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Cable-free charge-up – Inductive charging stations for electric vehicles

by Prof. Dr. Benedikt Schmülling / schmuelling@uni-wuppertal.de

Wireless battery charging is a far more convenient solution for the driver of an electric vehicle than a conventional cable link-up. You park your car in the charging bay and the automatic charge-up process begins immediately. No more plugs and sockets; no more dirty or weathered cables; no more risk from cables dangling between charging station and vehicle. These and other advantages make contact-free inductive battery charging technology an attractive alternative to conventional cable systems.

The Federal Government’s energy concept sees Germany as developing into a lead market for electromobility. Key advantages of raising the market share of electric vehicles are the potential reduction of greenhouse gas emissions – on the assumption that the electricity used for charging the vehicles is derived from renewable sources – and the use of off-peak electricity to even out fluctuations in daily production-consumption ratios. In this respect, however, the economic and ecological impact of electromobility depends on the length of the charging period, so significant impact presupposes a large number of vehicles using the system at the same time.

Conventional battery charging systems use cables to connect the vehicle to the electricity supply. This has several disadvantages, including the risk of vandalism, as well as extra tasks involving inconvenience for the driver. Lack of care in handling the connecting cables may result in flat batteries, and loose or damaged cables can be a safety risk.

An alternative is contact-free battery charging with energy transfer by an electromagnetic field . This innovative technology provides simple, reliable and safe battery charging and is thus geared to significantly raising consumer acceptance of electromobility. This, in turn, will enhance the beneficial impact on the increasingly unpredictable production-consumption curve of electrical energy. Inductive charging avoids the drawbacks of cable systems: it is barrier-free, with no plug-in or other manual interventions and no dangling cables to tempt vandals. In fact the stationary side of the system can even be hidden beneath the asphalt. Moreover, wireless technology of this sort will likely be less susceptible to wear and tear than plug-in systems.

Fig. 1: Stationary (below) and mobile (above) inductive charging plates for electric vehicles developed by Paul Vahle GmbH & Co. KG, Kamen.

No inductive charging system for electric vehicles has yet come on the market, but several companies and research institutes worldwide are working on projects that promise successful market launch. The following paragraphs will outline the various components of an inductive charging system and describe some successfully functioning prototypes.

Inductive charging systems for electric vehicles consist of a mobile element attached, for example, to the base of the vehicle, and a stationary element over which the vehicle, in this example, must be positioned for charging. The stationary element can be situated in a publicly accessible parking space either above or immediately below the asphalt surface.

The electromagnetic coupling between the mains energy supply and the vehicle is via a stationary charging plate and a mobile pickup plate. The stationary plate is connected (conductively) to a field regulator – a high frequency (HF) inverter equipped with a field regulating system . This converts the grid source (at 230 V and 50 Hz) into high frequency AC in the low 3-figure kHz region. Powered by this HF source, the stationary charging plate produces an HF alternating electromagnetic field. The plate assembly consists of a flat copper coil with several turns set in a robust envelope of e.g. fiberglass-reinforced plastic (FRP), which is then mounted on a matching aluminum sheet covered with a layer of soft magnetic ferrites . The mobile pickup plate is similarly constructed, possible differences consisting only in its external measurements, the number of turns in the coil, and perhaps the distribution of the windings.

Fig. 2: Electric vehicle at a Vahle charging station. Photo: Paul Vahle GmbH & Co. KG.

Fig. 1 shows typical stationary and mobile charging plates, and Fig. 2 shows a vehicle parked at an above-ground stationary plate. In line with the safety limits for electromagnetic fields prescribed by the Regulations for Applications ( VDE-AR-E-2122-4-2) of the Verband der Elektrotechnik, Elektronik, Informationstechnik (VDE: Association of German Electrical Engineers), the dimensions of prototype charging plates are very large. Thus the stationary plate may have an area of up to A ≈ 1 m2, which – combined with its weight – represents a major marketing obstacle for the system. The ideal inductive charging system would combine low costs with high efficiency, and high power density with conformity to electromagnetic field safety limits.

The HF electromagnetic field of the stationary charging plate is linked inductively with the coil of the mobile plate as in a conventional transformer –the circuit drawing is technically equivalent. However, the high operating frequency causes high system reactances , which must be compensated in order to ensure adequate transfer of active power to the vehicle. So the primary coil is connected, either in series or in parallel, with condensers. The secondary coil receives similar treatment, and the HF current induced there is then fed through a rectifier into the on-board battery charger. The nominal power rating of systems developed to date is PN = 3.3 kW, for a transfer distance between charging and pickup plate of up to 210 mm.

The choice of electromagnetically compatible components with suitable performance that work smoothly together is a major challenge in the development of these systems. In particular, the design and manufacture of a reliable field regulator presents a problem, as the use of 140 kHz technology in devices of this sort is not common at present. Fig. 3 shows the entire inductive charging system chain.

Fig. 3: Inductive charging system chain.

An alternative to the floor-mounted charging plate is inductive charging through the vehicle’s front registration (‘number’) plate. The difference lies only in the positioning, design and, above all, size of the two plates. The pickup plate – again containing a flat coil – will have the same dimensions as the vehicle’s front registration plate and be mounted directly behind it. The registration plate itself must in this case be made of material that possesses low values of electric and magnetic conductivity, e.g. plastic.

The stationary charging plate will be slightly wider than the registration plate and be mounted on a specially designed stand that is permanently installed at a public or private parking site. To start the charging process, the vehicle will be parked at the stand in such a way that the front registration plate is in direct physical contact with the stationary charging plate. This is mounted on an arm that can retract into the stand if the vehicle exerts too much mechanical pressure on it. Indicator lamps on the charging stand tell the driver when the correct parking position has been reached. Fig. 4 shows a charging stand in use.

A great advantage of this system, compared with the floor-mounted solution, is its significantly smaller charging plates. Due to the immediate contact between the plates, the distance to be bridged is far shorter and the magnetic link-up correspondingly stronger. Moreover, no foreign bodies can enter the intermediate space between the plates, so the magnetic flux density can be a good deal higher without incurring safety problems, and the plates are therefore significantly smaller for the same nominal power transfer rating. A disadvantage is the complexity of the charging stand, with mechanically movable components and indicator lamps. Conversely, an advantage of the floor-mounted system for many potential areas of application is its simplicity for the driver, who can approach and leave it in any direction.

Fig. 4: Electric vehicle at a Vahle charging stand.

Current systems from both German and international providers show that inductive charging stations for electric vehicles are a viable commercial option, but they must be optimized and extended in scope before they can be introduced widely on the automotive market. UW’s Electromobility Research Group is working intensively on the further development of these systems with a view to achieving early market launch.

One question, for instance, is how to ensure the correct positioning of the vehicle over the stationary charging plate. There is some lateral tolerance in this respect, but serious misalignment would lead to loss of efficiency and power transfer. What is needed in practice, therefore, is an automatic positioning indicator at eye-height for the driver. A further critical issue is the limit to which the charging and pickup plates can be reduced in size and weight. Here there are several approaches. One is to increase the magnetic flux density so that smaller plates can transmit the same power. This would, however, violate legal restrictions on magnetic fields in publicly accessible areas. A solution to this problem would be a reliable detection system and automatic cut-out in the event of foreign bodies straying into the electromagnetic field. An alternative approach seeks to achieve some reduction in plate size without exceeding legal limits by changing the geometry of the coils and/or using new magnetic materials. A third approach currently under investigation at UW’s Electromobility Research Group concerns the raising of the nominal power transfer rating so that electric vehicle batteries can in future be charged more rapidly at inductive charging stations.

Further pre-launch challenges include

• development of suitable production technologies for the various components of the inductive charging system
• establishment of appropriate norms to ensure operational compatibility among charging stations and mobile elements from different manufacturers
• limitation of interference between the charging system and other on-board systems (electromagnetic compatibility).

The high costs of the additional technology will predictably confine these systems at first to the premium automobile segment. But market launch will bring economies of scale that will gradually open inductive charging to a wider band of users, and in the foreseeable future the technology will offer a viable alternative to conventional conductive charging. The major advantages of barrier-free access and – especially in publicly accessible space – of security from vandalism will promote the spread of this technology.

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