1. A power converter, provided with: an input transformer whose primary winding is connected to a main power supply and which has a plurality of secondary windings and a tertiary winding connected to an auxiliary power supply; a main switch connected between said main power supply and said primary winding; an auxiliary switch connected between said auxiliary power supply and said tertiary winding; a current restriction part connected between said auxiliary power supply and said auxiliary switch; a first unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a first gate pulse signal and, also, has one end connected to a load; a second unit inverter connected in series with said first unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said first gate pulse signal; and a control device performing control for closing said auxiliary switch, closing said main switch after a prescribed time has elapsed, and opening said auxiliary switch after a prescribed time has elapsed.

2. The power converter according to Claim 1, wherein: said main power supply and said auxiliary power supply are three-phase alternating currents; said tertiary winding of said input transformer is connected to said auxiliary power supply in a wiring pattern in which the phases coincide with those of said main power supply connected to said primary winding; and said current restriction part is an initial charging resistor

3. The power converter according to Claim 2, further provided with: a third unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a second gate pulse signal, differing from said first gate pulse signal, and, also, has one end connected to a load; a fourth unit inverter connected in series with said third unit inverter and making -14- a conversion into a single-phase alternating current with variable frequency by means of control of said second gate pulse signal; a fifth unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a third gate pulse signal, differing from said first gate pulse signal and said second gate pulse signal, and, also, has one end connected to a load; and a sixth unit inverter cormected in series with said fifth unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said third gate pulse signal.

4. The power converter according to Claim 1, further provided with: a winding connection part connected between said auxiliary switch and said tertiary winding and making the phases of said main power supply and said auxiliary power supply coincide; a first phase detector detecting the phases of said main power supply; a second phase detector detecting the phases of said auxiliary power supply; and wherein: said main power supply and said auxiliary power supply are three-phase alternating currents; said current restriction part is an initial charging resistor; said second unit inverter, together with having the same configuration as said first unit inverter, outputs a single-phase alternating current having exactly the same frequency and phase as those of said first unit inverter; and said control device, after controlling said winding connection part so that the phase of said main power supply and said auxiliary power supply coincide in said primary winding on the basis of a phase comparison signal obtained from said first phase detector and said second phase detector, performs control for closing the auxiliary switch and, after a prescribed time has elapsed, closing the main switch, and opening said auxiliary switch after a prescribed time has elapsed.

5. The power converter according to Claim 4, ftxrther provided with: a third unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a second gate pulse signal, different from said first gate pulse signal, and, also, has one end connected to a load; -15- a fourth unit inverter connected in series with said third unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said second gate pulse signal; a fifth unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a third gate pulse signal, different from said first gate pulse signal and said second gate pulse signal, and, also, has one end connected to a load; and a sixth unit inverter connected in series with said fifth unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said third gate pulse signal.

6. The power converter according to Claim 5, wherein: said auxiliary power supply is generated from a single-phase AC power supply or a DC power supply on the basis of an output signal of said first phase detector.

7. A power converter, substantially as herein described with reference to accompanying drawings. Dated this le*"" day of May 2012 of Anand & Anand Advocates Agents for the applicants

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BACKGROUND OF THE INVENTION
Field of the Invention
The present invention pertains to a power converter.
Stated in greater detail, it pertains to technology for safely charging a smoothing capacitor included in an inverter and effectively reducing the rush current arising when connecting a main high-voltage power supply thereto, in a power converter for controlling the variable speed of an induction motor Description of the Related Art
As is generally known, in a country which is lacking in resources, energy conservation is an imperative position. In particular, strict energy conservation is demanded for large-sized electric motors.
E.g., the IDF (Induced Draft Fan: a fan sucking out combustion gases from a boiler furnace) or the forced draft fan (a fan bringing in the outside air for burning fiiel) of a thermal power plant uses a three-phase AC (alternating current) motor in the range of several thousand kilowatts.
As representative technology making existing high-voltage three-phase electric motors (3.3 kV, 6.6 kV, and the like) energy-efficient, there can be cited variable speed control based on a muhiplexing power converter using the multiplex inverter method.
E.g., since it is difficult to make semiconductor elements, represented by the IGBT (Insulated Gate Bipolar Transistor), constituting an inverter, tolerant to high voltages, high-voltage outputs are accommodated by connecting low-voltage output inverters in series and multiplexing the same. Also, since the muhiplex inverter method multiplexes low-voltage inverters, there are also the effects of reducing the distortion of the inverter output voltage and preventing isolation degradation due to harmonics of the motor winding.
Since individual low-voltage inverters constituting a multiplexing power converter such as this have a need for mutually isolated power supplies, there is in most cases provided, on the input side, a transformer in which the secondary windings are divided up into the number of low-voltage inverters only.
If a three-phase AC power supply is supplied from the secondary windings of a transformer, each of the low-voltage inverters briefly rectifies this into direct current with a rectifying circuit such as a diode bridge. On that occasion, well-known smoothing capacitors

-2-
are connected in parallel on the output side of the rectifying circuit. Because, even if they are low-voltage inverters, what is concerned is capacitors into which several hundred volts are input, the electric charge accumulated in the smoothing capacitors works out to a considerable electric charge quantity. Since a number of low-voltage inverters such as these are provided and multiplexed, the total electrostatic capacitance of the smoothing capacitors incorporated in all of the low-voltage inverters, and then the total quantity of electric charge accumulated in the same smoothing capacitor groups, become enormous.
Because of this, if high-voltage three-phase AC power is suddenly supplied to the primary winding with respect to a transformer in which low-voltage inverters containing the smoothing capacitors are connected to a number of secondary windings, there is generated an enormous rush current in order to charge the smoothing capacitors. There is a risk that this rush current reaches several times, or several tens of times, the rated current of the constituent components of the transformer and the like and ends up destroying circuit elements.
In order to prevent the destruction of circuit elements such as these, there is the method of charging the smoothing capacitors in advance, before supplying the high-voltage three-phase AC power to the primary winding. In order to initially charge the smoothing capacitors together, there is put into practical use a method of adding a winding for initial charging to the power supply transformer. This method is useful from the viewpoint of not providing initial charging circuits to the individual low-voltage inverters and from the viewpoint of making a reduction in cost.
SUMMARY OF THE INVENTION
As for conventional multiplex inverters for the drive of a high-vohage motor, initial charging in the stage before impressing the high voltage is carried out with respect to the smoothing capacitors included in a plurality of low-voltage inverters in order to reduce the rush current arising when a high voltage is impressed on the primary winding of an input transformer In this way, it is possible to effectively suppress the charging current with respect to the smoothing capacitors concerned by rush currents.
This initial charging procedure has been implemented in the order of (1) carrying out initial charging through impedance, (2) halting the initial charging, and (3) impressing a high voltage on the primary winding.
However, even in this conventional example, a rush current due to magnetic induction saturation flows at the time of impressing the high voltage in the excitation circuit of the input transformer Regarding the induction rush current at the time of directly connecting

-3-
the transformer to the high-voltage AC power supply, it is widely commented upon in various references. Since there are cases in which this induction rush current attains as much as five to ten times the rated current, there is a risk that fluctuations in the power supply voltage are triggered together with the generation of the rush current and that adverse effects are exerted on other devices connected to the same grid.
Regarding this problem, in JP-A-59-11730, there is proposed a method of providing an auxiliary excitation device for impressing a power supply on the primary side while exciting the secondary side from a separate power supply via an impedance. However, in this case, the auxiliary excitation device is configured as an additional device, so the cost increases.
The present invention has for an object to solve such problems, reduce rush currents eiFectively without adding complex configuration circuits, and furnish a safe and stable power converter
In order to solve the aforementioned problems, the power converter of the present invention is provided with:
a plurality of secondary windings that are connected to the main power supply of a primary winding;
an input transformer having a tertiary winding connected to an auxiliary power supply;
a main switch connected between the main power supply and the primary winding;
an auxiliary switch connected between the auxiliary power supply and the tertiary winding;
a current restriction part connected between the auxiliary power supply and the auxiliary switch;
a first unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied fi-om said secondary windings into direct current, converts the same into a single-phase alternating current with variable fi^equency by means of a first gate pulse signal and, also, has one end connected to a load;
a second unit inverter connected in series with the first unit converter and making a conversion into single-phase alternating current with variable fi-equency by means of control of the first gate pulse signal; and
a control device performing control for closing the auxiliary switch and, after a prescribed time has elapsed, closing the main switch.
Upon making in advance the configuration of the tertiary winding and the input

-4-
transformer so that the phase due to the auxiliary power supply coincides with the phase of the main power supply, the power converter is able, by opening an auxiliary switch connected to the auxiliary power supply and the tertiary winding of the input transformer, after throwing the main switch to which the main power supply is connected, to effectively suppress the excitation rush current of the input transformer.
According to the present invention, it is possible to reduce the rush current effectively and fiimish a safe and stable power converter, without adding complex configuration circuits.
BRffiF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram of a power converter which is an embodiment of the present invention.
Figs. 2 A and 2B are, respectively, a diagram describing a connection mode of a unit inverter and a circuit diagram of the unit inverter
Fig. 3 is a block diagram of a control device.
Fig. 4 is a flow chart indicating the flow of operation of a control device.
Fig. 5 is an explanatory diagram of the current waveforms of each part in a conventional example power converter
Fig. 6 is an explanatory diagram of voltage waveforms of the primary winding of an input transformer and the magnetic induction generated in the input transformer, in a conventional example power converter.
Fig. 7 is an explanatory diagram of the current waveforms of each part, in a power converter being an embodiment of the present invention.
Fig. 8 is an explanatory diagram of voltage waveforms of the primary winding of an input transformer and the magnetic induction generated in the input transformer, in a power converter being an embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
System Configuration
Fig. 1 is a block diagram of a power converter which is an embodiment of the present invention.
A main power supply 102, which is a three-phase AC power supply, supplies electric power for driving an induction motor 103. A primary winding 106 of an input transformer 105 is connected, via a main switch 104, to main power supply 102. A plurality of

-5-
secondary windings 107a, 107b, 107c, 107d, 107e, and 107f are provided in input transformer 105. To each of secondary windings 107a to 107f, there are respectively connected unit inverters 108a, 108b, 108c, 108d, 108e, and 108f In power converter 101 of Fig. 1, for each phase, two unit inverters are connected in series and two unit inverters are three-phase star connected to output three-phase AC voltage.
On the other hand, an auxiliary power supply 109, which is a three-phase AC power supply, is connected, separately from main power supply 102, to a tertiary winding 113 of input transformer 105 via an initial charging resistor 110, an auxiliary switch 111, and a winding connection part 112.
Auxiliary power supply 109 supplies three-phase AC power at a low voltage, compared to main power supply 102.
Initial charging resistor 110 is a current restriction resistance for restricting the current flowing in the smoothing capacitors that are provided in the interior of unit inverters 108atol08f
Winding connection part 112 is a switch for matching the phases of the three-phase AC voltage that is output from main power supply 102 and the three-phase AC voltage that is output from auxiliary power supply 109 and also carries out delta star conversion, as the need arises.
Unit inverter 108a is an example of a first unit inverter, has one end connected to a load called induction motor 103, and generates a single-phase AC voltage with variable frequency.
Unit inverter 108b is an example of a second unit inverter, is connected in series with unit inverter 108a, and generates a single-phase AC voltage that is equal in frequency and phase to that of unit inverter 108a.
Unit inverter 108c is an example of a third unit inverter, has one end connected to the load called induction motor 103, and generates a single-phase AC voltage that has exactly the same frequency as, and a phase that differs from, that of unit inverter 108a.
Unit inverter 108d is an example of a fourth unit inverter, is connected in series with unit inverter 108c, and generates a single-phase AC voltage that is equal in frequency and phase to that of unit inverter 108c.
Unit inverter 108e is an example of a fifth unit inverter, has one end connected to the load called induction motor 103, and generates a single-phase AC voltage that has exactly the same frequency as, and a phase that differs from, that of unit inverter 108a and unit inverter 108c.

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Unit inverter 108f is an example of a sixth unit inverter, is connected in series with unit inverter 108e, and generates a single-phase AC voltage that is equal in frequency and phase to that of unit inverter 108e.
Between main power supply 102 and main switch 104, there is connected a first phase detector 116. Similarly, between auxiliary power supply 109 and initial charging resistor 110 as well, there is connected a second phase detector 114.
First phase detector 116 and second phase detector 114 are connected to a control device 115. Control device 115 controls main switch 104, auxiliary switch 111, winding connection part 112, and unit inverters 108a to lOSf
In a steady state, main switch 104 of power converter 101 is closed and auxiliary switch 111 is open. And then, due to the fact that control device 115 supplies a gate pulse signal to unit inverters 108a to 108f, a three-phase AC voltage is impressed on induction motor 103, and induction motor 103 is driven rotatably.
Figs. 2Aand 2B are, respectively, a diagram describing a connection mode of a unit inverter and a circuit diagram of the unit inverter
As shown in Fig. 2A, unit inverters 108h, 108i, ..., and 108n outputting the same AC voltage are able, by being connected in series, to collectively output high AC voltages. On that occasion, the first output terminal 201 of a certain unit inverter and the second output terminal 202 of a neighboring unit inverter are cormected. This has precisely the same meaning as joining the polarities of batteries outputting a DC voltage.
In order to make the respective unit inverters 108h, 108i, ..., and 108n connected in series match the respective fi-equencies and phases of the AC voltages that are output by the unit inverters, there is supplied a common gate pulse signal from control device 115.
Fig. 2B is a circuit diagram of a unit inverter 108.
The three-phase AC voltage supplied from secondary winding 107 of input transformer 105 is impressed on a bridge recfifying circuit composed of diodes D203a, D203b, D203c, D203d, D203e, and D204f and is converted into the DC voltage of a pulsating current. In the bridge rectifying circuit, smoothing capacitor C204 is connected in parallel. The DC voltage obtained in this way is supplied to a full bridge inverter composed of IGBTs 205a, 205b, 205c, and 205d. A gate pulse signal is supplied from control device 115 to the IGBT gates and an AC voltage based on PWM (Pulse Width Modulation) control is output.
Further, diodes D206a, D206b, D206c, and D206d that are fi-eewheeling diodes or also called reflux diodes are connected in parallel between the collectors and emitters of the IGBTs in order to prevent breakdown incidents of circuit elements due to a reverse electromotive

-7-force generated from induction motor 103.
In this manner, there is generated a single-phase AC voltage for which the output power and frequency are controlled by means of a gate pulse signal from first output terminal 201 and second output terminal 202.
As appreciated from the aforementioned description, there are supplied gate pulse signals having a common phase to the unit inverters connected in series.
If Fig. 1 is viewed on fiirther reflection, unit inverter 108a and unit inverter 108b are connected in series, making up the first phase of a star-type three-phase AC power supply.
Similarly, unit inverter 108c and unit inverter 108d are connected in series, making up the second phase of a star-type three-phase AC power supply.
Similarly, unit inverter 108e and unit inverter 108f are connected in series, making up the third phase of a star-type three-phase AC power supply.
In order to form each of the phases of the three-phase alternating current, gate pulse signals with differing phases are supplied to the phase-forming unit inverters.
In other words, a first gate pulse signal supplied to unit inverter 108a and unit inverter 108b, a second gate pulse signal supplied to unit inverter 108c and unit inverter 108d, and a third gate pulse signal supplied to unit inverter 108e and unit inverter 108f respectively have differing phases.
Fig. 3 is a block diagram of control device 115.
The output signals of first phase detector 116 and second phase detector 114 are respectively supplied to a phase difference detection part 301. Phase difference detection part 301 detects the phase difference between the three-phase AC voltage output by main power supply 102 and the three-phase AC voltage supplied by auxiliary power supply 109. A connection control part 302 outputs a control signal controlling the connection state of wind connection part 112.
A control part 303 receives a phase difference detection signal output by phase difference detection part 301 and outputs information about an optimal connection pattern to connection control part 302. Also, while carrying out timing with a timer 304, it carries out on/off control of auxiliary switch 111 and main switch 104. And then, after closing main switch 104, it outputs control command information for appropriate power generation to a gate pulse generation part 305.
Gate pulse generation part 305 generates and outputs PWM control signals to IGBTs 205a, 205b, 205c, and 205d that constitute unit inverters 108a to 108f
Control part 303 receives an operation command by means of operation part 306

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and displays the operating state via a display part 307.
Flow of the throw processing of main power supply 102 of control device 115
Fig. 4 is a flow chart indicating the flow of operation of control device 115.
When control part 303 receives a launch command from the operator via operation part 306 and starts processing (Step S401), phase difference detection part 301 detects the present phase difference between the three-phase AC voltage that is output by main power supply 102 and the three-phase AC voltage that is output by auxiliary power supply 109 and provides the same phase difference information to control part 303 (Step S402). Control part
303 controls winding connection part 112 via connection control part 302 on the basis of the
phase difference and carries out the phase matching in input transformer 105 (Step S403).
When phase matching has been completed, control part 303 next controls auxiliary switch 111 to be ON (Step S404). And then, it launches timer 304 (Step S405) and checks whether a prescribed time (e.g. 1 s) needed for the current flowing from auxiliary power supply 109 to become sufficiently small has elapsed (Step S406).
If it is confirmed in Step S406 that the prescribed time has elapsed ("Yes" in Step S406), control part 303 resets timer 304 and controls it to be OFF (Step S407) and next controls main switch 104 to be ON (Step S408). And then, timer 304 is launched once again (Step S409) and checks whether a prescribed time (e.g. 20 ms) needed for main power supply 102 and auxiliary power supply 109 to be simultaneously connected (overlapping) to input transformer 105 has elapsed (Step S410).
If it is confirmed in Step S410 that the prescribed time has elapsed ("Yes" in Step S410), control part 303 controls auxiliary switch 111 to be OFF (Step S411) and next resets timer
304 and controls it to be OFF (Step S412). And next, it controls gate pulse generation part 305
and starts control of the induction motor (Step S413) and terminates the string of processes (Step
S414).
In other words, power converter 101 of the present embodiment, after connecting auxiliary power supply 109 to input transformer 105 (Step S404) and charging smoothing capacitors C204 inside unit inverters 108a to lOSf, connects main power supply 102 to input transformer 105 (Step S408) without disconnecting auxiliary power supply 109 and, if the prescribed time needed for the overlap has elapsed ("Yes" in Step S410), disconnects auxiliary power supply 109 (Step S411). Operation of power converter 101
Fig. 5 is an explanatory diagram of the current waveforms of each part in a conventional example power converter 101.

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Fig. 6 is an explanatory diagram of voltage waveforms of the primary winding 106 of an input transformer 105 and the magnetic induction generated in the input transformer 105, in conventional example power converter 101.
In power converter 101 of the conventional example, after opening auxiliary switch 111, time was set aside until main switch 104 got closed (P501). Because of this, if main switch 104 was closed, there flowed a big excitation rush current the same instant in input transformer 105 (P502).
This excitation rush current flows as a result of the magnetic induction saturation of input transformer 105. The principle thereof will be described with reference to Fig. 6. The upper section of Fig. 6 is a voltage waveform appearing in primary winding 106 of input transformer 105 and the lower section is a waveform of the magnetic induction generated in input transformer 105.
As for the time interval during which auxiliary switch 111 is closed, the voltage of auxiliary power supply 109 (P601) is set to be somewhat lower, compared with the voltage of main power supply 102 (P602), in order to make an excitation via initial charging resistor 110. The induction waveform lags the voltage waveform by 90°.
When auxiliary switch 111 is opened (P603), after the magnetic induction has diminished a little with an internal time constant, the held fixed value is maintained (P604).
When main switch 104 is closed (P605) in this state with magnetic induction remaining, there is, from the same moment, magnetic induction (P606) due to the voltage of main power supply 102 is generated and added to the remaining magnetic induction (P604).
The combined magnetic induction arising from adding the residual magnetic induction and the magnetic induction due to the voltage of main power supply 102 shows a maximum value (P607) when main switch 104 is thrown with a timing at which the voltage of main power supply 102 is zero. In the example of Fig. 6, it works out to being equal to or greater than twice that of the normal value. If the magnetic induction becomes great like this, a magnetic induction saturation is triggered in an input transformer 105 created with a regular design. If so, as a result, the rush current becomes greater in order for the impedance of the excitation circuit to decrease rapidly.
Fig. 7 is an explanatory diagram of the current waveforms of each part, in a power converter 101 of the present embodiment.
Fig. 8 is an explanatory diagram of the voltage waveforms of the primary winding 106 of an input transformer 105 and the magnetic induction generated in input transformer 105, in a power converter 101 of the present embodiment.

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A point of difference of power converter 101 of the present embodiment with the conventional example is the point that the timing of opening auxiliary switch 111 is chosen to be after main switch 104 has been closed (P701). By proceeding in this way, it is possible to reduce the increase in magnetic induction after main switch 104 has been closed. The principle thereof will be described with reference to Fig. 8.
Power converter 101 of the present embodiment, at the moment (P801) when main switch 104 has been closed, has magnetic induction (negative value) due to auxiliary power supply 109 which is synchronized with the voltage and phase of main power supply 102 (P802). For this reason, even if there is generated magnetic induction (P803) due to main power supply 102, it is possible to suppress the combined magnetic induction (P804) to be lower than in the conventional example. Consequently, the rush current also becomes smaller (P702).
In power converter 101 of the present embodiment, when auxiliary power supply 109 is impressed on tertiary winding 113, it is necessary for the voltage phase appearing in primary winding 106 to coincide with the phase of the voltage of main power supply 102. This is possible, if the phase relationship of auxiliary power supply 109 and main switch 102 on the equipment side is investigated in advance, by making the two coincide by means of the winding configuration of tertiary winding 113. Because of this, first phase detector 116, second phase detector 114, and winding connection part 112 are provided and phase matching is conducted in advance in control device 115, before connecting auxiliary power supply 109 to input transformer 105. Also, from the viewpoint of making the phases match, it is necessary, as a prerequisite that the frequencies also coincide.
In the present embodiment, the following application examples are possible.
(1) Main power supply 102 may be a single-phase alternating current. In this case, there results that a winding connection part 112 is added between auxiliary power supply 109 and input transformer 105 and a three-phase to two-phase conversion or a three-phase to single-phase conversion is carried out in a well-known Scott transformer or the like.
(2) It is possible to generate auxiliary power supply 109 from main power supply 102. In addition to input transformer 105, an auxiliary transformer is prepared separately and tertiary winding 113 of input transformer 105 is connected via initial charging resistor 110 to the secondary winding of the auxiliary transformer.
(3) In power converter 101 related to the aforementioned embodiment, before connecting tertiary winding 113 of input transformer 105 to auxiliary power supply 109, the difference in phase between main power supply 102 and auxiliary power supply 109 was detected from first phase detector 116 and second phase detector 114 and winding connection

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part 112 was controlled in order to align the phases of main power supply 102 and auxiliary power supply 109 in input transformer 105.
In case main power supply 102 and auxiliary power supply 109 are generated from a common power supply and the load being interposed up to reaching input transformer 105 does not vary substantially, it can be expected that the difference in phase between main power supply 102 and auxiliary power supply 109 is usually fixed. In a case like this, a winding connection part 112 modifying the connection state of tertiary winding 113 in response to the phase difference is unnecessary and auxiliary power supply 109 and tertiary winding 113 may be connected in a wiring pattern adapted to the phase difference known in advance.
In a case like this, winding connection part 112 in Fig. 1, phase difference detection part 301 and connection control part 302 in Fig. 3, as well as Step S402 and Step S403 in Fig. 4, become unnecessary (omission object A120 in Fig. 1, omission object A320 in Fig. 3, and omission object A420 in Fig. 4).
(4) Instead of initial charging resistor 110, it is possible to provide a current restriction circuit based on chopper control. It is also possible to call initial charging resistor 110 and the current restriction circuit based on chopper control "current restriction parts".
(5) As is clear from the description this far, it is a condition that auxiliary power supply 109 has a phase that substantially matches that of main power supply 102. As long as this condition is met, it is also possible to generate auxiliary power supply 109 from a single-phase AC power supply or a DC power supply. If what is concerned is a single-phase AC power supply, it is possible to use unit inverter 108 in Fig. 2B as is. If what is concerned is a DC power supply, it is possible to use a configuration in which the rectifying bridge circuit and smoothing capacitor C204 of unit inverter 108 in Fig. 2B have been omitted. On that occasion, there is created, on the basis of the output signal obtained from first phase detector 116, an auxiliary power supply 109 which is a three-phase AC power supply for which the phases are equal to those of main power supply 102.
In the present embodiment, power converter 101 has been disclosed.
As for power converter 101 of the present embodiment, it is possible, upon aligning in advance the phase of auxiliary power supply 109 with that of main power supply 102, to effectively suppress the excitation rush current of input transformer 105 by opening auxiliary switch 111 connected to input transformer 105 after connecting main switch 104.
Above, there has been given a description regarding an embodiment of the present invention, but the present invention is not one limited to the aforementioned embodiment and includes other variations and applications, as long as the same do not depart from the substance

-12-
of the present invention, set out in the patent claims. Reference Signs List
101 power converter
102 main power supply
103 induction motor
104 main switch
10 5 input transformer
106 primary winding
107 secondary winding
107a secondary winding
108, 108a, 108b, 108c, 108d, 108e, 108f unit inverter
109 auxiliary power supply
110 initial charging resistor
111 auxiliary switch
112 winding connection part
113 tertiary winding
114 second phase detector
115 control device
116 first phase detector

201 first output terminal
202 second output terminal 205a, 205b, 205c, 205d IGBT

301 phase difference detection part
302 connection control part
303 control part
304 timer
305 gate pulse generation part
306 operation part
307 display part
C204 smoothing capacitor
D203a, D203b, D203c, D203d, D203e, D203f, D206a, D206b, D206c, D206d diode

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CLAIMS:
1. A power converter, provided with:
an input transformer whose primary winding is connected to a main power supply and which has a plurality of secondary windings and a tertiary winding connected to an auxiliary power supply;
a main switch connected between said main power supply and said primary winding;
an auxiliary switch connected between said auxiliary power supply and said tertiary winding;
a current restriction part connected between said auxiliary power supply and said auxiliary switch;
a first unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a first gate pulse signal and, also, has one end connected to a load;
a second unit inverter connected in series with said first unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said first gate pulse signal; and
a control device performing control for closing said auxiliary switch, closing said main switch after a prescribed time has elapsed, and opening said auxiliary switch after a prescribed time has elapsed.
2. The power converter according to Claim 1, wherein:
said main power supply and said auxiliary power supply are three-phase alternating currents;
said tertiary winding of said input transformer is connected to said auxiliary power supply in a wiring pattern in which the phases coincide with those of said main power supply connected to said primary winding; and
said current restriction part is an initial charging resistor
3. The power converter according to Claim 2, further provided with:
a third unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a second gate pulse signal, differing from said first gate pulse signal, and, also, has one end connected to a load;
a fourth unit inverter connected in series with said third unit inverter and making

-14-
a conversion into a single-phase alternating current with variable frequency by means of control of said second gate pulse signal;
a fifth unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a third gate pulse signal, differing from said first gate pulse signal and said second gate pulse signal, and, also, has one end connected to a load; and
a sixth unit inverter cormected in series with said fifth unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said third gate pulse signal.
4. The power converter according to Claim 1, further provided with:
a winding connection part connected between said auxiliary switch and said tertiary winding and making the phases of said main power supply and said auxiliary power supply coincide;
a first phase detector detecting the phases of said main power supply;
a second phase detector detecting the phases of said auxiliary power supply; and wherein:
said main power supply and said auxiliary power supply are three-phase alternating currents;
said current restriction part is an initial charging resistor;
said second unit inverter, together with having the same configuration as said first unit inverter, outputs a single-phase alternating current having exactly the same frequency and phase as those of said first unit inverter; and
said control device, after controlling said winding connection part so that the phase of said main power supply and said auxiliary power supply coincide in said primary winding on the basis of a phase comparison signal obtained from said first phase detector and said second phase detector, performs control for closing the auxiliary switch and, after a prescribed time has elapsed, closing the main switch, and opening said auxiliary switch after a prescribed time has elapsed.
5. The power converter according to Claim 4, ftxrther provided with:
a third unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a second gate pulse signal, different from said first gate pulse signal, and, also, has one end connected to a load;

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a fourth unit inverter connected in series with said third unit inverter and making a conversion into a single-phase alternating current with variable frequency by means of control of said second gate pulse signal;
a fifth unit inverter which, after temporarily converting the alternating-current (AC) power supply supplied from said secondary windings into direct current, converts the same into a single-phase alternating current with variable frequency by means of a third gate pulse signal, different from said first gate pulse signal and said second gate pulse signal, and, also, has one end connected to a load; and
a sixth unit inverter connected in series with said fifth unit inverter and making a
conversion into a single-phase alternating current with variable frequency by means of control of
said third gate pulse signal.
6. The power converter according to Claim 5, wherein:
said auxiliary power supply is generated from a single-phase AC power supply or a DC power supply on the basis of an output signal of said first phase detector.
7. A power converter, substantially as herein described with reference to accompanying drawings.
Dated this le*"" day of May 2012
of Anand & Anand Advocates Agents for the applicants