Abstract:
Inductive power transfer (IPT) is an emerging technology which achieves power trans-fer between two points without the use of physical connections. While there are many potential applications, the focus of this thesis is its use in electric vehicle (EV) charging to enable battery charging when the EV is stationary (parked over a designated area) or moving slowly in a taxi rank scenario. The thesis investigates the development of the magnetic pads with an interest on their leakage flux, power transfers and interoperability characteristics given that at the onset of this work, little had been done that combined all these aspects. While prior work had looked at optimizations for non-polarized circular pads to en-hance power transfer and reduce leakage, little work had been undertaken on polarized and multi-coil pad structures. As such, a study involving the polarized Double D pad (DDP) topology was performed and investigated how changing the sizes of the ferrite, copper, and aluminium from the original design proposed by previous researchers can af-fect the performance metrics of the IPT system. By extending the ferrite past the return windings, it was found to be possible to both reduce the peak coupling when the primary and a secondary pad were aligned, whilst also reducing the leakage flux when the pads are misaligned. A similar result is also true if the primary pad was a multi-coil Bi-polar pad (BPP). The work shows that such improvements, while effective, must be carefully designed since extending the ferrite too far can cause the leakage flux to rise as it begins to attract the main flux. An analysis was then performed to determine the interoperability for a range of mixed pad topologies, shaping and sizing these pads using best practice design approaches and practical limitation by car manufacturers. The chosen primary and secondary pad pairs had different topologies, and different sizes and used desirable air gaps. This study looked at the limitations of the Z- range and its impacts on the pad sizes required to transfer the a required VA to the secondary pad. The variations in the sizes and topologies required to achieve suitable power transfer directly impacted the coupling variations, unloaded pad quality factors, efficiency and leakage flux generated by the IPT system. The study also showed that if a smaller variation in primary inductance is the design goal, then non-polarized pads should be series tuned, whereas polarized pads should be parallel tuned. Otherwise, if the design parameter chosen is to minimise the variation in coupling factor as the secondary pad moves, then a BPP primary or secondary pad is the most desirable topology. Solenoid pads were considered but found to be undesirable as a primary pad because of the high levels of flux outside a vehicle where humans may be present, as well as large variations in coupling with movement The computationally intensive work of evaluating these interoperable systems and analysing them for design led to the development of a mathematical model to help reduce the total number of simulations to achieve a better understanding of how k varies in IPT systems. The work shows that the mathematical model developed reduces the relationship of k between a pair of pads down to a function based on the ratio between the areas of the primary and secondary pads, and is validated against simulations and practical design. The role of the BPP as a primary pad in an IPT system is analysed in detail, par-ticularly for applications where controlled alignment may not be possible, such as EV charging when parked at lights, or in taxi stands. The BPP primary is shown to perform better than either polarized or non-polarized options given it is more tolerant to misalign-ment providing its primary coils are properly controlled. Furthermore, if both primary and secondary control exist, then the system efficiency, as well as the leakage flux, can be managed despite large misalignment. If the BPP is properly designed, the coils will have minimal intercoupling with one another. However, as the secondary pad is placed too close to the primary, or is improperly designed, then its presence will cause this intercoupling to rise. The intercoupling impacts the effective coupling factor of the system, which describes the VA transfer between the primary and the secondary pads. An expression was derived to describe how all the coupling factors in an IPT system with a BPP primary and a single coil secondary can affect the effective coupling factor. This expression shows that series and parallel systems can have different effective couplings despite being in the same position. It also shows that the VA transfer can be controlled by varying not only the magnitude, but also the phases of the primary currents.