Vehicle wireless power transfer using metal member with high permeability to improve charging efficiency

A bottom portion of a vehicle includes a metal member formed of a metal having a higher magnetic permeability than aluminum, and a power reception coil including a low-turn-count portion with a smaller number of turns and a high-turn-count portion with a larger number of turns than the low-turn-count portion. In plan view as seen from below the metal member and the power reception coil, the low-turn-count-portion is located in at least a part of a facing portion of the power reception coil, the facing portion facing the metal member.

This nonprovisional application is based on Japanese Patent Application No. 2015-154616 filed on Aug. 4, 2015 with the Japan Patent Office, the entire contents of which are hereby incorporated by reference.

BACKGROUND

Technical Field

The present disclosure relates to a vehicle including a wireless power reception apparatus.

Description of the Background Art

A wireless power transfer system wirelessly transmitting electric power from a power transmission apparatus to a power reception apparatus has been known (Japanese Patent Laying-Open Nos. 2013-154815, 2013-146154, 2013-146148, 2013-110822, 2013-126327). The power transmission apparatus includes a power transmission coil and the power reception apparatus includes a power reception coil.

A vehicle disclosed in WO2013/076870 includes a power reception apparatus provided on the bottom surface of the vehicle, and a muffler provided on the bottom surface of the vehicle and located at a position adjacent to the power reception apparatus. A power transmission apparatus includes an inverter for adjusting the voltage and the frequency of electric power supplied from a power supply. The muffler includes a metal cover.

SUMMARY

When the vehicle disclosed in WO2013/076870 is to receive electric power from the power transmission apparatus, the vehicle is stopped at a position where the power reception apparatus faces the power transmission apparatus in the vertical direction. Then, electric power is supplied to a power transmission coil of the power transmission apparatus to form magnetic flux around the power transmission coil. The magnetic flux formed around the power transmission coil passes through a power reception coil, and the power reception coil thus receives electric power.

The present inventor studied the influence of the metal cover on the coupling coefficient between the power transmission coil and the power reception coil, in the case where the metal cover of the muffler is provided on the right side of the power reception coil.

The inventor found that, in a system where a muffler made of iron or stainless steel is provided in the vicinity of the power reception apparatus, a part of the magnetic flux emitted from the power transmission coil is guided by the metal cover to the power reception coil, when electric power is transmitted from the power transmission coil to the power reception coil in the state where the power transmission coil and the power reception coil are positionally aligned accurately.

The inventor then conducted a study on different types of metal covers, and found that more magnetic flux is guided to the power reception coil in the case where the metal cover is formed of a metal having a higher magnetic permeability such as iron and stainless steel. Meanwhile, the inventor found that in the case where the metal cover is formed of a metal having a lower magnetic permeability such as aluminum, the magnetic flux from the power transmission coil is reflected by the metal cover.

Based on the newly found phenomenon as described above, the inventor then conducted a study on variation of the coupling coefficient between the power transmission coil and the power reception coil under conditions that the power transmission coil is positionally displaced from the power reception coil in different manners.

Consequently, the inventor found that in the case where the metal cover is disposed in the vicinity of the power reception coil, a large difference arises between the coupling coefficient when the power transmission coil is positionally displaced toward the metal cover and the coupling coefficient when the power transmission coil is positionally displaced in the direction away from the metal cover.

The inventor further found that the manner of variation of the coupling coefficient varies depending, for example, on the type of the metal forming the metal cover.

Variation of the coupling coefficient causes variation of the voltage received by the power reception apparatus. Moreover, if the power transmission apparatus performs constant-power control, the current flowing in the power transmission apparatus is also varied considerably.

As a result, there arises the necessity of a higher breakdown voltage of the power reception apparatus, or a higher current-carrying capacity of the power transmission apparatus.

The inventor found that the above-described problems arise not only in the case where the metal cover of the muffler is provided but also in the case where a metal member is provided in the vicinity of the power reception coil.

The embodiments have been made in view of the above-described problems, and an object of the embodiments is to provide a vehicle which can reduce variation of the coupling coefficient between a power transmission coil and a power reception coil, even when a metal member is provided in the vicinity of the power reception coil and the power transmission coil is positionally displaced from the power reception coil.

A vehicle according to one aspect of the present disclosure includes: a power reception apparatus including a power reception coil configured to wirelessly receive electric power from a power transmission coil, the power reception apparatus being provided at a bottom surface of the vehicle; and at least one metal member provided at a position horizontally adjacent to the power reception apparatus in plan view as seen from below the bottom surface of the vehicle. The at least one metal member is formed of a metal having a higher magnetic permeability than a magnetic permeability of aluminum. The power reception coil includes a low-turn-count portion with a smaller number of turns, and a high-turn-count portion with a larger number of turns than the low-turn-count portion. The low-turn-count portion is located in at least a part of a facing portion of the power reception coil, the facing portion facing the at least one metal member in plan view as seen from below the at least one metal member and the power reception coil.

When the power reception apparatus of the vehicle receives electric power transmitted from the power transmission apparatus, a part of magnetic flux from the power transmission apparatus enters the metal member. Since the metal member is formed of a metal having a higher magnetic permeability than aluminum, the magnetic flux passes into the metal member. The magnetic flux entering the metal member passes through the metal member and is emitted to the outside of the metal member. A part of the magnetic flux emitted from the metal member passes through the power reception coil.

In some cases, positional displacement of the power transmission coil from the power reception coil may increase the amount of magnetic flux entering the metal member. As the amount of the magnetic flux entering the metal member increases, the amount of magnetic flux guided by the metal member to the power reception coil increases.

In the facing portion of the power reception coil, the low-turn-count portion is located, and the number of turns of the facing portion is small. Therefore, even when the amount of magnetic flux passing through the facing portion increases, the influence on the coupling coefficient between the power reception coil and the power transmission coil is small.

As a result, even when the power transmission coil is positionally displaced, variation of the coupling coefficient between the power reception coil and the power transmission coil can be reduced.

Preferably, the at least one metal member includes a first metal member and a second metal member provided at a position closer to the ground beneath the vehicle than a position where the first metal member is provided. In plan view as seen from below the first metal member, the second metal member, and the power reception coil, the power reception coil includes a first facing portion facing the first metal member and a second facing portion facing the second metal member. The number of turns of the second facing portion is smaller than the number of turns of the first facing portion.

During power transfer, the amount of magnetic flux entering the second metal member is larger than the amount of magnetic flux entering the first metal member, because the magnetic field strength of magnetic flux formed around the power transmission coil is higher as the position is closer to the power transmission coil.

Therefore, during power transfer, the amount of magnetic flux guided by the second metal member to the power reception coil is larger than the amount of magnetic flux guided by the first metal member to the power reception coil.

The magnetic flux guided by the second metal member enters the second facing portion of the power reception coil, and the magnetic flux guided by the first metal member enters the first facing portion of the power reception coil.

The number of turns of the second facing portion is smaller than the number of turns of the first facing portion. Therefore, the influence which is exerted on the coupling coefficient by the magnetic flux passing through the second facing portion can be made small.

Accordingly, even when the power transmission coil is positionally displaced from the power reception coil, variation of the coupling coefficient between the power reception coil and the power transmission coil can be reduced.

A vehicle according to another aspect of the present disclosure includes: a power reception apparatus including a power reception coil configured to wirelessly receive electric power from a power transmission coil, the power reception apparatus being provided at a bottom surface of the vehicle; and at least one metal member provided at a position horizontally adjacent to the power reception apparatus in plan view as seen from below the bottom surface. The at least one metal member has a magnetic permeability equal to or less than a magnetic permeability of aluminum. The power reception coil includes a low-turn-count portion with a smaller number of turns, and a high-turn-count portion with a larger number of turns than the low-turn-count portion. The high-turn-count portion is located in at least a part of a facing portion of the power reception coil, the facing portion facing the at least one metal member in plan view as seen from below the at least one metal member and the power reception coil.

In the power reception apparatus, the magnetic permeability of the metal member is equal to or less than that of aluminum. Therefore, magnetic flux is hindered from passing through the metal member, and consequently is converted to eddy current, and likely to generate heat.

As a large amount of eddy current flows inside the surface of the metal member, the eddy current creates a magnetic field. The magnetic field is distributed in a direction of reducing the entering magnetic flux. Consequently, magnetic flux which is to enter the metal member is reflected. The magnetic flux reflected by the metal member hinders progress of the magnetic flux flowing from the power transmission coil toward the power reception coil. As a result, there is a decrease in the amount of magnetic flux that passes through the facing portion which is a part of the power reception coil and which faces the metal member.

Depending on the direction of the positional displacement of the power transmission coil, the distance between the power transmission coil and a metal-side portion of the power reception coil located relatively closer to the metal member may decrease while the distance between the power transmission coil and the other portion of the power reception coil may increase. As the distance between the power transmission coil and the metal-side portion of the power reception coil decreases, the amount of magnetic flux directed toward the metal-side portion of the power reception coil increases. Accordingly, the amount of magnetic flux entering the metal member increases and eddy current formed in the metal member also increases.

As the eddy current generated in the metal member increases, the amount of magnetic flux reflected by the eddy current also increases. The reflected magnetic flux hinders progress of magnetic flux flowing from the power transmission coil toward the power reception coil.

Due to this, regardless of the fact that the distance between the power transmission coil and the metal-side portion of the power reception coil decreases when the positional displacement occurs as described above, the amount of magnetic flux flowing through the facing portion of the power reception coil increases merely slightly, relative to the amount of magnetic flux flowing through the facing portion when the power reception coil and the power transmission coil are positionally aligned with each other.

Meanwhile, the number of turns of the facing portion of the power reception coil is larger than that of the other portion. Therefore, the slight increase of the amount of magnetic flux flowing through the facing portion causes an increase of an electromotive voltage induced at the facing portion to a certain extent.

Thus, even when the power transmission coil is positionally displaced, variation of the coupling coefficient between the power reception coil and the power transmission coil can be reduced.

Preferably, the at least one metal member includes a first metal member and a second metal member which is provided at a position closer to the ground beneath the vehicle than a position where the first metal member is provided. In plan view of the first metal member, the second metal member, and the power reception coil as seen from below the first metal member, the second metal member, and the power reception coil, the power reception coil includes a first facing portion facing the first metal member and a second facing portion facing the second metal member. The number of turns of the second facing portion is larger than the number of turns of the first facing portion.

Regarding the power reception apparatus, depending on the direction of positional displacement of the power transmission coil, the distance between the power transmission coil and a portion of the power reception coil located relatively closer to the first metal member may decrease, or the distance between the power transmission coil and a portion of the power reception coil located relatively closer to the second metal member may decrease.

When the distance between the power transmission coil and the portion of the power reception coil located relatively closer to the first metal member decreases, the amount of magnetic flux entering the first metal member increases and eddy current formed in the surface of the first metal member also increases. Accordingly, even when the distance between the power transmission coil and the portion of the power reception coil located relatively closer to the first metal member decreases, the amount of magnetic flux passing through the first facing portion of the power reception coil increases merely slightly.

Moreover, as the distance between the power transmission coil and the portion of the power reception coil located relatively closer to the second metal member decreases, the amount of magnetic flux entering the second metal member increases and eddy current formed inside the surface of the second metal member also increases. Thus, the amount of magnetic flux entering the second facing portion of the power reception coil decreases. Accordingly, even when the distance between the power transmission coil and the portion of the power reception coil located relatively closer to the second metal member decreases, the amount of magnetic flux passing through the second facing portion of the power reception coil increases merely slightly.

A comparison is made between the increase of the amount of magnetic flux passing through the first facing portion when the aforementioned positional displacement occurs and the increase of the amount of magnetic flux passing through the second facing portion when the aforementioned positional displacement occurs. As a result, the increase of the amount of magnetic flux passing through the second facing portion is smaller.

The reason is that because the second metal member is closer to the ground than the first metal member, the amount of magnetic flux entering the second metal member is larger and the amount of eddy current formed in the surface of the second metal member is larger, when the positional displacement occurs.

In view of the above, the number of turns of the second facing portion of the power reception coil is made larger than the number of turns of the first facing portion to suppress occurrence of a difference in coupling coefficient between the power reception coil and the power transmission coil, even when the above-described positional displacement occurs.

The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a schematic diagram showing a wireless charging system1. As shown inFIG. 1, wireless charging system1includes a power transmission apparatus9and a vehicle3having a power reception unit2.

Power reception unit2includes a power reception apparatus4provided at a bottom surface14of vehicle3, a rectifier5converting AC power received by power reception apparatus4to DC power, and a battery6storing the DC power from rectifier5. Power reception apparatus4includes a power reception coil7and a capacitor8. Power transmission apparatus9includes a power transmission coil10, a capacitor11, and a frequency converter12, and power transmission apparatus9is connected to a power supply13.

FIG. 2is a circuit diagram schematically showing a circuit of wireless charging system1. As shown inFIG. 2, capacitor8is connected in series to power reception coil7, and power reception coil7and capacitor8form a series LC resonant circuit. Capacitor11is connected in series to power transmission coil10, and power transmission coil10and capacitor11form a series LC resonant circuit.

The resonant circuit formed by power transmission coil10and capacitor11and the resonant circuit formed by power reception coil7and capacitor8are configured to have respective resonant frequencies which are equal or substantially equal to each other.

Moreover, the resonant circuit formed by power transmission coil10and capacitor11and the resonant circuit formed by power reception coil7and capacitor8are configured to have respective Q factors which are both 100 or more.

In this way, the transmitter's resonant circuit and the receiver's resonant circuit are configured to enable electric power to be transferred at high efficiency even when the distance between power reception apparatus4and power transmission apparatus9is large.

FIG. 3is an exploded perspective view showing power transmission apparatus9. As shown inFIG. 3, power transmission apparatus9includes a housing20, a coil unit21housed in housing20, and a frequency converter12. Housing20includes a case body22having an opening formed to open upward, and a lid23provided to close the opening of case body22.

Case body22includes a partition member24that separates a space housing coil unit21and a space housing frequency converter12from each other. Case body22is formed of a metal material.

Lid23includes a resin lid25closing the space housing coil unit21, and a metal lid26closing the space housing frequency converter12.

FIG. 4is a cross-sectional view along a line IV-IV inFIG. 3. As shown inFIG. 4, coil unit21includes power transmission coil10, a ferrite core30having an upper surface on which power transmission coil10is disposed, a metal support plate31provided on the lower surface of ferrite core30, and capacitor11provided at the lower surface of metal support plate31and connected to power transmission coil10.

Ferrite core30includes an annular core32having an upper surface on which power transmission coil10is disposed, and a central core33disposed to overlap the inner peripheral edge of annular core32.

FIG. 5is a plan view as seen from above power transmission coil10and ferrite core30. As shown inFIG. 5, annular core32includes a plurality of division core members50arranged annularly and spaced apart from one another. Central core33includes a plurality of division core members51arranged annularly and spaced apart from one another.

As shown inFIG. 4, power transmission coil10includes a lower coil34disposed on the upper surface of annular core32, and an upper coil35disposed on lower coil34.

Lower coil34is formed of a coil wire36wound to surround a winding axis O1, and upper coil35is also formed of coil wire36wound to surround winding axis O1. Lower coil34and upper coil35are formed to surround central core33.

Metal support plate31includes: a flat portion40formed in an annular shape and having an upper surface on which annular core32is disposed; and a protruding portion41formed in a central region inside flat portion40and having an upper surface on which central core33is disposed. Protruding portion41includes an annular peripheral wall portion42extending upward from the inner peripheral edge of flat portion40, and a ceiling portion43formed on the upper end of peripheral wall portion42. On the upper surface of ceiling portion43, central core33is disposed.

On the bottom surface of case body22, a plurality of wall portions45and a plurality of wall portions46are formed, and the lower surface of flat portion40is supported by wall portions45and wall portions46.

Between the lower surface of metal support plate31and the bottom surface of case body22, capacitor11and a plurality of electrical devices47are disposed.

FIG. 6is a plan view of bottom surface14as seen from below bottom surface14. As shown inFIG. 6, vehicle3includes a floor panel52which forms the bottom surface of vehicle3, and an exhaust unit53disposed at the lower surface of floor panel52. Exhaust unit53includes an exhaust pipe54connected to an engine (not shown), and a muffler55to which exhaust pipe54is connected. Muffler55is disposed in a back region of vehicle3.

In the example shown inFIG. 6, muffler55is disposed at a position which is horizontally adjacent to power reception apparatus4in plan view as seen from below bottom surface14. Specifically, muffler55is disposed on the right R side of power reception apparatus4.

FIG. 7is an exploded perspective view showing power reception apparatus4. As shown inFIG. 7, power reception apparatus4includes a housing60and a power reception coil unit61housed in housing60.

Housing60includes a case body62having an opening formed to open downward, and a lid63provided to close the opening. Case body62is formed of a metal material. Lid63is formed of a material which can transmit magnetic flux therethrough. For example, the lid is formed of resin.FIG. 8is a cross-sectional view along a line VIII-VIII shown inFIG. 7. As shown inFIG. 8, power reception coil unit61includes power reception coil7, a ferrite core64having a lower surface on which power reception coil7is disposed, a metal support member65having a lower surface on which ferrite core64is provided, and capacitor8disposed at the upper surface of metal support member65.

Power reception coil7includes a lower coil66and an upper coil67disposed at the upper surface of lower coil66.

Ferrite core64includes an annular core68formed in an annular shape, and a central core69disposed at the lower surface of annular core68. Central core69is disposed to contact the inner peripheral edge of annular core68. On the upper surface of power reception apparatus4, a fixture member70is provided. Fixture member70fixes power reception apparatus4to the lower surface of floor panel52. It should be noted that “power reception apparatus4is disposed at the bottom surface” encompasses both the case where “power reception apparatus4is directly fixed to floor panel52” and the case where “power reception apparatus4is disposed below floor panel52and located at a distance from floor panel52” as shown inFIG. 8.

Metal support member65includes an annular flat portion85having a lower surface on which annular core68is disposed, and a protruding portion86having a lower surface on which central core69is disposed. Protruding portion86includes a peripheral wall portion87extending downward from the inner peripheral edge of flat portion85, and a bottom portion88formed on the lower end of peripheral wall portion87.

Muffler55is fixed at the lower surface of floor panel52by means of a fixture member (not shown). Muffler55includes an inner pipe77, a silencing material78provided to surround the periphery of inner pipe77, and a metal cover71which covers inner pipe77and silencing material78. Metal cover71is formed for example of a metal material containing at least one of iron and stainless steel as a main component, and the magnetic permeability of the metal material of metal cover71is higher than the magnetic permeability of aluminum.

Specifically, when a main component of the metal material of metal cover71is iron, the metal material may be any of 99.95% pure iron, 99.8% pure iron, iron-cobalt alloy, Permalloy (registered trademark) (Fe—Ni alloy), silicon steel (alloy containing iron and a small amount of silicon), and the like. Alternatively, when a main component of the metal material of metal cover71is stainless steel, the metal material may be any of ferrite-based stainless steel and martensite-based stainless steel.

Aluminum has a magnetic permeability of 1.256×10−6[H/m]. Iron (99.95% pure iron) has a magnetic permeability of 2.5×10−1[H/m], and iron (99.8% pure iron) has a magnetic permeability of 6.3×10−3[H/m]. Iron-cobalt alloy has a magnetic permeability of 2.3×10−2[H/m], and Permalloy (Fe—Ni alloy) has a magnetic permeability of 1.0×10−2[H/m]. Silicon steel (alloy containing iron and a small amount of silicon) has a magnetic permeability of 5.0×10−3[H/m].

Ferrite-based stainless steel has a magnetic permeability on the order of 1.26×10−3[H/m] to 2.26×10−3[H/m]. Martensite-based stainless steel has a magnetic permeability on the order of 9.42×10−4[H/m] to 1.19×10−3[H/m].

Thus, the metal material containing at least one of iron and stainless steel as a main component has a higher magnetic permeability than aluminum, and allows magnetic flux to more easily pass through the inside than aluminum.

As shown inFIG. 8, metal cover71includes a lower portion72and an upper portion73. Lower portion72is located at the same level as power reception coil7or located lower than power reception coil7. Upper portion73is located higher than power reception coil7.

Floor panel52has a groove56formed thereon. A part of exhaust pipe54is located within groove56, and exhaust pipe54is located higher than power reception coil7. Exhaust pipe54is also formed of a metal material similar to the metal material for metal cover71.

FIG. 9is a plan view showing components such as lower coil66and ferrite core64. InFIG. 9, components such as upper coil67are not shown. As shown inFIG. 9, annular core68includes a plurality of division core members75arranged annularly and spaced apart from one another, and central core69includes a plurality of division core members76disposed annularly and spaced apart from one another.

Specifically, annular core68includes division core members75A to75L. Along the right R side of annular core68, division core members75A to75D are aligned in order from the back to the front of the vehicle.

Along the front F side of annular core68, division core members75D to75G are aligned in order from the right to the left of the vehicle.

Along the left L side of annular core68, division core members75G to75J are aligned in order from the front to the back of the vehicle.

Along the back B side of annular core68, division core members75J to75A are aligned in order from the left to the right of the vehicle.

Lower coil66includes an end80connected to rectifier5and an end81connected to upper coil67.

Lower coil66includes a coil body83and a crossover wire84, and coil body83forms most of lower coil66.

Coil body83is formed of a coil wire82wound to surround a winding axis O2, and the distance from winding axis O2increases as the wire extends from end81toward end80.

FIG. 10is a cross-sectional view along a line X-X shown inFIG. 9, illustrating one end of a crossover wire84and its surroundings. As shown inFIGS. 9 and 10, crossover wire84extends through a gap between division core member75A and division core member75B, penetrates a hole formed in peripheral wall portion87, and enters the inside of protruding portion86.FIG. 11is a cross-sectional view along a line XI-XI shown inFIG. 9, illustrating the other end of crossover wire84and its surroundings. As shown inFIGS. 9 and 11, crossover wire84penetrates a hole formed in peripheral wall portion87and is drawn from inside protruding portion86. Then, crossover wire84extends through the gap between division core member75D and division core member75E and is connected to coil body83.

Thus, crossover wire84extends in metal support member65, and therefore, as shown inFIG. 9, coil body83of lower coil66includes a low-turn-count portion92with a smaller number of turns and a high-turn-count portion93with a larger number of turns than low-turn-count portion92. Of lower coil66, the portion located on the lower surface of division core members75B,75C,75D is low-turn-count portion92. Of lower coil66, the portion located at the lower surface of division core members75E to75A is high-turn-count portion93.

Here, as shown inFIG. 9, in plan view as seen from below lower coil66and muffler55, a portion of coil body83of lower coil66that faces lower portion72of metal cover71is defined as a facing portion90. Specifically, inFIGS. 8 and 9, it is supposed that lower portion72is projected in the left direction L. Then, the portion that overlaps lower coil66corresponds to facing portion90. Namely, the portion where lower portion72and lower coil66are adjacent to each other is facing portion90.

In the example shown inFIG. 9, low-turn-count portion92is located in facing portion90. Specifically, low-turn-count portion92is disposed so that facing portion90is located in a part of low-turn-count portion92. Low-turn-count portion92may be located in a part of facing portion90.

FIG. 12is a plan view of components such as upper coil67and ferrite core64as seen from below upper coil67and ferrite core64. InFIG. 12, components such as lower coil66are not shown.

As shown inFIG. 12, upper coil67includes an end95connected to end81of lower coil66, an end96connected to capacitor8, a coil body102, and a crossover wire101. Coil body102forms most of upper coil67.

Coil body102is formed of coil wire82wound to surround winding axis O2, and the distance from winding axis O2decreases as the wire extends from end95toward end96.

FIG. 13is a cross-sectional view along a line XIII-XIII shown inFIG. 12. As shown inFIGS. 12 and 13, one end of crossover wire101extends through a gap between division core members75, penetrates a hole formed in peripheral wall portion87, and is drawn to the inside of protruding portion86. Specifically, from the gap between division core member75A and division core member75B, crossover wire101is drawn to the inside of metal support member65.

FIG. 14is a cross-sectional view along a line XIV-XIV shown inFIG. 12. InFIG. 14, the other end of crossover wire101also penetrates a hole formed in peripheral wall portion87, extends through the gap between division core members75, and is connected to coil body102. Specifically, crossover wire101is drawn out from the gap between division core member75D and division core member75E.

Thus, crossover wire101extends in metal support member65, and therefore, as shown inFIG. 12, coil body102of upper coil67includes a low-turn-count portion103with a smaller number of turns and a high-turn-count portion104with a larger number of turns than low-turn-count portion103.

Low-turn-count portion103is located at the lower surface of division core members75B,75C,75D, and high-turn-count portion104is located at the lower surface of division core members75E,75F,75G,75H,75I,75J,75K,75L,75A.

Here, as shown inFIG. 12, in plan view of upper coil67and muffler55as seen from below upper coil67and muffler55, a portion of coil body102of upper coil67facing lower portion72of metal cover71(supposing that metal cover71is projected in the left direction L on upper coil67, the portion of metal cover71that is projected on upper coil67) is defined as a facing portion110.

In the example shown inFIG. 12, low-turn-count portion103is located in facing portion110. Specifically, low-turn-count portion103is disposed so that facing portion110is located in a part of low-turn-count portion103. Low-turn-count portion103may be located in a part of facing portion110.

FIG. 15is a plan view as seen from below power reception coil7and power transmission coil10. InFIG. 15, a tolerable range T1represents a tolerable range of positional displacement of winding axis O1of power transmission coil10, from winding axis O2of power reception coil7.

When electric power is transmitted from power transmission coil10to power reception coil7in the state where winding axis O1of power transmission coil10is located in tolerable range T1, the power transmission efficiency of power transmission coil10is a predetermined value or more. When electric power is transmitted in the state where winding axis O1of power transmission coil10is located outside tolerable range T1, the power transmission efficiency is smaller than the predetermined value.

In the present embodiment, power transmission from power transmission coil10is stopped when the power transmission efficiency of power transmission coil10is smaller than the predetermined value.

When winding axis O1is moved along the outer peripheral edge of tolerable range T1, power reception coil7is located in power transmission coil10. Namely, in plan view of power transmission coil10and power reception coil7as seen from below power reception coil7and power transmission coil10, power reception coil7receives electric power from power transmission coil10in the state where power reception coil7is located in power transmission coil10.

Further, when winding axis O1is moved along the outer peripheral edge of tolerable range T1, at least a part of muffler55is located in the outer peripheral edge of the trajectory formed by the outer peripheral edge of power transmission coil10. In other words, in the present embodiment, a metal member to be provided is a metal member which is at least partially located in the outer peripheral edge of the trajectory of the outer peripheral edge of power transmission coil10when power transmission coil10is moved along the outer peripheral edge of tolerable range T1. This is because a metal member provided at a position further away from power transmission coil10or power reception coil7has a small influence on the electric power transfer.

As to electric power transfer by means of power reception apparatus4and power transmission apparatus9configured in the above-described manner, a description will be given usingFIG. 16for example.

FIG. 16is a cross-sectional view showing a state where electric power is transmitted from power transmission apparatus9to power reception apparatus4with winding axis O1coinciding with winding axis O2. As shown inFIG. 16, in the state where power transmission apparatus9and power reception apparatus4are positionally aligned (the state where winding axis O1coincides with winding axis O2in plan view of power reception coil7and power transmission coil10), the distance between the right side portion of power reception coil7and the right side portion of power transmission coil10is identical to the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10.

Moreover, the distance between the front side portion of power reception coil7and the front side portion of power transmission coil10is also identical to the distance between the back side portion of power reception coil7and the back side portion of power transmission coil10.

The amount of magnetic flux MF emitted from power transmission coil10and passing through power reception coil7is basically inversely proportional to the distance between power transmission coil10and power reception coil7. Therefore, in the case where muffler55is absent, the amount of magnetic flux passing through the right side portion of power reception coil7is identical to the amount of magnetic flux passing through the left side portion of power reception coil7.

In contrast, in the case where muffler55is disposed in the vicinity of power reception coil7like the present embodiment, a change occurs to the distribution of magnetic flux passing through power reception coil7.

As shown inFIG. 16, a part of magnetic flux MF emitted from power transmission coil10enters metal cover71of muffler55. As described above, metal cover71is formed of a metal having a high magnetic permeability. Therefore, magnetic flux MF entering metal cover71flows well in metal cover71.

Then, after flowing in metal cover71, magnetic flux MF is emitted from the surface of metal cover71to the outside. Here, in the horizontal direction, lower portion72of metal cover71is located at the same position as or lower than power reception coil7. Thus, a part of magnetic flux MF entering lower portion72is emitted from lower portion72toward power reception coil7and passes through power reception coil7.

Here, magnetic flux MF which is to enter metal cover71would hardly pass through power reception coil7if metal cover71is absent. As metal cover71is present, a part of the magnetic flux is guided toward power reception coil7.

In particular, iron has an electrical resistance of 1.00×10−7(Ωm) and stainless steel has an electrical resistance of 7.2×10−7(Ωm). The electrical resistance of a metal containing iron or stainless steel as a main component is higher than the electrical resistance (2.65×10−8(Ωm)) of aluminum.

Therefore, when magnetic flux enters metal cover71, eddy current is less prone to flow in the surface of metal cover71. Because eddy current is less prone to flow, the strength of magnetic field generated by the eddy current is weak and the magnetic flux reaching metal cover71is less prone to be reflected.

Thus, magnetic flux MF from power transmission coil10is prone to enter metal cover71. Since metal cover71has a high magnetic permeability, magnetic flux MF entering metal cover71flows well in metal cover71and more magnetic flux MF is guided to power reception coil7.

Therefore, in a part of power reception coil7that is located in the vicinity of metal cover71, more magnetic flux MF passes as compared with the remaining part.

Because upper portion73is located higher than power reception coil7, magnetic flux MF entering metal cover71and thereafter emitted from upper portion73hardly passes through power reception coil7.

Exhaust pipe54is also formed of a metal material similar to that for metal cover71. Magnetic flux MF entering exhaust pipe54flows well in exhaust pipe54. In contrast, exhaust pipe54is located higher than power reception coil7, and it hardly occurs that magnetic flux MF flowing through exhaust pipe54is emitted toward power reception coil7.

Thus, metal cover71includes lower portion72which is located lower than power reception coil7. Therefore, in a part of power reception coil7that is adjacent to lower portion72, more magnetic flux passes.

FIG. 17is a plan view as seen from below power transmission coil10and power reception coil7, illustrating a state where power transmission coil10is positionally displaced in the left direction L from power reception coil7.FIG. 18is a cross-sectional view showing a state where power transmission coil10is positionally displaced in the left direction L from power reception coil7, relative to the state where power reception coil7and power transmission coil10are positionally aligned with each other.

As shown inFIG. 18, when power transmission coil10is positionally displaced in the left direction L, the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7decreases. Meanwhile, the distance between the left side portion of power transmission coil10and the left side portion of power reception coil7increases.

As a result, while more magnetic flux MF flows through the right side portion of power transmission coil10and the right side portion of power reception coil7, less magnetic flux MF flows through the left side portion of power transmission coil10and the left side portion of power reception coil7. Most of magnetic flux MF emitted from the left side portion of power transmission coil10does not flow toward power reception coil7, but flows to surround the left side portion of power transmission coil10and thus form a self-closing loop of magnetic flux.

Thus, as the amount of magnetic flux flowing through the right side portion of power transmission coil10and the right side portion of power reception coil7increases, magnetic flux MF entering metal cover71increases. Consequently, magnetic flux MF guided by metal cover71to power reception coil7also increases. Consequently, more magnetic flux MF passes through the right side portion of power reception coil7.

InFIGS. 9 and 12, the magnetic flux guided by lower portion72to lower coil66and upper coil67flows through facing portion90of lower coil66and facing portion110of upper coil67.

Therefore, even when the magnetic flux is guided by lower portion72of metal cover71, an electromotive voltage induced at facing portions90and110can be made small, since the number of turns of the coil wire of facing portions90and110is small.

As a result, even when power transmission coil10is positionally displaced in the left direction L from power reception coil7, it can be suppressed that the coupling coefficient between power transmission coil10and power reception coil7varies largely from the coupling coefficient in the state where power transmission coil10and power reception coil7are positionally aligned to each other.

FIG. 19is a plan view as seen from below power transmission coil10and power reception coil7, illustrating a state where power transmission coil10is positionally displaced in the right direction R from power reception coil7.FIG. 20is a cross-sectional view of a state where power transmission coil10is positionally displaced in the right direction R from power reception coil7.

As shown inFIG. 20, as power transmission coil10is positionally displaced in the right direction R from power reception coil7, the distance between the left side portion of power transmission coil10and the left side portion of power reception coil7decreases. Meanwhile, the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7increases.

Due to this, the amount of magnetic flux flowing between the right side portion of power transmission coil10and the right side portion of power reception coil7is smaller than the amount of magnetic flux flowing between the left side portion of power transmission coil10and the left side portion of power reception coil7.

At this time, magnetic flux MF formed in the right side portion of power transmission coil10does not flow toward power reception coil7but flows to surround the right side portion of power transmission coil10and thus forms a self-closing loop of magnetic flux. Accordingly, the amount of magnetic flux entering metal cover71decreases and the amount of magnetic flux guided by metal cover71to power reception coil7also decreases.

Since the amount of magnetic flux guided from metal cover71decreases, the amount of magnetic flux passing through power reception coil7when power transmission coil10is positionally displaced in the left direction L from power reception coil7is larger than the amount of magnetic flux passing through power reception coil7when power transmission coil10is positionally displaced in the right direction R from power reception coil7.

Meanwhile, in the left side portion of power reception coil7, high-turn-count portions93,104are located, and the amount of magnetic flux flowing through these portions increase. Consequently, the difference between the coupling coefficient when power transmission coil10is positionally displaced in the right direction R from power reception coil7and the coupling coefficient when power transmission coil10is positionally displaced in the left direction L from power reception coil7decreases.

Thus, even when power transmission coil10is positionally displaced in the left direction L and the amount of magnetic flux guided by metal cover71increases, the number of turns of the coil wire of facing portions90,110of power reception coil7can be reduced to thereby suppress increase of an electromotive voltage induced at power reception coil7, and suppress variation of the coupling coefficient when power transmission coil10is positionally displaced in the left direction L or right direction R.

In the case where power transmission coil10is positionally displaced in the frontward direction F or backward direction B, the amount of magnetic flux flowing through the left side portion and the right side portion of power reception coil7is almost the same as the amount of magnetic flux flowing through the right side portion and the left side portion of power reception coil7when power transmission coil10and power reception coil7are positionally aligned with each other.

In the case where power transmission coil10is positionally displaced in the frontward direction F, the distance between the back side portion of power reception coil7and the back side portion of power transmission coil10decreases. Meanwhile, the distance between the front side portion of power reception coil7and the front side portion of power transmission coil10increases.

Therefore, the amount of magnetic flux flowing through the back side portion of power transmission coil10and the back side portion of power reception coil7increases, while the amount of magnetic flux flowing through the front side portion of power transmission coil10and the front side portion of power reception coil7decreases.

Thus, it is suppressed that the coupling coefficient when power transmission coil10is positionally displaced in the frontward direction F from power reception coil7varies largely from the coupling coefficient in the state where power transmission coil10is positionally aligned with power reception coil7.

It should be noted that the magnetic flux distribution and the coupling coefficient in the case where power transmission coil10is positionally displaced in the backward direction B from power reception coil7are substantially identical to those in the case where power transmission coil10is positionally displaced in the frontward direction F.

In this way, variation of the coupling coefficient depending on the direction of positional displacement can be suppressed. Therefore, when constant-power transmission is done from power transmission coil10to power reception coil7, variation of the amount of current flowing through power transmission coil10depending on the direction of positional displacement can be reduced.

According to the above description of the examples shown inFIGS. 1 to 20, metal cover71of muffler55is provided as a metal member adjacent to power reception apparatus4, by way of example. However, the metal member adjacent to power reception apparatus4is not limited to metal cover71of muffler55. For example, it may be a part of floor panel52.

FIG. 21is a cross-sectional view illustrating a modification of the vehicle on which the power reception apparatus is mounted according to the first embodiment. In the example shown inFIG. 21, floor panel52has a protruding portion111formed to protrude downward. Protruding portion111includes an upper portion112located higher than power reception coil7, and a lower portion113which is horizontally at the same position as power reception coil7or located lower than power reception coil7. Floor panel52and protruding portion111are each made of a metal material containing iron or stainless steel as a main component.

Such a protruding portion111also induces magnetic flux MF, like the above-described metal cover71. The present disclosure is also applicable to the case where power reception apparatus4is mounted on a vehicle in which such protruding portion111is formed.

Second Embodiment

Regarding the first embodiment, the description is given above of the example where one metal member is provided in the vicinity of power reception apparatus4. In the following, a description will be given, by means ofFIG. 22for example, of an example where two (a plurality of) metal members are arranged in the vicinity of power reception apparatus4. Any feature identical or substantially identical to the corresponding one shown inFIGS. 1 to 21is denoted by the same reference character, and the description thereof may not be repeated in some cases.

FIG. 22is a plan view showing a bottom surface14of a vehicle3. As shown inFIG. 22, a muffler55is disposed on the right R side of power reception apparatus4. On the left L side of power reception apparatus4, a protruding portion111of floor panel52is formed.

FIG. 23is a cross-sectional view showing power reception apparatus4, muffler55, and protruding portion111. As shown inFIG. 23, the lower end of metal cover71is located lower than the lower end of protruding portion111.

Protruding portion111includes an upper portion112located higher than power reception coil7and a lower portion113located lower than upper portion112.

FIG. 24is a plan view showing a lower coil66and a ferrite core64, where un upper coil67is not shown. As shown inFIG. 24, a coil wire82is wound in such a manner that the distance from winding axis O2decreases as the wire extends from an end80toward an end81.

Coil wire82passes on the lower surface of division core members75B,75C,75D in the direction from division core member75B to division core member75D. After passing multiple times on the lower surface of division core members75B,75C,75D, coil wire82is drawn from the gap between division core member75A and division core member75B to the inside of a metal support member65. Then, coil wire82is drawn out from the gap between division core member75D and division core member75E. Thus, a crossover wire84A passing inside metal support member65is formed.

Subsequently to the drawn-out crossover wire84A, coil wire82passes on the lower surface of division core members75E to75G from division core member75E to division core member75G. Then, from the gap between division core member75G and division core member75H, coil wire82is drawn to the inside of metal support member65. Then, coil wire82is drawn out of metal support member65from the gap between division core member75I and division core member75J. Thus, a crossover wire84C is formed.

Subsequently to the drawn-out crossover wire84C, coil wire82passes on the lower surface of division core members75J to75A from division core member75J to division core member75A. Then, coil wire82is drawn to the inside of metal support member65from the gap between division core member75A and division core member75B. After this, coil wire82is drawn out from the gap between division core member75D and division core member75E. Thus, a crossover wire84B is formed. Then, coil wire82passes on the lower surface of division core member75E and division core member75F to reach end81.

In this way, coil wire82is wound so that the number of turns of the portion located on the lower surface of division core member75B, division core member75C, and division core member75D and the number of turns of the portion located on the lower surface of division core member75H and division core member75I are smaller.

Namely, a portion of lower coil66located on the lower surface of division core member75B, division core member75C, and division core member75D is formed as a low-turn-count portion92A. A portion of lower coil66located on the lower surface of division core member75H and division core member75I is also formed as a low-turn-count portion92B.

In the example shown inFIG. 24, the number of turns of low-turn-count portion92A is smaller than the number of turns of low-turn-count portion92B.

Here, inFIG. 24, a portion of lower coil66facing lower portion72of metal cover71is defined as a facing portion90A and a portion of lower coil66facing lower portion113of protruding portion111is defined as a facing portion90B.

In facing portion90A, low-turn-count portion92A is located. In facing portion90B, low-turn-count portion92B is located. Low-turn-count portion92A may be located in a part of facing portion90A, and low-turn-count portion92B may be located in a part of facing portion90B.

FIG. 25is a plan view showing an upper coil67and ferrite core64. InFIG. 25, coil wire82is formed in such a manner that the distance from winding axis O2decreases as the wire extends from an end95toward an end96.

Coil wire82passes on the lower surface of division core members75B,75C,75D in the direction from division core member75B to division core member75D. After passing multiple times on the lower surface of division core members75B,75C,75D, coil wire82is drawn from the gap between division core member75A and division core member75B to the inside of metal support member65. Then, coil wire82is drawn out from the gap between division core member75D and division core member75E. Thus, a crossover wire101A passing inside metal support member65is formed.

Subsequently to the drawn-out crossover wire101A, coil wire82passes on the lower surface of division core members75E to75G from division core member75E to division core member75G. Then, coil wire82is drawn to the inside of metal support member65from the gap between division core member75G and division core member75H. Then, coil wire82is drawn out from the gap between division core member75I and division core member75J. Thus, a crossover wire101C is formed.

Subsequently to the drawn-out crossover wire101C, coil wire82passes on the lower surface of division core members75J to75A from division core member75J to division core member75A. Then, coil wire82is drawn to the inside of metal support member65from the gap between division core member75A and division core member75B. After this, coil wire82is drawn out from the gap between division core member75D and division core member75E. Thus, a crossover wire101B is formed. Then, coil wire82passes on the lower surface of division core member75E and division core member75F to reach end96.

In this way, coil wire82is wound so that the number of turns of the portion located on the lower surface of division core member75B, division core member75C, and division core member75D and the number of turns of the portion located on the lower surface of division core member75H and division core member75I are smaller.

Namely, a portion of upper coil67located on the lower surface of division core member75B, division core member75C, and division core member75D is formed as a low-turn-count portion103A. A portion of upper coil67located on the lower surface of division core member75H and division core member75I is also formed as a low-turn-count portion103B.

In the example shown inFIG. 25, the number of turns of low-turn-count portion103A is smaller than the number of turns of low-turn-count portion103B.

Here, inFIG. 25, a portion of upper coil67facing lower portion72of metal cover71is defined as a facing portion110A. Low-turn-count portion103A is located in facing portion110A. Specifically, facing portion110A is located in a part of low-turn-count portion103A. Low-turn-count portion103A may be located in a part of facing portion110A.

A portion of upper coil67facing lower portion113of protruding portion111is defined as a facing portion110B. Low-turn-count portion103B is located in facing portion110B. Low-turn-count portion103B may be located in a part of facing portion110B.

FIG. 26is a cross-sectional view illustrating power transfer in a state where power reception apparatus4and power transmission apparatus9are positionally aligned accurately.

As shown inFIG. 26, a part of magnetic flux MF from power transmission coil10enters metal cover71. Then, a part of magnetic flux MF entering lower portion72of metal cover71is guided toward power reception coil7.

Meanwhile, a part of magnetic flux MF from power transmission coil10enters lower portion113of protruding portion111. Magnetic flux MF entering lower portion113flows in lower portion113. Then, a part of magnetic flux MF entering lower portion113is emitted toward power reception coil7.

Here, metal cover71is located closer to the ground than protruding portion111. Therefore, the amount of magnetic flux entering metal cover71is larger than the amount of magnetic flux entering protruding portion111. This is because the strength of the magnetic field formed around power transmission coil10is higher as closer to power transmission coil10. Namely, since metal cover71is located closer to the ground than protruding portion111, the strength of the magnetic field to which the lower end of metal cover71is exposed is higher than the strength of the magnetic field to which the lower end of protruding portion111is exposed.

FIG. 27is a cross-sectional view illustrating power transfer in a state where power transmission apparatus9is positionally displaced in the left direction L.

As shown inFIG. 27, as power transmission coil10is thus positionally displaced, the distance between the right side portion of power reception coil7and the right side portion of power transmission coil10decreases. Accordingly, magnetic flux MF flowing between the right side portion of power reception coil7and the right side portion of power transmission coil10increases.

Therefore, more magnetic flux MF enters metal cover71and the amount of magnetic flux guided from metal cover71to power reception coil7increases. Meanwhile, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10increases.

Therefore, less magnetic flux MF flows through the left side portion of power reception coil7and the left side portion of power transmission coil10, and less magnetic flux MF enters protruding portion111. Thus, almost no magnetic flux MF enters protruding portion111.

Even when the amount of magnetic flux guided by lower portion72increases, low-turn-count portions92A and103A are located respectively in facing portions90A and110A that are respective portions of lower coil66and upper coil67and that are adjacent to lower portion72, as shown inFIGS. 24 and 25.

Therefore, a counter-electromotive force generated at facing portions90A and110A of lower coil66and upper coil67can be reduced. Accordingly, the difference between the coupling coefficient in the state where power transmission coil10and power reception coil7are positionally aligned with each other and the coupling coefficient when power transmission coil10is positionally displaced in the left direction L from power reception coil7can be made small.

FIG. 28is a cross-sectional view illustrating a state where power transmission coil10is positionally displaced in the right direction R from power reception coil7. It should be noted that the amount of positional displacement of power transmission coil10inFIG. 28is identical to the amount of positional displacement of power transmission coil10inFIG. 27.

As shown inFIG. 28, the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7increases, while the distance between the left side portion of power transmission coil10and the left side portion of power reception coil7decreases.

Therefore, the amount of magnetic flux flowing through the left side portion of power reception coil7is larger than the amount of magnetic flux flowing through the right side portion of power reception coil7. Because of this, the amount of magnetic flux guided by protruding portion111to power reception coil7is larger than the amount of magnetic flux guided by protruding portion111to power reception coil7in the state where power transmission coil10and power reception coil7are positionally aligned.

InFIGS. 24 and 25, low-turn-count portions92B and103B are located in facing portions90B and110B of lower coil66and upper coil67. Therefore, increase of a counter-electromotive force generated at facing portions90B and110B is suppressed.

Accordingly, occurrence of a large difference between the coupling coefficient in the state where power transmission coil10is positionally displaced in the right direction R and the coupling coefficient in the state where power transmission coil10and power reception coil7are positionally aligned can be suppressed.

The amount of magnetic flux guided by protruding portion111to power reception coil7in the case where power transmission coil10is positionally displaced in the right direction R is smaller than the amount of magnetic flux guided by lower portion72to power reception coil7in the case where power transmission coil10is positionally displaced in the left direction L.

As shown inFIGS. 24 and 25, the number of turns of facing portions90B,110B is larger than the number of turns of facing portions90A,110A.

Therefore, the counter-electromotive force generated at facing portions90B,110B when power transmission coil10is positionally displaced in the right direction R is identical to or almost identical to the counter-electromotive force generated at facing portions90A,110A when power transmission coil10is positionally displaced in the left direction L.

Accordingly, the coupling coefficient when power transmission coil10is positionally displaced in the left direction L from power reception coil7is identical to or almost identical to the coupling coefficient when power transmission coil10is positionally displaced in the right direction R from power reception coil7.

Thus, in the case where metal members located at different heights are disposed with power reception coil7interposed therebetween, the number of turns of a portion of power reception coil7adjacent to one metal member which is relatively closer to the ground is made smaller than the number of turns of a portion thereof that is adjacent to the other metal member which is relatively further from the ground. Accordingly, even when power transmission coil10is positionally displaced toward any one of the metal members, occurrence of a difference in coupling coefficient between power reception coil7and power transmission coil10can be suppressed.

Third Embodiment

Regarding the first and second embodiments, the description is given above of the example where the metal member (metal cover71) disposed in the vicinity of power reception coil7is formed of a metal containing iron or stainless steel as a main component. A description will be given next of a third embodiment in which the metal member is formed of a material having a low magnetic permeability like aluminum.

Any feature identical or substantially identical to the corresponding one inFIGS. 1 to 27is denoted by the same reference character, and the description thereof may not be repeated in some cases.

FIG. 29is a cross-sectional view showing a power reception apparatus and a muffler55according to the third embodiment, illustrating a state where a power reception coil7and a power transmission coil10are positionally aligned. The magnetic permeability of a metal material which forms a metal cover71B is equal to or higher than the magnetic permeability of aluminum. As the metal material forming metal cover71B, aluminum or copper for example may be used.

The magnetic permeability of aluminum is 1.256×10−6[H/m] and the magnetic permeability of copper is 1.256629×10−6[H/m]. The electrical conductivity of aluminum is 2.65×10−8[(Ωm)] and the electrical conductivity of copper is 1.68×10−8[(Ωm)] which are higher than respective electrical conductivities of stainless steel and iron.

Metal cover71B also includes an upper portion73B located higher than power reception coil7and a lower portion72B located at the same level or lower than power reception coil7in the vertical direction.

FIG. 30is a plan view of power reception apparatus4according to the third embodiment, illustrating a lower coil66and a ferrite core64.

As shown inFIG. 30, a coil wire82is wound in such a manner that the distance from a winding axis O2decreases as the wire extends from an end80toward an end81.

Coil wire82extends on the lower surface of division core members75B,75C,75D from division core member75B to division core member75D.

After passing several times on the lower surface of division core members75B to75D, coil wire82is drawn from the gap between division core member75D and division core member75E to the inside of a metal support member65. Then, coil wire82is drawn out from the gap between division core member75A and division core member75B and passes again on the lower surface of division core members75B,75C,75D.

Thus, the number of turns of coil wire82is larger than that of the remaining portion on the lower surface of division core members75B to75D.

Namely, a high-turn-count portion93is formed in a portion located on the lower surface of division core member75B to division core member75D, while a low-turn-count portion92is formed in a portion located on the lower surface of division core members75E to75A.

Here, a portion of lower coil66facing lower portion72B of metal cover71B is defined as a facing portion90. High-turn-count portion93is located in facing portion90. Specifically, facing portion90is located in a part of high-turn-count portion93. Arrangement may be done so that a part of facing portion90is located in high-turn-count portion93.

FIG. 31is a plan view showing an upper coil67and ferrite core64. As shown inFIG. 31, upper coil67is wound in such a manner that the distance from winding axis O2decreases as the wire extends from an end95toward an end96.

Coil wire82extends on the lower surface of division core members75B,75C,75D from division core member75B to division core member75D.

After passing several times on the lower surface of division core members75B to75D, coil wire82is drawn from the gap between division core member75D and division core member75E to the inside of metal support member65. Then, coil wire82is drawn out from the gap between division core member75A and division core member75B and passes again on the lower surface of division core members75B,75C,75D.

Thus, the number of turns of coil wire82is larger than that of the remaining portion on the lower surface of division core members75B to75D.

Namely, a high-turn-count portion104is formed in a portion located on the lower surface of division core member75B to division core member75D, while a low-turn-count portion103is formed in a portion located on the lower surface of division core members75E,75F,75G,75H,75I,75J,75K,75L, and75A.

Thus, as shown inFIGS. 30 and 31, high-turn-count portion93is located in a portion located in facing portion90of lower coil66, and high-turn-count portion104is located in a portion located in facing portion110of upper coil67.

InFIG. 29, a part of magnetic flux MF emitted from the right side portion of power transmission coil10enters lower portion72B of metal cover71B. Since the magnetic permeability of the metal forming metal cover71B is low, the entering magnetic flux is hindered from flowing in metal cover71B. Most of the entering magnetic flux becomes eddy current and is thereafter converted to heat. Therefore, a large amount of eddy current flows in metal cover71B. In particular, since the electrical conductivity of aluminum or copper which forms metal cover71B is higher than the electrical conductivity of stainless steel or iron, particularly a large amount of eddy current flows.

As the amount of eddy current formed in the surface of metal cover71B increases, the strength of a magnetic field formed by the eddy current also increases. The magnetic field formed by the eddy current is formed in the direction of reducing the amount of the entering magnetic flux, the magnetic flux incident on metal cover71B is reflected.

Thus, the magnetic flux reflected by metal cover71B hinders the magnetic flux from flowing from power transmission coil10toward power reception coil7.

Consequently, the amount of magnetic flux flowing through the right side portion of power transmission coil10and the right side portion of power reception coil7is smaller than that in the case where metal cover71B is not provided.

FIG. 32is a cross-sectional view showing a state where power transmission coil10is positionally displaced in the left direction L from power reception coil7. As shown inFIG. 32, as power transmission coil10is positionally displaced in the left direction L, the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7decreases.

Therefore, the amount of magnetic flux flowing from the right side portion of power transmission coil10toward the right side portion of power reception coil7is to increase. Accordingly, the amount of magnetic flux flowing toward metal cover71B also increases and the amount of magnetic flux reflected by metal cover71B also increases. The magnetic flux reflected by metal cover71B hinders flow of the magnetic flux from power transmission coil10toward power reception coil7.

As a result, while the amount of magnetic flux flowing through the right side portion of power reception coil7in the state where power transmission coil10is positionally displaced in the left direction L is slightly larger than that in the state where power transmission coil10and power reception coil7are positionally aligned, the difference between these amounts of magnetic flux is small.

Meanwhile, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10increases. Therefore, the amount of magnetic flux flowing through the left side portion of power reception coil7in the state where power transmission coil10is positionally displaced in the left direction L is smaller than that in the state where power transmission coil10and power reception coil7are positionally aligned.

As a result, the amount of magnetic flux flowing through power reception coil7in the state where power transmission coil10is positionally displaced in the left direction L is smaller than that in the state where power transmission coil10and power reception coil7are positionally aligned.

Meanwhile, as shown inFIGS. 30 and 31, the number of turns of facing portions90,110is large. Therefore, even when power transmission coil10is positionally displaced in the left direction L to cause decrease of the amount of magnetic flux passing through the left side portion of power reception coil7, the amount of magnetic flux passing through facing portions90,110slightly increases to thereby suppress occurrence of a difference in counter-electromotive force generated at power reception coil7.

Namely, the difference of the coupling coefficient is small between the state where power transmission coil10and power reception coil7are positionally aligned and the state where power transmission coil10is positionally displaced.

FIG. 33is a cross-sectional view showing a state where power transmission coil10is positionally displaced in the right direction R from power reception coil7. As shown inFIG. 33, as power transmission coil10is positionally displaced in the right direction R, the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7increases. Therefore, the amount of magnetic flux flowing through the right side portion of power reception coil7decreases.

Meanwhile, the distance between the left side portion of power transmission coil10and the left side portion of power reception coil7decreases and thus the amount of magnetic flux flowing through the left side portion of power reception coil7increases.

Therefore, the difference of the coupling coefficient is small between the case where power transmission coil10is positionally displaced in the right direction R and the case where power transmission coil10and power reception coil7are positionally aligned.

As seen from the above, even when power transmission coil10is positionally displaced, variation of the coupling coefficient can be reduced in the third embodiment as well.

Fourth Embodiment

Regarding the third embodiment, the description is given above of the example where metal cover71is provided as a metal member. The disclosure is also applicable to an example where a plurality of metal members are provided.

Any feature identical or substantially identical to the corresponding one shown inFIGS. 1 to 33is denoted by the same reference character, and the description thereof may not be repeated in some cases.

FIG. 34is a cross-sectional view showing a power reception apparatus4, a power transmission apparatus9, and its surroundings. In the example shown inFIG. 34, an on-board device130is located along a portion of a floor panel52that is on the left L side of power reception apparatus4. A muffler55is also provided along a portion of floor panel52that is on the right R side of power reception apparatus4.

On-board device130includes a metal cover133, and metal cover133includes an upper portion131located higher than a power reception coil7and a lower portion132located lower than upper portion131.

Both metal cover133of on-board device130and a metal cover71B are each formed of a metal containing at least one of aluminum and copper as a main component.

FIG. 35is a plan view showing a part of power reception apparatus4. InFIG. 35, an upper coil67is not shown.

Here, a coil wire82forming a lower coil66is wound so that the distance from winding axis O2decreases as the wire extends from an end80toward an end81. After wound multiple times to surround the periphery of a central core69, coil wire82is drawn from a gap between division core members75D and75E to the inside of a metal support member65. After this, coil wire82is drawn out from the gap between division core members75A and75B.

Accordingly, a crossover wire is formed to extend in metal support member65. The crossover wire is drawn out again from inside metal support member65at the rear side in the direction in which the wire is wound. Thus, the number of turns of a specific portion is made larger.

Then, coil wire82is wound again to surround the periphery of central core69.

After this, coil wire82is drawn from the gap between division core member75I and division core member75J to the inside of metal support member65. After this, coil wire82is drawn out from the gap between division core member75G and division core member75H to surround the periphery of central core69toward end81.

Thus, a crossover wire extending in metal support member65is formed. The crossover wire is drawn out at the rear side in the direction in which coil wire82is wound, and thereafter wound in the winding direction to thereby form a high-turn-count portion93B.

After this, coil wire82is wound around the periphery of central core69toward end81. Again, coil wire82is drawn from the gap between division core member75D and division core member75E to the inside of metal support member65. Then, coil wire82is drawn from the gap between division core member75A and division core member75B.

After this, coil wire82is wound to surround the periphery of central core69toward end81.

In this way, coil wire82is wound to form a high-turn-count portion93A as a part of lower coil66, on the lower surface of division core members75B to75D, and high-turn-count portion93B is also formed on the lower surface of division core member75H and division core member75I. On the lower surface of division core members75E,75F and the lower surface of division core members75J,75K,75L,75A, low-turn-count portions92A,92B are formed as a part of lower coil66.

The number of turns of high-turn-count portions93A,93B is larger than that of portions92A,92B, and the number of turns of high-turn-count portion93A is larger than the number of turns of high-turn-count portion93B.

Here, a portion of lower coil66facing lower portion72B of metal cover71B is defined as a facing portion90A, and a portion of lower coil66facing a metal cover133of installed device130is defined as a facing portion90B. As shown inFIG. 35, high-turn-count portion93A is located in facing portion90A, and high-turn-count portion93B is located in facing portion90B.

FIG. 36is a plan view showing upper coil67and its surrounding features. InFIG. 36, lower coil66is not shown. Upper coil67is also formed by winding coil wire82similarly to lower coil66.

Thus, a high-turn-count portion104A is formed in a portion of upper coil67located on the lower surface of division core members75B,75C,75D. A high-turn-count portion104B is formed in a portion of upper coil67located on the lower surface of division core members75H,75I.

Of upper coil67, a portion located on the lower surface of division core members75E,75F and a portion located on the lower surface of division core members75K,75L are formed as low-turn-count portions103A,103B, respectively.

The number of turns of high-turn-count portions104A,104B is larger than the number of turns of low-turn-count portions103A,103B, and the number of turns of high-turn-count portion104A is larger than the number of turns of high-turn-count portion104B.

Here, a portion of upper coil67facing lower portion72B is defined as a facing portion110A, and a portion of upper coil67facing lower portion132of metal cover133is defined as a facing portion110B. In facing portion110A, high-turn-count portion104A is located. In facing portion110B, high-turn-count portion104B is located.

Namely, inFIGS. 35 and 36, high-turn-count portions93A,104A are located in respective portions of power reception coil7facing lower portion72B of metal cover71B, and high-turn-count portions93B,104B are located in respective portions of power reception coil7facing lower portion132of metal cover133.

A description is now given of the functions and effects of power reception apparatus4configured in the above-described manner. As shown inFIG. 34, in the state where power reception apparatus4and power transmission apparatus9are positionally aligned, electric power is transmitted from power transmission apparatus9to power reception apparatus4.

Magnetic flux flows from power transmission apparatus9to power reception apparatus4, and a part of the magnetic flux enters metal cover71B and metal cover133. Since metal cover71B is located closer to the ground than metal cover133, the amount of magnetic flux entering lower portion72B is larger than the amount of magnetic flux entering metal cover133.

Therefore, more eddy current is generated in lower portion72B than eddy current formed in the surface of metal cover133. As a result, the amount of magnetic flux flowing through a portion of power reception coil7located on the side of metal cover71B is smaller than the amount of magnetic flux flowing through a portion of power reception coil7located on the side of metal cover133.

FIG. 37is a cross-sectional view showing a state where power transmission apparatus9is positionally displaced in the left direction L from power reception apparatus4. As shown inFIG. 37, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10increases, and the amount of magnetic flux flowing between the left side portion of power reception coil7and the left side portion of power transmission coil10decreases. The amount of magnetic flux entering metal cover133also decreases and eddy current formed in the surface of metal cover133also decreases. Accordingly, the influence of the eddy current decreases.

Meanwhile, the distance between the right side portion of power reception coil7and the right side portion of power transmission coil10decreases. Therefore, the amount of magnetic flux flowing through the right side portion of power reception coil7and the right side portion of power transmission coil10is to increase. Accordingly, the amount of magnetic flux entering metal cover71B increases and more eddy current is formed in the surface of metal cover71B. The increased eddy current restricts the amount of magnetic flux flowing toward the right side portion of power reception coil7. As a result, the amount of magnetic flux flowing from the right side portion of power transmission coil10toward power reception coil7merely slightly increases.

Specifically, inFIGS. 35 and 36, the portions of power reception coil7that are influenced greatly by the eddy current formed in metal cover71B are facing portions90A,110A.

Meanwhile, the number of turns of the portions where facing portions90A,110A are located is larger than the number of turns of the remaining portion. Therefore, a slight increase of the magnetic flux passing through facing portions90A,110A causes an increase, to a certain extent, of a counter-electromotive force generated at the portions.

As a result, even when power transmission apparatus9is positionally displaced in the left direction L, a large difference of the coupling coefficient between power reception coil7and power transmission coil10is suppressed between the state where power transmission apparatus9is displaced in the left direction L and the state where these coils are positionally aligned.

FIG. 38is a cross-sectional view showing a state where power transmission apparatus9is positionally displaced in the right direction R. As shown inFIG. 38, as power transmission apparatus9is positionally displaced in the right direction R, the distance between the right side portion of power reception coil7and the right side portion of power transmission coil10increases. Accordingly, the amount of magnetic flux flowing between the right side portion of power reception coil7and the right side portion of power transmission coil10decreases, the amount of magnetic flux entering metal cover71B also decreases, and the influence of eddy current formed in the surface of metal cover71also decreases.

Meanwhile, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10decreases, and the amount of magnetic flux flowing through the left side portion of power reception coil7and the left side portion of power transmission coil10is to increase.

Therefore, the amount of magnetic flux entering metal cover133also increases and eddy current formed in the surface of metal cover133also increases. As the eddy current increases, the influence on the magnetic flux flowing from the left side portion of power transmission coil10toward the left side portion of power reception coil7increases.

Because of this, as compared with the case where on-board device130is absent, the amount of magnetic flux flowing through the left side portion of power reception coil7is smaller.

It is supposed here that the magnitude of positional displacement of power transmission apparatus9in the left direction L shown inFIG. 37is equal to the magnitude of positional displacement of power transmission apparatus9in the right direction R shown inFIG. 38.

Since metal cover133is located higher than metal cover71B, the amount of magnetic flux entering metal cover133inFIG. 38is smaller than the amount of magnetic flux entering metal cover71B inFIG. 37. Therefore, the amount of magnetic flux flowing through the left side portion of power reception coil7inFIG. 38is larger than the amount of magnetic flux flowing through the right side portion of power reception coil7inFIG. 37.

Thus, when power transmission apparatus9is positionally displaced in the right direction R, the amount of magnetic flux entering the left side portion of power reception coil7is decreased relative to that in the case where on-board device130is absent. Meanwhile, this decrease of the amount of magnetic flux is smaller than the decrease of the amount of magnetic flux flowing through the right side portion of power reception coil7inFIG. 37.

Here, the number of turns of the portions of power reception coil7where facing portions90B,110B are located is larger than the number of turns of low-turn-count portions92A,92B,103A,103B. Therefore, the increase to some extent of the amount of magnetic flux entering the left side portion of power reception coil7can cause a large increase of an electromotive voltage induced at facing portions90B,110B. Since the amount of magnetic flux flowing through the left side portion of power reception coil7inFIG. 38is larger than the amount of magnetic flux flowing through the right side portion of power reception coil7inFIG. 37, the number of turns of facing portions90B,110B is smaller than the number of turns of facing portions90A,110A.

Therefore, even when power transmission apparatus9is positionally displaced in the right direction R and the amount of magnetic flux passing through the right side portion of power reception coil7decreases, a large decrease of the coupling coefficient can be suppressed.

As a result, even when power transmission apparatus9is positionally displaced in the right direction R, a large difference of the coupling coefficient can be suppressed between the state where power transmission apparatus9and power reception apparatus4are positionally aligned and the state where power transmission apparatus9is positionally displaced as shown inFIG. 38.

As seen from the above, wireless charging system1in the fourth embodiment can reduce variation of the coupling coefficient between power reception coil7and power transmission coil10even when power transmission apparatus9is positionally displaced.

Fifth Embodiment

Regarding the foregoing embodiments, the description is given above of the case where a plurality of metal members are provided in the vicinity of power reception apparatus4and the metal members are the same type of metal members. The present disclosure, however, is also applicable to the case where the metal members are different types of metal members.

Any of the features shown inFIG. 39for example that is identical or substantially identical to the corresponding one shown inFIGS. 1 to 38is denoted by the same reference character, and the description thereof will not be repeated.

FIG. 39is a cross-sectional view showing a wireless charging system1according to a fifth embodiment. InFIG. 39, a metal cover71B of a muffler55is formed of a metal containing at least one of aluminum and copper as a main component.

An on-board device130B includes a metal cover133B, and metal cover133B includes an upper portion131B located higher than a power reception coil7and a lower portion132B located closer to the ground than upper portion131B. Lower portion132B includes a portion horizontally located at the same position as power reception coil7, and a portion located lower than power reception coil7.

The magnetic permeability of a metal of metal cover133B is higher than the magnetic permeability of aluminum. Specifically, metal cover133B is formed of a metal containing at least one of stainless steel and iron as a main component, for example.

Namely, in metal cover71B, eddy current is likely to be formed due to magnetic flux entering metal cover71B. Magnetic flux entering metal cover133B flows well in metal cover133B.

FIG. 40is a plan view showing a lower coil66and a ferrite core64. As shown inFIG. 40, lower coil66is formed by winding a coil wire82from an end80toward an end81to surround winding axis O2. Specifically, coil wire82is wound to successively pass across division core members75A,75B,75C,75D,75E,75F,75G,75H,75I,75J,75K, and75L.

Coil wire82of lower coil66is drawn from the gap between division core member75G and division core member75H to the inside of a metal support member65. Then, coil wire82is drawn out from the gap between division core member75I and division core member75J, and thereafter extends in the direction of winding across the division core members as described above. The gap between division core member75I and division core member75J is located downstream, in the winding direction of coil wire82, of the gap between division core member75G and division core member75H.

The part of coil wire82located downstream, in the winding direction, of the gap between division core member75D and division core member75E, is drawn from the gap between division core member75D and division core member75E to the inside of metal support member65. Then, coil wire82is drawn out from the gap between division core member75A and division core member75B. After this, coil wire82is wound in the aforementioned winding direction. The gap between division core member75A and division core member75B is located upstream, in the winding direction, of the gap between division core member75D and division core member75E.

Coil wire82is wound in this way. Accordingly, lower coil66includes a high-turn-count portion93A which is located on the lower surface of division core members75B,75C,75D, a low-turn-count portion92A located on the lower surface of division core members75E,75F,75G, a low-turn-count portion92C located on the lower surface of division core members75H,75I, and a low-turn-count portion92B which is located on the lower surface of division core members75J,75K,75L,75A.

Here, the number of turns of high-turn-count portion93A is larger than the number of turns of low-turn-count portions92A,92B,92C, and the number of turns of low-turn-count portions92A,92B is larger than the number of turns of low-turn-count portion92C.

Low-turn-count portion92C is located in a portion of lower coil66facing lower portion132B of metal cover133B. High-turn-count portion93A is located in a portion of lower coil66facing lower portion72B of metal cover71B, namely a facing portion90A.

FIG. 41is a plan view showing an upper coil67. Upper coil67is formed similarly to lower coil66.

Upper coil67includes a high-turn-count portion104A located on the lower surface of division core members75B,75C,75D, a low-turn-count portion103A located on the lower surface of division core members75E,75F,75G, a low-turn-count portion103C located on the lower surface of division core members75H,75I, and a low-turn-count portion103B located in the lower surface of division core members75J,75K,75L,75A.

The number of turns of high-turn-count portion104A is larger than the number of turns of low-turn-count portions103A,103B,103C, and the number of turns of low-turn-count portions103A,103B is larger than the number of turns of low-turn-count portion103C.

Low-turn-count portion103C is located in a portion of upper coil67, namely a facing portion110B facing lower portion132B of metal cover133B. High-turn-count portion104A is located in a portion of upper coil67, namely a facing portion110A facing lower portion72B of metal cover71B.

Thus, power reception coil7includes low-turn-count portions92C,103C formed in the portion facing lower portion132B of metal cover133B, and high-turn-count portions93A,104A formed in the portion facing lower portion72B of metal cover71B.

As shown inFIG. 39, when electric power is transmitted from power transmission apparatus9to power reception apparatus4with power reception apparatus4and power transmission apparatus9positionally aligned with each other, magnetic flux reaches lower portion72B of metal cover71B. Since metal cover71B is formed of a metal containing at least one of aluminum and copper as a main component, the magnetic flux reaching metal cover71B is reflected by metal cover71B.

Due to the reflected magnetic flux, the amount of magnetic flux flowing from power transmission coil10toward the right side portion of power reception coil7decreases.

Meanwhile, magnetic flux enters metal cover133B of on-board device130B. Since metal cover133B is formed of a metal containing at least one of iron and stainless steel as a main component, the magnetic flux entering metal cover133B flows well in metal cover133B. A part of the magnetic flux flowing in metal cover133B enters the left side portion of power reception coil7.

FIG. 42is a cross-sectional view showing a state where power transmission apparatus9is positionally displaced in the left direction L. InFIG. 42, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10increases. Therefore, the amount of magnetic flux entering metal cover133B decreases. The amount of magnetic flux flowing directly from the left side portion of power transmission coil10toward the left side portion of power reception coil7also decreases.

As a result, almost no magnetic flux flows in metal cover133B toward power reception coil7.

The distance between the right side portion of power reception coil7and the right side portion of power transmission coil10decreases. Therefore, the amount of magnetic flux reaching metal cover71B increases and the amount of magnetic flux reflected by metal cover71B also increases. Thus, the amount of magnetic flux which is hindered from flowing by the reflected magnetic flux increases.

As a result, even when the distance between the right side portion of power transmission coil10and the right side portion of power reception coil7decreases, the amount of magnetic flux flowing through the right side portion of power reception coil7merely slightly increases, relative to the amount of magnetic flux flowing through the right side portion of power reception coil7in the state where power reception apparatus4and power transmission apparatus9are positionally aligned.

Meanwhile, as shown inFIGS. 40 and 41, the number of turns of facing portions90A,110A of power reception coil7facing lower portion72B of metal cover71B is large, and therefore, even when magnetic flux slightly increases, a large electromotive force can be generated.

Therefore, even when power transmission apparatus9is positionally displaced in the left direction L, it can be suppressed that the coupling coefficient between power reception coil7and power transmission coil10largely varies from the coupling coefficient in the state where power reception apparatus4and power transmission apparatus9are positionally aligned.

FIG. 43is a cross-sectional view showing a state where power transmission apparatus9is positionally displaced in the right direction R. As shown inFIG. 43, when power transmission apparatus9is positionally displaced in the right direction R, the distance between the left side portion of power reception coil7and the left side portion of power transmission coil10decreases. As a result, the amount of magnetic flux guided by metal cover133B to power reception coil7increases.

Meanwhile, as shown inFIGS. 40 and 41, the number turns of facing portions90B,110B of power reception coil7is small, and therefore, even when the amount of magnetic flux guided by metal cover133B increases, a large increase of the electromotive force generated at facing portions90B,110B can be suppressed.

As a result, it can be suppressed that the coupling coefficient in the case of positional displacement as shown inFIG. 43largely varies from the coupling coefficient in the case where power reception apparatus4and power transmission apparatus9are positionally aligned.

Regarding the first to fifth embodiments, the description is given above of the system in which power reception coil7and power transmission coil10are formed so that power reception coil7is located within power transmission coil10in plan view of power transmission coil10and power reception coil7as seen from below power transmission coil10and power reception coil7, in the state where power transmission coil10and power reception coil7are positionally aligned. The present disclosure, however, is not limited to the system as described above.

FIG. 44is a plan view showing a modification of wireless charging system1. In the example shown inFIG. 44, power transmission coil10and power reception coil7are positionally aligned with each other. In plan view of power transmission coil10and power reception coil7as seen from below power transmission coil10and power reception coil7, power transmission coil10is formed to be located within power reception coil7.

Metal cover71of muffler55is formed of a metal containing at least one of iron and stainless steel as a main component, similarly to the first embodiment.

Power reception coil7includes a facing portion120facing muffler55, and the number of turns of coil wire in facing portion120is smaller than the number of turns in the remaining portion.

FIG. 45is a plan view showing a state where power transmission coil10is positionally displaced to be closer to muffler55. As shown inFIG. 45, as power transmission coil10is positionally displaced, the distance between power transmission coil10and an opposite portion121increases, opposite portion121being a portion of power reception coil7and located opposite to muffler55. As a result, the amount of magnetic flux passing through opposite portion121of power reception coil7decreases.

Meanwhile, the distance between facing portion120of power reception coil7and a portion of power transmission coil10that is located on the side of muffler55decreases.

The amount of magnetic flux flowing through the portion of power transmission coil10that is located on the side of muffler55and facing portion120of power reception coil7increases, and the amount of magnetic flux entering metal cover71increases.

Further, the amount of magnetic flux guided by metal cover71to facing portion120of power reception coil7also increases.

Here, the number of turns of the coil wire in facing portion120is small. Therefore, even when the amount of magnetic flux passing through facing portion120is excessively large, increase of an electromotive voltage induced at facing portion120can be suppressed.

Therefore, even when power transmission coil10is positionally displaced toward muffler55, variation of the coupling coefficient between power transmission coil10and power reception coil7is reduced.

If power transmission coil10is positionally displaced away from muffler55, the distance between power transmission coil10and opposite portion121of power reception coil7decreases, and the amount of magnetic flux passing through opposite portion121increases. Meanwhile, the distance between facing portion120of power reception coil7and transmission coil10increases, and the amount of magnetic flux passing through facing portion120of power reception coil7decreases.

Therefore, even when power transmission coil10is positionally displaced away from muffler55, decrease of the coupling coefficient between power transmission coil10and power reception coil7can be suppressed.

According to the description of the example shown inFIGS. 39 and 40, metal cover71is formed of a metal containing at least one of aluminum and copper as a main component.

In the case where metal cover71is formed of a metal such as iron and stainless steel having a magnetic permeability equal to or more than that of aluminum, the number of turns of facing portion120of power reception coil7is made smaller than that of the remaining portion.

Thus, the embodiments are applicable to systems in which various coil shapes are employed.

Although the embodiments have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation.