Modeling and Control of Inductive Power Transfer System with Misalignment and Load Independence

Abstract

Abstract Inductive Power Transfer (IPT) presents a promising solution for electric vehicles (EVs), yet challenges persist in achieving high tolerance against coupling and load variations. To address this issue, new model and control scheme for IPT system are required. This thesis is dedicated to the modeling and control of an IPT system with a focus on misalignment and load tolerance. To understand the properties and performance of the IPT system, this thesis begins with an investigation into the conventional model of an IPT system with a seriesseries resonant tank. Using the Finite Element Method (FEM), the electromagnetic field around the magnetic coupler is studied, providing insights into the relationship between misalignment and mutual inductance. Subsequently, an equivalent circuit is constructed for the IPT system. It becomes evident that under the conventional control scheme, the system’s output power and efficiency are vulnerable to misalignment variations, posing a significant obstacle in power flow management. To address this challenge, a novel digital control strategy is proposed. By synchronizing the inverter output voltage with the transmitter current, constant output power can be achieved in high coupling conditions. The control circuit is simple, as it only captures the zero crossing of the current in the transmitting coil. However, inherent propagation delays in the measurement and control circuit can lead to frequency drift and high switching losses, thereby compromising system efficiency and output power. In response, a digital control scheme with delay compensation is introduced, and the boundary of soft switching operation is investigated. It is concluded that soft switching operation can be achieved by applying the new control strategy. Experimental results reveal that within a misalignment range of 0−160mm, the system maintains nearly constant output power. A peak efficiency of 92% can be attained when soft switching is realized in the inverter. This proposed control scheme enhances the robustness of the IPT system against misalignment variations, facilitating cost-effective hardware implementation and obviating the need for dualside communication. However, the variation of load during power transfer can still cause change in output power. Further analysis of power flow under various load conditions reveals the system’s sensitivity to load variations. Leveraging the grid-connected rectifier, the DC-link voltage, crucial for the IPT system’s output power, can be controlled to address the issue. Thus, the voltage controller of a three-phase Vienna rectifier is designed. Simulation results demonstrate that the rectifier has less than 1V output voltage ripple under 800V DC-link output voltage and 50kW output power, with a total harmonic distortion of input current of 2.67%. Notably, the stability of the IPT system remains unaffected by variations in the DC-link voltage. Furthermore, the proposed control strategy provides more flexibility to the charging system without requiring additional DC-DC converters. This thesis addresses the key challenges in IPT system design and control, offering innovative solutions to achieve robust, efficient, and adaptable EV charging systems. The proposed control schemes provide constant output power despite changes in coupling and load conditions. Therefore, the works in this thesis pave the way for the development of versatile, compact, and lightweight IPT systems.