What is Litz wire? The word "Litz" is derived from the German word "Litzendraht" meaning woven wire. It refers to wire consisting of a number of individually insulated magnet wires twisted or braided into a uniform pattern, so that each strand tends to take all possible positions in the cross-section of the entire conductor.
This multi-strand configuration or Litz construction is designed to minimize the power losses exhibited in solid conductors due to "skin effect". Skin effect refers to the tendency of current flow in a conductor to be confined to a layer in the conductor close to its outer surface. At low frequencies, skin effect is negligible, and current is distributed uniformly across the conductor. However, as the frequency increases, the depth to which the flow can penetrate is reduced. Litz wire constructions counteract this effect by increasing the amount of surface area without significantly increasing the size of the conductor.
Even properly constructed Litz wires will exhibit some skin effect due to the limitations of stranding. Wires intended for higher frequency ranges require more strands of a finer gauge size than Litz wires of equal cross-sectional area but composed of fewer and larger strands. Skin Effect Calculator.
Proximity effect is the tendency for current to flow in loops or concentrated distributions due to the presence of magnetic fields generated by nearby conductors. In transformers and inductors, proximity effect losses are generally more significant than skin effect losses. In Litz wire windings, proximity effect may be sub-divided into internal proximity effect (the effect of other currents within the bundle) and outer proximity effect ( the effect of the current in other bundles). The reason for twisting or weaving Litz wire, rather than just grouping fine conductors together, is to ensure that the strand currents are equal. Simple twisted bunched conductor wire can accomplish this adequately where proximity effect would be the only significant problem with solid wire. Where skin effect would also be a problem, more complex Litz wire constructions can be used to ensure equal strand currents. Therefore, in a well-designed construction, strand currents are nearly equal.
Litz wire sizes are often expressed in abbreviated format: N / XX, where N equals the number of strands and XX is the AWG (American Wire Gauge) size of each strand. For example, a typical size of a Litz-wire would be expressed as "12 / 38" or twelve strands of 38 AWG (0.100 mm). Insulation and serving/jacketing options are listed after the size, for example, 12/38 Single Nylon Served. For more information on this subject Click Here.
Since the primary benefit of a Litz conductor is the reduction of AC losses, the first consideration in any Litz design is the operating frequency. The table next to this shows frequencies versus AWG strand sizes:
Typical applications for Litz wire conductors include high-frequency inductors and transformers, motors, relays, inverters, power supplies, DC/DC converters, communications equipment, ultra-sonic equipment, sonar equipment, television equipment, and heat induction equipment.
Twisting Tightness, Pitch
The standard twist configuration is twelve twists per foot (TPF) on most Litz-wires (also referred to as pitch); however, non-standard twisting is available on request. Sometimes the twisting is expressed as twists per inch (TPI). The maximum number of twists in a given length is limited by the size of the strands.
Litz wire may be quoted as served or unserved. "Served" simply means that the entire Litz construction is wrapped with a nylon textile, yarn or silk for added strength and protection. Another option is to have the Litz wire construction taped or extruded with NOMEX®, Mylar ®, Kapton ®, Mica, FEP Teflon ®, or PVC. Total Litz Wire Area Calculator
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The outer insulation and the insulation on the component conductors may be serving of nylon, cotton, NOMEX ®, fiberglass or ceramic. Heat-sealed polyester, rubber, vinyl and Teflon ® tape wraps along with most thermoplastic insulations are also available as outer insulation if the applications dictate special requirements for voltage breakdown or environmental protection.
To read Charles Sullivan's "Optimal Choice for Number of Strands in a Litz-Wire Transformer Winding" Click Here. For a more technical discussion of power losses due to copper loss, skin effect, proximity effect, hysteresis and eddy currents, you may find an article "Power losses in wound components" interesting.