AC-AC converter with open phase detection based on second harmonic

A power conversion device of an embodiment includes a rectifier that full-wave rectifies alternating current of a plurality of phases supplied from a power supply side, a capacitor that smoothes an output voltage of the rectifier, a voltage detection unit that detects the smoothed voltage, and an open phase detection unit that detects that an open phase has occurred in the alternating current of the plurality of phases based on a component having a frequency that is twice as high as a fundamental frequency of the alternating current of the plurality of phases included in frequency components of the smoothed voltage.

TECHNICAL FIELD

Embodiments of the present invention relate to a power conversion device and an open phase detection device.

BACKGROUND ART

There has been known an open phase detection device that detects an open phase (broken phase) occurring on a secondary side of a transformer based on a voltage on an AC input side. It is desired to detect the open phase based on a voltage between terminals of a capacitor, which is detected by a voltage detector.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

An object of the present invention is to provide an open phase detection device capable of detecting an open phase based on a voltage between terminals of a capacitor, which is detected by a voltage detector.

Solution to Problem

A power conversion device according to the present invention includes a rectifier, a capacitor, a voltage detection unit, and an open phase detection unit. The rectifier full-wave rectifies alternating current of a plurality of phases supplied from a power supply side. The capacitor smoothes an output voltage of the rectifier. The voltage detection unit detects the smoothed voltage. The open phase detection unit detects that an open phase has occurred in the alternating current of the plurality of phases based on a component having a frequency that is twice as high as a fundamental frequency of the alternating current of the plurality of phases included in frequency components of the smoothed voltage.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an open phase detection device and an open phase detection method of a power conversion device of embodiments will be described with reference to the drawings. In the following description, elements having the same or similar functions are denoted by the same reference numerals.

The term “connection” in the specification is not limited to a case where connection is made physically and also includes a case where connection is made electrically.

In the present specification, an open phase means that power of a phase is not supplied due to disconnection of windings of electric wires or a transformer, connection failure, melting of a fuse, open circuit, and the like.

In the present specification, a frequency of a fundamental wave in the waveform of three-phase AC power is referred to as a fundamental frequency.

First Embodiment

FIG.1is a configuration diagram of a power conversion system2of a first embodiment.FIG.1illustrates an AC power supply1, the power conversion system2, and an electric motor3.

The AC power supply1is a commercial power supply, a power generator and the like, and supplies three-phase AC power to the power conversion system2.

The electric motor3is, for example, an AC variable speed electric motor such as an induction motor. The electric motor3is driven by AC power supplied from the power conversion system2and outputs rotational driving force to an output shaft (not illustrated).

The power conversion system2includes, for example, a transformer20, a rectifier32, a capacitor35, an open phase detection device40, an inverse conversion device50, and a control unit55. The power conversion system2forward-converts the three-phase AC power, which is supplied from the AC power supply1, by using the rectifier32and the capacitor35to generate DC power, inversely converts the DC power by using the inverse conversion device50to generate three-phase AC power, and supplies the three-phase AC power to the electric motor3.

The transformer20is, for example, a three-phase transformer. The transformer20includes, for example, a primary winding22and a secondary winding24. The transformer20receives the supply of the three-phase AC power from the AC power supply1through the primary winding22, transforms the received three-phase AC power, and outputs the transformed three-phase AC power from the secondary winding24. One end of each of transformer output lines25R,25S, and25T is connected to each of the R, S, and T phases of the secondary winding24. The other ends of the transformer output lines25R,25S, and25T are connected to AC input terminals of the rectifier32. The three-phase AC power output from the secondary winding24is output to the rectifier32via the transformer output lines25R,25S, and25T. The secondary winding24and the transformer output lines25R,25S, and25T are a part of an object whose open phase is detected by the open phase detection device40.

The rectifier32includes, for example, an AC input terminal, a three-phase full-bridge diode rectifier circuit, a rectifier positive electrode terminal33A, and a rectifier negative electrode terminal33B. The rectifier32full-wave rectifies the three-phase AC power, which is supplied to the AC input terminal of the rectifier32from the transformer20, by the three-phase full-bridge diode rectifier circuit, and outputs the rectified DC power from the rectifier positive electrode terminal33A and the rectifier negative electrode terminal33B.

The rectifier positive electrode terminal33A is connected to one end of a positive electrode line60. The other end of the positive electrode line60is connected to a positive electrode input terminal52of the inverse conversion device50. The rectifier negative electrode terminal33B is connected to one end of a negative electrode line70. The other end of the negative electrode line70is connected to a negative electrode input terminal54of the inverse conversion device50.

The capacitor35is connected between the positive electrode line60and the negative electrode line70and smoothes the DC power that is output from the rectifier32. For example, the rectifier32and the capacitor35form a forward conversion device30. The forward conversion device30is an example of a power conversion device.

The open phase detection device40includes, for example, a voltage detector42and an open phase detection unit44.

The voltage detector42, for example, detects a voltage applied between a positive electrode terminal35A and a negative electrode terminal35B of the capacitor35(hereinafter, referred to an inter-terminal voltage), and outputs a detection value representing the inter-terminal voltage to the open phase detection unit44. The voltage detector42, for example, detects the inter-terminal voltage via a DC voltage converter and the like whose input and output are insulated, quantizes the inter-terminal voltage by an analog-to-digital (AD) converter (not illustrated), and outputs the quantized voltage as a voltage detection value representing the inter-terminal voltage. The voltage detector42is an example of a voltage detection unit.

The open phase detection unit44acquires the voltage detection value representing the inter-terminal voltage from the voltage detector42and determines, based on the acquired voltage detection value, whether an open phase has occurred in an open phase detection target range. The open phase detection unit44and the open phase detection target range detected by the open phase detection unit44will be described below.

The inverse conversion device50is, for example, a two-level inverter including a switching element such as an insulated gate bipolar transistor (IGBT). The switching element of the inverse conversion device50is pulse width modulation (PWM)-controlled by the control unit55. The inverse conversion device50converts the DC power, which is supplied from the rectifier32via the positive electrode line60and the negative electrode line70, into three-phase AC power having a variable frequency and a variable voltage. The inverse conversion device50supplies the converted three-phase AC power to the electric motor3via a load power line58.

The control unit55outputs a gate pulse signal to the switching element of the inverse conversion device50by feedback control based on a detection value and the like of a current detector (not illustrated) that detects a load current flowing through the load power line58, thereby PWM-controlling the switching element. Moreover, the control unit55receives an open phase detection signal that is output in an open phase determination process by the open phase detection unit44, which will be described below, and changes the control state of the PWM control based on the received open phase detection signal.

Next, the configuration of the open phase detection unit44and the content of the open phase determination process will be described. In the open phase determination process, the open phase detection unit44determines whether an open phase has occurred in the open phase detection target range, based on a component having a frequency that is twice as high as the fundamental frequency of the three-phase AC power supplied by the AC power supply1, which is included in frequency components of a voltage smoothed by the capacitor35. The open phase detection target range will be described below.

FIG.2is a configuration diagram of the open phase detection device40of the first embodiment. As described above, the open phase detection device40includes, for example, the voltage detector42and the open phase detection unit44.

The open phase detection unit44includes, for example, an acquisition section404, a fast Fourier transform section406, an extraction section408, a determination section410, and a storage section420. The storage section420stores, for example, the voltage detection value acquired by the acquisition section404and representing the inter-terminal voltage, a frequency spectrum generated by the fast Fourier transform section406, an extracted frequency component Fext, a threshold voltage VTH, a program for the open phase determination process, and the like. Each of the acquisition section404, the fast Fourier transform section406, the extraction section408, and the determination section410is implemented by, for example, a hardware processor such as a central processing unit (CPU) executing a program (software). Furthermore, some or all of these constituent elements may be implemented by hardware (circuit section; including a circuitry) such as a large scale integration (LSI), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), and a graphics processing unit (GPU), or may be implemented by cooperation of software and hardware. The storage section420is implemented by, for example, a hard disk drive (HDD), a flash memory, an electrically erasable programmable read only memory (EEPROM), a read only memory (ROM), a random access memory (RAM) and the like.

The acquisition section404acquires the detection value (voltage detection value) detected by the voltage detector42and representing the inter-terminal voltage, and stores the detection value in the storage section420. Furthermore, the acquisition section404acquires a detection value stored in the storage section420and representing a predetermined number of inter-terminal voltages and outputs the detection value to the fast Fourier transform section406. Note that the acquisition section404may perform the acquisition of the detection value from the voltage detector42and the reading of the detection value from the storage section420in parallel.

The fast Fourier transform section406generates the frequency spectrum by performing FFT processing (fast Fourier transform) on the voltage value representing the predetermined number of inter-terminal voltages, which is received from the acquisition section404, and stores the generated frequency spectrum in the storage section420. The frequency spectrum obtained by the FFT processing indicates frequency components of the predetermined number of inter-terminal voltages received from the acquisition section404. The number of inter-terminal voltages, which are received in the fast Fourier transform section406from the acquisition section404for the FFT processing, can be set to any number such that frequency components desired to be extracted are included with sufficient resolution in the frequency spectrum obtained by the FFT processing. As an example, in order to obtain frequency spectrums illustrated inFIG.6BandFIG.6Dto be described below, the fast Fourier transform section406acquires data of inter-terminal voltages as illustrated inFIG.6AandFIG.6B.

The extraction section408extracts a component having a frequency that is twice as high as the fundamental frequency of the three-phase AC power supplied by the AC power supply1from the frequency spectrum generated by the fast Fourier transform section406and stored in the storage section420, and stores the extracted component in the storage section420as the extracted frequency component Fext.

The determination section410determines whether a voltage VFextof the extracted frequency component Fextextracted by the extraction section408and stored in the storage section420is larger than the threshold voltage VTH. When the voltage VFextis larger than the threshold voltage VTH, the determination section410outputs the open phase detection signal to the control unit55. For example, a voltage of an extracted frequency component when there is no open phase and a voltage of an extracted frequency component when there is an open phase can be measured in advance, and an average value thereof or a value in a predetermined range with respect to the average value can be set as the threshold voltage VTH.

Next, the open phase determination process of the open phase detection device40of the first embodiment will be described with reference to simulation results. In examples to be described below, it is assumed that the open phase detection device40sets the secondary winding24and the transformer output lines25R,25S, and25T as the open phase detection target range and performs open phase detection.

With reference toFIG.3AtoFIG.3C, the reason why a component having a frequency 2f that is twice as high as the fundamental frequency f of the AC power supply1appears in the inter-terminal voltage when there is an open phase will be described.FIG.3Ais a diagram for explaining an example of a voltage waveform of the three-phase AC power supplied to the transformer20from the AC power supply1.FIG.3Bis a diagram for explaining an example of a pulsating current full-wave rectified by the rectifier32when there is no open phase on any of the secondary winding24and the transformer output lines25R,25S, and25T.FIG.3Cis a diagram for explaining an example of a pulsating current full-wave rectified by the rectifier32when there is an open phase on the transformer output line25R.

As illustrated inFIG.3A, the length of time of one cycle in the three-phase AC power output from the AC power supply1is represented by the reciprocal 1/f of the fundamental frequency f of the AC power supply1. The voltage waveform of a pulsating current after such three-phase AC power is full-wave rectified by the rectifier32includes six peaks in one cycle 1/f.

In a case where there is no open phase in the open phase detection target range, when the full-wave rectified pulsating current is smoothed by the capacitor35, the waveform of an inter-terminal voltage Vc including a ripple component is obtained as illustrated inFIG.3B. The waveform of the inter-terminal voltage Vc has a shape such that the peaks of a full-wave rectified voltage are connected, and the period of the ripple, which is a time interval between sags, is ⅙f.

On the other hand, in the following description, it is assumed that there is an open phase on the transformer output line25R. In such a case, the capacitor35is charged in the phases (S-phase and T-phase) with no open phase and is discharged in the phase (R-phase) with an open phase. This is because, in the phase (R-phase) with an open phase, power consumption by the inverse conversion device50exceeds a current supplied from the rectifier32and charge/discharge balance of the capacitor35becomes negative. Therefore, as illustrated inFIG.3C, in the waveform of the inter-terminal voltage Vc of the capacitor35, there are sags S1and S2(hereinafter, referred to as large sags), in which a voltage greatly drops compared with other sags, in the phase (R-phase) with an open phase. Since each phase voltage of the full-wave rectified pulsating current has two peaks per cycle 1/f, the large sag occurs twice per cycle 1/f. Therefore, when there is an open phase, the inter-terminal voltage Vc includes the component having the frequency 2f that is twice as high as the fundamental frequency f of the AC power supply1. Accordingly, the aforementioned open phase detection device40detects the component having the frequency 2f that is twice as high as the fundamental frequency f of the AC power supply1, thereby enabling open phase detection.

FIG.4is a flowchart of the open phase determination process of the open phase detection device40of the first embodiment. Although not illustrated in the flowchart, it is assumed that the voltage acquisition process, in which the voltage detector42detects the voltage value representing the inter-terminal voltage and the acquisition section404acquires the voltage value and stores the voltage value in the storage section420, is performed in parallel with the process illustrated inFIG.4.

The power conversion system2performs the open phase determination process at predetermined time intervals or when a specific condition is satisfied (when a specific event occurs).

The open phase determination process will be described below. First, the acquisition section404acquires the voltage value stored in the storage section420and representing the predetermined number of inter-terminal voltages and outputs the voltage value to the fast Fourier transform section406(step S100). Next, the fast Fourier transform section406generates the frequency spectrum by performing FFT processing on the voltage value representing the predetermined number of inter-terminal voltages received from the acquisition section404, and stores the generated frequency spectrum in the storage section420(step S110) Next, the extraction section408extracts the component having the frequency that is twice as high as the fundamental frequency of the three-phase AC power supplied by the AC power supply1, from the frequency spectrum stored in the storage section420, and stores the extracted component in the storage section420as the extracted frequency component Fext(step S120).

Next, the determination section410determines whether the voltage VFextof the extracted frequency component Fextstored in the storage section420is larger than the threshold voltage VTH(step S130). When the voltage VFextis larger than the threshold voltage VTH, the determination section410advances the process to step S140and outputs the open phase detection signal to the control unit55. On the other hand, when the voltage VFextis equal to or less than the threshold voltage VTH, the determination section410ends the open phase determination process.

According to the first embodiment, based on a component having a frequency that is twice as high as a fundamental frequency of an alternating current of a plurality of phases included in frequency components of a voltage smoothed by the capacitor35, it is detected that an open phase has occurred in the alternating current of the plurality of phases. In this way, it is possible to detect an open phase based on the inter-terminal voltage of the capacitor35detected by the voltage detector42, thereby facilitating open phase detection.

Furthermore, according to the first embodiment, the frequency spectrum of the voltage smoothed by the capacitor35is derived by the FFT processing, and the component having the frequency that is twice as high as the fundamental frequency is extracted from the frequency spectrum. In this way, it is possible to detect an open phase simply by performing arithmetic processing on the inter-terminal voltage, so that it is possible to implement open phase detection with a minimum addition of hardware.

Furthermore, according to the first embodiment, when the voltage of the component having the frequency that is twice as high as the fundamental frequency from the frequency spectrum is larger than the predetermined threshold voltage VTH, it is determined that an open phase has occurred in the open phase detection target range. In this way, it is possible to detect an open phase by the voltage level of the frequency that is twice as high as the fundamental frequency of the AC power supply1in the frequency spectrum derived from the inter-terminal voltage, thereby enabling appropriate open phase detection.

Second Embodiment

FIG.5is a configuration diagram of a power conversion system2A of a second embodiment. The forward conversion device30of the power conversion system2of the first embodiment is what is called a six-pulse diode converter (forward conversion device30). A forward conversion device group30A of the power conversion system2A of the second embodiment is what is called a 36-pulse diode converter.

FIG.5illustrates the AC power supply1, the power conversion system2A, and the electric motor3.

The power conversion system2A includes, for example, a transformer group20A, the forward conversion device group30A, open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN, and an inverse conversion device group50A Similarly to the power conversion system2, the power conversion system2A forward-converts three-phase AC power, which is supplied from the AC power supply1, to generate DC power, inversely converts the DC power to generate three-phase AC power, and supplies the three-phase AC power to the electric motor3. However, the power conversion system2A is different from the power conversion system2in that the U-phase, V-phase, and W-phase AC power to be supplied to the electric motor3is supplied from separate forward conversion devices31U,31V, and31W and inverse conversion devices50U,50V, and50W, respectively. Note that, in the following description, when the open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN are not particularly distinguished from one another, they are simply referred to as the open phase detection device40.

The transformer group20A includes, for example, a transformer20U, a transformer20V, and a transformer20W. Each of the transformers20U,20V, and20W is a three-winding transformer in which a connection method on a secondary side is different. Since the transformers20U,20V, and20W have the same configuration, the transformer20U will be described below as a representative.

The transformer20U includes, for example, a primary winding22U, a secondary winding24UP, and a tertiary winding24UN. The transformer20U receives the supply of the three-phase AC power from the AC power supply1through the primary winding22U, transforms the received three-phase AC power, and outputs the transformed three-phase AC power from the secondary winding24UP and the tertiary winding24UN. Each of the secondary winding24UP and the tertiary winding24UN has, for example, a start connection and a delta connection, so that the three-phase AC power output from the secondary winding24UP has a phase advanced by 30° with respect to the three-phase AC power output from the tertiary winding24UN.

The three-phase AC power output from the secondary winding24UP and the tertiary winding24UN is output to the forward conversion device31U via transformer output lines25UPR,25UPS,25UPT,25UNR,25UNS, and25UNT. Phase currents of R-phase, S-phase, and T-phase flow through the transformer output lines25UPR,25UPS, and25UPT, respectively, as line currents. The line currents of R-phase, S-phase, and T-phase, which have potentials different from those of the transformer output lines25UPR,25UPS, and25UPT and phases shifted by 30° from the transformer output lines25UPR,25UPS, and25UPT, flow through the transformer output lines25UNR,25UNS, and25UNT. The transformer20V and the transformer20W are configured similarly to the transformer20U. Regarding the transformer20V and the transformer20W, a description thereof is incorporated by replacing U in the reference numerals of the constituent elements in the description of the transformer20U with V and W, respectively.

The forward conversion device group30A includes, for example, forward conversion devices31U,31V, and31W. Since the forward conversion devices31U,31V, and31W have the same configuration, the forward conversion device31U will be described below as a representative. The forward conversion device31U includes, for example, a rectifier32UP, a rectifier32UN, a capacitor35UP, and a capacitor35UN.

Each of the rectifiers32UP and32UN is a three-phase full-bridge diode rectification circuit. An AC side of the rectifier32UP is connected to the secondary winding24UP via the transformer output lines25UPR,25UPS, and25UPT. An AC side of the rectifier32UN is connected to the tertiary winding24UN via the transformer output lines25UNR,25UNS, and25UNT. Load sides of the rectifiers32UP and32UN are connected in series with each other. DC power rectified by the rectifiers32UP and32UN connected in series with each other is output from a rectifier positive electrode terminal33UP and a rectifier negative electrode terminal33UN.

The rectifier positive electrode terminal33UP is connected to one end of a positive electrode line60U. The other end of the positive electrode line60U is connected to a positive electrode input terminal52U of the inverse conversion device50U. The rectifier negative electrode terminal33UN is connected to one end of a negative electrode line70U. The other end of the negative electrode line70U is connected to a negative electrode input terminal54N of the inverse conversion device50U.

Since each of the rectifiers32UP and32UN receives the supply of AC power having different potentials from the secondary winding24UP and the tertiary winding24UN of the transformer20U, the outputs of the rectifiers32UP and32UN are connected in series, so that the total voltage of an output voltage of the rectifier32UP and an output voltage of the rectifier32UN is output from the rectifier positive electrode terminal33UP and the rectifier negative electrode terminal33UN.

The capacitor35UP and the capacitor35UN are connected in series with each other and are connected between the positive electrode line60U and the negative electrode line70U, thereby smoothing the DC power output from the forward conversion device31U.

A connection point65UA between the capacitor35UP and the capacitor35UN is connected to a neutral line65U. The capacitor35UP and the capacitor35UN have substantially the same capacitance such that the potential of the connection point65UA to the neutral line65U is an intermediate potential between the positive electrode line60U and the negative electrode line70U.

That is, in the forward conversion device31U, the rectifiers32UP and32UN output a pulsating current by rectifying the AC power supplied from the transformer20U, and the capacitor35UP and the capacitor35UN output DC power by smoothing the pulsating current output from the rectifiers32UP and32UN and supply the DC power to the inverse conversion device50U.

The forward conversion devices31V and31W are also configured similarly to the forward conversion device31U. Regarding the forward conversion devices31V and31W, a description thereof is incorporated by replacing U in the reference numerals of the constituent elements in the description of the forward conversion device31U with V and W, respectively. Note that, in the following description, when the capacitors35UP,35UN,35VP,35VN,35WP, and35WN are not particularly distinguished from one another, they are referred to as the capacitor35.

The open phase detection device40UP is connected to a positive electrode terminal35UPA and a negative electrode terminal35UPB of the capacitors35UP. The open phase detection device40UN is connected to a positive electrode terminal35UNA and a negative electrode terminal35UNB of the capacitor35UN.

The open phase detection device40VP is connected to a positive electrode terminal35VPA and a negative electrode terminal35VPB of the capacitors35VP. The open phase detection device40VN is connected to a positive electrode terminal35VNA and a negative electrode terminal35VNB of the capacitor35VN.

The open phase detection device40WP is connected to a positive electrode terminal35WPA and a negative electrode terminal35WPB of the capacitor35WP. The open phase detection device40WN is connected to a positive electrode terminal35WNA and a negative electrode terminal35WNB of the capacitor35WN.

In the following description, when the open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN are not particularly distinguished from one another, they are referred to as the open phase detection device40.

Each of the open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN has the same configuration as that of the open phase detection device40of the first embodiment; however, an open phase determination process of the second embodiment is different from the open phase determination process of the first embodiment in that the determination section410performs determination based on a first threshold voltage VTH1and a second threshold voltage VTH2. The open phase determination process of the second embodiment will be described below.

The inverse conversion device group50A includes, for example, the inverse conversion devices50U,50V, and50W. Since the inverse conversion devices50U,50V, and SOW have the same configuration, the inverse conversion device50U will be described below as a representative. The inverse conversion device50U is, for example, a full-bridge NPC (neutral-point-clamped) five-level inverter.

The inverse conversion device50U is PWM-controlled to convert the DC power, which is supplied from the forward conversion device30U via the positive electrode line60U and the negative electrode line70U, into three-phase AC power having a variable frequency and a variable voltage. The inverse conversion device50U supplies the converted U-phase AC power to the electric motor3via a load power line58U. The inverse conversion devices50V and50W are also configured similarly to the inverse conversion device50U. Regarding the inverse conversion devices50V and50W, a description thereof is incorporated by replacing U in the reference numerals of the constituent elements in the description of the inverse conversion device50U with V and W, respectively.

Next, the open phase determination process of the open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN in the second embodiment will be described. In the following description, as an example, a description will be given assuming that the open phase detection device40WP detects an open phase on a transformer output line25WPR of the transformer20W. Therefore, in the following description, “when there is an open phase” means that there is an open phase on the transformer output line25WPR of the transformer20W.

In the following description, “when there is no open phase” means that there is no open phase on any of the secondary winding24UP, the tertiary winding24UN, a secondary winding24VP, a tertiary winding24VN, a secondary winding24WP, a tertiary winding24WN, the transformer output lines25UPR,25UPS,25UPT,25UNR,25UNS, and25UNT of the transformer20U, transformer output lines25VPR,25VPS,25VPT,25VNR,25VNS, and25VNT of the transformer20V, and transformer output lines25WPR,25WPS,25WPT,25WNR,25WNS, and25WNT of the transformer20W.

FIG.6Ais a diagram illustrating a simulation result of an inter-terminal voltage detected by the open phase detection device40WP of the second embodiment when there is no open phase.FIG.6Bis a diagram illustrating a simulation result of a frequency spectrum generated by the open phase detection device40WP of the second embodiment when there is no open phase.FIG.6Cis a diagram illustrating a simulation result of an inter-terminal voltage detected by the open phase detection device40WP of the second embodiment when there is an open phase.FIG.6Dis a diagram illustrating a simulation result of a frequency spectrum generated by the open phase detection device40WP of the second embodiment when there is an open phase.

FromFIG.6AtoFIG.6D, it is assumed that the fundamental frequency of the AC power supply1is 50 Hz and the output frequency of the inverse conversion device group50A is 100 Hz.

InFIG.6AandFIG.6C, a horizontal axis denotes time [seconds]. A vertical axis inFIG.6Adenotes a voltage [V] generated between the positive electrode terminal35WPA and the negative electrode terminal35WPB of the capacitor35WP. A vertical axis inFIG.6Cdenotes a voltage [V] generated between the positive electrode terminal35WPA and the negative electrode terminal35WPB of the capacitor35WN.

InFIG.6BandFIG.6D, a horizontal axis denotes a frequency [Hz] and a vertical axis denotes a voltage ratio [%] of voltage values of frequency components of an inter-terminal voltage based on the result of the FFT. For example, the voltage ratio is a ratio of the voltage values of frequency components when the voltage value of a voltage fundamental wave of the AC power supply is set to 100% as a reference. Note that since the voltage ratio is a ratio of voltage values, a reference voltage value may be an effective value or a peak value. Furthermore, as illustrated in the drawings, the voltage value of an effective value of the fundamental wave of the AC power supply appears at the frequency of 0 Hz. However, inFIG.6BandFIG.6D, only a part of the voltage values of the effective value of the fundamental wave of the AC power supply are illustrated because the scale is shown by enlarging and displaying the range of the voltage ratio of 0% to 20%.

When there is no open phase, since the full-wave rectified voltages output from the capacitors32WP and32WN are DC voltages including ripples with a period of ⅙f as described inFIG.3B, the voltage detector42detects a DC voltage having a waveform as illustrated inFIG.6A.

The fast Fourier transform section406generates a frequency spectrum as illustrated inFIG.6Bby performing a fast Fourier transform (FFT transform) on the detection value of the waveform as illustrated inFIG.6A, which is detected by the voltage detector42. The frequency spectrum as illustrated inFIG.6Bincludes a component FBIShaving a frequency of 200 Hz that is twice as high as an output frequency 100 Hz of the inverse conversion device50. However, the frequency spectrum does not include a component having a frequency of 100 Hz that is twice as high as the fundamental frequency 50 Hz of the AC power supply1.

When there is an open phase, since the full-wave rectified voltages output from the capacitors32WP and32WN are DC voltages including large ripples with a period of ½f as described inFIG.3C, the voltage detector42detects a DC voltage having a waveform as illustrated inFIG.6C.

The fast Fourier transform section406generates a frequency spectrum as illustrated inFIG.6Dby performing a fast Fourier transform (FFT transform) on the detection value of the waveform as illustrated inFIG.6C, which is detected by the voltage detector42. The frequency spectrum as illustrated inFIG.6Dincludes the component FBIShaving the frequency of 200 Hz that is twice as high as the output frequency 100 Hz of the inverse conversion device50. Moreover, the frequency spectrum includes a component FBPShaving a frequency of 100 Hz that is twice as high as the fundamental frequency 50 Hz of the AC power supply1.

Next, with reference toFIGS.7A to7F, in the open phase determination process of the open phase detection device40of the second embodiment, a simulation result when the output frequency of the inverse conversion device50and the fundamental frequency of the AC power supply1are changed will be described.

In the following description, in the frequency spectrum generated by performing the fast Fourier transform on the inter-terminal voltage, a component having a frequency of 100 Hz that is twice as high as the fundamental frequency 50 Hz of the AC power supply1is referred to as a “double power supply frequency component”.

FIG.7Ais a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 50 Hz and the power supply frequency is 25 Hz in the power conversion system2A of the second embodiment.FIG.7Bis a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 50 Hz and the power supply frequency is 50 Hz in the power conversion system2A of the second embodiment.FIG.7Cis a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 50 Hz and the power supply frequency is 60 Hz in the power conversion system2A of the second embodiment.FIG.7Dis a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 50 Hz and the power supply frequency is 100 Hz in the power conversion system2A of the second embodiment.

That is, the simulation conditions ofFIGS.7A to7Dare that the output frequency of the inverse conversion device group50A is fixed at 50 Hz and the fundamental frequency of the AC power supply1is set to 25 Hz inFIG.7A, is set to 50 Hz inFIG.7B, is set to 60 Hz inFIG.7C, and is set to 100 Hz inFIG.7D. In each ofFIGS.7A to7D, a black bar (left) indicates the double power supply frequency component when there is no open phase and a white bar (right) indicates the double power supply frequency component when there is an open phase on the transformer output line25WPR.

In the simulation results ofFIGS.7A to7D, the simulation results ofFIGS.7A,7C, and7Dshow a similar trend. Hereinafter,FIG.7Awill be described as an example. When the fundamental frequency of the AC power supply1is set to 25 Hz, the voltage of the double power supply frequency component detected by all the open phase detection devices increases when there is an open phase on the transformer output line25WPR and becomes larger than the first threshold voltage VTH1. Particularly, the voltage of the double power supply frequency component detected by the open phase detection devices40WP and40WN increases significantly as compared with other cases there is an open phase on the transformer output line25WPR and becomes larger than the second threshold voltage VTH2. The first threshold voltage VTH1and the second threshold voltage VTH2will be described below.

The simulation result ofFIG.7Bshows a trend different from in the simulation results ofFIGS.7A,7C, and7Ddescribed above. In the simulation conditions ofFIG.7B, the fundamental frequency 50 Hz of the AC power supply1is the same as the output frequency 50 Hz of the inverse conversion device group50A. Therefore, in the simulation result ofFIG.7B, even when there is no open phase, the voltage of the double power supply frequency component is higher than those of the simulation results ofFIGS.7A,7C, and7D. When the state in which there is no open phase is changed to the state in which there is an open phase, the double power supply frequency component increases, but the change is slight. Accordingly, it may be difficult to distinguish the state in which there is no open phase and the state in which there is an open phase. For example, in the double power supply frequency component detected by the open phase detection devices40UP and40UN, there is almost no difference between the state in which there is no open phase and the state in which there is an open phase. Furthermore, in the double power supply frequency component detected by the open phase detection devices40VP and40VN, there is a difference between the state in which there is no open phase from the state in which there is an open phase, but the magnitude thereof is smaller than the difference in the simulation results ofFIGS.7A,7C, and7D.

Therefore, in the simulation ofFIG.7B, the voltage of the double power supply frequency component detected by the open phase detection device40WP increases when there is an open phase on the transformer output line25WPR and becomes larger than a third threshold voltage VTH3. Accordingly, in the case of the simulation ofFIG.7B, it is possible to detect an open phase based on the inter-terminal voltages vcWP and vcWN of the capacitors35WP and35WN (capacitors of their own phases) receiving the supply of power from the transformer output line25WPR in which an open phase has occurred, but it may be difficult to detect an open phase based on inter-terminal voltages of the other capacitors35UP,35UN,35VP, and35VN (capacitors other than their own phases). The third threshold voltage VTH3will be described below.

FIG.7Eis a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 100 Hz and the power supply frequency is 50 Hz in the power conversion system2A of the second embodiment.FIG.7Fis a diagram illustrating a simulation result of the double power supply frequency component when the output frequency of the inverse conversion device group50A is 200 Hz and the power supply frequency is 50 Hz in the power conversion system2A of the second embodiment.

That is, the simulation conditions ofFIGS.7E and7Fare that the fundamental frequency of the AC power supply1is fixed at 50 Hz and the output frequency of the inverse conversion device group50A is set to 100 Hz inFIG.7Eand is set to 200 Hz inFIG.7F. In each ofFIGS.7E and7F, a black bar (left) indicates the double power supply frequency component when there is no open phase and a white bar (right) indicates the double power supply frequency component when there is the open phase on the transformer output line25WPR.

The simulations ofFIGS.7E and7Fshow a similar trend. Hereinafter,FIG.7Ewill be described as an example. When the output frequency of the inverse conversion device group50A is set to 100 Hz, the voltage of the double power supply frequency component detected by all the open phase detection devices increases when there is an open phase on the transformer output line25WPR and becomes larger than the first threshold voltage VTH1. Particularly, the voltage of the double power supply frequency component detected by the open phase detection devices40WP and40WN increases significantly when there is an open phase on the transformer output line25WPR and becomes larger than the second threshold voltage VTH2.

Next, setting of the first threshold voltage VTH1, the second threshold voltage VTH2, and the third threshold voltage VTH3will be described.

The first threshold voltage VTH1and the second threshold voltage VTH2are threshold voltages used for determination by the determination section410when the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device50W are not substantially the same. The first threshold voltage VTH1is a threshold voltage used to determine whether an open phase has occurred in phases other than its own phase. The second threshold voltage VTH2is a threshold voltage used to determine whether an open phase has occurred in its own phase.

For example, the first threshold voltage VTH1may be an average value VavgTH1of an average value Vavg_allof the voltages of the double power supply frequency components of all phases when there is no open phase and an average value Vavg_nonopof the voltages of the double power supply frequency components of phases other than a phase having the open phase when there is the open phase, or may be a value in a predetermined range with respect to the average value VavgTH1. Specifically, the first threshold voltage VTH1may be the average value VavgTH1of the average value Vavg_allof the voltages of the double power supply frequency components of the inter-terminal voltages vcUP, vcUN vcVP, vcVN, vcWP, and vcWN when there is no open phase and the average value Vavg_nonopof the voltages of the double power supply frequency components of the inter-terminal voltages vcUP, vcUN vcVP, and vcVN when there is the open phase on the transformer output line25WPR, or may be the value in the predetermined range with respect to the average value VavgTH1.

For example, the second threshold voltage VTH2may be an average value VavgTH2of an average value Vavg_allof the voltages of the double power supply frequency components of all phases when there is no open phase and an average value Vavg_opof the voltages of the double power supply frequency components of a phase having the open phase when there is the open phase, or may be a value in a predetermined range with respect to the average value VavgTH2. Specifically, the second threshold voltage VTH2may be the average value VavgTH2of the average value Vavg_allof the voltages of the double power supply frequency components of the inter-terminal voltages vcUP, vcUN vcVP, vcVN, vcWP, and vcWN when there is no open phase and the average value Vavg_opof the voltages of the double power supply frequency components of the inter-terminal voltages vcWP and vcWN when there is the open phase on the transformer output line25WPR, or may be the value in the predetermined range with respect to the average value VavgTH1. The second threshold voltage VTH2is a value larger than the first threshold voltage VTH1.

The third threshold voltage VTH3is a threshold used for determination by the determination section410when the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device50W are substantially the same.

For example, the third threshold voltage VTH3may be an average value VavgTH3of an average value Vavg_allof the voltages of the double power supply frequency components of all phases when there is no open phase and an average value Vavg_opof the voltages of the double power supply frequency components of a phase having the open phase when there is the open phase, or may be a value in a predetermined range with respect to the average value VavgTH3. More specifically, the third threshold voltage VTH3may be the average value VavgTH3of the average value Vavg_allof the voltages of the double power supply frequency components of the inter-terminal voltages vcUP, vcUN vcVP, vcVN, vcWP, and vcWN when there is no open phase and the average value Vavg_opof the voltages of the double power supply frequency components of the inter-terminal voltages vcWP and vcWN when there is the open phase on the transformer output line25WPR, or may be the value in the predetermined range with respect to the average value VavgTH3.

Furthermore, the voltage of the double power supply frequency component of each phase in the calculation of the first threshold voltage VTH1, the second threshold voltage VTH2, the third threshold voltage VTH3, and the threshold voltage VTHmay be an average value of voltages in a certain period, or an average value obtained by performing processing such as exclusion of an outlier and an abnormal value and weighting may be used.

In the open phase determination process of the second embodiment, the open phase detection device40performs determination as follows according to the voltage of the double power supply frequency component. Note that, in the present specification, “frequencies are substantially the same” means that frequencies are so close that they are not separable in the frequency spectrum obtained by the FFT processing.A. Case where the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A do not substantially coincide with each other(A1) Case where the voltage of the double power supply frequency component is equal to or less than the first threshold voltage VTH1:the open phase detection device40determines that there is no open phase in all phases.(A2) Case where the voltage of the double power supply frequency component is larger than the first threshold voltage VTH1and is equal to or less than the second threshold voltage VTH2:the open phase detection device40determines that there is an open phase in phases other than a phase to which the open phase detection device40is connected (other than its own phase) and there is no open phase in the phase (own phase) to which the open phase detection device40is connected.(A3) Case where the voltage of the double power supply frequency component is larger than the second threshold voltage VTH2:

the open phase detection device40determines that there is the open phase in the phase (own phase) to which the open phase detection device40is connected. Note that, in such a case, it is not possible to determine whether there is the open phase in the phases other than the phase to which the open phase detection device40is connected (other than its own phase).B. Case where the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A substantially coincide with each other(B1) Case where the voltage of the double power supply frequency component is equal to or less than the first threshold voltage VTH3:the open phase detection device40determines that there is no open phase in the phase (own phase) to which the open phase detection device40is connected and determines that it is not possible to determine whether there is the open phase in the phases other than the phase to which the open phase detection device40is connected (other than its own phase).(B2) Case where the voltage of the double power supply frequency component is larger than the first threshold voltage VTH3:the open phase detection device40determines that there is the open phase in the phase (own phase) to which the open phase detection device40is connected. Note that, in such a case, it is not possible to determine whether there is the open phase in the phases other than the phase to which the open phase detection device40is connected (other than its own phase).

FIG.8is a diagram illustrating the simulation result of the double power supply frequency component when the current of the electric motor3is changed in the power conversion system2A of the second embodiment. In the power conversion system2A of the second embodiment, the voltage of the double power supply frequency component varies depending on the magnitude of power consumption of a load (for example, the electric motor3). For example, among ripples of a smoothed voltage, a ripple at the timing corresponding to a phase in which an open phase has occurred is larger than a ripple at the timing corresponding to other phases, so that the ripple increases as the voltage of the double power supply frequency component is larger than the magnitude of power consumption of the load (for example, the electric motor3).

FIG.8illustrates the simulation result of the voltage of the double power supply frequency component of the inter-terminal voltage vcWP when the fundamental frequency of the AC power supply1is set to 50 Hz, the output frequency of the inverse conversion device group50A is set to 100 Hz, the DC output voltage of the inverse conversion device group50A is set to 5,700 V, the current of the electric motor3is changed to 1,000 A, 1,500 A, and 2,000 A. As illustrated inFIG.8, when there is no open phase, even though the current of the electric motor3is changed, there is almost no change in the voltage of the double power supply frequency component. However, when there is the open phase on the transformer output line25WPR, the voltage of the double power supply frequency component increases as the current of the electric motor3increases.

FIG.9is a flowchart of the open phase determination process by the open phase detection device of the second embodiment. Since the flowchart ofFIG.9is the same as the flowchart ofFIG.4, except that step S130is changed to step S130A and steps S135and S136are added, the same steps are denoted by the same reference numerals and a description thereof will be omitted.

After the process of step S120, the determination section410determines whether the voltage VFextof the extracted frequency component Fextstored in the storage section420is larger than the first threshold voltage VTH1(step S130A). When the voltage VFextis larger than the first threshold voltage VTH1, the determination section410advances the process to step S135and determines whether the voltage VFextof the extracted frequency component Fextstored in the storage section420is larger than the second threshold voltage VTH2(step S135). When the voltage VFextis larger than the second threshold voltage VTH2, the determination section410outputs a second open phase detection signal to the control unit55(step S136). On the other hand, in step S135, when the voltage VFextis equal to or less than the second threshold voltage VTH2, the determination section410outputs a first open phase detection signal to the control unit55(step S140).

In step S130A, when the voltage VFextis equal to or less than the first threshold voltage VTH1, the determination section410ends the open phase determination process.

According to the second embodiment, in the power conversion system2A including the 36-pulse diode inverter (the forward conversion device group30A) and the five-level converter (the inverse conversion device50), it is possible to detect an open phase based on the inter-terminal voltage of the capacitor35detected by the voltage detector42.

According to the second embodiment, under the condition (the above A.) that the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A do not substantially coincide with each other, it is possible to detect an open phase in the following first and second cases (the above A2 and A3).

As the first case (A2), when the voltage of the double power supply frequency component is larger than the first threshold voltage VTH1and is equal to or less than the second threshold voltage VTH2, the open phase detection devices40UP,40UN,40VP, and40VN determine that an open phase has occurred in a phase (for example,25WPR) other than phases to which the open phase detection devices40UP,40UN,40VP, and40VN are connected among a plurality of phases. In this way, when the power supply frequency and the inverter frequency do not substantially coincide with each other, it is not possible to detect an open phase in a phase other than its own phase.

As the second case (A3), when the voltage of the double power supply frequency component is larger than the second threshold voltage VTH2, the open phase detection devices40WP and40WN determine that an open phase has occurred in the phase (for example,25WPR) to which the open phase detection devices40WP and40WN are connected among the plurality of phases. In this way, when the power supply frequency and the inverter frequency do not substantially coincide with each other, it is possible to detect an open phase in its own phase.

Furthermore, according to the second embodiment, under the condition (the above B.) that the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A substantially coincide with each other, it is possible to detect an open phase in the third case (the above B2).

As the third case (B2), when the voltage of the double power supply frequency component is larger than the third threshold voltage VTH3, the open phase detection device40WP determines that an open phase has occurred in a phase to which the open phase detection device40WP is connected among the plurality of phases. In this way, even when the power supply frequency and the inverter frequency substantially coincide with each other, it is possible to detect an open phase in its own phase.

Modified Example of Second Embodiment

A power conversion system of the modified example of the second embodiment has the same configuration as that of the power conversion system2A of the second embodiment, except that the open phase detection devices40UP and40VP are provided, but the open phase detection devices40UN,40VN,40WP, and40WN are not provided. As described above, in the case (the above A.) where the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A do not substantially coincide with each other, each of the open phase detection devices40UP and40VP can make three types of determinations of (A1) no open phase, (A2) open phase in phases other than its own phase, and (A3) open phase in its own phase, by using the first threshold voltage VTH1and the second threshold voltage VTH2.

Since the open phase detection devices40UP and40VP are provided for the U-phase and the V-phase, respectively, it is possible to detect whether an open phase has occurred in any of the U-phase, the V-phase, the W-phase, based on the determination result of the open phase detection device40UP and the determination result of the open phase detection device40VP. That is, even though the open phase detection device40is not provided in the W-phase, for example, when both the open phase detection devices40UP and40VP determine that there is an open phase in phases other than its own phase (A2), it is possible to detect that there is the open phase in the W-phase. That is, when the open phase detection device40is provided for at least two of the U-phase, the V-phase, the W-phase, it is possible to determine whether the open phase has occurred in any of the U-phase, the V-phase, the W-phase.

Third Embodiment

FIG.10is a configuration diagram of a power conversion system2B of a third embodiment. The power conversion system2B has the same configuration as that of the power conversion system2A of the second embodiment, except that an open phase detection device40W is provided for the capacitor35WP and no open phase detection device40is provided for the other capacitors. In the second embodiment, the open phase detection devices40UP,40UN,40VP,40VN,40WP, and40WN are connected to the capacitors35UP,35UN,35VP,35VN,35WP, and35WN, respectively; however, in the third embodiment, the open phase detection device40is not connected to the capacitors35UP,35UN,35VP,35VN,35WP, and35WN and the open phase detection device40W is connected to the capacitor35WP. The open phase detection device40WP of the third embodiment has the same configuration as that of the open phase detection device40of the first embodiment.

As described in the second embodiment, the open phase detection device40W determines whether the voltage VFextof the extracted frequency component Fextis larger than the first threshold voltage VTH1and the second threshold voltage VTH2, thereby detecting an open phase of the secondary winding24UP, the tertiary winding24UN, the secondary winding24VP, the tertiary winding24VN, the transformer output lines25UPR,25UPS,25UPT,25UNR,25UNS, and25UNT of the transformer20U, and the transformer output lines25VPR,25VPS,25VPT,25VNR,25VNS, and25VNT of the transformer20V, in addition to detecting an open phase of the secondary winding24WP, the tertiary winding24WN, and the transformer output lines25WPR,25WPS,25WPT,25WNR,25WNS, and25WNT of the transformer20W. However, in such a case, the condition is that the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device50W are not substantially the same.

In this way, in the power conversion system2B including the 36-pulse diode inverter (the forward conversion device group30A) and the five-level converter (the inverse conversion device50), even though no open phase detection device40is provided for all the capacitors35UP,35UN,35VP,35VN,35WP, and35WN, the open phase detection device40WP is provided for the capacitor35WP, so that it is possible to detect an open phase in all the phases.

First Modified Example Common to Embodiment

FIG.11is a configuration diagram of an open phase detection device40A of a first modified example of the first to third embodiments. The open phase detection device40A includes, for example, the voltage detector42and an open phase detection unit44A. The open phase detection unit44A has the same configuration as that of the open phase detection unit44, except that a band pass filter section406A is provided instead of the fast Fourier transform section406and the extraction section408in the open phase detection device40of the first to third embodiments.

A pass frequency band of the band pass filter section406A includes a frequency that is twice as high as the frequency of the AC power supply1. For example, the center frequency of the pass frequency band of the band pass filter section406A coincides with the frequency that is twice as high as the frequency of the AC power supply1. The band pass filter section406A allows a component (double power supply frequency component) having a frequency corresponding to the pass frequency band among voltage detection values to pass therethrough and supplies the double power supply frequency component to the determination section410. The determination section410determines that an open phase has occurred when the voltage of the double power supply frequency component is larger than a predetermined threshold voltage.

According to the open phase detection device40A of the first modified example, the band pass filter section406A extracts the double power supply frequency component by a frequency component selection characteristic of a band pass filter. In this way, even though the fast Fourier transform section406and the extraction section408are not provided, it is possible to extract a component in a desired pass frequency band and to detect an open phase.

Second Modified Example Common to Embodiment

FIG.11is a configuration diagram of an open phase detection device40B of a second modified example of the first to third embodiments. The open phase detection device40B includes, for example, the voltage detector42, a band pass filter circuit43, and an open phase detection unit44B. The open phase detection unit44B has the same configuration as that of the open phase detection device40, except that the fast Fourier transform section406and the extraction section408are not provided in the open phase detection device40of the first to third embodiments.

A pass frequency band of the band pass filter circuit43includes a frequency that is twice as high as the frequency of the AC power supply1. For example, the center frequency of the pass frequency band of the band pass filter circuit43coincides with the frequency that is twice as high as the frequency of the AC power supply1. The band pass filter circuit43allows a component (double power supply frequency component) having a frequency corresponding to the pass frequency band among voltage detection values output from the voltage detector42to pass therethrough and supplies the double power supply frequency component to the acquisition section404. The acquisition section404acquires the double power supply frequency component and stores the double power supply frequency component in the storage section420as a voltage detection value. The determination section410reads a predetermined number of voltage detection values from the storage section420and determines whether an average value of the predetermined number of voltage detection values is larger than the threshold voltage VTH. When the average value of the predetermined number of voltage detection values is larger than the threshold voltage VTH, the determination section410outputs the open phase detection signal to the control unit55. Note that the setting of the threshold voltage VTHis as described above.

According to the open phase detection device40B of the second modified example, the band pass filter circuit43extracts the double power supply frequency component. In this way, even though the fast Fourier transform section406and the extraction section408are not provided, it is possible to extract a component in a desired pass frequency band and to detect an open phase.

Third Modified Example Common to Embodiment

In the first to third embodiments, the open phase detection unit44outputs the open phase detection signal to the control unit55in the open phase determination process, and the control unit55changes the control state of PWM control based on the open phase detection signal. In the third modified example of the first to third embodiments, in addition to or instead of the configurations of the first to third embodiments, the open phase detection unit44may transmit the open phase detection signal to a high-level device (not illustrated). Furthermore, the open phase detection unit44may output the open phase detection signal from a display device or a speaker as an image, voice and the like, thereby notifying a worker of the occurrence of an open phase.

According to at least one embodiment described above, the power conversion device includes the rectifier, the capacitor, the voltage detection unit, and the open phase detection unit. The rectifier full-wave rectifies alternating current of a plurality of phases supplied from a power supply side. The capacitor smoothes an output voltage of the rectifier. The voltage detection unit detects the smoothed voltage. The open phase detection unit detects that an open phase has occurred in the alternating current of the plurality of phases based on a component of a frequency that is twice as high as a fundamental frequency of the alternating current of the plurality of phases included in frequency components of the smoothed voltage.

While certain embodiments of the invention have been described, these embodiments have been presented by way of examples only and are not intended to limit the scope of the invention. These embodiments can be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes can be made without departing from the spirit of the present invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are also included in the invention described in the claims and equivalents thereof.

In the embodiments described above, the examples of the 6-pulse and 36-pulse diode converters have been described; however, the present invention is not limited thereto. The present invention can also be applied to diode converters of 12 pulses, 24 pulses and the like.

In the embodiments described above, the output frequency of the inverse conversion device group50A is fixed to a predetermined value; however, the output frequency of the inverse conversion device group50A may be changed in real time to accelerate or decelerate the electric motor3. In such a case, a process such as adding and discarding a flag may be performed such that an inter-terminal voltage at the timing, at which the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A substantially coincide with each other, is not employed in the open phase determination process by the open phase detection device40. In this way, even when the timing, at which the fundamental frequency of the AC power supply1and the output frequency of the inverse conversion device group50A substantially coincide with each other, occurs by changing the output frequency of the inverse conversion device group50A to accelerate or decelerate the electric motor3, it is possible to exclude the inter-terminal voltage at such a timing from the open phase determination process by the open phase detection device40.

REFERENCE SIGNS LIST