High resistance to thermal shock

Undulated polyester or DMD (Dracon - Mylar - Dracon) film presents an excellent alternative with respect to the use of fibreglass mesh for various reasons that wil be outlined below. tI is in fact well known that cast-resin windings must have meshes that enclose the internal and external part of the resin as otherwise they would not withstand the thermal shocks that normally occur during their use. The fibreglass meshes present their own mechanical resistance, which allows the coils to resist thermal shocks, however the main force that avoids breakage of the coils lies in the formation of a cross-linking of the resin. The Araldite resin normally used in the casting of the coils is in itself a powerful adhesive sealant, which, fi suitably crosslinked, is able to allow the windings to resist thermal shocks which generate considerable mechanical stress. The corrugated polyester or DMD has precisely this aim and, that is, to cross-link the resin, which, as previously mentioned, is in itself an excellent adhesive sealant capable of withstanding thermal shocks. The figure 1 show the use of corrugated films. SILTECH has carried out various experiments to verify this characteristic by means of cooling and subsequently heating the coils, and providing a current that is twice as strong with respect to the rated current according to the IEC 726 standards.

Coils subjected to the test procedure at -60 ° did not suffer any breackage.

High resistance to sliding disgharge

The term 'sliding discharge' refers to the longitudinal perforabili- ty of a layer of resin that incorporates the fibreglass mesh or the

undulated elements in polyester or DMD or fiberglass net. See figure 2 One of the main characteristics of the undulated polyester or DMD material is that ti presents a remarkable degree of resistance to surface sliding discharges. The undula- ted elements embedded in the resin winding have discharge voltages (flashover) 4-5 times higher than those of the fibreglass mesh. This applies, in particular, to the DMD, which is formed by two outer layers in Dacron and an inner layer of polyester. The three strips of film are glued together using a thermo-hardening adhesive presenting a high level of adhesiveness and heat resistance. The Dacron sheets outside the set of layers have a matt, fibrous surface which facilitates the attachment of the

resin. This characteristic increases resistance to sliding dischar- ges as demonstrated by numerous tests conducted in the SILTECH laboratories.

Experimentation with sliding discharges

The verification process involved the use of disks obtained from a resin cylinder with the material to be tested placed inside.

Three cylinders were produced which contained fibreglass mesh (1st cyl.), undulated polyester film (2nd cyl.) and DMD (3rd cyl.). See photo 3.

Performing precision-cutting operations, disks measuring 2;5 and 10 mm were then created. See photo 4. The disks thus obtained

were exposed to an electric field by means of two electrodes as shown in figure 3. As the perforation voltage reached high values, the electrodes and disks had to be immersed in a tank of insulating oil (see photo 5).

Perforation voltage sequence

The diagram below provides an overview of test results and shows the levels of resistance of the materials with respect to the sliding discharges.