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This blog post suggests that apple dropped their promised AirPower wireless charging mat because it didn't pass EMI standards, specifically due to the (unconfirmed) idea that they were using a large number of coils to make the product simpler for consumers to charge multiple devices without having to carefully position them over individual coils.

One quote stood out to me:

We asked an engineer with experience building wireless charging systems what obstacles Apple was working to overcome. “Over time, these harmonics add up and they become really powerful signals in the air,” explains William Lumpkins, VP of Engineering at O & S Services. “And that can be difficult—that can stop someone’s pacemaker if it’s too high of a level. Or it could short circuit someone’s hearing aid.” If Apple’s multi-coil layout was spinning off harmonics left and right, it’s possible AirPower couldn’t pass muster with US or EU regulations.

Specifically the bolded part surprised me, and I'm not sure if it's just because he's simplifying for a lay audience. I would have thought that the resultant field due to harmonics at any reasonable distance would be established near-instantly since it's just EM waves travelling in free space around the speed of light.

Is there something I'm missing that causes this to be different in the case of wireless charging devices?

Secondly, is this just a power conditioning issue, or do the coils themselves generate the harmonics? That is, if you had a perfect sine generator at the resonant coil frequency, would it also generate harmonics in the emitted EM?

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    \$\begingroup\$ If driven by a perfect sinusoid, then the resonator has to have some nonlinearity if any harmonics appear. This nonlinearity could be in the iPHONE's rectifiers. \$\endgroup\$ – analogsystemsrf Mar 31 at 2:28
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Yes it is a serious challenge to shield pacemakers and shield radiation from Wireless Power Transfer (WPT) coils for small mobiles and large units to charge Electric Vehicles.

WPT is already used in EU with 4kW, 8kW and R&D with 16kW coils @ 100kHz or 200kHz for Wireless power transfer to EV cars and trucks. Although there are standards for stray radiation at some distance with designs to absorb stray MMF with a ferrite, the risks can increase with failures or non-compliance so fault detection and shutdown must be included in the overall design with stringent qualifications.

Harmonic currents are generated when diodes conduct into low ESR batteries or caps.

But in high-frequency WPT systems the operating frequency must stay within one of the designated, very restrictive industrial, scientific, and medical (ISM) bands (e.g., 6.78 MHz, 13.56 MHz, and 27.12 MHz; FCC 2014).

These higher frequencies might be used for smaller mobiles.

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We asked an engineer with experience building wireless charging systems what obstacles Apple was working to overcome. “Over time, these harmonics add up and they become really powerful signals in the air,” explains William Lumpkins, VP of Engineering at O & S Services. “And that can be difficult—that can stop someone’s pacemaker if it’s too high of a level. Or it could short circuit someone’s hearing aid.” If Apple’s multi-coil layout was spinning off harmonics left and right, it’s possible AirPower couldn’t pass muster with US or EU regulations.

You are correct in that the field at any distance from any one charging coil will be established more or less instantly, at the speed of light. I don't think the person quoted was using 'time' in that sense.

Over time, as the years go on, the number of charging coils will increase. Several lower power ones in the home for portable electronics. One in the garage for the car, several outside in the road and ones strewn across carparks for public charging. The pacemaker or hearing-aid wearer may have a tough time negotiating a route from A to B, avoiding high-field areas.

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  • \$\begingroup\$ Hmm, I think you're correct on the physics and potential future situation, but my cynicism makes me doubt that an individual company would care enough about contributing a fraction of the power that leads to a regulation-exceeding field 5 years from now to make them cancel a product. It doesn't seem likely to me that the quoted engineer intended that meaning. I think one of the reasons why the regulatory level is so low is to ensure that a few devices close together don't add up to a truly dangerous level \$\endgroup\$ – llama Mar 31 at 22:05

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