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Temperature's
"Shocking" Effect on Electrical Control Systems
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By Richard A. Kirkpatrick,
TB Wood's Inc., Chambersburg, PA
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The worst cold spell of the last three years has hit the West Coast of
America. Furnaces in buildings up and down the coast are being run at capacities
they have not seen since their factory testing. HVAC Systems go down for
a myriad of reasons, and service personnel are out on call all hours of
the night. Many components are replaced on-site and the systems are again
functional. Some experience repeat failures, suggesting a system design
flaw. |
| What is Occurring in
these Situations |
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Electronic components of many commercial and industrial systems are temperature
dependent. Inside these electronic components (including power supplies,
electronic motor drives, and PLC's), are electronic parts which only function
properly within specified temperature ranges. These ranges are considered
by the manufacturer of the electronic component in obtaining a temperature
rating for the component. Common temperature ranges include 0° to 40°C
(32° to 104°F), 0° to 50°C (32° to 122°F), and
0° to 60°C (32° to 140°F). Exceeding these temperature
ranges (either above or below the specified range) can cause unexplained
problems. For example, the component may produce a fault code suggesting
a component failure like internal error, memory error, or processor fault.
However, upon returning the component to the manufacturer and subsequent
testing by manufacturer, the device is reported as "NO PROBLEM FOUND" by
the test department (when tested at room temperature - - 22°C [72°F]).
This is because many of these problems are 'temporary' or 'self-curing'
and vanish when the component is returned to the proper temperature range. |
| Solutions |
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In many cases, to understand the problems associated with temperature,
one must first consider the system enclosure. Enclosures are very commonly
used to shield the electronic components from the environment. For example,
the situation described above has the air handling controls physically
mounted in a watertight enclosure on the roof top of the building in question.
Enclosure options are varied, but many share some common problems with
respect to temperature. |
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To better understand the temperature problems of enclosures, one must first
have an idea of the basic types of enclosures. The following is not a complete
listing of enclosure types, but it does cover some of the most common used
by electronic motor drive manufacturers. The associations between NEMA
ratings and IP as implied here is an oversimplification and should not
be taken literally. For example, NEMA 1 and IP20 are generally considered
equivalent in the marketplace. Technically this is not true. Both NEMA
1 and IP20 allow for a ventilated enclosure; however, NEMA I does not allow
holes in the top of the enclosure where IP20 does. |
| NEMA
Type |
IP
Type |
Properties |
| NEMA
1 |
IP20
or IP21 |
Designed
to provide protection against user contact with the enclosed equipment intended
for indoor use. |
| NEMA
12 |
IP54 |
In
addition to NEMA 1, provide protection against dust, falling dirt and dripping
noncorrosive liquids intended for indoor use. |
| NEMA
4 |
IP66 |
In
addition to NEMA 1, provide protection against dust and rain, hose directed
and splashing water intended for outdoor use. |
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The problems related to enclosures deal primarily with how to get heat
out of the enclosure. For enclosures mounted outdoors, the issue is often
complicated by radiated heat from the sun. That is, enclosures that are
mounted in the outdoors must be sealed from the environment (a NEMA 4 environment).
This seal keeps the elements from intruding into the enclosure and the
components within the enclosure. Unfortunately, this seal also creates
an oven type effect when exposed to the sun. |
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Many solutions exist to these common problems. Forced ventilation of the
enclosure is perhaps the simplest solution. However, this solution is only
possible if the enclosure is mounted indoors and if the environment is
below a certain percentage of dust and particulate contaminant (a NEMA
1 environment). In high particulate environments, NEMA 12 enclosures must
be used. NEMA 12 enclosures are sealed (typically with a gasket) which
can cause heat problems. In this case as well as the case of outdoor enclosures
(a NEMA 4), enclosure temperature controls (A/C and/or heaters) are commonly
used. Some manufacturers support what is known as a NEMA 12 - Ventilated
design. These designs contain filters to clean the air before entering
the enclosure. These enclosures may not meet the intended meaning of NEMA
12 and have the difficulty of filters that must be cleaned or temperature
problems will still result. |
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In addition to the above listed common solutions, some more innovative
answers exist. For example, some electronic drive manufacturers support
what's known as a 'Fins-out' arrangement. In this arrangement the drive
is mounted with the electronics inside the enclosure and the heat sink
fins protruding out the back of the enclosure. The majority of the heat
generated by a drive is dissipated via the heat sink; therefore, the heat
is never introduced into the enclosure and does not have to be removed.
This 'fins-out' design is usually available at the rating the drive manufacturer
supports for the main unit. A NEMA 4 enclosure with the 'fins-out' design
is a possibility. |
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Another innovative solution to this problem is just starting to emerge
via the power module design. This is a solution for large OEM's who can
afford to custom engineer a drive into their system. In this case, the
power module of the drive is supplied without heat sink or cover. The power
module has an aluminum plate on the bottom of the module which needs to
contact some means of dissipating that heat. This can simply be a large
piece of metal or, in more creative solutions, forced air used or created
by the system itself can actually be used to cool the power module. |
| Conclusion |
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Many problems exist with respect to operating electronic components of
systems at temperature extremes. There are may conventional and innovative
ways to address these issues depending upon your application needs. |
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This article is provided
courtesy of PTDA.
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to Electrical Reference Articles Index
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