Patent Publication Number: US-8542018-B2

Title: Power transmitting apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-87596, filed on Mar. 31, 2009, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a power transmitting apparatus transmitting power through magnetic resonance. 
     BACKGROUND 
     Hitherto, technologies for supplying power wirelessly through electromagnetic induction and/or radio waves have been available. Further, in recent years, technologies for supplying power wirelessly through magnetic resonance achieved to make a magnetic field resonate have been developed. The magnetic resonance is a phenomenon in which magnetic fields are coupled to each other between two resonating coils and energy transmission occurs. 
     SUMMARY 
     According to an aspect of the invention, a power transmitting apparatus includes: a first magnetic resonance coil that externally transmits power as magnetic field energy through magnetic resonance; a first power transmitting-and-receiving unit that supplies power to at least the first magnetic resonance coil; a second magnetic resonance coil that accepts the magnetic field energy through magnetic resonance occurring between the first magnetic resonance coil and the second magnetic resonance coil; a second power transmitting-and-receiving unit that accepts the power at least with reference to the second magnetic resonance coil; a main power supply; and a power supply-management unit configured to select either the power accepted by the second power transmitting-and-receiving unit or power transmitted from the main power supply, and transmit the selected power to the first power transmitting-and-receiving unit and/or transmit the power accepted by the second power transmitting-and-receiving unit and the power transmitted from the main power supply to the first power transmitting-and-receiving unit in combination. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an exemplary schematic configuration of a power transmitting apparatus according to a first embodiment of the present invention; 
         FIG. 2  illustrates power transmission and backflow that are attained through magnetic resonance; 
         FIG. 3  is a flowchart illustration processing operations performed through a power transmission-control unit; 
         FIG. 4  is a flowchart illustrating the details of power receiving device-detection processing; 
         FIG. 5  illustrates the separation of driving frequencies corresponding to the maximum power feeding efficiency; 
         FIG. 6  illustrates magnetic resonance achieved when a driving frequency obtained when a current value I reaches its peak is separated into a driving frequency f 0 +d and a driving frequency f 0 −d; and 
         FIG. 7  illustrates an exemplary schematic configuration of a power transmitting apparatus according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     For transmitting power wirelessly through magnetic resonance, the power transmission efficiency is increased with an increase in the degree of coupling between magnetic fields occurring between two coils. Therefore, it is preferable that the degree of coupling between magnetic fields occurring between coils be increased. 
     However, if an area specified on a power receiving coil to perceive variations in a magnetic flux is smaller than that specified on a power transmitting coil, it becomes difficult to increase the degree of coupling between magnetic fields and the transmission efficiency is decreased. For example, when the cabinet of a device including the power receiving coil is small in size, it is difficult to specify a sufficiently large loop area on the power receiving coil. Likewise, if it is difficult to provide the loop face of the power receiving coil so that the loop face is opposed to that of the power transmitting coil, the transmission efficiency is decreased. 
     In the past, when the transmission efficiency decreased due to a factor occurring on the power receiving-side, part of the energy transmitted from the power transmitting coil was entirely lost, because that part of the energy was not accepted by a device provided on the power receiving-side. 
     Accordingly, technologies disclosed in this application have been achieved to provide a power transmitting apparatus configured to increase the transmission efficiency by reducing losses of energy for transmission during the wireless power transmission achieved through magnetic resonance. 
     &lt;Means for Solving the Problems&gt; 
     A power transmitting apparatus disclosed in this application includes a first magnetic resonance coil configured to externally transmit power as magnetic field energy through magnetic resonance, and a second magnetic resonance coil configured to accept the magnetic field energy through the magnetic resonance occurring between the first magnetic resonance coil and the second magnetic resonance coil. The magnetic field energy accepted by the second magnetic resonance coil flows back to the first magnetic resonance coil. 
     &lt;Advantages&gt; 
     A power transmitting apparatus disclosed in this application may increase the transmission efficiency by reducing losses of energy for transmission during the wireless power transmission achieved through magnetic resonance. 
     &lt;Description of the Embodiments&gt; 
     Hereinafter, a power transmitting apparatus according to an embodiment of the present invention, that is, a power transmitting apparatus disclosed in this application will be described in detail with reference to the attached drawings. The above-described embodiment does not limit the disclosed technologies. 
     First Embodiment 
       FIG. 1  illustrates an exemplary schematic configuration of a power transmitting apparatus according to a first embodiment of the present invention. A power transmitting apparatus  1  illustrated in  FIG. 1  wirelessly supplies power to each of power receiving devices  5   a  and  5   b . The power transmitting apparatus  1  includes a main power transmitting device  2  and an auxiliary power receiving device  3 . 
     The main power transmitting device  2  includes a power transmission-control unit  21 , an alternating current-power supply  22 , a power switching circuit  23 , a power supply coil  24 , a power transmitting coil  25 , and a sensor  26 . Further, the auxiliary power receiving device  3  includes a power receiving coil  31  and a power extracting coil  32 . Each of the power receiving devices  5   a  and  5   b  includes a power receiving coil  51 , a power extracting coil  52 , and a load circuit  53 . 
     Each of the power transmitting coil  25 , and the power receiving coils  31  and  51  is an LC resonant circuit and functions as a magnetic resonance coil. The capacitor component of the LC resonant circuit may be achieved through an element and/or stray capacitance achieved by releasing both the ends of the coil. When an inductance is expressed by the sign L and the capacitor capacity is expressed by the sign C, a resonance frequency expressed by the sign f is determined as illustrated by the following expression: 
     
       
         
           
             
               
                 
                   
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     When the resonance frequency of the power transmitting coil  25  and that of the power receiving coil  31  are sufficiently close to each other, and the distance between the power transmitting coil  25  and the power receiving coil  31  is appropriately small, magnetic resonance may occur between the power transmitting coil  25  and the power receiving coil  31 . 
     Likewise, when the resonance frequency of the power transmitting coil  25  and that of the power receiving coil  51  are sufficiently close to each other and the distance between the power transmitting coil  25  and the power receiving coil  51  is appropriately small, magnetic resonance may occur between the power transmitting coil  25  and the power receiving coil  51 . 
     Therefore, when the magnetic resonance occurs while the power transmitting coil  25  resonates, magnetic field energy may be transmitted from the power transmitting coil  25  to each of the power receiving coils  31  and  51 . 
     The power supply coil  24  is a power transmitting-and-receiving unit configured to supply power to the power transmitting coil  25  through electromagnetic induction. The power supply coil  24  and the power transmitting coil  25  are arranged at a distance from each other, where the distance is long enough to cause electromagnetic induction. The power transmitting coil  25  is caused to resonate through the electromagnetic induction via the power supply coil  24 . Accordingly, the power transmitting coil  25  need not be electrically connected to a different circuit and the resonance frequency of the power transmitting coil  25  may be designed arbitrarily with high precision. 
     In each of the power receiving devices  5   a  and  5   b , the power extracting coil  52  is arranged at a position specified so that electromagnetic induction occurs between the power receiving coil  51  and the power extracting coil  52 . When the power receiving coil  51  resonates through magnetic resonance, energy is moved from the power receiving coil  51  to the power extracting coil  52  due to electromagnetic induction. The power extracting coil  52  is electrically connected to the load circuit  53 , and the energy moved to the power extracting coil  52  due to the electromagnetic induction is supplied to the load circuit  53  as power. That is to say, the power extracting coil  52  functions as a power transmitting-and-receiving unit. The load circuit  53  may be an arbitrary circuit such as a battery. 
     Thus, power is extracted from the power receiving coil  51  through electromagnetic induction via the power extracting coil  52 . Accordingly, the power receiving coil  51  may not be electrically connected to a different circuit and the resonance frequency of the power receiving coil  51  may be arbitrarily designed with high precision. 
     The power extracting coil  32  of the auxiliary power receiving device  3  is arranged at a position specified so that electromagnetic induction occurs between the power receiving coil  31  and the power extracting coil  32 . When the power receiving coil  31  resonates through magnetic resonance, energy is moved from the power receiving coil  31  to the power extracting coil  32  due to the electromagnetic induction. The power extracting coil  32  is electrically connected to the power switching circuit  23  via a power feedback path  32   a . The energy moved to the power extracting coil  32  due to the electromagnetic induction is supplied to the power switching circuit  23  as power. That is to say, the power extracting coil  32  functions as a power transmitting-and-receiving unit. 
     Thus, power is extracted from the power receiving coil  31  through electromagnetic induction via the power extracting coil  32 . Accordingly, the power receiving coil  31  may not be electrically connected to a different circuit and the resonance frequency of the power receiving coil  31  may be arbitrarily designed with high precision. 
     The alternating current-power supply  22  is a main power supply configured to externally transmit an alternating current with a frequency and an amplitude that are specified through the power transmission-control unit  21 . Hereinafter, the above-described frequency of the alternating current-power supply  22  will be referred to as a driving frequency. The power supply coil  23  electrically connected to the alternating current-power supply  22  vibrates at the driving frequency. 
     The power switching circuit  23  is a power management unit configured to select either the power transmitted from the auxiliary power receiving device  32  or that transmitted from the alternating current-power supply  22  and transmit the selected power to the power supply coil  24 , and/or transmit the power transmitted from the auxiliary power receiving device  32  and that transmitted from the alternating current-power supply  22  in combination to the power supply coil  24 . 
     The power supply coil  24  is excited by the driving frequency through the power switching circuit  23 . Therefore, the power transmitting coil  25  resonates at the driving frequency. Likewise, the power receiving coil  31  resonates at the driving frequency. 
     Thus, in the power transmitting apparatus  1 , the power of the alternating current-power supply  22  is transmitted to the load circuit  53  via the electromagnetic induction attained between the power supply coil  24  and the power transmitting coil  25 , the magnetic resonance attained between the power transmitting coil  25  and the power receiving coil  51 , and the electromagnetic induction attained between the power receiving coil  51  and the power extracting coil  52 . 
     Further, due to the magnetic resonance occurring between the power receiving coil  31  of the auxiliary power receiving device  3  and the power transmitting coil  25 , the auxiliary power receiving device  3  accepts part of the energy transmitted from the power transmitting coil  25 , and returns part of the energy back to the main power transmitting device  2  through electromagnetic induction occurring between the power receiving coil  31  and the power extracting coil  32 . 
     The efficiency of the power transmission attained through the magnetic resonance occurring between the power transmitting coil  25  and each of the power receiving coils  31  and  51  depends on a performance indicator expressed by the following expression: 
     
       
         
           
             
               
                 
                   
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     Here, the sign κ denotes the magnitude of the energy flow attained per unit time, that is, the coupling efficiency indicating the degree of coupling between magnetic fields occurring between the two coils. Further, the sign Γ 1  indicates the energy loss occurring per unit time of the power transmitting coil  25 , and the sign Γ 2  indicates the energy loss occurring per unit time of the power receiving coil  31 . 
     As indicated by the above-described numerical expression, the efficiency of power transmission attained through the magnetic resonance may be increased with an increase in the coupling efficiency K. 
     However, if an area specified on the power receiving coil  51  of each of the power receiving devices  5   a  and  5   b  to perceive variations in a magnetic flux is smaller than that specified on the power transmitting coil  25 , it becomes difficult to increase the degree of coupling between the magnetic fields and the transmission efficiency is decreased. For example, when the housing of each of the power receiving devices  5   a  and  5   b  is small in size, it is difficult to specify a sufficiently large loop area on the power receiving coil  51 . Likewise, if it is difficult to arrange the loop face of the power receiving coil  51  of each of the power receiving devices  5   a  and  5   b  so that the loop face is opposed to that of the power transmitting coil  25 , the transmission efficiency is decreased. 
     For avoiding the entire loss of part of the energy transmitted from the power transmitting coil  25 , where the part is not accepted by the power receiving devices  5   a  and  5   b , the power transmitting apparatus  1  collects the energy transmitted from the power transmitting coil  25  through the auxiliary power receiving device  3 . Consequently, it becomes possible to reduce losses of the energy for transmission during the wireless power transmission attained through the magnetic resonance so that the transmission efficiency is increased. 
       FIG. 2  illustrates power transmission and backflow that are attained through the magnetic resonance. In  FIG. 2 , the energy loss of the power transmitting coil  25  is expressed by the sign Γ 1 , the energy loss of the power receiving coil  31  of the auxiliary power receiving device  3  is expressed by the sign Γ 2 , and the energy loss of the power receiving coil  51  of each of the power receiving devices  5   a  and  5   b  is expressed by the sign Γ 3 . Further, the coupling between the magnetic field of the power transmitting coil  25  and that of the power receiving coil  31  of the auxiliary power receiving device  3  is expressed by the sign κ 12 , and the coupling between the magnetic field of the power transmitting coil  25  and that of the power receiving coil  51  of each of the power receiving devices  5   a  and  5   b  is expressed by the sign κ 13 . 
     The shape and the arrangement of the power receiving coil  31  of the auxiliary power receiving device  3  may be designed in advance based on the power transmitting coil  25 . Therefore, it becomes possible to decrease the energy loss Γ 2  and increase the magnetic field coupling κ 12  to obtain high transmission efficiency. 
     Therefore, even though the energy loss Γ 3  of the power receiving coil  51  of the power receiving device is large and the magnetic field coupling κ 13  is small, the magnetic field energy transmitted from the power transmitting coil  25  may be collected through the power receiving coil  31  with efficiency and partially flowed back to the power transmitting coil  25 . 
     Returning to  FIG. 1 , the sensor  26  measures the magnitude of a magnetic field near the power transmitting coil  25  and transmits the current corresponding to the magnetic field magnitude. The power transmission-control unit  21  includes a power receiving device-detecting unit  41 , a frequency sweep processing unit  42 , and a power driving unit  43 . 
     The power receiving device-detecting unit  41  is a processing unit configured to detect that each of the power receiving devices  5   a  and  5   b  approaches and enters an area where the power receiving device may receive power wirelessly transmitted from the power transmitting apparatus  1 . When the alternating current-power supply  22  is driven at a constant frequency and a constant amplitude, and the power transmitting coil  25  resonates, the magnetic field energy transmitted from the power transmitting coil  25  through the magnetic resonance is increased with a decrease in the distance between the power transmitting coil  25  and the power receiving coil  51 . Therefore, when the alternating current-power supply  22  is driven at a constant frequency and a constant amplitude and the magnitude of the magnetic field near the power transmitting coil  25  is measured through the sensor  26 , the approach of the power receiving coil  51 , that is, the approach of each of the power receiving devices  5   a  and  5   b  may be detected. More specifically, the power receiving device-detecting unit  41  transmits data indicating the detection of the power receiving device when the value of a current transmitted from the sensor  26  exceeds a threshold value Th. 
     The frequency sweep processing unit  42  is a processing unit configured to change the driving frequency and acquire data of a change in the value of the current transmitted from the sensor  26 . The change in the driving frequency sweeps a given area. The current transmitted from the sensor  26  indicates the magnitude of the magnetic field near the power transmitting coil  25  and the magnetic field magnitude is increased as the power receiving coil  51  approaches the power transmitting coil  25 . Namely, the magnetic field magnitude is increased with an increase in the power feeding efficiency. Therefore, the result of processing performed through the frequency sweep processing unit  42  indicates the distribution of power feeding efficiencies with reference to the driving frequency. 
     The power driving unit  43  selects a driving frequency obtained when the power feeding efficiency is maximized based on the frequency sweep result and drives the alternating current-power supply  22  so that energy is transmitted between the power transmitting coil  25  and the power receiving coil  51 . 
       FIG. 3  is a flowchart illustrating processing operations performed through the power transmission-control unit  21 . The power transmission-control unit  21  executes processing operations that are illustrated in  FIG. 3  at regular intervals. When the processing operations are started, the power receiving device-detecting unit  41  executes power receiving device-detection processing in the first place (operation S 101 ). 
     If data indicating the detection of the power receiving device is not transmitted as a result of the power receiving device-detection processing (when the answer is No at operation S 102 ), the power transmission-control unit  21  finishes the processing as it is. If the data indicating the power receiving device-detection is transmitted as a result of the power receiving device-detection processing (when the answer is Yes at operation S 102 ), the frequency sweep processing unit  42  executes the frequency sweep processing (operation S 103 ). The frequency sweep processing unit  42  executes peak detection processing to detect the peak of the power feeding efficiency from the distribution of the power feeding efficiencies with reference to the driving frequency, the distribution being obtained as a result of the frequency sweep processing (operation S 104 ). 
     When the power feeding efficiency-peak is not detected as a result of the peak detection processing (when the answer is No at operation S 105 ), the processing returns to the power receiving device-detection processing performed through the power receiving device-detecting unit  41  (operation S 101 ). On the other hand, when the peak is detected (when the answer is Yes at operation S 105 ), the power driving unit  43  selects the driving frequency of the peak (operation S 106 ), drives the alternating current-power supply  22  (operation S 107 ), and causes magnetic resonance between the power transmitting coil  25  and the power receiving coil  51  and supplies power to the power receiving device  3 . 
     After that, when power requirements are satisfied (when the answer is Yes at operation S 108 ), the power transmission-control unit  21  stops supplying power and finishes the processing. The requirements for finishing the power supply may be arbitrarily determined, that is to say, the power supply may be stopped upon receiving an instruction to stop supplying power and/or when the value of the power feeding efficiency becomes equivalent to a given value or less, for example. When the requirements for finishing the power supply are not satisfied (when the answer is No at operation S 108 ), the power supply driving (operation S 107 ) is continued so that the power supply is continued. 
       FIG. 4  is a flowchart illustrating the details of the power receiving device-detection processing. When the power receiving device-detection processing is started, the sensor  26  measures the magnetic field magnitude (operation S 201 ), and the power receiving device-detecting unit  41  determines whether or not the value of the magnetic field magnitude is equivalent to the threshold value or more (operation S 202 ). Since the magnetic field magnitude is obtained as the value of the current transmitted from the sensor  26 , the power receiving device-detecting unit  41  determines the magnetic field magnitude by comparing the current value to the threshold value. 
     If the result of the determination made by the power receiving device-detecting unit  41  illustrates that the value of the magnetic field magnitude is less than the threshold value (when the answer is No at operation S 202 ), the power receiving device-detecting unit  41  finishes the power receiving device-detection processing as it is. On the other hand, when the value of the magnetic field magnitude is equivalent to the threshold value or more (when the answer is Yes at operation S 202 ), the power receiving device-detecting unit  41  transmits data indicating that the power receiving device is detected (operation S 203 ), and finishes the processing. 
     Here, it should be noted that the power receiving device-detection processing illustrated in  FIG. 4  has been exemplarily described. Namely, the detection of approach of each of the power receiving devices  5   a  and  5   b  may be achieved through arbitrary technologies. For example, each of the power receiving devices  5   a  and  5   b  may be detected through an optical sensor provided separately. Further, in the case where the magnetic resonance is detected, the output of the alternating current-power supply may be reduced so as to be lower than that obtained at the power supply time during the power receiving device-detection processing, for example. 
     Next, the separation of the peak of a driving current will be described. The driving frequency corresponding to energy at its peak, the energy being transmitted through magnetic resonance occurring between the power transmitting coil  25  and the power receiving coil  31 , that is, the driving frequency corresponding to the maximum power feeding efficiency is obtained near the resonance frequency of the coil. However, when the distance between the power transmitting coil  25  and the power receiving coil  31  is reduced to a degree, the separation of the driving frequency corresponding to the maximum power feeding efficiency is confirmed. 
       FIG. 5  illustrates the separation of each of several driving frequencies corresponding to the maximum power feeding efficiency. In  FIG. 5 , the sign f denotes a driving frequency. Further, the sign I denotes a current transmitted from the sensor  26 , which corresponds to the power feeding efficiency. The threshold value Th is determined to detect the approach of the power receiving coil  51 . Each of the signs D 1 , D 2 , D 3 , D 4 , D 5 , and D 6  denotes the distance between the power transmitting coil  25  and the power receiving coil  31 , where the relationship D 1 &lt;D 2 &lt;D 3 &lt;D 4 &lt;D 5 &lt;D 6  holds. 
     As illustrated in  FIG. 5 , when each of the distances D 6  to D 4  is attained, the current value I reaches its peak at a driving frequency f 0 . The peak value is increased with a decrease in the distance. On the other hand, when each of the distances D 1  to D 3  is attained, the peak is separated into two peaks including a peak attained on the low frequency-side of the driving current f 0  and a peak attained on the high frequency-side of the driving current f 0 . Of the separated peaks, the peak attained on the high frequency-side denotes a driving frequency f 0 +d and that attained on the low frequency-side denotes a driving frequency f 0 −d. 
       FIG. 6  illustrates magnetic resonance occurring when a driving frequency attained when the current value I reaches its peak is separated into the driving frequency f 0 +d and the driving frequency f 0 −d. When magnetic resonance occurs at the driving frequency f 0 −d on the low frequency-side, the resonance of the power transmitting coil  25  and that of the power receiving coil  31  are in phase with each other so that the directions of the magnetic fields agree with each other. Therefore, while the power supply is performed through the magnetic resonance, strong magnetic fields occur between the power transmitting coil  25  and the power receiving coil  31 . 
     On the other hand, when magnetic resonance occurs at the driving frequency f 0 +d on the high frequency-side, the resonance of the power transmitting coil  25  and that of the power receiving coil  31  are opposite in phase to each other so that the directions of the magnetic fields are reversed. Therefore, the magnetic fields occurring between the power transmitting coil  25  and the power receiving coil  31  while the power supply is performed through the magnetic resonance are weaker than in the case where the magnetic resonance occurs at the driving frequency f 0 −d, and cancel each other in some locations. 
     When the magnetic resonance occurs between the main power transmitting device  2  and the auxiliary power receiving device  3  at the driving frequency f 0 +d attained on the high frequency side, a place where the magnetic fields of the main power transmitting device  2  and the auxiliary power receiving device  3  cancel each other occurs. For attaining energy transmission with efficiency by achieving magnetic resonance between each of the power receiving devices  5   a  and  5   b  that are provided as the power transmission destinations and the main power transmitting device  2 , it is preferable that a stable magnetic field be produced between the main power transmitting device  2  and the auxiliary power receiving device  3 . Therefore, if the separation occurs in the peak of the driving frequency, the power transmission-control unit  21  achieves the magnetic resonance through the use of the driving frequency f 0 −d attained on the low-frequency side. 
     As described above, the power transmitting apparatus  1  of the first embodiment includes the power transmitting coil  25  configured to transmit power to each of the power receiving devices  5   a  and  5   b  through the magnetic resonance and the auxiliary power receiving device  3  configured to flow energy accepted through the magnetic resonance achieved between the power transmitting coil  25  and the power receiving coil  31  back to the power transmitting coil  25  as power. Therefore, it becomes possible to collect part of the energy that is not accepted by each of the power receiving devices  5   a  and  5   b  of the energy transmitted from the power transmitting coil  25  to decrease the energy losses and increase the transmission efficiency. 
     Second Embodiment 
       FIG. 7  illustrates an exemplary schematic configuration of a power transmitting apparatus  1   a  according to a second embodiment of the present invention. The power transmitting apparatus  1   a  illustrated in  FIG. 7  wirelessly supplies power to a power receiving device (not illustrated). The power transmitting apparatus  1   a  includes a main power transmitting device  2   a  and an auxiliary power receiving device  3   a.    
     The power transmitting apparatus  1   a  switches between a main power transmission operation performed to transmit energy from the main power transmitting device  2   a  to the auxiliary power receiving device  3   a  through magnetic resonance and a backflow operation performed to transmit energy from the auxiliary power receiving device  3   a  to the main power transmitting device  2   a  through magnetic resonance. 
     The auxiliary power receiving device  3   a  includes a power receiving coil  31 , a power extracting coil  32 , a battery  71 , a transmission driving unit  72 , and an operation control unit  73 . During the main power transmission operation, the power receiving coil  31  accepts magnetic field energy transmitted from the main power transmitting device  2   a  through magnetic resonance, and the power extracting coil  32  extracts power from the power receiving coil  31  through electromagnetic induction and accumulates the power in the battery  71 . 
     Then, during the backflow operation, the transmission driving unit  72  supplies power to the power extracting coil  32  as a power supply. The power extracting coil  32  passes the supplied power to the power receiving coil  31  through electromagnetic induction. The power receiving coil  31  transmits magnetic field energy to the power transmitting coil  25  through magnetic resonance occurring between the power receiving coil  31  and the power transmitting coil  25  of the main power transmitting device  2   a . The operation control unit  73  functions as an operation switching unit configured to switch between the main power transmitting operation and the backflow operation in the auxiliary power receiving device  3   a.    
     The main power transmitting device  2   a  includes the alternating current-power supply  22 , the power supply coil  24 , the power transmitting coil  25 , a battery  61 , a transmission driving unit  62 , and an operation control unit  63 . During the main power transmitting operation, the transmission driving unit  62  selects either power transmitted from the alternating current-power supply  22  or power transmitted from the battery  61  and transmits the selected power to the power supply coil  24  and/or transmits the power transmitted from the alternating current-power supply  22  and that transmitted from the battery  61  in combination to the power supply coil  24  that supplies power to the power transmitting coil  25  through electromagnetic induction. Then, the power transmitting coil  25  supplies power to the power receiving coil  31  through magnetic resonance. 
     Then, during the backflow operation, the power transmitting coil  25  accepts magnetic field energy transmitted from the auxiliary power receiving device  3   a  through magnetic resonance and the power supply coil  24  extracts power from the power transmitting coil  25  through electromagnetic induction and accumulates the power in the battery  61 . The operation control unit  63  functions as an operation switching unit configured to switch between the main power transmitting operation and the backflow operation in the main power transmitting device  2   a.    
     Thus, as is the case with the first embodiment, the power transmitting apparatus  1   a  of the second embodiment includes the power transmitting coil  25  configured to transmit power to each of the power receiving devices  5   a  and  5   b  through the magnetic resonance and the auxiliary power receiving device  3   a  configured to flow energy transmitted through the magnetic resonance achieved between the power transmitting coil  25  and the power receiving coil  31  back to the power transmitting coil  25  as power. Therefore, it becomes possible to decrease the energy losses and increase the transmission efficiency. 
     Further, in the power transmitting apparatus  1   a , power flows from the auxiliary power receiving device  3   a  back to the main power transmitting device  2   a  through the magnetic resonance. Therefore, the power backflow may be achieved wirelessly. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.