Electrical Power Transfer with “No Wires” - Energy Harvesting Autonomous Sensor Systems - page 196

of the resonator coil can become smaller in size to fit into the mobile gadget.
It is thus more viable to utilize WPT technology as a self-sustaining portable
energy source for the mobile gadgets.

6.2.1 Concept Principles of WPT with Magnetic Resonance

The concept of WPT is very much similar to an air-core transformer where no
ferromagnetic material is inserted between the primary and secondary sides
of the transformer. The source power of alternating frequency is connected
to the primary coil, setting up a constantly changing magnetic field, which
induces an electromotive force in the secondary coil according to Faraday's
law of induction. This electromotive force then drives an induced current in
the secondary coil. Efficiency of a transformer is maximized by having the
secondary coil tightly wound around the primary in order to maximize the
coupling between the coils. As the magnetic field strength, which is given by
the Biot-Savart equation as

|=

o Idl

| dB (6.1)

4 r 2

decreases with the square of the distance r separating the secondary coil from
the primary coil, the magnetic coupling between the coils is greatly reduced,
hence the efficiency of the transformer or the WPT system. To strengthen the
magnetic coupling, magnetic resonance for WPT has been discussed by Kurs
et al. [163] such that both the primary (transmitting) and secondary (receiv-
ing) coils are tuned to the same resonant frequency, making use of magnetic
resonance to compensate for the weakened coupling due to the increase in sep-
aration distance. Magnetic resonance occurs when magnetic waves of certain
frequencies are absorbed by an object, causing that object to resonate. The fre-
quency absorbed by an object that causes that object to resonate is dependent
on many variables, including molecular structure, shape, and size/length of
the object. In an experiment conducted by Jorgensen and Culberson [171], the
authors had a primary coil emitting evanescent, magnetic waves that a sec-
ondary coil absorbed. Evanescent waves are different from ordinary waves
because evanescent waves oscillate in time but diminish over distance [171].
Assuming the coils are of the same size, shape, and mass, at a certain fre-
quency (the resonant frequency) the primary coil will resonate and cause the
secondary coil to resonate as well. An overview of the WPT system is shown
in Figure 6.15 .
According to Kurs et al. [163], intuitively, two resonant objects of the same
resonant frequency tend to exchange energy efficiently, while dissipating rel-
atively little energy in extraneous off-resonant objects. In systems of coupled
resonances, there is often a general “strongly coupled” regime of operation.
If one can operate in that regime in a given system, the energy transfer is
expected to be very efficient. Midrange power transfer implemented in this