"N_Cook"
PlainBill


Way back in my dark past I worked in a plant where we wound our own
transformers. Indeed, the doubling of the primaries is standard
practice. Note that if your primary voltage is 240VAC the voltage
between the adjacent wires will be approximately 180V maximum.

Of course, these were using 'I and E' laminates. We tested the wound
bobbin for the correct number of turns and leakage between the wires
of the paired windings. After the laminations were added the entire
transformer was vacuum impregnated with varnish. Not much chance for
movement between the windings after that!


Did you ever metal saw across any of them to actually check on the
impregnation. ? Not transformers, but I've seen such a check on a similar
process with much the same viscosity of fluid and most of the interior was
not impregnated, vacuum gave out to viscosity down fine pathways.


** Nevertheless, vacuum impregnation works very well with transformers.

The basic idea is to remove all the air spaces between windings and fill
them with insulation material - so the unit will never suffer from "corona"
discharge.

http://en.wikipedia.org/wiki/Corona_...a_dischar ges

Corona discharge is the biggest killer over time of transformers that handle
AC mains or higher voltages - particularly valve output transformers and
EHT transformers.

Until fairly recent times, vacuum impregnation was standard practice for all
mains voltage transformers - toroidal types were wound with porous cloth
tape insulation to allow this step to be done.

Now makers use polyester tape insulation along with tougher, flexible
enamels and impregnation is impossible.

IMO - a mains toroidal with dual primaries ought to have them wound
separately and with at least one layer of poly tape in-between.



...... Phil