JP5612956B2 - Non-contact power transmission device - Google Patents

Description

  The present invention relates to a non-contact power transmission device that transmits power from a primary circuit to a secondary circuit by electromagnetic induction.

  As a conventional non-contact power transmission apparatus, for example, there is a wireless transmission apparatus (indicated by reference numeral 901 in the figure) disclosed in Patent Document 1 and shown in FIG. The wireless transmission device 901 includes a primary side circuit device 930 and a secondary side circuit device 950. The primary side circuit device 930 includes a resonance capacitor 951 connected to one end of the power induction coil 903 and one output end of the power source 902. The secondary circuit device 950 includes a resonance capacitor 952 connected in parallel to the power receiving coil 904.

In the primary side circuit device 930, the following equation (i) is satisfied, and in the secondary side circuit device 950, the parameters of the respective circuit components are set so as to satisfy the following equation (ii). A resonance state was generated in the device, and power was supplied to the secondary circuit device 950.
ωL ₁ −1 / (ωC ₁ ) = 0 (ie, f = 1 / (2π√ (L ₁ C ₁ )) (i)
ωL ₂ −1 / (ωC ₂ ) = 0 (that is, f = 1 / (2π√ (L ₂ C ₂ )) (ii)
Where ω = 2πf, L ₁ is the self-inductance of the power induction coil 903, C ₁ is the capacitance of the resonant capacitor 951, L ₂ is the self-inductance of the power receiving coil 904, and C ₂ is the capacitance of the resonant capacitor 952. f is a power supply frequency.

Japanese Patent Laid-Open No. 10-322247

However, in the secondary side circuit device 950, a load composed of the rectifier circuit 905, the smoothing capacitor 913, the secondary side load 906, and the like is connected in series to the power receiving coil 904. In the above equation (ii), Since the impedance of these loads is not taken into consideration, even if the self-inductance L ₂ of the power receiving coil 904 and the capacitance C ₂ of the resonant capacitor 952 are set so as to satisfy this equation (ii), the secondary side There is a problem that the circuit device 950 does not resonate as a whole, that is, reactance of the power receiving coil 904 remains, so that impedance cannot be matched and power transmission efficiency is reduced.

  The present invention aims to solve the above problems. That is, an object of the present invention is to provide a non-contact power transmission device capable of increasing the power transmission efficiency by bringing the secondary side circuit into a resonance state.

In order to achieve the above object, the invention described in claim 1 receives a primary side circuit unit having a power transmission coil for transmitting power from an AC power source by electromagnetic induction and the power transmitted from the power transmission coil. A non-contact power transmission device having a power receiving coil and a secondary circuit unit having a load connected in series with the power receiving coil, wherein the secondary circuit unit exists in parallel with the power receiving coil. A capacitance C _P2 , wherein the capacitance C _P2 is due to a parasitic capacitance of the power receiving coil, and the capacitance C _P2 satisfies the following formula (A): This is a non-contact power transmission device.
C _P2 = (Z ₀ ± √ (Z ₀ ² -4ω ² L ₂ ² )) / 2ω ² Z ₀ L ₂ (A)
However, Z ₀ ≧ 2ωL ₂ , ω = 2πf, Z ₀ is the impedance [Ω] of the load, ω is the angular velocity [rad / sec], L ₂ is the self-inductance [H] of the power receiving coil, and f is the AC power source. [Hz].

The invention described in claim 2 is the invention described in claim 1 , wherein a winding ratio n between the number of turns of the power transmission coil and the number of turns of the power receiving coil satisfies the following formula (B). It is characterized by.
n = N ₁ / N ₂ ≧ 1 (B)
N ₁ is the number of turns of the power transmission coil [times], and N ₂ is the number of turns of the power receiving coil [ times].

According to the first aspect of the present invention, the secondary circuit unit has the capacitance C _P2 existing in parallel with the power receiving coil, and the capacitance C _P2 is _expressed by the above formula (A). Therefore, the entire secondary side circuit unit including the load connected in series with the power receiving coil can be brought into a resonance state, so that the reactance of the power receiving coil can be canceled and the impedance can be matched and the power can be reduced. Transmission efficiency can be increased.

Further, according to the invention described in claim 1, the capacitance C _P2 is, since due to the parasitic capacitance of the power receiving coil, without additional components, increasing the power transmission efficiency with a simple structure be able to.

According to the invention described in claim 2 , since the turn ratio between the number of turns of the power transmission coil and the number of turns of the power receiving coil satisfies the above formula (B), impedance matching is achieved by adjusting the turn ratio. This can be easily performed, and the number of turns of the power receiving coil can be reduced to reduce the weight of the secondary circuit unit.

It is a circuit diagram of one embodiment of a non-contact power transmission device concerning the present invention. It is a graph which shows the relationship between the winding number in a receiving coil used for the non-contact electric power transmission apparatus of FIG. 1, and a self-inductance. It is a circuit diagram which shows the structure of the modification of the non-contact electric power transmission apparatus of FIG. FIG. 2 is a circuit diagram showing a configuration in which a resonance capacitor included in a secondary circuit unit is connected in series between a power receiving coil and a load in the non-contact power transmission apparatus of FIG. 1. In the non-contact power transmission apparatus of FIG. 1, the resonance capacitor provided in the primary circuit unit is connected in parallel with the AC power supply, and the resonance capacitor provided in the secondary circuit unit is connected in series between the power receiving coil and the load. It is a circuit diagram which shows the structure connected to. It is a graph which shows the relationship between the space | interval of a power transmission coil and a receiving coil, and power transmission efficiency in the some non-contact electric power transmission apparatus from which the connection form of a resonant capacitor differs. It is a graph which shows the relationship between the space | interval of a power transmission coil and a receiving coil, and power transmission efficiency in the some non-contact electric power transmission apparatus from which the winding number of a coil differs. It is a circuit diagram of the conventional non-contact electric power transmission apparatus.

  Hereinafter, an embodiment of the non-contact power transmission apparatus of the present invention will be described with reference to FIGS.

  As shown in FIG. 1, the non-contact power transmission device 1 of the present invention includes a primary circuit unit 10 and a secondary circuit unit 20.

The primary circuit unit 10 includes an AC power supply 11 that outputs an AC voltage V ₁ [V] (that is, AC power) having a frequency f [Hz], a power transmission coil 12 that is connected in series to the AC power supply 11, and an AC power supply. 11 and a primary side resonance capacitor 13 inserted in series between the power transmission coil 12 and the power transmission coil 12.

The power transmission coil 12 is, for example, a planar uniform spiral coil in which fine wires made of copper are densely wound in an annular shape. Transmitting coil 12 includes an internal resistance 15 of the resistance R _1. When the AC voltage V ₁ is applied from the AC power supply 11, the power transmission coil 12 transmits power to the power receiving coil 22 described later by electromagnetic induction. For example, a plastic film type or ceramic type capacitor is used as the primary side resonance capacitor 13. In addition, the primary side resonance capacitor 13 may be connected in parallel to the AC power supply 11, or may be configured such that the primary side resonance capacitor is not provided.

The self-inductance L ₁ of the power transmission coil 12 and the capacitance C _S1 of the primary side resonance capacitor 13 are set so as to satisfy the following expression (1).

Thus, by setting the self-inductance L ₁ of the power transmission coil 12 and the capacitance C _S1 of the primary-side resonance capacitor 13, the primary-side circuit unit 10 can be brought into a resonance state.

  The secondary circuit unit 20 includes a power receiving coil 22, a load 24 connected in series to the power receiving coil 22, and a secondary side resonance capacitor 23 connected in parallel to the power receiving coil 22.

The power receiving coil 22 is, for example, a planar uniform spiral coil having the same configuration as that of the power transmitting coil 12 described above, in which fine wires made of copper are densely wound in a ring shape. Of course, the power receiving coil 22 may have a configuration (material, shape, number of turns, etc.) different from that of the power transmitting coil 12, and in particular, the number of turns N ₂ of the power receiving coil 22 is less than or equal to the number of turns N ₁ of the power transmitting coil (that is, It is desirable that the winding ratio n = N ₁ / N ₂ ≧ 1). Receiving coil 22 includes an internal resistor 25 of the resistance R _2. A mutual inductance M exists between the power receiving coil 22 and the power transmitting coil 12. The power receiving coil 22 is arranged opposite to the power transmission coil 12 to receive an electromagnetic induction effect and receive power transmitted from the power transmission coil 12 (that is, an induced voltage and an induced current are induced). In the present embodiment, the power transmission coil 12 and the power reception coil 22 are planar flat spiral coils. However, the present invention is not limited to this, and the power transmission coil 12 and the power reception coil are not limited to the object of the present invention. The configuration of the coil 22 is arbitrary.

As the secondary side resonance capacitor 23, for example, a plastic film type or ceramic type capacitor is used similarly to the above-described primary side resonance capacitor 13. The load 24 has an impedance Z ₀ set to 50Ω, for example.

  Here, in FIG. 1, when the impedance of the power receiving coil 22 viewed from the secondary resonance capacitor 23 and the load 24 side is Z, the impedance Z is expressed by the following equation (2).

  As shown in the equation (2), the impedance Z is generally given by the sum of the real term and the imaginary term (Z = Re [Z] + jIm [Z]), and the imaginary term (Im) included in the impedance Z [Z]) is set to 0 and the impedance Z is set to only a real term (Re [Z]), thereby reducing the impedance value and reducing the power loss. From this equation (2), the imaginary term Im [Z] of the impedance Z is expressed by the following equation (3).

Then, an equation is established in which the sum of the imaginary term Im [Z] and the reactance ωL ₂ of the receiving coil 22 becomes zero. This equation is expressed by the following equation (4).

Then, when this equation (4) is solved for the capacitance C _P2 of the secondary side resonance capacitor 23, the following equation (5) is obtained.

However, Z ₀ ≧ 2ωL ₂ , ω = 2πf, Z ₀ is the impedance [Ω] of the load 24, ω is the angular velocity [rad / sec], L ₂ is the inductance [H] of the receiving coil 22, and f is the AC power source 11. The frequency of the AC voltage V ₁ is [Hz].

Then, by setting the self-inductance L ₂ of the receiving coil 22, the capacitance C _P2 of the secondary resonance capacitor 23, and the impedance Z _{0 of the} load 24 so as to satisfy this equation (5), the secondary side The circuit unit 20 enters a resonance state, and the reactance ωL ₂ and the imaginary term Im [Z] cancel each other.

  Next, a method for setting values (parameters) of each circuit component of the secondary circuit unit 20 will be described.

The impedance Z ₀ of the load 24 of the secondary circuit unit 20 of the non-contact power transmission apparatus 1 described above is 50Ω, and the frequency of the AC voltage V ₁ of the AC power supply 11 is 100 kHz. At this time, from the proviso of the above formula (5), the self-inductance L ₂ of the power receiving coil 22 is expressed by the following formula: L ₂ ≦ Z ₀ / 2ω
If the above value is substituted into this equation, the self-inductance L ₂ of the power receiving coil 22 becomes a value less than 40 μH. The self-inductance L ₂ is preferably a large value in order to increase the transmission distance. Then, when the calculated self-inductance L ₂ is applied to a graph showing the relationship between the number of turns of the coil and the self-inductance shown in FIG. 2, it can be seen that the number of turns of the power receiving coil 22 is three. Then, the self-inductance L ₂ of the receiving coil 22 having three turns is obtained from the graph of FIG. 2, and the f-Z ₀ , L ₂ and the above equation (5) are used to determine the secondary resonance capacitor 23. The capacitance C _P2 is further obtained, and these values are set for each circuit component.

As described above, according to the present invention, the secondary side circuit unit 20 has the electrostatic capacity C _P2 present in parallel with the power receiving coil 22, and the electrostatic capacity C _P2 is _expressed by the above equation (5). As a result, the entire secondary side circuit unit 20 including the load 24 connected in series to the power receiving coil 22 can be brought into a resonance state, so that the reactance ωL ₂ of the power receiving coil 22 can be canceled and the impedance can be reduced. The power transmission efficiency can be increased by matching.

Further, since the capacitance C _P2 is due to the secondary resonance capacitor 23 connected in parallel to the power receiving coil 22, the capacitance C _P2 satisfying the above equation (5) can be easily set. .

Further, since the winding ratio n between the winding number N ₁ of the power transmission coil 12 and the winding number N ₂ of the power receiving coil 22 is 1 or more (that is, n = N ₁ / N ₂ ≧ 1), the winding ratio is adjusted. As a result, impedance matching can be easily performed, and the number of turns of the power receiving coil 22 can be reduced to reduce the weight of the secondary circuit unit 20.

According to the present embodiment described above, the capacitance C _P2 is due to the secondary resonance capacitor 23 connected in parallel to the power receiving coil 22, but the present invention is not limited to this. For example, instead of the secondary side resonance capacitor 23 in the above configuration, the capacitance C _P2 may be due to the parasitic capacitance of the power receiving coil 22, and in this way, without adding components, The power transmission efficiency can be increased with a simple configuration.

  Further, according to the present embodiment, the power transmission coil 12 of the primary side circuit unit 10 and the power reception coil 22 of the secondary side circuit unit 20 are arranged to face each other directly, thereby transmitting power. The present invention is not limited to this. For example, the non-contact power transmission device 2 having the configuration shown in FIG. 3 may be used.

  The non-contact power transmission device 2 includes a first circuit device 31 having a first circuit unit 40 and a relay circuit unit 60, and a second circuit device 32 having a second circuit unit 50.

The first circuit unit 40 includes a first coil 42, which is a planar uniform helical coil including an internal resistor 45, a first load 44 connected in series to the first coil 42, and a parallel connection to the first coil 42. The first resonance capacitor 43 and an AC power supply (not shown) for applying an AC voltage to the first coil 42 are provided. In the first circuit unit 40, the self-inductance L ₁ of the first coil 42, the capacitance C _P1 of the first resonance capacitor 43, and the impedance Z ₁ of the first load 44 are set to values that satisfy the above formula (5). (However, in the formula (5), L ₂ is read as L ₁ , C _P2 is read as C _P1 , and Z ₀ is read as Z ₁ ). That is, the first circuit unit 40 is in a resonance state at the frequency f of the AC power supply.

The relay circuit unit 60 includes a relay coil 62 that is a planar uniform helical coil including an internal resistor 65, and a resonance capacitor 63 connected in series to the relay coil 62. In the relay circuit unit 60, the self-inductance L ₃ of the relay coil 62 and the capacitance C ₃ of the resonant capacitor 63 are set to values that satisfy the above formula (1) (however, in the formula (1), L ₁ Is replaced with L ₃ and C _S1 is replaced with C ₃ ). That is, the relay circuit unit 60 enters a resonance state at the frequency f of the AC power supply. Instead of the resonant capacitor 63, the parasitic capacitance of the relay coil 62 may be set to be the capacitance C _3.

Similar to the first circuit unit 40, the second circuit unit 50 includes a second coil 52 that is a planar uniform spiral coil including an internal resistance 55, and a second load 54 connected in series to the second coil 52. The second resonance capacitor 53 connected in parallel to the second coil 52 and an AC power source (not shown) for applying an AC voltage to the second coil 52 are provided. In the second circuit unit 50, the self-inductance L ₂ of the second coil 52, the capacitance C _P2 of the second resonance capacitor 53, and the impedance Z ₀ of the second load 54 are set to values that satisfy the above equation (5). ing. That is, the second circuit unit 50 is in a resonance state at the frequency f of the AC power supply.

  The first coil 42, the second coil 52, and the relay coil 62 are arranged so as to be coaxially overlapped with each other and the relay coil 62 is sandwiched between the first coil 42 and the second coil 52. When power is transmitted from the first circuit device 31 to the second circuit device 32, the first coil 42 functions as a power transmission coil and the second coil 52 functions as a power reception coil. Further, when power is transmitted from the second circuit device 32 to the first circuit device 31, the first coil 42 functions as a power receiving coil, and the second coil 52 functions as a power transmitting coil.

As described above, the frequency f of the AC power source including the relay coil 62 disposed so as to be sandwiched between the first coil 42 and the second coil 52 and the resonant capacitor 63 connected in series to the relay coil 62. By providing the relay circuit unit 60 that is in a resonance state, the power transmitted between the first coil 42 and the second coil 52 is relayed by the relay coil 62 included in the relay circuit unit 60, and the first coil The transmission distance between 42 and the second coil 52 can be extended. Further, by increasing the number of windings of the relay coil 62, the mutual inductance M _13, M ₂₃ between the coils is increased, thereby further extend the transmission distance. Further, both the first circuit unit 40 and the second circuit unit 50 are provided with an AC power source, and bidirectional power transmission can be performed. In addition, by impedance Z ₀ of the impedance Z1 and the second load 54 of the first load 44 is adjusted to 50Ω respectively, typical 50Ω line can be used, impedance matching can be easily.

(Verification 1)
The inventor verified the relationship between the connection form of each resonance capacitor provided in the primary side circuit unit 10 and the secondary side circuit unit 20 of the contactless power transmission device 1 and the power transmission efficiency.

Example 1
In the non-contact power transmission apparatus 1 of FIG. 1, the primary circuit unit 10 is composed of a copper wire having a diameter of 0.5 mm, the number of windings is 10 times, and the average side length (that is, the length of one side of the outer edge) An average side length Davg of Dout and the side length Din of the inner edge Dav; Davg = (Dout + Din) / 2) is a square annular closely wound flat plate-shaped uniform spiral coil of 5 cm, and an AC output from the AC power supply 11 is used. The frequency f of the voltage V1 was set to 100 kHz, and the respective values were set so that the self-inductance L ₁ of the power transmission coil 12 and the capacitance C _P1 of the primary side resonance capacitor 13 satisfied the above formula (1). In the secondary side circuit unit 20, a flat plate-shaped uniform coil having the same configuration as that of the power transmission coil 12 is used as the power reception coil 22, and the self-inductance L ₂ of the power reception coil 22 and the static resonance of the secondary side resonance capacitor 23 are used. The values were set so that the capacitance C _P2 and the impedance Z _{0 of the} load 24 satisfied the above formula (5). In the circuit configuration of FIG. 1, the resonant capacitor is provided in series on the primary side and in parallel on the secondary side, and is hereinafter referred to as “SP resonant circuit”.

(Example 2)
In Example 1, it was set as the same structure as Example 1 except having changed the average one side length of the power transmission coil 12 and the receiving coil 22 into 10 cm.

(Comparative Example 1)
A contactless power transmission device 4 shown in FIG. 4 is arranged between a power receiving coil 22 and a load 24 instead of the secondary resonance capacitor 23 provided in the secondary circuit unit 20 of the contactless power transmission device 1 of FIG. A secondary resonance capacitor 27 is provided in series. Other than this, the circuit configuration is the same as in FIG. 1, and the same members are denoted by the same reference numerals. In the circuit configuration of FIG. 4, the resonant capacitors are provided in series on the primary side and in series on the secondary side, and are hereinafter referred to as “SS resonant circuit”. In this SS resonance circuit, the primary side circuit unit 10 has the same configuration as that of the first embodiment, and the secondary side circuit unit 20 has the same configuration of the power receiving coil 22 and the load 24 as those of the first embodiment. The respective values were set so that the self-inductance L ₂ and the capacitance C _S2 of the secondary resonance capacitor 27 satisfy the following expression (6).

(Comparative Example 2)
In the comparative example 1, it was set as the same structure as the comparative example 1 except having changed the average one side length of the power transmission coil 12 and the receiving coil 22 into 10 cm.

(Comparative Example 3)
A contactless power transmission device 5 shown in FIG. 5 is replaced with a primary side resonance capacitor 13 provided in the primary side circuit unit 10 of the contactless power transmission device 1 of FIG. In place of the secondary side resonance capacitor 23 provided with the capacitor 17 and included in the secondary side circuit unit 20 of the non-contact power transmission device 1 of FIG. 1, it is inserted in series between the power receiving coil 22 and the load 24. A secondary resonance capacitor 27 is provided. Other than this, the circuit configuration is the same as in FIG. 1, and the same members are denoted by the same reference numerals. In the circuit configuration of FIG. 5, the resonant capacitor is provided in parallel on the primary side and in series on the secondary side, and is hereinafter referred to as “PS resonant circuit”. In this PS resonance circuit, the AC power supply 11 and the power transmission coil 12 of the primary side circuit unit 10 have the same configuration as in the first embodiment, and the self-inductance L ₂ of the power transmission coil 12 and the capacitance C _P1 of the primary side resonance capacitor 17 However, the respective values are set so as to satisfy the following formula (7), and the power receiving coil 22 and the load 24 of the secondary side circuit unit 20 have the same configuration as that of the first embodiment, and the self inductance of the power receiving coil 22 is set. The respective values were set so that L ₂ and the capacitance C _S2 of the secondary resonance capacitor 27 satisfy the above formula (6).

(Comparative Example 4)
In the comparative example 3, it was set as the same structure as the comparative example 3 except having changed the average one side length of the power transmission coil 12 and the receiving coil 22 into 10 cm.

  About Example 1, 2 mentioned above, and Comparative Examples 1-4, a wattmeter is connected to each of the primary side circuit unit 10 and the secondary side circuit unit 20, and the power transmission coil 12 and the receiving coil 22 are concentrically and densely. The power in each circuit unit when the interval (ie, transmission distance) was gradually increased from the overlapped state (ie, interval 0) was measured, and the power transmission efficiency was calculated from the ratio of the measured power. The result is shown in FIG.

  From the graph shown in FIG. 6, the SP resonance circuit in which the secondary side resonance capacitor 23 is connected in parallel to the power receiving coil 22 in the secondary side circuit unit 20 has the secondary side resonance capacitor connected in series to the power receiving coil 22. It was found that the power transmission efficiency was higher than that of the SS resonance circuit and the PS resonance circuit (Examples 1 and 2 and Comparative Examples 1 to 4). In addition, since the graphs of Comparative Example 1 and Comparative Example 3 and Comparative Example 2 and Comparative Example 4 are respectively overlapped, for the primary side circuit unit 10, the primary side resonance capacitor is connected either in series or in parallel. Even if it provided, power transmission efficiency did not change (Comparative Examples 1 to 4). It was also found that the power transmission efficiency can be maintained high when the one side length of each coil is large even when the interval between the coils is widened (Examples 1 and 2). From these results, the effect of the present invention could be confirmed.

(Verification 2)
In addition, the inventor has verified the relationship between the number of turns of the power transmission coil 12 and the power reception coil 22 and the power transmission efficiency.

  In the contactless power transmission device 1 described above, in a configuration in which the number of turns of the power transmission coil 12 and the power reception coil 22 is different, the result of measuring the power transmission efficiency by changing the interval between the coils (that is, the transmission distance) is shown. As shown in FIG.

P1 in the graph of FIG. 7, the number of turns N ₁ to 10 times of the transmitting coil 12, the number of turns N ₂ three times of the power receiving coil 22, indicates when the impedance of the load 24 was 50 [Omega, P2 is the power transmission coil 12 The number of turns N ₁ of the power receiving coil 22 is 3 times, the number of turns N ₂ of the power receiving coil 22 is 3 times, and the impedance of the load 24 is 50Ω. P3 is the number of turns N ₁ of the power transmitting coil 12 15 times. 22 shows the case where the number of turns N ₂ is 3 times and the impedance of the load 24 is 50Ω. P4 is the number of turns N ₁ of the power transmission coil 12 is 10 times and the number of turns N ₂ of the power receiving coil 22 is 10 times. The case where the impedance of 24 is set to 350Ω is shown. In each configuration, the average side length of the power transmission coil 12 and the power reception coil 22 was 50 cm.

From the graph of FIG. 7, if the number of turns N ₂ are the same on the receiving coil 22, it has been found that can be the more number of turns N1 of the power transmission coil 12 increases the power transmission efficiency (P1 to P3). Further, it has been found that even when the impedance Z _{0 of the} load 24 is high, the power transmission efficiency can be increased by increasing the number of turns of each of the power transmission coil 12 and the power reception coil 22 (P2, P4).

  In addition, embodiment mentioned above only showed the typical form of this invention, and this invention is not limited to embodiment. That is, various modifications can be made without departing from the scope of the present invention.

DESCRIPTION OF SYMBOLS 1, 2 Non-contact electric power transmission apparatus 10 Primary side circuit unit 11 AC power supply 12 Power transmission coil 13 Primary side resonance capacitor 20 Secondary side circuit unit 22 Power receiving coil 23 Secondary side resonance capacitor (capacitor)
24 Load

Claims (2)

A secondary circuit unit having a primary circuit unit having a power transmission coil for transmitting power from an AC power supply by electromagnetic induction, a power reception coil for receiving the power transmitted from the power transmission coil, and a load connected in series to the power reception coil In a non-contact power transmission device having a side circuit unit,
The secondary side circuit unit has a capacitance C _P2 that exists in parallel with the power receiving coil,
The capacitance C _P2 is due to the parasitic capacitance of the receiving coil; and
The non-contact power transmission device, wherein the capacitance C _P2 satisfies the following formula.
C _P2 = (Z ₀ ± √ (Z ₀ ² -4ω ² L ₂ ² )) / 2ω ² Z ₀ L ₂
However, Z ₀ ≧ 2ωL ₂ , ω = 2πf, Z ₀ is the impedance [Ω] of the load, ω is the angular velocity [rad / sec], L ₂ is the self-inductance [H] of the power receiving coil, and f is the AC power source. [Hz]. The contactless power transmission device according to claim 1, wherein a winding ratio n between the number of turns of the power transmission coil and the number of turns of the power receiving coil satisfies the following expression.
n = N ₁ / N ₂ ≧ 1
N ₁ is the number of turns of the power transmission coil [times], and N ₂ is the number of turns of the power receiving coil [ times].

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