Chuck Yung
EASA Technical Support Specialist 

When rewinding motors rated 6 kV and above, there are certain steps beyond the normal rewind procedures used for 2.3 kV/4 kV machines. Whether a machine is to be VPI processed makes a difference in how the winding should be treated. 

Aside from the obvious issues of insulation and higher voltages, there is also the possibility of partial discharge (PD), which brings its own unique set of problems. Air is an electrical insulator, albeit one of inconsistent quality. Increased humidity lowers the dielectric breakdown voltage of air, so an air gap that might be adequate under dry conditions may prove inadequate when the humidity is high (Table 1). Even though a form coil is fully taped, sealed and processed, the presence of air in voids within the coils, or between the coil and ground (i.e., in the slot) can cause problems with PD. 

LOGIN TO VIEW AND DOWNLOAD THE ARTICLE