Electromagnetic Shaft Currents inTurbomachinery: An Update

Part I: Status Reviewand Shaft Brush Development

Unscheduled shutdowns and serious damage to the machinery can be prevented Dydemagnetization of all rotating and stationary components of the entire train.

U.S. SohreTurbomachinery Consultant

Ware, MA

i .oBRIEF REVIEW OF THE PRINCIPLESINVOLVED

Reference (l_) or Reference (5) should beconsulted for a detailed explanation of shaftcurrent generation. In a nutshell, theprocess works as follows:

Prerequisites For ElectromagneticGeneration:

The machine must be relatively large,and/or have a rotor running at high sur-face speeds, and/or have tight clear-ances between rotating and stationaryelements.

A major portion of the machine must bemade of a. material which can retainmagnetism.

Residual magnetism must be present, orthe unit must be located in a magneticfield. The residual magnetism usually

results from arc-welding, use of mag-netic tools, lifting magnets, magneticparticle inspection, chrome-plating, etc.If the components are not thoroughlydemagnetized after such operations,residual magnetism will be present andthis can set up self-magnetization andshaft current generation when the machineis operating. See (6) for a very good,simple explanation of magnetic principles.

.2Shaft Current Generation

0149-3701/81/4861-1 $02.00 © 1981 AlChE

The principle of operation of a generator inany power plant is the fact that a conductortraveling through a magnetic field willgenerate a voltage at its two ends, one endbeing positive, the other negative. If theends are electrically connected, a currentwill flow. This current will be proportionalto the dimensions of the conductor, thestrength, of the mangetic field, and thevelocity with which the conductor is travel-ing through the fields. Another mechanisminvolves generation of eddy-currents instator or rotor, in the same fashion aswith ah eddy-current brake. If the above

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prerequisites are met, any machine willgenerate internal currents. In a turbine-compressor unit these currents are short-circuited, the electric energy being trans-formed into heat in the rotor and stator.Where the currents travel across the inter-face between parts -- for example the verythin oil film at bearings and seals -- or atany other contact point offering an electricalresistance, there will be sparking and sur-face damage. If currents are high -- andthey can reach very high values, of severalthousand amperes -- there can be massivearc-welding and the machine may destroyitself.

As contacting points (for exampletouching labyrinth points) are consumed byarcing, clearances will increase. Thisraises the electrical resistance at thisgiven location until another location offers alower resistance, and the current will thenchange to another path. This process con-tinues until vibration or leakage becomesexcessive, or a failure occurs in the thrustbearings, journals, or seals. If the telltalesigns of sparking, welding, electrodischargemachining, and spark tracks are not observed,ultimately every seal and bearing in amachine may be severely damaged, and theunit may wreck.

The final stage of electric currentgeneration is reached when s elf-excitationoccurs, i.e. the currents flow through themachine in a manner which intensifies theexisting magnetic fields, which then causehigher current to flow, and so on, to thepoint of short-circuited self-excitation. Thisprocess may take years to develop, or it mayoccur within seconds. The culmination of theprocess is reached during the last secondsor so, when the rotor crashes into thestator at full speed and with the throttlevalve wide open, for example when a thrustbearing failure.occurs. The magnetic gapbetween the generating components is thenzero and -- sinc'e magnetic field strength isinversely proportional to the gap -- enor-mous currents are generated at this time.The evidence of such extreme currents canbe seen in Part II of this paper.

Self-excitation is often initiated by ashock, or by vibration of the machine, forexample during an episode of oil whirl,compressor surge, fluid slugging, quicktemperature changes and consequent rotorbow, etc.

A machine having a small degree ofresidual magnetism can build up its mag-netic fields without any external prompting-- just sitting there and running steadily.This' has been documented by field measure-ments. The process may finally level out ata certain field strength (which may or maynot cause objectionable damage), or self-excitation may occur at any time, due to anunrelated incident, such as shock.

Severe shaft current damage is some-times experienced with new machines, butthe majority of problems which have beenencountered were with machines which hadbeen operating several years, most of themaround 6 to 12 years.

The machines most frequently involvedare:

- Ammonia plants: Syngas train; air train;refrigeration train; CO2 train.

- Methanol plants: Syngas train.

- Ethylene plants: Cracked gas train;propylene train.

- Oil refining: Air train; H2 compressors;wet gas compressors.

- Nitric-acid trains.

- Gas turbines.

The list is by no means limited tothese processes. The problem can occuranywhere if the prerequisites are given.During the last two years we have seen it inall kinds of turbomachinery, including oilfield gas reinjection units, oxygen com-pressors, even a small cryogenic gasexpander which was no more than a fewinches long, having an impeller of

186

approximately 5" diameter, and running at50, 000 RPM. This machine was badlydamaged several times. The reason for theproblem was very strong magnetism result-ing from spot welding, and the very smallclearances in this machine.2. 0HISTORY AND STATUS OF WORK

The phenomenon of electromagnetic shaftcurrent generation has been understood fora long time, see Reference (5) and thereferences listed in ( I ) . However, com-pressor designers and operators were gen-erally not aware of the problem. My per-sonal involvement dates to 1971, when avery severe case occurred with an air com-pressor in a 1, 000 ton ammonia plant. Atthat time I abstracted some of the referencesin (2). More problems of this sort occurredsporadically, getting more and more severeand more frequent. It became apparent thata more organized approach was required,and this prompted me to get together withMr. Nippes (a well-known electricalmachinery designer) and to present the paperat the 1978 Turbomachinery Symposium (1).

Many of the machines involved hadshaft current problems as long as peoplecould remember (decades), some of themachines since they were first installed. Thereal cause was never identified, and the partswere just periodically replaced. The primi-tive shaft brushes which were sometimesinstalled (because electrostatic currentswere suspected) failed relatively rapidly,and then the people just accepted "wear" inbearings, seals, governor drive gears, etc. ,as a fact of life.

In the late seventies, with the in-creased use of on-deck welding, magnetic-particle inspection, magnetic tools andlifting devices, etc. , the failure frequencyin compressor trains reached epidemic pro-portions. Some of these failures wereextremely severe, and repetitive to thedegree that they would recur within days oreven hours after an overhaul.

The paper ( I ) is quite detailed. Any-one not requiring so much technical detail

will find simpler descriptions in (3) or (4).An excellent description of the electromag-netic principles involved and of tests whichhad been performed was presented bySchier in Reference (5). When we publishedpaper ( I ) , we were not aware of thisreference.

During the last two years, Mr. Nippeshas become involved in the investigation ofmany shaft current problems, and he hasdeveloped apparatus and procedures todetect shaft currents and magnetic fields,and to demagnetize machines, as he willexplain himself.

As experience was accumulated, itbecame quite apparent that relapses couldoccur after demagnetization -- and es-pecially after partial demagnetization -- ifthe cur r ent-generation of a machine was notmonitored in some fashion. This was trueespecially after partial demagnetization,when the unit was not completely dis-assembled, or if magnetism was againintroduced into the train (spare parts,welding, magnetic particle inspection).Also, some minor residual magnetismsometimes survived, and a slowly pro-gressing damage would develop in thebearing and seals. Evidently, a good shaftbrush was needed to allow monitoring ofcurrent as well as grounding of such smallresidual currents. The brushes which wereavailable at the time turned out to be un-suitable and/or unreliable for the purpose.Most were not adaptable to existingmachinery and/or did not have the featureswhich the plant engineers required, forpurpose of maintenance, wear detection,and plant reliability. These brushes hadexcessive contact resistance and quitworking altogether after a relatively shortoperating time.

Plant operators asked me to design asuitable brush. To find out exactly whatwas required I conducted an extensiveseries of tests with brushes of many dif-ferent materials and configurations. Itbecame apparent that a new design wasrequired, which had to be specificallytailored for the monitoring of voltage and

187

currents, and for removal of residualcurrents. Brushes of this new design havebeen in continuous operation since 1979.

The function and features of variousshaft-riding brushes will be described below.3. 0SHAFT-RIDING BRUSHES

. 1General Considerations

Brushes cannot eliminate strong shaftcurrents and/or prevent self-magnetizationand s e If-excitation of a unit. Their functionis to allow monitoring of the voltages andcurrents, and to protect the machine fromdamage resulting from residual magnetismafter demagnetization. The important pointsto remember when applying shaft brushesare:

Currents of the electrostatic type willbe effectively eliminated by the applicationof one brush per shaft. Although certainlocations are better suited than others, thebrush can in most cases be located whereverit is most convenient.

Electromagnetic currents: There areso many possibilities of electromagneticexcitation that it is impossible to predictthe strength of the currents and the pathswhich they may take. There are no brusheswhich can protect a machine against allpossible combinations of such currents. Also,if magnetization is high and currents arevery strong, any brush will deterioratefairly rapidly, and frosting and other sparkdamage may occur on the shaft surface underthe brush. Heating of the brush and of theshaft may then occur because of currents.This is one reason for a cooling-oil require-ment. If shaft frosting due to excessivecur rent-transfer has progressed beyond acertain point, electrical contact of thebrush with the shaft will be lost, the brushbecoming ineffective. This can be detectedby measuring current transfer through thebrush, as will be described later. If theshaft currents are strong enough to causeloss of contact within a relatively shorttime, it will be necessary to demagnetizethe machine.

Summarizing, we can say that thefunction of a shaft brush in a magneticallyactive machine is to allow monitoring andto prevent damage to bearings, seals,couplings, gears, etc. This will beaccomplished in most cases. However, itis possible that more current is generatedthan the brush can handle. In this casebrush wear will be excessive, heating mayoccur, a'nd the brush will cease to functionproperly.

The above pattern of behavior hasbeen observed during extensive testing,with several different types of brushes andwith several different materials. Thetests were run at 10, 000 RPM, on a 5 inchshaft surface. The shaft material was4340, which is a common material used forturbomachinery shafting.

JJDefinition Of Function

The brush must remove electricity fromshaft surfaces running with 250 ft. I see.to approximately 500 ft. /sec. , with aminimum of electrical resistance, andwithout interruption of current flow duringa revolution, for example because of thebrush jumping off the surface. Acetylizingmaterials may be unacceptable. Environ-ment of the brush and its holder may be ahazardous atmosphere. Malfunction orfailure of this device must not precipitatean accident or a shutdown of the unit.

The degree of wear of the brush mustbe determined from the outside, and thebrush must be capable of being raised and/or removed and replaced without the riskof a spark or rub against rotating com-ponents. Brush wear must be limited to apoint where no shaft damage can occur,and there must be provisions for a wearindicator and alarm, as even short periodsof operation without the brush could causesevere damage or failure of the machinery,and a worn brush will cause sparking,excessive heat, and shaft damage.

TTie brush itself may be exposed to astrong oil flow which discharges from thebearings and seals, or it may be working

188

dry, or in an oil vapor environment. Leak-age of significant amounts of gas or oilthrough the brush mounting to the outsidemay not be acceptable.

The operating life of the brush shouldpreferably be long enough to allow replace-ment at scheduled shutdown only. Thismeans at least one year of continuousoperation.

Surface irregularities and slight pittingor surface roughness should not have asignificant effect on brush performance,although it will obviously accelerate wearof any brush.

The brush assembly should be capableof operating in environments from -60°F toabout 300°F (to be specified by user). Pro-truding brush components may see 500°F(gas and steam turbines).

In most cases there is only approxi-mately 1 inch of axial shaft surface accessi-ble for the brush, for example between thecoupling and bearing case, or on a (integral)coupling hub, or inside a bearing. For thesereasons, axial movement of the brush mustbe extremely limited, to avoid contact withrotating or stationary parts during service.This would cause either a rub (spark,damage to rotating parts), or a short-circuitof the rotor against the casing (spark, un-controllable electrical loops with possibilityof self-excitation).

The accessible shaft location deter-mines the mounting requirements of thebrush. Since many installations are onexisting machinery, there is usually onlyone choice of location, and this is often in avery inaccessible area, for example in theend cover of a barrel-type compressor.Also, the type of mounting varies with eachmachine, and so does the distance betweenthe mounting surface of the brush and thecenterline of the rotor, which determined thelength of the brush assembly. Some mountingsurfaces are cylindrical, others are flat(either in a horizontal or vertical plane) orthe brush assembly may have to be mountedagainst the face of a bearing case or seal.

The mounting area is often obstructed bypiping and other mechanical and electricalcomponents, and the space availability forremoving the brush assembly may beextremely limited.

For the above reasons, a design wasselected which has sufficient flexibility toaccommodate these varying conditions. Thedevice shown in Figure 1* consists of abrush with wire bristles of a specialmaterial, which are protruding from a verynarrow, rectangular cartridge which can bemounted in a variety of ways, and which canbe modified in length to suit individualconditions. The brush is removable fromthe outside of the machine.. 3Grounding of Shaft Brushes

-1 General considerations and safety.Shaft currents may reach high voltages andhigh current densities. This can occursuddenly and unexpectedly. Arcing andelectrical shock to personnel may occur ifproper precautions are not taken. There-fore a switch should be provided to dis-connect the brush terminal from the groundbefore the brush is raised from the shaftor before it is lowered, to prevent arcing.A suitable grounding arrangement is shownin Figure 2.

The suitability of the switch should bechecked for the given environment, and forthe voltage and current to be used. If thereis a risk of fire or explosion, an inert-gaspurge or oil purge may be provided at eachbrush casing,

-2 Grounding arrangements. If mag-netically induced currents are present, itwill be necessary to have a brush on eachend of each magnetically active machine.

*Patents are applied for. Therefore nodetailed illustration can be published atthis time. If additional information isdesired, please contact the author.

189

This means that each body in the train needstwo brushes once magnetism has permeatedthe entire train. The question is now how toclose the electrical loop across the brushesin a way which will minimize current recir-culation, magnetization, spark damage,power loss, and local heating. Obviously,the brush will not prevent the currents frombeing generated, but it can prevent sparkdamage to critical parts.

The grounding arrangement will bedifferent for each installation, depending onwhere and how the currents are generated.For this reason it would be advisable toconsult an electrical expert to prescribe thebest arrangement for each individual case.At the present time it seems that in mostcases where the entire unit is generatingcurrents, the most effective way of groundingwill be as shown in Figure 2, connecting allbrushes to a common lead, with a groundlead near the middle of the unit. This groundlead should be connected to a separate, good,and well-maintained ground. Each brushshould also have a switch in its lead to permitdisconnecting individual brushes from thegrounding system, for check-out and formaintenance.

Needless to say, it is necessary toreview all casing grounds in the train and inthe baseplate and concrete foundation rein-forcements. Make sure that they are pro-perly designed and in good operating condi-tion and that no voltage differential existsbetween any of the grounding points.

-3 Installation checkout.1, Before the unit is started, the brushes

should be checked with the ground switchopen for:- Resistance to casing with brush raised.

This should be infinite.- Resistance from terminal to shaft with

brush down. This should be zero.With ground switch closed, check forproper operation of all switches.

2. Startup:- Start with all brushes raised.- At speed, with ground switches open,

lower one brush at a time and, using

an oscilloscope, measure the voltageacross the ground switch. This is theshaft voltage being generated. It willusually show up as a series of near-vertical spikes across the screen ofthe scope, often giving indications ofrunning frequency, or multiplesthereof (see Ref. (1_) ).

_._4Measuring Shaft Voltage and Current

Measure current, as shown in Figure 3,using a non-induetive resistance, ofapproximately 1 ohm from the brush tothe ground, and measuring voltage-dropacross this resistance using the oscillo-scope. This voltage drop can then beconverted into current, using the for-mula I (amp) = voltage drop/O, whereÎÏ = resistance. The measured currentis the current discharged by the voltagespikes from the rotor to the ground. Itis obviously not possible to measure thecurrent strength of these spikes, whichlast only a few milliseconds, by meansof conventional ammeters.

If voltage and current are within reasonand no signs of distress are noted, theground switch can be closed. However,if current reaches several amperes,shutdown and demagnetization should bescheduled, because brushes and machin-ery may deterioTate rapidly.

- Observe effect of each brush upon voltageand current of all other brushes. Ofcourse, it is necessary to keep detailedrecords of the readings made at startupand at periodic intervals.

- Watch for any evidence of excessive heatbeing generated in the vicinity of thebrush, due to eddy-currents in the shaft.Feel the casing area, watch for oilsmoke from breathers, oil temperaturerise, etc. Pull the brush and feel forheat if the situation looks questionable.Often, a good idea of the heat beingdeveloped can be had by feeling the areaof the casing where the oil throw-offfrom the brush hits the casing walls.

190

Brush Checkout

-1 Ohmmeter readings.While an ohmmeter is not a suitable instru-ment to check brush contact resistance (theinterface between the brush and a rotatingshaft is of a quite different nature than anelectrical resistance), it can be used to geta fair idea of brush effectiveness. This canbe done as follows (see Figure 4):

- Open ground switches of the two brusheson a rotor.

Connect the ohmmeter across the terminalof the two brushes, which must be on thesame shaft, with no couplings or otherresistance in between. The meter willprobably fluctuate, but if the averagereading is below 50 ohm, the brushes areoperating properly. Reverse meter leads.Probably one reading is quite high,because a semiconductor effect seems toexist across the brush interfaces. Thelower reading should be used for judgingbrush conditions.

- Raise each brush briefly and make surethat the resistance goes to infinity, indi-cating there is no short against the ground.Observe the reading after the brush hasbeen lowered again. It will take a fewminutes to stabilize the reading.

-2 Voltage drop.Using a DC voltmeter or, preferably, anoscilloscope, check the voltage of a smallbattery with the rotor in series with theinstrumentation, as shown in Figure 5.Repeat this test with the leads reversed. Ifthe measured voltage drop across thebrushes and rotor is about 2 to 3 volts, bothbrushes are in good condition.

Caution: Do not connect the abovebattery circuit or the ohmmeter to the brushterminal for any longer than necessary(about 5 seconds per reading), because evena small battery current can generate sub-stantial eddy currents in the shaft under-neath the brush, with consequent shaftheating and brush contact surface deteriora-

tion can occur quite rapidly, within a fewminutes.

Brush condition and wear should bechecked daily at first, later in weeklyintervals, once satisfactory operation hasbeen established.4.0TESTING OF SHAFT BRUSHES

Note: For space reasons, most of thischapter had to be eliminated for this edition.Refer to Ref. (8) for a detailed descriptionof the tests.±l_Description of Test Rig

5 inch shaft section driven by 10, 000RPM AC/DC motor. The test brushrides on this surface.

- Shaft brushes for voltage input to therotor: one "spark plug" type ("Bendix")stranded copper-wire brush (Figure 1,Reference ( I ) ) at the shaft-end face,approximately 1/8" from shaft center-line. This brush deteriorated duringtesting, resistance exceeding 150 ohm,and a second input brush had to beinstalled at the other end of the shaft.This brush was of the metal-bristle typeand accessible from the outside formanipulation, so good contact could beverified during operation.

Oil supply: To simulate the environmentof a bearing case, a large, high-pressureoil pump supplied approximately 15 GPMof turbine oil into the enclosure, splash-ing against adjacent shields on both sidesof the brush, but not directly contactingthe brush.

A separate, small oil pump supplieda 1/8" stream of oil directly to thebrush, approximately 1. 5 inches up-stream of the shaft center line.

The oil being used on most tests wasa new turbine oil. This oil was identicalto the oil being used in an ammoniaSyngas compressor train.

191

During the test the oil temperature atthe tank was normally between 90°F and120°F, which is' approximately the inlettemperature of oil used on turbomachinery.

_^Test Results

-1 General observations and conclusions.Over a period of several months, hundredsof test runs were performed, with differentbrush arrangements, different test condi-tions, different types of currents, and dif-ferent current densities. Thirty-two basictest series were performed, evaluatingpromising arrangements and their perfor-mance versus time. The criteria to evaluatebrush performance were:

1. Contact resistance, using the criteriadescribed above.

2. Contact-pressure requirements.

3. Deterioration of shaft surface.

4. Deterioration of brush contact area.

5. Brush wear rate.

6. Brush friction.

7. Brush and shaft heating. Oil heating.

We had hoped that such an extensivetest program would not be necessary, feelingthat we had a promising design which shouldfunction well, perhaps with some minorfine-tuning. Although this finally turned outto be the case, the initial results wereextremely disappointing, as the contactbroke down within days or even hours ofoperation, and the cause of this was verydifficult to determine, requiring majormodifications of the test apparatus. Greatrefinement of the test procedures wasnecessary and, of course, major revisionsof brush design and testing of other types ofbrushes had to be performed.

Because of the difficulties initiallyexperienced, we contacted four highly re-garded experts in electrical brush design,and sample brushes were manufactured by

four manufacturers, each involving severalconfigurations and a variety of material(carbon, graphite, metal, metal-impreg-nated). To compound the difficulties, itwas found that the reference brush feedingthe currents into the rotor deterioratedand falsified the readings. This brush wasof the so-called "Bendix" type, or sparkplug-type, which is shown in Figure 1 (1).The operating conditions to which thebrushes were exposed were well withinstated limits.

^_3Conclusions (Preliminary)

-1 The type of shaft currents we are deal-ing with in this test setup will cause rapiddeterioration of any brush if the shaftcurrents get high, and/or if a given currentstarts generating secondary currents andmagnetic fields in the rotor. Such "fieldsand currents evidently have much moreeffect upon the brush than the passage of anAC or DC current of known characteristics.

-2 The behavior of the brushes is entirelydifferent from behavior of commonly usedbrushes in electric machinery.

-3 Within these limitations, the new brushdesign functioned very well.

-4 If currents exceed certain limits, rapiddeterioration of the contact surface willoccur and the brush will cease to function.This can be detected with the unit in opera-tion by using the methods described pre-viously, especially the ohmmeter methodor the battery voltage method. If currentsare so high that the brushes deterioraterapidly, there is no other remedy than todemagnetize the equipment.

-5 Commonly used solid brushes of carbon,graphite, metal, and metal-impregnationsfailed within a verv short time, causingshaft damage.

The test program will be continued,to find the most suitable materials andarrangements.

In the meantime, the wire bristle

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brushes are considered suitable for fieldinstallation. Within the stated limits, theyshould reduce or eliminate shaft currentdamage to the machinery, and they will per-mit accurate measuring of shaft voltages andcurrents, thereby allowing monitoring ofdetrimental magnetic activity.

^4Field OperationA set of 4 brushes was placed in continuousoperation in 1979, being periodically (every2 hours) checked for wear and contact resis-tance. The bristles are of two differentmaterials. Surface speed is up to 204 ft. /sec.This unit (33, 000 HP propylene compressor)had a 9 year history of severe shaft currentproblems, dating back to original startup.Had bearing failures in 3 to 5 month intervals,finally 1 month intervals. Babbitt was 20mils, exposing the bronze backing. The"sagging" of the shaft into the bearings wasmonitored with the gap voltage of the vibra-tion probes. See (I) for details. The unithad two spark plug-type brushes, at the shaftends, which were not capable of maintainingelectrical contact, see above. The unit wasdemagnetized and four brushes were installed.Results to date (1981):- No increase in contact resistance.- No noticeable brush wear. It is estimated

that no brush replacement will be re-quired for many operating years.

- No increase in gap voltage of the vibrationprobes due to spark-erosion of bearings.

- Bearings running significantly cooler.- Maximum currents going through brushes

are about 0. 065 amp AC and 0. 075 ampDC, measured by using the l ß resistor +oscilloscope method. Occasionally cur-rents go to 0. 12 A.

- No evidence of shaft current damage to theunit.

- The "phone pickup" signals are now muchlower. This is a result of the demagne-tization. There are definitely no more"crackling" signals in the bearing area.

There are a number of additionalbrushes in field operation, but it was diffi-cult to obtain meaningful operating data.5. 0^PERMISSIBLE SHAFT CURRENT LEVELS

There is practically no information availableon this subject -- as far as experience with

process equipment is concerned. Reference(T) contains a very good explanation of theshaft current problem experienced withelectric generating units, and it also des-cribes copper-braid grounding deviceswhich work well in these large machines.However, those machines have ample spacefor such devices and they can be shut downfor inspection and for replacement, whileprocess equipment is usually required torun continuously for several years. Thisvery useful instruction pamphlet containsthe following recommendations:

"Shaft Voltage Measurement;The voltage from shaft to ground shouldbe checked at least once a month, to besure that the grounding devices arefunctioning properly. Because knowingthe characteristic wave shape and peakvoltage are important troubleshootingtools, the recommended method ofmeasurement is an oscilloscope. Theleads should be short and the oscillo-scope should be grounded to the turbinepedestal where the shaft voltage is beingmeasured. Measurements are to be takenat the exposed shaft between the turbineand the generator and at the front stand-ard through the shaft stick hole. Theoscilloscope must be AC and DC coupledso that both voltage components can beobserved. The maximum instantaneousvoltage must not exceed 6 volts zero topeak while the oscilloscope is in the DCcoupled mode. Voltage levels in excessof 6 volts can dissipate enough energy ina point contact to damage bearings. Ascope camera is also a good method torecord the data if one is available.

If for some reason an oscilloscope isnot available, a vacuum tube voltmeteris better than not taking the requiredmeasurement. Voltage spikes will not beseen by a meter, however, and the de-gree of assurance is considerably dimin-ished. The voltage is to be measured at

the same locations listed above and beAC and DC coupled. The maximum volt-age must not exceed 1 volt RMS. Anyvoltage over the maximum establishedvalue will require corrective action. "

193

This may serve as a temporary guide-line on voltage potential, until informationon turbocompressors becomes available.

Concerning currents, we know evenless. The demagnetized propylene com-pressor mentioned above has approximately1 volt peak-to-peak voltage (with all brushesgrounded), as measured with the oscillo-scope, and up to 0. 15 amps peak-tó-peak(as meaaured across the 1 ohm resistor bymeans of oscilloscope) at the-#1 (turbinefront) bearing. This is for normal opera-tion, and it varies from day to day withoperating conditions and perhaps evenambient conditions. The #4 brush (com-pressor outboard) has 0. 04 amps. With thebrush at the #4 position raised, the #1brush current increases to 1.2 A, and the#2 brush goes from 0. 05 A to 0. 3 A. It canbe seen that the situation is far fromstraightforward.

While this example does not providemuch help, it does give an indication of theorders of magnitude to be expected,6.0CONCLUSIONS

While we are still very much in the darkconcerning electromagnetic shaft currents,we at least have learned how to deal withthem in a manner which can prevent un-scheduled shutdowns and serious damage tothe machinery. This is done by demagneti-zation of all rotating and stationary com-ponents of the entire train -- including thestructural steel baseplates (if used). Whenthe unit is put in service after demagneti-zation, shaft current buildup is monitoredby means of the telephone pickup techniquedescribed in (1), by measurement of staticmagnetic fields using the Gaussmeter, andby monitoring of shaft voltages and currentswith the shaft brush. Residual electromag-netic currents are grounded by means ofshaft brushes, one at each bearing, ifpossible.

Although there were a few notablerelapses, the experience so far has beenquite good where machinery was completely

demagnetized -- provided the demagne-tizing job was done by experienced special-ists, with special tools for the measure-ment of fields and for the demagnetizingoperation.7.0REFERENCES

*(l)Sohre, J.S. , and Paul I. Nippes,"Electromagnetic Shaft Currents andDemagnetization on Rotors of Turbinesand Compressors", presented December1978, at the 7th Turbomachinery Sym-posium, Texas A&M University, CollegeStation, Texas.

*(2)Sohre, J.S., "Shaft Currents in RotatingEquipment. Electromagnetic and MovingParticle Induced", an informal reviewof literature. October 19, 1971.

*(3)Sohre, J.S. , "Are Magnetic CurrentsDestroying Your Machinery?", Hydro-carbon Processing, April 1979, page207 to 212.

*(4)Sohre, J.S. , "Shaft Currents Can Des-troy Turbomachinery", Power, March1980, page 96 to 100.

(5) Schier, V., "Selbsterregte UnipolareGleichstrome in Maschinenwellen",Elektrotechnische Zeitschrift, November12, 1965 (in German). A translation ofthis article was prepared by the Asso-ciated Electrical Industries, Ltd. ;Power Group, Research Laboratory,Trafford Park, Manchester 17, UnitedKingdom. Translation #3925 "Self-excited homopolar direct currents inthe shafts of machines".

(6)"Magnetic Particle Flaw Detection",Part HI. VIGILANCE, the Journal ofNational Vulcan Engineering InsuranceGroup Ltd. , Winter 1979, Volume III,No. 11, pages 143 & 144.

(7) "Shaft Grounding Device -- CopperBraid. Instructions", General ElectricSteam Turbine Dept. , Schenectady, NewYork, Publication No. GEK-72291 (newinformation, August 1979).

194

WEAR ALARMSWITCH

BRUSH P1VOT\

(1 1 '

(g>— ALARh,

^J1! X . ! ^^jlita -u. v ^x_

UGROUNDING SWITCH(NORMALLY CLOSED)

PLUG-IN RECEPTACLESFOR SCOPE, TO MEASUREVOLTAGE, CURRENT AND

RESISTANCE.

PATENTS APPLIED FOR

Figure 1. Rotor grounding brush electrical arrange-ment.

BRUSH CUOUND SWITCH(NORMALLY CLOSED)

OPEN SWITCH TO MEASURE VOLTAGE.CLOSE TO MEASURE CURRENT.

OSCILLOSCOPE

NON-1NPUCT1VK RESISTANCE( 1 TO i

COMPRESSOR

'

COUPLING;

'

/ f

_f — î_-t ±Tt

{ i

-— — ̂ ^^TURBINE

r\L ~

r

LFigure 2. Grounding arrangement: switches all nor-mally closed.

COMPRESSOR

TO GROUND -«*-

COUPLING

BRUSH

Figure 3. Arrangement to measure flashing current. Figure 4. Checkout schematic for brush contactresistance.

COM PR ESS OR N

DC VOLTMETER(VACUUM-TUBE,OR OSCILLOSCOPE)

-HI -BRUSH

IMBATTERY J

' (9V EVEREAETY522)

^-TO GROUND

Figure 5. Checkout schematic for voltage-dropacross brushes.

SOHRE, J.S.

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