Power transmission system

A power transmission system transmits electric energy to a secondary resonator having a secondary resonator coil through an electromagnetic field of a predetermined frequency from a resonator having a resonator coil and reduces noise using two noise cancellation resonators with one predetermined frequency and its higher harmonic component set as noise reduction target frequency. The system includes, as two noise cancellation resonators, a first noise cancellation resonator including a first noise cancellation resonator coil and has a resonance frequency higher than the noise reduction target frequency by a first shift frequency determined according to a coupling degree between the resonator coil and first noise cancellation resonator coil and second noise cancellation resonator including a second noise cancellation resonator coil and has a resonance frequency lower than the noise reduction target frequency by a second shift frequency according to coupling degree between the resonator coil and second noise cancellation resonator coil.

TECHNICAL FIELD

The present invention relates to a power transmission system that wirelessly transmits/receives power using a magnetic resonance method.

BACKGROUND ART

A magnetic resonance wireless power transmission system makes a resonance frequency of a transmission-side antenna and a resonance frequency of a reception-side antenna equal to each other to thereby perform efficient energy transmission from the transmission-side antenna to reception-side antenna and has a particular feature in that it can realize a power transmission distance of several tens of centimeters to several meters.

When such a magnetic resonance wireless power transmission system is used in a power station for vehicles such as electric vehicles, the reception-side antenna is mounted on a bottom part of the vehicle, and power is supplied from the transmission-side antenna buried above the ground to the reception-side antenna. In such a power transmission form, it is difficult for the transmission-side antenna and reception-side antenna to be completely electromagnetically coupled to each other, and much noise may be radiated from the antenna to cause a temperature rise of a metal part at the vehicle bottom.

To cope with this problem, in the wireless power transmission system, it is necessary to discuss measures to reduce noise generated from the antenna.

For example, as a technology to reduce high-frequency noise, Patent Document 1 (JP 2010-87024A) discloses that a resonance circuit composed of a conductor having a loop-shaped closed path and a capacitor electrically connected to the closed path is provided near a noise source.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

In the conventional technology described in Patent Document 1, a resonance frequency of an LC resonator for noise cancellation is matched with a frequency of noise to be removed so as to increase a noise reduction effect.

However, particularly in a wireless power transmission system that uses a magnetic resonance antenna, even when a frequency of the noise cancellation resonator is matched with the noise source frequency, a high noise reduction effect cannot always be obtained.

Means for Solving the Problems

To solve the above problem, according to the present invention, there is provided a power transmission system that transmits electric energy to a secondary resonator having a secondary resonator coil through an electromagnetic field of a predetermined frequency generated from a primary resonator having a primary resonator coil and that reduces noise using two noise cancellation resonators with one of the predetermined frequency and its higher harmonic component set as a noise reduction target frequency. The power transmission system includes, as the two noise cancellation resonators, a first noise cancellation resonator that includes a first noise cancellation resonator coil and has a resonance frequency higher than the noise reduction target frequency by a first shift frequency determined according to a coupling degree between the primary resonator coil and the first noise cancellation resonator coil and a second noise cancellation resonator that includes a second noise cancellation resonator coil and has a resonance frequency lower than the noise reduction target frequency by a second shift frequency determined according to a coupling degree between the primary resonator coil and the second noise cancellation resonator coil.

In the power transmission system according to the present invention, the primary resonator coil includes a main coil formed by winding a conductive wire about a first reference axis parallel to the ground, and the first noise cancellation resonator coil is formed by winding a conductive wire, outside a space formed by an extended surface of a winding end face of the main coil, about a second reference axis parallel to the first reference axis.

Further, in the power transmission system according to the present invention, the primary resonator coil includes a main coil formed by winding a conductive wire about a first reference axis parallel to the ground, and the second noise cancellation resonator coil is formed by winding a conductive wire, inside a space formed by an extended surface of a winding end face of the main coil, about a third reference axis parallel to the first reference axis.

Further, in the power transmission system according to the present invention, the first shift frequency is represented by the following expression.

fs⁢⁢2=12⁢π⁢Lm⁢⁢2⁢Cn⁢⁢2[Numeral⁢⁢8]
where a mutual inductance component between the primary resonator coil and the first noise cancellation resonator coil is Lm1, and a capacitance component of the first noise cancellation resonator coil is Cn1.

Further, in the power transmission system according to the present invention,

the second shift frequency is represented by the following expression.

fs⁢⁢2=12⁢π⁢Lm⁢⁢2⁢Cn⁢⁢2[Numeral⁢⁢14]
where a mutual inductance component between the primary resonator coil and the second noise cancellation resonator coil is Lm2, and a capacitance component of the second noise cancellation resonator coil is Cn2.

Further, in the power transmission system according to the present invention, a Q-value of the noise cancellation resonator coil is 50 or more.

Advantages of the Invention

The power transmission system according to the present invention includes the noise cancellation resonator having the noise cancellation resonator coil and has a resonance frequency higher than the predetermined frequency by a shift frequency determined according to a coupling degree between the primary resonator coil and the noise cancellation resonator coil. Thus, according to the power transmission system of the present invention, it is possible to suppress occurrence of noise particularly in the wireless power transmission system using a magnetic resonance antenna (resonator) and thereby to reduce noise leakage.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.FIG. 1is a block diagram of a power transmission system according to an embodiment of the present invention. In the present embodiment, a primary resonator150and a secondary resonator250are used respectively as transmission- and reception-side antennas constituting the power transmission system.

For example, as a power transmission system using an antenna according to the present invention, a system for charging the battery of vehicles such as electric vehicles (EV) and hybrid electric vehicles (HEV) is assumed. The power transmission system transmits power to the vehicle of a type described above in a non-contact manner and is thus provided in a parking space of the vehicle. A user of the vehicle stops his or her own vehicle in the parking space where the power transmission system is provided and makes the secondary resonator250mounted on the vehicle and primary resonator150face each other to thereby receive power from the power transmission system.

In the power transmission system, a resonance frequency of the primary resonator150on a power transmission system100side and a resonance frequency of the secondary resonator250on a power reception system200side are made equal to each other for efficient power transmission from the primary resonator150to secondary resonator250so as to allow efficient energy transmission from a transmission-side antenna to a reception-side antenna.

An AC/DC conversion section101in the power transmission system100is a converter that converts an input commercial power source into a constant DC power. An output from the AC/DC conversion section101may be increased to a predetermined voltage in a voltage controller102. A value of the voltage generated in the voltage controller102can be set under the control of a main controller110.

An inverter section103generates a predetermined AC voltage from the voltage supplied from the voltage controller102and inputs the AC voltage to a matching device104.FIG. 2is a view illustrating the inverter section of the power transmission system. As illustrated inFIG. 2, the inverter section103is composed of four field-effect transistors (FETs) QAto QDconnected in a full-bridge configuration.

In the present embodiment, the matching device104is connected between a connection portion T1between the switching elements QAand QBconnected in series and a connection portion T2between the switching elements QCand QDconnected in series. When both the switching elements QAand QDare ON, both the switching elements QBand QCare OFF, and when both the switching elements QBand QCare ON, both the switching elements QAand QDare OFF, whereby an AC voltage of a rectangular wave is generated between the connection portions T1and T2. In the present embodiment, a frequency of the rectangular wave generated by switching the respective switching elements ranges from about 20 kHz to about several 100 kHz.

A drive signal to the switching elements QAto QDconstituting the inverter section103is input from the main controller110. Further, a frequency for driving the inverter section103is controlled by the main controller110.

The matching device104includes a passive element having a predetermined circuit constant and receives an output from the inverter section103. An output of the matching device104is supplied to the primary resonator150. The circuit constant of the passive element constituting the matching device104can be adjusted based on an instruction from the main controller110. The main controller110issues an instruction to the matching device104so as to make the primary resonator150and secondary resonator250to resonate with each other. The matching device104is not essential.

The primary resonator150includes a primary resonator coil160having an inductive reactance component and a primary resonator capacitor170having a capacitive reactance component. The primary resonator150resonates with the vehicle-mounted secondary resonator250disposed opposite thereto to make it possible to supply electric energy output from the primary resonator150to the secondary resonator250. The primary resonator150and secondary resonator250function as a magnetic resonance antenna section in the power transmission system100.

The main controller110of the power transmission system100is a general-purpose information processing section including a CPU, a ROM that stores a program operating on the CPU, and a RAM serving as a work area of the CPU. The main controller110collaborates with the illustrated components connected thereto.

A communication section120performs wireless communication with a communication section220on the vehicle side to exchange data with the vehicle side. The data received by the communication section120is transferred to the main controller110; conversely, the main controller110can transmit predetermined information to the vehicle side through the communication section120.

The following describes components provided on the vehicle side. In a power reception side system of the vehicle, the secondary resonator250resonates with the primary resonator150to thereby receive electric energy output from the primary resonator150. The secondary resonator250is mounted to a bottom surface portion of the vehicle.

The secondary resonator250includes a secondary resonator coil260having an inductive reactance component and a secondary resonance capacitor270having a capacitive reactance component.

The AC power received by the secondary resonator250is rectified in a rectifying section202, and the rectified power is transmitted to a battery204through a charge controller203and then stored in the battery204. The charge controller203controls the charging of the battery204based on an instruction from a main controller210. More specifically, an output from the rectifying section202is increased or reduced to a predetermined voltage value in the charge controller203, so as to be stored in the battery204. Further, the charge controller203is configured to manage a residual amount of the battery204.

The main controller210is a general-purpose information processing section including a CPU, a ROM that stores a program operating on the CPU, and a RAM serving as a work area of the CPU. The main controller210collaborates with the illustrated components connected thereto.

An interface section215is provided at a driver's seat portion of the vehicle and is configured to provide predetermined information to the user (driver) or receives operation/input from the user. The interface section215includes a display device, buttons, a touch panel, and a speaker. When a predetermined operation is performed by the user, data corresponding to the operation is transmitted as operation data from the interface section215to the main controller210and processed therein. When predetermined information is provided to the user, display instruction data for displaying the predetermined information is transmitted from the main controller210to interface section215.

A communication section220on the vehicle side performs wireless communication with the communication section120on the power transmission side to exchange data with the system on the power transmission side. The data received by the communication section220is transferred to the main controller210; conversely, the main controller210can transmit predetermined information to the power transmission system side through the communication section220.

A user who needs to receive power by using the above power transmission system stops his or her own vehicle in a parking space where the power transmission side system as described above is provided and inputs an instruction to perform charging through the interface section215. Upon reception of the instruction, the main controller210acquires a residual amount of the battery204from the charge controller203and calculates an amount of power required to charge the battery204. The calculated power amount and information requesting power transmission are transmitted from the communication section220on the vehicle side to the communication section120of the power transmission side system. Upon reception of the calculated power amount and information requesting power transmission, the main controller110of the power transmission side system controls the voltage controller102, inverter section103, and matching device104to thereby transmit power to the vehicle side.

The following describes the circuit constant (inductance component, capacitance component) of each of the thus configured primary and secondary resonators150and250.FIG. 3is a view illustrating an equivalent circuit of the power transmission system100according to the embodiment of the present invention.

In the power transmission system100according to the present invention, the circuit constant (inductance component, capacitance component) of the primary resonator150and that of the secondary resonator250are consciously made to differ from each other so as to improve transmission efficiency.

In the equivalent circuit illustrated inFIG. 3, an inductance component, a capacitance component, and a resistance component of the primary resonator150are L1, C1, and Rt1, respectively, those of the secondary resonator250are L2, C2, and Rt2, respectively, and a mutual inductance between the primary resonator150and the secondary resonator250is M. The resistance components Rt1and Rt2are each an internal resistance of a conductive wire and are not provided purposefully. Further, R denotes an internal resistance of the battery204. A coupling coefficient between the primary resonator150and the secondary resonator250is denoted by K.

Further, in the present embodiment, the primary resonator150constitutes a series resonator having the inductance component L1and capacitance component C1, and the secondary resonator250constitutes a series resonator having the inductance component L2and capacitance component C2.

First, in magnetic resonance power transmission, a resonance frequency of the primary resonator150on the power transmission system100side and that of the secondary resonator250on the power reception system200side are made equal to each other for efficient power transmission from the primary resonator150to secondary resonator250so as to allow efficient energy transmission from a transmission-side antenna (primary resonator150) to a reception-side antenna (secondary resonator250). The condition for this can be represented by the following expression (1).

When the above expression (1) is represented only by a relationship among the inductance component L1, capacitance component CDinductance component L2, and capacitance component C2, it can be simplified as the following expression (2).
[Numeral 1]
L1C2=L2C2(2)

An impedance of the primary resonator150can be represented by the following expression (3), and an impedance of the secondary resonator250can be represented by the following expression (4). In the present specification, values defined by the following expressions (3) and (4) are defined as the impedances of the respective resonators.

In the power reception side system of the magnetic resonance power transmission system100, when the battery204shifts to a constant voltage charging mode, an input impedance is varied by charging power since the voltage of the battery204is constant. When the charging power is large, the input impedance is low, while when the charging power is small, the input impedance is high. In terms of efficiency, it is desirable that the impedance of the secondary resonator250on the power reception side be set to a value close to the input impedance according to the charging power for the battery204.

On the other hand, the higher the input impedance to the primary resonator150as viewed from a power source on the power transmission side, the better in terms of efficiency. This is because a loss occurs in proportion to the square of current due to a power source internal resistance.

Thus, it is desirable that the impedance of the primary resonator150represented by the expression (3) and the impedance of the secondary resonator250represented by the expression (4) have a relationship of the following expression (5).

When the above expression (5) is represented only by a relationship among the inductance component L1, capacitance component C1, inductance component L2, and capacitance component C2, it can be simplified as the following expression (6).

In the power transmission system100according to the present invention, the circuit constant of the primary resonator150and the circuit constant of the secondary resonator250satisfy the above expressions (2) and (6). Thus, when the battery204is charged on the power reception side system, efficient power transmission can be achieved.

Here, an internal impedance of the battery204is taken into consideration. In the power reception side system, when the impedance of the secondary resonator250and the impedance of the battery204are matched with each other, the battery204can be charged efficiently.

That is, in the present embodiment, in addition to the conditions of the expressions (2) and (6), the impedance of the secondary resonator250represented by the expression (4) and an impedance R of the battery204are allowed to have a relationship of the following expression (7).

Thus, when the battery204is charged in the power reception side system, efficient power transmission can be achieved in the entire system.

The following describes noise leakage countermeasures taken in the thus configured power transmission system100.

As described above, in the power transmission system100according to the present embodiment is used for charging a battery mounted on an electric vehicle (EV) or a hybrid electric vehicle (HEV).FIG. 4is a view for explaining an installation form of the primary resonator150and secondary resonator250in the power transmission system according to the embodiment of the present invention.

As illustrated inFIG. 4, the primary resonator coil160and primary resonator capacitor170constituting the primary resonator150are housed in a primary resonator case140installed on the ground. On the other hand, the secondary resonator coil260and secondary resonance capacitor270constituting the secondary resonator250are housed in a secondary resonator case240attached to the vehicle bottom surface portion.

When the power transmission system100performs power transmission in the situation described above, an area where intensity of an electromagnetic field is high occurs in a peripheral portion where the primary resonator150and secondary resonator250do not face each other, resulting in leakage of noise.

Such an electromagnetic field leaking from the resonator during power transmission may enter a metal part of the vehicle to heat it or may leak from between the vehicle bottom surface and the ground to affect environment and human bodies.

To cope with this, in the power transmission system100according to the present invention, two noise cancellation resonators (a first noise cancellation resonator300and a second noise cancellation resonator340) are disposed in the vicinity of the primary resonator150serving as the power transmission side antenna. This can reduce the leakage described above, thus making it possible to suppress environment and human bodies from being affected by the leakage of the electromagnetic field.

FIG. 5is a view for explaining a layout of the primary resonator coil160and secondary resonator coil260.

FIG. 5is a view illustrating the primary resonator case140housing the primary resonator150and the secondary resonator case240housing the secondary resonator250in a state where they are extracted. InFIG. 5, only the primary resonator coil160of the primary resonator150is illustrated, and the primary resonator capacitor170is omitted. Similarly, only the secondary resonator coil260of the secondary resonator250is illustrated, and the secondary resonance capacitor270is omitted.

In the present embodiment, the primary resonator coil160is constituted of a ferrite substrate161and a coil winding162wound around the ferrite substrate161, and the secondary resonator coil260is constituted of a ferrite substrate261and a coil winding262wound around the ferrite substrate261. The primary resonator coil160is referred to also as a main coil. The primary resonator coil160(main coil) is defined as a coil formed by winding a conductive wire about a first reference axis parallel to the ground.

The first noise cancellation resonator300used in the present invention is formed by series connection of a first noise cancellation resonator coil310having an inductance component Ln1and a first noise cancellation resonator capacitor320having an inductance component Cn1. InFIG. 5, only the first noise cancellation resonator coil310is illustrated, and the first noise cancellation resonator capacitor320is omitted. The first noise cancellation resonator coil310is constituted of a ferrite substrate311and a coil winding312wound around the ferrite substrate311.

In the present embodiment, the first noise cancellation resonator coil310is formed by winding a conductive wire, outside a space formed by an extended surface of a winding end face of the primary resonator coil160(main coil), about a second reference axis parallel to the first reference axis.

The second noise cancellation resonator340used in the present invention is formed by series connection of a second noise cancellation resonator coil350having an inductance component Ln2and a second noise cancellation resonator capacitor360having an inductance component Cn2. InFIG. 5, only the second noise cancellation resonator coil350is illustrated, and the second noise cancellation resonator capacitor360is omitted. The second noise cancellation resonator coil350is constituted of a ferrite substrate351and a coil winding352wound around the ferrite substrate351.

In the present embodiment, the second noise cancellation resonator coil350is formed by winding a conductive wire, inside a space formed by the extended surface of the winding end face of the primary resonator coil160, about a third reference axis parallel to the first reference axis.

The first and second noise cancellation resonators300and340will be described more in detail. The first and second noise cancellation resonators300and340used in the present invention are suitable for noise countermeasures taken particularly in the wireless power transmission system using a magnetic resonance antenna; however, they can improve noise reduction efficiency with respect to various noise sources as well as the noise source of the above power transmission system.

FIG. 6is a view for explaining coupling between the first and second noise cancellation resonators300and340according to the embodiment of the present invention and the primary resonator150as the noise source.

InFIG. 6, the primary resonator150is used as a power transmission antenna of the power transmission system and generates an electromagnetic field of a predetermined fundamental frequency to transmit power to the secondary resonator250for power reception (not illustrated inFIG. 6) using the magnetic resonance method.

The primary resonator150is formed by series connection of the primary resonator coil160having the inductance component L1and the primary resonator capacitor170having the inductance component C1.

On the other hand, the first noise cancellation resonator300is formed by series connection of the first noise cancellation resonator coil310having the inductance component Ln1and the first noise cancellation resonator capacitor320having the inductance component Cn1and does not contribute to the power transmission to the secondary resonator250but removes the electromagnetic field (noise) leaking from the primary resonator150.

Further, the second noise cancellation resonator340is formed by series connection of the second noise cancellation resonator coil350having the inductance component Ln2and the second noise cancellation resonator capacitor360having the inductance component Cn2and does not contribute to the power transmission to the secondary resonator250but removes the electromagnetic field (noise) leaking from the primary resonator150.

InFIG. 6, Lm1denotes a mutual inductance between the primary resonator coil160and the first noise cancellation resonator coil310.

Further, Lm2denotes a mutual inductance between the primary resonator coil160and the second noise cancellation resonator capacitor360.

Actually, the first and second noise cancellation resonators300and340each have a closed structure at a terminal part (2); however, inFIG. 6, characteristics of the first and second noise cancellation resonators300and340will be described by recognizing the first and second noise cancellation resonators300and340as power transmission circuits of power from the primary resonator150to the first and second noise cancellation resonators300and340.

FIG. 7is a view illustrating, in an overlapping manner, a frequency dependence of transmission efficiency in a power transmission circuit formed by the primary resonator coil160and first noise cancellation resonator300and a frequency dependence of a noise emissivity. InFIG. 7, the horizontal axis represents a frequency, and S21of the vertical axis denotes power that passes through a terminal (2) when a signal is input to a terminal (1).

As illustrated inFIG. 7, in the frequency characteristics of power transmission efficiency in the power transmission circuit ofFIG. 6, two extreme values are given at two frequency points, respectively. InFIG. 7, a frequency exhibiting a lower extreme value is defined as a first extreme-value frequency fm1, and a frequency exhibiting a higher extreme value is defined as a second extreme-value frequency fe1. Further, inFIG. 7, fc1denotes a resonance frequency of the first noise cancellation resonator300.

When power transmission is performed with the primary resonator150driven at the first extreme-value frequency which is the lower extreme-value frequency, the primary resonator coil160of the primary resonator150and the first noise cancellation resonator coil310of the first noise cancellation resonator300are coupled under a magnetic wall condition.

On the other hand, when power transmission is performed with the primary resonator150driven at the second extreme-value frequency which is the higher extreme-value frequency, the primary resonator coil160of the primary resonator150and the first noise cancellation resonator coil310of the first noise cancellation resonator300are coupled under an electric wall condition.

FIG. 8is a view illustrating, in an overlapping manner, a frequency dependence of transmission efficiency in a power transmission circuit formed by the primary resonator coil160and second noise cancellation resonator340and a frequency dependence of a noise emissivity. InFIG. 8, the horizontal axis represents a frequency, and S31of the vertical axis denotes power that passes through a terminal (3) when a signal is input to a terminal (1).

As illustrated inFIG. 8, in the frequency characteristics of power transmission efficiency in the power transmission circuit ofFIG. 6, two extreme values are given at two frequency points, respectively. InFIG. 8, a frequency exhibiting a lower extreme value is defined as a first extreme-value frequency fm2, and a frequency exhibiting a higher extreme value is defined as a second extreme-value frequency fe2. Further, inFIG. 8, fc2denotes a resonance frequency of the second noise cancellation resonator340.

When power transmission is performed with the primary resonator150driven at the second extreme-value frequency which is the higher extreme-value frequency, the primary resonator coil160of the primary resonator150and the second noise cancellation resonator coil350of the second noise cancellation resonator340are coupled under an electric wall condition.

On the other hand, when power transmission is performed with the primary resonator150driven at the first extreme-value frequency which is the lower extreme-value frequency, the primary resonator coil160of the primary resonator150and the second noise cancellation resonator coil350of the second noise cancellation resonator340are coupled under a magnetic wall condition.

The second extreme-value frequency fe2inFIG. 8is equal to the first extreme-value frequency fm1, inFIG. 7.

The following describes concept of the electric wall and magnetic wall generated at a symmetry plane located between the primary resonator coil160of the primary resonator150and the first noise cancellation resonator coil310of the first noise cancellation resonator300.

FIG. 9is a view schematically illustrating the state of current and electric field at the first extreme-value frequency (frequency upon coupling under magnetic wall condition). At the first extreme-value frequency, a phase of the current flowing in the primary resonator coil160is substantially the same as that of the current flowing in the first noise cancellation resonator coil310, and vectors of magnetic fields are aligned at around a middle point of the primary resonator coil160and first noise cancellation resonator coil310. This state is regarded as generating a magnetic wall whose magnetic field is perpendicular to a symmetry plane located between the primary resonator coil160and the first noise cancellation resonator coil310.

As illustrated inFIG. 9, when the primary resonator150and first noise cancellation resonator300are coupled under the magnetic wall condition, the magnetic filed from the primary resonator coil160goes into the first noise cancellation resonator coil310.

FIG. 10is a view schematically illustrating the state of current and electric field at the second extreme-value frequency (frequency upon coupling under electric wall condition). At the second extreme-value frequency, a phase of the current flowing in the primary resonator coil160is substantially opposite to that of the current flowing in the first noise cancellation resonator coil310, and vectors of magnetic fields are aligned at around the symmetry plane located between the primary resonator coil160and the first noise cancellation resonator coil310. This state is regarded as generating an electric wall whose magnetic field is horizontal to the symmetry plane located between the primary resonator coil160and the first noise cancellation resonator coil310.

As illustrated inFIG. 10, when the primary resonator150and first noise cancellation resonator300are coupled under the electric wall condition, the magnetic filed from the primary resonator coil160and that from the first noise cancellation resonator300exclude each other at the symmetry plane.

As for the concept of the electric and magnetic walls described above, what is described in the following documents and the like is adopted herein: Takehiro Imura, Youichi Hori, “Transmission technology with electromagnetic field resonant coupling”, IEEJ Journal, Vol. 129, No. 7, 2009, and Takehiro Imura, Hiroyuki Okabe, Toshiyuki Uchida, Youichi Hori, “Research on magnetic field coupling and electric field coupling of non-contact power transmission in terms of equivalent circuits”, IEEJ Trans. IA, Vol. 130, No. 1, 2010.

Here, it can be seen from the frequency characteristics (denoted by a dashed-dotted line inFIG. 7) of the noise radiation from the primary resonator150that the minimum value appears at the first extreme-value frequency (frequency upon coupling under magnetic wall condition), and the maximum value appears at the second extreme-value frequency (frequency upon coupling under electric wall condition).

With the above characteristics, in the present invention, resonator300is set such that a frequency of the electromagnetic field (noise) radiated from the primary resonator150coincides with the first extreme-value frequency fm1(frequency upon coupling under magnetic wall condition).

More specifically, the resonance frequency fc1of the first noise cancellation resonator300is set to a resonance frequency higher than a predetermined frequency (fm1in the present embodiment, which is referred to also as a noise reduction target frequency) of the electromagnetic field generated from the primary resonator150by a shift frequency fs1determined according to a coupling degree k1(lower case) between the primary resonator coil160and the first noise cancellation resonator coil310.

The shift frequency fs1is determined according to the coupling degree k1between the primary resonator coil160and the first noise cancellation resonator coil310. That is, the shift frequency fs1is determined by the mutual inductance Lm1between the primary resonator coil160and the first noise cancellation resonator coil310, and the above shift frequency fscan be calculated by the following expression (8).

Thus, the resonance frequency fc1of the first noise cancellation resonator300should be calculated by the following expression (9).

The above setting of the resonance frequency fc1of the first noise cancellation resonator300allows the primary resonator coil160of the primary resonator150as the noise source and the first noise cancellation resonator coil310to be coupled to each other under the magnetic wall condition. Thus, as can be seen from the frequency characteristics of the noise radiation ofFIG. 7, the first noise cancellation resonator300can efficiently remove the noise radiated from the primary resonator150.

According to the first noise cancellation resonator300used in the present invention, a high noise reduction effect can be achieved in noise countermeasures taken particularly in the wireless power transmission system using a magnetic resonance antenna.

The first noise cancellation resonator300used in the present invention is passive with respect to noise. Therefore, it is desirable that the characteristics of the first noise cancellation resonator300be almost the same as the level of an opposite-phase wave of noise. Further, it is desirable that the loss in the first noise cancellation resonator300be reduced as much as possible. As a result of experiments, it was confirmed that the noise reduction effect is high when a Q-value of the first noise cancellation resonator300is 50 or more.

The frequency of the electromagnetic field generated from the primary resonator150as the noise source includes not only the fundamental wave but also noise components of higher harmonic waves of the fundamental wave. Accordingly, there is a need to remove the noise components by using the first noise cancellation resonator300.

As for the above higher harmonic waves, in a system that is likely to emit a higher harmonic wave whose frequency is an odd multiple of a frequency that is used to drive the primary resonator150, a higher harmonic wave calculated by the following expression (10) is generated from the primary resonator150. Therefore, the resonance frequency of the first noise cancellation resonator300should be determined by the following expression (11).
[Numeral 10]
f2n−1=(2n−1)fm1(10)

(n is a natural number)

In a system that is likely to emit a higher harmonic wave whose frequency is an even multiple of a frequency that is used to drive the primary resonator150, a higher harmonic wave that is calculated by the following expression (12) is generated from the primary resonator150. Therefore, the resonance frequency of the first noise cancellation resonator300should be determined by the following expression (13).
[Numeral 12]
f2n=2nfm1(12)

(n is a natural number)

Further, it can be seen from the frequency characteristics (denoted by a dashed-dotted line inFIG. 8) of the noise radiation from the primary resonator150that the maximum value appears at the first extreme-value frequency (frequency upon coupling under magnetic wall condition), and the minimum value appears at the second extreme-value frequency (frequency upon coupling under electric wall condition).

With the above characteristics, in the present invention, the resonance frequency fc2of the second noise cancellation resonator340is set such that the frequency of the electromagnetic field (noise) radiated from the primary resonator150coincides with the second extreme-value frequency fe2(frequency upon coupling under electric wall condition).

More specifically, the resonance frequency fc2of the second noise cancellation resonator340is set to a resonance frequency lower than a predetermined frequency (fe2in the present embodiment, which is equal to fm1, the fe2being referred to also as a noise reduction target frequency) of the electromagnetic field generated from the primary resonator150by a shift frequency fs2determined according to a coupling degree k2(lower case) between the primary resonator coil160and the second noise cancellation resonator coil350.

The shift frequency fs2is determined according to the coupling degree k2between the primary resonator coil160and the second noise cancellation resonator coil350. That is, the shift frequency fs2is determined by the mutual inductance Lm2between the primary resonator coil160and the second noise cancellation resonator coil350, and the above shift frequency fs2can be calculated by the following expression (14).

Thus, the resonance frequency fc2of the second noise cancellation resonator340should be calculated by the following expression (15).
[Numeral 15]
fc2=fm2−fs2(15)

The above setting of the resonance frequency fc2of the second noise cancellation resonator340allows the primary resonator coil160of the primary resonator150as the noise source and the second noise cancellation resonator coil350to be coupled to each other under the electric wall condition. Thus, as can be seen from the frequency characteristics of the noise radiation ofFIG. 8, the second noise cancellation resonator340can efficiently remove the noise radiated from the primary resonator150.

The second extreme-value frequency fe2inFIG. 8is equal to the first extreme-value frequency fm1inFIG. 7.

According to the second noise cancellation resonator340used in the present invention, a high noise reduction effect can be achieved in noise countermeasures taken particularly in the wireless power transmission system using a magnetic resonance antenna.

The second noise cancellation resonator340used in the present invention is passive with respect to noise. Therefore, it is desirable that the characteristics of the second noise cancellation resonator340be almost the same as the level of an opposite-phase wave of noise. Further, it is desirable that the loss in the second noise cancellation resonator340be reduced as much as possible. As a result of experiments, it was confirmed that the noise reduction effect is high when a Q-value of the second noise cancellation resonator340is 50 or more.

The frequency of the electromagnetic field generated from the primary resonator150as the noise source includes not only the fundamental wave but also noise components of higher harmonic waves of the fundamental wave. Accordingly, there is a need to remove the noise components by using the second noise cancellation resonator340.

As for the above higher harmonic waves, in a system that is likely to emit a higher harmonic wave whose frequency is an odd multiple of a frequency that is used to drive the primary resonator150, a higher harmonic wave calculated by the following expression (16) is generated from the primary resonator150. Therefore, the resonance frequency of the second noise cancellation resonator340should be determined by the following expression (17).
[Numeral 16]
f2n−1=(2n−1)fm2(16)

(n is a natural number)

In a system that is likely to emit a higher harmonic wave whose frequency is an even multiple of a frequency that is used to drive the primary resonator150, a higher harmonic wave that is calculated by the following expression (18) is generated from the primary resonator150. Therefore, the resonance frequency of the second noise cancellation resonator340should be determined by the following expression (19).
[Numeral 18]
f2n=2nfm2(18)

(n is a natural number)

As described above, the first noise cancellation resonator coil310of the first noise cancellation resonator300used in the present invention is coupled to the primary resonator coil160of the primary resonator150under the magnetic wall condition. In this manner, the first noise cancellation resonator coil310is aimed at obtaining the noise reduction effect. This principle will be schematically described.

FIGS. 11A and 11Bare conceptual views each explaining how the noise reduction effect is improved by the first noise cancellation resonator300according to the embodiment of the present invention.

FIG. 11Aillustrates a case where the primary resonator coil160of the primary resonator150is coupled to the first noise cancellation resonator coil310of the first noise cancellation resonator300under the magnetic wall condition. In this case, a magnetic field from the primary resonator coil160goes into the first noise cancellation resonator coil310. Based on this, a magnetic field that is generated at a point X is also expected to go into the first noise cancellation resonator coil310. As a result, as surrounded by a dotted line, the magnetic field coming from the primary resonator coil160and the magnetic field going into the first noise cancellation resonator coil310cancel each other, with the result that a noise cancellation effect occurs.

FIG. 11Billustrates a case where the primary resonator coil160of the primary resonator150is coupled to the first noise cancellation resonator coil310of the first noise cancellation resonator300under the electric wall condition. In this case, the magnetic field from the primary resonator coil160and the magnetic field from the first noise cancellation resonator coil310exclude each other. Based on this, a magnetic field is expected to occur in such a way as to go into a point X. As a result, as surrounded by a dotted line, the magnetic field coming from the primary resonator coil160and the magnetic field coming from the first noise cancellation resonator coil310boost each other, with the result that the noise will be amplified.

As described above, the second noise cancellation resonator coil350of the second noise cancellation resonator340used in the present invention is coupled to the primary resonator coil160of the primary resonator150under the electric wall condition. In this manner, the second noise cancellation resonator coil350is aimed at obtaining the noise reduction effect. This principle will be schematically described.

FIG. 12are conceptual views each explaining how the noise reduction effect is improved by the second noise cancellation resonator340according to the embodiment of the present invention.

FIG. 12Aillustrates a case where the primary resonator coil160of the primary resonator150is coupled to the second noise cancellation resonator coil350of the second noise cancellation resonator340under the magnetic wall condition. In this case, a magnetic field from the primary resonator coil160goes into the second noise cancellation resonator coil350. Based on this, a magnetic field that is generated at a point X is also expected to go into the second noise cancellation resonator coil350. As a result, as surrounded by a dotted line, the magnetic field coming from the primary resonator coil160and the magnetic field from the second noise cancellation resonator coil350boost each other, with the result that the noise will be amplified.

FIG. 12Billustrates a case where the primary resonator coil160of the primary resonator150is coupled to the second noise cancellation resonator coil350of the second noise cancellation resonator340under the electric wall condition. In this case, the magnetic field from the primary resonator coil160and the magnetic field from the second noise cancellation resonator coil350exclude each other. Based on this, a magnetic field is expected to occur in such a way as to go into a point X. As a result, as surrounded by a dotted line, the magnetic field coming from the primary resonator coil160and the magnetic field going into the second noise cancellation resonator coil350cancel each other, with the result that a noise cancellation effect occurs.

As described above, the power transmission system100according to the present invention includes the first noise cancellation resonator300and the second noise cancellation resonator340and is thus capable of suppressing occurrence of noise particularly in the wireless power transmission system using magnetic resonance antenna to thereby reduce noise leakage.

INDUSTRIAL APPLICABILITY

The power transmission system of the present invention is suitably used for a magnetic resonance wireless power transmission system to charge vehicles such as electric vehicles (EV) and hybrid electric vehicles (HEV), which have become increasingly popular in recent years. In a case where such a magnetic resonance wireless power transmission system is used for power supply to vehicles such as electric vehicles (EV) and hybrid electric vehicles (HEV), it is assumed that a resonator for power transmission is buried in the ground and that a resonator for power reception is mounted to a bottom portion of the vehicle.

In such a power transmission form, it is difficult for the transmission-side resonator and reception-side resonator to be completely electromagnetically coupled to each other, and much noise may be radiated from the resonator to cause a temperature rise of a metal part at the vehicle bottom. To cope with this problem, in the wireless power transmission system, it is necessary to discuss measures to reduce noise generated from the resonator. In the conventional technology, a resonance frequency of an LC resonator for noise cancellation is matched with a frequency of noise to be removed so as to increase a noise reduction effect. However, particularly in a wireless power transmission system that uses a magnetic resonance antenna, even when a frequency of the noise cancellation resonator is matched with the noise source frequency as a noise countermeasure, a high noise reduction effect cannot always be obtained.

The power transmission system according to the present invention includes the noise cancellation resonator having the noise cancellation resonator coil and has a resonance frequency higher than the predetermined frequency by a shift frequency determined according to a coupling degree between the primary resonator coil and the noise cancellation resonator coil. Thus, according to the power transmission system of the present invention, it is possible to suppress occurrence of noise particularly in the wireless power transmission system using a magnetic resonance antenna (resonator) and thereby to reduce noise leakage. Therefore, the industrial applicability is very high.

REFERENCE SIGNS LIST