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Re: Vegas pole pigs can't take the heat (fwd)
---------- Forwarded message ----------
Date: Sun, 8 Jul 2007 13:22:01 EDT
Subject: Re: Vegas pole pigs can't take the heat (fwd)
In a message dated 7/8/07 12:58:11 P.M. Eastern Daylight Time,
>The answer is simple. There is no "excessive current" that is causing the
Well, less current helps, and more current hurts. At some point the current
becomes "excessive" for the conditions, since as it increases it "exceeds"
the limitations of the system - specifications notwithstanding.
>Normal current under high temps --- the oil can not adequately handle the
>cooling requirements of the xmfr and the core gets hotter and hotter as the
>day progresses, especially if the xmfr is operating at full load but yet
>in overcurrent conditions. The xmfr oil and case can not dissipate the
>into the surrounding ambient air fast enough to prevent core overheating.
Then that would be either AB-normal ("excessive") current for the
conditions, or abnormal (excessive) conditions/application. Or maybe a poorly-defined
specification of what "normal" is.
>Excessive thermal problems under normal load conditions, aggrevated by high
>heat, produce problems.
Then the "normal" load needs to be de-rated for those conditions, or a
different device that can handle the same "normal" load needs to be used!
This is no different than choosing wire size and type for an application
- or engineering any other device in a system. The application tables in the
NEC are based on engineering data (which they give you, and you can use if
you want to). The tables are based on an 86 deg F ambient, but the NEC has
tables for de-rating the wire for "excessive" temperatures. You also have to
de-rate for things like packing wires together in conduit (raceways) or cables.
And you have to make sure the wire doesn't get hot enough to harm the
terminals that it's attached to. Voltage drop is generally important, and hotter
wire has more resistance.
Very simple - when you choose a piece of equipment for an application,
if it's not designed to survive the application, it probably won't.
How do you make a pole pig survive high ambient heat? Make it more
efficient at getting rid of heat (bigger and/or more expensive pig), make it
generate less heat(bigger and/or more expensive pig), or design it to withstand
higher temps(bigger and/or more expensive pig). See the trend?
So unless the Vegas pigs are the unlucky victims of occasional abuse or
quality control, the blame still rests squarely on those who are trying to
get those pigs to do more than they were designed to. Hence my previous post...
>A lot like the human body. Under "normal loads" it does fine. Throw it in
>Iraq or some other high-temp area and it can not function at "full-load"
>without encountering some problems. Extra precautions have to be taken.
Same goes for any other device, like pole pigs. The simplest extra
precaution to take would be to reduce the average loads.
>In this example, adding more water and more salt help. In the case of the
>pole xmfr it has to survive on what it has --- it doesn't get extra help.
In the human case, doing less work per time (power) produces less heat
issues. So does running at times of lower ambient temperatures. Or do the same
work at the same ambient, but with people that handle the heat better
(acclimated/trained/native). Same goes for pole pigs...
>As a former U.S. Marine in Vietnam during the late 60's I know all to well
>how these thermal effects can overload a human. Same applies to xmfrs.
Humans have lousy efficiency compared to xfrmrs, but we do have the
luxury of sentience. If only those Vegas pigs could speak...
Center for the Advanced Study of Ballistic Improbabilities
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