# Wireless Power Transfer

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## Wireless Power Transfer

### Introduction

Abstract:A proposed wireless power transfer system based on magnetic resonance is analyzed using 3D FEM simulation in EMPro, and compared to experimental results.The distribution of power without wires is a popular research topic. Many of the proposed systems are based on the principle of electromagnetic induction, and the main challenge is to meet both transmission efficiency and transmission distance requirements.

Recently, a method based on magnetic resonance was proposed at MIT (1) and was verified with a prototype. This highly effective method can achieve high power and long-distance transmission. Below is a photo and schematic the proposed system.

Figure 1: MIT wireless power transfer system prototype

Figure 2: Schematic of the proposed wireless power tranfer system

A is a single loop (radius = 25 cm) that is part of the driving circuit. S and D are the source and device coils (radius = 30 cm, length = 20 cm, 5.25 turns) with the same resonance frequency mutually. B is a single loop (radius = 25 cm) attached to the load, and like . The angle between coil D and the loop A is adjusted to ensure that their direct coupling is zero. Coils S and D are aligned coaxially. The loops/coils are all copper wire with a diameter of 3 mm.

With a 2.1 m distance between coils S and D, the efficiency of the system was measured to be 40%.

### Design Challenges

Practical systems need to transfer power across useful distances with high efficiency and need to be reasonably immune to objects that might appear between the source and device. We can use 3D EM simulation to characterize system efficiency versus transmission distance, and the effect of interfering objects.

To analyze the magnetic resonance method, a 3D model was created in EMPro. The distance between the S and D coils was initially set to 1.5 m.

Figure 3: 3D model of wireless power transfer system in EMPro

### Results

A 3D FEM simulation was performed on the system and the results are shown below.

Figure 4: Simulation results with coil distance = 1.5 m

The power efficiency can then be calculated from the S21 results.

Figure 5: Calculated power transfer efficiency with coil distance = 1.5 m

The distance between transmitter and receiver coils was parameterized, and a swept simulation was performed in EMPro. As shown below, the results of analysis matched well with the theory and the experimental results from MIT

Figure 6: Comparison of power transfer efficiency from MIT (left) and from EMPro simulations (right)

With the magnetic resonance method, there is less environmental influence compared to the electromagnetic induction method. As an example, a large metal plate is inserted between the helical coils. The distance between coils is set to 1 m, and another FEM simulation is performed in EMPro.

Figure 7: A metal plate is inserted between the source and device coils to analyze the impact of foreign objects on system efficiency

The transmission efficiency becomes 57% with an inserted metal plate compared with 90% transmitting efficiency without the metal plate. Although transmission efficiency was reduced, it still exceeded 50%, which is favorable compared to other transfer system to realize. The E-field and H-field plots confirm a resonant state even in the presence of a metal plate.

Figure 8: Simulation results with a plate inserted

Next, a repeater device is proposed to improve the transmission efficiency. An additional coil, identical to the helical coil of S and D is inserted and analyzed with EMPro.

Figure 9: A repeater is inserted to improve system efficiency at longer distances

The transmission efficiency without the repeater is 48.4% using a coil distance of 2 m. The efficiency improves to about 80% by inserting the repeater device. The E-field and H- field results confirm a resonant state in each helical coil when the repeater device is inserted.

Figure 10: Simulation results with a repeater device

After verifying electromagnetic models of wireless power transfer, analysis of a complete system, including the transmission model and a nonlinear circuit, can be accomplished with the combination of EMPro and the ADS circuit simulator, as shown below.

Figure 11: Full system simulation in ADS, utilizing the 3D EM results from EMPro

### Conclusion

With a 3D EM simulator, the wireless power transfer method of magnetic resonance can be easily analyzed and modeled. The results of EMPro FEM analysis match well with the MIT theoretical results. In addition to the basic wireless power transfer model, EMPro can analyze behavior with additional elements, such as a metal plate, or a repeater. EMPro & ADS make it possible to verify performance of a complete system, including the designed wireless power transfer system. This is an ideal solution for optimizing each component for maximizing whole system performance.

### Reference

(1) A. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P. Fisher, and M. Soljačić : “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science, 317, pp. 83-86 (2007)