Mica Tape Porosity and High Voltage Coils

We are obviously talking of VPI here?

What is the tape construction, Film backed, film faced, glass backed etc.

Various factors affect the impregnation, no matter how porous the tape is, if you can't get the resin to flow evenly throughout the coil and then porosity will be of little value (as there will be nothing to suck up). Due to the tighness of the tape and the viscosity of most VPI resins little to no "wicking" occurs. In my opinion successful impreganation will be determined by many other factors before poriosity becomes an issue. The amount of base resin in a tape is often largely detremined by how much resin is required to avoid mica flyoff when used in taping. The lower the resin content the better but there is a crossover point. So in short I suppose the more porous a tape the better as it absorbs the secondary insulation (the resin) more readily.

Some points to ponder on impregnation, not trying to teach you how to suck eggs here;

a) Voltage, the higher the voltage the more layers the more difficult the impregnation.

b) The viscosity of the resin used. The more viscous the harder to impregnate, the less viscous the more resin drains during curing cycle.

c) Static or rotate cure, rotate curing offering in my experience the better results and tan delta readings. Some forget to keep the resin in once you have got it in.

d) The preheat temperature, this affects the change in viscosity when the resin meets with the coil. It also removes excess moisture making way for more secondary insulation.

e) The levels of vacuum applied and then the pressure cycle at the end. Self explanitory.

f) The orientation of the job, vertical is best.

g) How the vessel is flooded, it is best to flood from below.

h) Type of impregnant, Epoxy vs. Polyester. The old Ford Chev debate. I am an Epoxy believer for many reasons. Tank stability, the fact the it is 100% solids (no volume loss during the curing cycle) and no additives.

i) If endwinding sealing tapes are used incorrectly. Some of these are Polyester based and when subject to temperatures in excess of some 80/90 deg C will shrink and provide a barrier to impregnants.

Hope this helps.

Finally, I have no stats on this but would bet that poor impregnation is more common that poor mica tape. A good epoxy resin with a good VPI cycle will offer excellent tan delta results, reduce partial discharge and provide a mechnically sound winding which in turn leads to an electrically sound winding.

Regards

Andrew