The Vacuum Pressure Impregnation (VPI) process involves pre-baking the workpiece to remove moisture, followed by cooling. The workpiece is then placed in a vacuum environment to expel air and volatiles from the interior of the unfinished coil. By utilizing the capillary action of the coil and the gravity of the varnish under vacuum conditions, and applying a certain pressure to the impregnating varnish using dry compressed air or inert gas after the vacuum is released, the varnish rapidly penetrates and fills the inner layers of the insulation structure. Currently, in China, VPI is still an intermittent insulation process. The drying process of the workpiece occurs within the impregnation tank, while the curing process generally takes place in a separate container or oven. Drying methods include vacuum drying, atmospheric drying, or rotational drying.
VPI Process Flow: ⊙ → Pre-baking and Dehumidification → Tank Insertion → Vacuum Evacuation → Vacuum Impregnation → Pressure Impregnation → Pressure Draining → Pressure Release and Dripping → Tank Removal → Curing and Drying → ⊙.
VPI surpasses other impregnation processes in terms of varnish penetration and impregnation effectiveness. In terms of application, VPI is more suitable for large high-voltage coils, multi-layer yoke coils, high-requirement large windings, and other high-voltage coils. Theoretically, with VPI, both vacuum and pressure levels can be significantly high, increasing costs. In contrast, the FGH process, due to its continuous operation and specific production and cost requirements, has a more limited application range.
In application experiments and practical work, we have observed that under a certain temperature condition, when the vacuum level of a varnish is below a specific absolute pressure value—reaching a corresponding "critical" vacuum—significant foam and mist can form in the varnish, leading to "foaming" and "atomization" phenomena. "Foaming" introduces numerous cavities in the varnish, hindering impregnation, while "atomization" causes a substantial loss of solvent or diluent, affecting the curing process.
The principle of applying pressure facilitates the varnish’s penetration into voids. The varnish delivery process inherently involves atmospheric pressure. Suppose the wetting of the capillaries within the insulation structure has reached equilibrium. In that case, increasing pressure will not significantly enhance the filling of the entire insulation structure, unless the increased pressure is maintained throughout the curing process. Thus, the effective way to enhance filling is to reduce varnish viscosity, decrease voids in the insulation structure, and increase capillary action, rather than merely increasing pressure. Based on the "Viscosity and Pressure on Penetration Rate" test, data indicates that when varnish viscosity is high, increasing pressure has a considerable effect on filling speed; however, when varnish viscosity is low, increasing pressure has a less significant effect on filling speed. Varnish viscosity has a highly significant influence on filling speed, with the two being inversely related.
In conclusion, in applying the VPI process, an overemphasis on and blind pursuit of high vacuum or high pressure is both misguided and counterproductive. Such practices can negatively impact impregnation efficiency and may even compromise impregnation quality.