**5. Conclusions**

Polyimide tapes have excellent properties that make them ideal choice to manufacture the turn insulation of form wound machines (both low-voltage hairpin machines used, e.g., in traction electrification and medium-voltage machines). Indeed, the final insulation class of the insulation system could be lower than that of the polyimide tapes, as it will be difficult (or extremely costly) to find a whole set of materials (enamels, slot liners, phase separators, wedges, sleeving, varnish or resin, conductive armor tape, stress control tapes) able to achieve the same thermal performance of the turn insulating. The results reported here confirm the capability of PI tapes to withstand extreme temperatures for long times. Exposing PI to temperatures above 300°C, moreover, has a positive effect as the acid and amine groups formed by hydrolysis reconnect, restoring the polymeric chains. Indeed, hydrolyzed PI displays a higher relative permittivity than the pristine tapes (4 versus 3.5 at 50 Hz), leading to a reduction of the partial discharge inception voltage (90%). A possible negative effect observed after thermal aging at 270°C is a general increase of the relative permittivity that would also lead to a reduction of PDIV, although more moderate than that induced by hydrolysis.

The electrical endurance tests highlight the excellent capability of PI coronaresistant (CR) materials to withstand partial discharge bombardment. The apparent short times (from 80 min at the beginning of the test to 40 min after the three stages of aging) should be weighed against the large electrical stress used for the tests (7 kV/100 μm = 140 kV/mm within the PI), a stress level hardly experienced in electrical machinery. Thermal aging seems to have a limited impact on the electrical endurance unless the PI is exposed to temperatures above 300°C for long times. By inspecting pristine and aged tapes using the scanning electron microscope, the electrical endurance of the CR PI tapes was explained by a three-layer structure of the tapes: a central layer of PI serves the purpose of conferring flexibility to the tapes, and two nanostructured outer layers ensure resistance to partial discharge bombardment. Aging at 300°C seems to have an impact on the morphology of the nanostructured layers, leading to a substantial (50%) reduction of the electrical endurance of the tapes.
