Contactless power transfer for electric vehicle charging application

Contactless Power Transfer (CPT) is the process of transferring power between two or more physically unconnected electric circuits or devices by means of magnetic induction. The potential application of CPT can range from power transfer to low power home and office appliances to high power industrial systems. Medical, marine, transportation, battery charging applications where physical connections are either dangerous or impossible or inconvenient are all prospective candidates for use of this technology. This thesis mainly concentrates on development of fundamental theory of CPT and application of CPT technology in achieving driving range extension of Electric Vehicles (EV). The work in this thesis can be divided into three broad categories which are, analysis and development of design criteria of a CPT system, experimental and practical implementation, and range extension studies with on-road charging of EVs using CPT technology. The main component of a CPT system is the specially constructed transformer with a large air gap between the primary and secondary winding. In order to achieve efficient power transfer through this large air gap, the principle of resonance is used. To increase the power transfer capability and to reduce the VA rating of the CPT system, capacitive compensation is used in both primary and secondary winding. In this regard, Series-Series (SS), Series-Parallel (SP), Parallel-Series (PS) and Parallel-Parallel (PP) topologies are analysed and design criteria for efficient and stable operation are presented. Constant current mode and constant voltage mode operation of SS compensated system are discussed and it is concluded that SS compensation topology is the most suitable topology for battery charging application. Power electronic requirements for efficient power transfer are investigated and it is concluded that use of active rectifiers in the output stage provide more controlling options and higher power transfer efficiency. To test the many analytically deduced design considerations and feasibility of a CPT system with respect to the efficiency of power transfer, an experimental setup is built. The efficiency of power transfer was measured to be close to 91%. But since DC power is required at the output to accomplish battery charging process, full bridge diode rectifier is employed in the output stage causing additional losses in the system which brings down the overall efficiency of system to 83.2%. To reduce losses in the rectifier stage, use of active rectifier is recommended. Later, CPT charging process is successfully demonstrated on MagIC (Magnetically Induced Charging) car, which is a radio controlled car with super capacitors acting as the on-board energy storage. Due to the multitude of advantages associated with CPT technology, the use of this technology in EV charging application is considered. In particular, application of CPT technology in on-road charge replenishment of EVs is discussed with the help of two case studies. In the first case study, urban driving scenario is considered in which it is assumed that CPT systems are installed at traffic signals and CPT for charge replenishment would occur whenever the EV stops at a red light. From this case study, it is inferred that, for the various urban driving cycles under consideration, considerable driving range enhancement is achievable i.e., the driving range of the EV would be more than doubled with CPT of 20kW. In the second case study, highway driving scenario is considered in which charge replenishment occurs when the EV drives over the primary winding buried underneath the highway. From this case study, it is concluded that to attain considerable range extension, large portion of the highway will have to be covered by primary winding of the CPT system. It is therefore recommended that economic and practical feasibility of such on-road charge replenishment systems have to be studied further in detail. A cost estimate of such CPT systems for stationary EV charging application is also presented.