Patent Publication Number: US-2012032521-A1

Title: Non-contact electric power feeding equipment and non-contact electric power feeding system

Description:
TECHNICAL FIELD 
     The present invention relates generally to non-contact electric power feeding equipment and non-contact electric power feeding systems, and particularly to non-contact electric power feeding equipment and non-contact electric power feeding systems having electric power feeding equipment and an electric power receiving device receiving electric power from the electric power feeding equipment, provided with resonators, respectively, caused to resonate through an electromagnetic field to feed the electric power receiving device with electric power in a non-contact manner. 
     BACKGROUND ART 
     Electric vehicles, hybrid vehicles and other electric motored vehicles are gaining large attention as ecologically friendly vehicles. These vehicles have mounted therein an electric motor generating force to drive and thus cause the vehicle to travel, and a rechargeable power storage device storing therein electric power supplied to the electric motor. Note that hybrid vehicles include a vehicle having mounted therein an electric motor and in addition an internal combustion engine together therewith as power sources, and a vehicle having mounted therein a power storage device and in addition a fuel cell together therewith as direct current power supplies for driving the vehicle. 
     A hybrid vehicle is also known that, as well as an electric vehicle, allows a power supply external to the vehicle to charge a power storage device mounted in the vehicle. For example, a plug-in hybrid vehicle is known. This vehicle allows the power storage device to be charged from a general household power supply through a charging cable connecting a receptacle of a power supply provided in premises and a charging port of the vehicle. 
     On the other hand, an electric power transfer method without using a power supply cord or an electric power transfer cable, i.e., wireless power transfer, is gaining attention in recent years. There are three wireless electric power transfer techniques known as being promising, which are power transfer through electromagnetic induction, power transfer by microwaves, and power transfer through resonance. 
     Of these three techniques, power transfer through resonance causes a pair of resonators (e.g., a pair of self resonant coils) to resonate in an electromagnetic field (a near field) to transfer electric power through the electromagnetic field in a non-contact manner, and can transfer large electric power of several kW over a relatively large distance (e.g., of several meters (see Patent Document 1 and Non Patent Document 1 for example). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: WO2007/008646 
     Non Patent Documents 
     Non Patent Document 1: Andre Kurs et al., “Wireless Power Transfer via Strongly Coupled Magnetic Resonances”, [online], Jul. 6, 2007, Science, volume 317, pp. 83-86, [searched on Sep. 12, 2007], Internet &lt;URL:http://www.sciencemag.org/cgi/reprint/317/5834/83.pdf&gt; 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention  
     When electric power is transferred, an electromagnetic field is generated with a strength, which affects the surroundings. Accordingly, an upper limit is set therefor by an EMC related standard. When resonance is utilized to feed electric power in a non-contact manner for example to feed electric power to an electric motored vehicle requiring large electric power to be fed thereto, feeding electric power using a single frequency (e.g., a resonant frequency) causes at the frequency a large peak in electromagnetic field strength and may fail to satisfy the standard. Feeding reduced electric power to satisfy the standard, however, invites feeding electric power over a long period of time and impairs the user&#39;s convenience. While reducing a peak in electromagnetic field strength without feeding significantly reduced electric power is desired, such is not particularly discussed in the above documents. 
     The present invention contemplates non-contact electric power feeding equipment and a non-contact electric power feeding system that can reduce a peak in electromagnetic field strength without feeding significantly reduced electric power. 
     Means for Solving the Problems 
     The present invention provides non-contact electric power feeding equipment including an electric power transferring resonator, a power supply device, and a control device. The electric power transferring resonator transfers electric power to an electric power receiving device in a non-contact manner by resonating, with an electric power receiving resonator of the electric power receiving device through an electromagnetic field. The power supply device is connected to the electric power transferring resonator for supplying the electric power transferring resonator with predetermined high frequency electric power. The control device sets a frequency range for a spread spectrum, as based on an S-parameter S 21  of a circuit constituted of the electric power transferring resonator and the electric power receiving resonator, and controls the power supply device to supply the electric power transferring resonator with high frequency electric power having the frequency range. 
     Preferably, the control device sets the frequency range for the spread spectrum, as based on a frequency band allowing the S-parameter S 21  to have a relatively increased amplitude characteristic. 
     Preferably, the control device alternately switches two resonant frequencies that appear in an amplitude characteristic of the S-parameter S 21  to provide the spread spectrum. 
     Furthermore, preferably, the control device sets the frequency range for the spread spectrum to be a frequency band including at least one of two resonant frequencies that appear in an amplitude characteristic of the S-parameter S 21 . 
     Furthermore, preferably, the control device sets the frequency range for the spread spectrum to be a frequency band between two resonant frequencies that appear in an amplitude characteristic of the S-parameter S 21  excluding the two resonant frequencies. 
     Furthermore, preferably, the control device sets the frequency range for the spread spectrum, as based on a Q factor calculated as based on a resonant frequency that appears in an amplitude characteristic of the S-parameter S 21 . 
     Preferably, the electric power transferring resonator includes a primary coil and a primary self resonant coil. The primary coil is connected to the power supply device. The primary self resonant coil is fed with electric power from the primary coil through electromagnetic induction and generates the electromagnetic field. 
     Furthermore, the present invention provides a non-contact electric power feeding system including: electric power feeding equipment capable of outputting predetermined high frequency electric power; and an electric power receiving device capable of receiving electric power from the electric power feeding equipment in a non-contact manner. The electric power receiving device includes an electric power receiving resonator for receiving electric power from the electric power feeding equipment in a non-contact manner through an electromagnetic field generated between the electric power feeding equipment and the electric power receiving device. The electric power feeding equipment includes an electric power transferring resonator, a power supply device, and a control device. The electric power transferring resonator transfers electric power to the electric power receiving resonator in a non-contact manner by resonating with the electric power receiving resonator through the electromagnetic field. The power supply device is connected to the electric power transferring resonator for supplying the electric power transferring resonator with predetermined high frequency electric power. The control device sets a frequency range for a spread spectrum, as based on an S-parameter S 21  of a circuit constituted of the electric power transferring resonator and the electric power receiving resonator, and controls the power supply device to supply the electric power transferring resonator with high frequency electric power having the frequency range. 
     Preferably, the electric power transferring resonator includes a primary coil and a primary self resonant coil. The primary coil is connected to the power supply device. The primary self resonant coil is fed with electric power from the primary coil through electromagnetic induction and generates the electromagnetic field. The electric power receiving resonator includes a secondary self resonant coil and a secondary coil. The secondary self resonant coil receives electric power from the primary self resonant coil by resonating with the primary self resonant coil through the electromagnetic field. The secondary coil extracts through electromagnetic induction the electric power received by the secondary self resonant coil. 
     EFFECTS OF THE INVENTION 
     Thus in the present invention a spread spectrum can be provided to have a frequency range set as based on an S-parameter S 21  of a circuit constituted of an electric power transferring resonator and an electric power receiving resonator, and the spread spectrum with that frequency range can be used to transfer electric power. Thus a frequency band with high (electric power) transfer efficiency can be used and a peak in electromagnetic field spectrum for a particular frequency can also be reduced. Thus in the present invention a peak in electromagnetic field strength can be reduced without feeding significantly reduced electric power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  generally shows in configuration a non-contact electric power feeding system according to an embodiment of the present invention. 
         FIG. 2  is an equivalent circuit diagram of a portion involved in transferring power through resonance. 
         FIG. 3  represents electromagnetic field strength observed when electric power feeding equipment feeds electric power to an electric power receiving device. 
         FIG. 4  represents an amplitude characteristic of an S-parameter S 21  of a circuit shown in  FIG. 2 . 
         FIG. 5  is a functional block diagram of a control device shown in  FIG. 1 , 
         FIG. 6  shows in configuration a hybrid vehicle indicated as one example of an electric motored vehicle having mounted therein an electric power receiving device shown in  FIG. 1 . 
         FIG. 7  is a diagram for illustrating a Q factor. 
         FIG. 8  shows a phase difference between a current passing through a primary self resonant coil and that passing through a secondary self resonant coil. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter reference will be made to the drawings to describe the present invention in embodiments. In the figures, identical or corresponding components are identically denoted and will not be described repeatedly in detail. 
       FIG. 1  generally shows in configuration a non-contact electric power feeding system according to an embodiment of the present invention. With reference to  FIG. 1 , the non-contact electric power feeding system includes electric power feeding equipment  1  and an electric power receiving device  2 . Electric power feeding equipment  1  includes a high frequency electric power supply device  10 , a primary coil  20 , a primary self resonant coil  30 , and a control device  40 . 
     High frequency electric power supply device  10  is connected to primary coil  20  and operative in response to a drive signal received from control device  40  to generate a predetermined a high frequency voltage (for example of approximately several MHz to less than 20 MHz). High frequency electric power supply device  10  is constituted for example of a sine wave inverter circuit and controlled by control device  40 . 
     Primary coil  20  is provided generally coaxially with primary self resonant coil  30  and configured to be capable of being magnetically coupled with primary self resonant coil  30  through electromagnetic induction, and receives high frequency electric power from high frequency electric power supply device  10  and feeds it to primary self resonant coil  30  through electromagnetic induction. 
     Primary self resonant coil  30  is an LC resonant coil having opposite ends open (or unconnected) and resonates with a secondary self resonant coil  60 , which will be described hereinafter, of electric power receiving device  2  through an electromagnetic field to transfer electric power to electric power receiving device  2  in a non-contact manner. Note that while C 1  denotes the stray capacitance of primary self resonant coil  30 , an actual capacitor may alternatively be provided. 
     Control device  40  generates a drive signal for controlling high frequency electric power supply device  10  and outputs the generated drive signal to high frequency electric power supply device  10  to control high frequency electric power supply device  10  to control electric power fed from primary self resonant coil  30  to the electric power receiving device  2  secondary self resonant coil  60 . 
     Note that control device  40  sets a frequency range for a spread spectrum, as based on an S-parameter S 21  of a circuit that is constituted of primary coil  20 , primary self resonant coil  30 , and the electric power receiving device  2  secondary self resonant coil  60  and a secondary coil  70 , which will be described hereinafter, and implements power transfer through resonance, and control device  40  controls high frequency electric power supply device  10  to output high frequency electric power having that frequency range. 
     More specifically, as has been described previously, when a single frequency is used to feed electric power, then, at that frequency, a large peak is caused in electromagnetic field strength, and a predetermined standard may not be satisfied. Accordingly in the present embodiment a peak in electromagnetic field strength caused in feeding electric power is reduced by employing spread spectrum techniques to transfer electric power spread over a predetermined frequency range. 
     To reduce a peak in electromagnetic field strength without feeding significantly reduced electric power, the present embodiment provides a spread spectrum having a frequency range set as based on a frequency band allowing a circuit implementing power transfer through resonance to provide an S-parameter S 21  having a relatively increased amplitude characteristic. More specifically, the S-parameter S 21  indicates a transfer factor from an input port of the circuit constituted of primary coil  20 , primary self resonant coil  30 , and the electric power receiving device  2  secondary self resonant coil  60  and secondary coil  70  (i.e., an input of primary coil  20 ) to an output port of the circuit (i.e., an output of secondary coil  70 ). Accordingly, a spread spectrum having a frequency range set as based on a frequency band allowing the S-parameter S 21  to have a relatively increased amplitude characteristic allows a peak in electromagnetic field strength to be reduced without feeding significantly reduced electric power. 
     What characteristic the S-parameter S 21  in the circuit implementing power transfer through resonance has and how control device  40  is configured in function will be described later. 
     Electric power receiving device  2  includes secondary self resonant coil  60  and secondary coil  70 . As well as primary self resonant coil  30 , secondary self resonant coil  60  is also an LC resonant coil having opposite ends open (or unconnected) and resonates with primary self resonant coil  30  of electric power feeding equipment  1  through an electromagnetic field to receive electric power from electric power feeding equipment  1  in a non-contact manner. Note that while C 2  denotes the stray capacitance of secondary self resonant coil  60 , an actual capacitor may alternatively be provided. 
     Secondary coil  70  is provided generally coaxially with secondary self resonant coil  60  and configured to be capable of being magnetically coupled with secondary self resonant coil  60  through electromagnetic induction, and secondary coil  70  extracts the electric power that is received by secondary self resonant coil  60  through electromagnetic induction, and outputs the extracted electric power to a load  3 . 
       FIG. 2  is an equivalent circuit diagram of a portion involved in transferring electric power through resonance. With reference to  FIG. 2 , power transfer through resonance allows two LC resonant coils having the same natural frequency to resonate, as two tuning forks do, in an electromagnetic field (a near field) to transfer electric power from one coil to the other coil through the electromagnetic field. 
     More specifically, high frequency electric power supply device  10  is connected to primary coil  20  and feeds high frequency electric power of approximately several MHz to less than 20 MHz to primary self resonant coil  30  magnetically coupled with primary coil  20  through electromagnetic induction. Primary self resonant coil  30  is an LC resonator provided by the coil&#39;s own inductance and stray capacitance C 1  and resonates with secondary self resonant coil  60  having the same resonant frequency as primary self resonant coil  30  through an electromagnetic field (a near field). This passes energy (electric power) from primary self resonant coil  30  to secondary self resonant coil  60  through the electromagnetic field. The energy (electric power) passed to secondary self resonant coil  60  is extracted by secondary coil  70  magnetically coupled with secondary self resonant coil  60  through electromagnetic induction and is supplied to load  3 . 
     Note that the above S-parameter S 21  corresponds to a ratio at which electric power input to a port P 1  (i.e., electric power output from high frequency electric power supply device  10 ) reaches a port P 2 , i.e., a transfer factor from port P 1  to port P 2 . Between ports P 1  and P 2  is formed the circuit constituted of primary coil  20 , primary self resonant coil  30 , secondary self resonant coil  60  and secondary coil  70 . 
       FIG. 3  represents electromagnetic field strength observed when electric power feeding equipment  1  feeds electric power to electric power receiving device  2 . With reference to  FIG. 3 , the axis of ordinates represents electromagnetic field strength in feeding electric power and the axis of abscissas represents high frequency electric power supplied from electric power feeding equipment  1  to electric power receiving device  2  in frequency. A curve k 1  represents an electromagnetic field strength provided when a single frequency f is used to feed electric power and a curve k 2  represents an electromagnetic field strength provided when a spread spectrum is used to feed electric power. 
     As shown in  FIG. 3 , when the single frequency f is used to feed electric power, then, at frequency f, a large peak rises in electromagnetic field strength, and the generated electromagnetic field strength would exceed a specification value. In contrast, when the spread spectrum is used, the peak in electromagnetic field strength is reduced, and the generated electromagnetic field strength can be reduced to the specification value or smaller. 
       FIG. 4  represents an amplitude characteristic of an S-parameter S 21  of a circuit shown in  FIG. 2 , With reference to  FIG. 4 , the axis of ordinates represents S-parameter S 21  in amplitude and the axis of abscissas represents high frequency electric power supplied from high frequency electric power supply device  10  to the circuit in frequency. 
     As shown in  FIG. 4 , the  FIG. 2  circuit implementing power transfer through resonance provides an S-parameter S 21  having an amplitude characteristic with two peaks at frequencies f 1  and f 2  and thus characterized in that it has an amplitude relatively increased over a wide frequency band. In other words, the S-parameter S 21  also increases at frequencies between frequencies f 1  and f 2 . Accordingly in the present embodiment a spread spectrum is provided to have a frequency range set to include a frequency band allowing the S-parameter S 21  to be increased in amplitude. Thus, a frequency band with high (electric power) transfer efficiency is used, while a spread spectrum reduces a peak in electromagnetic field strength. 
     Note that the spread spectrum&#39;s modulation system may be a direct sequence spread spectrum or a frequency hopping spread spectrum. The present embodiment adopts a frequency hopping spread spectrum employing resonant frequencies f 1 , f 2  in the amplitude characteristic of the S-parameter S 21 . 
       FIG. 5  is a functional block diagram of control device  40  shown in  FIG. 1 . With reference to  FIG. 5 , control device  40  includes oscillation circuits  110 ,  120 , an M-sequence random number generation circuit  130 , a selector switch  140 , a power supply control unit  150 , and a drive signal generation unit  160 . 
     Oscillation circuit  110  generates a signal having resonance frequency f 1  in the amplitude characteristic of the S-parameter  521  previously obtained. Oscillation circuit  120  generates a signal having resonance frequency f 2  in the amplitude characteristic of the S-parameter S 21 . M-sequence random number generation circuit  130  generates a random number signal constituted of 0 and 1. 
     Selector switch  140  receives the signal of frequency f 1  output from oscillation circuit  110  and the signal of frequency f 2  output from oscillation circuit  120 , and selector switch  140  operates in response to the random number signal received from M-sequence random number generation circuit  130  to select one of the signal of frequency f 1  received from oscillation circuit  110  and the signal of frequency f 2  received from oscillation circuit  120  and output the selected signal to power supply control unit  150 . 
     Power supply control unit  150  generates a control instruction for causing high frequency electric power supply device  10  (see  FIG. 1 ) to output high frequency electric power having the frequency of the signal received from selector switch  140 , and outputs the generated control instruction to drive signal generation unit  160 . Drive signal generation unit  160  operates in accordance with the control instruction received from power supply control unit  150  to generate a signal for driving high frequency electric power supply device  10 , and outputs the generated signal to high frequency electric power supply device  10 . 
     In control device  40  a signal having frequency f 1  and a signal having frequency f 2  are randomly switched by selector switch  140  and thus selectively provided to power supply control unit  150 . Power supply control unit  150  controls high frequency electric power supply device  10  to output high frequency electric power having the frequency of the received signal. High frequency electric power supply device  10  thus outputs spectrum spread high frequency electric power randomly switched in frequency between frequencies f 1  and f 2  (or hopped in frequency). 
     Note that the above control can also be implemented with an S-parameter S 21  calculated by using a technique employing a directional coupler such as a network analyzer. Furthermore, it can also be implemented with an S-parameter replaced with a Z-parameter, a Y-parameter or the like, 
       FIG. 6  shows in configuration a hybrid vehicle indicated as one example of an electric motored vehicle having mounted therein electric power receiving device  2  shown in  FIG. 1 . With reference to  FIG. 6 , a hybrid vehicle  200  includes a power storage device  210 , a system main relay SMR 1 , a boost converter  220 , inverters  230 ,  232 , motor generators  240 ,  242 , an engine  250 , a power split device  260 , and a drive wheel  270 . Furthermore, hybrid vehicle  200  also includes secondary self resonant coil  60 , secondary coil  70 , a rectifier  280 , a system main relay SMR 2 , and a vehicular ECU  290 . 
     Hybrid vehicle  200  has engine  250  and motor generator  242  mounted therein as power sources. Engine  250  and motor generators  240 ,  242  are coupled with power split device  260 , Hybrid vehicle  200  travels on driving force generated by at least one of engine  250  and motor generator  242 . Power generated by engine  250  is split by power split device  260  to two paths: one is a path transmitting power to drive wheel  270  and the other is a path transmitting power to motor generator  240 . 
     Motor generator  240  is an alternate current rotating electric machine and is for example a 3-phase alternate current synchronous electric motor having a rotor with a permanent magnet embedded therein. Motor generator  240  uses kinetic energy of engine  250  through power split device  260  to generate electric power. For example, when power storage device  210  has a state of charge (SOC) smaller than a predetermined value, engine  250  is started and motor generator  240  generates electric power to charge power storage device  210 . 
     Motor generator  242  is also an alternate current rotating electric machine and is, as well as motor generator  240 , for example a 3-phase alternate current synchronous electric motor having a rotor with a permanent magnet embedded therein. Motor generator  242  uses at least one of electric power stored in power storage device  210  and electric power generated by motor generator  240  to generate driving force which is in turn transmitted to drive wheel  270 . 
     Furthermore, when the vehicle is braked or travels downhill and its acceleration is reduced or the like, mechanical energy stored in the vehicle as kinetic energy, potential energy and the like is used via drive wheel  270  to drive motor generator  242  to rotate motor generator  242  to allow motor generator  242  to operate as an electric power generator, Motor generator  242  thus operates as a regenerative brake converting traveling energy to electric power and generating braking force. The electric power generated by motor generator  242  is stored in power storage device  210 . 
     Power split device  260  is constituted of a planetary gear including a sun gear, a pinion gear, a carrier, and a ring gear. The pinion gear engages with the sun gear and the ring gear. The carrier supports the pinion gear to be capable of revolving and is also coupled with a crankshaft of engine  250 . The sun gear is coupled with a shaft of rotation of motor generator  240 . The ring gear is coupled with a shaft of rotation of motor generator  242  and drive wheel  270 . 
     System main relay SMR 1  is provided between power storage device  210  and boost converter  220  and operates in response to a signal received from vehicular ECU  290  to electrically connect power storage device  210  to boost converter  220 , Boost converter  220  boosts the voltage on a positive electrode line PL 2  to a voltage equal to or larger than that output from power storage device  210 . Note that boost converter  220  is constituted for example of a direct current chopper circuit. Inverters  230 ,  232  drive motor generators  240 ,  242 , respectively. Note that inverter  230 ,  232  is constituted for example of a 3-phase bridge circuit. 
     Secondary self resonant coil  60  and secondary coil  70  are as has been described with reference to  FIG. 1 . Rectifier  280  rectifies alternate current electric power extracted by secondary coil  70 . System main relay SMR 2  is provided between rectifier  280  and power storage device  210  and operates in response to a signal received from vehicular ECU  290  to electrically connect rectifier  280  to power storage device  210 . 
     Vehicular ECU  290  in a traveling mode turns on and off system main relays SMR 1  and SMR 2 , respectively, and when the vehicle travels, vehicular ECU  290  operates in accordance with an accelerator pedal position, the vehicle&#39;s speed and other signals received from a variety of sensors to generate a signal for driving boost converter  220  and motor generators  240 ,  242  and output the generated signal to boost converter  220  and inverters  230 ,  232 . 
     Furthermore, when electric power feeding equipment  1  (see  FIG. 1 ) feeds electric power to hybrid vehicle  200 , vehicular ECU  290  turns on system main relay SMR 2 . This allows electric power that is received by secondary self resonant coil  60  to be supplied to power storage device  210 . Note that between rectifier  280  and power storage device  210  a DC/DC converter may be provided to receive direct current electric power rectified by rectifier  280  and convert it in voltage to the level in voltage of power storage device  210 . 
     Note that system main relays SMR 1  and SMR 2  can also both be turned on to receive electric power from electric power feeding equipment  1  while the vehicle travels. 
     Thus in the present embodiment a spread spectrum can be provided to have a frequency range set as based on an S-parameter S 21  of a circuit that is constituted of primary coil  20 , primary self resonant coil  30 , secondary self resonant coil  60  and secondary coil  70  and implements electric power transfer through resonance, and the spread spectrum with that frequency range can be used to transfer electric power. Thus a frequency band with high (electric power) transfer efficiency can be used and a peak in electromagnetic field spectrum for a particular frequency can also be reduced. Thus in the present embodiment a peak in electromagnetic field strength can be reduced without electric power feeding equipment  1  feeding significantly reduced electric power to electric power receiving device  2 . 
     Note that while in the above embodiment the spread spectrum&#39;s modulation system is a frequency hopping spread spectrum, it may be a direct sequence spread spectrum. When the direct sequence spread spectrum is adopted, the spread spectrum may have a frequency range set at a frequency band including both resonant frequencies f 1  and f 2  shown in  FIG. 4 , a frequency band including one of frequencies f 1  and f 2 , or a frequency band between frequencies f 1  and f 2 . 
     When a frequency range including one of frequencies f 1  and f 2  is adopted and a direct sequence spread spectrum is employed to provide a spread spectrum, the frequency range may be determined as based on a Q factor. 
       FIG. 7  is a diagram for illustrating a Q factor. With reference to  FIG. 7 , when electric power feeding equipment  1  transfers high frequency electric power of frequency f to electric power receiving device  2 , the Q factor is expressed as follows: 
         Q  factor= f/Δf   (1)
 
     where Δf represents a width of a frequency band allowing a voltage of Vm√2 when voltage applied to load  3  (see  FIG. 1 ) is represented as Vm. Accordingly, a spread spectrum is provided to have a frequency range set for example with Q 1  representing a Q factor obtained from a peak for the  FIG. 4  resonant frequency f 1  in a frequency characteristic of a voltage applied to the load, as follows: 
       Δ f 1=α( f 1/ Q 1)  (2)
 
     where α represents an adjustment coefficient. 
     Alternatively, the spread spectrum may be provided to have a frequency range set with Q 2  representing a Q factor obtained from a peak for the  FIG. 4  another resonant frequency f 2  in the frequency characteristic of the voltage applied to the load, as follows: 
       Δ f 2=β( f 2/ Q 2)  (3)
 
     where β represents an adjustment coefficient. 
     Note that α and β are set below the specification&#39;s upper limit value. 
     Thus the spread spectrum can have a frequency range appropriately set as based on a Q factor. 
     Furthermore, varying in frequency the high frequency electric power supplied from electric power feeding equipment  1  to electric power receiving device  2  can change a distribution in strength of an electromagnetic field generated therearound when electric power feeding equipment  1  feeds electric power receiving device  2  with electric power. 
       FIG. 8  shows a phase difference between a current passing through primary self resonant coil  30  and that passing through secondary self resonant coil  60 . With reference to  FIG. 8 , the axis of abscissas represents high frequency electric power supplied from electric power feeding equipment  1  to electric power receiving device  2  in frequency. As shown in  FIG. 8 , a current passing through primary self resonant coil  30  and that passing through secondary self resonant coil  60  have a phase difference varying with the fed electric power&#39;s frequency. 
     Note that an electromagnetic field caused around each coil has a distribution varying with a current passing through the coil, and when electric power feeding equipment  1  feeds electric power receiving device  2  with electric power, an electromagnetic field is generated with a strength having a distribution, which corresponds to an electromagnetic field generated around primary self resonant coil  30  by a current passing through primary self resonant coil  30  and an electromagnetic field generated around secondary self resonant coil  60  by a current passing through secondary self resonant coil  60  that are superposed on one another. 
     Accordingly, utilizing the fact that a current passing through primary self resonant coil  30  and that passing through secondary self resonant coil  60  have a phase difference varying with the fed electric power&#39;s frequency, as described above, to vary high frequency electric power supplied from electric power feeding equipment  1  to electric power receiving device  2  in frequency can change a distribution in strength of an electromagnetic field generated therearound when electric power feeding equipment I feeds electric power receiving device  2  with electric power, and feeding electric power having an appropriately selected frequency allows electromagnetic field strength to be reduced at a desired location. 
     In the above embodiment primary coil  20  is used to feed primary self resonant coil  30  with electric power through electromagnetic induction and secondary coil  70  is used to extract electric power from secondary self resonant coil  60  through electromagnetic induction. Alternatively, primary coil  20  may be dispensed with and high frequency electric power supply device  10  may directly feed primary self resonant coil  30  with electric power, and secondary coil  70  may be dispensed with and secondary self resonant coil  60  may have electric power extracted directly therefrom. 
     Furthermore in the above description a pair of self resonant coils is resonated to transfer electric power. Alternatively, resonators in the form of the pair of self resonant coils may be replaced with those in the form of a pair of high dielectric disks. Each disk is formed of a material of a high dielectric constant, such as TiO 2 , BaTi 4 O 9 , LiTaO 3 , or the like. 
     Furthermore, while in the above description an electric motored vehicle having electric power receiving device  2  mounted therein has been described by way of example as a series/parallel type hybrid vehicle employing power split device  260  to split and thus transmit power of engine  250  to drive wheel  270  and motor generator  240 , the present invention is also applicable to different types of hybrid vehicles. More specifically, the present invention is applicable for example to: a so called series type hybrid vehicle that employs engine  250  only for driving motor generator  240  and generates force only by motor generator  242  for driving the vehicle; a hybrid vehicle recovering only regenerated energy of kinetic energy that is generated by engine  250  as electrical energy; and a motor-assisted hybrid vehicle having an engine as a major power source and a motor as an assistant as required. Furthermore, the present invention is also applicable to an electric vehicle excluding engine  250  and traveling only on electric power, and a fuel cell vehicle including a direct current power supply implemented as power storage device  210  and in addition thereto a fuel cell. 
     Note that in the above description primary self resonant coil  30  and primary coil  20  correspond in the present invention to an embodiment of an “electric power transferring resonator” and secondary self resonant coil  60  and secondary coil  70  correspond in the present invention to an embodiment of an “electric power receiving resonator”. Furthermore, oscillation circuits  110 ,  120 , M-sequence random number generation circuit  130  and selector switch  140  correspond in the present invention to an embodiment of a “frequency setting unit”. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in any respect, The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     DESCRIPTION OF THE REFERENCE SIGNS 
       1 : electric power feeding equipment,  2 : electric power receiving device,  3 : load,  10 : high frequency electric power supply device,  20 : primary coil,  30 : primary self resonant coil,  40 : control device,  60 : secondary self resonant coil,  70 : secondary coil,  110 ,  120 : oscillation circuit,  130 : M-sequence random number generation circuit,  140 : selector switch,  150 : power supply control unit,  160 : drive signal generation unit,  200 : hybrid vehicle,  210 : power storage device,  220 : boost converter,  230 ,  232 : inverter,  240 ,  242 : motor generator,  250 : engine,  260 : power split device,  270 : drive wheel,  280 : rectifier,  290 : vehicular ECU, C 1 , C 2 : stray capacitance, SMR 1 , SMR 2 : system main relay, PL 1 , PL 2 : positive electrode line, NL: negative electrode line.