Litz wire is a type of cable used in electronics. The
wireA wire is a single, usually cylindrical, string of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various wire gauges...
is designed to reduce the
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
and
proximity effectIn a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions...
losses in conductors. It consists of many thin wires, individually coated with an insulating film and twisted or woven together, following one of several carefully prescribed
patterns often involving several levels (groups of twisted wires are twisted together, etc.).
Litz wire is a type of cable used in electronics. The
wireA wire is a single, usually cylindrical, string of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various wire gauges...
is designed to reduce the
skin effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
and
proximity effectIn a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions...
losses in conductors. It consists of many thin wires, individually coated with an insulating film and twisted or woven together, following one of several carefully prescribed
patterns often involving several levels (groups of twisted wires are twisted together, etc.). This winding pattern evens out the time that the conductors spend at the outside of the conductor.
The term litz wire originates from
litzendraht, German for bundled wire.
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 effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
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 ⅓ inch (8 mm). At 60 kHz, the skin depth of copper is about about 0.01 inch (0.254 mm). At 6 MHz, the skin depth is about 0.001 inch (25.4 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 effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
. 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
bundleBundle or Bundling may refer to:In marketing:* Product bundling, a marketing strategy that involves offering several products for sale as one combined product...
). 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 effectSkin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
and associated power losses when used in high-frequency applications are reduced. The ratio of distributed
inductanceInductance is the property in an electrical circuit where a change in the electric current through that circuit induces an electromotive force that opposes the change in current ....
to distributed
resistanceThe electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the...
is increased, relative to a solid conductor, resulting in a higher
Q factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
at these frequencies.
Proximity effect
In cases involving multiple wires, or multiple turns, such as windings in
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field...
s and
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s,
proximity effectIn a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions...
causes losses to increase at high frequency even sooner and more rapidly than does skin effect.
Effectiveness
Terman 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 .
Litz wire is very effective below 500 kHz; it is rarely used above 2 MHz as it is much less effective there.
Litz wire has a higher impedance per unit cross-section, but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies.
Applications
Litz wire is used to make
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and
transformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field...
s, especially for high frequency applications where the skin effect is more pronounced and
proximity effectIn a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions...
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
WWVBWWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks throughout North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic...
. 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
inductorAn inductor or a reactor is a passive electrical component that can store energy in a magnetic field created by the electric current passing through it. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
, not the aircraft vertical-velocity indicator also called a
variometerThe term Variometer also refers to a type of variable electrical inductorA variometer is an instrument in an aircraft used to inform the pilot of the instantaneous rate of descent or climb...
).
See also
- Proximity effect (electromagnetism)
In a conductor carrying current, if currents are flowing through one or more other nearby conductors, such as within a closely wound coil of wire, the distribution of current within the first conductor will be constrained to smaller regions...
- Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor, at an average depth...
- Skin depth
- Twisted pair
Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from Unshielded Twisted Pair cables, and crosstalk between neighboring pairs.-...
External links