Understanding
what WiTricity technology is—transferring electric energy or power over
distance without wires—is quite simple. Understanding how it works is a bit
more involved, but it doesn’t require an engineering degree. We’ll start with
the basics of electricity and magnetism, and work our way up to the WiTricity
technology.
Electricity:
The flow of electrons (current) through a conductor (like a wire), or charges
through the atmosphere (like lightning). A convenient way for energy to get
from one place to another!
Illustration
of earth's magnetic field
An
illustration representing the earth's magnetic field
Magnetism:
A fundamental force of nature, which causes certain types of materials to
attract or repel each other. Permanent magnets, like the ones on your
refrigerator and the earth’s magnetic field, are examples of objects having
constant magnetic fields.
Oscillating
magnetic fields vary with time, and can be generated by alternating current
(AC) flowing on a wire. The strength, direction, and extent of magnetic fields
are often represented and visualized by drawings of the magnetic field lines.
Electric
current flowing in a wire creates a magnetic field
As
electric current, I, flows in a wire, it gives rise to a magnetic field, B,
which wraps around the wire. When the current reverses direction, the magnetic
field also reverses its direction.
Representation
of the magnetic field created when current flows through a coil
The
blue lines represent the magnetic field that is created when current flows
through a coil. When the current reverses direction, the magnetic field also reverses
its direction.
Electromagnetism:
A term for the interdependence of time-varying electric and magnetic fields.
For example, it turns out that an oscillating magnetic field produces an
electric field and an oscillating electric field produces a magnetic field.
Magnetic
Induction: A loop or coil of conductive material like copper, carrying an
alternating current (AC), is a very efficient structure for generating or
capturing a magnetic field.
If
a conductive loop is connected to an AC power source, it will generate an
oscillating magnetic field in the vicinity of the loop. A second conducting
loop, brought close enough to the first, may “capture” some portion of that
oscillating magnetic field, which in turn, generates or induces an electric
current in the second coil. The current generated in the second coil may be
used to power devices. This type of electrical power transfer from one loop or
coil to another is well known and referred to as magnetic induction. Some
common examples of devices based on magnetic induction are electric
transformers and electric generators.
Energy/Power
Coupling: Energy coupling occurs when an energy source has a means of
transferring energy to another object. One simple example is a locomotive
pulling a train car—the mechanical coupling between the two enables the
locomotive to pull the train, and overcome the forces of friction and inertia
that keep the train still—and, the train moves. Magnetic coupling occurs when
the magnetic field of one object interacts with a second object and induces an
electric current in or on that object. In this way, electric energy can be
transferred from a power source to a powered device. In contrast to the example
of mechanical coupling given for the train, magnetic coupling does not require
any physical contact between the object generating the energy and the object
receiving or capturing that energy.
A
transformer uses magnetic induction to transfer power between its windings
An
electric transformer is a device that uses magnetic induction to transfer
energy from its primary winding to its secondary winding, without the windings
being connected to each other. It is used to “transform” AC current at one
voltage to AC current at a different voltage.
Resonance:
Resonance is a property that exists in many different physical systems. It can
be thought of as the natural frequency at which energy can most efficiently be
added to an oscillating system. A playground swing is an example of an
oscillating system involving potential energy and kinetic energy. The child
swings back and forth at a rate that is determined by the length of the swing.
The child can make the swing go higher if she properly coordinates her arm and
leg action with the motion of the swing. The swing is oscillating at its resonant
frequency and the simple movements of the child efficiently transfer energy to
the system. Another example of resonance is the way in which a singer can
shatter a wine glass by singing a single loud, clear note. In this example, the
wine glass is the resonant oscillating system. Sound waves traveling through
the air are captured by the glass, and the sound energy is converted to
mechanical vibrations of the glass itself. When the singer hits the note that
matches the resonant frequency of the glass, the glass absorbs energy, begins
vibrating, and can eventually even shatter. The resonant frequency of the glass
depends on the size, shape, thickness of the glass, and how much wine is in it.
Resonant
Magnetic Coupling: Magnetic coupling occurs when two objects exchange energy
through their varying or oscillating magnetic fields. Resonant coupling occurs
when the natural frequencies of the two objects are approximately the same.
Two
idealized resonant magnetic coils
Two
idealized resonant magnetic coils, shown in yellow. The blue and red color
bands illustrate their magnetic fields. The coupling of their respective
magnetic fields is indicated by the connection of the colorbands.
WiTricity
Technology: WiTricity power sources and capture devices are specially designed
magnetic resonators that efficiently transfer power over large distances via
the magnetic near-field. These proprietary source and device designs and the
electronic systems that control them support efficient energy transfer over
distances that are many times the size of the sources/devices themselves.
This
diagram shows how the magnetic field can wrap around a conductive obstacle
The
WiTricity power source, left, is connected to AC power. The blue lines
represent the magnetic near field induced by the power source. The yellow lines
represent the flow of energy from the source to the WiTicity capture coil,
which is shown powering a light bulb. Note that this diagram also shows how the
magnetic field (blue lines) can wrap around a conductive obstacle between the
power source and the capture device.
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