Once the enameled wire has undergone a bending test, it […]
Once the enameled wire has undergone a bending test, it can change considerably, creating stress in the enamel layer. When measured on a wet enameled wire, even a slight bend in the enameled wire can result in a considerable reduction in insulation resistance. If the enameled wire bends quickly, or bends on pins or sharp corners, the effect increases so much that the method is no longer suitable because the capacitor discharges too fast.
The following method is used to further investigate the problem. Connect the strong bending enameled wire to the negative terminal of the battery, and immerse the enameled wire in a saline solution containing a small amount of phenolphthalein. The other end of the battery is connected to a nickel rod placed in the same solution. Electrolytic effect occurs immediately: Hydrogen bubbles together with OH- ions are generated at the bend of the immersed wire. The latter can be observed from the presumed red color of phenolphthalein on the outside of each bend. This is expected if it is assumed that tensile stress can produce small cracks.
If the enameled wire is slightly bent, we are surprised that this method will produce negative results even after a long period of time. Careful microscopic investigation eventually produced a rather complete sc-defect image. When the enameled wire bent the pin quickly, it was observed that a crack suddenly formed in the glaze layer after some time. Even when the enameled wire is bent slowly and slightly, very fine cracks can still be observed after a period of time. However, the cracks in these hair do not extend directly to the core but are limited to several outer layers of the insulator.
It can be assumed that this crack did indeed cause a considerable reduction in resistance but did not allow electrolytic effects. If the enameled wire is heated, hair cracks often close again: they are no longer visible under the microscope, and the insulation resistance is observed to increase again. Hair cracks can be considered as relatively "innocuous" precursors due to, for example, the resulting cracks. Bend a sharp angle.