IEEE Spectrum May, 2010 - 40

emily cooper

cal transformer, the magnetic lines of force that link the primary and secondary are channeled through iron, maintaining
a tight coupling that keeps power losses to a minimum. If you
separate the primary and secondary coils by a distance that's
fi lled with nothing but air, those losses mount and the transfer becomes inefficient.
"Resonance enables efficient energy transfer," says Soljacˇic´,
describing the basic strategy his team used to get significant
amounts of power to flow. It's not a new idea: Tesla's eponymous coils use that very same principle.
A good way to understand why resonance helps is to imagine the mechanical analogue. Suppose you wanted to transfer
mechanical energy across a room, but all you had coupling the
power source with the load was a long and very weak spring.
You'd have to pump the end of the spring you were holding
vigorously, moving it back and forth as fast and as far as you
could until sweat poured down your brow. It wouldn't be very
efficient, but only with such effort would the far end of the
spring wiggle a bit.
To make life easier, you could attach your end of the spring
to a pendulum swinging in a wide arc, for instance. Now your
arm wouldn't hurt so much, and the far end of the spring would
still wiggle. But another difficulty would appear when you
tried to attach that far end of the spring to a mechanical load.
If you weren't careful, you'd find the waves of energy being sent
down the spring weren't being absorbed-most of what little
energy that got to the far end would just bounce back. To solve
this new problem, you could attach the far end of the spring to a
second pendulum, one that was built exactly like the first. Now,
all you would need to do is give the first pendulum some gentle rhythmic shoves until the amplitude of its swinging became
large enough to get the far end of the spring wiggling in time
with it. And those little wiggles would in turn have the right
timing to get the second pendulum swinging. Despite having
only a weak spring as the conduit, you would have transferred
power across the room. Then you could do something useful
with it-maybe smash a window.
This mechanical analogy may seem a bit loopy, but in fact
it provides a very good parallel for what goes on between
the coupled resonant electrical oscillators used to transfer
power inductively. The mechanical version even shows some
of the subtleties of wireless-power systems-for example,
that the coupling between the primary and secondary oscillators gives rise to a second, higher frequency of resonance. More
important, this thought experiment helps to illustrate a fundamental challenge: As the frequency and amplitude of the oscillations increase, the primary starts to experience significant
losses of power. Air resistance would sap the energy of a swinging pendulum, to take one example. For electrical oscillators,
most of the losses arise just from the resistance of the wires.
So when Soljacˇic´ calls his system "efficient," he's speaking
in relative terms. The actual plug-to-bulb efficiency in his demonstration would make an environmentalist cringe-it was
only 15 percent. Nevertheless, Soljacˇic´ and his colleagues were
so enthusiastic about the prospects of using such inductive systems to charge cellphones and laptops at a distance that they
founded a start-up to commercialize the technology. Dubbed
WiTricity Corp., the company, which is located in Watertown,
Mass., now has about 20 employees.
Strangely enough, even before Soljacˇic´'s work appeared in
print, others at MIT had been looking into the problem of how
to send power wirelessly over short distances. Jeff Lieberman,
spectrum.ieee.org

trANsFormiNg
A trANsFormer
A wireless-power system operates
much like an ordinary transformer-
but with only air between the coils

Primary coil

magneticﬁeld lines

secondary coil

iron core

AN ORDINARY TRANSFORMER (shown schematically above) contains
two coils-a primary and a secondary-which are wound around different
parts of a steel frame. the steel has high magnetic permeability and channels
magnetic-ﬁeld lines within it, so virtually all of the magnetic ﬁeld created by
powering the primary coil passes through the secondary. this tight magnetic
coupling allows power to ﬂow from primary to secondary with high efficiency.

magneticﬁeld lines

IF THE PRIMARY AND SECONDARY COILS do not share a common
steel core and are instead separated by nothing but air, the magnetic linkage
becomes far more tenuous. only a small fraction of the magnetic-ﬁeld lines
generated by the primary will pass through the secondary coil. the power source
would then have to drive very large currents in the primary to transfer the same
amount of power. But those larger currents would give rise to larger losses.

capacitance

resonant
primary

resonant
secondary

ADDING CAPACITANCE to the primary circuit causes a resonant
oscillation, with energy shifting back and forth between the magnetic ﬁeld
surrounding the coil and the electric ﬁeld within the capacitor. in this way, high
currents can be attained within the primary without suffering losses within the
source powering it. adding capacitance to the secondary so that it resonates at
the proper frequency further boosts the efficiency of the power transfer.
mAy 2010 * iEEE SpEctrum * NA

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Table of Contents for the Digital Edition of IEEE Spectrum May, 2010
