Look Ma, No Wires!

Inductive coupling could cut the power cord forever in music technology.
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It seems that everything is going wireless these days. Cell phones, Wi-Fi local area networks, even citywide networks like WiMAX all cut the cords that bind us. In the realm of music technology, wireless microphone systems are becoming ever more popular, especially as new technologies are used to circumvent old problems (see “Tech Page: What Was Old Is New Again” in the September 2007 issue of EM, available at www.emusician.com).

However, there's one cord that has yet to be cut — the power cord. All the tools and toys we love to use must get their power from somewhere, usually a wall outlet. Of course, many low-power devices can use rechargeable batteries, but they must be connected to AC power periodically to remain functional.

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FIG. 1: Sending a current through the primary coil sets up an electromagnetic field, which induces a current in the secondary coil without any electrical connection between them.
Illustration by Chuck Dahmer

One way to eliminate the need for power cords is a technique called inductive coupling, which has been around since 1880, when Nikola Tesla first used it to illuminate a vacuum tube with no direct electrical connection. The principle is relatively simple: sending a current through a coil of wire, called the primary, creates an electromagnetic (EM) field around it. If you place another coil of wire, called the secondary, near the primary, the EM field induces a current in the secondary with no electrical connection between the two (see Fig. 1).

So why isn't this technique being widely used to power devices without power cords? For one thing, the primary and secondary must be very close to each other, and the relative orientation of the coils must be precisely controlled. In addition, the primary and secondary must be carefully “tuned” to resonate with each other, and the amount of power that can be transferred is limited by the inefficiency of the process, which can lose more than 30 percent of the energy conveyed by a wired connection.

A company called Fulton Innovation has developed an inductive-coupling technology called eCoupled (www.ecoupled.com) that overcomes many of these limitations. The eCoupled system comprises a power supply, which plugs into the wall and houses the primary, and the device to be powered, which includes the secondary. These circuits dynamically tune themselves to the appropriate resonance frequency in the neighborhood of 100 kHz, allowing much greater flexibility in separation and orientation, though we're still talking inches apart, not feet.

The system achieves an efficiency of more than 98 percent, which is comparable to a wired connection. This allows much higher power levels to be conveyed — for example, Fulton expects to be able to power an electric skillet by transmitting 1,500W, though there is no theoretical limit.

Another hallmark of the eCoupled system is its ability to send data along with power. This is not enabled by inductive coupling directly, but it can be accomplished in a variety of ways. Piggybacking data on the power signal can transmit up to several hundred kilobits per second, but for greater bit rates, a separate set of coils must be used. The eCoupled system has achieved speeds up to 3.3 Mbps in this way, with a theoretical maximum of 200 Mbps.

In most applications, one power supply is paired with one secondary device. However, the system can also power several devices from one primary, in which case the ideal resonance frequency is calculated according to the combined inductance of all devices.

Clearly, this technology has great potential for electronic musicians who depend on power for their studio and stage equipment. With something like an eCoupled system, all you'd have to do is place power supplies around the area and put the gear where you want it rather than near a power outlet. Not only that, but the digital music data could also be transmitted between devices, providing an entirely wireless system, the ultimate in technological freedom.