Description du problème
Cette histoire de cas est un exemple classique de l'importance d'une approche systématique pour résoudre un problème.. Il s'agit d'un entrepreneur qui travaille avec plusieurs usines de fabrication de haute technologie.
L'affaire a commencé par une visite sur place chez un gestionnaire d'immeuble frustré par son système électrique.. Selon le gérant, rien d'électrique dans son immeuble ne semblait fonctionner correctement et personne n'avait été en mesure de déterminer pourquoi ni de proposer une solution. Le directeur a ajouté que trois de ses électriciens avaient démissionné et qu'il était désormais en grande difficulté..
L'entrepreneur a posé de nombreuses questions pour essayer d'avoir une image plus détaillée du problème.. Cet effort s'est avéré infructueux, l'entrepreneur a donc demandé à être conduit dans la partie du bâtiment où les problèmes étaient les plus graves. He was following the axiom, “When in doubt, start at the victim load.”
In a far corner, a large medical machine was going through a critical test procedure. The machine was equipped with a large display screen, a keyboard and a control panel with several cables and hoses leading to other pieces of equipment. The operator display screen showed the test procedure was in progress.
Next to the machine was a workbench set up for circuit board repair. The workbench had a soldering iron, a lighted magnifying lens and a fan. The workbench power strip plugged into the same receptacle as the large medical machine. As the contractor watched, the person at the workbench reached over and turned on the fan. At that moment, the operator display screen momentarily went blank and then came back on with the words “Program Reset” displayed in large letters.
Mesures
The contractor measured the voltage at the receptacle feeding both loads. His Fluke 87 true-rms DMM measured 115 En. The building manager repeated the measurement with his Fluke 27 average-responding DMM, which displayed 118 En. Why did the true-rms unit read lower?
True-rms instruments will give correct, but lower readings than average-responding instruments on square waves or waveforms that look like square waves or waveforms that look like square waves. The contractor connected his Fluke 43B and displayed the voltage waveform. The display showed that the waveform (simulated in Fig. 1) was severely clipped at the top — making it more like a square wave than a sine wave. The peak value measured only 135 En, rather than the expected 162 En.
The contractor then drew a one-line diagram of the system. The one-line showed that the transformer supplying the test area was at the opposite end of the building — almost 500 feet away (voir figure. 2). The majority of the loads on that transformer were non-linear, and they were drawing high peak currents at the peak of the voltage. The combination of high peak currents and high impedance of the long run combined to produce severe voltage clipping at the end of the circuit — right where the test area was.
Théorie et analyse
Since the medical machine’s internal circuits operated on low voltage dc, the internal power supply would have a diode/capacitor input circuit that required a certain minimum peak voltage for proper operation. The nameplate on the medical machine showed the machine needed a supply voltage between 100 et 135 V rms ac. The engineers who designed the machine and specified the nameplate assumed the supply voltage would be a sine wave, so the minimum peak would be 141 V peak (100 x 1.41). Since the measured value of the peak input voltage was only 135 En, the machine was running on a peak voltage that was already 6 V below the absolute minimum required. When the fan was turned on, the surge current drawn by the fan motor further reduced the voltage to a point where the power supply of the machine went out of regulation. This is what caused the machine to reset.
Solution
Le problème de l’écrêtage des pics de tension (à dessus plat) est courant dans les bâtiments de haute technologie. La plupart des bâtiments actuellement utilisés n'ont pas été conçus pour gérer la vaste gamme d'ordinateurs et de charges non linéaires si typiques aujourd'hui.. Dans ce cas,, un recâblage important serait nécessaire pour réduire la chute de tension entre le transformateur et la charge. Une alternative serait de rapprocher les charges les plus sensibles du transformateur..
Source:
Fluke Corporation
CP 9090, Everett, WA USA 98206
Pour plus de renseignements:
Dans le U.S.A. (800) 443-5853
En Europe / M-Orient / Afrique (31 40) 2 675 200
Au Canada (800)-36-FLUKE
D'autres pays +1 (425) 446-5500 ou
L'accès au Web: http://www.fluke.com/