Design Aspects of Inductive Power Transfer Systems for Electric Vehicle Charging

Licentiatavhandling, 2019

During the last decade the transition towards electric propelled vehicles has had an upswing and electric vehicle (EV) sales has increased steadily. This is much due to legislations on lowering emissions but also due to lower total cost of ownership. Eliminating tailpipe emissions is a major driver for the electrification of the transportation sector. In order to meet the goals on CO2 -emissions the shift towards EVs in the transportation sector needs to increase even faster. Two of the holdbacks for a more widespread penetration of EVs are the limited range and the frequent and slow recharging compared with internal combustion engine (ICE) vehicles. With technology advancements in energy storage and power electronics the energy storage and charging of EVs could instead become a strength for EVs.

An emerging technology for recharging EVs is by inductive power transfer (IPT). Having no contact between the vehicle and the charger makes it inherently safe with respect to electrical shocks. Furthermore, with no moving parts the maintenance requirement becomes minimal. This technology is especially appealing in automated charging applications and opportunity charging. Charging can be initiated automatically for buses at bus stops, delivery trucks when loading or unloading goods, taxis at taxi ranks and at traffic light intersections. By charging more frequently the life time of the battery is increased. Alternatively, a smaller battery pack can be used. IPT can also be integrated seamlessly in public parking places without obscuring the view and without any risk of getting unplugged.

The fundamental principle of IPT is based on power transfer by non-radiative electro-magnetic fields. Challenges with designing IPT systems involve trade-offs between efficiency, misalignment tolerance, gravimetric- and area related power density, and stray fields. In this thesis a thorough analysis of coil design is presented and the most common compensation topologies are evaluated. Two series-series compensated IPT chargers are designed and prototypes are developed and verified experimentally. Firstly, a home charger rated for 3.7 kW input power with an air gap of 210 mm is designed. The coil design is based on analytical results in combination with the finite element method. In this system, the current in the primary coil is constant, regardless of alignment and coupling between coils. At rated load with aligned coils, 94 % dc-to-dc efficiency is achieved. The second charger is a fast charger rated for 50 kW with an air gap of 180 mm. The dc-to-dc efficiency is above 95 %, down to 10 % of the rated load, including losses in the full-bridge inverter, transmitting- and receiving coil with compensation, and the output rectifier.

By using SiC MOSFETs a high switching frequency (85 kHz) for power transfer and a more compact coil design can be utilized. The area related power density of the vehicle assemblies of the two chargers are 20 kW/m2 and 148 kW/m2 respectively. A limiting factor for the maximum achievable power transfer capability is the stray fields around and inside the vehicle. A simulation model of the stray field is developed and verified with measurements. With the home charger mounted on a plug-in hybrid electric vehicle (PHEV), measurements show that the magnetic flux density is less than 10 % of the allowed emission limits at the most severe locations.