Abstract:
Electric Vehicles (EVs) are a viable option for future transportation, but readily available infrastructure to charge their batteries is still developing. It is possible to charge EVs while they are parked at home, on the side of a public road or even while in continuous motion on a roadway with conductive charging systems. However, EV charging can be improved with wireless charging, which removes the need for mechanical connections, improving safety and reliability. The leading technology for wireless EV charging is Inductive Power Transfer (IPT), which uses a magnetic coupler on the ground side (called a primary) and on the vehicle side (called a secondary) to produce and collect the magnetic field. A roadway is a very harsh environment as it is subject to extremes in temperature and vibration. Any roadway IPT system must operate for a long time without maintenance as taking it out of service for maintenance causes disruption. One key way to improve the mechanical robustness is to remove the most fragile element of the IPT magnetic coupler, ferrite, which has excellent magnetic properties but is a brittle ceramic that may break in the roadway. In addition, magnetic field leakage from the system should be as low as possible, as there may be people present near the system. This thesis explores ways to design IPT magnetic couplers for a roadway environment for improved mechanical robustness and a reduced leakage magnetic field. The magnetic topologies investigated use a stationary charging system, which has similar dimensions to those being standardised, as a basis of comparison. However a fully dynamic charging concept is explored at the end of the thesis. A background to IPT systems is presented with an emphasis on roadway environments and previous designs for magnetic couplers, including an axisymmetric circular magnetic coupler with a reflection winding. An optimisation of a circular reflection winding with a genetic algorithm is presented to investigate and improve on the reflection winding concept. A metric is then developed to fairly compare the leakage magnetic flux density from different primary geometries when they deliver the same power to a secondary. Analysis is also provided to show that modelling magnetic couplers as a solid block is a sensible approximation, which reduces simulation complexity and run time. Finally, a comparison of two of the leading magnetic topologies, the rectangular and the Double D (DD) magnetic coupler, is presented by comparing the effect on the normalised leakage magnetic flux density when altering their geometries. From this analysis the DD topology is selected as the preferable topology for roadway IPT, given the DD magnetic coupler has a greater tolerance to misalignment and lower leakages where people may be present. To investigate improving the mechanical robustness of the DD topology in a roadway environment, an analysis of the complete removal of ferrite and the optimal placement of a reflection winding to shape the field is presented. An optimised magnetic coupler was validated in the laboratory, and the reflection winding then further optimised with a genetic algorithm. A mathematical model is also presented which allows the parameters of the magnetic coupler to be calculated with variations of the reflection winding. These variations include changing the number of turns, and two different ways to drive current in the reflection winding; the reflection winding can be directly driven with the same current but in the opposite sense to the driven winding, or Lenz’ law can be exploited to produce a similar current in the reflection winding. These ferrite-less Double D magnetic couplers are validated in the laboratory. The most effective use of small volumes of ferrite in DD magnetic couplers is investigated by presenting the effect on coupling and leakage as the size of the ferrite is increased from nothing to its original size. A small practical size of ferrite is chosen, and a reflection winding added to further reduce the leakage field. Additional ferrite is then placed at the edge of the magnetic coupler to further reduce leakage, and a number of these magnetic couplers are validated in the laboratory. Finally, a primary topology is presented which provides a continuous power transfer with a number of discrete magnetic couplers to a moving vehicle. To evaluate this, a different secondary is used to that used for the stationary magnetics. This is a multi-winding bi-polar magnetic coupler, which is better able to assess interoperability to different secondaries. The primary can behave as either a rectangular or DD magnetic coupler, which is the mechanism that allows smooth transfer to the secondary bi-polar magnetic coupler while in motion. The primary can also be used with rectangular or DD secondary magnetic couplers but the power transfer is not as consistent while the secondary moves over the primary. A number of practical issues are discussed including the selection of copper area, the effect of cross coupling between the primaries, and the leakage magnetic field. This thesis explores the design of inductive power transfer magnetic couplers for roadway applications. The key areas focused on are the development of suitable metrics and techniques to evaluate roadway IPT systems, the investigation into reducing and removing ferrite in DD topology magnetic couplers, and the addition and optimisation of a reflection winding to help shape the field. It is completed with an idea to develop a continuous Double D highway that is both modular in installation and is able to provide a continuous power transfer.