Understanding Dielectric Breakdown
Dielectric failure in motors is rarely instantaneous; it is a progressive degradation. Standard PVC or generic polyester tapes often fail due to Partial Discharge (PD)—micro-sparks that occur within air voids in the adhesive or tape layers under high voltage stress. These sparks erode the insulation over time.
Thermal Class Mismatch
A primary cause of failure is using Class B (130°C) tapes in Class F (155°C) or Class H (180°C) motors. As the tape degrades thermally, it becomes brittle and cracks, compromising the dielectric barrier. Engineers must specify Glass Cloth or Polyimide (Kapton) tapes for high-temp windings.
Varnish Compatibility
Insulation tapes must be compatible with impregnation varnishes. Incompatible adhesives can dissolve or soften during the VPI (Vacuum Pressure Impregnation) process, contaminating the varnish tank and causing coil looseness. Thermosetting rubber or acrylic adhesives are essential for chemical resistance during VPI.