Litz wire is a particular type of multistrand wire or cable used in electronics to carry alternating current (AC) at radio frequencies. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies up to about 1 MHz. It consists of many thin wire strands, individually insulated and twisted or woven together, following one of several carefully prescribed patterns[better source needed] often involving several levels (groups of twisted wires are twisted together, etc.). The result of these winding patterns is to equalize the proportion of the overall length over which each strand is at the outside of the conductor. This has the effect of distributing the current equally among the wire strands, reducing the resistance. Litz wire is used in high Q inductors for radio transmitters and receivers operating at low frequencies, induction heating equipment and switching power supplies.
How Litz wire works
One technique to reduce the resistance is to place more of the conductive material near the surface where the current is by replacing the wire with a hollow copper tube. The larger surface area of the tube conducts the current with much less resistance than a solid wire with the same cross-sectional area would. The tank coils of high power radio transmitters are often made of copper tubing, silver plated on the outside, to reduce resistance. However tubing is not flexible and requires special tools to bend and shape.
Litz wire is another method, which employs a stranded wire with individually insulated conductors (forming a bundle). Each thin conductor is less than a skin-depth, so an individual strand does not suffer an appreciable skin effect loss. The strands must be insulated from each other—otherwise all the wires in the bundle would short together, behave like a single large wire, and still have skin effect problems. Furthermore, the strands cannot occupy the same radial position in the bundle over long distances: the electromagnetic effects that cause the skin effect would still disrupt conduction. The weaving or twisting pattern of the wires in the bundle is designed so that the individual strands are on the outside of the bundle for a distance (where the EM field changes are smaller and the strand sees low resistance), and are on the inside of the bundle for a distance (where the EM field changes are the strongest and the resistance is higher). If strands have a comparable impedance, current is distributed equally among every strand within the cable. This allows the interior of the litz wire to contribute to the overall conductivity of the bundle.
Another way to explain the benefit of litz braiding is as follows: the magnetic fields generated by current flowing in the strands are in directions such that they have a reduced tendency to generate an opposing electromagnetic field in the other strands. Thereby, for the wire as a whole, the skin effect and associated power losses when used in high-frequency applications are reduced. The ratio of distributed inductance to distributed resistance is increased, relative to a solid conductor, resulting in a higher Q factor at these frequencies.
Examples of skin depth in copper wire at different frequencies
- At 60 Hz, the skin depth of a copper wire is about 7.6 millimetres (0.30 in).
- At 60,000 Hz (60 kHz), the skin depth of copper wire is about 0.25 millimetres (0.0098 in).
- At 6,000,000 Hz (6 MHz)  the skin depth of copper wire is about 25 micrometres (0.00098 in).
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