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Implementing wireless electric vehicle charging

Posted: 30 Dec 2015     Print Version  Bookmark and Share

Keywords:wireless charger  electric vehicles  EV  J1773  J2954 

A tightly-coupled system gives the most efficient transfer of power but at the cost of high sensitivity to coil misalignment. Such systems are popular for consumer applications such as cell phones where the transmitter and receiver are within a few millimeters. EV wireless charging demands more flexibility in coil alignment and a longer range, so it uses a system with a resonant receiver-transmitter combination. This allows for looser coupling but is less efficient.

Figure 1 shows the block diagram of a resonant charging system. If two high-Q resonators are placed in close proximity such that there is coupling between them, the resonators can exchange energy.

Figure 2: Equivalent circuit for a coupled resonator system (source: Witricity)

The equivalent circuit for such a coupled resonator is shown in figure 2. The generator outputs a sinusoidal voltage with amplitude Vg and frequency ω with output resistance Rg. The source (transmitter) and device (receiver) resonator coils are represented by the inductors Ls and Ld, coupled through their mutual inductance M, where M = k √ (LsLd).

A resonator is formed by a coil and a capacitor in series. Rs and Rd are the parasitic resistances of the coil and resonant capacitor for the respective resonators. The load is represented by an equivalent AC resistance RL.

The maximum power transfer efficiency (PTE) occurs at the resonant frequency and is a function of the electromagnetic coupling of the two coils. The power transfer (PT) between them occurs when the source and device impedances are matched. If the two coils are strongly coupled, PT varies with frequency, with twin peaks above and below the resonant frequency.

Both PTE and PT cannot be optimised simultaneously; EV charger designs normally focus on improving the PTE first, then use techniques such as adaptive impedance matching to boost the PT.

Coil design
The design and shape of the transmitter and receiver resonator coils have a key effect on system performance. For stationary charging, the transmitter coil is in the form of a flat pad containing the coil to generate the field and a ferrite layer to guide it, plus an aluminium layer for shielding.

Figure 3: Resonator coil arrangement (Source: Vahle/coilwindingexpo.com)

Each design has different characteristics: for example sensitivity to coil rotation, or flux pattern. Figure 3 shows a transmitter coil in a "double-D" (DD) shaped configuration with a bipolar field where the flux in each coil flows in opposite directions. This design has high sensitivity to receiver coil orientation (rotation) but does not require underbody shielding.

Standardisation: SAE J2954
Although J1773 is dead, efforts are reaching fruition to develop an inductive charging standard for EVs, now renamed Wireless Power Transfer (WPT). The SAE has been working since 2012 on J2954, the standard that will govern wireless power transfer (WPT) for electric vehicles.

Participants include automotive OEMs such as GM, BMW, Ford, Nissan & Toyota, Tier 1 suppliers Delphi, Panasonic and Magna, WPT suppliers such as Qualcomm and LG, and a collection of other organisations such as the Argonne National Laboratory, the EPA, the DOT, UL and the University of Tennessee.

The goal is at least 90% efficiency for static charging in residential locations or car parks. J2954 also plans for future on-road dynamic charging with embedded systems. The standard defines three levels of charging for both light-duty and heavy-duty applications, WPT1 (residential: 3.7kW), WPT2 (private/public parking: 3.7kW) and WPT3 (fast charge: 22kW).

For light-duty vehicle use, the SAE team has settled on a charging frequency of 85kHz, which lies within an internationally available frequency band. The team hopes to finalise the J2954 standard by 2017 with recommendations to be released by the end of 2016.

Aftermarket systems and OEM activity
A number of suppliers are developing aftermarket inductive charging systems that can be added to EVs without voiding the warranty. Typically these make use of ground-based charging pads installed in a garage, parking facility or on the roadway.

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