Patent Publication Number: US-11043905-B2

Title: AC-AC power converter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2018-042376, filed on Mar. 8, 2018 is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an AC-AC power converter that converts electric power from a single-phase AC power source into three-phase AC power. 
     2. Description of the Related Art 
     Those that includes a rectifier circuit, a smoothing capacitor, and an inverter circuit are disclosed as AC-AC power converters that convert electric power from a single-phase AC power source into three-phase AC power (see, for example, Patent Document 1). 
     [Patent Document 1] Japanese Patent Application Publication No. H11-289769 
     [Patent Document 2] Japanese Patent Application Publication No. 2003-284387 
     Many power source devices that supply AC power of a several kW class convert AC power that is input from a single-phase power source into three-phase AC power and output the three-phase AC power. Such an AC-AC power converter is usually formed including a rectifier circuit for converting electric power from an AC power source into DC power and an inverter circuit for converting rectified DC power into AC power of a desired specification. 
     However, when single-phase to three-phase power conversion is performed, a difference occurs inevitably between input power and output power, and pulsations therefore occurs in the power. A buffer becomes necessary in order to absorb and compensate for the pulsations. Conventionally, it is a common practice to form this buffer using a large capacity DC link capacitor. When the AC-AC power converter operates at several kW and several 100 V, the capacity required for the DC link capacitor is of the order of mF. In order to realize this, it is necessary to use a large-capacity electrolytic condenser. However, since electrolytic condensers have a large volume and a short life, electrolytic condensers have a great disadvantage in terms of size, cost, device life, and the like. Therefore, there is a need for an AC-AC power converter that does not require an electrolytic condenser. 
     Examples of prior art that realize such an electrolytic condenser-less AC-AC power converter include those having a component composed of a small capacitor, an inductor, and a switch element that is added to a DC link portion so as to compensate only for power pulsations by the additional circuit. Due to this additional circuit, the role required for the DC link capacitor is limited to the removal of switching noise etc. Thus, a large capacity electrolytic condenser is unnecessary. However, this technique has problems such as an increase in component cost associated with the additional circuit and complication of control. 
     As another example of the prior art, there is a technique that realizes appropriate motor control by supplying pulsations of input power directly to a DC link voltage and controlling the switching of an inverter even when the DC voltage of a DC link has pulsations (see, for example, Patent Document 2). In other words, in this technique, the inertia of a motor or its load is used as a compensation means for power pulsations. According to this technique, electrolytic condensers can be omitted without requiring special additional components. However, in this technique, since the DC voltage of the DC link pulsates greatly, a DC booster circuit cannot be applied. Therefore, there is a problem that an output voltage higher than an input voltage cannot be obtained, which limits the applicability of the motor. 
     SUMMARY OF THE INVENTION 
     In this background, a purpose of the present invention is to reduce the capacity of a DC link capacitor of an AC-AC power converter without requiring additional components. 
     An AC-AC power convertor according to one embodiment of the present invention is an AC-AC power convertor for converting a first AC voltage to a second AC voltage, including: a rectifier circuit for rectifying the first AC voltage to generate a rectified voltage; an inverter for generating the second AC voltage from the rectified voltage; and a controller for controlling the rectifier circuit and the inverter, wherein the controller controls the rectifier circuit and the inverter such that power generated by the first AC voltage and the pulsations of power generated by the rectified voltage are output to an external device. 
     Optional combinations of the aforementioned constituting elements, and implementations of the invention in the form of methods, apparatuses, programs, transitory or non-transitory storage media, systems, and the like may also be practiced as additional modes of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings that are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several figures, in which: 
         FIG. 1  is a block diagram showing an AC-AC power converter according to an embodiment; 
         FIG. 2  is a block diagram showing a conventional AC-AC power converter; 
         FIG. 3  is a block diagram showing controllers of the AC-AC power converter of  FIG. 2 ; 
         FIG. 4A  is a diagram showing changes over time of an input voltage, an input current, and input power of the AC-AC power converter of  FIG. 2 ;  FIG. 4B  is a diagram showing changes over time of a DC link capacitor voltage, a DC link capacitor current, and DC link capacitor power of the AC-AC power converter of  FIG. 2 ;  FIG. 4C  is a diagram showing changes over time of motor rotation speed, motor torque, and motor power in  FIG. 2 ; 
         FIG. 5  is a block diagram showing a controller of the AC-AC power converter of  FIG. 1 ; 
         FIG. 6A  is a diagram showing changes over time of an input voltage, an input current, and input power of the AC-AC power converter of  FIG. 1 ;  FIG. 6B  is a diagram showing changes over time of a DC link capacitor voltage, a DC link capacitor current, and DC link capacitor power of the AC-AC power converter of  FIG. 1 ;  FIG. 6C  is a diagram showing changes over time of motor rotation speed, motor torque, and motor power in  FIG. 1 ; 
         FIG. 7  is a block diagram showing an exemplary variation of the controller of the AC-AC power converter of  FIG. 1 ; 
         FIG. 8  is a block diagram showing an exemplary variation of the controller of the AC-AC power converter of  FIG. 1 ; 
         FIG. 9  is a block diagram showing an exemplary variation of the controller of the AC-AC power converter of  FIG. 1 ; 
         FIG. 10  is a block diagram showing an exemplary variation of the AC-AC power converter according to the embodiment; and 
         FIG. 11  is a block diagram showing an AC-AC power converter according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
     In the following embodiments, like numerals represent like constituting elements, and duplicative explanations will be omitted. For the sake of ease of explanation, some constituting elements are appropriately omitted in the figures. Unless otherwise specified, with respect to a numerical value A, &lt;A&gt; represents the average value of A, and A* represents the target value of A. 
       FIG. 1  is a block diagram showing an example of an AC-AC power converter  100  according to an embodiment. The AC-AC power converter  100  functions as a power converter that generates three-phase power based on power from a single-phase power source  300 . As an example, the AC-AC power converter  100  can be used to drive a variety of devices such as pumps, compressors, electric actuators of ships and aircrafts, robotic arms, and the like. The AC-AC power converter  100  includes a PFC rectifier circuit  10 , a DC link  20 , an inverter  30 , and a controller  40 . The three-phase power generated by the AC-AC power converter  100  is output to an external device such as a motor  400 . The motor  400  is driven by the three-phase power that has been input and drives a load  500 . In this specification, the upstream side may be referred to as upstream or input, and the downstream side may be referred to as downstream or output in accordance with the flow of electric power or a signal flowing from the single-phase power source  300  to the output of three-phase power. 
     The single-phase power source  300  may be, for example, a commercial power source or a generator. The single-phase power source  300  outputs an input voltage v G  to the PFC rectifier circuit  10  of the AC-AC power converter  100 . 
     The PFC rectifier circuit  10  is a rectifier circuit having a PFC (Power Factor Correction) function and may be implemented using a publicly-known technique. The PFC rectifier circuit  10  performs full-wave rectification on the input voltage v G  that has been input from the single-phase power source  300  so as to generate a rectified voltage and then removes high frequencies from a current waveform using the PFC function. The PFC rectifier circuit  10  outputs the rectified voltage to the DC link  20 . 
     The DC link  20  is arranged downstream of the PFC rectifier circuit  10  and includes a DC link capacitor  22 . The DC link capacitor  22  is a small-capacity condenser constituted of, for example, a film or a ceramic condenser. The DC link capacitor  22  functions as a means for removing switching noise or the like. Since pulsations are removed by the control described later in the rectified voltage, the DC link capacitor  22  does not need to be a large-capacity electrolytic condenser. The DC link  20  outputs a DC link voltage v DC  to the inverter  30 . 
     The inverter  30  is arranged downstream of the DC link  20  and generates a three-phase AC voltage based on the DC link voltage v DC . The inverter  30  may be implemented using a publicly-known technique. The three-phase AC voltage is composed of, for example, a U phase, a V phase, and a W phase and may alternate with a phase difference of 2π/3. The inverter  30  supplies the AC voltage that has been generated to the motor  400 . 
     The controller  40  includes a DC link voltage controller  42 , a PFC rectifier circuit controller  44 , a speed controller  46 , and an inverter controller  48 . The controller  40  controls the PFC rectifier circuit  10  and the inverter  30  so as to adjust the input current, the rectified voltage, and the three-phase AC voltage that are generated. The details of the control by the controller  40  will be described later. 
     The motor  400  is driven by the three-phase power supplied from the inverter  30  and transmits motive power that has been obtained to the load  500  so as to move the load  500 . 
     The load  500  is a load such as a flywheel, which is moved by the motor  400 , and has inertia J TOT  (for example, the moment of inertia). 
     The rectifier circuit does not necessarily have to have a PFC function. In other words, the PFC rectifier circuit  10  may be replaced by any type of rectifier circuit that generates a DC voltage from an AC voltage. 
     The DC link  20  may be completely omitted. In this case, the PFC rectifier circuit  10  outputs the rectified voltage to the inverter  30 . 
     [Conventional Voltage Control] 
     Before describing voltage control by the AC-AC power converter according to the embodiment, voltage control by a conventional AC-AC power converter will be described.  FIG. 2  is a block diagram showing a conventional AC-AC power converter  200 . The first difference from the AC-AC power converter  100  of  FIG. 1  is that a DC link capacitor  24  is constituted of a large-capacity electrolytic condenser. This is for the purpose of buffering the difference between input power and output power so as to compensate for the difference as described later. The second difference is that a controller  40 , which is integrated in the AC-AC power converter  100  of  FIG. 1 , is separated into a first controller  50  and a second controller  60 . The first controller  50  includes a DC link voltage controller  52  and a PFC rectifier circuit controller  54 . The second controller  60  includes a speed controller  66  and an inverter controller  68 . The first controller  50  independently controls the PFC rectifier circuit  10 , and the second controller  60  independently controls the inverter  30 . Other configuration of the AC-AC power converter  200  is the same as the configuration of the AC-AC power converter  100 . In particular, it is to be noted that the controllers shown in  FIG. 1  and  FIG. 2  are abstract functional blocks, and any concrete implementation by hardware, software, or the like is not limited to these drawings. 
       FIG. 3  is a block diagram showing the first controller  50  and the second controller  60  of the AC-AC power converter  200  of  FIG. 2 . The DC link voltage controller  52  includes a first input terminal  52   b , a second input terminal  52   c , and an output terminal  52   d . The PFC rectifier circuit controller  54  includes an input terminal  54   b  and an output terminal  54   c . The speed controller  66  includes a first input terminal  66   b , a second input terminal  66   c , and an output terminal  66   d . The inverter controller  68  includes an input terminal  68   b , a first output terminal  68   c , a second output terminal  68   d , and a third output terminal  68   e . The first controller  50  includes a low pass filter  53  on the upstream side of the second input terminal  52   c  of the DC link voltage controller  52 . 
     The current DC link voltage v DC  is input to the low pass filter  53 . The low pass filter  53  removes high frequency components from v DC , generates an average DC link voltage &lt;v DC &gt;, and inputs the average DC link voltage &lt;v DC &gt; to the second input terminal  52   c  of the DC link voltage controller  52 . A target average DC link voltage &lt;v DC &gt;* is input to the first input terminal  52   b  of the DC link voltage controller  52 . The DC link voltage controller  52  calculates a target average capacitor power &lt;P C &gt;* based on the difference Δv DC  (not shown) between &lt;V DC &gt;* and &lt;v DC &gt; and outputs the target average capacitor power &lt;P C &gt;* from the output terminal  52   d.    
     A target average speed &lt;ω&gt;* of the motor  400  is input to the first input terminal  66   b  of the speed controller  66 . The current average speed &lt;ω&gt; of the motor  400  is input to the second input terminal  66   c . The speed controller  66  calculates target average inverter output &lt;P INV &gt;* based on the difference Δω (not shown) between &lt;ω&gt;* and &lt;ω&gt; and outputs the target average inverter output &lt;P INV &gt;* from the output terminal  66   d.    
     &lt;P INV &gt;* output from the output terminal  66   d  of the speed controller  66  is branched into two at a branch point v 1 , and one is added to &lt;P C &gt;* output from the output terminal  52   d  of the DC link voltage controller  52 . As a result, a target average rectified power &lt;P PFC &gt;* is calculated as &lt;P PFC &gt;*=&lt;P INV &gt;*+&lt;P C &gt;*. The calculated &lt;P PFC &gt;* is input to the input terminal  54   b  of the PFC rectifier circuit controller  54 . The other &lt;P INV &gt;* branched at the branch point v 1  is input to the input terminal  68   b  of the inverter controller  68 . 
     The PFC rectifier circuit controller  54  calculates a target input current i G * (not shown) based on the target average rectified power &lt;P PFC &gt;* that has been input, obtains a PFC output duty ratio d B  from the inductor current difference, and outputs the PFC output duty ratio d B  from the output terminal  54   c . The PFC output duty ratio d B  that has been output is input to the PFC rectifier circuit  10  via a pulse width modulator (not shown) such that desired control is realized. 
     The inverter controller  68  obtains inverter output duty ratios d U , d V  and d W  based on the target motor power &lt;P INV &gt;* that has been input and outputs the duty ratios d U , d V  and d W  to the first output terminal  68   c , the second output terminal  68   d , and the third output terminal  68   e , respectively. The inverter output duty ratios d U , d V  and d W  that have been output are input to the inverter  30  via a pulse width modulator (not shown) such that desired control is realized. 
       FIG. 4A  shows changes over time of an input voltage, an input current, and input power of the AC-AC power converter  200  of  FIG. 2 .  FIG. 4B  shows changes over time of a DC link capacitor voltage, a DC link capacitor current, and DC link capacitor power of the AC-AC power converter  200  of  FIG. 2 .  FIG. 4C  shows changes over time of motor rotation speed, motor torque, and motor power in  FIG. 2 . 
     In this specification, it is given that the input voltage (AC voltage supplied by the single-phase power source  300 ) v G  forms a sinusoidal wave having an amplitude V G  and a frequency f G  and is expressed as follows. 
     v G =V G *sin(2πf G t) In order to satisfy a condition where the power factor equals 1, the input current i G  that is input to the PFC rectifier circuit  10  is controlled so as to form a sinusoidal wave having the same frequency and the same phase as those of v G . That is, when the amplitude is denoted as I G , i G  is expressed as follows.
 
 i   G   =I   G *sin(2π f   G   t )
 
Therefore, input power p G  that is input to the PFC rectifier circuit  10  is as follows.
 
 p   G   =V   G   *i   G   =V   G *sin(2π f   G   t )* I   G *sin(2π f   G   t )= P   0 *(1−cos(2π*2 f   G   t ))
 
     Note that it is given that P 0 =V G *I G /2. As described, the input power p G  vibrates at a frequency 2f G , which is twice the frequency f G  of the input voltage v G . On the other hand, motor power p M  is controlled by the second controller  60  so as to be a temporally constant value P 0  (average value of input power p G ). 
     As shown in  FIG. 4A  and  FIG. 4C , the respective waveforms of the input power p G  and the motor power p M  do not match. The DC link capacitor  24  of the DC link  20  compensates for the difference between this input power p G  and the motor power p M  by buffering the difference. Regarding this point, an explanation will be given in the following. The DC link capacitor  24  accumulates electrostatic energy E C  inside thereof.
 
 E   C =½* C   DC   *v   DC   2  
 
     Note that the capacity of the DC link capacitor  24  is denoted as C DC . As a result, the condenser current is flows through the DC link  20 . Then, a DC link voltage pulsation Δv DC  (ripple) vibrating at a frequency 2f G , which is twice the frequency f G  of the input voltage v G , occurs in the DC link voltage v DC . The DC link voltage pulsation Δv DC  depends on the average output power P 0 , the average DC link voltage V DC , the frequency f G  of the input voltage v G , and the capacity C DC  of the DC link capacitor  24  and is expressed as follows. 
     
       
         
           
             
               
                 
                   
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                           V 
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                           C 
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     In other words, in order to compensate for this pulsation so as to suppress the pulsation, it is necessary to sufficiently increase the capacity C DC  of the DC link capacitor  24 . In general, in order to realize a normal inverter function, it is necessary to suppress the DC link voltage pulsation Δv DC  within several percent of the average DC link voltage V DC . According to Equation 5, for example, when P 0  is 5 kW, V DC  is 100 V, and f G  is 50 Hz, it is found that C DC  of about 3 mF is required when Δv DC /V DC  is suppressed to 5 percent. 
     [Voltage Control According to Embodiment] 
     Voltage control according to the embodiment of the present invention will be described.  FIG. 5  is a block diagram showing the controller of the AC-AC power converter  100  of  FIG. 1 . The DC link voltage controller  42  includes a first input terminal  42   b , a second input terminal  42   c , and an output terminal  42   d . The PFC rectifier circuit controller  44  includes an input terminal  44   b  and an output terminal  44   c . The speed controller  46  includes a first input terminal  46   b , a second input terminal  46   c , and an output terminal  46   d . The inverter controller  48  includes an input terminal  48   b , a first output terminal  48   c , a second output terminal  48   d , and a third output terminal  48   e . The controller  40  includes a low pass filter  43   a  between the output terminal  42   d  of the DC link voltage controller  42  and the input terminal  44   b  of the PFC rectifier circuit controller  44 . The controller  40  includes a low pass filter  43   b  on the upstream side of the second input terminal  46   c  of the speed controller  46 . 
     A target DC link voltage v DC * is input to the first input terminal  42   b  of the DC link voltage controller  42 . The current DC link voltage v DC  is input to the second input terminal  42   c . As will be described later, the DC link voltage v DC  is controlled so as to be a temporally constant value (having no pulsation). Therefore, unlike the AC-AC power converter  200  in  FIG. 2 , a low pass filter for eliminating high frequency components does not need to be placed upstream of the second input terminal  42   c . The DC link voltage controller  42  obtains target capacitor power p C * based on the difference Δv DC  (not shown) between v DC * and v DC  and outputs the target capacitor power p C * from the output terminal  42   d.    
     The target capacitor power p C * that is output from the output terminal  42   d  of the DC link voltage controller  42  is branched into two at a branch point v 2 , and one is input to the low pass filter  43   a . p C * generated by the DC link voltage controller  42  includes high frequency voltage noise that is caused by noise caused by the inductor of the motor, noise of the input power source, or the like. The low pass filter  43   a  removes these high frequency components from p C * to generate the target average capacitor power &lt;P C &gt;* and outputs the target average capacitor power &lt;P C &gt;*. The other p C * branched at the branch point v 2  is subtracted from target rectified power p PFC * so as to calculate target motor power p M * (p M *=p PFC *−p C *). The p M * that has been calculated is input to the input terminal  48   b  of the inverter controller  48 . 
     As described above, the target motor power p M * that is input to the inverter controller  48  is obtained by subtracting the target capacitor power p C * from the target rectified power p PFC *. In other words, the input power p G  and the pulsation Δp DC  of the DC link are input to the motor  400 . The motor  400  compensates for this pulsation by the inertia possessed by the load  500 . As a result, the pulsations of the DC link become zero, and p M =p G  is established. In other words, the motor power p M  agrees with the input power p G . 
     As described later, the speed ω of the motor pulsates at the frequency 2f G , which is twice the frequency f G  of the input power p G , due to compensation of the input power p G  by the motor  400 . Accordingly, high frequency components of ω are removed using a low pass filter as shown in the following. The current motor speed ω is input to the low pass filter  43   b . The low pass filter  43   b  removes the high frequency components from w to generate the current average speed &lt;ω&gt; of the motor and inputs the current average speed &lt;ω&gt; to the second input terminal  46   c  of the speed controller  46 . A target average speed &lt;ω&gt;* of the motor  400  is input to the first input terminal  46   b  of the speed controller  46 . The speed controller  46  obtains target average inverter output &lt;P INV &gt;* based on the difference Δω (not shown) between &lt;ω&gt;* and &lt;ω&gt; and outputs the target average inverter output &lt;P INV &gt;* from the output terminal  46   d.    
     The target average inverter output &lt;P INV &gt;* that has been output from the output terminal  46   d  of the speed controller  46  is added to the target average capacitor power &lt;P C &gt;* that has been output from the low pass filter  43   a . As a result, the target average rectified power &lt;P PFC &gt;* is calculated as &lt;P PFC &gt;*=&lt;P C &gt;*+&lt;P INV &gt;*. The calculated &lt;P PFC &gt;* is input to the input terminal  44   b  of the PFC rectifier circuit controller  44 . The PFC rectifier circuit controller  44  calculates a target input current i G * (not shown) based on the target average rectified power &lt;P PFC &gt;* that has been input, obtains a PFC output duty ratio d B  from the inductor current difference, and outputs the PFC output duty ratio d B  from the output terminal  44   c . The PFC output duty ratio d B  that has been output is input to the PFC rectifier circuit  10  via a pulse width modulator (not shown) such that desired control is realized. 
     The inverter controller  48  obtains inverter output duty ratios d U , d V  and d W  based on the target motor power p M * that has been input and outputs the inverter output duty ratios d U , d V  and d W  to the first output terminal  48   c , the second output terminal  48   d , and the third output terminal  48   e , respectively. The inverter output duty ratios d U , d V  and d W  that have been output are input to the inverter  30  via a pulse width modulator (not shown) such that desired control is realized. 
       FIG. 6A  shows changes over time of an input voltage, an input current, and input power of the AC-AC power converter  100  of  FIG. 1 .  FIG. 6B  shows changes over time of a DC link capacitor voltage, a DC link capacitor current, and DC link capacitor power of the AC-AC power converter  100  of  FIG. 1 .  FIG. 6C  shows changes over time of motor rotation speed, motor torque, and motor power in  FIG. 1 . 
     Since  FIG. 6A  is the same as  FIG. 4A , the explanation thereof will be omitted. As described above, since the motor power p M  and the input power p G  are controlled so as to coincide with each other, the respective waveforms of p M  and p G  coincide with each other ( FIG. 6A  and  FIG. 6C ). In other words, p M  vibrates at a frequency 2f G , which is twice the frequency f G  of the input voltage v G . Since p M =p G  is established, the DC link capacitor power p C  becomes zero ( FIG. 6B ). Therefore, the DC link capacitor current is does not flow, and the DC link capacitor voltage v DC  has a temporally constant value. As described above, according to the voltage control of the present embodiment, the pulsation Δv DC  does not occur in the DC link voltage. Therefore, a large-capacity electric field condenser for compensating for this is unnecessary. 
     Since the motor power p M  vibrates at the frequency 2f G , the motor torque t M  also vibrates at the frequency 2f G  ( FIG. 6C ). When the motor torque t M  is larger than the load torque t L =T 0 , the load  500  is accelerated, and the energy of the motor is converted into the following kinetic energy E KIN .
 
 E   KIN =½* J   TOT *ω 2  
 
     Conversely, when the motor torque t M  is smaller than the load torque t L =T 0 , the load  500  is decelerated, and the kinetic energy E KIN  of the load  500  is supplied to the motor  400 . In this way, the speed ω of the motor  400  has a pulsation (ripple) Δω vibrating at the frequency 2f G  around the average speed Ω. The pulsation Δω of this motor speed depends on the average motor power P 0 , the average speed Ω, the frequency f G  of the input voltage v G , and the inertia J TOT  and is expressed as follows. 
     
       
         
           
             
               
                 
                   
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                           J 
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     In other words, by applying a load having a sufficiently large inertia, the pulsation can be compensated for and suppressed. 
     As described above, according to the AC-AC power converter according to the embodiment, by compensating for the power pulsation using the motor or the inertia of the load, the DC link voltage can be set to a temporally constant value while the target average speed of the motor is being output. Thereby, the capacity of the DC link capacitor can be reduced without requiring additional components. 
     Described above is an explanation given based on the embodiment of the present invention. The embodiment is intended to be illustrative only, and it will be obvious to those skilled in the art that various modifications and changes can be developed within the scope of the claims of the present invention and that such modifications and changes are also within the scope of the claims of the present invention. Therefore, the descriptions and figures in the specification should be treated demonstratively instead of being treated in a limited manner. 
     An explanation will be given in the following regarding exemplary variations. In the figures and explanations of the exemplary variations, the same or equivalent constituting elements and members as those in the embodiment shall be denoted by the same reference numerals. Explanations that are the same as those in the embodiment are appropriately omitted, and an explanation will be given focusing on features that are different from those of the embodiment. 
     (First Exemplary Variation) 
     In the above-described embodiment, the pulsations of the input power and the pulsations of the DC link power are input to the motor, and the voltage pulsations of the DC link are compensated for using the motor or the inertia of the load. However, the present invention is not limited thereto. For example, only the pulsations of the DC link may be extracted and input to the motor. Also in this case, the voltage pulsations of the DC link can be absorbed and compensated for using the motor or the inertia of the load. 
       FIG. 7  is a block diagram showing a controller  4010 , which is an exemplary variation of the controller  40  of the AC-AC power converter  100  of  FIG. 1 . The component configuration of the controller  4010  is the same as the component configuration of the controller  40  of  FIG. 5 . In the following, an explanation will be given regarding the flow of control and a signal while focusing on differences from  FIG. 5 . 
     The target capacitor power P C * that is output from the output terminal  42   d  of the DC link voltage controller  42  is branched into two at a branch point v 3 , and one is input to the low pass filter  43   a . The low pass filter  43   a  removes high frequency components from P C * to generate the target average capacitor power &lt;P C &gt;* and outputs the target average capacitor power &lt;P C &gt;*. &lt;P C &gt;* output from the low pass filter  43   a  is branched into two at a branch point v 4 , and one is added to the target average inverter output &lt;P INV &gt;* output from the output terminal  46   d  of the speed controller  46 . As a result, the target average rectified power &lt;P PFC &gt;* is calculated as &lt;P PFC &gt;*=&lt;P C &gt;*+&lt;P INV &gt;*. The calculated &lt;P PFC &gt;* is input to the input terminal  44   b  of the PFC rectifier circuit controller  44 . The other &lt;P C &gt;* branched at the branch point v 4  is subtracted from the other P C * branched at the branch point v 3 , and an input power pulsation p C,AC  is generated. In other words, the input power pulsation p C,AC  is obtained by extracting only the pulsation part from the target capacitor power P C *. The input power pulsation p C,AC  is subtracted from the target rectified power p PFC * so as to calculate target motor power p M * (p M *=p PFC *−p C,AC ). p M * that has been calculated is input to the input terminal  48   b  of the inverter controller  48 . 
     As described above, the target motor power p M * that is input to the inverter controller  48  is obtained by subtracting the input power pulsation p C,AC  from the target rectified power p PFC . In other words, the pulsation Δp DC  of the DC link is input to the motor  400 . The motor  400  compensates for this pulsation by the inertia possessed by the load  500 . As a result, the pulsations of the DC link become zero, and p M =p G  is established. In other words, the motor power p M  agrees with the input power p G . 
     The PFC rectifier circuit controller  44  calculates a target input current i G * (not shown) based on &lt;P PFC &gt;* that has been input, obtains a PFC output duty ratio d B  from the inductor current difference, and outputs the PFC output duty ratio d B  from the output terminal  44   c . The PFC output duty ratio d B  that has been output is input to the PFC rectifier circuit  10  via a pulse width modulator (not shown) such that desired control is realized. 
     The inverter controller  48  obtains inverter output duty ratios d U , d V  and d W  based on the target motor power p M * that has been input and outputs the inverter output duty ratios d U , d V  and d W  to the first output terminal  48   c , the second output terminal  48   d , and the third output terminal  48   e , respectively. The inverter output duty ratios d U , d V  and d W  that have been output are input to the inverter  30  via a pulse width modulator (not shown) such that the control is realized. 
     As described above, also in this exemplary variation, by compensating for the voltage pulsations of the DC link by the motor, the motor power p M  is controlled to be equal to the input power p G  in the same way as in the above-described embodiment. Thereby, the DC link capacitor power p C  becomes zero. 
     According to this exemplary variation, the DC link voltage can be set to a temporally constant value while the target average speed of the motor is being output. Thereby, the capacity of the DC link capacitor can be reduced without requiring additional components. 
     (Second Exemplary Variation) 
     In the above-described embodiment, all the pulsations of the DC link voltage are compensated for by using the motor. However, compensation by the motor involves a tradeoff where the speed of the motor pulsates at a frequency that is twice the frequency of the input voltage as described above ( FIG. 6C ). This may be undesirable in some applications. In order to suppress the pulsations of the motor speed, not all but only a part of the input power pulsations may be compensated for by the motor, and the rest may be compensated for by the DC link capacitor. 
     Regarding the pulsation compensation of the DC link voltage, the proportion of the contribution by the motor is denoted as k (0&lt;k≤1) (hereinafter this k is referred to as a distribution coefficient). That is,
 
 p   M,AC   =k*P   PFC,AC  
 
     Note that p M,AC  and p PFC,AC  represent the fluctuation from the average value of motor power p M  and the fluctuation from the average value of rectified power P PFC , respectively (p M =&lt;p M &gt;+p M,AC , P PFC =&lt;P PFC &gt;+p PFC,AC ). Therefore, the motor output is as follows.
 
 p   M   =&lt;p   M   &gt;+p   M,AC   =&lt;p   M   &gt;+k*p   PFC,AC   =&lt;P   PFC   &gt;−&lt;p   C   &gt;+k *( P   PFC   −&lt;P   PFC &gt;)= K*p   PFC,AC   −&lt;p   C &gt;+(1− k )*&lt; P   PFC &gt;
 
     By selecting the distribution coefficient k with an appropriate value, it is possible to distribute compensation of the DC link voltage pulsation at a desired ratio between the motor and the DC link capacitor. 
       FIG. 8  is a block diagram showing a controller  4020 , which is an exemplary variation of the controller  40  of the AC-AC power converter  100  of  FIG. 1 . The configuration of the controller  4020  is different from the configuration of the controller  40  of  FIG. 5  in that the controller  4020  further includes an attenuator  45   a  on the upstream side of the PFC rectifier circuit controller  44  and an attenuator  45   b  on the upstream side of the inverter controller  48 . The attenuator  45   a  attenuates the intensity of the target average rectified power &lt;P PFC &gt;* by 1−k times. The attenuator  45   b  attenuates the intensity of the target rectified power P PFC * by k times. Other configuration of the controller  4020  is the same as the configuration of the controller  40 . 
     The target average capacitor power &lt;P C &gt;* from which high frequency components have been removed by the low pass filter  43   a  after the target average capacitor power &lt;P C &gt;* is output from the output terminal  42   d  of the DC link voltage controller  42  is branched into two at a branch point v 5 . One &lt;P C &gt;* branched at the branch point v 5  is added to the target average inverter output &lt;P INV &gt;* output from the output terminal  46   d  of the speed controller  46 , and the target average rectified power &lt;P PFC &gt;* is calculated. The calculated &lt;P PFC &gt;* is branched into two at a branch point v 6 , and one is input to the input terminal  44   b  of the PFC rectifier circuit controller  44 . The other &lt;P PFC &gt;* branched at the branch point v 6  is input to the attenuator  45   a . &lt;P PFC &gt;* is attenuated by 1−k times by attenuator  45   a , and then &lt;P C &gt;* branched at the branch point v 5  is subtracted. The power represented by this (1−k)*&lt;P PFC &gt;*−&lt;P C &gt;* is added to target rectified power k*p PFC * attenuated k times by the attenuator  45   b  and calculated as p M * and is then input to the input terminal  48   b  of the inverter controller  48 . 
     That is,
 
 p   M   *=k*p   PFC   −&lt;P   C &gt;*+(1− k )&lt; P   PFC &gt;*
 
     According to the present exemplary variation, it is possible to distribute compensation of the DC link voltage pulsation at a desired ratio between the motor and the DC link capacitor. Thereby, the capacity of the DC link capacitor can be reduced while suppressing the pulsations of the speed of the motor. 
     (Third Exemplary Variation) 
     In the above-described embodiment, smoothing of the DC link voltage is realized by compensating for the pulsations of the DC link voltage vibrating at a frequency that is twice the frequency of the input voltage by the motor. However, in the actual implementation, the pulsations of the DC link voltage may have high frequency components such as those of four times (second order high frequency), eight times (fourth order high frequency), or 12 times (sixth order frequency) the frequency of the input voltage. High frequency noise generated by resonance of these high frequency components cannot be completely suppressed by the above-described technique alone. In this case, it is difficult to completely smooth the DC link capacitor voltage. In order to solve this problem, resonance control for suppressing high frequency noise may be added to DC link voltage control. 
       FIG. 9  is a block diagram showing a controller  4030 , which is an exemplary variation of the controller  40  of the AC-AC power converter  100  of  FIG. 1 . The configuration of the controller  4030  is different from the configuration of the controller  40  of  FIG. 5  in that the controller  4030  further includes a resonance controller  49  on the upstream side of the PFC rectifier circuit controller  44 . The same number of resonance controllers  49  as the number of high frequencies of the orders to be controlled are arranged. For example, in  FIG. 9 , three types of resonance controllers  49  for suppressing the second order high frequency, the fourth order high frequency, and the sixth order high frequency are shown. Other configuration of the controller  4030  is the same as the configuration of the controller  40 . 
     Into the resonance controllers  49 , −v DC  obtained by inverting the polarity of the DC link voltage is input. Each of the resonance controllers  49  controls the high frequencies using the following function G R,n (s) and outputs target capacitor power relating to the high frequency of each order of the DC link voltage. 
     
       
         
           
             
               
                 
                   
                     
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     In the expression, the gain is denoted as K I , and the order of the high frequency is denoted as n. From the resonance controller  49 , the target capacitor power relating to the high frequency of each order is output. The target capacitor power relating to the high frequency of each order that has been output is added to the target capacitor power output from the DC link voltage controller  42 . Thereby, the target capacitor power P C * whose high frequency noise is suppressed is generated. Other flows for the control and the signal are the same as those in the controller  40  of  FIG. 5 . 
     According to this exemplary variation, high frequency noise of the voltage pulsations of the DC link can be suppressed, and the DC link voltage can be smoothed with higher accuracy. 
     (Fourth Exemplary Variation) 
     In the above-described embodiment, the DC link voltage is the same as the input voltage. According to the voltage control of the present invention, since the pulsations of the DC voltage of the DC link is suppressed, for example, by placing a DC booster circuit on the downstream side of the DC link, the target DC link voltage can be set to a value higher than that of the input voltage. 
       FIG. 10  is a block diagram showing an AC-AC power converter  110 , which is an exemplary variation of the AC-AC power converter  100  of  FIG. 1 . The AC-AC power converter  110  further includes a booster circuit  70  on the downstream side of the DC link  20 , and other configuration of the AC-AC power converter  110  is the same as the configuration of the AC-AC power converter  100 . 
     The booster circuit  70  boosts the DC link voltage smoothed due to pulsation compensation by the motor  400 . For example, with regard to a single-phase AC voltage of 200 V supplied from the single-phase power source  300 , the booster circuit  70  boosts a smoothed DC link voltage of 200 V by two times so as to generate a rectified voltage of 400 V and inputs the rectified voltage to the inverter  30 . The inverter  30  generates a three-phase AC voltage of 400 V. 
     According to the present exemplary variation, a three-phase AC voltage that is higher than an input single-phase AC voltage can be obtained. 
     (Fifth Exemplary Variation) 
     One exemplary variation of the present invention is a method of controlling an AC-AC power conversion system. In other words, a method according to one aspect of the present invention is a method of controlling an AC-AC power conversion system. The AC-AC power conversion system comprises: a rectifier circuit for rectifying a single-phase AC voltage so as to generate a rectified voltage; an inverter for generating a three-phase AC voltage from the rectified voltage; and a DC link capacitor as an intermediate stage between the rectifier circuit and the inverter, wherein the method comprises: generating a sinusoidal input current from a single-phase AC voltage; supplying average input power and the pulsations of input power to the output side of the inverter; and matching the rectified voltage to a reference rectified voltage. 
     (Sixth Exemplary Variation) 
     One exemplary variation of the present invention is a method of controlling an AC-AC power conversion system. In other words, a method according to one aspect of the present invention is a method of controlling an AC-AC power conversion system. The AC-AC power conversion system comprises: a rectifier circuit for rectifying a single-phase AC voltage so as to generate a rectified voltage; an inverter for generating a three-phase AC voltage from the rectified voltage; and a DC link capacitor for compensating for a part of the pulsations of input power, wherein the method comprises: generating a sinusoidal input current from a single-phase AC voltage; supplying average input power and an adjustable part of the pulsations of the input power to the output side of the inverter; and matching an average rectified voltage to a reference rectified voltage. 
     In a method according to the fifth exemplary variation or the sixth exemplary variation of the present invention, a three-phase external device is connected to an inverter, and the pulsations of output power are compensated for by the inertia of the three-phase external device and the load thereof, and the method may further comprise matching the average rotational speed of the three-phase external device to reference rotational speed. 
     In the method according to the fifth exemplary variation or the sixth exemplary variation of the present invention, an AC-AC power conversion system may include a three-phase rectifier instead of a rectifier, wherein the three-phase rectifier is connected to a three-phase power source, and the method may further comprise compensating for input power pulsations that occur when imbalance occurs in the three-phase power source. 
     In the method according to the fifth exemplary variation or the sixth exemplary variation of the present invention, the AC-AC power conversion system may further include a resonance controller for reducing low frequency distortion. 
     The method according to the sixth exemplary variation of the present invention may further comprise outputting the pulsations of the input power dispersively to the external device and the DC link in accordance with conditions for time and load. 
       FIG. 11  is a block diagram showing an AC-AC power converter according to an embodiment. In the embodiment, the three-phase PFC rectifier may be connected to three-phase AC mains.