Foil winding is a common manufacturing process for low-voltage windings in transformers, reactors and other electrical equipment. Copper or aluminum foil is used as the conductor and wound together with interlayer insulation to form a compact coil with a relatively uniform structure.
The manufacturing process generally includes material preparation, machine setup, synchronized winding, terminal connection, coil forming and quality inspection. The actual process depends on the winding design, conductor material, insulation class and product construction.
1. Material Preparation
Before winding, the conductive foil, interlayer insulation and end insulation materials must be prepared according to the product drawings and process requirements.
Copper or Aluminum Foil Inspection
Select copper or aluminum foil with the specified dimensions and material properties. Check its thickness, width, surface condition and edge quality.
The foil surface should be clean and free from oil, noticeable scratches, oxidation contamination and other defects that could affect electrical connections or insulation performance. The foil edges should also be inspected. Deburring may be required to prevent sharp edges from damaging the interlayer insulation.
Foil thickness and dimensional tolerances must meet the product drawings, material specifications and purchasing requirements. A single thickness range should not be treated as a universal requirement for all transformer low-voltage windings.
Insulation Material Preparation
Prepare interlayer insulation paper or film according to the winding insulation design. Common materials include DMD insulation paper, polyester film and other composite materials suitable for the specified insulation system.
For DMD insulation paper, check its width, thickness, surface condition and storage condition. Its width must match the foil width and the winding end-insulation design to provide the required insulation coverage after winding.
End insulation boards, insulation tapes or other materials should also be prepared if required by the product construction.
Foil Slitting and End Allowance
If the supplied foil is wider than required, use a slitting machine to cut it to the specified width. After slitting, inspect the edges for burrs, curling and deformation.
An appropriate length must also be reserved at both foil ends for terminal connections and welding. The required allowance depends on the winding drawing and welding fixture design.
2. Foil Coil Winding
Winding is the main manufacturing stage and is generally performed on a foil winding machine. Depending on the coil design, the machine may include foil decoilers, insulation decoilers, tension control units, edge alignment systems, guide rollers and the winding unit.
Winding Machine Setup
First, inspect the main shaft, clamping mechanism, decoilers, tension control system and edge alignment devices to confirm that they are operating properly.
Set the foil tension and insulation tension according to the foil thickness, width and winding design. Excessive tension may deform the foil or stretch the insulation material. Insufficient tension can result in loose winding and uneven layer arrangement.
The positions of the guide rollers and alignment devices should also be checked to ensure that the foil and insulation enter the winding area at the required positions.
Synchronized Feeding of Foil and Insulation
Load the copper or aluminum foil onto the decoiler and install the interlayer insulation material on its corresponding feeding unit.
Once the machine starts, the foil and insulation are fed into the winding area simultaneously according to the winding design. The insulation material should remain continuous and flat during winding, without wrinkles, displacement or localized stretching.
The foil edges must remain aligned. On machines equipped with automatic edge correction, sensors detect the material position and the control system adjusts the feeding position to reduce lateral deviation during winding.
Turn Count and Coil Dimension Control
Set the target turn count, winding direction and other process parameters according to the winding design. The machine follows the preset program, while its counting system controls the number of turns.
During winding, check the coil inner diameter, outer diameter, axial width and layer arrangement. Once the specified turn count is reached, the machine stops and the foil and insulation are cut according to the process requirements.
Some foil winding machines also include automatic pressing, end-insulation feeding and foil edge treatment units. The actual configuration depends on the machine model and product requirements.
3. Terminal Connection and Insulation
After winding, the conductor ends must be connected and insulated. This stage directly affects the electrical connection quality and the insulation structure at the coil ends.
Terminal Connection
According to the winding design, connect the foil ends to copper or aluminum terminal bars. Common joining methods include TIG welding, cold-pressure welding and other welding processes suitable for the specific materials and joint construction.
Each joining method has different requirements for material thickness, joint geometry and welding equipment. The process should follow validated manufacturing parameters.
After joining, inspect the weld appearance, connection position and joint integrity. Check for visible cracks, lack of fusion, burn-through and other defects that may affect connection reliability. Electrical performance checks should be performed when required by the product inspection plan.
End Insulation
Apply insulation to the inner and outer winding ends, terminal areas and other locations requiring additional insulation protection.
Common materials include DMD insulation paper, epoxy glass cloth tape and other materials compatible with the specified insulation system. The insulation should provide continuous coverage and be applied evenly, without looseness, wrinkles or exposed areas.
The insulation material, coverage and thickness must be determined according to the winding voltage rating, dimensions and insulation design. They should not be assigned a universal thickness or dielectric strength.
4. Coil Forming and Post-Processing
After terminal connection, the coil is formed and undergoes drying, impregnation or curing when required by its construction.
Mechanical Forming
Remove the winding from the winding machine and place it in the appropriate forming fixture according to the product design.
Apply controlled pressure and positioning to achieve the specified inner diameter, outer diameter, axial height and end geometry. The forming pressure must be controlled to prevent foil deformation, insulation damage or disruption of the layer structure.
After forming, inspect the coil dimensions, end-face flatness and conductor arrangement.
Drying and Insulation Treatment
The post-processing method depends on the insulation system used in the transformer or reactor.
For windings requiring drying, an oven or other suitable equipment may be used to remove moisture from the insulation materials. The drying temperature, duration and process conditions must be determined according to the heat resistance of the insulation materials and the manufacturing process.
For products using resin impregnation, impregnation and curing may be carried out according to the insulation system requirements. Some dry-type transformers use vacuum pressure impregnation, while others use resin casting or different insulation processes.
Vacuum pressure impregnation is not required for every foil coil. The impregnation material, vacuum conditions, curing temperature and holding time must follow the resin manufacturer's technical data and the product process documentation.
5. Finished Coil Inspection
After winding and post-processing, the coil must undergo visual, dimensional and electrical inspections to confirm compliance with the product drawings and inspection requirements.
Visual and Dimensional Inspection
Check the foil surface for scratches, deformation and visible contamination. Confirm that the layers are arranged evenly and that the insulation materials have no wrinkles, damage or displacement.
Measure the coil inner diameter, outer diameter, axial height and other critical dimensions. The terminal positions and welded areas must also meet the drawing requirements.
Insulation Inspection
Perform insulation resistance measurements and other specified insulation tests according to the product construction and applicable inspection requirements.
The test location, test voltage and duration must be clearly defined. An insulation resistance acceptance value should not be specified independently of the product type, insulation system, test temperature and test conditions.
DC Resistance Measurement
Use a DC resistance tester to measure winding resistance and compare the measured values with the design calculations or specified limits.
For three-phase windings, compare the DC resistance of each phase. The measured values may need to be converted or corrected to the specified reference temperature. Permissible deviations must follow the applicable product standards, design documents and acceptance requirements rather than a universal fixed percentage.
6. Safety Precautions During Foil Winding
The main shaft, decoilers and guide rollers of a foil winding machine can create entanglement and pinch hazards. Operators must follow the machine safety procedures and must not touch rotating components or adjust mechanisms that present a risk of injury while the machine is running.
Welding operations require suitable personal protective equipment, local exhaust ventilation and fire prevention measures according to the welding method. Equipment used for vacuum, pressurized or heated impregnation and curing must be operated according to the manufacturer's instructions and process documentation. Pressure, temperature and safety interlocks must be checked before operation.
Foil coil quality depends not only on the winding machine but also on conductor materials, insulation materials, tension control, terminal connections and post-processing. Production inspections should follow the product drawings and process documents, with critical dimensions and electrical properties checked to ensure that the winding meets the requirements for subsequent assembly and operation.
