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enamelled_wire

Enamelled wire

Stan Zurek, Enamelled wire, Encyclopedia Magnetica, E-Magnetica.pl

Enamelled wire1) (also referred to as magnet wire2), winding wire3), magnetic wire4), enamel wire5)) - a type of winding wire, usually made from copper or aluminium, insulated by enamel, and used for making windings and coils.

Enamelled copper wire with a round cross-section is widely used in windings of small and medium size electric motors and transformers, as well as other electromagnetic devices such as sensors and actuators.

Litz wire is typically made from enamelled copper wire.

Enamelled wire (0.43 mm outer and 0.375 mm copper diameter, enamel thickness 0.0275 mm) under a microscope: left - cut end with visible copper core (pink metal) and a layer of enamel (darker coating), right - crack in the enamel caused by bending the wire over too small a radius (bare copper visible underneath) enamelled_wire_e_magnetica.jpg
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Submersible wires, one with partially removed enamel x-default

Technical parameters

Enamelled wire must withstand automatic or manual coil winding operation rotor_winding_machine_magnetica.jpg

Despite its simple appearance, enamelled wire is a sophisticated product. The enamelled wire must withstand the mechanical stresses due to bending and stretching during coil winding, which is typically executed at the fastest speed possible in order to increase productivity.

Also, during nominal operation (and overload events) the mechanical forces, thermal stress, as well as chemical and environmental conditions require certain level of performance from the wire and its insulation, so that a reasonable life time can be expected.

The frequently used grades have their technical parameters standardised by international standards such as: IEC 603176), NEMA MW10007), and JIS C32028).

For example, for round enamelled wires the following parameters are defined or controlled: 9)

  • dimensions (diameter)
  • adherence
  • mechanical flexibility (resistance of enamel to cracking)
  • elongation (due to tension required during winding)
  • heat shock resistance
  • spring back (after bending)
  • dielectric breakdown strength
  • completeness of cure (of the enamel)
  • solderability (ease of applying solder directly through the enamel)
  • thermoplastic flow (cut-through)
  • solubility
  • transformer oil resistance and hydrolitic stability
  • bond
  • thermal endurance
  • scrape resistance of the enamel
  • loss of mass
  • electrical resistance
  • lap shear bond strength

Enamel properties

Apart from electrical insulation, the enamel also provides protection against corrosion or oxidation of the copper wire.

Typical electric strength of the enamel is around 170-220 V/μm, which is why a relatively thin layer of enamel can withstand significant voltage. For example, the 0.375 mm wire with the cracked enamel shown above has a voltage breakdown of 4.35 kV despite the enamel thickness being only 0.0275 mm.

The life span of electrical insulation is related to the temperature at which it is used. A typical minimum lifetime of enamel in its nominal temperature is at the order of 20 000 hours.

There are several nominal temperature ranges (thermal classes), listed with the basis on which the enamel is made, for example:

  • 155-180°C - polyurethane (good solderability at 370-390°C)
  • 180-200°C - polyesterimide (good thermal and chemical resistance, solderable above 470°C)
  • 220°C - polyamidimide (good thermal and mechanical resistance, not solderable through enamel)
  • 240°C - aromatic polyimide (very good thermal, chemical and radiation resistance, not solderable through enamel)

Soldering

Enamelled wire of this common-mode choke is soldered directly to the PCB by using the “self-fluxing” property of enamel, without first removing the coating by any mechanical way

The presence of enamel impedes soldering (as compared to bare copper). For this reason, the chemical properties of enamel are optimised in order to facilitate soldering (especially for the enamels rated up to 200°C), which can be achieved by direct application of heat and solder to the outer surface of the enamelled wire.

The enamel will dissolve or boil off, exposing the copper underneath, and to some extent providing even some fluxing or self-fluxing properties. This is sometimes indicated in the name of the product, for example “Polysol” of Elektrisola.10)

The enamel rated for higher temperatures does not support the action of direct soldering through to the copper. Instead, the coating has to be removed by some mechanical means first.11)

Self-bonding enamelled wire

There are also special grades of enamel for specific applications. For example, there is “self-bonding” type of enamel which has an additional layer of adhesive.12)

After a winding is made, the formed wires can be made to adhere to each other, by activating the adhesive with a solvent or warming up the assembly. With the thermal method the coil can be warmed up (depending on the type of the bonding agent), typically to around 120-220°C by: blowing hot air on the assembly, putting it in an oven, or by the “resistive method” (passing current through the coil to heat it up).

With the solvent method, the adhesive can be activated by applying (brushing, spraying or wicking) a suitable solvent such as denatured alcohol (ethanol or methanol), during the winding process. The solvent may be diluted with water, as required, but the strength of bonding is reduced.13)

Grades

Enamel wire is typically made in three “grades”, with the number related to the thickness of enamel (usually assumed to be the number of coating layers):

  • Grade 1 - thinnest insulation, single layer
  • Grade 2 - medium insulation, double layer
  • Grade 3 - thickest insulation, triple layer

Grade 3 of enamelled wire should not be confused with triple-insulated wire.

For example, a comparison of the dimensions for 0.5 mm enamelled wire is as shown in the table below. The higher grades have significantly increased voltage breakdown at the expense of thicker insulation and outer diameter of the wire (so fewer wires can fit into the same cross-section area of the winding).

Manufacturer data for 0.5 mm nominal diameter enamelled wire (circular cross-section)14)
Grade Min. diameter (mm) Max. diameter (mm) Elongation (%) Breakdown voltage (V) Tension (cN)
Grade 1 0.524 0.544 28 2400 1287
Grade 2 0.545 0.566 28 4600 1287
Grade 3 0.567 0.587 28 7000 1287

Production

The production process involves several stages:

  • drawing the wire - from a thicker gauge to the target diameter (in several steps)
  • annealing - to remove mechanical stress
  • coating - with raw enamel (liquid resin)
  • curing - drying off the volatile content of the resin and forming the polymer links
  • applying lubricant - for easing further mechanical processing
  • quality control - checking the diameter of the manufacture wire
  • final spooling - on standardised reels, ready for shipping to the final user
Production process of enamelled wire15)

Photos of typical enamelled wires and their applications

Enamelled wire on spools, as used in production of windings enamelled_wire_on_spools_2_magnetica.jpg
Enamelled wires on spools enamel_wires_magnetics.jpg
Multi-section coil for a motor stator winding coil_for_stator_winding_magnetica.jpg
Litz wire with strands made from enamelled wires, with serving served_litz_wire.jpg
Stator of an induction motor wound with enamelled wire stator_and_rotor_of_induction_motor_magnetica.jpg
Three-phase transformer with enamelled wire windings 3-phase_trafo_with_exposed_windings_-_magnetica.jpg

See also

References

enamelled_wire.txt · Last modified: 2021/08/05 23:41 by stan_zurek

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