INSULATING MATERIALS
3. Where the service life of existing equipment is
shortened by overload, heat moisture, or a
Electrical insulating materials are classified
combination of these factors
according to their temperature indices. The temperature
Silicone insulation is not a "cure all" for motor and
index of a material is related to the maximum
generator failures. Before deciding to use silicone
temperature at which the material will provide a
insulation, check the installation to determine the
specified life as determined by tests or by experience.
cause of failure. Misalignment of bearings, mounting
bolts dislocated, a bent shaft, failure or
Current-carrying conductors require insulation to
inoperativeness of overload devices, or similar causes
isolate them from electrically conductive parts of the
may have initiated the failure rather than the
unit and to separate points of unequal potential. An
insulation itself.
understanding of the different types of insulation used
will help you in the repair of electrical equipment. The
Consideration must be given to the conditions to
different classes of insulation materials are listed in table
which the windings will be subjected during winding,
7-6.
varnishing, drying, or baking. Class A, B, and F
materials are generally tough and will take a lot of abuse.
There are certain conditions when the rewinding of
Class H and N materials are considered somewhat
class A and class B insulated motors with class H
fragile and should be handled with care in order not to
insulation becomes necessary. This is done to prevent
damage the resin film.
a recurrence of insulation breakdown and ultimate
failure. Examples of such conditions areas follows:
Varnish
1. Where the location's ambient temperature
exceeds the equipment design ambient (usually
The process of varnishing new or reconditioned
50C or 122F)
windings helps to prolong the life of the machine by
preventing the introduction of water vapor and dirt or
2. Where excessive moisture (usually condensate)
dust once the machine is placed into operation. In
is present and the windings are exposed
Table 7-6.--Classes of Insulation
7-31
