presented by:
Divya.s
Aarthi V.G..

---

[attachment=9208]
Wireless power transfer
OVERVIEW
Imagine a future in which wireless power transfer is feasible: cell phones, household robots, mp3 players, laptop computers and other portable electronics capable of charging themselves without ever being plugged in, freeing us from that final, ubiquitous power wire. Some of these devices might not even need their bulky batteries to operate.
Introduction
Wireless energy transfer or wireless power transmissionis the process that takes place in any system where electrical energy is transmitted from a power source to an electrical load without interconnecting wires. Wireless transmission is useful in cases where instantaneous or continuous energy transfer is needed but interconnecting wires are inconvenient, hazardous, or impossible.
Wireless energy transfer is different from wireless transmission of information, such as radio, where the signal-to-noise ratio or the percentage of power received becomes critical only if it is too low to recover the signal successfully. With wireless energy transfer efficiency is the more important parameter.
The most common form of wireless power transmission is carried out using induction, followed by electro dynamic induction. Other present-day technologies for wireless power include those based upon microwaves and lasers.
Transmission of electrical energy from one object to another without the use of wires is called wireless transmission. Consider two self resonating copper coil of same resonating frequency with a diameter 20 inches each.
One copper wire is connected to the power source(transmitter), while the other copper wire is connected to the device(receiver).
Near field is Wireless transmission techniques over distances comparable to, or a few times the diameter of the device(s), and up to around a quarter of the wavelengths used. Near field energy itself is non radiative, but some radiative losses will occur. In addition there are usually resistive losses. Near field transfer is usually magnetic (inductive), but electric (capacitive) energy transfer can also occur.
 Induction
Main article: Inductive coupling
The action of an electrical transformer is the simplest instance of wireless energy transfer. The primary and secondary circuits of a transformer are not directly connected. The transfer of energy takes place by electromagnetic coupling through a process known as mutual induction. (An added benefit is the capability to step the primary voltage either up or down.) The battery charger of a mobile phone or the transformers on the street are examples of how this principle can be used. Induction cookers and many electric toothbrushes are also powered by this technique.
The main drawback to induction, however, is the short range. The receiver must be very close to the transmitter or induction unit in order to inductively couple with It.
 Electro dynamic induction
The "electro dynamic inductive effect" or "resonant inductive coupling" has key implications in solving the main problem associated with non-resonant inductive coupling for wireless energy transfer; specifically, the dependence of efficiency on transmission distance. Electromagnetic induction works on the principle of a primary coil generating a predominantly magnetic field and a secondary coil being within that field so a current is induced in the secondary. This results in a relatively short range because most of the magnetic field misses the secondary. Over greater distances the induction method is inefficient and wastes much of the transmitted energy.
The application of resonance improves the situation somewhat, moderately improving the efficiency by "tunneling" the magnetic field to a receiver coil that resonates at the same frequency. When resonant coupling is used the two inductors are tuned to a mutual frequency and the input current is modified from a sinusoidal into a non-sinusoidal rectangular or transient waveform so as to more aggressively drive the system. In this way significant power may be transmitted over a range of many meters. Unlike the multiple-layer windings typical of non-resonant transformers, such transmitting and receiving coils are usually single layer solenoids or flat spirals with series capacitors, which, in combination, allow the receiving element to be tuned to the transmitter frequency and reduce losses.
A common use of the technology is for powering contactless smartcards, and proposed systems exist to power and recharge laptops and cell phones.