High Efficiency Inductive Power Transfer Systems for Vehicle Charging

Doctoral thesis, 2021

Awareness of climate change due to greenhouse gas emissions and air pollution has led to a transition from internal combustion engine vehicles to electric vehicles. Wireless charging by inductive power transfer is a promising solution for charging electric vehicles. Inductive power transfer is safe, convenient and can be integrated in a non-obstructing way with very low need for maintenance. Concerns on system efficiency and power transfer of inductive power transfer hinders the further development and deployment of the technology. In this thesis, the research focus is on two important measures to achieve high system efficiency.

The first point is pad design. An analytical solution for the coils is proposed as a starting point for the pad design. The initial design is evaluated and further improved in finite element method simulations. A high switching frequency is desired to lower the flux density and hence reduce the amount of magnetic material in the pad.

The second point is operation and power flow control. Unsymmetric duty cycle reduces the switching losses and results in higher efficiency compared with symmetric duty cycle. Phase shift is suitable for single phase systems or unbalanced three-phase systems. Load angle control can be realized in both single phase and three-phase systems and can also be utilized for bi-directional power transfer.

In order to explore the performance of IPT systems in different power ranges, three systems are designed, built and tested. In the first setup, the focus is to realize a wireless charging system from the grid (230~V, 50~Hz) to 300 V dc. The system efficiency can reach 90~% at the nominal power of 3.3~kW with a 20~cm air gap. To achieve higher power and efficiency, a 50 kW system is designed and built. The dc-dc efficiency is above 95~% over a 20~cm air gap. A back-to-back setup is proposed which allows power circulating in the system and only the losses need to be provided. The third setup is a three-phase dual-active bridge topology with magnetically decoupled pads. The proposed topology has the advantages of employing commonly used three-phase inverters and provides a stable dc-link power flow. Magnetically decoupled pads also enables modularity of the transmitters and receiver pads and reduces the effects of interphase mutual inductance. The test results show that 252 kW can be transferred over a 12~cm airgap. The highest measured dc-dc efficiency for this setup is above 97~%.

Stray fields from the pads are a concern for inductive power transfer in public areas. From measurements on the first setup it is concluded that the stray fields are well below the international standard, both inside and in the surroundings of the tested vehicle.