Temperature's "Shocking" Effect on Electrical Control Systems
By Richard A. Kirkpatrick, TB Wood's Inc., Chambersburg, PA

 

     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
     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 40C (32 to 104F), 0 to 50C (32 to 122F), and 0 to 60C (32 to 140F). 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 - - 22C [72F]).

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
     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.
     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.
     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.

     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.

 

 
     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.

     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
     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.
This article is provided courtesy of PTDA.

 

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