Design Aspects of Inductive Power Transfer Systems for Electric Vehicle Charging

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

High Efficiency Inductive Power Transfer Systems for Vehicle Charging

Awareness of climate change due to greenhouse gas emissions and air pollution has led to a transition from internal combustion engine vehicles to electric vehicles. Wireless charging by inductive power transfer is a promising solution for charging electric vehicles. Inductive power transfer is safe, convenient and can be integrated in a non-obstructing way with very low need for maintenance. Concerns on system efficiency and power transfer of inductive power transfer hinders the further development and deployment of the technology. In this thesis, the research focus is on two important measures to achieve high system efficiency.The first point is pad design. An analytical solution for the coils is proposed as a starting point for the pad design. The initial design is evaluated and further improved in finite element method simulations. A high switching frequency is desired to lower the flux density and hence reduce the amount of magnetic material in the pad.The second point is operation and power flow control. Unsymmetric duty cycle reduces the switching losses and results in higher efficiency compared with symmetric duty cycle. Phase shift is suitable for single phase systems or unbalanced three-phase systems. Load angle control can be realized in both single phase and three-phase systems and can also be utilized for bi-directional power transfer.In order to explore the performance of IPT systems in different power ranges, three systems are designed, built and tested. In the first setup, the focus is to realize a wireless charging system from the grid (230~V, 50~Hz) to 300 V dc. The system efficiency can reach 90~\% at the nominal power of 3.3~kW with a 20~cm air gap. To achieve higher power and efficiency, a 50 kW system is designed and built. The dc-dc efficiency is above 95~\% over a 20~cm air gap. A back-to-back setup is proposed which allows power circulating in the system and only the losses need to be provided. The third setup is a three-phase dual-active bridge topology with magnetically decoupled pads. The proposed topology has the advantages of employing commonly used three-phase inverters and provides a stable dc-link power flow. Magnetically decoupled pads also enables modularity of the transmitters and receiver pads and reduces the effects of interphase mutual inductance. The test results show that 252 kW can be transferred over a 12~cm airgap. The highest measured dc-dc efficiency for this setup is above 97~\%.Stray fields from the pads are a concern for inductive power transfer in public areas. From measurements on the first setup it is concluded that the stray fields are well below the international standard, both inside and in the surroundings of the tested vehicle

Design and Stray Field Evaluation of Inductive Power Transfer in Electric Vehicle Charging

The influence of magnetic stray fields in the vicinity of a vehicle is a concern when inductive power transfer (IPT) is used for wireless charging for electric vehicles. To accurately evaluate the stray fields, the design of the coils and the chassis configuration should be considered. In this paper the analysis and design of a series-series compensated 3.3 kW IPT charger is presented. A prototype of the charger is constructed and mounted on a plug-in hybrid electric vehicle. The stray fields are measured at rated power and compared with finite element analysis simulations. The simulated and measured magnetic flux density are in good agreement. The highest measured magnetic flux density is 1.67 μT at the left tire with the ground assembly misaligned by 100 mm. It is only 6.2 % of the flux density allowed for the general public in the ICNIRP standard. It is concluded that for low power IPT chargers, the requirement on leakage flux density is easily fulfilled

Rotor Design of a Line-Start Synchronous Reluctance Machine with Respect to Induction Machine for Industrial Applications

This paper introduces a unique rotor design for line-start synchronous reluctance machines. Efficiency and synchronization capability are determined and a comparison between line-start synchronous reluctance machines and induction machines is drawn. For the design and simulation, FEM calculations were used and the results were experimentally verified. Within the frame of this paper a 4 kW rated induction machine with an efficiency class rating of IE3 was chosen as a benchmark motor. The new rotor-cage design of the line start reluctance machine decreases the motor losses at steady state by about 28%. This promising approach could be used to upgrade induction machines in industry environments instead of replacing them. Nevertheless, a challenge in the design is a good balance between the steady state performance and synchronization capability, whereby the range of industrial applications is limited for high efficient line-start synchronous reluctance machines

Start capability of industrial synchronous motor with high efficiency

This thesis includes an investigation of the starting capability of a high efficiency synchronous motor for direct on-line industrial applications. The approach is to use a hybrid rotor concept to achieve the starting properties of an induction motor and the steady state performance of a synchronous reluctance motor. The hybrid motor rated at 20 kW is compared and evaluated against a same size induction motor in aspect to parameter variations. Simulations showed that the rotor losses can be reduced by 80 % in the best case. The total motor losses decrease by 20-30 %. This gain in efficiency will improve the efficiency class of the motor from IE3 to IE4. Furthermore, the results from this thesis hopes to raise the interest in investigation of new line-start, high efficiency motors further

Common Mode Power Control of Three-Phase Inverter for Auxiliary Load without Access to Neutral Point

The purpose of this study is to investigate the potential of obtaining an auxiliary dc output from the common mode switching harmonics of a three-phase inverter without access to the neutral point of the ac load. To achieve this, the control of common mode switching harmonics with space vector modulation is proposed. With this control, common mode switching harmonics can be regulated independently from the differential mode power flow to the main three-phase load. To physically implement the common mode power flow, a harmonic extraction circuit is placed in parallel with the main three-phase load. The extraction circuit is formed by series-connected inductance and capacitance. Due to the LC series resonance and the mutual coupling of the inductance, only common mode switching harmonics can pass through the extraction circuit. The extracted harmonics are then rectified and delivered to the auxiliary dc output. The control and topology are verified in both simulations and experiments. In the end, it is shown in experiments that there is no significant difference of the inverter efficiency after the auxiliary dc load is connected. This method can be used for instance to drive an ac machine while charging a battery or powering a compressor simultaneously

Rotor Design of Line-Start Synchronous Reluctance Machine With Round Bars

Within the field of industry applications, the induction machine (IM) is the most used motor type, due to its robustness and line-start ability. However, the need to constrain global warming demands new sustainable technologies with high efficiency. Therefore, line-start synchronous reluctance machines could be an opportunity to achieve a high efficiency for several industry applications, especially for constant-speed drives. This paper introduces a unique rotor design for line-start synchronous reluctance machines (LSSynRMs). Additionally, the influence of the rotor bar material/resistance and the stator resistance in comparison to common IMs for industrial applications is investigated. Finite element method calculations were used for the design and the efficiency enhancement was verified using experiments. Within the context of this analysis, a 4-kW-rated IM with an efficiency rating of IE3 was chosen as a benchmark motor. The new rotor-cage design decreases the motor losses at steady state by about 28%. This promising approach could be used to upgrade IMs in industry environments instead of replacing them. Nevertheless, a challenge in the design is a good balance between the steady-state performance and synchronization capability, which in turn limits the range of industrial applications for highly efficient LSSynRMs

Loss Reduction by Synchronous Rectification in a 50 kW SiC-based Inductive Power Transfer System

With development in wide band-gap semiconductors, such as silicon carbide (SiC), inductive power transfer (IPT) has become a promising technology for charging electric vehicles (EV). To meet fast charging demands by consumers, higher power levels in IPT is required. The power in IPT is usually limited by thermal stress due to losses at the vehicle side. Synchronous rectification is an efficient way to reduce losses compared to passive rectification. In this paper, the loss reduction accounted to synchronous rectification is quantitatively evaluated. The level of reductions varies depending on the load level, primary voltage, and constant current or constant voltage (CCCV) operation. The analytical results support the simulation results for various operating points. The loss profiles are studied for different dc-link voltages and duty cycles. An 800 V, 50 kW dual-active bridge test setup with series-series compensation is constructed and experiments are done for verification. The setup is arranged in a back-to-back configuration with a common dc-link. Experimental result shows that losses are reduced by up to 60 % on the receiving side with synchronous rectification. At rated operation the losses are reduced by 100 W in the secondary side inverter

Zero Voltage Switching for High Power Three-Phase Inductive Power Transfer With a Dual Active Bridge

Inductive power transfer (IPT) technology used for charging electric vehicles faces challenges in transferring high power because the power capacity and switching losses of high-frequency semiconductor devices are limiting factors. A three-phase system is a common high-power solution and soft switching is crucial for efficiency-oriented applications with high switching frequencies. In addition, dual active bridge (DAB) topology is a suitable topology for soft switching and has advantages in controllability and high efficiency. Therefore, to obtain higher power and efficiency, this paper studies the zero voltage switching (ZVS) of a three-phase inductive power transfer system with a dual active bridge. The three-phase IPT system with a DAB is different from both the normal three-phase DAB converter and the single-phase IPT system with a DAB, so their ZVS conditions and ranges are also different and need to be studied. This paper investigates the conditions and operating range for realizing zero voltage switching of the three-phase IPT system with a DAB. Based on this, the efficiency of the system is improved by changing the load angle between the primary and secondary sides. Finally, a 60 kW three-phase IPT system with a DAB is built, and the experimental verification of the study is conducted

Study on filtering performance of soft magnetic composite inductors for switching harmonics in 3-phase 70 kW PWM converter systems

Soft magnetic composite (SMC) is considered as an alternative solution for core materials of inductors used in LCL filters in power converter applications. Performance of SMC inductors in damping a range of harmonics caused by voltage switching is an interesting topic. The method used in this paper is to test the inductors together with a 70 kW PWM inverter to examine the filtering effects for different harmonics and the harmonic losses as well as the temperature rise of the SMC inductors. The study is conducted with different inductor currents and switching frequencies (5 kHz, 10 kHz and 15 kHz). The harmonic spectra of voltages and currents as well as the harmonic losses calculated based on the measured values. A FE model is built to study distributions of magnetic fluxes, losses and temperature. The model is verified based on comparisons of computed and experimental results