Page 11 - Basic PD Theory

Partial Discharge for Stator Windings

of impregnation during the manufacturing process, only after the process is completed. Some manufacturers rotate the stator
while it is in oven to reduce resin runoff. Since mid 1960’s, Global VPI is the dominant manufacturing process for motors.

1.3.1.3 Resin Loaded Tapes
(B-stage or resin-rich)
Due to their size, it is often not practical to VPI bars for long-core stators. In these applications, manufacturers prefer to use
mica paper tapes that are directly impregnated with an epoxy resin. However, these tapes can be used on any size of coil or bar
and are used by several manufacturers. These epoxy-loaded tapes are applied directly to the coil and then the coils are placed in a
press or autoclave. Inside the press or autoclave, the coils are heated and pressed for curing. Heating of the tapes causes the
resins in them to flow and to fill all voids within the tape layers. Good quality controls are necessary to ensure that there is
sufficient resin in the tape that it flows readily when heated and has a well-defined “kick over” point or temperature at which the
resin cures fully. The timing of the application of the correct heat and pressure as well as control of the amount of ground
insulation compression, are critical. Coils can then be tested for quality of consolidation or curing.
Next the coils are inserted and wedged into the slot, and the complete winding is often dipped and baked to seal and bond
bracing components. If possible, the complete core/winding system is then heated to cure bonding resin in endwindings and
connections.

1.3.1.4 Sealed Windings
Sealed windings are used in the following applications;
 Outdoor motors with open type enclosures
 TEWAC motors – totally enclosed water air-cooled
 Indoor open type enclosures where conductive materials or moisture exist at motor location

Global VPI process is best for this type of winding, though a dip and bake may be adequate to meet the requirements. The
NEMA MG-1 Submergence or Spray Test verifies sealing.

 Manufacturing Defect
Sometimes during the impregnation process, small voids are inadvertently left within the groundwall. These can occur because
of inadequate vacuum or pressure, incorrect resin viscosity, improper temperatures, tape wrinkling or foreign objects imbedded
in the tapes. In the presence of voltage stress, partial discharges can occur across these voids and attack the organic resin. The
attack may lead to strand or turn shorts in multi-turn coils if the voids are near the copper conductors, and eventually failure of
the coils can occur.
If the voids are in the centre of the groundwall insulation thickness, they are more benign and low magnitude, so failure can take
many years to happen. An additional problem from manufacturing defect is that the internal voids create a thermal barrier
inhibiting the transfer of heat from the copper to the core resulting in higher thermal stresses.
Due to the complexities present during the impregnation process, manufacturing defect is more probable in global VPI stators
or in resin-rich coils made using old tapes.

1.3.2 Surface Coatings
In addition to the insulation materials - strand, turn, and groundwall - there is also a need for voltage stress control in machines
rated 6kV and above, and some lower voltage VSD motors. One surface coating, known as the “semi-conductive coating,” is
applied to each coil leg and creates a more uniform electric stress throughout the insulation in the stator core portion of the
winding Figure 7. A second material coating, known as the “stress control coating,” overlaps the semi-conductive layer at each
end of the coil leg and extends towards the endwinding [Figure 8].

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