Litz wire

All you want to know about Litz wire

Litz wire is a special type of wire used in electronics. The wire is designed to reduce the skin effect and proximity effect losses in conductors. It consists of many thin wires, individually coated with an insulating film and twisted or woven together, following a carefully prescribed pattern often involving several levels (groups of twisted wires are twisted together, etc.).

The term litz wire originates from Litzendraht, German for bundled wire.

Contents

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 AC currents.

For low frequencies, the effect is negligible. For AC current at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the conduction to happen at 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 1/3 inch (1 cm). At 60,000 Hz, the skin depth of copper is about about 0.01 inch. At 6,000,000 Hz, the skin depth is about 0.001 inch. 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 fat wire with the non-current carrying interior deleted. It is bulky, but it saves copper.

Litz wire uses some different tricks. Instead of using one big conductor, it 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. However, that is not the complete story. The strands must be insulated from each other -- otherwise all the wires in the bundle would short together, look 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 see 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 sees 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 e.m.f. 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, proximity effect causes losses to increase at high frequency even sooner and more rapidly than does skin effect.

Effectiveness

Terman [1] provides an expression for the ratio of resistance to alternating current to resistance to direct current for a single litz wire. It does not apply to windings with multiple turns. An expression for the resistance ratio in windings is given by Sullivan [2].

Litz wire is very effective below 500 kHz; it is rarely used above 2 MHz as it is much less effective there.[3]

Applications

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 magnet wire in a cable 0.75 inches in diameter. Litz wire is used in an impressive variometer (i.e., variable inductor, not the aircraft vertical-velocity indicator also called a variometer). [4]

Notes

  1. ^ Terman, Frederick E. (1943). Radio Engineers' Handbook. McGraw-Hill, 37, 74, 80.. 
  2. ^ Charles R. Sullivan, "Optimal Choice for Number of Strands in a Litz-Wire Transformer Winding", IEEE Transactions on Power Electronics, 14(2), 1999, http://thayer.dartmouth.edu/inductor/papers/litzj.pdf, Eqn. (2).
  3. ^ Terman, Frederick E. Radio Engineers' Handbook, 37. 
  4. ^ Report TR 1693.

See also

References

Terman, Frederick E. (1943). Radio Engineers' Handbook. McGraw-Hill, 37, 74, 80.. 

Peder Hansen and Darrell Gish, WWVB Antenna and Antenna Tuning System, TR 1693, February 1995. TR1693, DTIC ADA299078.

External links

Retrieved from "http://en.wikipedia.org/wiki/Litz_wire"

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