Impregnation is process of applying a coating or layers of chemically stable liquids to porous surfaces
(like casting) or objects with inherent voids (like motor coils) to prevent accumulation of undesired
elements like dust, dirt, chemicals or water. Impregnation of objects improves its physical properties
and also tends to improve overall parameters of the equipment/component wherein use. For Ex. a
varnish impregnated coil improves the overall insulation resistance of the motor and hence the life of
the motor is improved.
Different chemicals are used in the process of the impregnation. These chemicals can be classified
as epoxy, varnish and resins. Impregnation process itself can be classified in different types like
flood impregnation, dip impregnation, vacuum impregnation, vacuum pressure impregnation, trickle
impregnation, hot roll dipped impregnation. At NACH we have expertise in manufacturing all these types
of the impregnation systems.
Flood impregnation and dip impregnation are simple impregnation processes often used in the low
voltage small frame electrical motors. In flood impregnation the resin/varnish is flooded in the chamber
where the preheated coils (electric motor stator or wound rotor) are placed. The coils are kept for
soaking in varnish for a certain period and then taken off to the baking oven. The varnish is stored in
a cold chamber or vessel called the storage tank. Typically temperatures for maintaining the varnish/
resins is 15 deg C to 10 deg C. In dip impregnation, coils are dipped in a tank that contains varnish and
kept for a certain time.
Vacuum pressure impregnation is a special process designed to impregnate the motor coils or some
castings that are used in critical applications. Vacuum pressure Impregnation often called the VPI
process impregnates the coils or metal castings under vacuum. Impregnation under vacuum prevents
advent of external moisture that may be accumulated in the voids of the coil or may be absorbed in
insulation. Vacuum impregnated coils are free of moisture that result good electrical properties.
In case of metal impregnation, vacuum impregnation results good adhesion of the chemical with the
surface. Metal impregnation process that involves impregnation of castings is often accompanied by
additional processes like cleaning of castings jobs, chemical treatment for better adhesion etc. Often
epoxy is used for porosity sealing of the castings.
In a vacuum pressure impregnation plant, there are two tanks, one is the impregnation chamber and the
other is the storage tank used to store the varnish/resin. In resin impregnation system, often storage
tanks require cold storage of level up to 10 Deg C to 15 Deg C. Alternately a epoxy impregnation system
demands hot storage, since the curing of epoxy occurs at room temperature. Distinct manufacturers
for epoxy like Alantas (formerly Dr Beck), Huntsman (formerly CIBA), Von Roll, Vantico Ltd. suggest
methods for vacuum pressure impregnation with indication of different processes like vacuum, soaking,
maintaining pressure, amount of pressure etc for better results.
In a typical vacuum pressure impregnation system for motors, preheated coils are kept in the
impregnation tank and vacuum is drawn. After a certain period resin is introduced in the impregnation
tank in controlled manner and filled till the top of the coil stack. Dry air or nitrogen pressure is applied in
the chamber for better penetration of the resin in the voids and this is maintained for a stipulated time.
Later the resin is withdrawn to the storage tank. Almost in all the impregnation processes the resin or
varnish transfer from tank to tank is achieved by pressure difference in the two tanks or by gravity flow
of the varnish.
In trickle impregnation or a hot rolled impregnation process, the resin is introduced on the motor coil
in a very controlled fashion. The coils are heated by use of its own resistance. This type of methods are
used where resin costs are higher or usage of resins/chemicals have limitations.
At NACH we have the expertise to develop the vacuum impregnation systems for different resins. NACH
can manufacture VPI systems as per guidelines and specifications defined by resin manufacturers.