WPT breaks all connections, Part 1: Page 9 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).
energy is converted directly into heat through “eddy currents” generated naturally in response to the changing magnetic field (from the opposing flux). The resultant heating due to these currents occurs in a flat metal plate on top of the cooker, which also serves as the base of the cooking pan. The earliest patents for induction heating date from the early 1900s. In the mid-1950s, demonstration stoves were revealed by the Frigidaire division of General Motors. This was however never put into production. In the early 1970s, commercial models were released for the very first time, by Westinghouse.


The Idea Finder: The Microwave Oven       

Wikipedia: Induction Cooking
Fields or Waves?

This causes a lot of confusion, so we need to touch upon this briefly here before
carrying on. It will also help distinguish between MPT and inductive WPT. The
electromagnetic spectrum is shown in Figure 2. We can observe the frequencies used for MPT (2.45GHz) and also for modern inductive WPT (~100-350 kHz and 6.78 MHz).

The first thing we realize is the frequency for MPT is much higher than inductive WPT. But the high-frequency aspect of MPT itself is not the real problem per se. After all, visible light has even higher frequencies and it is safe. The effect of MPT is more related to certain biological effects at certain frequencies. For that reason, mobile devices are routinely tested for compliance to mandatory SAR emission limits over the range 30 MHz to 6 GHz as shown. Not against visible light obviously!

There is another concept we need to understand: far-fields and near-fields. Time-varying magnetic fields induce electric fields, and similarly, as Maxwell showed in 1865 by extending Faraday’s Law, time-varying electric fields generate magnetic fields. There is a certain duality between them. Further, as we go further from the magnetic or electric source, the electric and magnetic fields tend to become more and more tangential to

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