Vacuum
Impregnating Transformer Coils using Epoxy Compounds
Vacuum impregnation is used in many different manufacturing processes to enhance
mechanical strength, seal surfaces and to enhance electrical insulation
properties.
One of the most common application for impregnating compounds is in electrical
transformer manufacturing. Transformer coils are impregnated for some or all of
the following reasons:
> Increase the overall dielectric
strength
> Improve the structural integrity
of the coils
> Reduce or eliminate winding
noise
> Improve heat transfer
> Reduce partial discharge
> Provide increased environmental
protection
The main purpose of vacuum impregnating transformer coils is to remove as much
air as possible from the structure and replace it with epoxy which has
significantly better dielectric strength. In addition, epoxies are excellent
adhesives and will solidly bond the wires and other components of the structure
to form a strong, solid assembly. As a side benefit, this also reduces noise
during operation.
Material Requirements:
Impregnating compounds are not designed to provide high build rates. They are
designed to penetrate and seal the "nooks and crannies" of the structure.
The best products have the following features:
> Reasonably low viscosity
> Reasonably good shelf life
> Long working time (pot life)
> Good dielectric strength
> High vapour pressure
> Operating temperature rating to
match or exceed the rating for the coil
> Moderate cure temperature
Typical steps
in coil impregnation:
Although the
steps involved in coil impregnation may vary depending on the material used, the
following could be considered as general using a single component impregnating
compound such as our popular
CLS 9310 single component epoxy.
Recommended
Equipment:
> Oven capable of
reaching and maintaining the required cure temperature.
> Vacuum tank of
sufficient size (also capable of sustaining pressure for best results)
> Sheet metal
tray to hold the coil during the cure cycle in the oven
> Silicone based
mould release to prevent adhesion to the tray.
> A vacuum pump
of sufficient size to develop and maintain a minimum vacuum level of 29" (982.05
Millibars) or better.
-
Pour the CLS
9310, single component epoxy, into a suitable size vacuum vessel (large
enough to accommodate the coil to be impregnated with sufficient room to
allow the material to rise while de-airing)
-
Slowly
agitate the epoxy in the vacuum tank for 5 to 10 minutes to make sure that
the mix is uniform
-
Dry the coil to be dipped in an oven
(at a high enough temperature to drive off any residual moisture, oil
components and other volatiles. Minimum 1000C)
-
Allow the coil to cool back down to room
temperature (DO
NOT IMMERSE HOT WINDINGS IN SINGLE COMPONENT EPOXY!!!!!). The heat
activated hardener component is part of the mix and the heat from a hot
component will trigger the curing process.)
-
Immerse the
cool coil assembly into the epoxy contained in the vacuum tank.
-
Draw vacuum
(minimum 29" or 982.05 Millibars) for 5 to 10 minutes (until the material no
longer rises) (see note #4)
-
Once the vacuum is released,
pressure may be applied (approximately 10-15psi) to the tank to drive the
epoxy into all the hard to reach cavities. This step is not normally
necessary but the results will be improved especially on very complex
windings.
-
Withdraw the coil assembly from
the vacuum tank and let the excess material run off back into the tank. (see
note #1 below)
-
Place the coil assembly on a
steel (sheet metal) tray coated with mould release.
(see note #2 below)
-
Leave the coil assembly on the
tray and place it into the cure oven pre-heated to 1250C
-
Cure for at least 4hours
(making sure that the coil assembly reaches 1250C for a period of
4hrs). (see note #3 below)
-
Remove from the oven, let cool
to room temperature. The coil is ready for assembly.
Notes:
-
It is best to
hang the coil over the dipping tank on an angle towards the hard to drain
areas. This will ensure thorough draining of any excess material.
-
The tray must
be thoroughly mould released otherwise the epoxy will adhere to the tray. A
silicone based mould release such as our
CLA 8000 Mould Release
-
It is critically important that the coil assembly itself reaches 1250C
for a minimum of 4 hours. Do not trust the setting on the oven
temperature controller. Make sure that the coil reaches 1250C for
the duration of the cure schedule.
-
Excessive vacuum or extended periods of vacuuming can result in the
removal (stripping out) of the more volatile components from the
formulation. It is best to maintain the recommended vacuum levels, just low
enough to remove the air from the assembly.
| 
