In
the 'B' stage, the resin dots are dry and tack free, allowing
the material to be stored and used in a similar way to normal
presspaper. When the winding is heated for drying purposes, the
adhesive dots melt and cure, thus creating permanent bonding sites
that are unaffected by further heating cycles which may occur
in service.
Rise time
of temperature within the drying over is important, particularly
during the transition for 60°C up to 100°C (plus), at
which point the resin starts to melt. To obtain maximum bond strength,
this transition should be completed as swiftly as possible, ideally
within three hours.
The drying
requirements of the transformer normally provide for adequate
curing of the resin but a careful check is strongly recommended
on over performance at various loadings.
The ability
of the basepaper to dry readily is a crucial factor for a Diamond
Dotted presspaper, particularly when used in foil type LV windings.
ELEPHANTIDE® Grade 3
presspaper is unique in this respect due to the combination of
cotton and kraft fibres used in its furnish. Winders also appreciate
its flexibility and the ease with which it conforms to uneven
surfaces. |
