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The Necessity Of Vacuum Pressure Equipment Impregnation Varnish

Vacuum pressure impregnation  equipment is mainly suitable for the insulation impregnation  treatment of high-quality power capacitors and electrical materials such as high-voltage (traction, explosion-proof) motor test transformers. This series of equipment puts the workpiece to be dipped in a completely sealed container to vacuum, and injects the impregnation liquid into it through the differential pressure method. After a certain pressure is applied, the impregnation  liquid can completely penetrate all the gaps of the workpiece to achieve impregnation . The best effect of lacquer. This series of equipment is equipped with multiple safety protection devices. The cylinder mouth adopts the lip-tooth method and automatically locks, and the cylinder cover adopts hydraulic automatic opening and closing. In fact, vacuum pressure equipment is relatively expensive. Our topic today is why motor manufacturers choose this equipment ? Then, the drying after impregnation is briefly introduced.

  1. Introduction of vacuum pressure impregnation varnish

For low-voltage motor windings that require high insulation treatment, vacuum, pressure or repeated pressure impregnation methods should be used. After the winding is vacuum dried (or hot air circulation drying), it must be cooled to 60~70℃ in vacuum, and then varnish ed, pressurized or repeatedly pressurized. Air can be used as the pressurized gas. Nitrogen should be used for varnish with flammable solvents to avoid explosion. The pressurizing pressure is 0.2~0.8MN/m2. For tightly wound magnetic pole coils or varnish with higher viscosity (such as silicone varnish , solvent-free varnish ), repeated pressure should be used.

Vacuum pressure impregnation varnish is a better impregnation method. It can more thoroughly remove the moisture and volatiles in the winding; it can also avoid the phenomenon of impermeability; at the same time, the viscosity of the varnish  can be higher to improve the filling performance. Practice has proved that the effect of the first vacuum pressure impregnation varnish is better than that of the second ordinary impregnation  varnish . For some important motor windings, vacuum pressure impregnation is necessary. The cost of vacuum pressure impregnation equipment is relatively high and should be carried out when the coil is not embedded as much as possible; when vacuum pressure impregnation is required after the wire is embedded, the external pressure mounting structure should be used as much as possible. In this way, the utilization rate of the impregnation equipment can be improved, and dust and debris from the machine base can be prevented from polluting the impregnation varnish .

The process parameters of vacuum pressure impregnation varnish depend on the structure of the workpiece and the nature of the varnish ; below we give a set of parameters for reference.

Name Vacuum degree mmHg(min) Pressure MN/m2(time min)
Single-mature DC armature coil 720(5-10) 0.2(15-30)
Single ac, DC pole coil 720(10-15) 0.2(60-75)
Stator windings of low voltage AC motor 720(15-20) 0.2(60-90)
Dc motor winding 720(15-20) 0.2(60-120)
Moisture-proof DC armature winding and main pole coil 740(5-10) 0.7-0.8(120-180)

 

  1. Drying after impregnation

Drying after impregnation is more complicated than pre-baking, because at this time there are not only physical processes (solvent volatilization); but also chemical processes (oxidation and polymerization processes of resin and drying oil in the varnish base). The drying process is generally divided into two stages. The first stage is mainly the volatilization of the solvent. At this time, the temperature should be controlled slightly higher than the volatilization temperature of the solvent, but not higher than its boiling temperature, so as to avoid the formation of more micropores or bubbles on the winding surface; Premature formation of a hard film on the surface prevents the evaporation of internal solvents.

At the same time, ventilation should be carried out to prevent excessive accumulation of solvent gas and cause explosion accidents; and to accelerate the volatilization of solvents. The second stage is mainly the polymerization and curing of the varnish base, and a hard varnish film is formed on the surface of the workpiece. For this reason, the drying temperature is generally about 10°C higher than the pre-baking temperature, and the rate of temperature rise depends on the impregnation varnish , which is generally about 20°C/hour. The drying time is related to the structure of the immersed workpiece and the heating method. It is generally determined by experiment. The time of the first stage depends on the volatilization of the solvent, generally about 2 to 3 hours, and the time of the second stage should be determined according to the insulation resistance, generally until the insulation resistance reaches a continuous (about 2 to 3 hours) stable value . When impregnation for multiple times, the baking time for the first few times should be shorter to keep the varnish film sticky, so that it can be well bonded with the varnish film formed by the next few impregnation without delamination. The last drying time should be longer to make the varnish film hard and intact. The rotor or DC armature windings should be longer to avoid the phenomenon of varnish  slinging due to poor hardening and heating during operation.

According to the different process requirements of the two stages, the temperature and air volume are generally controlled in this way. At the beginning, dry for a period of time under the condition of slightly higher than the volatilization temperature of the solvent. At this time, the air volume should be larger, and more than 10% of the air should be constantly changed. New, then keep the air volume constant, and heat up to the highest drying temperature at a rate of about 20°C/hour. Finally, when it is about to dry, the air volume can be smaller. For polymer varnish , since oxygen is not needed, the air exchange volume should be smaller.

The rotor or DC armature windings should be placed upright when they are dry to prevent the varnish  flow from congesting on one side and affecting the balance. If the equipment can only be dried flat due to the limitation of the equipment, the first stage of drying should be rotated 180 degrees regularly to prevent the varnish  flow from gathering on one side; or after the second impregnation , the drying position is the same as the first On the contrary, make the effects of the two flow knots cancel each other out. But these two methods are not as good as standing dry.

For windings or coils that require high insulation treatment, vacuum can be used to assist drying. That is, when the low temperature is 70~80℃, use a vacuum pump to help extract the solvent, generally pumping to 700~730mm water column, when there is no solvent condensation in the condenser, release the vacuum, raise the temperature, and then keep it at the highest allowable drying temperature dry under.

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