Wireless Charging for Electric Vehicles
Grant Covic and John Boys
The development of wireless charging systems for electric vehicles has gained significant momentum over the past decade. Part of this is based on the desire of cities to push away from petrol and diesel powered vehicles to help provide cleaner cities, given the intense urbanization which is occurring globally, and partly because electric vehicles are becoming more efficient and cost competitive. With wireless charging systems properly integrated into vehicles, and situated strategically around a city as well as at owners’ homes there should be no need to ever plug in their vehicles. Drivers should simply park as usual over a coil placed on the ground or buried in it. However adopting this technology also has the potential to solve a number of real and perceived problems. One of these is that today’s younger generation expects to be unshackled. There is a natural desire to have the world at its finger tips and to be able to move and yet stay connected. Yet it was not that long ago that cities and the country side were filled with telephone boxes, and queuing to make a telephone call while being tethered to a wired receiver was not uncommon. Today tethering for most communication needs would be unpalatable for most applications except for high speed internet and certainly not for basic internet or communication needs, while the idea of having to queue to make a call would send most customers to another network provider. The analogy for electric vehicles is obvious. Today plug in charging is common but in the future will become the domain of high power fast charging where necessary. The idea of queuing for such power is generally undesirable, and with sufficient wireless power charging systems, should be unnecessary in future cities.
Today, vehicles are charged at home or, in the case of fleets, at the owner’s place of business. Tomorrow, they’ll be charged in a variety of additional places, including at work, at the store, on the street and in places of interest. The provision of wireless electric vehicle charging points at these locations may increase employee/customer loyalty, attract new customers, and encourage wider adoption of wirelessly charged vehicles in larger population centers, thus reducing air pollution. Ideally, vehicles can be charged whenever and wherever they are parked and EV owners should not have to concern themselves with the grid connection, which will happen automatically.
This constant connection helps reduce range anxiety since the battery is then better able to be kept in a good state of charge, and also helps the network, because private vehicles do not arrive home demanding high power beyond the utility network design. In developed countries, electric utilities use consumption history to assume that each household will use a certain amount of power—about 2 kW, on average—and they build their infrastructure accordingly. Present EVs can demand as much as 8-10kW so that one or two EVs in a neighborhood could demand more than the design capacity and overrate the street transformer. To accommodate that demand, utilities would have to upgrade their transformers and other infrastructure, however if EVs are better managed by constant connection throughout the day, the demand at night can be minimized. Thus the wireless home charger would only operate to top up the battery and may be cheaper and smaller as a result.
Extending the range of EVs is also critical for growing their market beyond a niche. While modern batteries are improving, still consumers want never to have range concerns, and fleet owners in particular have a concern to maximize the time each vehicle is in use. Dynamic charging gives far better vehicle utilization.
State of the charging technology:
Many, if not all, of the automotive manufacturers have spent the past several years working with their suppliers evaluating, developing and refining wireless charging technologies. Today’s wireless charging technologies have efficiencies north of 90 percent, which is just a percent or two less than plug-in systems. The magnetics of the wireless transformer are essentially split (the primary on the ground and the secondary on the vehicle), and power is coupled using fields that are shaped to exist in the gap as shown in Fig. 1. The power-transfer efficiency is further improved by turning on only when a vehicle is present and needing power.
The wireless architecture is designed to minimize fields outside the vehicle footprint, which reduces the risk of exposure to humans. Fields are shaped and controlled to remove potential interference or exposure to humans. Safety is incorporated in the ancillary systems covering foreign object detection (FOD) and living object protection (LOP). A FOD system identifies any metallic objects between the base pad (BP) and vehicle pad (VP). This is important as metal items, even small objects such as a paperclip, may heat up and could pose a burn risk during power transfer. A LOP system identifies the presence of humans or animals close to the power transfer system, where magnetic field levels may exceed exposure regulations. Examples include a child reaching for a ball under a vehicle or a cat sitting under a vehicle. Both the FOD and LOP systems are normally located in the BP to reduce complexity on the vehicle. Should either a FOD or LOP safety system be triggered, power transfer will be suspended. The driver will be notified via a phone or email alert, and charging will reinitiate once the metallic or living object has been removed or moves on.
Multi-vendor interoperability is another desirable goal. Interoperability is the ability of any VP to charge from any BP irrespective of their design, manufacturer or vehicle to which they are fitted. Wireless charging won’t be attractive to vehicle owners if they have to wonder whether their destination has a compatible system. The good news is that the automotive industry is working towards standardization of WEVC. Over the past couple of years, automakers and others have been working to iron out these differences. They have a vested interest in interoperability because it eliminates the expense of developing country- or region-specific wireless charging solutions. Although most are still a work-in-progress, various standards like SAE J2954 are beginning to define criteria for safety and electromagnetic limits, testing, and efficiency and interoperability targets [RP].
In both high and low power applications -  the secondary systems which are desired for the vehicle are simple coil systems with minimal power electronics – although as shown ,  having the ability to control the Volt-Amps on both the primary and secondary coils and the associated electronics ends up with the best efficiency and also better emission profiles. For primary side charging both rectangular and Double-D (DD) designs (Fig. 2) have been considered by standards bodies, however one obvious pad in its omission is the solenoid, which was in favour early because of its simplicity . However unlike the rectangular and DD designs which have their main flux entering and exiting the magnetic structure orthogonal to the ground pad and directly toward the vehicle pad, the solenoid has its main flux exiting horizontally towards the sides of the vehicle, before this field bends around towards the secondary. Shaping the flux is difficult and consequently the emissions are also challenging. However using it as a secondary (particularly with a DD primary) results in good coupling factors ,  and since in light duty vehicles the secondary is usually much smaller and operates with a lower Volt-Ampere product, it’s operation does not impact the emissions significantly.
Increasing power transfer levels, especially of the in-ground system, will be essential if range concerns are to be completely eliminated. These base system will need to work with not only private vehicles but also goods trucks and other electric powered vehicles. As the vehicle power demand and distance between the couplers increase, there is a need to increase the size of the vehicle magnetics to help keep the coupling within a suitable design range (ideally in the range 0.1-0.25).
A simple rule of thumb for designers  is:
Where P_ois the output power, k¬ is the coupling factor between the ground and vehicle magnetic pads, and VA represents the Volt-Ampere product.
Thus a k=0.1 requires the product of the primary and secondary VA’s to be 100 times higher than the desired output power. At 10kW power transfer and matched sizes of magnetics and power electronics this requires the primary and secondary to be rated around 100kVA, but if the primary magnetics and electronics have higher ratings then the burden may fall on this design requiring perhaps 300kVA to lower the ratings of the secondary. In the end the primary VA is limited by the field emissions around the car and on the surface of the pad where small foreign objects may lie and get heated if not detected. Typically these surface fields are kept to around 3mT, while fields on the side of the car where humans may be present are kept to below that which may impact a pacemaker device.
Having too low a coupling means that the VA of the primary and secondary electronics have to be large to transfer the power. So it may seem like higher couplings are a good idea, but a higher k means the primary and secondary inductances are more sensitive to misalignment which means detuning can arise, and it also limits how large an operating resonant tuning factor (Q) can be achieved in the circuit before bifurcation occurs . Having circuit resonance factors in the range 3-10 is important to help reduce the emissions and ensure sinusoidal waveforms in the resonant tanks on both the primary and secondary.
The assumptions around the design of the magnetics plays a big part in the system’s optimization. While most systems assume sufficient tolerance for manual parking, the future of stationary parking alignment is likely to be handled by automatic guidance systems that will ensure operation is within design parameters [x,y] = [±75mm, ±100mm].
In higher power applications for public transport or trucks, the secondary magnetics may be generally matched to the primary or made bigger. As shown  mismatches of up to 2:1 are good design choices, but if there is a greater mismatch then the absolute coupling reduces severely. As a trade-off however having a large secondary  helps limit the sensitivity to misalignment and emissions which can be useful in the transition between stationary and moving applications.
Providing charging to meet a future city’s electric transport needs.
In future cities transport may be dominated by electric vehicles within the inner urban environment for public transport, goods services and taxis.
While public transport services like bus and rail have in the past had proprietary parking and corridors through cities, this is now being challenged by new light rail systems under development which run on wheels and therefore utilize the same road infrastructure. Presently most installed bus charging systems operate at a different frequency (20kHz or 50kHz) compared with private vehicle charging which is targeting 85kHz. This difference has been acceptable because of the proprietary nature of the public systems. Lower frequencies help constrain the reactance of the ground pad and therefore the voltages and core losses in the magnetics, and the pads are generally bigger because of the power levels under consideration. There is a general dislike to include lowering mechanisms on private vehicles, however the size of the air-gap between the ground and vehicle pads is dictated by the size of the magnetics and some of the vehicles in the mid-high power class are very high off the ground, thus if the vehicle system is not to become too large for these applications some approaches have to be considered. In public vehicles these gaps can be somewhat constrained by, for example, kneeling a bus.
Dynamic electric vehicle charging (DEVC) or charging on the move, whilst moving or driving ¬¬is a potential future application of this technology. This could be applicable in slow moving traffic, for instance at taxi ranks or at lights and also at higher speeds, such as lanes on a highway. It also may be used to provide additional power up hills, thereby essentially flattening the roads around cities, making EV battery design for public transport needs somewhat easier in hilly environments. However if the roadside space or roads are shared in future, doubling up on infrastructure that supports light duty private vehicles or heavy duty trucks and buses needs to be carefully considered in terms of its economics for a city. To prevent rapid wear-out of the road vehicles should meander slightly. This discussion is at the heart of research and standards today.
Taxi companies have been among the first and biggest buyers of hybrids. Wireless charging stations on streets is an ideal fit for taxis allowing taxi companies to buy EVs removing concerns over range and limited operation time. Another reason, particularly in hot climates, is that EV taxis could use public wireless charging to keep their air conditioning going while waiting for customers.
Presently off road above ground and flush mounted systems are being considered by standards groups. Below ground systems have not been looked at yet. As the gap between the ground and vehicle pad increases the coupling decreases rapidly, and smaller vehicle pads which are suited for stationary off-road charging may not be suitable for on road charging because of these larger gaps.
For vehicles used in applications where the alignment that is being targeted for stationary charging is not guaranteed, simple primary pad structures may also not be ideal. As an example, Taxi’s queuing in ranks are likely to need higher misalignment tolerances and higher power levels when waiting for customers. To achieve this, the primary pad in the ground needs to shape the field under the vehicle to enable the power transfer to be met without producing emissions despite non-ideal parking alignment. This requires more complex magnetics, using two or three coils , , -, and is presently the focus of research. Examples of two such multi-coils with two and three coil decoupled structures are shown below in Fig 3. These allow single phase, two phase and three phase operation as desired given all coils are independent allowing independent energization.
As the power increases it is also likely that the secondary electronics will need to improve. Today vehicle manufacturers prefer simple electronics, but bidirectional systems and active electronics that support multi-coil architectures are starting to show real promise . Like motor drive systems that evolved from single phase to three phase systems, wireless power transfer is evolving. Wireless power systems of the future need to link to the grid and should support clean energy initiatives. To do this they need to be connected more frequently and easily (which is possible with this technology), and may need to be available to support fluctuating energy sources such as solar and wind.
Future Research Directions
Technology requirements for roadway systems, particularly for dynamic charging and powering, will likely require changes to all parts of the present technology. An Inductive Power Transfer (IPT) Roadway is without doubt the most demanding IPT application. Here there are two conceptually different pads in road. For taxi-rank systems high power charging is needed using a ground pad with a strong cover buried to a depth where the roadway surface can be flat with no blisters indicating the presence of a charging pad in a thermally constrained environment. In dynamic applications the pad and any associated electronics are buried in a roadway with limited service opportunities and must be strong enough to withstand the forces of any type of vehicle running over it, without compromising the system or the roadway integrity. This in-ground pad must be even stronger than the taxi-rank system and may need a thick cover – perhaps 25 or even as large as 100mm – making good coupling difficult. The road material varies widely between US highways and Europe thus understanding the design constraints and expectations of road manufacturers amongst cost and performance is the target of research for the next 5-10 years to find suitable designs which meet cost and performance expectations, and are economically viable. Present research is focused on design options for pads or tracks  , , and also options with and without ferrite ,  to try and improve robustness. While eliminating ferrite reduces the coupling, it significantly improves robustness and may improve long term quality of the pad since there are no core losses and the copper may be able to be closer to the road surface.
Over the next decade, there will be a rapid roll-out of wireless charging infrastructure, initially in off-road parking applications soon after standardizations completes. However the pressure to extend this to on-road applications such as taxi-rank systems will follow soon after. Researchers must focus on improving the options and reducing the cost of the electronics to enable on-road stationary and dynamic power transfer and along with this there will be careful analysis of what may be economically viable, however the economics are not just in terms of getting a return on infrastructure, but also on the health benefits of electrifying within cities. What is also likely is some compromise between powered sections in and around cities and DC fast charging, with fast charging systems likely being preferred intercity and interstate.
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John Talbot Boys was born in Invercargill in 1940 and completed a BE in EEE in 1962, and a PhD in Physics in 1968 before going overseas for 5 years, in England with Redac Software Ltd and in The Republic of Ireland with a US Multinational, SPS Technologies. He returned to New Zealand in 1974 to a lectureship at the University of Canterbury transferring to the University of Auckland in 1977, where he is currently a Distinguished Professor Emeritus in the Department of Electrical and Computer Engineering.
Professor Boys’ research interests are Power Electronics, AC Motor Control, and Inductive power Transfer and in all of these he has published widely had Patents granted, and had successful collaborations with National and International Industries. He has won a significant number of prizes culminating in The Prime Minister’s Prize (with Grant Covic) for work with IPT. Professor Boys’ Professional interests are with Engineering New Zealand where he is a Distinguished Fellow, and more academically he is a Fellow of the Royal Society of New Zealand (FRSNZ).
In other interests Professor Boys is a keen golfer and a regular Church attender, and takes a real interest in the effect of technology on our standard of living and happiness.
|Grant Covic is a full professor at The University of Auckland, began working on inductive power transfer in the mid 90’s, and by early 2000’s was jointly leading a team focused on AGV and EV charging solutions. He has published more than 150 international refereed papers in this field, worked with over 30 PhDs and filed over 40 patents, all of which are licensed to various global companies in specialised application fields. Together with John Boys he co-foundered HaloIPT and was awarded the NZ Prime Minister’s Science Prize, amongst others for successful scientific and commercialization of this research. He is a senior member of IEEE, and a fellow of both Engineering New Zealand, and the Royal Society of New Zealand. Presently he heads inductive power research at the UoA, co-leads the interoperability sub-team within the SAE J2954 wireless charging standard for EVs, and is an IEEE Distinguished lecturer for the transportation electrification community.|