Litz wire
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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.
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[edit] Principle of Operation
The resistance of a conductor at DC (0 Hz) is dependent upon the area of the conductor. Larger conductors have lower resistance. 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 surface of conductor. At high enough frequencies, the interior of a large conductor does not carry much current. Thus, skin effect causes the resistance of a wire to become higher at higher frequencies. 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.
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.
[edit] 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].
[edit] 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.
Litz wire is very effective below 500 kHz; it is rarely used above 2 MHz.[3]
[edit] 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]
[edit] Notes
- ^ Terman, Frederick E. (1943). Radio Engineers' Handbook. McGraw-Hill, 37, 74, 80..
- ^ 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).
- ^ Terman, Frederick E. Radio Engineers' Handbook, 37.
- ^ Report TR 1693.
[edit] See also
[edit] 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.
