Insulation cure monitoring on global VPI large AC motors

Epoxy resins and micaceous insulation systems are used to insulate high voltage large rotating machine components. Composite insulation systems undergo marked changes in dielectric response as the cure of the system progresses. It is possible to exploit these changes to monitor and optimize the curing process. This paper presents the first of some work done on the global insulation cure monitoring of large AC motor stator windings processed through global vacuum pressure impregnation (VPI). Changes in the measured permittivity and dissipation factor in the range of 1 Hz to 10 kHz for laboratory samples, as well as capacitance test results on full size stators at 100 Hz, are reported. It is shown that permittivity (capacitance) appears to be ideal for cure monitoring purposes. It is also shown that the best frequencies for monitoring of this type are in the 100 Hz to 500 Hz range. Changes in the slope of the natural logarithm of permittivity (capacitance) with time (d[Ln(ε´)]/dt) are shown to be promising as an indicator of the level of cure. By terminating the curing when d[Ln(ε´)]/dt reaches a certain terminal value, it is believed that a high level of reproducibility in the level of cure of the resulting insulation system can be achieved. Discussion is given on analyses of d[Ln(ε´)]/dt, both in the laboratory and in the factory. The level of cure should also be correlated to other relevant physical properties such as the glass transition temperature or other global mechanical properties. It is shown how dielectric properties monitoring optimizes global VPI processing by producing a more reliable level of cure and reducing cure time.