Non-contact power transmission apparatus and method for designing non-contact power transmission apparatus

A non-contact power transmission apparatus having a resonance system is disclosed. The resonance system includes a primary coil to which an alternating-current voltage from an alternating-current source is applied, a primary-side resonance coil, a secondary-side resonance coil, and a secondary coil to which a load is connected. The impedance of the primary coil is set such that the output impedance of the alternating-current source and the input impedance of the resonance system are matched to each other.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2008-283563 filed Nov. 4, 2008.

BACKGROUND

The present invention relates to a non-contact power transmission apparatus and a method for designing a non-contact power transmission apparatus.

FIG. 6schematically shows a non-contact power transmission apparatus that transmits power from a first copper coil51to a second copper coil52placed at a distance from the first copper coil51by means of resonance of electromagnetic fields. Such apparatuses are disclosed, for example, in NIKKEI ELECTRONICS 2007.12.3 Issue, pages 117 to 128, and International Patent Publication WO/2007/008646. InFIG. 6, a magnetic field generated at a primary coil54connected to an alternating-current source53is intensified by means of magnetic field resonance of the first and second copper coils51,52. The effect of electromagnetic induction of the intensified magnetic field around the second copper coil52generates power in a secondary coil55. The generated power is then supplied to a load56. It has been confirmed that, when first and second copper coils51,52having a diameter of 30 cm were placed 2 m away from each other, 60-watt light as the load56was turned on.

To effectively supply output power of the alternating-current source53to the load56in this non-contact power transmission apparatus, it is necessary to efficiently supply the output power of the alternating-current source53to a resonance system (the first and second copper coils51,52and the primary and secondary coils54,55). However, the above listed documents only schematically disclose non-contact power transmission apparatuses, but have no concrete description as to how to obtain a non-contact power transmission apparatus that satisfies such requirements.

SUMMARY

Accordingly, it is an objective of the present invention to provide a non-contact power transmission apparatus that efficiently supplies output power of an alternating-current source to a resonance system, and a method for designing such an apparatus.

To achieve the foregoing objective and in accordance with one aspect of the present invention, a non-contact power transmission apparatus comprising a resonance system is provided. The resonance system includes a primary coil to which an alternating-current voltage from an alternating-current source is applied, a primary-side resonance coil, a secondary-side resonance coil, and a secondary coil to which a load is connected. The impedance of the primary coil is set such that the output impedance of the alternating-current source and the input impedance of the resonance system are matched to each other.

In accordance with another aspect of the present invention, a method for designing a non-contact power transmission apparatus including a resonance system is provided. The resonance system includes a primary coil to which an alternating-current voltage from an alternating-current source is applied, a primary-side resonance coil, a secondary-side resonance coil, and a secondary coil to which a load is connected. The method includes: setting the frequency of the alternating-current voltage of the alternating-current source; and setting the impedance of the primary coil such that the output impedance of the alternating-current source and the input impedance of the resonance system are matched to each other at the set frequency.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A non-contact power transmission apparatus10according to one embodiment of the present invention will now be described with reference toFIGS. 1 to 4.

As shown inFIG. 1, the non-contact power transmission apparatus10includes a resonance system12, which transmits power supplied from an alternating-current source11to a load17without contact. The resonance system12includes a primary coil13connected to the alternating-current source11, a primary-side resonance coil14, a secondary-side resonance coil15, and a secondary coil16. The secondary coil16is connected to the load17. The alternating-current source11supplies alternating-current voltage to the primary coil13. The alternating-current source11may receive direct-current voltage supplied by a direct-current source, convert the direct-current voltage to an alternating-current voltage, and supply the alternating-current voltage to the primary coil13.

The non-contact power transmission apparatus10applies alternating-current voltage from the alternating-current source11to the primary coil13, thereby generating a magnetic field at the primary coil13. The non-contact power transmission apparatus10intensifies the magnetic field generated at the primary coil13by means of magnetic field resonance of the primary-side resonance coil14and the secondary-side resonance coil15, thereby generating power in the secondary coil16through the effect of electromagnetic induction of the intensified magnetic field around the secondary-side resonance coil15. The generated power is then supplied to the load17.

The primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16are each formed by an electric wire. The diameter and the number of turns of each coil13,14,15,16are suitably set in accordance with, for example, the intensity of the power to be transmitted. In the present embodiment, the primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16have the same diameter.

The frequency of the alternating-current voltage outputted by the alternating-current source11can be arbitrarily changed. Accordingly, the frequency of the alternating-current voltage applied to the resonance system12can be changed freely.

A method for designing the non-contact power transmission apparatus10will now be described.

First, the specifications of the primary-side resonance coil14and the secondary-side resonance coil15, which form the resonance system12, are determined. In addition to the material of the electric wires forming the primary-side and secondary-side resonance coils14,15, the specifications include values that need to be determined when producing and installing the resonance coils14,15, such as the size of the wires, the diameter of the coils, the number of turns, the distance between the resonance coils14,15. Next, the specifications of the primary coil13and the secondary coil16are determined. In addition to the material of the electric wires forming the coils13,16, the specifications include the size of the wires, the diameter of the coils, and the number of turns. Normally, copper wires are used as the electric wires.

Then, in accordance with the determined specifications, the primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16are formed, and the resonance system12is assembled. The load17is connected to the secondary coil16. Subsequently, the input impedance Zin of the resonance system12is measured while changing the frequency of the alternating-current voltage of the alternating-current source11applied to the primary coil13. The input impedance Zin of the resonance system12refers to the impedance of the entire resonance system12measured at both ends of the primary coil13. Then, while changing the alternating current frequency of the alternating-current source11applied to the primary coil13, the required power transmission characteristics of the resonance system12, such as the power transmission efficiency and the transmitted power with respect to the frequency of the alternating-current voltage of the alternating-current source11are measured. A frequency of the alternating-current voltage of the alternating-current source11at which desired power transmission characteristics are obtained is set as a drive frequency.

Next, the impedance Z1of the primary coil13is adjusted such that the output impedance of the alternating-current source11and the input impedance of the resonance system12are matched to each other at the drive frequency. Specifically, the diameter, the number of turns, and the space between the turns of the primary coil13are adjusted.

When matching the output impedance of the alternating-current source11and the input impedance of the resonance system12to each other, it is most preferably that the impedances are exactly equal to each other. However, for example, the input impedance of the resonance system12and the output impedance of the alternating-current source11are permitted to be different in a range where a desired performance as a non-contact power transmission apparatus is achieved, for example, in a range where the power transmission efficiency of the non-contact power transmission apparatus10is 80% or higher or in a range where the reflected power from the primary coil13to the alternating-current source11is 5% or lower. For example, the difference between the input impedance of the resonance system12and the output impedance of the alternating-current source11is permitted as long as the difference is within ±10% range, and preferably ±5% range relative to the output impedance of the alternating-current source11. When the difference within the range, the impedances are deemed to be matched to each other.

The method for designing is based on findings of experiments by the inventors that the power transmission characteristics of the whole resonance system12are not influenced by the impedance of the primary coil13. The design of the power transmission characteristics of the resonance system12and the matching of the output impedance of the alternating-current source11and the input impedance of the resonance system12can be executed independently from each other.

Each of the coils13,14,15, and16, which form the resonance system12is formed by a thin vinyl insulated low voltage wire for automobiles (AVS wire) having a size (cross-sectional area) of 0.5 sq (square mm) Also, the primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16are formed in accordance with the following specifications:

The distance between the primary-side resonance coil14and the secondary-side resonance coil15: 200 mm

A resistor of 50Ω serving as the load17is connected to the secondary coil16, and a sine wave alternating current voltage of 10 Vpp (amplitude of 5 V) and frequency of 1 to 7 MHz was supplied as an input voltage to the primary coil13from the alternating-current source11. Then, the impedance Z1of the primary coil13, the input impedance Zin of the resonance system12, and the power transmission efficiency η were measured. To check the influence of the impedance Z1of the primary coil13on the input impedance Zin of the resonance system12and the power transmission efficiency η, the number of turns of the primary coil13was changed to a single turn and four turns without changing the specifications of the coils other than the primary coil13, and the measurement was performed on the resonance system12with the changed specifications.FIGS. 2,3, and4show the results of the measurements. InFIGS. 2 to 4, the horizontal axis represents the frequency of the alternating current voltage of the alternating-current source11, and the vertical axis represents the impedances Zin, Z1and the power transmission efficiency η. InFIGS. 2 to 4, the power transmission efficiency η is simply shown as the efficiency η. The power transmission efficiency η is represents the ratio of the power consumption at the load17to the input power to the primary coil13, and is obtained according to the following equation when expressed as a percent.
The power transmission efficiency η=(power consumption at the load 17)/(input power to the primary coil 13)×100[%]

1. The impedance Z1of the primary coil13is monotonically increased as the frequency of the alternating current voltage of the alternating-current source11is increased from 1 MHz to 7 MHz regardless of the number of turns of the primary coil13. The lower the frequency, the higher the rate of increase of the impedance Z1becomes.

2. The input impedance Zin of the resonance system12changes substantially in agreement with the impedance Z1of the primary coil13in a range where the frequency of the alternating current voltage of the alternating-current source11is 2 MHz or lower and in a range where the frequency is 6 MHz or higher. In the vicinity of the resonance frequency, parallel resonance and series resonance subsequently occurs, so that the input impedance Zin of the resonance system12changed to create a local maximum point Pmax and a local minimum point Pmin.

3. The power transmission efficiency η has a maximum value substantially at the same frequency regardless of the number of turns of the primary coil13. The frequency at which the power transmission efficiency η has the maximum value is defined as the resonance frequency of the resonance system12.

4. The frequency at which the local maximum point Pmax and the local minimum point Pmin appear in the input impedance Zin of the resonance system12is substantially constant regardless of the number of turns of the primary coil13.

5. When the frequency of the alternating current voltage of the alternating-current source11is set to a frequency between the frequency at which the local maximum point Pmax appears in the input impedance Zin of the resonance system12(the first frequency) and the frequency at which the local minimum point Pmin appears in the input impedance Zin of the resonance system12(the second frequency), the power transmission efficiency η is raised. Particularly, the power transmission efficiency η is maximized when the frequency of the alternating current voltage of the alternating-current source11is set to be close to a frequency that is between the frequency corresponding to the local maximum point Pmax and the local minimum point Pmin, and in which the input impedance Zin of the resonance system12is equal to the impedance Z1of the primary coil13.

6. The power transmission efficiency η is raised in an impedance decrease range where the input impedance Zin of the resonance system12is decreased as the frequency is increased. Particularly, the power transmission efficiency η is maximized in a range where the input impedance Zin of the resonance system12is decreased as the frequency is increased and at about the frequency at which the input impedance Zin of the resonance system12is equal to the impedance Z1of the primary coil13.

The present embodiment has the following advantages:

(1) The non-contact power transmission apparatus10has the resonance system12, which includes the primary coil13, which receives alternating-current voltage from the alternating-current source11, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16, to which the load17is connected. The output impedance of the alternating-current source11and the input impedance Zin of the resonance system12are matched to each other. Power is thus efficiently supplied from the alternating-current source11to the resonance system12. When matching the input impedance Zin of the resonance system12and the output impedance of the alternating-current source11to each other, it is sufficient to measure only the impedance Z1of the primary coil13instead of the input impedance Zin of the resonance frequency12. Therefore, the input impedances Zin of the resonance system12and the output impedances alternating-current source11are easily matched to each other.

(2) The alternating-current source11applies to the primary coil13an alternating current voltage of which the frequency is between the frequency corresponding to the local maximum point Pmax of the input impedance Zin in the resonance system12(the first frequency) and the frequency that corresponds to the local minimum point Pmin of the input impedance (the second frequency). Therefore, the power transmission efficiency η is raised. Particularly, the power transmission efficiency η is maximized when the primary coil13receives an alternating-current voltage the frequency of which is in the range between the first frequency and the second frequency and equalizes the input impedance Zin of the resonance system12and the impedance Z1of the primary coil13to each other.

(3) The alternating-current source11applies to the primary coil13an alternating-current voltage the frequency of which is in an impedance decrease range where the input impedance Zin of the resonance system12is decreased as the frequency is increased. Therefore, the power transmission efficiency η is raised. Particularly, the power transmission efficiency η is maximized when the primary coil13receives an alternating-current voltage the frequency of which is in the range set in the above described manner and equalizes the input impedance Zin of the resonance system12and the impedance Z1of the primary coil13to each other.

(4) The resonance frequency of the resonance system12remains unchanged even if the impedance Z1of the primary coil13is changed. Therefore, only by adjusting the impedance Z1of the primary coil13, the output impedance of the alternating-current source11and the input impedance Zin of the resonance system12can be matched to each other without changing the resonance frequency. Accordingly, the design and adjustment of the non-contact power transmission apparatus10are facilitated.

(5) The primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16have the same diameter. Therefore, the primary coil13and the primary-side resonance coil14can be easily produced by winding the coils13,14around a single cylinder, and the secondary-side resonance coil15and the secondary coil16can be easily produced by winding the coils15,16around a single cylinder.

(6) The non-contact power transmission apparatus10is designed in the following manner. That is, the frequency of an alternating-current voltage that the alternating-current source11applies to the primary coil13is set. Then, the impedance Z1of the primary coil13is set such that the output impedance of the alternating-current source11and the input impedance Zin of the resonance system12are matched to each other at the drive frequency. At this time, the power transmission characteristics of the resonance system12are not changed even if the impedance Z1of the primary coil13is changed. Thus, after the output impedance of the alternating-current source11and the input impedance Zin of the resonance system12are matched to each other, the alternating current frequency of the resonance system12and the alternating-current source11do not need to be adjusted again. It is thus possible to provide the non-contact power transmission apparatus10that efficiently supplies power from the alternating-current source11to the resonance system12. The output impedance of the alternating-current source11and the input impedance Zin of the resonance system12are easily matched to each other.

The present invention is not limited to the above embodiment, but may be modified as follows.

When needed, the impedance Z1of the primary coil13can be adjusted by changing the number of turns of the primary coil13, the diameter of the coil13, the size of the electric wire of the primary coil13, or the material of the electric wire of the primary coil13. However, the easiest way is to change the number of turns and the diameter of the coil.

When forming the coils13,14,15,16by winding electric wires, the coils13,14,15,16do not need to be cylindrical. For example, the coils may have a tubular shape with a simple cross-sectional shape such as a polygon including a triangle, a rectangle, and a hexagon. The coils may also have a cross-section of an asymmetrical figure.

The primary-side resonance coil14and the secondary-side resonance coil15are not limited to coils formed by winding an electric wire into a cylindrical shape, but may be formed by winding an electric wire into a spiral in a single plane as shown inFIG. 5.

The coils13,14,15, and16may be configured such that an electric wire is closely wound so that each turn contacts the adjacent turn, or may be configured such that the electric wire is wound with a space between each adjacent pair of turns.

The primary coil13, the primary-side resonance coil14, the secondary-side resonance coil15, and the secondary coil16do not need to have the same diameter. For example, the primary-side resonance coil14and the secondary-side resonance coil15may have the same diameter, and the primary coil13and the secondary coil16may be different from each other. Alternatively, the primary and secondary coils13,16may have a different diameter from the diameter of the primary-side and secondary-side resonance coils14,15.

The method for designing the non-contact power transmission apparatus10is not limited to the one in which, after the specification so the primary-side resonance coil14and the secondary-side resonance coil15, forming the resonance system12, are set, the specification of the alternating-current source11is set, and then the impedance Z1of the primary coil13is set such that the output impedance of the alternating-current source11and the input impedance Zin of the resonance system12are matched to each other. For example, the specification of the alternating-current source11may be set first, and, in accordance with the specification of the alternating-current source11, the specification of the primary-side resonance coil14and the secondary-side resonance coil15, which form the resonance system12, and the impedance Z1of the primary coil13may be set. Setting the specification of the alternating-current source11prior to the specification of the resonance system12means that, after the resonance frequency is determined when setting the specification of the resonance system12, the material and the size of the electric wires forming the primary-side resonance coil14and the secondary-side resonance coil15, the diameter of the coils, the number of turns, the distance between the resonance coils14,15are determined.