Wireless Induction Mat: Why Not Nix Power Cords Altogether?

You may have seen those “wireless charging mats” in stores: You place your (i)phone on this little mat with a thing plugged into its power port and it charges without interconnecting wires. The iPhone 5 is said to have this built-in by default according to a variety of sources, some public some not.

But why can’t everything charge like that? If we have that technology, why not nix the dreaded AC power cord altogether and just make everything wireless? The answer is simple: physics doesn’t allow it, and I explain why (in layman’s terms) below.

Power Supplies 101

First of all, lets backtrack: the outlet is at point (a), your device is at point (b), some distance away, and between the two are cables to deliver electricity that you wish were not there (and believe there is a means given the advent of these wireless inductance mats).

Here’s how it works: your outlet puts out 110-125 volts of AC (alternating current; here in the US, at least; line/mains voltages differ elsewhere but the concept is the same). Your digital device, whatever it may be, usually operates internally at roughly 3-12 volts of direct current DC (direct current, like a battery: electricity goes from + to – without alternating or anything funny).

So somewhere between your device and the wall there is going to be a little cube that converts the voltage from the mains/line voltage to the device’s voltage, usually in the form of a little “power brick” or “wall wart” as is the case with many phone chargers. Simple concept: take the standard voltage, convert it to the voltage you need to charge or run something. Devices like TVs are big enough to house the converter within themselves.

Cutting the cord

So where do we eliminate the cord? Let’s see:

Getting rid of the cord between the outlet/receptacle and converter is not feasible. For one, the AC alternates (hence the name) at 50 or 60 cycles per second depending on region. The lower the cycles per second, the longer the wireless wavelength is – hence why lower bass notes require sub-woofers to accommodate the low/long sound in sound theory.

Those long wavelengths going long distances would interfere with both each other and other devices unrelated to their purpose, and at such high voltages and currents used by the line receptacle this is not something you want going around the air if you have metal in your home.

However, there is a much larger consideration with wirelessly transmitting voltages through the air that is even more prevalent when sending off the lower-voltage (post-converter) power wirelessly as well: efficiency.

The longer the power goes from its source to where it is consumed wirelessly, the more it drops out. In addition, the power doesn’t magically radiate from its source to its destination in a straight line: all that power has to be radiated in a circular fashion, further multiplying the required power (and expense), lowering overall efficiency further.

Wireless Limitations

While the many types of wireless vary beyond the scope of this article, they all operate on the same general principles. The most common one is magnetism: see that thing on the paper at the top of this site with the copper wound around the black thing with two leads coming off the opposite ends of the black cylinder? That’s an inductor, and it plays a major role in wireless communication.

You see, wireless works in communication because efficiency doesn’t matter: the receiver doesn’t care how powerful the signal is by the time it gets there, as long as it can figure out where the signal is up and where the signal is down so it can translate that information into some other representation, usually binary ones and zeros. Furthermore, Wi-Fi operates at a fairly large (billions of cycles per second) frequency, and power-wise is capped at a single watt of power, which for all practical purposes is low when compares to the power needed by electronics to do stuff.

Back to the inductor, it is half of an example of wireless power transfer often-overlooked as such: the transformer. Whether on utility poles or within devices, a transformer converts one voltage to another by wrapping two sets of wires around a common iron core, with the number of turns per wire determining how much voltage it gets if it is on the receiving end.

The power from the “live” winding is wirelessly transferred to the iron core in the form of magnetism, and the other wire winding picks it up without making direct (uninsulated) contact with the iron core, which would cause a short-circuit. The amount of voltage received on the other non-live end depends on the turn ratio of the primary winding to the secondary winding and the input voltage on the primary wire.

The transformer’s windings are both tightly wrapped around the core so as much energy is transferred as possible – but if you remove the core and force the magnetic energy to flow from primary wire to secondary wire through air and pull them apart on top of that, the efficiency drops exponentially over the distance.

Leave it at the mats

This is why it is okay to transmit wireless energy in close-quarters, such as a (i)phone in direct physical (though not electrical/conductive) contact with a charging mat or a transformer’s wire windings in close proximity to an iron core, but not okay with respect to longer distances due to efficiency drop.

There are special cases of it working that may encourage or drive some away, however: one example is at my home, where both my (non-underground) power meter’s cables and my cable TV line were dropped from the utility pole in parallel about a foot and half apart from each other on their way to the side of my house. Every time I would go to plug/unplug my cable from a TV or something else, I would get a shock that clearly was AC voltage.

The line bringing power to my home (carrying all of the electric current my house is using, from the grid and back to it via the neutral connection) was inducing AC current into my cable line, causing an electric shock everytime I touch it. The lines were a fair distance apart and the power is only at ~120v, but the excessive current carried in the power lines is the reason why the efficiency drop over the distance was compensated for – ~50-200 amps per wireless connection is excessive for more purposes.

Another example is holding a fluorescent light bulb below transmission-level (~250,000-500,000 volt) power lines, where it will light up due to the electric field caused by the high voltage lines (as opposed to the magnetic field). But making your hair stand up on end in light of high voltage/power makes overcoming the efficiency drop not worth it, once again, in this case.

People who have made or experimented with Tesla coils may observe some of this as well, but all Tesla coils do it trade off current for high voltage before transmitting it wirelessly (with the help of store-and-release capacitors) at high frequencies to achieve the lightning effects. Not suitable for safely transmitting power wirelessly in the home.

It’s worth mentioning that a company named WiTricity is experimenting with safely transmitting wireless electricity, following a famous TED talk on the matter – but that was years ago and nothing has come from them since then, so they are more than likely a failed startup or else in project hell.

MIT also did research involving sending wireless power through directional antennas so that the power is concentrated in the desired direction instead of radiated circularly and wasting energy in the process – but having satellite dishes at either end to barely light up a 40W bulb makes this once again unfeasible.

So, for safety and power conservation (and a lower bill), leave the wireless charging at the mats and be happy with it.

Have any thoughts, insights or questions on this? Leave a comment and I’ll get back to you!