Design And Analysis Of Wireless Power Transfer Using High Frequency Resonant Inductive Coupling By Changing The Coil Geometry

Examensarbete för masterexamen
Master's Thesis
Electric power engineering (MPEPO), MSc
Gouthamas, Nagendra
Bandi, Adarsh
Abstract In this thesis, we have developed a wireless power transfer system for a space of dimensions (15cm x 15cm x 15cm), utilizing a series-connected transmitter (Tx) coil to generate an equally distributed electromagnetic field inside the space. This field can be picked up by a solenoid receiver (Rx) coil placed anywhere within the space to power a DC load. To develop the proposed system, we have studied a conventional wireless power transfer system fromWurth Electronics and investigated various coil geometries for both Tx and Rx. The conventional wireless power system has been examined regarding the influence of coil geometry, alignment between the transmitter (Tx) and receiver (Rx), as well as the coil parameters on efficiency. The wireless power system from Wurth Electronics is tested with different Rx coil geometries to evaluate interoperability between various Tx and Rx coil geometries, as well as to compare the coupling coefficient and magnetic field density of different Rx coil geometries with misalignment in order to evaluate most suitable Rx coil geometry for the proposed wireless power system. The different Rx coil geometries investigated are: concentric, solenoid, and center-tapped circular coil. The comparison is based on different alignment conditions between the transmitter and receiver coils. We find, from the results obtained by investigating different Rx coil geometries with the conventional system, that the solenoid coil geometry with a core provides a stable coupling coefficient under various misalignment conditions. Thus, the solenoid Rx geometry can be optimized to enhance coupling between the transmitter and receiver coils. Additionally, test results show that the use of center-tapped circular coils in wireless power power system helps in improving range from 30mm to 180mm compared to conventional wireless power system with concentric coils. However, the improved range is only achievable with significantly higher number of coil turns compared to concentric coils which reduces efficiency. Also, a unique BJT based automatically switching transmitter circuit is required for the center-tapped circular coils which makes it a non-viable solution due to inefficiency of the BJTs. Further a suitable transmitter coil geometry is investigated for use in combination with the solenoid receiver to demonstrate the proposed wireless power system within a space of dimensions (15 cm x 15 cm x 15 cm). Three different transmitter coils are investigated: the concentric coil, 2-series connected concentric coil, and 8-series connected concentric coil. A half-bridge transmitter circuit is designed to drive the various transmitter coils. The experimental and simulation results indicate that the concentric transmitter coil with a large diameter enhances high power transfer and better range due to a more effective distribution of the magnetic field. However, when using a solenoid receiver coil, the power transfer efficiency drops by 28% compared to the efficiency of 75% under aligned operation. Consequently, the concentric transmitter coil is designed with the coil turns distributed into smaller areas, forming a 2-series connected coil. Test results reveal that it was possible to transfer 10W of power at an efficiency of 75% when the coils were perfectly aligned. In contrast, when the coils were misaligned, power transfer efficiency of up to 48% was achieved. This is in contrast to the power transfer efficiency of 6% under misaligned conditions in the conventional wireless power system. The 2-series connected concentric Tx coil is then extended to an 8-series connected concentric Tx coil to form a space of dimensions (15 cm x 15 cm x 15 cm) in combination with a solenoid Rx coil. Simulation results indicate that the coupling coefficient between Tx & Rx improved by approximately 11% and measurement results show that power transfer efficiency improved by 4% compared to the 2-series connected concentric Tx coil. Similarly, when the solenoid Rx coil was rotated or misaligned with the Tx coil, the efficiency improved by approximately 33% compared to the 2-series connected concentric Tx coil. The increase in efficiency and coupling with Tx and Rx is mainly due to distribution of coil turns into smaller areas and then connecting them in series which improved the magnetic field distribution compared to other coil geometries. Hence, the 8-series connected concentric coil as Tx and solenoid as Rx are selected as the suitable Tx and Rx coils, respectively, for the proposed wireless power system. In comparison to the conventional wireless power system, the efficiency remains approximately the same at 80% when the coils were perfectly aligned and efficiency improved by more than 60% when the coils were misaligned.
Keywords: Wireless power transfer, coil geometry, coil parameter, alignment, coupling coefficient, series connected concentric coil. , wireless power transfer , coil geometry , coil parameter , alignment , coupling coefficient , series connected concentric coil
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