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is to achieve a better life of the repaired motor, on the one hand,
and facilitate an easy rewinding of the machine, on the other, as
class F is a thinner insulation which is easy to rewind at site.

9.5 Maintenance of insulation

9.5.1 Against insulation failure
The properties of an insulating material are greatly affected
by moisture, temperature, repeat overvoltages, and
chemical vapour. Care must be taken to avoid these
harmful effects to achieve prolonged life of the machine.
Whenever a motor is installed in a humid atmosphere
and is switched on after a long shutdown, insulation
resistance must be checked before energizing the motor.
As a precaution, insulation resistance must be checked
Figure 9.3(b) Stator core after vacuum pressure
impregnation (VPI) before a restart after a long shutdown, even in temperate
conditions. If the insulation level is found to be below
the recommended level as shown in equation (9.1) it
must be made up as noted below.
9.5.2 Making-up the insulation level
This can be done by homogeneously warming and drying
the windings in the following way:
A crude method can be to set up two lamps at both
ends of the stator housing after removing the end
shields. This can be done in smaller motors, say, up
to ‘355’ frame sizes.
By circulating roughly 50-60% of the rated current
under a locked rotor condition, at a reduced voltage.
To facilitate easy escape of the moisture to the
Figure 9.3(c) Exploded view of a VPI stator core atmosphere, the bearing covers may be removed.
At important installations, built-in heaters are recom-
mended which can be switched ON when the motor
Note For HT motor windings, ordinary dipping and baking, as is not in use. The rating of these heaters, known as
for LT motors, is not recommended in view of the very high stresses space heaters, is such that a continuous ‘ON’ heater
to which the windings may be subject during switching or while will not have any harmful effects on the motor
clearing a fault, or in the event of a system disturbance. See also windings, nor will they increase the winding tempera-
Section 17.5. An ordinary cured motor winding may not be able to ture beyond the safe limits. The rule of thumb is to
withstand these conditions. set the space heater rating so that it is able to raise the
inside temperature of the motor by 8-10°C higher
Repairing technique than the ambient. Its main function is to keep the
Generally, no electrical fault would cause damage to the moisture condensation away from the windings by
windings except in the event of failure of the protective maintaining a higher temperature inside the motor
system. Mechanical surface damage is, however, possible housing.
during transportation or installation or due to penetration
of foreign matter into the windings. It is possible to In certain cases, where the motor is too large and it is
repair such damage at site without affecting the per- idle for a long period before it is installed and electrically
formance or quality of the winding insulation. The connected (the space heaters are therefore OFF) even
excellent electrical, mechanical and thermal properties these heaters may not be sufficient to absorb moisture
of the end turn windings localize such damage. Repair is which might have condensed deep in the slots, unless, of
possible by slicing and cleaning the damaged part, and course, the heaters are kept ON for a considerably long
applying to it the pre-impregnated insulating tapes and period. In such cases, it is advisable to heat the motor by
other resins. It is recommended to request the manufacturer methods 1 and 2 in addition to using the built-in heaters,
to carry out such repairs. until the insulation level of the windings reaches the
required level. Once the motor has been installed, these
Note For motors that are insulated other than by vacuum space heaters, when provided in the windings, are switched
impregnation, it is recommended to rewind a damaged stator with ON automatically as soon as the motor is idle, and thus
class F insulation, even if the original insulation was E or B. This eliminate deep moisture condensation.