Oil-immersed transformers
Distribution and small power transformer windings, where the enamel works alongside paper insulation in mineral oil.
Winding wire
Class 120 Acetal Enamelled Aluminium Round Wire
Acetal enamelled aluminium round wire is soft electrical-grade aluminium wire coated with a polyvinyl acetal (PVA) enamel rated for thermal class 120 °C. The enamel is tough, resists abrasion during winding and is highly compatible with transformer oil, which is why it is a long-standing choice for oil-immersed transformer coils and low-temperature-rise motors. Metline supplies acetal enamelled aluminium wire in grade 1 and grade 2 insulation to coil makers worldwide.
- Thermal class
- 120 °C (class E)
- Enamel
- Polyvinyl acetal
- Conductor
- Annealed aluminium, ≥ 61 % IACS
- Standards
- IEC 60317-0-3 · GB/T 23312.2
What acetal enamelled aluminium wire is
Winding wire is the conductor of every coil, motor and transformer. It needs a thin, continuous insulation film that survives winding, varnish impregnation and years at temperature. Polyvinyl acetal enamel, often known by the older trade name Formvar, was one of the first synthetic wire enamels and is still valued for its toughness, flexibility and resistance to hydrolysis in sealed oil systems.
The thermal class of 120 °C (class E under IEC 60085) is modest compared with modern polyester-imide and amide-imide enamels, which reach 180 – 220 °C. But oil-immersed distribution transformers run well below that limit, and in that service acetal's compatibility with mineral oil and paper insulation, its mechanical toughness and its cost make it a sound choice.
Using aluminium instead of copper as the conductor cuts coil weight and material cost. For the same resistance, an aluminium winding needs about 1.6 times the cross-section of copper (roughly 1.27 times the diameter), yet weighs only about half as much. Designers accept a slightly larger coil in return.
Specifications and supply range
Typical specification
- Conductor
- Annealed electrical-grade aluminium (1350 or 1070 class), resistivity ≤ 0.028264 Ω·mm²/m at 20 °C
- Nominal diameter
- About 0.25 – 5.00 mm
- Enamel
- Polyvinyl acetal, single coat system
- Insulation build
- Grade 1 (thin) or grade 2 (thick) to IEC 60317-0-3 or GB/T 23312.2
- Thermal class
- 120 °C
- Heat shock
- Tested at 155 °C
- Packing
- Plastic or steel spools by diameter, in cartons or on pallets
Exact increase in diameter due to enamel, breakdown voltage and pinhole limits depend on nominal diameter and grade; they are set in the tables of the order standard.
Grade 2 enamel roughly doubles the film build of grade 1 and gives higher breakdown voltage and fewer pinholes. It is the usual choice for transformer windings; grade 1 is used where space factor in the coil matters more.
Conductor and enamel properties
| Property | Typical value or requirement | Why it matters |
|---|---|---|
| Conductor tensile strength (annealed) | ≈ 70 – 90 MPa | Low strength means care with winding tension |
| Conductor elongation | Typically 10 – 25 %, minimum set by diameter | Allows tight winding without fracture |
| Conductor conductivity | ≥ 61 % IACS | Determines winding resistance and losses |
| Thermal class | 120 °C | Long-term temperature limit of the enamel |
| Heat shock | No cracking after test at 155 °C | Film survives thermal stress on a stretched or wound wire |
| Flexibility and adhesion | No cracks after elongation and mandrel winding tests | Film stays intact through forming |
| Breakdown voltage | By diameter and grade, per the standard | Turn-to-turn insulation strength |
Test methods follow the IEC 60851 series. Values above are indicative; the certificate reports the tests required by the order standard.
Winding, impregnation and jointing
- Pre-bake before varnish dipping: acetal film under winding stress can craze (fine cracking) on contact with solvents. Heating the wound coil at about 120 °C for an hour or more relieves the stress before impregnation.
- Control winding tension: annealed aluminium is soft; excessive tension thins the conductor and raises resistance.
- Stripping: acetal is not solderable through the film. Remove it mechanically or with a suitable stripping agent before jointing.
- Jointing aluminium: use crimp connectors rated for aluminium, cold-pressure welding, TIG or ultrasonic welding. Avoid plain soldered joints to copper without suitable flux and process control.
- Storage: keep spools dry and in their packaging until use; aluminium oxide forms quickly on bare ends.
Where acetal enamelled aluminium wire is used
Motors with low temperature rise
Small and medium motors and micro-motors whose hot-spot temperature stays within class 120.
Oil-filled motors
Coils in medium and large oil-filled machines that benefit from oil compatibility.
Instrument coils and reactors
Coils for meters, chokes and reactors.
Relays and solenoids
Electromagnetic coils where cost and weight savings from aluminium are valued.
How acetal compares with other aluminium winding-wire enamels
| Enamel | Thermal class | Strengths | Typical use |
|---|---|---|---|
| Polyvinyl acetal | 120 °C | Oil compatibility, toughness, hydrolysis resistance | Oil-immersed transformers |
| Polyester | 130 / 155 °C | Good all-round electrical and thermal properties | General motors and coils |
| Polyester-imide | 180 °C | Higher thermal endurance | Motors and dry-type transformers |
| Polyester-imide or polyester with amide-imide overcoat | 200 – 220 °C | Highest thermal and mechanical performance | Hermetic motors, high-temperature coils |
Choose acetal when the equipment is oil-filled and runs within class 120. For dry-type transformers or motors running hotter, move to polyester, polyester-imide or amide-imide overcoated wire.
References and standards
- IEC 60317-0-3 – Winding wires: general requirements, enamelled round aluminium wire
- IEC 60851 series – Winding wires: test methods
- GB/T 23312.2-2009 – Enamelled round aluminium winding wire, Part 2: polyvinyl acetal, class 120
- IEC 60085 – Electrical insulation: thermal evaluation and designation
- IEC 60317-68 – Polyvinyl acetal enamelled rectangular aluminium wire, class 120
Acetal enamelled aluminium (aluminum) wire: frequently asked questions
What does "class 120" mean for enamelled wire?
It is the thermal class of the insulation: the enamel is rated for long-term operation at a hot-spot temperature up to 120 °C (class E under IEC 60085). Short excursions above this are tolerated, but sustained higher temperatures shorten insulation life.
Why is acetal enamel used in oil-immersed transformers?
Polyvinyl acetal is highly compatible with mineral transformer oil and paper, resists hydrolysis in sealed oil systems and forms a tough film that survives winding. Transformer windings in oil normally run within its class 120 rating.
Can acetal enamelled aluminum wire be soldered directly?
No. Acetal is not a solderable enamel. Strip the film mechanically or chemically first, and use jointing methods suited to aluminium such as crimping, cold welding or TIG welding.
Why does acetal wire sometimes craze during varnishing?
Stressed acetal film can develop fine cracks when exposed to solvents. Pre-baking the wound coil at about 120 °C for an hour or more before impregnation relieves the stress and prevents crazing.
Which insulation grade should I choose?
Grade 2 (thicker film) is normal for transformer windings because it gives higher breakdown voltage and fewer pinholes. Grade 1 saves space where the voltage between turns is low.
Do you supply enamelled aluminium wire internationally?
Yes. Metline supplies enamelled aluminium winding wire worldwide on standard spools, with test certificates to the order standard.
Sourcing winding wire?
Send the diameter, grade, enamel type, standard and quantity. Our team will reply with a specification and delivered quotation.
