Using capacitance measurements to determine fill of VPI coils

There are several ways to impregnate motor windings. Although it requires an initial capital investment, vacuum pressure impregnation (VPI) using a solventless resin is generally recognized as the best way to fill a coil. Perhaps the most common concern to VPI quality assurance is the question of whether void-free coils are actually achieved. Voids in a high voltage coil can lead to excess vibration, heat build-up, corona, and motor failure. Efficient and effective quality assurance of VPI processed units can be maintained through several means: process control; coil construction; maintenance of VPI resin; and process monitoring, especially capacitance measurements