Power transmission device and power transmission system

A power supply circuit supplies AC power to a power-transmitting coil. A power-transmitting coil unit includes the power-transmitting coil and transmits AC power to a power receiving device. The power-transmitting coil unit executes first power transmission and second power transmission in which AC power received by the power receiving device is less than the AC power in the first power transmission and executes the second power transmission before executing the first power transmission.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Japanese Patent Application No. 2019-206041, filed on Nov. 14, 2019, the entire disclosure of which is incorporated by reference herein.

FIELD

This application relates generally to a power transmission device and a power transmission system.

BACKGROUND

A wireless power transmission technology transmitting (feeding) electric power without using a power cord is receiving attention. The wireless power transmission technology allows wireless transmission (feeding) of electric power from a power-transmitting-side device to a power-receiving-side device. Accordingly, application of the wireless power transmission technology to transportation equipment such as electric trains and electric vehicles, and various products such as consumer electronics, electronic equipment, wireless communication equipment, and toys is expected.

Part of electric members or mechanical members constituting a power transmission system, such as an electrolytic capacitor, requires a certain temperature or higher in order to exhibit performance thereof. Accordingly, the aforementioned electric member or mechanical member may not sufficiently exhibit the performance thereof when used in a state of the temperature of the use environment being too low. Consequently, for example, there is a risk that electric power containing a large number of ripples and a large amount of noise is transmitted from a power-transmitting-side unit to a power-receiving-side unit. In such a case, a part an operating characteristic of which is degraded at a low temperature needs to be heated, and the temperature of the part needs to be raised to a temperature at which the characteristic is kept excellent. For such a purpose, Unexamined Japanese Patent Application Publication No. 2010-268664 discloses a system raising the temperature of equipment being a power-receiving-side device by an electromagnetic field generated at power transmission.

SUMMARY

The system disclosed by Unexamined Japanese Patent Application Publication No. 2010-268664 can raise the temperature of equipment being a power-receiving-side device, such as a power storage device, but cannot raise the temperature of a part in a power-transmitting-side device. Consequently, there is a risk that electric power containing a large number of ripples and a large amount of noise is fed from the power-transmitting side to the power-receiving side, as described above.

The present disclosure has been made in view of the aforementioned problem, and an objective of the disclosure is to enable a rise in the temperature of a part constituting a power-transmitting-side device in a power transmission system wirelessly transmitting electric power.

In order to solve the aforementioned problem, a power transmission device according to an aspect of the present disclosure

is a power transmission device wirelessly transmitting electric power to a power receiving device including a power-receiving coil equipped on a moving body and

includes:a power-transmitting coil unit including a power-transmitting coil and transmitting AC power to the power receiving device; anda power supply circuit supplying AC power to the power-transmitting coil,

wherein the power-transmitting coil unit executes first power transmission and second power transmission in which AC power received by the power receiving device is less than the AC power in the first power transmission and executes the second power transmission before executing the first power transmission.

Further, in order to solve the aforementioned problem, a power transmission system according to an aspect of the present disclosure

includes a power receiving device equipped on a moving body and a power transmission device wirelessly transmitting electric power to the power receiving device, wherein

the power transmission device includes:a power supply circuit supplying AC power to a power-transmitting coil; anda power-transmitting coil unit including the power-transmitting coil and transmitting AC power to the power receiving device,

the power receiving device includes:a power-receiving coil unit including a power-receiving coil and receiving AC power; anda rectifier circuit rectifying AC power received by the power-receiving coil unit, and

the power-transmitting coil unit executes first power transmission and second power transmission in which AC power received by the power receiving device is less than the AC power in the first power transmission and executes the second power transmission before executing the first power transmission.

The power transmission device and the power transmission system with the aforementioned configurations can raise the temperature of a member by performing the second power transmission and then can perform the first power transmission.

DETAILED DESCRIPTION

A power transmission system according to embodiments of the technology according to the present disclosure will be described below referring to drawings. Note that, in the following embodiments, the same components are given the same sign. Further, a ratio between sizes of and shapes of components illustrated in each drawing do not necessarily represent an actual ratio and actual shapes.

A power transmission system1according to the present embodiment can be used for charging a secondary battery in various devices such as mobile equipment such as a smartphone, an electric vehicle (EV), and industrial equipment. A case of the power transmission system1executing charging of a storage battery in an EV will be illustrated below.

FIG. 1is a diagram illustrating a configuration of the power transmission system1used for charging a storage battery80included in an electric vehicle8. The electric vehicle8runs with a motor driven by electric power charged in the storage battery80such as a lithium ion battery or a lead storage battery as a power source. The electric vehicle8is an example of a moving body in the present disclosure.

As illustrated inFIG. 1, the power transmission system1includes a power transmission device10wirelessly transmitting (wirelessly feeding) electric power from an AC or DC commercial power source100to the electric vehicle8and a power receiving device2receiving electric power transmitted by the power transmission device10and charging the storage battery80. Note that it is assumed in the following description that the commercial power source100is an AC power source.

The power transmission device10includes a power supply circuit12supplying AC power to a power-transmitting coil unit18, a control circuit102controlling an operation of the power supply circuit12, and the power-transmitting coil unit18transmitting AC power to the electric vehicle8.

The power supply circuit12generates AC power at a frequency within a predetermined range from electric power supplied from the commercial power source100and supplies the AC power to the power-transmitting coil unit18. The control circuit102is a device controlling the power supply circuit12in such a way that a frequency of AC power supplied to the power-transmitting coil unit in first power transmission to be described later and the frequency in second power transmission to be described later are different from each other. For example, the control circuit102controls the power supply circuit12in such a way that a frequency fdof AC power takes a value between 75 kHz and 90 kHz. For example, in response to reception of a charging start instruction signal, the control circuit102starts charging of the storage battery80by controlling the power supply circuit12.

The power-transmitting coil unit18executes the first power transmission and the second power transmission to be described later. The power-transmitting coil unit18includes a magnetic plate182made of a magnetic substance such as an iron material or ferrite, and a power-transmitting coil180being wound on the magnetic plate182, being applied with AC voltage, and inducing magnetic flux1. For example, the power-transmitting coil unit18is fixed on a parking lot floor. Further, the frequency fdof AC power transmitted toward the electric vehicle8by the power transmission device10can be varied by control over the power supply circuit12by the control circuit102.

The power receiving device2is fixed to the electric vehicle8. The power receiving device2includes a power-receiving coil unit20receiving AC power transmitted by the power transmission device10and a rectifier circuit24rectifying received AC power to DC power and supplying the DC power to the storage battery80. The power-receiving coil unit20includes a magnetic plate202and a power-receiving coil200wound on the magnetic plate202. The power-receiving coil200faces the power-transmitting coil180in a state of the electric vehicle8stopping at a preset position.

When AC voltage is applied to the power-transmitting coil unit18by the power supply circuit12in a state of the power-transmitting coil180in the power-transmitting coil unit18and the power-receiving coil200in the power-receiving coil unit20facing each other, AC current flows in the power-transmitting coil180, and the power-transmitting coil unit18induces alternating magnetic flux1. By the alternating magnetic flux Φ being interlinked with the power-receiving coil200, a counter electromotive force is induced in the power-receiving coil200. The rectifier circuit24rectifies and smoothes the counter electromotive force induced in the power-receiving coil200and supplies DC power to the storage battery80. Further, a charging circuit converting DC power supplied from the rectifier circuit24into DC power for charging the storage battery80may be included between the rectifier circuit24and the storage battery80.

FIG. 2is a circuit diagram of the power supply circuit12and the power-transmitting coil unit18. As illustrated inFIG. 2, the power supply circuit12includes a power factor improvement circuit120improving a power factor of commercial AC power supplied by the commercial power source100and an inverter circuit160generating AC power supplied to the power-transmitting coil unit18.

The power factor improvement circuit120operates as a boosting converter circuit rectifying and boosting commercial AC power supplied from the commercial power source100to DC power in accordance with control by the control circuit102. The power factor improvement circuit120supplies thus acquired DC power to the inverter circuit160.

The power factor improvement circuit120includes a full-wave rectifier circuit122, a capacitor124, an inductor126, a metal-oxide-semiconductor (MOS)-field-effect transistor (FET)128, a diode130, and an electrolytic capacitor132. For example, the electrolytic capacitor132is configured with a large-capacity electrolytic capacitor. A characteristic of an electrolytic solution filled in an electrolytic capacitor is affected by temperature. Accordingly, a usable temperature range is defined for an electrolytic capacitor; and there is a risk that a sufficient characteristic may not be acquired when the electrolytic capacitor is used at a temperature equal to or lower than the lower limit of the usable temperature range. While the lower limit of the usable range varies by product, it is assumed here that the lower limit is −20° C. Accordingly, the electrolytic capacitor132corresponds to a member the temperature of which needs to be raised when the temperature is lower than −20° C.

Then, when the temperature of the electrolytic capacitor132is lower than −20° C., it is required to raise the temperature of the electrolytic capacitor132to a temperature equal to or higher than −20° C. before charging the storage battery80. For this purpose, prior to power transmission for charging, the power transmission system1causes the power transmission device10to generate heat and raise the temperature of the electrolytic capacitor132. A method of causing the power transmission device10to generate heat by use of loss caused by power transmission in which AC power received by the power receiving device2is reduced by decreasing efficiency of power transmission from the power-transmitting coil unit18to the power-receiving coil unit20is employed in the present embodiment.

The power transmission device10includes a temperature detection circuit134detecting the temperature of the electrolytic capacitor132. For example, the temperature detection circuit134includes a thermistor installed in the neighborhood of the electrolytic capacitor132and detects a temperature measured by the thermistor as the temperature of the electrolytic capacitor132. Measuring temperature is hereinafter referred to as detecting temperature as appropriate. The temperature detection circuit134is an example of a temperature detector in the present disclosure.

The inverter circuit160is configured with a full-bridge circuit including N-channel power MOS-FETs162,164,166, and168. The inverter circuit160converts DC power supplied from the power factor improvement circuit120into AC power by the N-channel power MOS-FETs162,164,166, and168being turned on and off in accordance with control by the control circuit102and supplies the AC power to the power-transmitting coil unit18.

FIG. 3is a diagram illustrating an equivalent circuit of the power-transmitting coil unit18and the power-receiving coil unit20. Further,FIG. 3explicitly illustrates DC resistance Rtof the power-transmitting coil180and DC resistance Rfof the power-receiving coil200that are not illustrated inFIG. 1andFIG. 2.

As illustrated inFIG. 2, the power-transmitting coil unit18includes power-transmitting-side capacitors184-1and184-2each of which is connected between each of two terminals connecting the inverter circuit160and the power-transmitting coil unit18, and each of two terminals of the power-transmitting coil180. Note that the two power-transmitting-side capacitors184-1and184-2are illustrated as one power-transmitting-side capacitor184in the equivalent circuit of the power-transmitting coil unit18illustrated in and afterFIG. 3. The power-transmitting-side capacitor184is an example of a power-transmitting-side capacitor in the present disclosure. Further, the power-transmitting coil180constitutes a resonance circuit along with the power-transmitting-side capacitors184-1and184-2.

The power-receiving coil unit20also has a configuration similar to that of the power-transmitting coil unit18and includes two power-receiving-side capacitors. The two power-receiving-side capacitors are illustrated as one power-receiving-side capacitor204in the equivalent circuit of the power-receiving coil unit20illustrated in and afterFIG. 3. The power-receiving-side capacitor204is an example of a power-receiving-side capacitor in the present disclosure. Further, the power-receiving coil200constitutes a resonance circuit along with the power-receiving-side capacitor204.

In a state of the electric vehicle8stopping at a predetermined parking position, the power-transmitting coil180and the power-receiving coil200do not mechanically interfere with each other and face each other in parallel at a distance Dv. In this state, the power-transmitting coil180and the power-receiving coil200are most tightly coupled through magnetic flux Φ and can most efficiently achieve power transmission.

Note that a coupling coefficient k indicating a degree of magnetic coupling between the power-transmitting coil180and the power-receiving coil200varies with change in a relative positional relation between the power-transmitting coil180and the power-receiving coil200. Accordingly, when a relative positional relation between the power-transmitting coil unit18and the power-receiving coil unit20changes, reactance of an entire circuit including the magnetically coupled power-transmitting coil unit18and power-receiving coil unit20viewed from the power supply circuit12changes, and a resonance frequency fonalso changes. Note that k=0 to 1. Further, “the entire circuit including the magnetically coupled power-transmitting coil unit18and power-receiving coil unit20” viewed from the power supply circuit12is simply referred to as a circuit including the power-transmitting coil unit18and the power-receiving coil unit20in the following description.

FIG. 4is a diagram illustrating a frequency-F-versus-admittance-A characteristic of the power-transmitting coil unit18and the power-receiving coil unit20when a relative positional relation between the circuits is optimized (the coupling coefficient k is maximized). Note that a unit of frequency F is Hz, and a unit of admittance A is siemens (S=1/Ω) inFIG. 4. The frequency-F-versus-admittance-A characteristic is hereinafter described as a frequency F-admittance A characteristic. As illustrated inFIG. 4, a resonance frequency of a circuit including the power-transmitting coil unit18and the power-receiving coil unit20becomes the resonance frequency fonwhen the relative positional relation between the circuits is as illustrated inFIG. 3.

Further, when the frequency of AC power supplied to the power-transmitting coil unit18matches the resonance frequency fon, the power-transmitting coil unit18and the power-receiving coil unit20as a whole exhibits a maximum value Amaxof admittance A, and efficiency of power transmission from the power-transmitting coil unit18to the power-receiving coil unit20is maximized. Such a power transmission method is called a magnetic field resonance method and is characterized by a small decrease in power transmission efficiency from the power-transmitting coil unit18to the power-receiving coil unit20relative to a decrease in a value of the coupling coefficient k between the power-transmitting coil180and the power-receiving coil200.

However, the frequency of AC power applied to the power-transmitting coil unit18may not necessarily be matched to the resonance frequency fon. For example, in consideration of stability of an operation or the like of the power transmission system1, the frequency may be shifted to a frequency fdin a direction on the higher side of or in a direction on the lower side of the resonance frequency fonby Δfd.

Further, in a case of not transferring electric power much to the power-receiving coil unit20and heating a circuit element in the power factor improvement circuit120when the temperature is low, the control circuit102controls the inverter circuit160in such a way as to set the frequency of AC power supplied to the power-transmitting coil180to a frequency fdlor a frequency fdhat which power transmission efficiency is decreased, instead of the frequency fd.

Specifically, the control circuit102controls the inverter circuit160and causes the inverter circuit160to generate AC power at the frequency fdhdistant from the resonance frequency fonby Δfdhon the higher side or AC power with the frequency fdldistant from the resonance frequency fonby Δfdlon the lower side and supply the generated AC power to the power-transmitting coil unit18. The absolute values |Δfdh| and |Δfdl| of Δfdhand Δfdlare set in such a way as to be greater than the absolute value |Δfd| of Δfd. In other words, the frequencies fdhand fdlare more distant from the resonance frequency fonthan the frequency fd. At this time, a value of the admittance A of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20decreases from Amaxto Afd. Accordingly, AC power received by the power receiving device2decreases; and in this state, the power-transmitting coil unit18generates heat due to the DC resistance Rtof the power-transmitting coil180and iron loss (eddy-current loss) of the magnetic plate182, and the temperatures of the case and the internal atmosphere of the power-transmitting coil unit18rise. Furthermore, each element inside the power supply circuit12generates heat, and the temperatures of the case and the internal atmosphere of the power supply circuit12rise. The heat generated in the power-transmitting coil unit18is used for raising the temperature of a member in a surrounding area. Further, the electrolytic capacitor132is heated by the heat generated in the power supply circuit12.

Next, power transmission processing executed by the power transmission system1with the configuration described above will be described with reference to a flowchart inFIG. 5. Note that, in the following description, an operation of transmitting electric power with a relatively high degree of efficiency from the power-transmitting coil unit18to the power-receiving coil unit20for charging the storage battery80by magnetic coupling between the power-transmitting coil unit18and the power-receiving coil unit20is referred to as “first power transmission,” and an operation of raising the temperature of a part without transferring electric power much from the power-transmitting coil unit18to the power-receiving coil unit20by intentionally decreasing power transmission efficiency is referred to as “second power transmission”; and the two are distinguished as such.

First, an operation of the power transmission system1when only the first power transmission is to be executed without the need to execute the second power transmission will be described. When receiving a charging start instruction, the control circuit102starts the power transmission processing illustrated inFIG. 5and first detects the temperature of the electrolytic capacitor132by the temperature detection circuit134(Step S101). Note that the instruction to start charging is given by a charging start instruction signal.

Next, the control circuit102determines whether the detected temperature of the electrolytic capacitor132is lower than a threshold temperature (Step S102). The threshold temperature is a temperature set based on the lower limit of an allowable operating range and is, for example, −20° C. It is assumed here that the temperature detected by the temperature detection circuit134is equal to or higher than the threshold temperature (Step S102: No).

In this case, the control circuit102executes the first power transmission (Step S103). Specifically, the control circuit102controls the inverter circuit160and causes the inverter circuit160to generate AC power at the frequency fdthat can be stably transmitted from the power-transmitting coil unit18to the power-receiving coil unit20with a substantially highest degree of efficiency and supply the AC power to the power-transmitting coil unit18. In other words, the control circuit102causes the inverter circuit160to output an AC signal at the frequency fdpredetermined close to the resonance frequency fonof the circuit including the power-transmitting coil unit18and the power-receiving coil unit.

Consequently, current at the frequency fdflows in the power-transmitting coil180in the power-transmitting coil unit18, and alternating magnetic flux Φ at the frequency fdis induced. The alternating magnetic flux Φ is interlinked with the power-receiving coil200in the power-receiving coil unit20, and a counter electromotive force is induced in the power-receiving coil200. The rectifier circuit24rectifies and smoothes voltage induced in the power-receiving coil200and supplies the resulting power to the storage battery80. The control circuit102continues the first power transmission until the storage battery80is sufficiently charged.

The control circuit102determines whether to end the first power transmission (Step S104). For example, the control circuit102determines whether the storage battery80is sufficiently charged by detecting the voltage of the storage battery80through a voltage detection circuit (unillustrated) or the like. When the charging is not sufficient, the control circuit102determines not to end the first power transmission (Step S104: No) and continues the first power transmission. When the charging is sufficient, the control circuit102determines to end the first power transmission (Step S104: Yes) and ends this round of power transmission processing. For example, when the voltage of the storage battery80is higher than a reference value, the control circuit102determines that the storage battery80is sufficiently charged and ends this round of power transmission processing.

Next, an operation of the power transmission system1when the second power transmission is required before the first power transmission will be described. When receiving a charging start instruction, the control circuit102starts the power transmission processing illustrated inFIG. 5, detects the temperature of the electrolytic capacitor132by the temperature detection circuit134(Step S101), and compares the detected temperature with the threshold temperature (Step S102). It is assumed here that the detected temperature is lower than the threshold temperature (Step S102: Yes).

In this case, the control circuit102executes the second power transmission (Step S105). Specifically, the control circuit102controls the inverter circuit160and sets the frequency of AC power supplied to the power-transmitting coil180by the inverter circuit160to the frequency fdlor the frequency fdhat which power transmission efficiency is decreased, instead of the frequency fd.

In other words, the control circuit102controls the inverter circuit160and causes the inverter circuit160to generate AC power at the frequency fdhdistant from the resonance frequency fonby Δfdhon the higher side or AC power at the frequency fdldistant from the resonance frequency fonby Δfdlon the lower side and supply the AC power to the power-transmitting coil unit18.

Since the frequencies fdhand the fdlare shifted from the resonance frequency fonin the second power transmission, electric power is hardly transferred from the power-transmitting coil unit18to the power-receiving coil unit20. However, each element inside the power supply circuit12, such as a field effect transistor (FET) constituting a circuit or a ferrite core in a filter circuit, generates heat, and the temperatures of the case and the internal atmosphere of the power supply circuit12rise. Consequently, the electrolytic capacitor132is heated, and the temperature of the electrolytic capacitor132rises. Accordingly, the capacitance of the electrolytic capacitor132gradually increases. Further, when the power factor improvement circuit120and the inverter circuit160constituting the power supply circuit12are accommodated in separate cases, each element inside the power factor improvement circuit120generates heat and the temperatures of the case and the internal atmosphere of the power factor improvement circuit120rises; and thus the electrolytic capacitor132is heated.

Further, the value of the admittance A of the power-transmitting coil unit18and the power-receiving coil unit20as a whole decreases from Amaxto Afd. Accordingly, AC power received by the power receiving device2from the power transmission device10decreases; and in this state, the power-transmitting coil unit18generates heat due to the DC resistance Rtof the power-transmitting coil180in the power-transmitting coil unit18and the iron loss (eddy-current loss) of the magnetic plate182, and the temperatures of the case and the internal atmosphere of the power-transmitting coil unit18rises. The heat generated in the power-transmitting coil unit18is used for raising the temperature of a member in a surrounding area.

While executing the second power transmission, the control circuit102periodically detects the temperature of the electrolytic capacitor132by the temperature detection circuit134(Step S101) and determines whether the detected temperature is lower than the threshold temperature (Step S102).

When determining that the detected temperature is lower than the threshold temperature (Step S102: Yes), the control circuit102continues execution of the second power transmission (Step S105). On the other hand, when determining that the detected temperature is equal to or higher than the threshold temperature (Step S102: No), a determination is made that the capacitance of the electrolytic capacitor132nearly reaches a rated value, and the control circuit102advances the processing to aforementioned Step S103and thereafter causes the power-transmitting coil unit18to execute the first power transmission until the storage battery80is sufficiently charged.

As described above, when the detected temperature is lower than the threshold temperature, the second power transmission for raising the temperature of a part is executed before the first power transmission being a normal power transmission (feeding) operation is started, according to the present embodiment. Although electric power is hardly transferred from the power-transmitting coil unit18to the power-receiving coil unit20in the second power transmission, each element inside the power supply circuit12, such as a FET constituting a circuit or a ferrite core in a filter circuit, generates heat similarly to during execution of the first power transmission, and the temperatures of the case and the internal atmosphere of the power supply circuit12rise.

Further, when the power factor improvement circuit120and the inverter circuit160constituting the power supply circuit12are accommodated in separate cases, each element inside the power factor improvement circuit120generates heat similarly to during execution of the first power transmission, and the temperatures of the case and the internal atmosphere of the power factor improvement circuit120rise. Further, AC power received by the power receiving device2decreases; and in this state, the power-transmitting coil unit18generates heat due to the DC resistance Rtof the power-transmitting coil180and the iron loss of the magnetic plate182, and the temperatures of the case and the internal atmosphere of the power-transmitting coil unit18rise. The heat generated in the power-transmitting coil unit18raises the temperature of a member in a surrounding area. Accordingly, the temperature of the electrolytic capacitor132also rises, and the capacitance of the electrolytic capacitor132is restored nearly to the rated value; and the electrolytic capacitor132enters a state in which the function thereof can be exhibited. Accordingly, electric power with a small number of ripples and a small amount of noise can be transmitted in the subsequent first power transmission. Further, since the second power transmission is for raising the temperature of a part, power transmission efficiency of the second power transmission is intentionally decreased compared with during execution of the first power transmission in such a way that electric power containing a large number of ripples and a large amount of noise is not received by the power receiving device2during execution of the second power transmission.

Further, while the second power transmission is executed when the temperature of the electrolytic capacitor132detected by the temperature detection circuit134is lower than the threshold temperature in the present embodiment, the control circuit102may execute the second power transmission without performing temperature detection of the electrolytic capacitor132by the temperature detection circuit134when receiving a charging start instruction. In this case, the temperature detection circuit134may not be provided.

A technique of decreasing an amount of electric power transmitted from the power-transmitting coil unit18to the power-receiving coil unit20by shifting the frequency of AC power supplied from the power supply circuit12to the power-transmitting coil unit18from the frequency fdin the first power transmission has been described in Embodiment 1. The technique of decreasing an amount of electric power transmitted from the power-transmitting coil unit18to the power-receiving coil unit20is not limited to the above, and any technique capable of removing the circuit including the power-transmitting coil unit18and the power-receiving coil unit20as a whole from a resonant state is applicable. Embodiment 2 employing a technique of decreasing an amount of electric power transmitted from a power-transmitting coil unit18to a power-receiving coil unit20by changing impedance will be described below.

A basic configuration of a power transmission system according to the present embodiment is similar to the configuration of the power transmission system1according to Embodiment 1. However, as illustrated inFIG. 6, a power transmission device10further includes an impedance adjustment circuit32. Note that the impedance adjustment circuit32may be included in a power supply circuit12, may be included in the power-transmitting coil unit18, or may be placed in another part.

As illustrated inFIG. 6, the impedance adjustment circuit32is configured with a series circuit including a MOS-FET322performing an on/off operation in accordance with control by a control circuit102and a power-transmitting-side capacitor320and is connected in parallel with a power-transmitting-side capacitor184-1. As illustrated inFIG. 7, the MOS-FET322operates as a switch, connects the power-transmitting-side capacitor184-1to the power-transmitting-side capacitor320in parallel by being turned on (continuous), and changes the impedance of a circuit including the power-transmitting coil unit18and the power-receiving coil unit20from the impedance under normal operation when the MOS-FET322is turned off. Further, for example, the frequency of electric power supplied to the power-transmitting coil unit18by the power supply circuit12is a frequency fdin the first power transmission as well as in the second power transmission in the present embodiment.

FIG. 8is a diagram illustrating a frequency F-admittance A characteristic of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20when a relative positional relation between the power-transmitting coil unit18and the power-receiving coil unit20is optimized. InFIG. 8, a broken line represents a characteristic in a state of the MOS-FET322being turned off (cutoff), and a solid line represents a characteristic in a state of the MOS-FET322being turned on. As illustrated inFIG. 8, when the control circuit102turns the MOS-FET322off, the circuit including the power-transmitting coil unit18and the power-receiving coil unit20as a whole resonates at a resonance frequency fon. The control circuit102controls the inverter circuit160and causes the inverter circuit160to supply AC power at the frequency fdshifted from the resonance frequency fonby Δfdto the power-transmitting coil180and then causes the power-transmitting coil180to execute the first power transmission.

On the other hand, when raising the temperature of a part in the power supply circuit12in a state of an amount of electric power transmitted to the power-receiving coil unit20being decreased, the control circuit102turns the MOS-FET322on. Then, the capacitance of the power-transmitting-side capacitor320is added to the capacitance of the power-transmitting-side capacitor184-1, and thus the impedance changes. Accordingly, the resonance frequency of the entire circuit including the power-transmitting coil unit18and the power-receiving coil unit20decreases from the frequency fonby Δfoland becomes a resonance frequency fol. The difference (Δfol−Δfd) between the frequency fdof AC power and the resonance frequency folis greater than the difference Δfdbetween the frequency fdof the AC power and the resonance frequency fon, and thus the following inequality holds: Δfd<(Δfol−Δfd), thereby leading 2×Δfd<Δfol. In other words, the power-transmitting coil unit18and the power-receiving coil unit20as a whole may become a resonance circuit with the resonance frequency fonor a resonance circuit with the resonance frequency folin accordance with control by the control circuit102. Thus, the control circuit102controls the MOS-FET322in such a way that the impedance of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20as a whole during execution of the first power transmission and impedance during execution of the second power transmission are different from each other.

Accordingly, when the control circuit102turns the MOS-FET322on, a value of admittance A of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20as a whole at the frequency fddecreases from Amaxbeing a nearly maximum value to Afol, as illustrated inFIG. 8. When the value of the admittance A decreases, power transmission efficiency from the power-transmitting coil unit18to the power-receiving coil unit20decreases.

In other words, by causing the power-transmitting coil unit18to supply AC power at the frequency fdto the inverter circuit160in a state of the MOS-FET322being turned on, the control circuit102can cause the power-transmitting coil unit18to execute the second power transmission. Note that control for switching between the first power transmission and the second power transmission by the control circuit102according to Embodiment 2 is similar to the control described above with reference toFIG. 5. Further, change of the resonance frequency by adjusting impedance is not limited to the technique of changing capacitance of a circuit and may be achieved by changing inductance. For example, the change can be achieved by changing a relative position between a magnetic plate182and a power-transmitting coil180constituting the power-transmitting coil unit18.

The technique of adjusting the impedance of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20may be any technique without being limited to the technique described in Embodiment 2. For example, either one or both of the power-transmitting-side capacitors184-1and184-2may be configured with one or more variable capacitors, and capacitance thereof may be changed. Embodiment 3 employing such a configuration will be described below.

A basic configuration of a power transmission system according to the present embodiment is similar to the configuration of the power transmission system1according to Embodiment 1 illustrated inFIG. 2. However, as illustrated inFIG. 9, a power-transmitting coil unit34is used in place of the power-transmitting coil unit18. The power-transmitting coil unit34has a configuration in which the fixed capacitance power-transmitting-side capacitor184-1in the power-transmitting coil unit18is replaced by a variable capacitance power-transmitting-side capacitor340a capacitance value of which changes continuously or in stages. For example, the power-transmitting-side capacitor340is configured with a variable capacitor or a varicap diode, and a capacitance value of the power-transmitting-side capacitor340can be adjusted by a control circuit102.

The control circuit102is a device controlling an impedance adjustment circuit32in such a way that the impedance of a power-transmitting-side circuit during the first power transmission and the impedance of the power-transmitting-side circuit during the second power transmission are different from each other. For example, when executing the first power transmission, the control circuit102sets the capacitance value of the power-transmitting-side capacitor340to a predetermined first capacitance value. At this time, a circuit including the power-transmitting coil unit34and a power-receiving coil unit20resonates at a resonance frequency fonas indicated by a broken line inFIG. 10.

On the other hand, when executing the second power transmission, the control circuit102sets the capacitance value of the power-transmitting-side capacitor340to a second capacitance value smaller than the first capacitance value. At this time, the resonance frequency of the circuit including the power-transmitting coil unit34and the power-receiving coil unit20becomes a resonance frequency fohhigher than the resonance frequency fonby Δfohas indicated by a solid line inFIG. 10. The difference (Δfoh+Δfd) between a frequency fdof AC power supplied to the power-transmitting coil unit34by a power supply circuit12and the resonance frequency fohis greater than the difference Δfdbetween the frequency fdand the resonance frequency fon, and thus the following inequality holds: Δfd<(Δfoh+Δfd), thereby leading 0<Δfon.

Further, control for switching between the first power transmission and the second power transmission by the control circuit102may be similar to the control described above with reference toFIG. 5.

Further, impedance of a circuit may be changed by configuring capacitance in the power-receiving coil unit20with a variable capacitor and changing the capacitance value or by making inductance of an inductor included in a power-transmitting coil unit18variable and changing the inductance.

A technique of changing the impedance of a circuit including a power-transmitting coil unit and a power-receiving coil unit may be any technique without being limited to the techniques described in Embodiments 2 and 3. For example, a power-transmitting coil unit35may be used in place of the power-transmitting coil unit18, as illustrated inFIG. 11. The power-transmitting coil unit35has a configuration in which a power-transmitting-side capacitor350is connected to a power-transmitting-side capacitor184-1in parallel, and one of the capacitors is selected by a switch323. For example, the switch323is a changeover switch configured with a semiconductor switch.

The control circuit102controls the switch323in such a way that the power-transmitting-side capacitor184-1is connected to a power-transmitting-side capacitor184-2when the resonance frequency of a circuit including the power-transmitting coil unit35and a power-receiving coil unit20is set to, for example, fonindicated inFIG. 8. On the other hand, the control circuit102controls the switch323in such a way that the power-transmitting-side capacitor350is connected to the power-transmitting-side capacitor184-2when the resonance frequency is set to folindicated inFIG. 8. The configuration can also change the impedance of the circuit including the power-transmitting coil unit35and the power-receiving coil unit20.

The impedance may also be changed by switching connection of inductors.

While an example of placing an impedance adjustment circuit32including the power-transmitting-side capacitor350and the switch323in the power-transmitting coil unit35has been described in the above description, the impedance adjustment circuit may be placed anywhere in the power transmission device10. For example, the impedance adjustment circuit may be placed in a power supply circuit12.

Examples of changing the impedance of a power-transmitting-side circuit have been described in Embodiments 2 to 4. An example of changing the impedance of a power-receiving-side circuit will be described in the present embodiment. As illustrated inFIG. 12, a power transmission system according to the present embodiment has a configuration in which a transmission circuit400is added to the power transmission device10in the power transmission system1according to Embodiment 1, an impedance adjustment circuit configured with a series circuit including a power-receiving-side capacitor406and a MOS-FET322is connected in parallel with a power-receiving-side capacitor204in a power-receiving coil unit20, and a reception circuit402and a control circuit404are added to the power receiving device2. Note that the MOS-FET322is represented by an electric circuit symbol indicating a switch inFIG. 12.

The transmission circuit400receives a detected-temperature signal being a signal indicating the temperature of the electrolytic capacitor132detected by the temperature detection circuit134in the power factor improvement circuit120from the control circuit102and transmits the detected-temperature signal to the reception circuit402. The reception circuit402supplies the detected-temperature signal received from the transmission circuit400to the control circuit404. The transmission circuit400wirelessly communicates with the reception circuit402.

The control circuit404determines whether the temperature detected by the temperature detection circuit134is in a range in which the capacitance of the electrolytic capacitor132decreases, such as equal to or lower than −20° C. When the temperature detected by the temperature detection circuit134is equal to or lower than −20° C., the control circuit404turns the MOS-FET322on and connects the power-receiving-side capacitors204and406in parallel. Further, when the temperature detected by the temperature detection circuit134is higher than −20° C., the control circuit404turns the MOS-FET322off and detaches the power-receiving-side capacitor406from the power receiving device2.

When the MOS-FET322is turned on, the capacitance of the power-receiving-side capacitor406is added to the capacitance of the power-receiving-side capacitor204in the power-receiving coil unit20, and the impedance changes. Accordingly, for example, the resonance frequency of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20is decreased by Δfolfrom the resonance frequency fonwhen the first power transmission is performed, the frequency being indicated inFIG. 8, and becomes fol. In other words, the circuit including the power-transmitting coil unit18and the power-receiving coil unit20becomes either a resonance circuit with a resonance frequency fonor a resonance circuit with a resonance frequency foldepending on the on/off state of the MOS-FET322.

Thus, the control circuit404can cause the power-transmitting coil unit18to execute the first power transmission by turning the MOS-FET322off and can cause the power-transmitting coil unit18to execute the second power transmission by turning the MOS-FET322on.

Further, the control circuit102may transmit a power transmission mode instruction signal in place of a detected-temperature signal to the control circuit404through the transmission circuit400and the reception circuit402. The power transmission mode instruction signal is a signal giving an instruction to execute either of the first power transmission and the second power transmission. In this case, the control circuit102(i) determines, from the temperature of the electrolytic capacitor132detected by the temperature detection circuit134, which of an instruction to execute the first power transmission and an instruction to execute the second power transmission should be given and (ii) supplies a power transmission mode instruction signal indicating the determination result to the transmission circuit400.

The control circuit404turns the MOS-FET322off when a power transmission mode instruction signal gives an instruction to execute the first power transmission and turns the MOS-FET322on when a power transmission mode instruction signal gives an instruction to execute the second power transmission. Thus, a determination entity determining whether to execute the first power transmission or the second power transmission may be the control circuit102included in the power transmission device10or the control circuit404included in the power receiving device2.

Further, the impedance adjustment circuit may be placed anywhere in the power receiving device2. The impedance adjustment circuit may be placed in the power-receiving coil unit20, may be placed in the rectifier circuit24, or may be placed at another position.

An example of changing the impedance of the power-receiving-side circuit by use of a circuit connecting the power-receiving-side capacitor406to the MOS-FET322in series has been described in Embodiment 5. The technique of changing the impedance of the power-receiving-side circuit is not limited to the example. For example, the impedance adjustment circuit including the power-receiving-side capacitor406and the MOS-FET322as illustrated inFIG. 12may be replaced by an impedance adjustment circuit configured with a variable capacitance power-receiving-side capacitor412a capacitance value of which changes in accordance with control by the control circuit404, as illustrated inFIG. 13.

The control circuit404switches the capacitance of the power-receiving-side capacitor412between capacitance for the first power transmission and capacitance for the second power transmission, based on a detected-temperature signal or a power transmission mode instruction signal received from the reception circuit402. The configuration can change power-receiving-side reactance similarly to Embodiment 5.

Further, the resonance frequency may be changed by adjusting the impedance by a technique such as replacing the power-receiving-side capacitor204with a variable capacitor and changing the capacitance of the variable capacitor.

Further, the resonance frequency may be changed by adjusting the impedance by changing the inductance of the power-receiving coil200by adjusting a relative position between the power-receiving coil200and the magnetic plate202.

Further, similarly to Embodiment 4 illustrated inFIG. 11, the impedance of the power receiving device2may be adjusted by switching between a plurality of capacitors having different capacitance values to be connected in the power receiving device2.

Further, the impedance adjustment circuit may be placed anywhere in the power receiving device2. The impedance adjustment circuit may be placed in the power-receiving coil unit20, may be placed in the rectifier circuit24, or may be placed at another position.

Examples of switching between the first power transmission and the second power transmission to be executed by adjusting the impedance of a circuit including a power-transmitting coil unit and a power-receiving coil unit by electric control have been described in Embodiments 1 to 6. The present disclosure is not limited to the above, and the second power transmission may be executed by reducing power supply from a power-transmitting coil unit to a power-receiving coil unit by weakening magnetic coupling between the power-transmitting coil unit and the power-receiving coil unit.

A technique of switching between the first power transmission and the second power transmission by adjusting a coupling coefficient k indicating a degree of magnetic coupling between a power-transmitting coil unit18and a power-receiving coil unit20will be described below in the present embodiment.FIG. 14is a diagram illustrating a configuration of a power transmission system1capable of changing a relative positional relation between the power-transmitting coil unit18and the power-receiving coil unit20. In the power transmission system1illustrated inFIG. 14, a power supply circuit12and the power-transmitting coil unit18are placed on a moving device360.

The moving device360is configured with an electric jack or the like and is installed on a floor surface362or the like. The moving device360moves positions of the power supply circuit12and the power-transmitting coil unit18in a vertical direction in accordance with control by a control circuit102and changes the distance between the power-transmitting coil unit18and the power-receiving coil unit20between a distance Dvfor the first power transmission and a distance (Dv+Δfv) for the second power transmission. When the distance between the end of a power-transmitting coil180in the power-transmitting coil unit18and the end of a power-receiving coil200in the power-receiving coil unit20in the vertical direction is increased from the distance Dvto the distance (Dv+Δfv) in a direction of magnetic flux1, the value of the coupling coefficient k between the power-transmitting coil180and the power-receiving coil200decreases. The moving device360is an example of a positional relation adjustment device in the present disclosure.

As the distance between the power-transmitting coil unit18and the power-receiving coil unit20increases, the value of the coupling coefficient k indicating strength of magnetic coupling between the power-transmitting coil unit18and the power-receiving coil unit20decreases. When the coupling coefficient k decreases, a counter electromotive force in the power-receiving coil220decreases even when the power supply circuit12transmits electric power under the same condition, and efficient power transmission cannot be performed.

Accordingly, by setting the distance between the power-transmitting coil unit18and the power-receiving coil unit20to Dv, the control circuit102can cause the power-transmitting coil unit18to execute the first power transmission. Further, by increasing the distance between the power-transmitting coil unit18and the power-receiving coil unit20to (Dv+Mv), the control circuit102can cause the power-transmitting coil unit18to execute the second power transmission.

Note that control for switching between the first power transmission and the second power transmission by the control circuit102is similar to the control described above with reference toFIG. 5. Further, the power-transmitting coil unit18does not necessarily need to be moved together with the power supply circuit12, and the moving device360may move only the power-transmitting coil unit18as long as the position of the power-transmitting coil unit18can be changed independently of other components in the power transmission system1.

Further, the configuration itself for moving the power-transmitting coil unit18may be any configuration without being limited to the electric jack.

A configuration for changing a coupling coefficient k between a power-transmitting coil unit18and a power-receiving coil unit20is not limited to the configuration described in Embodiment 7. A technique of changing the coupling coefficient k by moving positions of a power supply circuit12and the power-transmitting coil unit18in a direction perpendicular to magnetic flux Φ will be described in the present embodiment. As illustrated inFIG. 15, a power-transmitting coil unit18and a power supply circuit12according to the present embodiment are placed on a moving device370.

The moving device370is configured with a horizontally moving table, is installed on a floor surface362or the like, and is slidable in a horizontal direction parallel with the surface of the page. The moving device370moves the positions of the power supply circuit12and the power-transmitting coil unit18in a direction perpendicular to the magnetic flux Φ by 0 to a maximum of Mhin accordance with control by a control circuit102. Further, in the present embodiment, the power-transmitting coil unit18and the power-receiving coil unit20are in a state of directly facing each other when an amount of movement is 0, and the distance in a vertical direction between the power-transmitting coil unit18and the power-receiving coil unit20is fixed to Dv.

When a distance M between the center of a power-transmitting coil180in the power-transmitting coil unit18and the center of a power-receiving coil200in the power-receiving coil unit20is 0, a coupling coefficient k between the power-transmitting coil180and the power-receiving coil200is maximized. On the other hand, as the distance M between the center of the power-transmitting coil180and the center of the power-receiving coil200increases, the value of the coupling coefficient k decreases. As described in Embodiment 7, as the coupling coefficient k decreases, a counter electromotive force in the power-receiving coil unit20decreases, and efficient power transmission cannot be performed.

By setting the distance M between the center of the power-transmitting coil180in the power-transmitting coil unit18and the center of the power-receiving coil200in the power-receiving coil unit20to0, the control circuit102maximizes the coupling coefficient k between the power-transmitting coil180and the power-receiving coil200and causes the power-transmitting coil unit18to execute the first power transmission at a frequency fd. On the other hand, by setting the distance M to a predetermined value greater than 0, the control circuit102decreases the coupling coefficient k and causes the power-transmitting coil unit18to execute the second power transmission at the frequency fd.

Note that control for switching between the first power transmission and the second power transmission by the control circuit102according to the present embodiment is similar to the control described above with reference toFIG. 5. Further, the power-transmitting coil unit18does not need to be moved together with the power supply circuit12, and the moving device370may move only the power-transmitting coil unit18as long as the position of the power-transmitting coil unit18can be changed independently of other components in the power transmission system1.

Further, the configuration itself for horizontally moving the power-transmitting coil unit18may be any configuration without being limited to the horizontally moving table.

While techniques of changing the coupling coefficient k between the power-transmitting coil unit18and the power-receiving coil unit20by changing the distance between the power-transmitting coil unit18and the power-receiving coil unit20have been described in Embodiments 7 and 8, another technique may be employed. For example, the coupling coefficient k may be controlled by changing the parallelism between the power-transmitting coil180and the power-receiving coil220.

For example, as illustrated inFIG. 16, a tilt angle of a power-transmitting coil unit18and a power supply circuit12placed on a stage may be changed by making a supporting shaft of a moving device380rotatable around an axis by any angle θ. In this case, for example, by controlling the moving device380in such a way that the stage is horizontal during execution of the first power transmission, the control circuit102makes the central axis of a power-transmitting coil180parallel with the central axis of a power-receiving coil220and increases the coupling coefficient k. Further, by tilting the stage during execution of the second power transmission, the control circuit102makes the central axis of the power-transmitting coil180tilted relative to the central axis of the power-receiving coil220and decreases the coupling coefficient k.

Note that control for switching between the first power transmission and the second power transmission by the control circuit102according to the present embodiment is similar to the control described above with reference toFIG. 5. Further, the power-transmitting coil unit18and the power supply circuit12do not need to be moved integrally, and the moving device380may swing only the power-transmitting coil unit18as long as the position of the power-transmitting coil unit18can be changed independently of the power supply circuit12.

The technique of changing the tilt angle of the power-transmitting coil unit18may be any technique without being limited to swinging.

Examples of manipulating the power-transmitting coil unit18for making the second power transmission executable have been described in aforementioned Embodiments 7 to 9. The present disclosure is not limited to the above. For example, the second power transmission may be enabled by manipulating the power-receiving coil unit20and the rectifier circuit24.

FIG. 17illustrates a configuration allowing a power-receiving coil unit20and a rectifier circuit24to be moved in a vertical direction by the moving device360illustrated inFIG. 14. The moving device360supports the power-receiving coil unit20and the rectifier circuit24, moves the unit and the circuit in the vertical direction in accordance with control by a control circuit404, and controls a coupling coefficient k between a power-transmitting coil unit18and the power-receiving coil unit20.

As described in Embodiment 5, a control circuit102transmits a detected-temperature signal or a power transmission mode instruction signal to a control circuit404through a transmission circuit400and a reception circuit402. In accordance with information indicated by the detected-temperature signal or the power transmission mode instruction signal, the control circuit404controls the moving device360and causes the power-receiving coil unit20and the rectifier circuit24to move in the vertical direction. The control circuit102controls a power supply circuit12in such a way as to execute the second power transmission in a state of the coupling coefficient k being relatively small and execute the first power transmission in a state of the coupling coefficient k being relatively large.

Note that control for switching between the first power transmission and the second power transmission is similar to the control described above with reference toFIG. 5. Further, the power-receiving coil unit20and the rectifier circuit24do not need to be moved integrally, and the moving device360may move only the power-receiving coil unit20as long as the position of the power-receiving coil unit20can be changed independently of the rectifier circuit24.

Further, the technique of moving the power-receiving coil unit20may be any technique without limitation.

A configuration enabling movement of the power-receiving coil unit20and the rectifier circuit24in a vertical direction by the moving device360has been described in Embodiment 10. An example of enabling movement of a power-receiving coil unit20and a rectifier circuit24in a horizontal direction by the moving device370illustrated inFIG. 15will be described with reference toFIG. 18in the present embodiment.

The moving device370supports the power-receiving coil unit20and the rectifier circuit24, moves the unit and the circuit in the horizontal direction in accordance with control from a control circuit404, and controls a coupling coefficient k between a power-transmitting coil unit18and the power-receiving coil unit20. As described in Embodiment 10, the control circuit404can receive a detected-temperature signal or a power transmission mode instruction signal from a control circuit102through a transmission circuit400and a reception circuit402. In accordance with information indicated by the detected-temperature signal or the power transmission mode instruction signal, the control circuit404moves the power-receiving coil unit20and the rectifier circuit24in the horizontal direction. The control circuit102controls a power supply circuit12in such a way as to execute the second power transmission in a state of the coupling coefficient k being relatively small and execute the first power transmission in a state of the coupling coefficient k being relatively large.

Note that control for switching between the first power transmission and the second power transmission is similar to the control described above with reference toFIG. 5. Further, the power-receiving coil unit20and the rectifier circuit24do not need to be moved integrally, and the moving device370may move only the power-receiving coil unit20as long as the position of the power-receiving coil unit20can be changed independently of the rectifier circuit24.

Further, the technique of sliding the power-receiving coil unit20may be any technique without limitation.

Further, similarly to Embodiment 9, the first power transmission and the second power transmission may be executed by adjusting the coupling coefficient k between the power-transmitting coil unit18and the power-receiving coil unit20by adjusting the tilt angle of the power-receiving coil unit20.

The moving devices360,370, and380are examples of a positional relation adjustment device adjusting a relative positional relation between the power-transmitting coil180and the power-receiving coil220and have only to be capable of singly or compositely changing a position in a vertical direction, a position in a horizontal direction, and a tilt angle of either one or both of the power-transmitting coil and the power-receiving coil. Further, the control circuits102and104control the positional relation adjustment device in such a way that magnetic coupling between the power-transmitting coil and the power-receiving coil is weaker during execution of the second power transmission than during execution of the first power transmission.

Furthermore, the electric control illustrated in Embodiments 1 to 6 may be performed in combination with the mechanical control illustrated in Embodiments 7 to 11.

Modified Example

In a case of an electric vehicle8returning from outside, it is efficient to start the second power transmission at a point when the electric vehicle8approaches a parking position and start the first power transmission at a point when the electric vehicle8is parked at a predetermined position. When such a configuration is provided, for example, a detection device detecting that a garage shutter is opened or the electric vehicle8is approaching and supplying a charging start instruction signal to a control circuit102is placed. In response to receiving the charging start instruction signal, the control circuit102starts processing of causing a power supply circuit12to supply electric power at a resonance frequency fdof a circuit including a power-transmitting coil unit18and a power-receiving coil unit20to the power-transmitting coil unit18. However, the power-transmitting coil unit18is not facing the power-receiving coil unit20in this stage, and therefore the power-transmitting coil unit18is not in a resonant state; and the second power transmission is started and the temperature of a circuit part and the like are raised. Subsequently, by the electric vehicle8being parked or stopped at the predetermined position, the resonance frequency of the circuit including the power-transmitting coil unit18and the power-receiving coil unit20nearly matches the frequency fd, and an automatic changeover to the first power transmission is performed. The configuration enables efficient charging of a storage battery80in the electric vehicle8by electric power with a small number of ripples.

In the above description, power transmission efficiency is intentionally decreased during execution of the second power transmission compared with during execution of the first power transmission in such a way that AC power received by the power receiving device2in the second power transmission is less than AC power received by the power receiving device2in the first power transmission. The AC power received by the power receiving device2in the second power transmission may be zero. Making an amount of electric power transmitted from the power-transmitting coil unit18to the power-receiving coil unit20zero in the second power transmission can be achieved by, for example, changing a frequency of AC power, impedance, and a coupling coefficient in such a way that a counter electromotive force induced in the power-receiving coil200is equal to or less than the voltage of the storage battery80. At this time, the storage battery80being a load has high impedance, and current is not supplied from the power receiving device2to the storage battery80; and therefore AC power received by the power receiving device2becomes zero. On the other hand, when levels of ripples and noise contained in electric power received by the power receiving device2during execution of the second power transmission are within allowable ranges, an amount of electric power transmitted from the power-transmitting coil unit18to the power-receiving coil unit20in the second power transmission may not need to be zero.

While an example of using a MOS-FET as a switch has been described above, a switch configured with another semiconductor element may be used for either one or both of an on/off switch and a changeover switch. Further, a switch other than a semiconductor switch may be used.

While an electrolytic capacitor has been illustrated as a member requiring a temperature rise in the above description, a member requiring a temperature rise may be any electric or electronic part or any mechanical part as long as the part cannot sufficiently exhibit a function thereof when the temperature is low.