Quantcast Chapter 3 Transformers

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
(3) Ambient temperature readings. Dry-type
transformers are cooled by free circulation of sur-
Transformers referred to herein are limited to those
rounding air over their surfaces. In a totally en-
having a primary voltage under 600V usually of
closed transformer, all heat is transferred by the
dry-type construction and are used for lighting, con-
exterior surfaces; an encased transformer depends
trol power, and small power applications. These
upon air to enter the case at the bottom, flow up-
small power transformers sometimes supply power
ward over core and coil surfaces, and flow out of the
to loads where continuity of service is critical and
case at the opening near the top. These transform-
therefore a greater degree of attention is justified.
ers will perform satisfactorily at rated output when
While the percentage of transformer failures is low,
surrounding air does not exceed 40 degrees C (104
failures that do occur are serious and result in ex-
degrees F) and adjacent structures permit free
tensive downtime and expense. The best assurance
movement of cooling air. Dry-type transformers are
of continued high reliability is regular maintenance
designed to reach rated temperature rise above am-
procedures. A transformer is a device usually used
bient air temperature when operating continuously
to transform, or step down a higher distribution
at rated voltage, frequency, or load. Serious over-
level voltage to a lower utilization level. Although
heating may result if the unit is operated for sus-
among the most reliable components in an electrical
tained periods at above rated voltage, above rated
system, proper transformer maintenance is still a
current, or at lower than rated frequency. Operating
necessity. While removal of a transformer from ser-
a transformer above the recommended temperature
vice cannot always be accomplished, visual inspec-
will shorten the life of the solid insulation and sub-
tions and testing can be performed with the trans-
sequently increase the risk of a failure. Therefore,
former in service. Transformers require very little
it's important that ambient temperature readings
attention when compared to most electrical appara-
be taken at the transformer to verify that it is
tus. The extent of the inspection and maintenance
within its design limits. If these limits are exceeded,
will be governed by the size, the importance of ser-
simply moving the transformer to a cooler environ-
vice community, the location on the system, and
ment or providing additional ventilation or remov-
operating conditions such as, ambient temperature
ing structures that prohibit the flow of cool air
and the surrounding atmosphere. In general, a two-
around the transformer may correct overtempera-
year maintenance cycle is appropriate (see para
ture conditions. If these changes are not feasible
15-3 for transformers).
then the load on the transformer needs to be re-
3-2. Dry-type transformers.
duced or a higher rating transformer installed.
b. Special inspections. Before any work, more ex-
Dry-type transformers are of open-or-ventilated
tensive than a visual inspection is performed on a
type construction with either air or gas serving as
transformer, it must be de-energized, tagged and
the insulation medium.
locked-out (para 122). This is to ensure the safety
a. Routine inspections. All measurements should
of both personnel and equipment. In general, dry-
be taken at the time of peak load and recorded so
that a means of comparing existing versus previous
type transformers have no moving parts (fig 31).
The only maintenance required is periodic tighten-
transformer conditions is available. Routine inspec-
ing of connections and removal of accumulated dust,
tions of dry-type transformers should include load
current readings, voltage readings and ambient
dirt and lint as outlined below:
temperature readings.
(1) Check for dirt accumulation on windings,
internal leads and insulating surfaces.
(1) Load current readings. If load current read-
ings exceed the rated full load current of the trans-
(2) Check for dirt accumulation that impedes
former, then steps should be taken to reduce the
the flow of cool air.
load to within design limits.
(3) Check for tracking and carbonization over
insulating surfaces.
(2) Voltage readings. Either undervoltages or
overvoltages can be detrimental to a load and/for the
(4) Check for cracked or loose insulators or coil
transformer. If one of these conditions exists, then
its cause should be determined and corrected to
(5) Check deterioration of the turn insulation
and barrier cylinders.
within nominal nameplate values.


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