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.