978-1-4799-4455-2/14/$31.00 ©2014 IEEE 225 37th Int. Spring Seminar on Electronics Technology

Wireless Energy Transfer Using Resonant Induction

Dan–Sebastian Filip, Dorin Petreus

Applied Electronics Department, Technical University of Cluj–Napoca, Romania

Abstract: In this paper an idea of wireless energy transfer using resonant inductors is presented. Comsol Multiphysics is used in order to simulate the energy transfer depending of the geometry of the resonant coil, parasitic components and position of the coils. Using different positions of the emitter coil and more than 1 resonant coil we can analyze the best case for transferring energy to the receptor.

1. INTRODUCTION

As technology progresses, more and more electronic devices will dictate our daily lives. Making these devices portable requires the use of batteries or other energy storing component, and the number of functions and computation power of that device will establish the autonomy of the device. In essence, the more powerful the device, the more energy will consume and the less time the battery will hold. This battery discharging would not be a problem if charging it would not require the device to become tied with a “wire to the wall”.

Wireless power transmission has been studied and experimented with since the first experiments by N. Tesla, end of the 19th century and the first successful laboratory scale demonstrations during the 1960s by Brown et al.[1] Since then, the wireless transfer energy has evolved and other methods of energy transfer have been discovered. For medium distances the most used is the resonant coupled inductors method. On the market there are already the charging pads, in which you charge your electronic device by placing it on the pad. However, while there is no cable from the device to the power source, the device needs to be immobile on the pad in order to charge.

In this article a series of simulation are presented showing the advantages and disadvantages of transferring wireless energy using resonant inductors placed at different distances and angles to one another. The main idea is transferring energy from one coil to another regardless of the angle at which they are placed to one another.

2. DEFINING THE PARAMETERS

In the first step we will consider a primary coil of 50 turns constructed in air. The inner diameter of the coil is considered to be 100mm and the length is 8mm. This will be the primary coil, which will be driven by a sinusoidal voltage of 25V and frequency between 10 and 80 kHz depending on the resonant frequency of the system. This coil is placed on the xOy plane. Two similar coils will be placed on the xOz and yOz planes in such a way that the center of these 3 coils will be the same. In order for the cooper windings to not intersect one another we will have one of the coil radius reduced with 5mm while the other will be increase 5mm. This change in radius will have an influence on the inductors value. This can be compensated by the value of a capacitor placed in parallel with the inductor. In this way the 3 resonant tanks will have the same resonant frequency.

The secondary coil will have 50 turns with an inner diameter of 20mm also constructed in the air. Angle and position of this coil will change with respect to the primary coil in order to simulate the energy transfer from a primary fixed system to a mobile device.

Having these parameters it is easy to calculate the induction of the coils, but during wireless transfer the parasitic components of the coils are also important. Also there is the need of a capacitor in order to create the resonant tank. Because it is difficult to construct the capacitor and simulate it as an electromagnetic component we will add an ideal capacitor in parallel

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with the coil. In this way the simulation will be faster and the influence of the parasitic components of the capacitor will be ignored. We will also add a series resistor with the primary coil to simulate the losses on the wire of the coil.

In Comsol we create the coils as part of the electro-magnetic and we will add the additional components using the electric circuit model. The circuit is as shown in Fig. 1. The netlist was done in LTSpice and imported in Comsol. The inductors components are changed with the inductor model from the electromagnetic module. The rest of the components behave without considering any additional parasitic components.

Fig. 1. Equivalent schematic of the circuit to be simulated in Comsol.

2.1. Secondary Coil Parallel and Centered to Primary Coil

In the first test we will consider the secondary coil placed on a parallel plane with one of the primary coils. The distance between the 2 coils will be with 15mm greater than the maximum radius of the primary coil, this would mean at a distant of 120mm. Considering that the secondary coil is placed perpendicular to the other 2 primary coils, the coupling factor will be low and therefore the energy transferred from this two coils can be neglected. Having this in mind it is better to eliminate the supply for these coils. This is done by increasing the corresponding series resistor to a higher value. In the system the coils are still present but little current is supplied trough them.

In Fig. 2 it is possible to observe the induced current on the secondary coil, with doted green, and the currents trough the primary coil. In order to see this better the primary current was scaled with a factor of 100. Due to the distance between the two parallel coils the energy transfer is small.

Fig. 2. Currents trough the coils when the secondary coil is

parallel and centered to primary coil.

In Fig. 3 it is possible to see the magnetic flux lines generated by the primary coil.

Fig. 3. Magnetic flux lines generated from the primary coil and closing trough the secondary coil.

Fig. 4. Currents trough the coils when the supplied coil is

perpendicular to the secondary coil.

Supplying one of the primary coils placed on a perpendicular plane to the secondary coil will not

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result in any energy transfer between the 2 coils. In Fig. 4 you can see the current trough the primary coil marked with doted red and the currents induced in the other 3 coil, the 2 primary coils and the secondary one. At this point the coupling factor between the primary coil and the secondary coil is close to 0 meaning there is almost no energy transferred from the primary side to the secondary side.

Fig. 5. Magnetic flux lines generated from the primary coil and closing trough the secondary coil.

This is also confirmed by Fig. 5. Because the secondary coil is placed perpendicular to the primary coil the flux lines do not cross trough the receiving coil and therefore the energy transfer can be neglected.

2.2. Secondary Coil Parallel with the Primary Coil, Not Centered

We will now proceed to move the secondary coil away from the center of the primary coil, but we will keep it on the same plane. The coil was moved 60mm on the x axis.

Fig. 6. Currents trough the primary and secondary coil.

The coupling factor is lower than in the previous case, which was expected, but because the coil is moved away also from the coils perpendicular to the secondary coil plane, the coupling factor between the secondary coil and one of the primary coil has increased. The current trough the primary and secondary coils are presented in Fig. 6.

Fig. 7. Magnetic flux lines generated from the primary coil

and closing trough the secondary coil.

Because the secondary coil is moved, the coupling factor between the secondary coil and the primary coil situated on the xOz plane has increased. Supplying only this coil and measuring the current trough bough the coils reveals that there is, indeed, some energy transfer. This can be seen in Fig. 8.

Fig. 8. Currents trough the coils when the secondary coil is

perpendicular to the primary coil and at 60mm distance from the center.

In Fig. 9 it is possible to notice how some of the flux lines close trough the secondary coil. The next step is to supply both the coils in order to transfer the maximum energy to the secondary coil.

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Fig. 9. Magnetic flux lines generated from the primary coil

and closing trough the secondary coil.

We will supply the primary coil situated in the xOy plane, which is parallel to the secondary coil, and we will also supply the coil in the yOz plane. This coil although perpendicular to the plane of the secondary coil because of its relative position will transfer some of the energy. Adding the energy induced from the parallel coil will increase the received energy, which will actually be close to the energy received when the secondary coil was in the center of the primary coil.

Fig. 10. Currents trough the coils when the secondary coil is perpendicular to one of the primary coil and at 60mm

distance from the center.

How the two primary coils couple with the secondary coil can be seen better from Fig. 11. It is clear that the flux lines from the parallel primary coil will pass through the receiver coil and because the 60mm position variation, the coupling factor will be lower, but because this 60mm positioning, some of the flux lines from the yOz coil will pass through the secondary coil and induce a current in this coil.

Fig. 11. Magnetic flux lines generated from the two

primary coils, closing trough the secondary coil.

Because there are 2 supplied coils, and the position of the secondary coil has also changed, the coupling factor has changed and therefore the resonant frequency of the whole system has also changed. To find this we will increase the working frequency until we find the maximum point where the energy is transferred. Because the position of the coil changes the coupling factor, it is difficult to calculate this due to many parameters. Finding the resonant frequency using the maximum transferred energy is less efficient but much easier.

In the figure below are shown the currents trough the secondary coil at different working voltages. It is noticed that at a frequency of 65 kHz the energy transfer is at maximum meaning that this is the resonant frequency.

Fig. 12. Induced current in the secondary coil at different frequencies.

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2.3. Secondary Coil at 45deg Angle to the Primary Coil and Centered

The next step is to place the secondary coil at a 45deg angle from the original position. The coupling factor was reduced because of this change in angle. This can be seen in Fig. 13, where the current induced in the secondary coil is lower than in previous cases. Looking at Fig. 14 we noticed the field lines are blocked by one of the primary coils. This acts as a shield and therefore the energy transfer is reduced.

Fig. 13. Current trough the 3 primary coils and the trough

the secondary coil.

Fig. 14. Magnetic flux lines generated from the two primary coils, closing trough the secondary coil.

The secondary coil is placed at 45deg from the xOy and xOz coil and perpendicular to the plane of yOz coil. This is a difficult position from the point of the energy transfer, however if the two primary coils are supplied there still is some energy transfer. This can be seen in the figures below.

Fig. 15. Current trough the 2 primary coils and the induced

current trough the secondary and the third primary coils.

Comparing with the previous case, the current is almost the same in the secondary coil, meaning that we obtain almost the same energy transfer regardless of the secondary coil position to the yOz coil. The magnetic flux lines are also shown below.

Fig. 16. Magnetic flux lines generated from the two

primary coils, closing trough the secondary coil.

The secondary coil is rotated at 90deg angle with respect to its original position. In this case the position is helping the energy transfer by increasing the coupling factor. In the previous cases the secondary coil was at 45deg angle to one of the primary coils. Rotating the coil with 90deg, means placing the coil at 45deg angle to two of the primary coils. Because of this coupling factor we can obtain the best energy transfer.

In Fig. 17 it is possible to see the current trough the secondary coil much larger than the currents trough the primary coils. If we compare this value with the values in the previous situation, we can see that the value is almost doubled. This is expected, as in this

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case, two primary coils have a good coupling factor with the secondary coil. In Fig. 18 the flux lines generated from the primary coils can be observed.

Fig. 17. Current trough the 3 primary coils and the trough

the secondary coil.

Fig. 18. Magnetic flux lines generated from all three

primary coils, closing trough the secondary coil.

3. CONCLUSIONS

Having all this measurements we can assume that all the critical cases have been covered. If the distance between the secondary coil and the primary system consisting of 3 coils placed at 90deg to one another is constant, the energy transferred from the primary coils to the secondary coils will be somewhere between the

values shown. There will always be an energy transfer between the primary system and the secondary coil, regardless of the angle at which the secondary coil is placed relative to the primary coil.

In order to optimize the energy transfer it is necessary to supply only the primary coils that actually help in the transfer without dissipating energy on a primary coil that has a low coupling factor with the primary coil.

The resonant frequency also plays an important role in the energy transfer. Because the coupling factor varies with the angle at which the coils are placed it is necessary to always find the resonant frequency and ensure the maximum energy transfer.

REFERENCES

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[2] S. Pawade, T. Nimje, D. Diwase „Goodbye Wires: Approach to Wireless Power Transmission”, International Journal of Emerging Technology and Advanced Engineering, ISSN 2250-2459, Volume 2, Issue 4, April 2012.

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[4] V. Choudhary, A. Kadir, S. pal Singh, „Wireless power transmission “a novel idea””, International Journal of Scientific & Engineering Research, ISSN 2229-5518, Volume 2, Issue 9, September-2011.

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[7] http://www.wirelesspowerconsortium.com/ technology/ coupling-factor.html.