analysis on wireless energy transfer based on magnetic www.daneshpajooh.ir

IEEE Vehicle Power and Propulsion Conference (VPPC), September 3-5, 2008, Harbin, China

978-1-4244-1849-7/08/$25.00C 2008 IEEE

Simulation and Experimental Analysis on Wireless Energy Transfer Based on Magnetic

Resonances

Chunbo Zhu, Kai Liu*, Chunlai Yu, Rui Ma, Hexiao Cheng School of Electrical Engineering and Automation, Harbin Institute of Technology, Harbin, China, 150001

*Corresponding Author, Email: [email protected]

AbstractWireless energy transfer based on coupled magnetic resonances is a new technology which energy can be transferred via coupled magnetic resonances in the non-radiative near-field. A simple energy transfer system structure was proposed in this paper. Based on this structure, the back electromotive force (back-EMF) in the receiving coil related with different transfer distance and with driving frequency was simulated and analyzed. Experimental results shows that the power can be transmitted is up to 50W, the transfer efficiency is more than 60%, the distance of energy transfer is longer than 1 meter. The energy transferring is able to go through various non-metallic objects. The prospective applying fields are wide, such as EV charging, industrial robot, aerospace, wireless sensor networks, and RFID technology.

KeywordsEnergy transfer; Wireless; Magnetic resonance

I. INTRODUCTION For the electric vehicles (EVs) which using electric

energy as the energy source, the power supplementary equipment (charger) is one of indispensable subsystems; its function is converting the energy of power grid into stored driving power of the EVs. Most of the energy supply of EVs is realized by the electrical wire. American inventors use the microwave beams and laser beams to realize the remote energy supply using the directional antenna. However, when we use this method to transfer energy, any obstruction in the transfer path is not allowed. It is dangerous for human bodies or other organisms which in the energy transfer path [3, 4].

The basic principle of this technology is that two separate coils with same resonance frequency are possible to form a resonant system based on high frequency magnetic coupling and exchange energy in a high efficiency, while the coupling effect is weak between those objects with different resonance frequency. The medium of energy transfer is an alternating magnetic field. This technology can be used to provide energy for electricity equipments wirelessly in a certain distance. MIT (Massachusetts Institute of Technology) in American also did some research on principle simulation and experimental verification about this technology [1, 2]. This technology can be used to provide energy wirelessly for EVs charging, it will be a convenient and rapid method in the future, it can realize the economical running way of the battery using low depth charge and low depth

discharge to prolong the life of the battery. If we can make further improvement on the transfer power and efficiency, this technology will have a wider application prospect, such as industrial robot, aerospace, wireless sensor networks, and RFID technology.

II. THE POSSIBILITY OF ENERGY TRANSFER VIA COUPLED MAGNETIC RESONANCES

Resonance phenomenon is widely existed in nature. Different kind of resonance contains different kind of energy. The sound of tuning fork is produced by resonance and the earthquake is also produced by resonance, while the energy of earthquake is much higher than the sound of tuning fork.

Resonance is a trend that one physical system in its natural frequency tends to absorb more energy from the environment. In other words, it is a phenomenon that one object vibrates which cause the other one with the same frequency vibrates. Resonances can transfer energy. There is a simple example: when two tuning forks A and B with the same frequency are placed not far apart, hit fork A to make it phonating, when we hold fork A to stop its phonation, we find that fork B without hitting is phonating. This is the resonance phenomenon in acoustics. The energy which makes fork B phonating is come from the sound wave generated by fork A, the energy-transfer medium is sound field. It can be said that the essence of vibration propagation is the transfer of energy. Similarly to sound field, it is also possible for electromagnetic field.

Electromagnetic resonance exists widely in electromagnetic system. The electromagnetic field itself is an energy field which can provide energy to used electric apparatus. Considering its danger to people and other organisms in electric field, the magnetic field is safe and more suitable to be used as the energy-transfer medium in magnetic resonances energy transfer.

The radiating electromagnetic wave itself contains energy, no matter whether there is a receiver or not, the energy of electromagnetic wave is continuously consumed. If we can make a non-radiative magnetic field with a certain resonance frequency, when the resonator, such as the LC oscillating circuit, with the same resonance frequency in it, the electromagnetic resonance is generating, energy in the inductance coil continues gathering, the voltage is increasing, the receiving energy


can be used by the load after being converted by follow-up circuits.

Generally speaking, electromagnetic systems with same resonance frequency are weak couplings apart in a certain distance. Two systems with same inherent resonance frequency will generate strong magnetic resonance and form a magnetic resonances system. If there are more than two resonators in effective range, they can also join the resonances system. One resonator can be connected with continuously power supply to serve as the energy source and others consume the energy, so the energy transfer system realized. In other words, we can transfer energy from one place to another via invisible magnetic field instead of the visible electrical wires.

III. STRUCTURE OF ENERGY TRANSFER SYSTEM VIA COUPLED MAGNETIC RESONANCE

Figure 1. Schematic diagram of energy transfer system via coupled

magnetic resonance

As shown in figure 1, a simple structure of energy transfer system via coupled magnetic resonance is proposed. The energy supply of source is provided by power convert module, inductor sL and capacitor sC constitute a resonance source circuit to generate an alternative non-radiative magnetic field. The resonance frequency of LC circuit is sf . The control signal for power switch tube T is generated by switch drive circuit, and its frequency is kf . In theory, when tf is close or equal to sf , the oscillation of source resonance circuit is strongest, the value of resonance current is highest, and the magnetic field intensity is also strongest. Inductor tL and capacitor tC constitute the receiving resonance circuit to produce resonance with the magnetic field which generated by source resonance circuit to receive energy. The frequency of receiving resonance circuit is tf , the parameters of tL and tC neednt be in full accord with the source resonance circuit. What the receiving resonance circuit must need is to ensure sf = tf , that is the necessary condition for energy transfer.

In figure 1, sC and tC are capacitors in resonance circuit. If there are more than 2 capacitors, the equivalent capacitance should be calculated according to the specific circuit and the relationship between the capacitors (series

or parallel). sL and tL are the values of coil inductance in resonance circuits. For different shape and structure of coils, the formula to calculate the coil inductance is also different. The circular loop coil is used in this resonance system proposed in this paper, and the coil inductance can be calculated in formula (1) [5]:

]75.1)8[ln(02

=

aRRNL (1)

Where, N coil turn;

70 104

= permeability of vacuum (H/m); Rradius of coil (m); aradius of conductor section (m).

If the inductor L and capacitor C in resonance circuit is determined, the circuit resonance frequency f could be calculated from formula (2):

LCf

21

= (2)

The frequency kf of driving signal for power switch tube T can be called driving frequency. It can be determined according to sf . The driving signal can be generated by signal generation circuit or device. The closer kf to sf , the stronger is the magnetic field. The stronger is the magnetic fields, the longer is the energy transfer distance and also higher is the transfer efficiency. Through the onoff control of power switch tube by driving signal, resonance current is emerging in the source resonance coil. Then this resonance current generates the alternating non-radiative magnetic field.

The working frequency of energy receiver tf is also determined by sf . When the receiving resonance coil enters into the alternating magnetic field produced by the energy transmitting source, it begins to oscillate because it has the same resonance frequency with the alternating magnetic field. As have been said before, energy in the inductance coil continues gathering, the voltage is increasing, and the receiving energy can be used by the load after being converted by follow-up high-frequency rectifier circuit.

IV. FINITE ELEMENT SIMULATION ANALYSIS

Figure 2. Model of the finite simulation analysis

As shown in figure 2, a model of finite element simulation for magnetic resonances energy transfer system is established. An ideal AC voltage source is used in simulation as the power supply of energy source, its can


be expressed by formula )sin(0 += tVV . The voltage amplitude 0V and frequency can be adjusted according to different simulation conditions. The frequency equivalent to the driving frequency kf . The supply power can be loaded to source resonance circuit which constituted by the inductor sL and capacitor sC . The simulation model of inductance coil is established by the experiment parameters. By using the circuit function of finite element analysis software, the capacitor and inductance coil are connected to form the LC resonance circuits. The simulation of energy transferred according to different distance between sL and tL can be done.

In this paper, the back electromotive force (back-EMF) in the receiving coil related with different transfer distance and with driving frequency is simulated and analyzed.

Curves in figure 3 show the impact on the back-EMF caused by different transfer distance. It is obvious that the longer is the distance between the two coils, the lower the back-EMF of the receiving coil is.

0

5

10

15

20

0 5 10 15 20 25 30 35 40 45 50 55

Distance between Source and Receive Coil (cm)

Back

-EM

F o

f R

ece

ive C

oil

(V

)

f=310kHzf=630kHz

Figure 3. Relationship between the transfer distance and back-EMF of

the receiving coil

0

10

20

30

40

50

60

70

80

90

600 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Frequence of K (kHz)

Back

-EM

F o

f R

ece

ive C

oil

(V

)

fs=ft=1150kHz(V)

fs=ft=1100kHz(V)

Figure 4. Relationship between different driving frequency kf and

back-EMF of the receiving coil

When LC circuit has been set, its inherent resonance frequency can be calculated by formula (2). Changing the frequency of AC source equivalent to changing the driving frequency kf , which change the magnetic field generated by LC circuit. The curves in Figure 4 show the relationship between different driving frequency fk and back-EMF of the receiving coil. The simulation result

shows that when the back-EMF reaches the maximum, the driving frequency fk is not completely consistent with the inherent resonance frequency of the LC resonant circuit, there existed a certain deviation. For example, we can see the inherent resonance frequency of LC resonance circuit is 1150 kHz, while the back-EMF of receiving coil reaches the maximum when the driving frequency is 1450 kHz. The reason for this phenomenon needs to be studied in further research.

V. EXPERIMENTAL RESULTS According to the structure proposed before, the

experimental device has been made and the experimentally study of energy transfer shows a broad application prospect for this technology in the future.

Figure 5. The schematic diagram of Energy transfer experiment.

The energy transfer experimental device is showed by figure 5. It is a small diameter resonator, the parameters of source and receiving resonance coils are same. The diameter of inductance coil is 160mm, the diameter of conductor section is 2.1mm and the coil turn is 4. The parameter of capacitor connected with the inductance coil is 6.6nF with withstand voltage of 630V. The power is an 110V DC power and its maximum output current is 3A. The on-off control frequency of power switch tube T is 630 kHz. Experimental result shows that the power can be transferred is up to 50W, the transfer efficiency is more than 60%, and the maximum distance of energy transfer can reach to 300mm.

In another energy transfer experiment, the larger diameter resonance coils are used. The parameters of receiving resonance coil are same with the source resonance coil. The diameter of inductance coil is 500 mm. The conductor is multiple wires whose cross section area is 0.75mm2 and the coil turn is 2. The capacitor is a 4.4nF film capacitor with a withstand voltage of 630V. The power is a 25V/120W DC power. The driving frequency is 310 kHz. The experimental result shows that the maximum transfer power is also 50W, and the transfer efficiency can still be more than 60%. The transfer distance is 1 meter which is longer than that by using small diameter coil resonator.

Experiments of energy transfer show that transfer efficiency varies with the distance between the source and


receiving resonance coils. The longer is the distance between source and receiving coil, the lower the transfer power and efficiency is.

One advantage of this technology is energy transfer can go though various objects. Many different kinds of obstacles were placed between the source and receiving coils respectively to test the ability of going through objects. Experiment result shows that energy can still be transferred even the receiver is sheltered in most conditions. Non-metallic objects such as walls, books, wooden products, organic glass panels, leather, and textiles have no impact on power transfer. The impact of metallic objects on the system depends on different characteristics of metal conductor. It would have slight impact if the object with size less than the diameter of coil, or which cannot generate a larger eddy current; If the metallic objects which can generate larger eddy current or form a close loop is close to this system, the impact will be greater even block energy transfer.

Figure 6. The schematic diagram of Energy transfer experiment

adding enhanced coil

In addition, as shown in figure 6, it is found that when a LC resonance circuit with the same parameters and sizes of the source resonance circuit is placed between the source and receiving resonance coil but close to the source one, the energy transfer distance and transfer power can be improved both. More analysis and experimental study still need to be done to analyze the root of this phenomenon.

VI. CONCLUSION Based on the experimental result and analysis, it is

proved that energy transfer is completely feasible using the technology of magnetic coupling resonance. Nevertheless, there are still a lot of research and experimental work to be done to make this technology more practical.

REFERENCES [1] Andre Kurs, Aristeidis Karalis, Robert Moffatt, J. D.

Joannopoulos, Peter Fisher, Marin Soljacic. Wireless Power Transfer via Strongly Coupled Magnetic Resonances. Science. 2007, July 6th, Vol. 317, 83-86.

[2] Aristeidis Karalis, J. D. Joannopoulos, and Marin Soljacic. Wireless Non-Radiative Energy Transfer. The AIP Industrial Physics Forum, 2006.11.

[3] W. C. Brown. The History of Power Transmission by Radio Waves. IEEE Trans. Microwave Theory Tech, 1984, Vol. 32, 1230-1242.

[4] Lin, J.C. Space Solar-power Stations, Wireless Power Transmissions, and Biological Implications. Microwave Magazine, IEEE Volume 3, Issue 1, March 2002 Page(s):36-42.

[5] Daqian Fang. Handbook of Electrical Calculations. Shandong Science and Technology Press, 1994.

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