Form coils are preformed windings brought into their final geometry in a defined tool or winding fixture before being installed in the armature, stator or transformer. In contrast to random or layered windings, form coils produce highly precise, reproducible windings with defined dimensions.
Simply put: form coils are tailor-made windings that are fully shaped before assembly. This saves time, increases quality and enables dense winding packages with high electrical and thermal load capacity.
Structure and characteristics of form coils
A form coil typically consists of:
- Conductor wire: usually copper (enamel-insulated, flat or round wire), less often aluminium.
- Turn insulation: by enamel coating or additional interlayers.
- Coil former: for smaller coils, plastic or pressboard carriers.
- Layer insulation: Nomex, polyester films (HOSTAPHAN, Mylar) or laminate composites such as NMN or DMD.
- End and connection design: solder tabs, stranded wires or cable connections.
- Impregnation: varnishes or resins for mechanical stabilisation and electrical consolidation.
Form coils differ by design: single coils, double form coils,hairpin coils (e-mobility), and banded high-voltage coils with mica insulation.
Materials and insulation systems
The choice depends on voltage level, temperature class and field of application:
- Aramid paper (Nomex): high voltage and temperature resistance, standard in motors and generators.
- Polyester films (HOSTAPHAN, Mylar): economical, good electrical strength.
- Polyimide films (Kapton): for high-temperature applications (up to class H and beyond).
- Mica tapes (mica tape, mica composite): for high-voltage coils with partial discharge resistance.
- DMD/NMN laminates: combinations for medium to high voltages.
- Glass fabric: as reinforcement and for banding.
- Resins and varnishes: for impregnation (trickle, VPI or resin-rich).
Manufacturing processes
- Wire preparation: enamelling or coating, cutting to length.
- Winding in forms: defined winding machines and coil forms produce the final geometry.
- Inserting insulation: interlayers, edge reinforcements, layer insulation.
- Banding: with mica or glass fabric tapes.
- Impregnating: depending on process, dip, VPI (Vacuum Pressure Impregnation) or resin-rich.
- Curing: thermally or chemically.
- Finishing: forming, insulating and marking connections.
- Testing: dielectric withstand strength, dimensional accuracy, resistance and partial discharge.
Form coil manufacturing requires high precision, since even small dimensional deviations can negatively affect the fit in stator or armature slots.
Requirements for form coils
- Electrical: high dielectric strength, partial discharge resistance, creepage distances.
- Thermal: temperature rating according to insulation class (B 130 degrees C, F 155 degrees C, H 180 degrees C or higher).
- Mechanical: abrasion resistance during installation, stability against vibration, durability in continuous operation.
- Process reliability: clean fit into slots, reproducible geometry.
- Environmental resistance: resistance to moisture, oils and chemicals.
Fields of application
- Electric motors: synchronous and asynchronous motors, hairpin technology in e-mobility.
- Generators: large machines with high voltages and thermal load.
- Transformers: medium and high-voltage windings with demanding insulation requirements.
- Rail drives: coils in traction motors with strong vibrations and continuous load.
- Special machinery: induction systems, high-frequency engineering.
Benefits and challenges
Benefits
- Precise geometry, easy assembly.
- High packing density and efficiency.
- Long service life through matched insulation.
- Good automatability in series production.
Challenges
- Tight tolerances in manufacturing and assembly.
- Ensuring minimum bending radii during winding.
- Avoiding partial discharges at high voltage.
- Extensive testing (PD, high-voltage tests, ageing tests).
GOBA Takeaway
Form coils are central components in modern electrical machine manufacturing. Their benefits lie in precise prefabrication, reproducible quality and high operational reliability. Decisive for performance are material selection, insulation system and manufacturing precision. Whether in motors, generators or transformers: form coils contribute significantly to ensuring that machines operate efficiently, reliably and with long service life.