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naperlou
User Rank
Blogger
Maybe warmer is better
naperlou   8/20/2012 8:51:04 AM
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Sinc e the equipment was working before the A/C arrived and since the ambient was not a problem, maybe running at a higher temperature would be better.  Nromally,  I would expect, that the A/C units were cycling so often for reasons of humidity control, not temperature control.  This may be all wrong. 

On the other hand, I have seen studies done on large scale data centers that show that most of our modern electronics can run at higher temperatures and humidity than is usually specified for data centers.  So, perhaps you don't need so much A/C.

rathomas
User Rank
Iron
This takes me back
rathomas   8/21/2012 1:15:27 PM
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In the early '90s, we built hydraulic pump test stand for a Chinese customer.  It had a 300 HP 3-phase motor as the prime mover and that was driven by a variable speed drive.  Since industrial 3 phase mains in China is 380V and the drive only accepted 480V, we supplied the machine with a large step-up transformer assembly.  On a Friday, we commissioned the machine at the customer site outside of Beijing, but the drive faulted on undervoltage as the mains input was running at about 350V.  No problem, we just moved the output leads to a different set of taps on the transformer and all was fine.  Until the next day, Saturday... when the drive faulted for overvoltage.  With the industrial load on the power grid reduced on the weekend, the mains voltage rose to around 410V!  A few months later, the customer got the supply stabilized enough that it wasn't a problem, but man, what a pain in the meantime.

Rob Spiegel
User Rank
Blogger
Re: This takes me back
Rob Spiegel   8/22/2012 11:56:02 AM
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Hey Rathomas,

Your story here would make a terrific Sherlock Ohms posting. Is there a chance you could write it up? We would need a bit more detail, since we try to make sure these postings are at least 350 words.

You can send it to: rob.spiegel@ubm.com

GlennA
User Rank
Gold
duty cycle
GlennA   8/21/2012 2:10:24 PM
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Was the A/C too large capacity ?  Or were the setpoints too close?  I would expect an A/C unit to run for longer than 45 seconds.  And if there were 2 units, one could be the primary, probably running continuously, and the other cycling as needed, which might reduce the inrush voltage drop.  The units would have to be programmed to alternate primary / secondary to balance the working hours.

akili
User Rank
Iron
A/C cycling
akili   8/21/2012 6:30:07 PM
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Yes GlennA, the A/C engineer told us it was to equalise the running hours but he didn't explain why the cycling interval was so short.  We needed every scrap of help with the average afternoon temperature of around 40 deg C (104 F) and regularly exceeding 120 F in the shade for short periods.  Combined with humidity usually over 80% it was like working in a Turkish Bath!  And naperlou, the A/C was set to keep indoors at a chilly 80 F.  Even so the contrast between inside and outside could literally make you feel queasy and quite unwell and we consumed many packets of Lomotil for our upset stomachs.  Most of the steel erecting had to be done at night under floodlights because steel placed in the sun was too hot to touch within minutes.  And yes, we did try the trick of leaving a steel girder in the sun and breaking an egg onto it, and it really did sizzle.  The dish and backing structure were painted with a special dazzling matt white two-part paint to reflect heat.

rdelaplaza
User Rank
Silver
Thermostat and Room Insulation
rdelaplaza   9/12/2012 3:28:38 AM
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Several things come to my mind, one is the differential temperature setting (or hysteresis) of the control thermostat, if it's too low, like 0.5 degrees and the AC is set to ie: 80F the Compressor will stop at 80 F and will start at 80.5 F, this problem can be worsened by bad room insulation, as soon the AC stops the room temperature shoots up really fast, or the AC fan blowing into the thermostat, but something else was not right, the "short cycle protection timer" was not doing his job, this timer prevents a AC or refrigeration compressor from starting before the internal pressures equalize, otherwise the compressor will try to start under pressure and the motor will stall and burn.

It works in your home AC, somebody fiddles with the termostat while the compressor is running, raises the temp and the compressor turns off, and then immediately lowers the temp setting, and the AC compressor will remain silent until the "short cycle protection timer" resets in 4 or 5 minutes.

The change in the regulator settings was like turning the radio volume up when the engine makes a strange noise. the problem was in the AC system.

The bigger the compressors the higher the "inrush current", it could be up to 10 times or more than the running Amperage, the start of big motors generates lots of heat and the duty cycle is not usually designed to start that many times per hour either, so probably the next thing to burn was going to be one of the AC compressors and then the other.

Besides the energy consumption (generators fuel) and the efficiency with all those start cycles per hour becomes a miserable waste of money.. but I gues that nobody will care, ... I'm not paying the power bill.

You mention the Middle East, so I will assume that it was a military installation?

Was there an engineer in charge of the installation?

Did anybody called the AC contractor when the AC problem was discovered?

We where paying the bill with OUR tax money and acumulated national debt to be paid for our kids and grandkids in the future.

Bah don't tell me, Another case where engineering becomes politics.

 

akili
User Rank
Iron
Re: Thermostat and Room Insulation
akili   10/25/2012 7:21:05 PM
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A couple of points to add after rdelaplaza's post.  Firstly it wasn't military, but it was the first commercial earth-station in the Gulf, built for the local telecomm utility company.  The local agents for the American A/C contractors were happy with their kit and as far as I recall it was reliable despite the frequent start-stop of the main compressors.  We had found a cheap solution to the twitching problem by changing a couple of components in the control circuit and we gained kudos by doing so and keeping the station on traffic and earning revenue.  That was what mattered, no "politics" or "point scoring" at all.  Learn the lessons, move on.

However, my experiences on military stations did sometimes see politics come into it.  I cen remember having to change the front panels on a whole rack-full of plug-in modules because some "jobs-worth" Ministry inspector spotted that the engraved markings for the part numbers were the wrong font size.  Being a military contract, the British tax-payer eventually picked up the bill - crazy!



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