It has been pointed out earlier that considerable defec […]
It has been pointed out earlier that considerable defects in the insulation are detected during the routine inspection of the enameled wire. Research laboratories in the past few years has been carrying out research work in order to develop more sophisticated mechanical stress under the enameled wire research methods. It has been found that if the insulation resistance of the enamel layer is measured by a sensitive method, even small cracks caused by such stress are also present. Although this survey. Wire to electrometer. The capacitor thus formed is charged to a known voltage and the time required for it to discharge to a certain lower voltage is measured. This time, it depends on the unknown insulation resistance and capacitance, which is unknown. In order to eliminate the latter unknown, the second measurement is to include a calibration resistor in the circuit, in parallel with the capacitor.
Measurement on dry enameled wire provides high insulation resistance values for all types of wires, as long as the enamel layer has no actual holes. Measurements on moist enameled wire generally indicate that this reduction in resistance is relatively small and that the exact value of the reduction shows the type of feature change.
Once the wire has undergone a bend test, the picture can change considerably, creating stress in the enamel layer. When measured on damp wires, even a slight bend in the wire can result in a considerable reduction in insulation resistance. If the wire bends quickly, or bends at a pin or a sharp corner, the effect is greatly increased so that this 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 wire to the negative terminal of the battery, and immerse the 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.