Impregnation refers to placing a transformer in insulating oil (also known as varnish) and creating negative pressure through vacuuming, so that the insulating oil fills all gaps within the transformer.

At this point, the internal state of the equipment is in a vacuum negative pressure state, so we also refer to this process as vacuum impregnation. (Some small manufacturers adopt non-vacuum impregnation processes, which yield relatively poor results, and are not mentioned here)

Vacuum impregnation is primarily aimed at enhancing the insulation strength and moisture resistance of transformers. It can also improve the heat resistance and heat dissipation of transformers, as well as enhance their mechanical properties, chemical stability, and aging resistance.

In addition, insulating oil itself possesses a certain viscosity, which can enhance the firmness of the bond between the magnetic core and the frame. For products with dimensions smaller than EE13, due to the difficulty in operating glue dispensing at the edge posts, we often use impregnation process instead of glue dispensing process.

Generally, the insulating oil we use is melamine alkyd resin paint, with toluene or xylene as the solvent. Since both toluene and xylene are harmful to human health, some foreign manufacturers, considering environmental protection, do not use impregnated transformers.

Currently, some domestic transformer manufacturers have switched to water-based solvents by adjusting the formula, using water to adjust the proportion of insulating oil according to the ratio, which can reduce the damage of toxic solvents to humans. However, the impregnation effect is slightly inferior to that of traditional xylene solvents.

In terms of temperature resistance, insulating oil is classified into E grade (120°C), B grade (130°C), F grade (155°C), H grade (180°C), and R grade (200°C). Currently, B grade and F grade are commonly used.

It is worth noting that the impregnation of transformers can easily lead to poor inductance, so special attention should be paid during the manufacturing process:

1. Impregnation can easily lead to changes in air gap volume, which in turn can cause variations in inductance. Therefore, the transformer core assembly must be properly executed in the early stages;

2. Due to the high vacuum negative pressure during impregnation, if the magnetic core tape (steel clip) is not fixed tightly, it is easy to cause the magnetic core to be misplaced or move, resulting in changes in inductance. Therefore, the magnetic core wrapping (steel clip) must be in place;

3. Foreign objects on the assembly surface of the magnetic core can also lead to changes in inductance after impregnation; therefore, it is essential to ensure that there are no foreign objects on the bonding surface during magnetic core assembly;

4. A reasonable baking temperature needs to be selected based on the characteristics of insulating oil; some high-permeability cores (filter products) have a low Curie temperature and are greatly affected by baking. To avoid this impact, they can be dried at a low temperature of 80°C or naturally air-dried.