Litz wires consist of multiple single insulated wires bunched together to form a twisted construction. They are used in a wide range of applications requiring good flexibility and high frequency performance.
High frequency litz wires are produced using multiple single wires electrically isolated from each other and are typically used in applications operating within a frequency range of 10 kHz to 5 MHz. High frequencies cause the current to flow in the perimeter area or even along the surface of each conductor - this is known as the "skin effect". Because of this effect, the cross section carrying the current is reduced and causes losses at high frequency. In addition, losses at higher frequencies are caused by the "proximity effect", where the neighboring electrical field negatively affects the electromagnetic distribution of the other strands within the bundle. Litz wires are engineered with a twisted construction can effectively minimize these high frequency losses.
Subsequent sections give an overview of basic terms and variants of litz wire.
The basic component of a litz wire is the insulated single wire. Conductor material and enamel insulation can be combined optimally to meet the demands of specific applications.
The length of lay describes the distance which a single wire needs for one complete rotation around the litz wire circumference (360 degrees).
The pitch direction indicates the twist or bunching direction of the bundled wire construction. Z-lay is bunched in a clockwise direction while S-lay is the opposite, or counter-clockwise twist direction.
Any number of single wires are directly bunched so each individual wire is freely located. The single wires can take any position within the litz wire cross section.
The individual wires are positioned in one or more layers concentrically around the litz wire center conductor. In this design configuration each single wire naturally moves into its predefined position during the twisting operation resulting in consistent dimensions and working properties.
Depending on the specified profile, litz wires can be twisted in several steps. Any number of pre-twisted bundles are bunched with each other in several bunching steps. The electrical and mechanical properties of the final product depend upon the designed litz wire construction.
Litz wires twisted in one or several steps without any additional coating such as serving, extruding or other functional coating.
Litz wires can be spirally served with different materials such as nylon or natural silk. During the serving process the litz wire is covered by a textured yarn in one or more layers. Serving of the litz wire improves dimensional stability, flexibility, and impregnation performance.
An additional insulating coating of the litz wire can be added by taping the outer surface with one or more tapes. Increased dielectric breakdown voltage, thermal endurance, and flexibility can be achieved by a suitable combination of taping material, number of tapes and the degree of overlapping of the tapes.
The degree of overlapping of two adjacent tape windings is defined by the gradient angle between the tape and the litz wire during the taping process. The overlapping percentage determines the number of tape layers located on top of each other and thus the insulation thickness of the litz wire.
Coating litz wires with extruded thermoplastic materials offers additional possibilities to insulate the litz wire. Extruded coatings are highly flexible and can provide additional protection against moisture and chemical attack.
Basic litz wires and some types of served or taped litz wires can be given a square or rectangular cross section by a profiling process. The compacted profile provides an optimized copper filling factor for more efficient electrical properties in the wound coil.
Ultra-fine litz wires or litz wires requiring increased tensile strength or flex life performance can be improved with the addition of strain relief from high-strength mono- or multifilament materials. To achieve an optimal force absorption, the strain relief materials are arranged in the center of concentric litz wire constructions. For bunched litz wires, a non concentric arrangement using a bare conductor wire as the strain relief member is also possible.
Self supporting coils can be produced using litz wires with a 'Smartbond' adhesive in automated winding processes. Very thin coils can be produced with Smartbond's unique construction providing additional space for designers or to help achieve miniaturization goals.