Vacuum pressure impregnation equipment is mainly suitable for the insulation impregnation of high-requirement, high-quality power capacitors and electrical materials, such as high-voltage (traction, explosion-proof) motors and test transformers. This series of equipment places the workpiece to be impregnated in a completely sealed container, creates a vacuum, and injects the impregnation solution through a pressure differential method. Applying a certain pressure ensures the impregnation solution thoroughly penetrates all gaps in the workpiece, achieving the best impregnation effect. This series of equipment is equipped with multiple safety protection devices. The cylinder mouth uses a lip-tooth method for automatic locking, and the cylinder cover uses hydraulic automatic opening and closing. In fact, vacuum pressure equipment is relatively expensive. Today's topic is why motor manufacturers choose this equipment, followed by a brief introduction to the drying process after impregnation.
1. Introduction to Vacuum Pressure Impregnation
For low-voltage motor windings requiring high insulation, vacuum, pressurized, or repeatedly pressurized impregnation methods are recommended. After vacuum drying (or hot air circulation drying), the windings must be cooled to 60-70°C in a vacuum before being coated with varnish and pressurized or repeatedly pressurized. Air can be used as the pressurizing gas; for varnishes containing flammable solvents, nitrogen is recommended to prevent explosions. The pressurization pressure is 0.2-0.8 MN/m². For tightly wound magnetic pole coils or varnishes with high viscosity (such as silicone varnish or solvent-free varnish), repeated pressurization is preferable.
Vacuum pressure impregnation is a highly effective impregnation method. It can thoroughly remove moisture and volatiles from the windings, avoid incomplete impregnation, and allows for higher varnish viscosity to improve filling performance. Practice has shown that a single vacuum pressure impregnation is more effective than a two-stage ordinary impregnation. For certain critical motor windings, vacuum pressure impregnation is essential. However, vacuum pressure impregnation equipment is expensive and should ideally be performed before the coils are embedded. If vacuum pressure impregnation is required after embedding, an external pressure mounting structure should be used whenever possible. This improves the utilization rate of the impregnation equipment and prevents dust and debris from contaminating the impregnation varnish.
The parameters for vacuum pressure impregnation process depend on the workpiece structure and the properties of the paint; below we provide a set of parameters for reference.
2. Drying after impregnation
Drying after impregnation is more complex than pre-baking because it involves not only physical processes (solvent evaporation) but also chemical processes (oxidation and polymerization of resins and drying oils in the varnish base). The drying process generally consists of two stages. The first stage primarily involves solvent evaporation. During this stage, the temperature should be controlled slightly above the solvent's evaporation temperature, but not above its boiling point, to avoid creating excessive micropores or bubbles on the winding surface; and to prevent premature formation of a hard film on the varnish surface, hindering the evaporation of the internal solvent.
Simultaneously, ventilation should be carried out to prevent excessive accumulation of solvent gas and potential explosions; it also accelerates solvent evaporation. The second stage mainly involves the polymerization and curing of the paint base, forming a hard paint film on the workpiece surface. For this purpose, the drying temperature is generally about 10°C higher than the pre-baking temperature, and the heating rate depends on the impregnating paint, generally about 20°C/hour. The drying time is related to the structure of the impregnated workpiece and the heating method, and is generally determined through testing. The time for the first stage depends on the solvent evaporation, generally about 2-3 hours. The time for the second stage should be determined based on the insulation resistance, generally until the insulation resistance reaches a stable value (about 2-3 hours). For multiple impregnations, the initial baking time should be shorter to maintain the viscous nature of the paint film, ensuring good adhesion with the paint film formed by subsequent impregnations and preventing delamination. The final drying time should be longer to ensure proper hardening of the paint film. For rotors or DC armature windings, the drying time should be even longer to prevent paint splattering during operation due to poor hardening and heat.
Depending on the different process requirements of the two stages, the temperature and airflow are generally controlled as follows: Initially, drying is carried out for a period of time at a temperature slightly higher than the solvent evaporation temperature. During this period, the airflow should be relatively large, with at least 10% of the air being constantly replaced. Then, the airflow is kept constant, and the temperature is increased to the maximum baking temperature at a rate of approximately 20°C/hour. Finally, when drying is about to be achieved, the airflow can be reduced. For polymeric paints, since no oxygen supply is required, the air exchange rate should be smaller.
Rotors or DC armature windings should ideally be dried upright to prevent varnish from flowing and caking on one side, affecting balance. If equipment limitations necessitate horizontal drying, they should be rotated 180 degrees periodically during the first drying stage to prevent varnish from flowing and caking on one side; alternatively, after the second varnish impregnation, the drying position should be reversed to offset the effects of the two caking processes. However, neither of these methods is as effective as vertical drying.
For windings or coils requiring high insulation, vacuum-assisted drying can be used. This involves first using a vacuum pump to extract the solvent at a low temperature of 70-80°C, typically pumping to 700-730 mm of water column. Once no solvent condenses in the condenser, the vacuum is released, the temperature is increased, and then drying is carried out at the maximum permissible drying temperature.
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