Page 243 - Industrial Power Engineering and Applications Handbook

9/222 Industrial Power Engineering and Applications Handbook

Operating temperature above - End winding (over-hang)
+11.0
+5.5
+22.0
+I65
the rated (B,,,) in "C
Figure 9.1 Reduction in life expectancy of a motor with a rise
in operating temperature

(ii) For larger ratings: by phenol or synthetic resin
insulation on one or both sides. Although insulation
on one side is common, by ensuring that the
laminations are always punched and stacked in
one direction only to avoid contact between the
punching burrs, insulation on both sides is always Figure 9.2 View of a formed coil
preferred.
Inisulation of the windings
Different manufacturers adopt different practices of
insulating the coil or the windings. Practices generally tates easy removal of an individual coil at site in case of
used may be one of the following. a damage and replacement with a spare coil. The user
can stock spare coils for such eventualities.
9.3.1 LT motors
9.3.2 HT motors
Wound stator To wind HT motors two methods are adopted:
By simple impregnation in a recommended insulating
varnish, normally synthetic or epoxy, followed by 1 Formed wound machines
baking (curing), in a temperature-controlled oven, at a This is a resin-rich system. The stator is wound with
specified temperature for a specific period. pre-formed coils on similar lines as noted above. After
For powerhouse insulation treatment the stator may thorough testing on each coil for the polarization index
be dipped in varnish for a minimum two to three times, and dissipation factor (tan 4, as discussed in Sections
each dipping being followed by backing. Sometimes 9.5 and 9.6, and the impulse voltage withstand test,
one immersion of the entire stator and two additional as discussed in Section 11.4.9, the individual coils
immersions of the overhangs followed by backing may are completely cured and toughened before inserting
be sufficient. them into the slots. The rest of the process is as noted
earlier.
Formed wound machines 2 Vacuum pressure impregnation (VPI)
For HT motors, the latest practice is to have the stator
For large motors, the practice is to wind the stator with vacuum pressure impregnated (VPI) in insulating resins
formed coils (Figure 9.2). The coils are pre-formed and as a standard procedure, not only to meet the
cured before insertion into the stator slots. They are requirements of 'powerhouse insulation' but to also
insulated with resin-rich glass and mica paper tapes. The develop a more simplified insulating process, to cure
process of impregnation is therefore termed 'resin-rich' and toughen the stator windings and to meet the
insulation. The completed formed wound stator is then severities of all operating conditions a motor may
heated to remove trapped moisture and finally impregnated have to encounter. As described later, this is termed a
in varnish class F or H as required, under vacuum and resin-poor insulating process because the insulating
pressure. The stator is then cured in an oven as described tapes now have a low resin content as they are later to
above. The process of insulation and curing conforms to be impregnated in resin. Performance and field data
powerhouse insulation requirements. This practice facili- of this insulating system have revealed excellent results,