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What is Magnetic Coil Wire?
Magnetic coil wire, or magnet wire, is a copper or aluminium wire that has been coated with insulation or enamel. The magnetic coil wire is designed to convert electric energy into magnetic energy.
Magnetic coil wire can be found in applications which require tight coils of wire. Applications include motors , transformers, hard disk drives, loudspeakers, electromagnets and inductors.
Magnetic coil wire will almost always be made of either copper or aluminum, though silver wire is also sometimes available.
Given aluminum’s lower conductivity, it requires a cross-section that is over sixty times larger than a copper wire would require in order to achieve equivalent ratings. For this reason, aluminum wire takes up more space than copper wire.
Transformer wire is usually made from electrolytic-tough pitch (ETP) copper, also called oxygen-free copper. Larger, lower voltage, transformers that contain kilovolt-amperes (kVA) greater than 15 will use aluminum wire in some cases.
The wire in the voice coil of a loudspeaker can be made of silver, but this is rare. Silver is the highest conducting metal, but it is much more expensive than copper since it is considered a precious metal. Copper or aluminum is much more widely used. Aluminum wire helps to raise the resonant frequency of the voice coil, which allows the coil to more easily respond to higher frequencies.
AC induction motors consist of a stator, a rotor and an electromagnet, each with its own coil.
The stator coil will be made out of either copper or aluminum. The higher conductivity of copper helps to increase the motor’s electrical efficiencies, so the majority of stator coils are copper.
Rotor conductors are usually die-cast in the shape of a squirrel cage. They can, however, be designed as wound-rotor motors too. Die-cast rotors are generally aluminum, with copper only becoming common in recent years, but winding rotors are made of either aluminum or copper.
Square, round and rectangular are all common shapes for magnetic coil wire. The wire shape is the determinant for how tightly the wire can be wound.
Rectangular wires lay flat and will, therefore, create the tightest coils. Rectangular wire can be used in larger power transformers that operate at high voltages.
Square wires will create tight coil as well, since they will have less space between the turns. These wires are available for confined spaces and applications that require higher power and a larger wire gauge.
Round wires will create coils that are the least tight because of large gaps between the turns.
Voice-coils will use a hexagonal wire in some cases.
Pictured above: Rectangular wire (left), round wire (right)
Solid vs. Stranded Cu Wire
Solid wire is made of one strand of insulated wire that is not easily bent, so it will be used in applications where the wires do not have to be moved often.
Stranded wire is a number of wires that are braided together into a larger wire. Stranded wires are more flexible than solid wires; the wire flexibility increases with the number of strands. Stranded wire has at least six wires wrapped around one in the middle, a minimum of seven total wires.
Stranded wire helps to slow down the effects of metal fatigue, which occurs when a wire is continuously subjected to loading and unloading. Eventually the wire material begins to crack; the flexibility of stranded wire reduced the effect.
Due to having more surface area, stranded wire is more susceptible to corrosion.
Pictured above: Solid copper wire (left), stranded wire (right)
Litz wire is another form of stranded wire. It contains multiple separately insulated stranded wires braided together into one larger, insulated wire.
Radio frequency inductors, and transformers that are high-frequency, are among the applications that use litz wire to minimize the skin-effect and proximity effect losses.
The skin effect occurs when alternating current pools in the top layer of a wire. This causes an increase in the wire’s effective resistance. The skin depth is how deep the current is flowing in the wire. It will be approximately a centimeter for a 60 Hz wire.
An alternating current creates an alternating magnetic field around a conductor when current flows through it. The field can cause swirling electric currents, or eddy currents, in adjacent conductors. This is known as the proximity effect.
The distribution of current in the adjacent wire can be altered by the eddy currents, causing the current to concentrate in the parts of the wire that are furthest away from nearby wires carrying current in the same direction.
Wire insulations are going to be either a yarn made of fabric, a thin varnish, or both. The insulation increases the wire’s thermal endurance.
What insulation really does is protect the wire from itself. The turns of bare wires, which do not have any insulation, cannot touch each other. If they did, the wire would short out. Insulated wire can be wound so that the wires do touch, since the bare copper windings are no longer in danger of coming in contact. Wires can have hundreds or thousands of turns, so insulation for a wound wire is almost always required.
A high-temperature tape, made out of polyimide or fiberglass, is sometimes applied to square or rectangular wires that have thick insulation. A varnish, which provides extra insulation, can be placed on the windings as well. It is meant to help improve both the wire’s strength, as well as its endurance.
The thermal capacity and diameter of the wire depend on the type of insulation used.
Oil-impregnated paper or pressboard insulations are found in wires in large power transformers that operate at high voltages.
TEMCo sells wires with a polyurethane insulation overcoated with polyamide, a wire with a thermal rating of 155°C. We also sell polyester-imide insulated wires overcoated with polyamide-imide, which have a thermal rating of 200°C.
Pictured above: Insulated wire (left), bare wire (right)
Wire diameter can be determined by the American wire gauge (AWG) rating of the wire. AWG is used in the U.S. and Canada to represent specific characteristics, such as diameter, resistance and current, of round, solid wire.
In total, there are 44 standard wire gauges, the smallest being 0000 and the largest being 40. As diameter decreases, the gauge increases, so a wire that has a diameter of 5.827 millimeters will have a correlating AWG size of 3, while a wire with a diameter measuring 0.101 millimeters will have an AWG size of 38.
The smallest gauge wire will have a diameter of 11.684 mm, while the largest gauge wire has a diameter of 0.0799 mm.
Like the wire diameter, wire weight will be based on AWG standards and will have an inverse relationship with the gauge, meaning as the gauge becomes greater, the weight of the wire will decrease. That is why the largest wire weight is 640.5005 lbs per 1000 feet, for a gauge of 0000. A 40 gauge wire has the smallest weight, 0.0299 lbs per 1000 feet.
Temperature Rating/Thermal Class
The thermal class, or temperature rating, is the maximum temperature at which a wire will have a 20,000 hour service life, which can be extended if the wire is used at a lower temperature.
The thermal class is determined by the insulation that is used. Thermal classes of 130°, 155°, 180° and 200° are common. The maximum thermal class is 250°.
Thermal class is measured in degrees celsius.
Bondable Magnetic Coil Wire
Bondable wire contains an extra adhesive film on top of the usual insulation that activate when heated up. It will bond the turn to turn windings of the wire into a self-supporting coil, eliminating the need for bobbins, the spindles around which the wire is wound.
Bonding agents include epoxy, polyester and polyamide.
There are three main bonding techniques:
- Solvent bonding: Applied while the wire is winding. If not, then the coiled wire will be dipped in the solvent once the winding is finished.
- Oven bonding: the wire will be heated up in an oven after it is fully wound. Oven bonding time can be anywhere from 10 to 30 minutes, with larger coiled wires taking longer to bond.
- Resistance bonding: the wire will be heated up with an electric current. The amount of voltage, as well as the time necessary to complete the process, both depend on the size of the wire and the coil design. Resistance bonding is used for wires that have a gauge of 34 or higher.
Soldering is when a wire is fused to another wire by melting another metal, typically lead or tin, that will join the other two together after it becomes solid again.
Wire insulation does need to be taken off before the wire can be soldered in some cases. The GP/MR-200 wire does need its insulation to be removed, while the Soderon 155 has insulation that acts as a flux so it doesn’t need to be removed.
Aluminum is not easily soldered, and so connections can corrode and eventually fail.
Breakdown voltage indicates the dielectric strength of the enamel insulation of the wire.
The breakdown voltage can be of 3 types: Grade 1, Grade 2 and Grade 3. Insulations with higher grades will have higher breakdown voltages. Higher ambient temperatures result in a reduction of breakdown voltage.
The measurement of the enamel is called the insulation thickness, or build. To get the measurement of the enamel, subtract the total diameter of the wire from the diameter of the insulation and wire put together.
Insulation build can come as single, heavy, triple or quadruple; single and heavy are the most popular builds.
Wires with smaller gauges come with higher breakdown voltages.
Where can I purchase my Magnetic Coil Wire?
Click to view the magnetic coil wire selection below from TEMCo. Wires are available in different gauges, temperature ratings and insulations. Buy your magnetic coil wire from TEMCo!
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