Introduction to Wireless Power

Unless you are particularly organized and good with tie
wrap, you probably have a few dusty power cord tangles around
your home. You may have even had to follow one particular
cord through the seemingly impossible snarl to the outlet,
hoping that the plug you pull will be the right one. This is
one of the downfalls of electricity. While it can make
people's lives easier, it can add a lot of clutter in the
process.

For these reasons, scientists have tried to develop methods
of wireless power transmission that could cut the clutter or
lead to clean sources of electricity. While the idea may
sound futuristic, it isn't particularly new. Nicola Tesla
proposed theories of wireless power transmission in the late
1800s and early 1900s. One of his more spectacular displays
involved remotely powering lights in the ground at his
Colorado Springs experiment station.

Tesla's work was impressive, but it didn't immediately lead
to widespread, practical methods for wireless power
transmission. Since then, researchers have developed several
techniques for moving electricity over long distances
without wires. Some exist only as theories or prototypes,
but others are already in use. If you have an electric
toothbrush, for example, you probably take advantage of one
method every day.

The wireless transmission of energy is common in much of the
world. Radio waves are energy, and people use them to send
and receive cell phone, TV, radio and WiFi signals every day.
The radio waves spread in all directions until they reach
antennae that are tuned to the right frequency. A similar
method for transferring electrical power would be both
inefficient and dangerous.

For example, a toothbrush's daily exposure to water makes
a traditional plug-in charger potentially dangerous. Ordinary
electrical connections could also allow water to seep into
the toothbrush, damaging its components. Because of this,
most toothbrushes recharge through inductive coupling.

Inductive Coupling

Inductive coupling uses magnetic fields that are a natural
part of current's movement through­ wire. Any time electrical
current moves through a wire, it creates a circular magnetic
field around the wire. Bending the wire into a coil amplifies
the magnetic field. The more loops the coil makes, the bigger
the field will be.

If you place a second coil of wire in the magnetic field
you've created, the field can induce a current in the wire.
This is essentially how a transformer works, and it's how
an electric toothbrush recharges. It takes three basic steps:

1. Current from the wall outlet flows through a coil
inside the charger, creating a magnetic field. In
a transformer, this coil is called the primary winding.
2. When you place your toothbrush in the charger, the
magnetic field induces a current in another coil, or
secondary winding, which connects to the battery.
3. This current recharges the battery.

You can use the same principle to recharge several devices
at once. For example, the Splashpower recharging mat and
Edison Electric's Powerdesk both use coils to create
a magnetic field. Electronic devices use corresponding
built-in or plug-in receivers to recharge while resting on
the mat. These receivers contain compatible coils and the
circuitry necessary to deliver electricity to devices'
batteries.

A newer theory uses a similar setup to transmit electricity
over longer distances.