The earth station supporting our satellites was in the Middle East. It was situated some miles from the main city in the desert at the far end of a long power line that fed other industrial sites. To meet the mains voltage stability criteria for the duplicated 10kW transmitters, we had to install two AVRs that incorporated huge motorized three-phase variacs. The stability of transmitter power up to the satellite is really vital, and the local mains were too unreliable.
All went well for some time. Then suddenly, we found that the small-geared motors driving the variacs were overheating. In both units, the plastic ventilation grilles had melted. Before long, both motors burned out. Replacement motors quickly showed the same symptoms. We heard the cry, "This has never happened before," from our AVR supplier.
Finally, we took all the covers off the units to see if the ambient temperature was the problem, although by now, the air-conditioning plant was up and running and the equipment rooms were cool and comfortable. We quickly spotted that, every 45 seconds, the motors twitched to and fro. It was only a few brisk revolutions of the motor shaft. The variac itself hardly moved.
It was driven through a worm gearbox, and it came back to the same point each time. We then realized that the twitch coincided with a change of hum from the air-conditioner plant on the lower floor of the tower.
The two large compressors cycled every 45 seconds, and the momentary drop in voltage caused the AVRs to twitch. This constituted 80 motor starts each way per hour, well outside the duty cycle rating of the motor. Ironically, the motors had survived in the high ambient temperatures before the A/C plant was commissioned, simply because they weren't continually twitching.
Changing the response of the regulator circuit prevented the twitch, and the transmitters themselves could handle the very small regular dip in supply voltage. The variacs then only moved when gross changes of voltage occurred or when we switched over to the standby diesel generators. I never did understand why the A/C compressors cycled so frequently. We didn't use that particular A/C supplier again.
This entry was submitted by Rod Hine and edited by Rob Spiegel.
Rod Hine graduated from Churchill College in Cambridge, England. He worked in satellite communications, meteorological telecoms, and then general automation, machine tools, and industrial control systems. He has also lectured in electronic engineering and cybernetics.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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.
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.
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.
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.
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.
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.
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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