In the vacuum‑pressure impregnation (VPI) process, workpieces are pre‑baked for dehumidification and then cooled, followed by placement under vacuum to remove air and volatiles inside bare coils. Relying on gravity of varnish under vacuum, capillary effect of coils, as well as certain pressure exerted by dry compressed air or inert gas on impregnating varnish after vacuum release, the varnish penetrates rapidly and fills inner layers of the insulation structure. Domestically, VPI is currently an intermittent insulation process. The dripping step of workpieces is completed inside the impregnation tank, while drying is generally carried out in separate vessels or ovens via vacuum drying, atmospheric static drying or rotary drying.

VPI Process Flow: ⊙ → Pre‑baking & dehumidification → Tank loading → Vacuum degassing → Vacuum varnish impregnation → Pressure impregnation → Pressurized varnish draining → Pressure relief & varnish dripping → Tank unloading → Curing & drying → ⊙. Comparison: Obviously, VPI delivers far better performance in varnish penetration and impregnation than other varnish‑dipping processes. In practical application, VPI is more suitable for large high‑voltage coils, multi‑layer neatly‑wound yoke coils, large windings with high requirements and other high‑voltage coils. Theoretically, fairly high vacuum and pressure can be achieved in VPI, which inevitably brings higher costs.

During application tests and actual operation, it is found that for a certain impregnating varnish under fixed temperature condition, when vacuum degree drops below a certain absolute pressure value (reaching the corresponding "critical vacuum"), massive foam inside varnish and heavy mist above liquid surface will occur, namely "foaming" and "atomization". Foaming generates numerous voids within varnish and hinders impregnation. Atomization causes massive escape of solvents or diluents and adversely affects curing performance.