Title

Wireless Electric

ty Transmissi

on

via Inductive coupling

Alican Çeviker 9/K

2

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. Bend-ing 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:

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.

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.

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 magnet-ic 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 circuit-ry necessary to deliver electricity to devices' batteries.

3

Household devices produce

relatively small magnetic fields. For this reason, chargers hold devices at the distance neces-sary to induce a current, which can only happen if the coils are close together. A larger, stron-ger field could induce current from farther away, but the process would be extremely inefficient. Since a magnetic field spreads in all directions, mak-ing a larger one would waste a lot of energy.

Researchers at MIT re-ported that

they had discov-ered an efficient way to transfer power between coils separated by a few meters. The team, led by Marin Soljacic, theorized that they could ex-tend the distance between the coils by adding resonance to the equation.

As long as both coils are out

of range of one another, noth-ing will happen, since the fields around the coils aren’t strong enough to affect much around them. Similarly, if the two coils resonate at dif-ferent frequen-cies, nothing will happen. But if two resonating coils with the same frequency get within a few meters of each other, streams of energy move from the trans-mitting coil to the receiving coil.

4

The MIT team’s pre-liminary

work suggests that this kind of setup could pow-er or recharge all the devices in one room. Some mod-ifications would be necessary to send power over long distances, like the length of a build-ing or a city. The team is making progress -- in June 2007, the MIT team published a paper detailing a successful demon-stration of their prototype. They used resonating coils to power a light bulb over a distance of about seven feet (two meters)

Whether or not it incorporates resonance, induction gen-erally sends power over

relatively short distances. But some plans for wireless power involve moving electricity over a span of miles. A few proposals even involve sending power to the Earth from space.

5

Inductance (measured in henry) is an effect which results from the magnetic field that forms around a current carrying conductor. Current flowing through

the inductor creates a magnetic field which has an as-sociated electromotive field which opposes the applied voltage. This counter electromotive force (emf) is gen-erated which opposes the change in voltage applied to the inductor and current in the inductor resists the change but does rise. This is known as inductive reac-tance. It is opposite in phase to capacitive reactance. Inductance can be increased by looping the conductor into a coil which creates a larger magnetic field.

As electrical current can be modeled by fluid flow, much like water through pipes; the inductor can be modeled by the flywheel effect of a turbine

rotated by the flow. As can be demonstrated intuitively and mathematically, this mimics the behavior of an elec-trical inductor; current is the integral of voltage, in cases of a sudden interruption of flow it will generate a high pressure across the blockage, etc. Magnetic interac-tions such as transformers, however, are not modeled.

When a sinusoidal alternating current (AC) flows through an inductor, a sinusoidal alternating voltage (or electromotive force (emf) ) is in-

duced. The amplitude of the emf is equal to the ampli-tude of the current and to the frequency of the sinusoid by the following equation. The phase of the current lags that of the voltage by 90 degrees. In a capacitor the current leads voltage by 90 degrees.

6

In physics, resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others. Frequencies at which the response

amplitude is a relative maximum are known as the sys-tem’s resonant frequencies, or resonance frequencies. At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.

Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy

and potential energy in the case of a pendulum). How-ever, there are some losses from cycle to cycle, called damping. When damping is small, the resonant fre-quency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibra-tions. Some systems have multiple, distinct, resonant frequencies.

Resonance phenomena occur with all types of vi-brations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance,

nuclear magnetic resonance (NMR), electron spin res-onance (ESR) and resonance of quantum wave func-tions. Resonant systems can be used to generate vibra-tions of a specific frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies (e.g. filters).