How Wireless Power Transfer Works

In this post we discuss regarding how wireless power transfer works or the transfer of electricity through air without using wires.

You might have already come across this technology and might have gone through many related theories on the Internet.
Although the Internet may be full of such articles explaining the concept with examples and videos, the reader mostly fails to understand the core principle governing the technology, and its future prospects.
In this article we’ll roughly try to get an idea regarding how a wireless electricity transfer happens or works or conduction takes place and why the idea is so difficult to implement over large distances.
The most common and classic example of wireless power transfer is our old radio and TV technology which works by sending electrical waves (RF) from one point to the other without cables, for the intended data transfer.
However the drawback behind this technology is that it is unable to transfer the waves with high current such that the transmitted power becomes meaningful and usable on the receiving side for driving a potential electrical load.
This problem becomes difficult since the resistance of air could be in the range of millions of mega Ohms and thus extremely difficult to cut through.
Another hassle that makes the long distance transfer even more difficult is the focusing feasibility of the power to the destination.
If the transmitted current is allowed to disperse over a wide angle, the destination receiver might not be able to receive the sent power, and could possibly acquire just a fraction of it, making the operation extremely inefficient.
However, transferring electricity over short distances without wires looks much easier and has been successfully implemented by many, simply because for short distances the above discussed constraints never become an issue.
For a short distance wireless power transfer, the air resistance encountered is much smaller, within a range of a few 1000 meg ohm (or even lesser depending on the proximity level), and the transfer becomes feasible rather efficiently with the incorporation of high current and high frequency.
In order to acquire an optimal distance-to-current efficiency, the frequency of transmission becomes the most important parameter in the operation.
Higher frequencies enable larger distances to be covered more effectively, and therefore this is one element that needs to be followed while devising a wireless power transfer apparatus.
Another parameter that helps the transfer easier is the voltage level, higher voltages allow involving lower current, and in keeping the device compact.
Now let’s try to grasp the concept through a simple circuit set up:

Parts List
R1 = 10 ohm
L1 = 9-0-9 turns, that is 18 turns with a center tap using a 30 SWG super enameled copper wire.
L2 = 18 turns using 30 SWG super enameled copper wire.
T1 = 2N2222
D1—-D4 = 1N4007
C1 = 100uF/25V
3V = 2 AAA 1.5V cells in series
The image above shows a straightforward wireless power transfer circuit consisting of the transmitter stage on the left and the receiver stage on the right side of the design.
Both the stages can be seen separated with a significant air gap for the intended shift of electricity.
The power transmitter stage looks like an oscillator circuit made through a feedback network circuit across an NPN transistor and an inductor.
Yes that’s right the transmitter indeed is an oscillator stage which works in a push-pull manner for inducing a pulsating high frequency current in the associated coil (L1).
The induced high frequency current develops a corresponding amount of electromagnetic waves around the coil.
Being at a high frequency this electromagnetic field is able to tear apart through the air gap around it and reach out to a distance that be permissible depending upon its current rating.
The receiver stage may be seen consisting of only a complimenting inductor L2 quite similar to L1, which has the sole role of accepting the transmitted electromagnetic waves and converting it back to a potential difference or electricity albeit at a lower power level due to the involved transmission losses through the air.
The electromagnetic waves generated from L1 is radiated all around, and L2 being somewhere in the line is hit by these EM waves. When this happens, the electrons inside the L2 wires are forced to oscillate at the same rate as the EM waves, which finally results in an induced electricity across L2 too.
The electricity is rectified and filtered appropriately by the connected bridge rectifier and C1 constituting an equivalent DC output across the shown output terminals.
Actually, if we carefully see the working principle of wireless power transfer we find it’s nothing new but our age old transformer technology that we ordinarily use in our power supplies, SMPS units etc.
The only difference being the absence of the core which we normally find in our regular power supply transformers. The core helps to maximize (concentrate) the power transfer process, and introduce minimum losses which in turn increases the efficiency to a great extent
The core also allows the use of relatively lower frequencies for the process, to be precise around 50 to 100 Hz for iron core transformers while within 100kHz for ferrite core transformers.
However in our proposed article regarding how wireless power transfer functions, since the two sections need to be entirely aloof from each other, the use of a core becomes out of question, and the system is compelled to work without the comfort of an assisting core.
Without a core it becomes essential that a relatively higher frequency and also higher current is employed so that the transfer is able to initiate, which may be directly dependent on the distance between the transmitting and the receiving stages.
To Summarize, from the above discussion we can assume that to implement an optimal power transfer through air, we need to have the following parameters included in the design:
A correctly matched coil ratio with respect to the intended voltage induction.
A high frequency in the order of 200kHz to 500kHz or higher for the transmitter coil.
And a high current for the transmitter coil, depending on how much distance the radiated electromagnetic waves is required to be transferred.

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