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Insulation papers and paper-like materials are the unsung heroes inside electric motors. They manage electrical stress, survive mechanical wear, and help define the motor’s thermal and service-life limits. This guide explains the most commonly used families of insulating papers, maps them to typical motor locations, and gives a pragmatic checklist and example specification you can copy into supplier requests.
Cellulosic (kraft / crepe) paper
Natural-cellulose sheet stock. Low cost, flexible, and easy to conform. Best suited to low-temperature, low-voltage machines and applications where affordability and mechanical pliability are prioritized.
Pressboard and thick cellulose laminates
Dense and mechanically robust; used where structure and thickness are required (wedges, spacers, structural partitions). Offers good compressive strength but limited high-temperature capability compared to engineered papers.
Mica-based papers and laminates
Mica provides high dielectric strength and excellent endurance in hot environments. Mica products are typically combined with backing materials (glass, aramid) and impregnated to form the primary dielectric in high-voltage or high-temperature stators.
Aramid fiber papers (e.g., Nomex-style)
High thermal resistance and tear strength with good dielectric behavior. Often used as slot liners, interlayer insulation, or backing for mica systems where thermal and mechanical performance are required.
Glass-fiber papers and cloths
Strong, moisture-resistant, and compatible with many resin systems. Chosen where dimensional stability and higher thermal class operation are needed.
Polyester film laminated papers (PET / Mylar composites)
Thin, electrically stable layers used for inter-turn barriers, thin phase separators, or where consistent thickness is critical.
Polyimide films and papers (Kapton-style)
Ultra-high temperature capability and thin form factor make these suitable for aerospace-grade motors or localized high-temperature zones (lead exits, terminal insulation).
Speciality-resin-treated papers
Papers finished with silicone, phenolic, or other treatments that alter surface energy, varnish-wetting, or moisture uptake for specific manufacturing processes.
Slot liners: thin aramid or polyester-film composites, or mica-backed laminates for higher temperatures. They protect winding turns from sharp stator-iron edges and reduce abrasion.
Inter-turn insulation: very thin aramid or polyester layers, sometimes combined with thin polyimide, depending on voltage per turn and thermal class.
Phase-to-phase / phase-to-ground barriers: thicker mica laminates or aramid/glass stacks that block higher voltages and prevent partial-discharge inception.
Ground-wall systems (stator winding insulation): multi-ply constructions where mica is usually the principal dielectric supported by aramid/glass backings and typically vacuum-impregnated.
Wedges, spacers, end-turn supports: pressboard or phenolic-treated cellulose for robust mechanical support and vibration resistance.
Lead/terminal protection and flexible connections: polyimide, thin aramid films, or silicone-impregnated papers where heat exposure is localized.
When selecting insulating paper, prioritize these material attributes:
Thermal rating (continuous temperature class). Ensure the chosen paper’s rating exceeds the motor’s expected hottest-spot temperature plus margin for aging.
Dielectric strength and PD (partial discharge) resistance. Important for medium- and high-voltage designs.
Mechanical durability. Tear, tensile and abrasion resistance matter in slots and at end-windings.
Dimensional stability and conformability. Crepe vs. flat paper choices affect ease of insertion and the uniformity of insulation thickness.
Impregnation/varnish compatibility. Confirm the paper bonds, doesn’t blister, and retains properties after VPI or epoxy impregnation.
Moisture absorption and chemical stability. These influence long-term dielectric life and corrosion risk for conductors.
Manufacturability. Can it be die-cut, slit, or wrapped on automated equipment without unacceptable scrap rates?
Thin inter-turn / film layers: 20–100 µm
Standard slot liners: 100–400 µm
Pressboard / structural parts: 0.5–3.0 mm (or thicker as needed)
Mica laminates (finished ground-wall stacks): 0.2–1.5 mm (depending on voltage and required safety margin)
Always confirm tolerances and thickness distribution with your supplier — tight control matters for automated winding and insulation clearance design.
Ask for up-to-date datasheets and test results for the specific batch or product family, including:
Dielectric breakdown voltage and dielectric strength measurements.
Thermal classification and thermal ageing test data.
Tensile, tear and elongation figures.
Moisture absorption or water uptake.
Compatibility test results for your chosen impregnation method (VPI, epoxy, silicone).
Partial-discharge inception voltage (for medium/high-voltage systems).
Relevant approvals or test-standard references (e.g., IEC/UL material designations where applicable).
Define operating voltages (turn-to-turn, phase-to-phase) and the worst-case field stress.
Define continuous and transient temperature targets and pick material class accordingly.
Identify mechanical stresses (slot edge sharpness, vibration levels, handling during assembly). Add protective layers if needed.
Select materials compatible with your varnish/impregnation process and validate by lab soak tests.
Confirm cutting, punching, or winding performance for automated assembly lines.
Build a small validation run and perform dielectric and thermal ageing tests on complete insulated winding samples — not just the dry-paper tests.
Application: stator ground-wall insulation for three-phase motor.
Operating conditions: 400 V line-to-line; continuous ambient 40°C; estimated hottest-spot 140°C.
Thermal class: F (155°C) minimum.
Construction: mica-based primary dielectric laminated to aramid backing; total finished thickness 0.8 ±0.05 mm.
Process compatibility: vacuum-pressure impregnation (epoxy resin system — specify resin grade).
Electrical requirements: PD inception > 2× operating phase-to-ground stress; dielectric withstand ≥ specified kV (define value).
Mechanical: tensile strength and tear resistance to meet supplier’s standard for motor-winding applications.
Documentation: batch test reports for dielectric strength, thermal class verification, moisture absorption, and impregnation compatibility.
