8/13/2019 2103501_6003_ENG_A_W

1/2

App lic a t ion N ote

Problem description

This case history is a classicexample of the importance ofa systematic approach to solv-ing a problem. It involves acontractor who works withseveral high- technologymanufacturing plants.

The case began w ith anon-site visit to a buildingmanager who was frustratedwith his electrical system.According to the manager,nothing electrical in his build-ing seemed to work properlyand no one had been able todetermine why or offer asolution. The manager wenton to say that three of hiselectricians had quit and thathe was now in real trouble.

The contractor asked manyquestions trying to get a moredetailed picture of the problem.This effort proved fruitless, sothe contractor asked to betaken to that part of the build-ing where the problems werethe worst. He was followingthe axiom, When in doubt,start at the victim load.

In a far corner, a largemedical machine was goingthrough a critical test proce-dure. The machine wasequipped w ith a large display

screen, a keyboard and acontrol panel with several

The m a lfunc tioning

m e dic a l m a c hine

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

cables and hoses leading toother pieces of equipment. Theoperator display screen showedthe test procedure was inprogress.

Next to the machine was aworkbench set up for circuit

board repair. The workbenchhad a soldering iron, a lightedmagnifying lens and a fan. Theworkbench power strip pluggedinto the same receptacle as thelarge medical machine. As thecontractor watched, the personat the workbench reached overand turned on the fan. At thatmoment, the operator displayscreen momentarily went blankand then came back on withthe words Program Resetdisplayed in large letters.

MeasurementsThe contractor measuredthe voltage at the receptaclefeeding both loads. His Fluke 87true-rms DMM measured 115 V.The building manager repeatedthe measurement with hisFluke 27 average-respondingDMM, which displayed 118 V.Why did the true-rms unit readlower?

True-rms instruments w illgive correct, but lower readingsthan average-responding

instruments on square waves orwaveforms that look like square

Operator: Electrical contractor

Measuring tools: Fluke 43B PowerQuality Analyzer, Fluke 87 True-rmsDMM, Fluke 27 DMM

Features used: True-rms voltage,average voltage, voltage waveform

PowerQualityC a s eS t u d y

8/13/2019 2103501_6003_ENG_A_W

2/2

Fluke. Keeping your worldup and running.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more inf ormation call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M- East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800)-36- FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446- 5116

Web access: http:/ /w ww.fluke.com/2003 Fluke Corporation. All rights r eserved.Printed in U.S.A. 8/2003 2103501 A-ENG- N Rev A

voltage between 100 and135 V rms ac. The engineerswho designed the machine andspecified the nameplateassumed the supply voltagewould be a sine wave, so theminimum peak would be 141 Vpeak (100 x 1.41). Since themeasured value of the peakinput voltage was only 135 V,the machine was running on apeak voltage that was already6 V below the absolute mini-mum required. When the fanwas turned on, the surgecurrent drawn by the fan motorfurther reduced the voltage to apoint where the power supplyof the machine went out ofregulation. This is what causedthe machine to reset.

Solution

The problem of voltage peakclipping (flat-topping) iscommon in high-tech buildings.Many of the buildings now inuse were not designed tohandle the vast array ofcomputers and non-linear loadsso typical today.

In this case, extensiverewiring would be necessaryto reduce the voltage dropbetween the transformer andthe load. An alternative wouldbe to move the most sensitiveloads closer to the transformer.

waves. The contractorconnected his Fluke 43B anddisplayed the voltage wave-form. The display showed thatthe waveform (simulated inFig. 1) was severely clipped atthe top making it more likea square wave than a sinewave. The peak value measuredonly 135 V, rather than theexpected 162 V.

The contractor then drew aone-line diagram of the system.The one-line showed that thetransformer supplying thetest area was at the oppositeend of the building almost500 feet away (see Fig. 2). Themajority of the loads on thattransformer were non-linear,and they were drawing high

peak currents at the peak ofthe voltage. The combinationof high peak currents and highimpedance of the long runcombined to produce severevoltage clipping at the end ofthe circuit right where thetest area was.

Theory and analysis

Since the medical machinesinternal circuits operated onlow voltage dc, the internalpower supply would have adiode/capacitor input circuitthat required a certain mini-mum peak voltage for properoperation. The nameplate onthe medical machine showedthe machine needed a supply

Volts 0

-100

-200

200

100

Fig. 1 Waveform of voltage supplying the medical machine (simulated)

Utility

120/208

277/480

FinalTestArea

500ft

Fig. 2 Partial one-l ine diagram of the high- tech manufacturing plant

2 Fluke Corporation Power Qualit y Case Study: The malfunct ioning medical machine