Electric machine and power supply system having battery pack

A battery pack includes a first receiving antenna including a first inductor and a first capacitor for receiving electric power from a first resonant magnetic field generated by a power supply source, at least one secondary battery charged by the electric power received by the first receiving antenna, an oscillator for producing radio-frequency power by DC power discharged from the secondary battery, and a transferring antenna including a second inductor and a second capacitor for generating a second resonant magnetic field from the radio-frequency power. A primary surface of the first inductor is parallel to a first plane of the battery pack, and a primary surface of the second inductor is parallel to a second plane of the battery pack. The second plane intersects with the first plane at an angle of a range of between 45° and 90° including 45° and 90°.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electric machine and a power supply system having a battery pack for wirelessly transferring electric power through a coupling by a resonant magnetic field.

2. Description of the Related Art

An electric machine such as an electric vehicle is driven by an electric motor. For example, an electric vehicle runs by using an electric motor as the power source, as opposed to a car whose power source is an internal-combustion engine. An electric vehicle has a power battery installed therein, and gains driving force by transferring energy stored in a power battery to the electric motor. The power battery may be, for example, a secondary battery such as a lithium-ion battery, a nickel hydrogen battery or a lead battery. In many cases, a power battery is installed in a vehicle body in the form of a battery pack in which a plurality of “modules” are packaged together with a charge-discharge control circuit, etc., wherein each module includes a plurality of “cells” (the minimal form of a battery including an electrode and an electrolyte) connected together in series. When the remaining amount of electricity of the power battery becomes low, the battery pack is charged by being connected to an external power supply so that the electric vehicle can run again. The external power supply may be the commercial power supply (100 V/200 V), charging equipment installed in charging stations, etc., capable of high-power charging, etc.

The power battery of a conventional electric vehicle is charged at home or at a charging station (see, for example, Japanese Laid-Open Patent Publication No. 11-146504) each time the remaining amount of electricity of the power battery becomes low.FIG. 14shows a power supply system for a conventional electric vehicle described in Japanese Laid-Open Patent Publication No. 11-146504. A power battery72, which is a rechargeable secondary battery, is packaged together with a charge-discharge control circuit61into a battery pack62. The battery pack62is secured inside the body of an electric vehicle70so that the battery pack62cannot be removed during normal use. The body of the electric vehicle70is provided with a vehicle-side connector64for receiving the electric power supplied from an external power supply63. The battery pack62is provided with a battery pack-side charging connector65, and a charging-side connector65and the vehicle-side connector64are connected together by a cable. The battery pack62is provided with a battery pack-side power output connector66, and the battery pack-side power output connector66is connected to a power source67in the electric vehicle70by a cable. When charging, the user connects a power supply connector69that is provided at the end of a cable68of the external power supply63to the vehicle-side connector64. The power battery72is charged by receiving the electric power from the external power supply63through the vehicle-side connector64, the charging-side connector65and the charge-discharge control circuit61.

Charging the power battery72takes some hours by normal charging, and some tens of minutes even by fast charging at higher voltages and currents. In addition to the large amount of time required for charging, there is another problem that the power battery72deteriorates when fast-charged repeatedly. In order to solve this problem, proposals have been made in which the power battery72whose battery level has become low is replaced with a fully-charged power battery (see, for example, Japanese Laid-Open Patent Publication No. 9-98518 and “2002 Report of research and study on car sharing systems using cars with replaceable battery” (March 2003, Mechanical Social Systems Foundation)).

In some fields other than electric machine such as electric vehicle, it has also been proposed to wirelessly charge a battery pack (see, for example, Japanese Laid-Open Patent Publication No. 10-248171). Japanese Laid-Open Patent Publication No. 10-248171 relates to a power supply device for use in a portable terminal device, in which electric power is transferred by an electromagnetic induction method. With this method, however, it is not possible to realize efficient transfer when there is a long distance between the power-transmitting antenna and the power-receiving antenna, or when the antennas are not well aligned with each other.

On the other hand, United States Patent Application Publication No. 2008/0278264 (FIGS. 12 and 14) discloses a new type of wireless energy transfer system for transferring energy from one of two resonators to the other, and vice versa, through the space between them. That wireless energy transfer system couples those two resonators with each other via the evanescent tail of the oscillation energy of the resonant frequency that is produced in the space surrounding those two resonators, thereby transferring the oscillation energy wirelessly (i.e., by a non-contact method).

With a conventional power supply system for an electric machine such as an electric vehicle, when an old battery pack is removed from the vehicle body to be replaced with another battery pack, it is necessary to unplug cables from the battery pack-side charging connector and the battery pack-side power output connector. When installing the new battery pack, it is necessary to plug the cables to these connecters. Moreover, it is necessary to establish an electric path by fitting together connecter terminals, and the connecter fitting operation requires a relatively large force. Moreover, as described in “2002 Report of research and study on car sharing systems using cars with replaceable battery” (March 2003, Mechanical Social Systems Foundation), it is necessary to provide measures to prevent electric shock during the operation or electric leak due to droplets such as rainwater.

On the other hand, with a technique for transmitting power to a household electric appliance by an electromagnetic induction method, it is not possible to realize efficient transfer when there is a long distance between the power-transmitting antenna and the power-receiving antenna, or when the antennas are not well aligned with each other.

SUMMARY OF THE INVENTION

The present invention has been made in order to solve the above problems, and provides an electric machine including a battery pack and a power supply system such that electric power can be transferred therebetween in a non-contact manner through a magnetic coupling between antennas, thus enabling safe and easy battery replacement. Moreover, the present invention also enables efficient electric power transfer even when there is a long distance between antennas, as compared with a conventional wireless power transfer method using electromagnetic induction.

An electric machine of the present invention includes: a driving electric motor; a battery pack for supplying electric power to the driving electric motor; and an energy transfer section for transferring an electric energy output from the battery pack to the driving electric motor, wherein: the battery pack includes: a first antenna for receiving electric power from a power supply source located outside the electric machine by coupling with a first resonant magnetic field generated by the power supply source; at least one secondary battery charged by the electric power received by the first antenna; an oscillator for producing radio-frequency power by DC power discharged from the secondary battery; and a second antenna for generating a second resonant magnetic field by the radio-frequency power; and the energy transfer section includes a third antenna that couples with the second resonant magnetic field generated by the second antenna, thereby transferring the radio-frequency power received by the third antenna to the driving electric motor.

A battery pack of the present invention includes: a first antenna for receiving electric power from a power supply source located outside by coupling with a first resonant magnetic field generated by the power supply source; at least one secondary battery charged by the electric power received by the first antenna; an oscillator for producing radio-frequency power by DC power discharged from the secondary battery; and a second antenna for generating a second resonant magnetic field by the radio-frequency power and magnetically coupling with a third antenna located outside.

A power supply system of the present invention includes: a battery pack for supplying electric power to a load; a first energy transfer section for transferring the electric power output from the battery pack to the load; and a second energy transfer section for transferring the electric power supplied from outside to the battery pack, wherein the battery pack includes: a first antenna for receiving electric power by coupling with a first resonant magnetic field generated by the second energy transfer section; at least one secondary battery charged by the electric power received by the first antenna; a first oscillator for producing first radio-frequency power by DC power discharged from the secondary battery; and a second antenna for generating a second resonant magnetic field by the first radio-frequency power; the first energy transfer section includes a third antenna that couples with the second resonant magnetic field generated by the second antenna; the radio-frequency power received by the third antenna is transferred to the load; and the second energy transfer section includes: a second oscillator for producing second radio-frequency power by using the electric power supplied from the outside; and a fourth antenna for generating the first resonant magnetic field by the second radio-frequency power.

According to an electric machine of the present invention, it is possible, without lowering the transfer efficiency, to supply electric power to a battery pack and to output electric power from a battery pack in a non-contact manner and with no contact points, thus enabling safe and easy battery replacement. Moreover, according to a power supply system of the present invention, it is possible to transfer energy efficiently to not only an electric machine but also a household electric appliance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A battery pack, an electric machine and a power supply system according to preferred embodiments of the present invention will now be described with reference to the drawings.

First, referring toFIG. 1, the first embodiment of the present invention will be described.

The present embodiment is directed to an electric vehicle, as an example of an electric machine having a battery pack of the present invention.FIG. 1(a) shows a configuration of the electric vehicle of the present embodiment, andFIG. 1(b) shows a configuration of a power supply system used in the electric vehicle. Note that the configuration of the electric vehicle and the power supply system shown inFIG. 1is merely one example of possible configurations of the present embodiment, and the configuration of the present embodiment is not limited to that shown inFIG. 1.

An electric vehicle9shown inFIG. 1runs using a driving electric motor18as the power source. The driving electric motor18serves as the power source of the electric vehicle9by receiving the electric power from a secondary battery2installed in the electric vehicle9. In a preferred example, the secondary battery2is installed in a vehicle body in the form of a battery pack1in which a plurality of secondary batteries are connected together, and is charged by an external power supply11shown inFIG. 1(b).

In the electric vehicle9, electric power is wirelessly transferred between the battery pack1and the external power supply11through a magnetic coupling between antennas by a resonant magnetic field. Moreover, electric power is also wirelessly transferred between the battery pack1and the driving electric motor18through a magnetic coupling between antennas by a resonant magnetic field.

The battery pack1includes a first antenna (battery pack-side power-receiving antenna)6, and a second antenna (battery pack-side power-transmitting antenna)7. A third antenna (vehicle-side power-receiving antenna),13is placed in the body of the electric vehicle9so as to oppose the second antenna7. The second antenna7wirelessly transfers electric power to the third antenna13. On the other hand, the first antenna6wirelessly transfers electric power to a fourth antenna (power supply-side power-transmitting antenna)14placed outside the electric vehicle9. These antennas are elements for transferring energy from one of two objects to the other by using a coupling phenomenon that has been produced by the evanescent tail of the electromagnetic field of the resonator.

The present embodiment eliminates the need for the connector plugging operation between the battery pack1and the driving electric motor18, and between the battery pack1and the external power supply11, which is necessary with conventional techniques. It is also possible to prevent the shorting of the connecter. Moreover, since electric power is wirelessly transferred through a magnetic coupling at a resonant frequency (a coupling by a resonant magnetic field), energy loss, which would otherwise be caused when an electromagnetic wave is transferred to a distant location, will not be caused. Therefore, the power can be transmitted with very high efficiency. Such an energy transmitting technique that uses the coupling phenomenon of a resonant electromagnetic field (i.e., a near field) will cause much less loss than a known non-contact power transmission that uses the Faraday's law of electromagnetic induction. Moreover, even when the distance between antennas is long, an efficient energy transfer can be achieved by such a technique. For example, in a preferred embodiment of the present invention, energy can be transmitted between two resonators (or antennas), which have an interval of as much as several meters between them. The transfer efficiency can be kept high even if the antennas are somewhat misaligned with each other.

Referring now toFIGS. 2 to 5, the first embodiment of the present invention will be described in greater detail.

In the figures referred to below, like elements to those shown inFIG. 1are denoted by like reference numerals.FIGS. 2 and 3show main elements of an electric vehicle of the present embodiment and an equivalent circuit thereof, respectively.FIGS. 4 and 5show main elements of a power supply system of the present embodiment and an equivalent circuit thereof, respectively. Note that the spatial arrangement of the various elements shown in the figures is merely illustrative, and the spatial arrangement is not limited to this.

As shown inFIG. 2, the electric vehicle9of the present embodiment includes the driving electric motor18as the power source, a drive control section16for controlling the electric power to be transmitted to the driving electric motor18, the battery pack1for supplying the electric power for driving the driving electric motor18, a first energy transfer section22for receiving the electric power from the battery pack1and transferring the electric power to the driving electric motor18, and a battery pack holding section54for securing the battery pack1to the vehicle body. The driving electric motor18may be, for example, an AC motor such as an induction motor or a permanent magnet synchronous motor, or a DC motor, or any other motor. Where an AC motor is used, the drive control section16transduces the electric power received from the battery pack1into an appropriate AC power, which is supplied to the driving electric motor18.

The battery pack1includes a vehicle attachment structure8. By means of the vehicle attachment structure8and the battery pack holding section54, the battery pack1is attached to the body of the electric vehicle9, and is held so that it can be removed. The vehicle attachment structure8and the battery pack holding section54may be placed at any position as long as the battery pack1can be held stably. The battery pack holding section54is placed under a seat of the electric vehicle9, for example, and holds the battery pack1in a stable position. The vehicle attachment structure8and the battery pack holding section54may be formed by any material, and the shape thereof is also a matter of design choice.

The battery pack1includes at least one secondary battery2. The secondary battery2may be any battery that can be charged/discharged, and may be, for example, a lithium-ion battery, a nickel hydrogen battery, a lead battery, etc. Preferably, the secondary battery2is placed in the battery pack1and includes a plurality of “modules” connected together, wherein each module includes a plurality of “cells” (the minimal form of a battery including an electrode and an electrolyte) connected together in series. When used in an electric vehicle for ordinary use, the total electric energy of the battery pack1is set to be 10 kWh or more, for example. The configuration of the secondary battery2in the battery pack1may be any configuration as long as it is possible to output enough electric power to allow the electric vehicle9to run for a long period of time.

The battery pack1includes the first antenna6for receiving radio-frequency power transmitted from outside, a battery pack-side rectifier5for transducing the electric power received by the first antenna6into DC power, a control section3for switching the secondary battery2between charging and discharging and controlling the secondary battery2so as to optimize the charging current and voltage according to the state-of-charge, a battery pack-side oscillator4for transducing the electric power from the secondary battery2into radio-frequency power, and the second antenna7for producing a magnetic field from the radio-frequency power received from the battery pack-side oscillator4.

The first energy transfer section22includes the third antenna13magnetically-coupled with the second antenna7to receive radio-frequency power, and a drive-side rectifier15for transducing the radio-frequency power received by the third antenna13into DC power and outputting the DC power to the drive control section16. The third antenna13is placed so as to oppose the second antenna7.

As shown inFIG. 3, the first antenna6is an LC resonant circuit including a first inductor111and a first condenser112connected together in series. The second antenna7is an LC resonant circuit including a second inductor113and a second condenser114connected together in parallel, and the third antenna13is an LC resonant circuit including a third inductor115and a third condenser116connected together in series. The capacitances of the condensers and the inductance values of the inductors are set so that the resonant frequency of the second antenna7and that of the third antenna13are of an equal value fa. The battery pack-side oscillator4is set so as to generate a sinusoidal voltage whose frequency is equal to the resonant frequency fa. In the present embodiment, the resonant frequency fa is set to 0.5-10 MHz, for example.

As shown inFIG. 4, a power supply system17of the present embodiment includes, in addition to the elements of the electric vehicle9, a second energy transfer section24for transferring electric power supplied from the external power supply11to the battery pack1. The second energy transfer section24includes a power supply-side oscillator12and the fourth antenna14.

As shown inFIG. 5, the fourth antenna14is an LC resonant circuit including a fourth inductor117and a fourth condenser118connected together in parallel. The capacitance of the fourth condenser118and the inductance value of the fourth inductor117are set so that the resonant frequency of the fourth antenna14is of a value fb equal to the resonant frequency of the first antenna6. The power supply-side oscillator12is set so as to generate a sinusoidal voltage whose frequency is equal to the resonant frequency fb. In the present embodiment, the resonant frequency fb is set to 0.5-10 MHz, for example.

As the battery pack-side oscillator4and the power supply-side oscillator12, a class D, E or F amplifier that would realize high efficiency and low distortion or a Doherty amplifier may be used. Optionally, a sinusoidal wave may be produced with high efficiency by arranging either a low-pass filter or a band pass filter after a switching element that generates an output signal with a distortion component.

Preferably, the external power supply11, the power supply-side oscillator12and the fourth antenna14are provided in a charging station, in a parking lot, in a house, etc. A plurality of fourth antennas14may be buried under a street along a certain extent of the street. In such a case, the electric vehicle9can charge the secondary battery2via any one of the fourth antennas14. During charging, the electric vehicle9is placed so that the first antenna6substantially opposes the fourth antenna14.

In the power supply system17, the antennas are preferably arranged so that the antennas are not covered by metal in the space between the fourth antenna14and the first antenna6, in the space between the second antenna7and the third antenna13, and near each antenna. If an antenna is covered by metal, the resonant magnetic field is blocked to thereby hinder the power transfer. The external power supply11may be a common power supply of AC 100 V or AC 200 V, or may be a power supply of a higher voltage.

The power transfer during charging in the power supply system17will now be described in detail. The power supply-side oscillator12receives the electric power from the external power supply11, and transduces the electric power into radio-frequency power whose frequency is equal to the resonant frequency fa of the first antenna6and the fourth antenna14. The radio-frequency power output from the power supply-side oscillator12is input to the fourth antenna14. The fourth antenna14and the first antenna6are coupled together by a resonant magnetic field formed between the resonant circuits. Thus, the first antenna6can efficiently receive the radio-frequency power transmitted from the fourth antenna14. The radio-frequency power received by the first antenna6is transduced by the battery pack-side rectifier5into DC power, and then input to the control section3, thereby charging the secondary battery2. The control section3performs a control so as to optimize the charging current and voltage according to the state-of-charge of the secondary battery2. For example, the control section3holds the charging current at a constant level until the voltage of the secondary battery2reaches a predetermined voltage, and then gradually decreases the charging current so that the charging voltage is constant.

The power transfer while the electric vehicle9is running will now be described in detail.

The power discharged from the secondary battery2is input to the battery pack-side oscillator4by the control section3. The battery pack-side oscillator4transduces the input discharge power into radio-frequency power whose frequency is equal to the resonant frequency fa of the second antenna7and the third antenna13. The radio-frequency power output from the battery pack-side oscillator4is input to the second antenna7. The second antenna7and the third antenna13are coupled together by a resonant magnetic field formed between the resonant circuits. Thus, the third antenna13can efficiently receive the radio-frequency power transmitted from the second antenna7. The radio-frequency power received by the third antenna13is transduced by the drive-side rectifier15into DC power, and transmitted to the drive control section16. The drive control section16appropriately transduces the received DC power, and transmits the transduced power to the drive system including the driving electric motor18.

According to the present embodiment, electric power can be wirelessly transferred between the battery pack1and the first energy transfer section22and between the battery pack1and the second energy transfer section24. Since there is no longer the need for the battery pack plugging operation using a cable and a connecter, which is necessary with conventional electric vehicles, it is possible to eliminate the need for plugging/unplugging the cable to/from the connecter when replacing the battery pack. It is also possible to avoid electric shock during the operation or electric leak due to rainwater. As a result, the battery pack1can be replaced easily and safely.

According to the present embodiment, since electric power is wirelessly transferred through a coupling by a resonant magnetic field, the transfer efficiency can be kept high, as compared with the conventional method by the electromagnetic induction, even if the distance between antennas is long (e.g., when the antenna gap is about several times the length of the short side of the antennas) or even if the antennas are misaligned with each other.

Each inductor is made of, for example, a coil. Although each inductor has a spiral structure whose number of turns is more than one and a rectangular shape in the present embodiment, they may have a structure and a shape other than these. Each inductor may have a loop structure whose number of turns is one, and may have a circular shape, an elliptical shape, or the like. These inductors do not have to be made of a single-layer conductor pattern, but may include a plurality of layered conductor patterns connected together in series.

A plane defined by the outline of a layer with the largest area of each inductor is herein referred to as the “primary surface” of the inductor. The primary surface of an inductor is herein referred to also as the “primary surface” of the antenna. For example, the primary surface of the first antenna6refers to the primary surface of the first inductor111.

Although the shape of the first antenna6and the shape of the second antenna7are shown inFIGS. 2 and 4to be equal to those of the fourth antenna14and the third antenna13, respectively, the advantages of the present invention will be obtained even when they have different shapes from each other. The first antenna6and the fourth antenna14are placed so as to oppose each other during charging, and the second antenna7and the third antenna13are placed so as to oppose each other. However, the first antenna6and the fourth antenna14do not have to be strictly opposing each other, and it is only required that they are placed so as not to be orthogonal to each other. Similarly, the second antenna7and the third antenna13do not have to be strictly opposing each other, and it is only required that they are placed so as not to be orthogonal to each other.

Although the first antenna6and the third antenna13are series resonant circuits while the second antenna7and the fourth antenna14are parallel resonant circuits in the present embodiment, the circuit configurations of the antennas are not limited to this. Each antenna may be either a series resonant circuit or a parallel resonant circuit as long as the resonant frequency of the antennas is determined appropriately. Although it is stated above that each antenna includes a condenser, a magnetic coupling by two inductors of an equal self resonant frequency may be used without using condensers.

Referring now toFIGS. 6 and 7, preferred orientations of the antennas will be described.FIG. 6shows preferred arrangements of planes parallel to the primary surfaces of the antennas of the battery pack1.FIG. 6(a) shows a configuration where a first plane121parallel to the primary surface of the first antenna6and a second plane122parallel to the primary surface of the second antenna7intersect with each other at an angle of 45° or more.FIG. 6(b) shows a configuration where the first plane121and the second plane122are orthogonal to each other.

If the first antenna6and the second antenna7of the battery pack1share an equal resonant frequency or have resonant frequencies close to each other, there may occur an unnecessary coupling between the first antenna6and the second antenna7, thus lowering the transfer efficiency. According to a research by the present inventors, the strength of coupling by a resonant magnetic field is higher as the direction of the magnetic flux generated by a power-transmitting antenna is closer to that generated by a power-receiving antenna. That is, as the arrangement between a power-transmitting antenna and a power-receiving antenna is closer to orthogonal, there is less induced current on the side of the power-receiving antenna, hence a weaker coupling.

Therefore, while the first plane121and the second plane122are most preferably orthogonal to each other, they do not have to be strictly orthogonal to each other. In the present embodiment, the angle between the first plane121and the second plane122is preferably 45° or more, more preferably 60° or more, and even more preferably 75° or more.

Referring now toFIG. 7, a preferred arrangement of antennas will be described.

Through an electromagnetic analysis, the present inventors found an antenna arrangement with which it is possible to realize a high transfer efficiency between the first antenna6and the fourth antenna14and between the second antenna7and the third antenna13.FIG. 7(a) shows the shapes and arrangement of the antennas (inductors) used in the analysis. In this analysis, the inductor of each antenna has a rectangular shape, and the long side of each inductor lies in the same direction. The primary surfaces of the second inductor113and the third inductor115are orthogonal to the primary surface of the first inductor111, and the primary surface of the fourth inductor117is parallel to the primary surface of the first inductor111. The center of the second inductor113and the center of the third inductor115, as vertically projected onto a plane that includes the primary surface of the first inductor111, are located within the area defined by the primary surface of the first inductor111.

FIG. 7(b) shows a cross-sectional view of the inductors taken along a plane indicated by a two-dot chain line inFIG. 7(a) as viewed from the direction of the arrows. InFIG. 7(b), the “size” of the first inductor111is denoted as L1. The “size” of the first inductor111as used herein refers to the length of the primary surface of the first inductor111in the direction vertical to the primary surface of the second inductor113. For example, if the first inductor111has a rectangular shape as shown inFIG. 7(a), the “size” of the first inductor111is defined to be the length of its shorter sides. If the inductor has a circular shape, the “size” is defined to be the diameter of the inductor. Ls denotes the distance between the center of the first inductor111and the middle point of the line segment extending between the center of the second inductor113and the center of the third inductor115as vertically projected onto the primary surface of the first inductor111.FIG. 7(c) shows the transfer efficiency and the degree of separation between antennas with respect to the displacement (Ls/L1) between a pair of the first antenna6and the fourth antenna14(the first antenna pair) and a pair of the second antenna7and the third antenna13(the second antenna pair). InFIG. 7(c), Path1represents the transfer efficiency between the second antenna7and the third antenna13, and Path2represents the transfer efficiency between the first antenna6and the fourth antenna14. The degree of separation represents the degree of separation between the first antenna6and the second antenna7. The conditions of this analysis are as follows.

(1) Conditions Regarding First Inductor111and Fourth Inductor117

(2) Conditions Regarding Second Inductor113and Third Inductor115

(3) Distance Between Uppermost Surface of First Inductor111and Lowermost Surface of the Second Inductor113

As shown inFIG. 7(c), a transfer efficiency of 90% or more, a preferable level in practice, is achieved for both of the first antenna pair and the second antenna pair, when the following inequality (1) is satisfied:
Ls<0.3×L1  (1)
The transfer efficiency can be further improved when the following inequality (2) is satisfied:
Ls<0.2×L1  (2)

Even if inequality 1 above is not satisfied, the transfer efficiency can be kept to be 80% or more and the advantages of the present invention can be sufficiently realized, as long as the condition of this analysis (Ls<0.5×L1) is satisfied.

FIG. 8shows a configuration of a power supply system having the preferred antenna arrangement shown inFIG. 7(b).

Note that while the resonant frequency of the first antenna pair and that of the second antenna pair are set to the same value in this analysis, it is possible to further suppress the unnecessary coupling by setting the resonant frequencies to different values.

The above shows that the second antenna pair is preferably positioned as close to the center of the first antenna pair as possible. It is preferred that the antennas in the battery pack1are arranged so as to satisfy inequality 1 or 2 above.

The method of wireless power transfer of the electric vehicle9of the present embodiment can be widely applicable to electric machines other than electric vehicles. For example, the method is applicable to hybrid electric vehicles that drive the axle by using a combination of the driving electric motor18and an internal-combustion engine, buses, trains, elevators, etc.

Referring now toFIG. 9, a second embodiment of the present invention will be described.

The electric vehicle9shown inFIG. 9includes a seat134and a plurality of wheels so that a passenger132can be seated as shown in the figure. The battery pack holding section54holds the battery pack1so that the primary surface of the first antenna6is placed parallel to the ground, and the primary surface of the second antenna7is placed vertical to the ground. The primary surface of the fourth antenna14is placed parallel to the primary surface of the first antenna6, and the primary surface of the third antenna13is placed parallel to the primary surface of the second antenna7. Note that only one wheel is illustrated inFIG. 9, but the electric vehicle9of the present embodiment has four wheels. These wheels are driven by the driving electric motor18.

The battery pack1is placed so that the distance from the seat134to the second antenna7is longer than the distance from the seat134to the center of the battery pack1. The center of the battery pack1as used herein refers to the spatial center thereof rather than the center of gravity thereof. Moreover, the primary surface of the first antenna6has a shape that is shorter in the vehicle traveling direction and longer in the vehicle lateral direction, and the primary surface of the second antenna7has a shape that is shorter in the vehicle vertical direction and longer in the vehicle lateral direction. The first antenna6and the second antenna7are arranged so that their longitudinal directions coincide with each other. Except for the above, the present embodiment is similar in configuration to the first embodiment. Note that “traveling direction” means the direction the electric vehicle9goes to by revolving the wheels.

Where the fourth antenna14is buried under a parking lot or a street, the fourth antenna14and the first antenna6of the electric vehicle9of the present embodiment can easily be magnetically coupled together since the primary surface of the first antenna6is parallel to the ground. Even if the first antenna6is misaligned with the fourth antenna14in the vehicle lateral direction, it is still possible to keep a large overlap between the primary surface of the first antenna6and the primary surface of the fourth antenna14. Therefore, with the configuration of the present embodiment, it is possible to suppress a decrease in the transfer efficiency. Moreover, although the magnetic field generated by the second antenna7will be present at the position of the passenger132, the influence of the magnetic field on the passenger132can be kept low since the second antenna7is spaced away from the seat134.

As described above, in the present embodiment, it is possible to realize charging with even higher efficiency, while keeping low the influence of the magnetic field on the passenger, realizing a higher level of safety.

Note that in the present embodiment, the direction of the magnetic field can be deflected by providing a material with a high magnetic permeability between the passenger132and the second antenna7. Then, it is possible to further lower the influence of the magnetic field on the passenger132.

The present embodiment relates to an electric vehicle having four wheels, but the number of wheels in an electric machine of the present invention is not always four.

Referring now toFIGS. 10 and 11, a third embodiment of the present invention will be described.

FIG. 10shows a basic configuration of an electric vehicle of the present embodiment.FIG. 11shows an equivalent circuit diagram of the electric vehicle of the present embodiment. A main difference between the present embodiment and Embodiment 1 is that the electric vehicle of the present embodiment includes a second secondary battery52that is different from the secondary battery2in the battery pack1.

As shown inFIG. 10, the electric vehicle includes the second secondary battery52which is a driving battery that can be charged/discharged, and a charge-discharge control section51which is a circuit for controlling the charging/discharging of the second secondary battery52. The capacity of the secondary battery2is set so that the battery pack1is light enough to be carried around by a person, and a carry handle53is provided.

With the electric vehicle of the present embodiment, the charge-discharge control section51outputs the electric power of the second secondary battery52to the drive control section16. The drive control section16performs a control such that the electric power from the charge-discharge control section51is used preferentially, while the electric power from the battery pack1is used when the battery level of the second secondary battery52has become low. When the battery level of the second secondary battery52and that of the secondary battery2in the battery pack1both become low, the passenger can hold the handle53of the battery pack1and remove the exhausted battery pack1from the electric vehicle to replace it with a fully-charged battery pack1. After the replacement, the driving electric motor18obtains electric power from the fully-charged battery pack1.

In the present embodiment, since the battery pack1is light-weighted and can easily be carried around, the battery pack1can be easily and safely replaced when the battery level of the secondary battery2becomes low. Therefore, no waiting time for charging is required, and the vehicle can resume running in a relatively short period of time. Note that in the present embodiment, the fourth antenna14does not need to be buried underground in a parking lot or a street, and may be in the form of an independent charging pad capable of charging the battery pack1placed thereon.

Referring now toFIG. 12, a fourth embodiment of the present invention will be described.

FIG. 12shows main elements of a power supply system for an automatic guided robot, as an example of the power supply system of the present invention. The basic configuration of the present embodiment is similar to that of the electric vehicle of Embodiment 1.

An automatic guided robot146shown in the figure may be, for example, a carrier robot used in a production line of a factory, and includes the driving electric motor18which is the power source, and the battery pack1. The battery pack1includes the secondary battery2for supplying electric power to the driving electric motor18, the battery pack-side oscillator4for transducing the electric power from the secondary battery2into radio-frequency power, and the first antenna6and the second antenna7for wirelessly transferring the electric power by a resonant magnetic field. The automatic guided robot146further includes the third antenna13which opposes, and magnetically couples with, the second antenna7, and the electric power from the second antenna7is received by the third antenna13and transferred to the driving electric motor18.

The secondary battery2is charged from the external power supply11. The power supply-side oscillator12transduces the electric power from the external power supply11into radio-frequency power whose frequency is equal to the resonant frequency of the fourth antenna14and the first antenna6. The fourth antenna14can transmit electric power to the first antenna6by generating a resonant magnetic field from the radio-frequency power from the power supply-side oscillator12so as to magnetically couple with the first antenna6. The electric power received by the first antenna6is transferred to the secondary battery2, thus charging the secondary battery2.

Referring now toFIG. 13, a fifth embodiment of the present invention will be described.

FIG. 13shows main elements of a power supply system for a household electric appliance as an example of the power supply system of the present invention. The power transfer method of the present embodiment is similar to that of the electric vehicle of Embodiment 1.

A household electric appliance148shown in the figure may be, for example, a mobile telephone or a personal computer, i.e., any battery-operated device. The wireless power transfer can be used for charging the secondary battery2and for supplying electric power to a load150from the secondary battery2.

The power supply system for the household electric appliance148includes the electrically-operated load150, the battery pack1for supplying electric power to the load150, and a power supply-side energy transfer section152. The battery pack1includes the secondary battery2for supplying electric power to the load150, the battery pack-side oscillator4for transducing the electric power from the secondary battery2into radio-frequency power, and the first antenna6and the second antenna7for wirelessly transferring the electric power by a resonant magnetic field. The household electric appliance148further includes the third antenna13which opposes, and magnetically couples with, the second antenna7, and the electric power from the second antenna7is received by the third antenna13and transferred to the load150.

The secondary battery2is charged from the external power supply11. The external power supply11transmits electric power to the power supply-side oscillator12. The power supply-side oscillator12transduces the electric power from the external power supply11into radio-frequency power whose frequency is equal to the resonant frequency of the fourth antenna14and the first antenna6. The fourth antenna14can transmit electric power to the first antenna6by generating a resonant magnetic field from the radio-frequency power from the power supply-side oscillator12so as to magnetically couple with the first antenna6. The electric power received by the first antenna6is transferred to the secondary battery2, thus charging the secondary battery2.

The electric machine of the present invention is not limited to electric vehicles, but is applicable to electric mobilities such as electric motorcycles, electric bicycles, electric wheelchairs and electric stand-up scooters, and automatic guided robots, etc. The battery pack and the power supply system of the present invention can be used as such, not only for electric machines described above but also for various electronic devices/apparatuses that require replacement of secondary batteries.