor Tr1 turns off. At this time, the transistor Tr3 becomes conductive and the switching element Tr
The ig gate signal is positively biased, and the switching element Trig is closed. At this time, when the switching element Trig is normal, the cathode voltage of the Zener diode D2 does not become a voltage sufficient to drive the transistor Tr3 that drives the failure signal Sf because the switching element Trig is turned on. Signal Sf
Is not output. However, the switching element Tr
If the ig does not turn on within a certain period of time or is disconnected, the cathode voltage of the Zener diode D2 rises, and the time constant determined by the delay circuit connected to the base of the transistor Tr2 is used. The base voltage of Tr2 increases. When the base voltage of the transistor Tr2 is thus established, the transistor Tr2 is turned on, and outputs the failure signal Sf. The failure signal Sf functions as a gate block signal in the gate driver 104 and the failure signal insulated photocoupler 22
The signal is output as a drive signal of the overvoltage protection circuit 109 via 0.

Next, FIG. 3, FIG. 4, FIG. 5, and FIG. 6 show specific examples of the configuration of various overvoltage protection circuits 109, respectively. First, the overvoltage protection circuit 109a shown in FIG. 3 includes a bidirectional switch Trig1 as a second switching unit.
Drive circuit 301 that controls opening and closing of bidirectional switches Trig1 and Trig2 based on Trig2 and failure signal Sf
And a resistor R10 connected in series to the bidirectional switches Trig1 and Trig2. The overvoltage protection circuit 109b of FIG. 4 includes a switching element Trig3 as second switching means, a drive circuit 401 that controls the switching element Trig3 to open and close based on the failure signal Sf, and a voltage between the output terminals A and B. This configuration includes diode bridges D5 to D8 that rectify and apply the voltage to both ends of the switching element Trig3. FIG.
The overvoltage protection circuit 109c includes a switching element Trig4 as second switching means and a failure signal Sf.
, A driving circuit 501 that controls the opening and closing of the switching element Trig4, a resistor R11 connected in series to the switching element Trig4, and a resistor R11 and a switching element Trig4 connected in series by rectifying the voltage between the output terminals A and B. Diode bridge D1 applied to both ends
1 to D14. Further, the overvoltage protection circuit 109d in FIG. 6 includes a switching element Trig5 as a second switching means, a drive circuit 601 for opening and closing the switching element Trig based on the failure signal Sf, and a resistor connected in series to the switching element Trig5. R12 and a resistor R12 and a switching element Tr connected in series by rectifying the voltage between the output terminals A and B.
diode bridges D15 to D applied to both ends of ig5
18 and a capacitor C3 connected in parallel. In any circuit configuration, the second switching means is driven by the failure signal Sf to operate so as to lower the voltage between the output terminals A and B of the PWM cycloconverter. As described above, in the PWM cycloconverter of the present embodiment, the failure detection circuit 110 detects the failure of the bidirectional switches 112 to 117 that are controlled to open and close by the PWM control, and outputs the failure signal Sf. Based on this, the voltage at the output terminals A and B of the PWM cycloconverter is suppressed by the overvoltage protection circuit 109, so that even if a failure occurs in the bidirectional switches 112 to 117, the PWM cycloconverter is prevented from being damaged by the overvoltage. be able to.

[Second Embodiment] FIG. 7 shows a second embodiment of the present invention.
FIG. 3 is a configuration diagram of an AC motor drive control device to which the PWM cycloconverter according to the embodiment is applied. In the figure, the same reference numerals are given to the same parts as those in FIG. 1 (first embodiment), and the description is omitted. In the figure, the PWM cycloconverter of the present embodiment includes an AC power supply 101, an AC line filter 102, a power supply voltage detection circuit 103, a gate driver 604, a commutation sequence switching circuit 60
5, a controller 606, a current direction detection circuit 607, an AC motor 108 as a load, a failure detection circuit 610, a logic circuit 631, and a bidirectional switch group 611. The bidirectional switch group 611 has a configuration including bidirectional switches 112 to 117 as switching means, as in the first embodiment. The logic circuit 631 performs ignition control at the time of failure. The output of the AC power supply 101 is supplied to the bidirectional switches 112 to 112 connected to the output terminal A via the AC line filter 102.
114 and the bidirectional switch 1 connected to the output terminal B side
15 to 117. Bidirectional switch 11
The other side of 2-114 is connected to an AC motor 108 through an A terminal.
, And the other side of the bidirectional switches 115 to 117 is connected to the other terminal of the AC motor 108 via a terminal B. The voltage detection circuit 103 includes an AC power supply 101
Then, the power supply voltage detection value is passed to the controller 606 after the output voltage is taken, insulated and stepped down. Further, the controller 606 determines and outputs a PWM command based on the output voltage command and the power supply voltage detection value of the voltage detection circuit 103. The current direction detection circuit 607 includes the bidirectional switch 112
The switch current direction is detected and output from the difference between the voltages at both ends. The commutation sequence switching circuit 605 determines the commutation order based on the PWM command from the controller 606 and the current direction detection value from the current direction detection circuit 607, and converts the PWM command into a gate signal and outputs it. The gate driver 604 insulates the gate signal and outputs a drive signal for the bidirectional switches 112 to 117. The bidirectional switches 112 to 117 receive the drive signal and supply power to the AC motor 108. Further, the failure detector 610 detects a failure from the voltage at both ends of the bidirectional switches 112 to 117 and the state of the gate signal, and outputs a failure signal. Upon receiving the failure signal, the logic circuit 631 turns off the gate signal of the phase of the switch in which the failure has occurred, and outputs one of the switches of the other phases that are operating normally.
One of them is selected and its gate signal is turned on.

FIG. 8 is a conceptual diagram in which the configuration of FIG. 7 is simplified to explain the operation of the PWM cycloconverter of the present embodiment. FIG. 8 shows a detailed configuration of the AC line filter 102, and the coil L
1 to L3 and capacitors C4 to C6. Further, the bidirectional switches 112 to 117 are respectively replaced with the bidirectional switches Sra, Ssa, Sta, Srb, S
sb and Stb. In FIG. 8, assuming that the bidirectional switch Sra has failed, the failure detection circuit 610 outputs a failure signal Sf as in the first embodiment. Since the failure detection circuit 610 is provided for each switch, it is possible to detect that the bidirectional switch Sra has failed. At this time, the bidirectional switches Ssa and Ssb are fired, and the bidirectional switches Sra, Sta, Srb and St are activated.
When the arc b is extinguished, the voltage between the output terminals A and B becomes "0", and destruction due to overvoltage can be prevented. As described above, in the PWM cycloconverter of the present embodiment, the PW
Bidirectional switches 112-1 controlled to open and close by M control
17 is detected by the failure detection circuit 610 and a failure signal Sf is output. Based on the failure signal Sf, the bidirectional switch of the phase in which no failure has occurred is controlled to be closed, and the P
The voltage between the output terminals A and B of the WM cycloconverter is
Since it is set to "0", even if a failure occurs in the switching means, it is possible to prevent the PWM cycloconverter from being destroyed due to overvoltage.

[0012]

As described above, according to the PWM cycloconverter of the present invention, the failure detecting means detects the failure of the switching means which is controlled to open and close by the PWM control, and outputs a failure signal. On the basis of this, the voltage across the output terminal of the PWM cycloconverter is suppressed by the overvoltage protection means. Therefore, even if a failure occurs in the switching means, the voltage across the output terminal of the PWM cycloconverter can be suppressed. PWM that can prevent destruction of PWM cycloconverter
A cycloconverter can be provided. Further, according to the PWM cycloconverter of the present invention, the failure detecting means detects a failure of the switching means which is controlled to open and close by the PWM control, and outputs a failure signal. Since the switching means is controlled to be closed and the voltage across the output terminal of the PWM cycloconverter is forcibly set to 0, the voltage across the output terminal of the PWM cycloconverter can be reduced even if a failure occurs in the switching means. Can be suppressed, and P
It is possible to provide a PWM cyclo converter capable of preventing the WM cyclo converter from being destroyed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an AC motor drive control device to which a PWM cycloconverter according to a first embodiment of the present invention is applied.

FIG. 2 is a detailed circuit configuration diagram of a gate drive circuit and a failure detection circuit according to the first embodiment.

FIG. 3 is a configuration diagram (part 1) illustrating a specific configuration of an overvoltage protection circuit;

FIG. 4 is a configuration diagram (part 2) illustrating a specific configuration of the overvoltage protection circuit;

FIG. 5 is a configuration diagram (part 3) illustrating a specific configuration of the overvoltage protection circuit;

FIG. 6 is a configuration diagram (part 4) illustrating a specific configuration of the overvoltage protection circuit;

FIG. 7 is a configuration diagram of an AC motor drive control device to which a PWM cycloconverter according to a second embodiment of the present invention is applied.

FIG. 8 is a conceptual diagram illustrating the operation of the PWM cycloconverter according to the second embodiment.

FIG. 9 is a configuration diagram of an AC motor drive control device to which the PWM inverter of the first conventional example is applied.

FIG. 10 is a configuration diagram of an AC motor drive control device to which a multiple PWM inverter of a second conventional example is applied.

FIG. 11 is a configuration diagram of a single-phase output PWM inverter in a second conventional example.

[Explanation of symbols]

Reference Signs List 101 AC power supply 102 AC line filter 103 Power supply voltage detection circuit 104 Gate driver 105 Commutation sequence switching circuit 106, 606 Controller 107, 607 Current direction detection circuit 108 AC motor 109 Overvoltage protection circuit 110, 610 Failure detection circuit 111, 611 Bidirectional Switch module 112-117 Bidirectional switch (switching means) A, B Output terminal of PWM cycloconverter Trig Switching element in bidirectional switch 219 Gate signal insulated photocoupler 220 Fault detection signal insulated photocoupler R1-R12 Resistance B1, B2 Bias power supply Tr1 To Tr5 Transistor D1 Collector catch diode D2 Zener diode C1 to C6 Capacitor Trig1, Trig2 Bidirectional switch (No. Switching means) Trig3, Trig4, Trig5 Switching elements (second switching means) D5 to D8, D11 to D14, D15 to D18 Diode bridge 631 Logic circuit Sra Bidirectional switch (between R phase and A) Ssa Bidirectional switch ( S phase bi-directional switch (between T phase and A) Srb bi-directional switch (between R phase and B) Ssb bi-directional switch (between S phase and A) Stb bi-directional switch (between T phase and A) 801 AC power supply 806, 906 Controller 810 Failure detection circuit 832 Diode module 833 Smoothing capacitor 834 Transistor module 835, 935 AC motor 836 Gate driver 837 Gate block circuit 940 Single phase output PWM inverter 1041 Single phase output transistor module Le

Claims (6)

[Claims] 1. An AC power source and each phase on the output side are directly connected by a switching means having a self-extinguishing ability, and the switching of the switching means is controlled by PWM control in accordance with an output voltage command. In a PWM cycloconverter that outputs arbitrary AC and DC voltages, a failure detection unit that detects a failure of the switching unit and outputs a failure signal, and is connected to both ends of an output terminal of the PWM cycloconverter. Overvoltage protection means for suppressing the voltage of the output terminal based on the
M cyclo converter. 2. The P switch according to claim 1, wherein the overvoltage protection unit includes a second switching unit that is controlled to open and close based on the failure signal.
WM cyclo converter. 3. The overvoltage protection unit includes: a second switching unit that is controlled to open and close based on the failure signal; a resistor connected in series to the second switching unit;
The PWM cycloconverter according to claim 2, comprising: 4. The over-voltage protection means includes a diode bridge for rectifying a voltage between output terminals of the PWM cycloconverter and applying the rectified voltage to both ends of the series-connected resistor and a second switching means. The PWM cycloconverter according to claim 3. 5. The PWM cyclo-converter according to claim 4, wherein said overvoltage protection means includes a resistor connected in series with said capacitor and a capacitor connected in parallel with said second switching means. 6. The phase of the AC power supply and each phase on the output side are directly connected by switching means having a self-extinguishing ability, and the switching of the switching means is controlled by PWM control according to an output voltage command. A PWM cyclo-converter that outputs an arbitrary AC and DC voltage, comprising: a failure detection unit that detects a failure of the switching unit and outputs a failure signal; The output terminal voltage of the PWM cycloconverter is set to 0
A PWM cycloconverter, characterized in that: