Litz wire
Encyclopedia
Litz wire is a type of cable used in electronics to carry alternating current
. The wire
is designed to reduce the skin effect
and proximity effect
losses in conductors used at frequencies up to about 1 MHz. It consists of many thin wire strands, individually insulated 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 equalizes the proportion of the overall length over which each strand is at the outside of the conductor.
The term litz wire originates from Litzendraht, German for braided/stranded wire or woven wire.
and the proximity effect
.
For low frequencies, the effect is negligible. For AC at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the current to flow near 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 in. At 60 kHz, the skin depth of copper is about 0.01 inch (0.254 mm). At 6 MHz, 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 large-diameter wire with the non-current carrying interior deleted. It is bulky, but it saves copper.
To combat the skin effect, litz wire 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. The strands must be insulated from each other -- otherwise all the wires in the bundle would short together, behave 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 provides 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 provides 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 electromagnetic field
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.
s and inductor
s, the proximity effect causes losses to increase at high frequency even sooner and more rapidly than does skin effect.
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,area but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies.
Litz wire is used to make inductor
s and transformer
s, 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.
. 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 inductor
, not the aircraft vertical-velocity indicator also called a variometer
).
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
. The wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod 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...
is designed to reduce the skin effect
Skin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
and proximity effect
Proximity effect (electromagnetism)
In a conductor carrying alternating 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. The resulting current crowding is termed the...
losses in conductors used at frequencies up to about 1 MHz. It consists of many thin wire strands, individually insulated 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 equalizes the proportion of the overall length over which each strand is at the outside of the conductor.
The term litz wire originates from Litzendraht, German for braided/stranded wire or woven wire.
Principle of operation
Litz wire reduces the impact of the skin effectSkin effect
Skin effect is the tendency of an alternating electric current to distribute itself within a conductor with the current density being largest near the surface of the conductor, decreasing at greater depths. In other words, the electric current flows mainly at the "skin" of the conductor, at an...
and the proximity effect
Proximity effect (electromagnetism)
In a conductor carrying alternating 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. The resulting current crowding is termed the...
.
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 alternating currents (AC).For low frequencies, the effect is negligible. For AC at frequencies high enough that the skin depth is small compared to the conductor size, the skin effect causes most of the current to flow near 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 in. At 60 kHz, the skin depth of copper is about 0.01 inch (0.254 mm). At 6 MHz, 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 large-diameter wire with the non-current carrying interior deleted. It is bulky, but it saves copper.
To combat the skin effect, litz wire uses lots of little conductors (strands) in parallel (forming a bundle
Bundle
Bundle 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. The strands must be insulated from each other -- otherwise all the wires in the bundle would short together, behave 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 provides 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 provides 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 electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
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
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
to distributed resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
is increased, relative to a solid conductor, resulting in a higher Q factor
Q factor
In 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 transformerTransformer
A 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 inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s, the proximity effect causes losses to increase at high frequency even sooner and more rapidly than does skin effect.
Effectiveness
provides an expression for the ratio of resistance to alternating current to resistance to direct current for an isolated litz wire. It does not apply to windings with multiple turns. An expression for the resistance ratio in windings is given by at Eqn 2 and Appendix A (page 289).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,area but litz wires can be used at thicker cable sizes, hence reducing or maintaining cable impedance at higher frequencies.
Applications
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and transformer
Transformer
A 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 effect
Proximity effect (electromagnetism)
In a conductor carrying alternating 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. The resulting current crowding is termed the...
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
WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks in most of 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 inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. 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 variometer
Variometer
The term variometer also refers to a type of variable transformer or an instrument for measuring the magnitude and direction of a Magnetic field....
).