Magnetic coupler design in wireless power transfer for vehicular applications

Abstract

Wireless power transfer (WPT) is increasingly attractive because of its simplicity and user-friendliness. As a soul part, magnetic coupler design is of great importance in WPT systems with beauty to improve the wanted couplings for energy transfer and to reduce or even eliminate unwanted couplings for stable operation. Furthermore, impressive characteristics, e.g., load-independent outputs, high efficiency, and high misalignment tolerance, are all highly related to magnetic coupler design. Hence, this thesis investigates several special magnetic couplers for wirelessly charging vehicular applications. Firstly, a special magnetic coupler based on three-coil WPT is proposed to charge electric scooters (ESs). Unipolar Q coil, mixed QDD coil, and bipolar DD coil are designed for the source coil, the transmitter coil (TX), and the receiver coil (RX), respectively. This coupler utilizes unipolar coils and bipolar coils, thereby magnetically decoupling the receiver coil from the source coil to realize load-independent output characteristics. Similarly, Q coils, DD coils and DDQ coils can also be utilized in double-receiver WPT systems for automatic guided vehicles (AGVs) to eliminate the unwanted cross-couplings. Unwanted cross-couplings are the bottleneck to hinder the development of double-receiver WPT systems, which dramatically disturb the system stability because receivers affect each other when the air gap changes, or misalignment happens. Two types of unwanted couplings can be reduced by presented structure. The first-type unwanted couplings between receivers can be eliminated, contributing to independent-work receivers. The second-type unwanted couplings among the source coil and receivers can be eliminated. Thus, load-independent constant voltage (CV) output can be realized. The inverter achieves zero phase angle (ZPA), degrading the volt-ampere rating and increasing efficiency. Most importantly, the coupler design process can become efficient since the proposed structure ensures an easy way to reduce the cross-couplings because the decoupling is from the shape rather than turn numbers. Then, a novel resonator scheme has been studied for wireless battery charging systems of Electric Bicycles (EBs). The Helmholtz coils are adopted to ensure a stable mutual inductance between the transmitting and receiving coils. The switched S-SP/S compensation scheme can be used to implement load-independent constant current (CC) and constant voltage (CV) charging for the battery loads instead of using bulky LCL or LCC compensation schemes. The advancements of the proposed design, as compared to the conventional methods, are more than constant mutual coupling and elimination of compensated inductors. The communication channel, additional user-end converter, and complicated control algorithms can also be annihilated. After investigating stational wireless chargers, dynamic wireless power transfer (DWPT) is also presented for wireless charging electric trains (ETs). An innovative topology using parallel multi-inverters, together with an improved magnetic coupler design, is introduced to increase the power capacity. A segmented transmitter technology, in which several transmitters mounted on the rail track are energized according to the position of the onboard pickups, is investigated to realize continuously stable power supply. Overall, several vehicular-application-oriented WPT systems, including the special magnetic coupler designs, associated power electronics and compensation topologies, are all analyzed in detail, developed, and validated by simulation and experimental tests.