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.

dual-active bridge

power flow control


electric vehicles

electromagnetic fields

inductive power transfer

high efficiency

high power


Opponent: Professor Peter Omand Rasmussen, Aalborg University


Daniel Pehrman

Chalmers, Electrical Engineering, Electric Power Engineering

A Full Power Range ZVS Control Technology for Bidirectional Inductive Power Transfer System

IECON Proceedings (Industrial Electronics Conference),; Vol. 2020-October(2020)p. 3861-3865

Paper in proceeding

Loss Reduction by Synchronous Rectification in a 50 kW SiC-based Inductive Power Transfer System

IECON Proceedings (Industrial Electronics Conference),; Vol. 2020-October(2020)p. 3907-3912

Paper in proceeding

Design and Stray Field Evaluation of Inductive Power Transfer in Electric Vehicle Charging

2019 14th International Conference on Ecological Vehicles and Renewable Energies, EVER 2019,; (2019)

Paper in proceeding

Pehrman, D. Liu, Y. Cui, C. Power Flow Control of Bi-directional Three-Phase IPT System

Cui, C. Pehrman, D. Liu, Y. Zhang, Q. Zero Voltage Switching for High Power Three-Phase Inductive Power Transfer with Dual Active Bridge

The transition from internal combustion engine vehicles to electric vehicles is an important step to reduce greenhouse gas emissions. The main challenges with this transition are charging and range of electric vehicles. The human interaction required for charging an electric vehicle is similar to refueling a combustion engine vehicle, yet charging is orders of magnitudes slower than refueling.

In this thesis the possibility of charging electric vehicles wirelessly is studied. Wireless charging can be realized by inductive power transfer between one or several transmitting pads placed on the ground and one or several recieving pads mounted on the vehicle. The principle is based on resonant operation between coils and capacitors which are tuned to a resonant frequency. Inductive power transfer is a safe and convenient way of charging which can also be implemented in urban areas in a non-obstructing way. By charging electric vehicles wirelessly the need for large batteries can also be reduced since charging can take place more frequent.

Within the frame of this work three different test setups are designed, tested, and evaluated. The focus point on this has been to demonstrate high power and high efficiency operation. System efficiency ranges from 90% - 97% and power levels are from 3 kW up to 250 kW. The air gap between the pads has a big influence on the performance. In the different systems, the air gap is selected to be between 10 cm – 25 cm which is considered reasonable for the application.

From the study it is concluded that inductive power transfer is suitable and promising technology for charging both passenger electric vehicle and larger vehicles such as trucks or buses. It is also shown from measurements on a vehicle that the stray fields from pads is well below the international limit.

Modular inductive power transfer (IPT) for high power vehicle charging

Swedish Energy Agency (46356-1), 2018-11-01 -- 2021-06-30.

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Electrical Engineering, Electronic Engineering, Information Engineering

Other Electrical Engineering, Electronic Engineering, Information Engineering



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4980





Opponent: Professor Peter Omand Rasmussen, Aalborg University

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