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What Is Litz Wire?

Litz wire

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Litz wire made out of 8 thinner isolated copper wires
Litz wire consists of multiple strands insulated electrically from each other. Ordinarily the strands are twisted or woven, but no twisting is shown in this diagram.

Litz wire is a type of cable used in electronics to carry alternating current. The wire is designed to reduce the skin effect and proximity effect losses in conductors used at frequencies up to about 1 MHz.[1] It consists of many thin wire strands, individually insulated and twisted or woven together, following one of several carefully prescribed patterns[2] often involving several levels (groups of twisted wires are twisted together, etc.). This winding pattern equalizes the proportion of the overall length over which each strand is at the outside of the conductor.

The term litz wire originates from Litzendraht, German for braided/stranded wire[3] or woven wire[4].

Contents

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Principle of operation

Litz wire reduces the impact of the skin effect and the proximity effect.

Skin effect

The resistance of an isolated conductor at DC (0 Hz) depends on its cross sectional area. A conductor with a larger area has a lower resistance. The skin effect causes that resistance to increase for alternating currents (AC).

For low frequencies, the effect is negligible. For AC at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the current to flow near the conductor's surface. At high enough frequencies, the interior of a large conductor does not carry much current. At 60 Hz, the skin depth of a copper wire is about 13 inches (8.5 mm). At 60 kHz, the skin depth of copper is about 0.01 inches (0.25 mm). At 6 MHz[5], the skin depth is about 0.001 inches (25 µm). Round conductors larger than a few skin depths don't conduct much current near their axis, so that material isn't used effectively.

When larger area conductors are needed, tricks are used to minimize the skin effect. The goal is to increase the conducting surface area. One trick is to use a hollow conductor with a wall that is about a skin-depth thick. It is essentially a large-diameter wire with the non-current carrying interior deleted. It is bulky, but it saves copper.

To combat the skin effect, litz wire uses lots of little conductors (strands) in parallel (forming a bundle). Each little 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: the electromagnetic effects that cause the skin effect would still disrupt conduction. The bundle is constructed so the individual strands are on the outside of the bundle (and provides low resistance) for a time, but also reside in the interior of the bundle (where the EM field changes are the strongest and the resistance is higher). If each strand provides about the same average resistance, then each strand will contribute equally to the conduction of the entire cable.

The weaving or twisting pattern of litz wire is designed so individual wires will reside for short intervals on the outside of cable and for short intervals on the inside of the cable. This allows the interior of the litz wire to contribute to the cable's conductivity.

Another way to explain the same effect 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.

Proximity effect

In cases involving multiple wires, or multiple turns, such as windings in transformers and inductors, the proximity effect causes losses to increase at high frequency even sooner and more rapidly than does skin effect.

Effectiveness

Terman (1943, p. 37, Table 18, 78) provides an expression for the ratio of resistance to alternating current to resistance to direct current for an isolated litz wire[6]. It does not apply to windings with multiple turns. An expression for the resistance ratio in windings is given by Sullivan (1999) at Eqn 2 and Appendix A (page 289).

Litz wire is very effective below 500 kHz; it is rarely used above 2 MHz as it is much less effective there. (Terman 1943, p. 37)

Litz wire has a higher impedance per unit cross-section,area but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies.[7]

Applications

Disassembled induction cooker showing litz wire coil.

Litz wire is used to make inductors and transformers, especially for high frequency applications where the skin effect is more pronounced and proximity effect can be an even more severe problem. Litz wire is one kind of stranded wire, but, in this case, the reason for its use is not the usual one of avoiding complete wire breakage due to material fatigue.

WWVB transmitting station

The NIST uses litz wire in the time code broadcasting station WWVB. The station transmits on 60 kHz. The litz wire consists of 6075 strands of #36 (0.127 mm) magnet wire in a cable ¾ inch (19 mm) in diameter. Litz wire is used in an impressive variometer (i.e., variable inductor, not the aircraft vertical-velocity indicator also called a variometer). (Hansen & Gish 1995)

Notes

References