WPT breaks all connections, Part 1: Page 8 of 12

March 16, 2015 //By Sanjaya Maniktala
WPT breaks all connections, Part 1
Sanjaya Maniktala offers the first part of a high-level review of the current state of wireless power transfer (WPT).
we use our microwave oven regularly to deliver thousands of Watts effortlessly “through air” (yes, wirelessly!) into a succulent breast of (former) chicken. But in the process, we have wisely learned to keep ourselves, and extraneous metals, pointedly out of the active space. And therein lies the basic problem with MPT: certain frequencies can be harmful to living (water-based) tissues. That includes our precious pets. We certainly would not relish a hot dog prepared in this fashion. That is why there are mandatory SAR (Specific Absorption Rate) limits imposed even on the tiny mobile phones in our hands — because these also utilize the same microwave frequencies for communication purposes.

No surprise that today, though a few methods of charging commercial electric vehicles (EVs) use focused microwave energy waves typically operating at 2.45 GHz (MPT), most others seem to be turning to safer methods based on Faraday’s Law of induction (Figure 1).  The penalty everyone faces by avoiding MPT, is that the separation between transmitter and receiver coils is necessarily small, typically only a few mm. This will be explained in the next section.

Note: Many molecules, such as water, consist of electric dipoles. These rotate as they try to align themselves with the alternating electric field of microwaves. The rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy manifests itself as heat. Microwave heating is often erroneously explained as a resonance of water molecules, but this is incorrect because those resonances occur only at above 1 Terahertz (THz). That is 1000 GHz, whereas microwaves are 300 MHz to 300 GHz.

Our kitchen microwave oven works at 2.45 GHz.Induction cookers, another fairly common kitchen item, do not use microwaves, but utilize low-frequency time-varying magnetic fields, i.e. Faraday’s Law. However, instead of
harnessing the transmitted energy in a copper coil and then using that to charge
batteries as we do it today in WPT, the

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