JP2013187963A - Power transmission system and power transmission device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that there are safety and transmission performance limitations with the background technology since adjusting an operational frequency so as to maintain series resonance causes increase of a voltage applied to transmission electrodes, and to provide power transmission system and a transmission device which can improve transmission performance.SOLUTION: A power transmission device 10 comprises power transmission electrodes E1 to E2 for exciting electric fields on the basis of AC voltage applied through a transformer 20, where the transformer 20 and the power transmission electrodes E1 to E2 forms a resonance circuit. A power reception device 30, on the other hand, comprises power reception electrodes E3 to E4 performing electric field coupling to the power transmission electrodes E1 to E2, a transformer 32 and rectifier circuit 34 which supply AC voltage excited at the power reception electrodes E3 to E4 to a battery 36. A CPU 16 repeatedly changes a frequency of the AC voltage applied to the power transmission electrodes E1 to E2, and measures impedance in parallel with the change. The CPU 16 also retrieves a frequency at a time point when the measured impedance shows a decrease tendency satisfying a predetermined condition toward a resonance frequency of the resonance circuit, and sets the retrieved frequency as a frequency of AC voltage at power supply time.

Description

  The present invention relates to a power transmission system, and more particularly to a power transmission system that transmits power from a power transmission device to a power reception device by electric field coupling between a plurality of electrodes provided in the power transmission device and a plurality of electrodes provided in the power reception device.

  The present invention also relates to a power transmission device, and relates to a power transmission device applied to the above-described power transmission system.

  An example of this type of power transmission system is disclosed in Patent Document 1. According to this background art, the fixed body including a plurality of power transmission electrodes is disposed in the power supply area, and the movable body including the plurality of power reception electrodes is disposed in the power supply area. The plurality of power transmission electrodes are arranged in the vicinity of the boundary surface between the power supply region and the power supply region. In addition, the plurality of power receiving electrodes are arranged in the vicinity of the boundary surface so as to face the plurality of power transmitting electrodes in a non-contact manner. The fixed body and the movable body further include a series resonance circuit and a parallel resonance circuit, respectively, and the operating frequency of the fixed body is adjusted so that at least series resonance is maintained. Thereby, power supply with high efficiency is realized.

JP 2009-89520 A

  However, since adjustment of the operating frequency so that series resonance is maintained causes an increase in the voltage applied to the power transmission electrode, there is a limit in safety, that is, transmission performance in the background art.

  Therefore, a main object of the present invention is to provide a power transmission system and a power transmission device that can enhance power transmission performance.

  A power transmission system according to the present invention (100: reference numeral corresponding to the embodiment; the same applies hereinafter) includes a plurality of first electrodes (E1, E2) that excite an electric field based on an AC voltage, and a plurality of first electrodes in series. A power transmission device (10) including a first inductor (L2) that forms a resonance circuit, a plurality of second electrodes (E3, E4) that are electrically coupled to a plurality of first electrodes, and a plurality of second electrodes are excited. A power transmission system (100) formed by a power receiving device (30) provided with a supply means (32, 34) for supplying power based on the electric field to a load (36), wherein the power transmission device has a frequency of an AC voltage. Change means (S33, S47 to S51) that repeatedly change, measurement means (S35) that measures impedance in parallel with the process of the change means, and the impedance measured by the measurement means is predetermined toward the resonance frequency of the series resonant circuit Change the frequency at the point of time when the decrease trend that satisfies the conditions is shown. First search means (S41) for searching among a plurality of frequencies designated by the means, and first setting means (S45) for setting the frequency detected by the first search means as the frequency of the AC voltage during power feeding Further prepare.

  Preferably, the predetermined condition includes a condition that the magnitude of the impedance is lower than the first threshold value.

  Preferably, the predetermined condition includes a condition that the decreasing rate of the impedance exceeds the second threshold value.

  Preferably, the power transmission device restarts the change unit when the approach of the foreign object is detected during the power supply, and the limiting unit (S43) that limits the search range of the first search unit to a range narrower than the change range of the change unit Restarting means (S9, S13).

  More preferably, the changing means changes the frequency by a predetermined width from the high frequency side to the low frequency side.

  Preferably, the power transmission device periodically sets a second inductor (L1) having a number of turns smaller than that of the first inductor and inductively coupled to the first inductor, and a current amount for conducting the second inductor. Switching means (14, 18) for switching is further provided.

  Preferably, the supply unit includes a third inductor (L3) that forms a parallel resonance circuit or a series resonance circuit together with the plurality of second electrodes, and the power transmission device includes the parallel resonance circuit out of the plurality of frequencies specified by the changing unit. Second search means (S37) for executing the process of searching for the resonance frequency of the first frequency based on the impedance measured by the measurement means, and second setting means for setting the frequency detected by the second search means as the frequency of the AC voltage (S39) is further provided, and the first setting means performs the process in an alternative manner after the process of the second setting means.

  More preferably, the supply means includes a fourth inductor (L4) having a number of turns smaller than that of the third inductor and inductively coupled to the third inductor.

  A power transmission device (10) according to the present invention includes a power receiving device (30) provided with supply means (32, 34) for supplying electric power based on an electric field excited by a plurality of first electrodes (E3 to E4) to a load (36). A power transmission device for transmitting electric power to a plurality of second electrodes (E1, E2) coupled to a plurality of first electrodes to excite an electric field based on an alternating voltage, and a plurality of second electrodes in series resonance Inductor (L2) forming the circuit, changing means (S33, S47 to S51) for repeatedly changing the frequency of the AC voltage, measuring means (S35) for measuring impedance in parallel with the processing of the changing means, and measuring means Search means (S41) for searching for a frequency at a point in time when the impedance shows a decreasing tendency satisfying a predetermined condition toward the resonance frequency of the series resonance circuit from among a plurality of frequencies specified by the change means, and detection by the search means AC voltage when feeding the measured frequency Setting means for setting the frequency comprises (S45).

  According to this invention, the frequency at the time of showing a decreasing tendency that satisfies the predetermined condition toward the resonance frequency (= series resonance frequency) of the series resonance circuit is set as the frequency of the AC voltage during power feeding. By setting to a frequency that tends to decrease toward the series resonance frequency (= a frequency different from the series resonance frequency), a situation in which the AC voltage applied to the plurality of first electrodes is excessive is avoided, thereby ensuring safety. Is secured. Further, by using the series resonance frequency as a reference, it is possible to reliably set the frequency of the AC voltage during power feeding even in a state where the degree of coupling between the power transmitting device and the power receiving device is low. Thus, power transmission performance is improved.

  The above object, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

It is a block diagram which shows the structure of one Example of this invention. It is an illustration figure which shows an example of the external appearance of FIG. 1 Example. It is a block diagram which shows the equivalent circuit of FIG. 1 Example. It is a graph which shows an example of the change of the impedance with respect to a frequency. It is a graph which shows another example of the change of the impedance with respect to a frequency. It is a flowchart which shows a part of operation | movement of CPU applied to the FIG. 1 Example. It is a flowchart which shows a part of other operation | movement of CPU applied to the FIG. 1 Example. It is a flowchart which shows a part of other operation | movement of CPU applied to the FIG. 1 Example. FIG. 12 is a flowchart showing yet another portion of behavior of the CPU applied to the embodiment in FIG. 1. It is a block diagram which shows a part of structure of the other Example of this invention.

  1 and 2, a power transmission system 100 according to this embodiment includes a power transmission device 10 having an upper surface in which power transmission electrodes E1 and E2 are embedded, and a power reception device having a lower surface in which power reception electrodes E3 and E4 are embedded. Formed by the device 30. When the lower surface of the power receiving device 30 is brought close to the upper surface of the power transmitting device 10 so that the power receiving electrodes E3 and E4 face the power transmitting electrodes E1 and E2 (see FIG. 2), the power receiving device 30 is electrically coupled to the power transmitting device 10. As a result, the power of the power transmission device 10 is transmitted to the power reception device 30.

  As shown in FIG. 1, the DC power supply 12 applies a DC voltage to the input terminal of the switch SW1 connected to either one of the terminals T1 and T2. The terminal T1 is directly connected to the inverter 18, and the terminal T2 is connected to the inverter 18 via the resistor R1. Therefore, a direct current voltage is supplied to the inverter 18 when the switch SW1 is connected to the terminal T1, and a direct current is supplied to the inverter 18 when the switch SW1 is connected to the terminal T2.

  The inverter 18 is turned on while the PWM signal output from the PWM generation circuit 14 is at the H level, and is turned off when the PWM signal output from the PWM generation circuit 14 is at the L level. Inverter 18 is also connected to inductor L1 among inductors L1 and L2 that form transformer 20 and are inductively coupled.

  Therefore, when inverter 18 is turned on / off as described above, an AC voltage is induced in each of inductors L1 and L2. However, the number of turns of the inductor L2 is larger than the number of turns of the inductor L1, and the AC voltage induced in the inductor L2 is higher than the AC voltage induced in the inductor L1. Further, the frequency and height of the AC voltage induced in each of the inductors L1 and L2 depend on the frequency (= PWM frequency) of the PMW signal and the duty ratio, respectively.

  The AC voltage induced in the inductance L2 is applied to the power transmission electrodes E1 and E2. An AC voltage having a frequency that corresponds to the frequency of the applied AC voltage and a height that depends on the degree of electric field coupling is excited in the receiving electrodes E3 and E4 that are field-coupled with the power transmission electrodes E1 and E2.

  The alternating voltage thus excited is supplied to the rectifier circuit 34 via inductors L3 and L4 that form a transformer 32 and are inductively coupled. However, the number of turns of the inductor L4 is smaller than the number of turns of the inductor L3, and the AC voltage supplied to the rectifier circuit 34 is lower than the AC voltage excited by the power receiving electrodes E3 and E4. The rectifier circuit 34 rectifies such an AC voltage into a DC voltage and supplies the rectified DC voltage to the battery 36. Thereby, the battery 36 is charged.

  FIG. 3 shows an equivalent circuit of the power transmission system 100 shown in FIG. According to FIG. 3, the power transmission side resonance circuit SR1 formed by the inductor having the inductance Lt and the capacitor having the capacitance Ct is provided in the power transmission device 10, and the power reception is formed by the inductor having the inductance Lr and the capacitor having the capacitance Cr. The side resonance circuit PR1 is provided in the power receiving device 30. Further, a coupling capacitance Cm is generated between the two capacitors.

Based on this, the anti-resonance frequency and the resonance frequency of the power transmission side resonance circuit SR1 are defined according to the equations 1 and 2, respectively, and the anti-resonance frequency and the resonance frequency of the power reception side resonance circuit PR1 are defined according to the equations 3 and 4, respectively. The
[Equation 1]
Ft_L = 1 / (2π√ (Lt (Ct + Cm)))
Ft_L: Anti-resonance frequency [Equation 2] of the power transmission side resonance circuit SR1
Ft_H = 1 / (2π√ (Lt (Ct−Cm)))
Ft_H: Resonance frequency of transmission side resonance circuit SR1 [Equation 3]
Fr_L = 1 / (2π√ (Lr (Cr + Cm)))
Fr_L: Anti-resonance frequency of the power receiving side resonance circuit PR1 [Equation 4]
Fr_H = 1 / (2π√ (Lr (Cr−Cm)))
Fr_H: Resonance frequency of the power receiving side resonance circuit PR1

  If the electric field coupling degree between the power transmitting device 10 and the power receiving device 30 is high, the impedance Z viewed from the inverter 18 changes in the manner shown in FIG. 4 with respect to the PWM frequency. On the other hand, if the electric field coupling degree between the power transmitting device 10 and the power receiving device 30 is low, the impedance Z viewed from the inverter 18 changes in the manner shown in FIG. 5 with respect to the PWM frequency. In addition, when a foreign object approaches the power transmission device 10 in a state where the electric field coupling degree is low, the impedance Z changes along a one-dot chain line shown in FIG.

  According to FIG. 4 where the electric field coupling degree is high, the impedance Z not only shows the maximum value and the minimum value corresponding to the anti-resonance frequency Ft_L and the resonance frequency Ft_H of the power transmission side resonance circuit SR1, but also the power reception side resonance circuit PR1. Corresponding to the anti-resonance frequency Fr_L and the resonance frequency Fr_H, respectively, show a maximum value and a minimum value. On the other hand, according to FIG. 5 where the electric field coupling degree is low, the impedance Z only shows the maximum value and the minimum value corresponding to the anti-resonance frequency Ft_L and the resonance frequency Ft_H of the power transmission side resonance circuit SR1, respectively.

  Since the magnitude of the coupling capacitance Cm depends on the electric field coupling degree, the values of the anti-resonance frequency Ft_L and the resonance frequency Ft_H of the power transmission side resonance circuit SR1 are different between FIGS. Further, in this embodiment, a point where the impedance Z has a minimum value is a resonance point, and a point where the impedance Z has a maximum value is an anti-resonance point.

  In order to charge the battery 36 provided in the power receiving device 30, the CPU 16 of the power transmitting device 10 connects the connection destination of the switch SW1 to the terminal T2, and fixedly sets the duty ratio indicating the initial value in the PWM generation circuit 14. At the same time, the PWM generation circuit 14 is repeatedly set with a PWM frequency that sequentially decreases from the maximum value to the minimum value of the sweep range shown in FIGS.

  PWM generation circuit 14 provides inverter 18 with a PWM signal having a set duty ratio and PWM frequency. Thus, an AC voltage having a height and frequency depending on the duty ratio and the PWM frequency is applied to the power transmission electrodes E1 to E2, and the impedance Z is measured based on the voltage at the input terminal of the inverter 18.

  If the measured impedance Z shows a maximum value, the CPU 16 considers that the anti-resonance frequency Fr_L of the power receiving side resonance circuit PR1 has been detected, and determines the current frequency as the PWM frequency at the time of power feeding. On the other hand, if the measured impedance Z falls below the threshold THz and the current PWM frequency belongs to the assumed range, the CPU 16 considers that a frequency around the resonance frequency Ft_H of the power transmission side resonance circuit SR1 has been detected. The current frequency is determined as the PWM frequency during power feeding.

  That is, when the electric field coupling degree between the transmission device 10 and the power reception device 30 is high, both the resonance of the power reception side resonance circuit PR1 and the resonance of the power transmission side resonance circuit SR1 are reflected in the frequency characteristic of the impedance Z (see FIG. 4). , The anti-resonance frequency Fr_L of the power receiving side resonance circuit PR1 is set as the PWM frequency during power feeding.

  On the other hand, when the electric field coupling between the transmission device 10 and the power reception device 30 is low and the resonance of the power reception side resonance circuit PR1 is not reflected in the frequency characteristic of the impedance Z (see FIG. 5), the power transmission side series resonance circuit SR1. The frequency around the resonance frequency Ft_H is set as the PWM frequency during power feeding. Furthermore, the process of setting the frequency around the resonance frequency Ft_H is performed in an alternative manner after the process of setting the anti-resonance frequency Fr_L of the power receiving resonance circuit PR1.

  When the PWM frequency during power supply is thus determined, the CPU 16 switches the connection destination of the switch SW1 to the terminal T1. Thereby, power supply to the power receiving device 30 is started.

  The impedance Z is repeatedly measured even after feeding is started. When the approach of a foreign object to the power transmission device 10 is detected based on the change in the impedance Z, the CPU 16 switches the connection destination of the switch SW1 to the terminal T2 and searches for the PWM frequency in the same manner as described above.

  As described above, when a foreign object approaches the power transmission device 10 in a state where the electric field coupling degree is low, the impedance Z changes along the alternate long and short dash line shown in FIG. In this state, the frequency at which the impedance Z falls below the threshold THz is out of the assumed range. For this reason, the PWM frequency is repeatedly updated within the sweep range. When the frequency characteristic of the impedance Z returns to the original state due to the foreign object moving away from the power transmission device 10, the frequency at the time when the impedance Z falls below the threshold THz falls within the assumed range. As a result, the PWM frequency during power feeding is set to a frequency around the resonance frequency Ft_H. When the frequency setting is completed, the connection destination of the switch SW1 is switched to the terminal T1, and power feeding is resumed.

  When the full charge of the battery 36 is detected based on the impedance Z, the CPU 16 switches the connection destination of the switch SW1 to the terminal T2, and turns off the PWM generation circuit 14 to stop the power supply to the power receiving device 30. When a designated time (= for example, 1 minute) elapses, the PWM generation circuit 14 is turned on, whereby the power supply to the power receiving device 30 is resumed. The PWM generation circuit 14 generates a PWM signal having a frequency and a duty ratio before shifting to the off state. When the power supply is resumed, the CPU 16 sweeps the PWM frequency and measures the frequency characteristic of the impedance Z in order to determine whether or not the power receiving device 30 and the power transmitting device 10 are coupled. Such a series of processes of stopping power feeding, waiting for a specified time, restarting power feeding, and measuring impedance is repeatedly executed as long as the connection between the power receiving device 30 and the power transmitting device 10 is detected.

  Specifically, the CPU 16 executes processing according to the flowcharts shown in FIGS. A control program corresponding to this flowchart is stored in the flash memory 16m.

  Referring to FIG. 6, in step S1, switch SW1 is connected to terminal T2, and in step S3, power feeding pre-processing is executed to determine the PWM frequency during power feeding. When the PWM frequency is determined, the connection destination of the switch SW1 is returned to the terminal T1 in step S5. Thereby, power supply to the power receiving device 30 is started.

  In step S7, the impedance Z is measured based on the voltage at the input terminal of the inverter 18. In step S9, it is determined based on the measured impedance Z whether or not a foreign object has approached the power transmission device 10, and in step S11, whether the battery 36 is fully charged is determined based on the measured impedance Z. To do. If the determination result of step S9 is YES, it will return to step S3, after performing the process similar to step S1 by step S13. On the other hand, if the determination result of step S11 is YES, it will progress to step S17 after performing the process similar to step S1 by step S15.

  In step S17, the PWM generation circuit 14 is turned off to stop the power supply to the power receiving device 30, and in step S19, the system waits for a specified time (= for example, 1 minute). When the designated time elapses, the process proceeds to step S21, and the PWM generation circuit 14 is turned on to restart the power supply to the power receiving device 30. The PWM generation circuit 14 generates a PWM signal having a frequency and a duty ratio before shifting to the off state.

  In step S23, the PWM frequency is swept to measure the frequency characteristic of the impedance Z. In step S25, it is determined based on the measured frequency characteristics whether or not the power receiving device 30 has been detached from the power transmitting device 10. If the determination result is NO, the process returns to step S17, while if the determination result is YES, the process returns to step S3.

  The power feeding pre-processing in step S3 is executed according to the subroutine shown in FIGS. First, in step S31, the duty ratio is set to an initial value, and in step S33, the PWM frequency is set to “fmax” (= maximum value of the sweep range). PWM generation circuit 14 provides inverter 18 with a PWM signal having a set duty ratio and PWM frequency.

  In step S35, the impedance Z is measured based on the voltage at the input terminal of the inverter 18, and in step S37, it is determined whether or not the measured impedance Z shows a maximum value. If the determination result is YES, it is considered that the resonance frequency Fr_L of the power receiving side resonance circuit PR1 has been detected, and the process proceeds to step S39. In step S39, the current frequency is set as the deterministic PWM frequency, and then the process returns to the upper layer routine.

  If the determination result in step S37 is NO, it is determined in step S41 whether or not the impedance Z measured in step S35 is lower than the threshold THz, and whether or not the current PWM frequency belongs to the assumed range is determined in step S43. In step S47, it is determined whether or not the current PWM frequency corresponds to “fmin” (= minimum value of the sweep range).

  If the determination result in step S41 or S43 is NO and the determination result in step S47 is NO, the PWM frequency is reduced by a predetermined width in step S49, and then the process returns to step S35. If the determination result of step S41 or S43 is NO and the determination result of step S47 is YES, the same process as step S33 is executed in step S51, and then the process returns to step S35. If the determination result of step S41 and S43 is YES, it will consider that the frequency around the antiresonance frequency Ft_H of the power transmission side resonance circuit SR1 was detected, and will progress to step S45. In step S45, the current frequency is set as the deterministic PWM frequency, and then the process returns to the upper layer routine.

  As can be seen from the above description, the power transmission device 10 includes power transmission electrodes E <b> 1 to E <b> 2 that excite an electric field based on an AC voltage applied via the transformer 20. Here, the transformer 20 and the power transmission electrodes E1 to E2 form a power transmission side resonance circuit SR1. On the other hand, the power receiving device 30 includes power receiving electrodes E3 to E4 that are electric field coupled to the power transmitting electrodes E1 to E2, and a transformer 32 and a rectifier circuit 34 that supply an AC voltage excited by the power receiving electrodes E3 to E4 to the battery 36. .

  Based on this, the CPU 16 repeatedly changes the frequency (= PWM frequency) of the AC voltage applied to the power transmission electrodes E1 to E2 (S33, S47 to S51), and measures the impedance Z in parallel with this (S35). ). The CPU 16 also shows a time point when the measured impedance Z shows a decreasing tendency that satisfies a predetermined condition (a predetermined condition: a condition that the magnitude of the impedance Z is lower than the threshold THz) toward the resonance frequency Ft_H of the power transmission side resonance circuit SR1. The frequency is searched (S41), and the detected frequency is set as the frequency of the AC voltage during power feeding (S45).

  By setting the frequency at the time of showing a decreasing tendency toward the resonance frequency Ft_H (= a frequency different from the resonance frequency Ft_H), a situation in which the AC voltage applied to the power transmission electrodes E1 to E2 becomes excessive is avoided. Safety is ensured. Further, by using the resonance frequency Ft_H as a reference, it is possible to reliably set the frequency of the AC voltage during power feeding even in a state where the degree of coupling between the power transmitting device 10 and the power receiving device 30 is low. Thus, power transmission performance is improved.

  In this embodiment, it is set as a predetermined condition that the impedance Z is lower than the threshold value THz. However, it may be set as a predetermined condition that the decrease rate of the impedance Z exceeds another threshold value.

  In this embodiment, the impedance Z is measured in order to determine the PWM frequency (see steps S25, S27, and S31). However, instead of the impedance Z, a current that conducts the inverter 18 may be measured.

  Furthermore, in this embodiment, the power receiving side resonance circuit PR1 is formed by a capacitor and an inductor connected in parallel. However, as shown in FIG. 10, the power receiving side resonance circuit PR1 may be formed by a capacitor and an inductor connected in series.

DESCRIPTION OF SYMBOLS 10 ... Power transmission apparatus 14 ... PWM generation circuit 16 ... CPU
18 ... Inverter 20, 32 ... Transformer 34 ... Rectifier circuit E1-E2 ... Power transmission electrode E3-E4 ... Power reception electrode

Claims (9)

A power transmission device including a plurality of first electrodes for exciting an electric field based on an alternating voltage, a first inductor that forms a series resonance circuit with the plurality of first electrodes, and a plurality of electric fields coupled to the plurality of first electrodes. A power transmission system formed by the second electrode and a power receiving device including a supply unit that supplies power based on an electric field excited by the plurality of second electrodes to a load,
The power transmission device is:
Changing means for repeatedly changing the frequency of the AC voltage;
Measuring means for measuring impedance in parallel with the processing of the changing means,
A first search for searching for a frequency at a point in time when the impedance measured by the measuring means shows a decreasing tendency satisfying a predetermined condition toward the resonance frequency of the series resonant circuit from among the plurality of frequencies specified by the changing means. And a first setting means for setting the frequency detected by the first searching means as the frequency of the AC voltage during power feeding.   The power transmission system according to claim 1, wherein the predetermined condition includes a condition that a magnitude of the impedance is lower than a first threshold value.   The power transmission system according to claim 1, wherein the predetermined condition includes a condition that a reduction rate of the impedance exceeds a second threshold value.   The power transmission device includes a limiting unit that limits a search range of the first search unit to a range narrower than a change range of the change unit, and a restart unit that restarts the change unit when an approach of a foreign object is detected during power feeding. The power transmission system according to claim 1, further comprising an activation unit.   The power transmission system according to claim 4, wherein the changing unit changes the frequency from a high frequency side to a low frequency side by a predetermined width.   The power transmission device has a number of turns smaller than the number of turns of the first inductor, a second inductor that is inductively coupled to the first inductor, and a switching that periodically switches a current amount that conducts the second inductor. The power transmission system according to any one of claims 1 to 5, further comprising means. The supply means includes the third inductor that forms a parallel resonance circuit or a series resonance circuit with the plurality of second electrodes,
The power transmission device includes: a second search unit that executes a process of searching for a resonance frequency of the parallel resonant circuit from a plurality of frequencies specified by the change unit based on the impedance measured by the measurement unit; Further comprising second setting means for setting the frequency detected by the second search means as the frequency of the AC voltage;
The power transmission system according to any one of claims 1 to 6, wherein the first setting unit performs processing in an alternative manner after the processing of the second setting unit.   The power transmission system according to claim 7, wherein the supply unit includes a fourth inductor having a smaller number of turns than the third inductor and inductively coupled to the third inductor. A power transmission device that transmits power to a power receiving device including a supply unit that supplies power based on an electric field excited by a plurality of first electrodes to a load,
A plurality of second electrodes coupled to the plurality of first electrodes to excite the electric field based on an alternating voltage;
An inductor that forms a series resonant circuit with the plurality of second electrodes;
Changing means for repeatedly changing the frequency of the AC voltage;
Measuring means for measuring impedance in parallel with the processing of the changing means,
Search means for searching a frequency at a time point when the impedance measured by the measuring means shows a decreasing tendency that satisfies a predetermined condition toward the resonance frequency of the series resonant circuit, from among a plurality of frequencies designated by the changing means, And a power transmission device comprising setting means for setting the frequency detected by the search means as the frequency of the AC voltage during power feeding.

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