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Summary:
"How
Well Do You understand Apparatus Temperature Ratings?" With
trilingual summary
Heat shortens insulation life. All
electrical apparatus includes the same basic types of insulation
material. Don't the same thermal ratings therefore apply to
everything? Can apparatus be too cold as well as too hot? Do
standards govern surface temperatures? How is insulation life
numerically related to temperature, and what is a "hot spot
allowance"?
Some answers to such questions may be
surprising. For example: maximum ambient temperatures are not the
same for all electrical apparatus, but may range from 30 to 50
degrees C depending upon the standard involved. Whereas a single
value applies to rotating machinery, the corresponding limit for
transformers typically has a dual value -- a daily average plus a
peak.
Minimum ambients -- the lowest
allowable temperatures -- span an even broader range. One reason is
that insulation is not the only concern. Temperature affects
lubricants, metals, and semiconductors in various ways.
Temperature rises also vary. Different
limits apply to cables, switchgear, electronics, and other
apparatus.
Surface temperatures are not
standardized except for apparatus in a combustible atmosphere. The
limit -- applicable not only to external surfaces but to internal
stator and rotor surfaces in non-explosion-proof motors -- varies
with the nature of the atmosphere. For any rotating machine,
external temperature of a bearing chamber (typically by infrared
thermography) indicates bearing condition only roughly, by
comparison with other bearings or at other times. No standard limit
exists.
An often-quoted rule for estimating how
temperature affects insulation life states that life doubles with a
temperature decrease of 10 degrees C, and is cut in half by a 10
degree increase. Although fairly accurate for Class A insulation,
the figure for modern Class F or H insulation is closer to 12-15
degrees, which makes a large difference in the life estimate.
Furthermore, maximum insulation
temperature isn't directly measurable by either thermocouple or
change-of-resistance measurement. Rotating machinery standards
recognize a difference between design or nameplate temperature and
the insulation thermal limit. Although usually thought of as a "hot
spot allowance," that term does not appear in NEMA MG 1 standards.
Also, the margin is quite different for transformers using the same
classes of insulation.
From "How
Well Do You Understand Apparatus Temperature Ratings?"
...by
Richard L. Nailen, EA Engineering Editor - published in
Electrical Apparatus
July 2005
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