Refrigerant Chemical Attack on Motor WindingsThe refrigerant environment inside a hermetic or semi-hermetic compressor is not chemically neutral. Over time, the combination of refrigerant chemistry, lubricating oil degradation products, trace contaminants, and operating temperatures can chemically attack motor winding insulation materials in ways that are distinct from simple thermal degradation. Understanding these chemical mechanisms helps technicians select compatible materials, interpret system contamination indicators, and take corrective action before chemical attack compromises winding integrity.Polyester (PET) film insulation, which is widely used as slot liner, phase-to-phase insulation, and wire insulation in hermetic motor windings, is susceptible to hydrolytic degradation in the presence of moisture and acid. Hydrolysis cleaves the ester bonds in the polymer chain, reducing molecular weight and mechanical strength, causing the film to become brittle and lose its dielectric properties. This is distinct from purely thermal degradation and can occur at relatively low temperatures if the acid content of the refrigerant-oil mixture is elevated.Chlorinated refrigerants (CFCs and HCFCs such as R-22 and R-502) historically presented chloride-based chemical attack risks to motor insulation. Chloride ions in the presence of moisture form hydrochloric acid, which attacks copper and insulation materials. The industry transition to HFC and HFO refrigerants has largely eliminated chloride-based attack but introduced new compatibility considerations: some HFO refrigerants and their degradation products are more reactive with certain elastomers and varnish formulations than their predecessors.Varnish compatibility is a critical consideration in refrigerant transitions. When a system is converted from R-22 to an HFC blend, or from R-404A to an HFO-based refrigerant, the new refrigerant-oil combination must be chemically compatible with existing motor varnish formulations. Some polyester-imide varnishes that performed well with R-22 and mineral oil systems show increased swelling, softening, or extraction of plasticizers when exposed to POE oil and HFC refrigerants, reducing insulation resistance.Oil degradation products—organic acids, aldehydes, and ketones formed by oil oxidation at high discharge temperatures—circulate throughout the system and deposit on motor surfaces. These deposits can form conductive carbon layers on winding surfaces if overheating occurs, creating low-resistance paths between conductors. They can also block oil return passages and reduce the thermal conductivity between windings and the refrigerant stream, further elevating winding temperatures.Refrigerant purity is an underappreciated factor in motor chemical health. Refrigerants containing excessive moisture (above AHRI Standard 700 moisture specifications), non-condensable gases, or contaminants from off-specification production or cross-contamination in recovery cylinders introduce additional chemical stress. Field technicians should use only new or properly reclaimed refrigerant meeting AHRI 700 specifications and should store refrigerant cylinders in dry, temperature-controlled environments to prevent moisture ingress.The most effective defense against chemical attack is keeping the system dry and clean. This means rigorous evacuation procedures, immediate filter-drier replacement after system openings, regular oil acid number monitoring, prompt response to filter-drier pressure drop indicating drier saturation, and complete system cleanup following any motor burnout. Systems that have experienced burnout contamination require suction filter-driers in addition to liquid line driers to capture acid and carbon contamination before it reaches the replacement compressor’s motor.