Detecting Turn Shorts in Rotor Windings Detecting Turn Shorts in Rotor Windings

A new test using magnetic flux monitoring

By MLADEN SASIC, GREG C. STONE, JAN STEIN, & CHRIS STINSON

he rotor windings of syn- detect rotor-winding shorted turns, but they may be unreli- chronous motors and generators are usually able because the rotor is not spinning for the test, and, if very reliable. However, the turn insulation only a few shorts are present, the shorts may disappear once T in such machines will eventually degrade the rotor is spinning (or vice versa). With funding from the and puncture due to thermal aging, load cycling, and/or U.S. Electric Power Research Institute (EPRI), a new type contamination. Although turn shorts do not directly lead to of online magnetic flux test has been developed that is suit- machine failure, they can lead to high bearing vibration, able for salient pole machines, in particular the salient pole may damage synchronizing systems within brushless rotors in four- (or more) pole hydrogenerators and synchro- motors, and may limit output. Offline tests are available to nous compressor motors that are widely used for air separation. The test requires retrofitting a small magnetic flux Digital Object Identifier 10.1109/MIAS.2012.2215644 63 Date of publication: 26 December 2012 probe to the stator core and instrumentation to interpret the

1077-2618/13/$31.00©2013IEEE 64 IEEE Industry Applications Magazine • Mar|Apr 2013 • www.ieee.org/ias tesy ofIrisPower.) A rotorfromasalientpolesynchronousmotor.(Photocour- The copper strip-on-edge salient pole design. The copperstrip-on-edge salientpoledesign. The multilayerwire-wounddesign. Pole Face Turn Insulation S Po Spring Pole BodyInsulation Pole BodyInsulation Conductors P C Packing Conductors p Pole WasherorCollarP Packing Pole WasherorCollar Taped Turns Turn Insulation Pole WasherorCollar 2 1 3

the more serious the vibration from this unbalanced magnetic the moreserious thevibrationfromthisunbalanced magnetic per revolution.Thesmallerthenumberofpoles ontherotor, higher bearingvibrationatafrequency correspondingtoonce asymmetrical magneticforce across theairgapand,thus, one poleisweakerthantheother poles,thiswillleadtoan rotor polewillbediminished. Since themagneticfieldfrom ure ofthemachine.Instead,magneticfieldfrom that some ofthecopperturnstogetherbutdoesnotresultinafail- the turninsulationthatdegradesandfailsfirst.Thisshorts automatically shutdownthemachine.Morecommonly, itis If agroundfaultoccurs,ground-faultdetectorswillnormally tamination ontherotorpolesmayalsodegradeinsulation. and groundinsulation.Inaddition,partlyconductivecon- bond thevariouscomponentstogether. the entirepolemaybedippedinaninsulatingliquidto body byinsulatingwashersandstrips(Figure3).Often, design, the winding is isolated from the grounded pole nected inseriestomakethecoil.Aswithmultilayer turn insulation, andthecopper pictureframes arecon- the creepage distance. lating tapemaybeappliedtothecopperstripincrease copper frames,especiallythosenearthepoleface,aninsu- turn insulation to insulate each copper frame. from epoxyglasslaminatesorNomexmaterials)actas nations (Figure3).Laminatedinsulatingseparators(made frame shape,sothatthecanslideoverpolelami- In this case,a thin copper strip is formedintoa picture- since itcanbemadetobetterwithstandrotationalforces. and thelaminationstoactasgroundinsulation. ing washersandstripsareplacedbetweenthemagnetwire support thewindingagainstcentrifugalforce.Insulat- (which is the part oftherotor pole closest to the stator) to direction, thelaminationsareshapedtohaveapoletip tion isthemagnetwireinsulation.Lookingfromaxial the pole,severalmagnetwirelayersdeep.Theturninsula- cross section,andmanyhundredsofturnsarewoundon pole (Figure2).Themagnetwireusuallyhasarectangular type. Inthisdesign,magnetwireiswrappedaroundthe under 1,000hporso)iscalledthemultilayerwire-wound The oldertypeoffieldpoledesign(stillusedonmotors 20-pole salientpolesynchronousmotor. copper windings.Figure1isaphotoofrotorfrom rotor axis.Aroundtheperipheryofeachpolecoreare steel core,whichlooksrectangularwhenviewedfromthe around therim.Eachfieldpoleconsistsofalaminated such awayastocreatealternatingnorthandsouthpoles the dcsupply(normallyuptoafewhundredvolts)in rotor rim.Thepolesarethenelectricallyconnectedto winding ismadebymountingthepolesinpairson poles. Eachfieldpoleisanelectromagnet,andtherotor tion businesshavesalientpolerotorwindingswith4–26 Most ofthesynchronousmotorsusedinairsepara- Rotor Winding has beenusedtodetectturnshorts. insulation andbrieflypresentstheoff article describestheagingprocessesofsalientpoleturn small voltagesignalsthataremeasuredfromtheprobe.This hermal and mechanical stresses gradually age the turn Thermal andmechanicalstressesgraduallyagetheturn For larger machines,thestrip-on-edgedesignisfavored There aretwobasictypesofsalientpoledesigns[1]. T he tape and separators form the line methodthat O n some pull. Usually one or more shorted turns ▪▪ the overexcitation of rotor wind- just cause higher bearing vibration, The smaller the ings for long periods which in many cases may be tolerable. the negative sequence currents in However, if more and more turn shorts number of ▪▪ rotor windings due to system occur over time, high vibration may force voltage unbalance. a shutdown of the motor. In addition, for poles on the motors with brushless exciters, the On–Off Cycling shorted turns may also lead to failure of rotor, the There are two heat sources when a the components within the motor synchronous motor is started. One intended to synchronize the motor to the more serious mainly applies to motors that are power system. Thus, although shorted started directly online, causing heat- turns on the rotor do not automatically the vibration ing due to currents flowing in the lead to a motor failure, they are an indica- from this pole tips of solid pole rotors or the tion of a problem that may cause prob- damper winding in those with lami- lems in the future. unbalanced nated poles. The other is heat gener- ated by the current in the windings Rotor Insulation magnetic once excitation is applied. Deterioration Processes Frequent starts and stops cause Rotor-winding insulation deteriorates pull. copper expansion and contraction as over time due to thermal aging, ther- the copper temperature increases or mal (or load) cycling, contamination decreases. Relative movement due to of the insulation, abrasion of the insulation due to parti- the different coefficients of thermal expansion in the cles in the airgap, and, of course, the high mechanical various components leads to relative movement force caused by high-speed rotation. between the copper and the turn insulation and, consequently, abrasion. Thermal Deterioration Thermal cycling resulting from frequent starts and Modern salient pole winding designs use Nomex ground stops leads to the cracking of the resin or varnish bond- and turn insulation in strip-on-edge windings, Dacron ing the insulation system components together. This and glass-covered high-temperature enamel turn insula- causes loosening and relative movement between these tion in wire-wound poles, and thermosetting bonding components, leading to increased looseness and abrasion. resins to provide an insulation system that has a thermal Also, if the windings are restrained from returning to IEEE Industry Appl i c a t ons M g z n e • r|Apr 2013 www. ieee .org/ ia s rating of at least Class F (155 °C). If these materials are their cold position, they may become distorted from this operated at Class B (130 °C) temperatures, they should process. The poor design or overly rapid or frequent load have a thermal life of 30 years or more. However, opera- cycles for the design are the root causes. Clearly, the tion above 130 °C will shorten the life of the insulation. more motor starts there are, the more likely it is that The materials most susceptible to thermal degradation there will be a problem. In addition, motors designed for are organic bonding and backing materials (epoxy resins few starts that are actually exposed to much more fre- and polyester films); the hotter the temperature and the quent starting due to operational or power system condi- longer the time at high temperature, the more brittle tions are more likely to suffer from this problem. and more susceptible the insulation becomes to mechanical force that will crack the insulation. Contamination The thermal life of insulation at hot spots in windings A likely cause of rotor-winding shorts in air separation is significantly reduced if the margin between the operat- applications is contamination. The strip-on-edge type is ing temperature and the thermal rating is significantly most susceptible to failure from contamination by partly reduced. This effect is more critical in older Class B insu- conducting materials. Such problems are not confined to lation systems, and the presence of such hot spots is very machines with open-type enclosures because oil leaking difficult to detect. from bearings and moisture from condensation, com- The following are the most common causes of thermal bined with particulates, can contaminate windings. This aging in salient pole windings: contamination has a tendency to accumulate near the overloading leading to operating temperatures well blocking between poles. Such problems are less likely in ▪▪ above expected the design values wire-wound types due to encapsulating the windings to high cooling-medium temperatures or inadequate keep contaminants out. ▪▪ cooling, which can be general, e.g., insufficient cool- When contaminants such as moisture, soot, dust, oil, ing air or cooling water, or local dead spots in the and dust mixtures cover the surfaces of salient pole cooling circuit due to poor design (especially at the windings, they can produce conducting paths between blocking between poles), manufacturing, or mainte- turns and to ground. This can lead to turn-to-turn fail- nance procedures ures (especially in strip-on-edge types) and ground the use of materials that have inadequate thermal faults due to electrical tracking. Certain chemicals can ▪▪ properties and consequently deteriorate at an unac- also attack insulating materials and cause them to ceptable rate when operated within design tempera- degrade. Earlier insulation systems containing materials ture limits such as asbestos, cotton fibers, and paper bonded by 65 66 IEEE Industry Applications Magazine • Mar|Apr 2013 • www.ieee.org/ias machines withoutinletairfiltersinlocationswhere eroded tocausegroundfaults.Theuseofopen-enclosure salient polewindingsandtheirinterconnectionsmaybe ground insulationinbothtypesof poles resultinginturnshorts.The ductors inmultilayerwire-wound This mayeventuallyexposethecon- the rotor-winding insulationsurfaces. of a motorby cooling air willabrade atmosphere carriedintotheinterior Abrasive dustfromthesurrounding Abrasive MaterialAttack absorption. ceptible tofailurefrommoisture organic varnishesaremuchmoresus- zontal axis is the circumferential position around the rotor. zontal axisisthe circumferentialpositionaround therotor. scale isthevoltageoutputof fluxprobe,whilethehori- The shapeofthefluxsignalat0 MW,0Mvar.Thevertical necting lead (white). (Photo courtesyofIrisPower.) A fluxprobe(black)mountedonastatorcoretooth,withcon-

Relative Flux Amplitude (%) -80 -60 -40 -20 20 40 60 80 0 Flux Signal 51 Flux Integrated 0 Direction ofRotorMovement 15 Time (ms) Raw Flux Flux Density 20 25 designed for designed for 30 ion Installaion or on astor The flexible The flexible probe is probe is tooh. 4 5

time torefurbish thedefectivepoles. plant turnaroundmustbeextended toaccommodatethe shut downduringaturnaround oftenmeansthatthe turn shortsbyapole-droptest onlywhenthemotoris Only anonlinetestcandothis. Inaddition,discovering the condition of the turn insulation at the present time. three orfouryearsagomaynot giveagoodindicationof bearing vibrationinonlyafewyears.Thustestdone turns canoccurandmultiplytothepointofveryhigh planned shutdownseveryfiveyearsorso.Y Finally, motorsusedforairseparationoftenonlysee disadvantages. drop acrossthatpole.Thistesthasthreesignificant present, therewillbeasmallerthanaveragevoltage across eachpoleisthenmeasured.Ifshortedturnsare slip ringswhenthemotorisshutdown.Thevoltage 120-V ac,isappliedbetweenthepositiveandnegative [1]. Inthepole-drop test, anacvoltage, forexample incipient ground faults) is to perform a pole-drop test The mostcommonwaytodetectshortedturns(and Offline DetectionofShortedTurns over speed. cause seriouswindingdamageiftherotorisoperated Mechanical windingstresseswillbecomeexcessiveand washer andintercoilconnectioncrackingfromfatigue. and stops.W from loosewindingswilloccurmainlyduringstarts machines willleadtocoildistortion,whileerosion tion. Inadequateinterpolebracinginlarge, high-speed will causeabrasionoftheconductorandgroundinsula- coil vibrationandmovementofthecoilsonpoles bracing isinadequateorbecomesloose,theresulting thermal agingoron–offcycling.Ifthepolewinding braced or the insulation has weakened as a result of crack thecoilinsulationiftheyarenotadequately components tend to distort the coil conductors and 2) 1) 3) It istimeconsumingtoperform,especiallyona Since therotorisnotrotating,centrifugal I large rotorwithmanypoles. is spinning. shut downbutmaynotbepresentwhentherotor Conversely, shortsmayoccurwhenthemachineis theless willbepresentatnormalrotatingspeeds. not be presentin the pole-drop test, which never forces arenotoccurring,andthussomeshortsmay down. t can only be performed with the motor shut inding loosenesscanalsoleadtopole rotor-winding insulationsystem and stopping. centrifugal forcesinducedbystarting windings byrotationandthecyclic includes theforcesimposedon is continuous centrifugal force. T failure insalientpolerotorwindings One ofthemostcommoncauses Centrifugal Forces environment canleadtothisproblem. there areabrasive materials in the The centrifugalforcesimposedon et shorted his - Principles of Magnetic past 30 years, electric power utilities Flux Monitoring The most widely have been implementing magnetic flux monitoring in the air gap Airgap Magnetic Flux applied method between the rotor and stator to detect Companies are trying to minimize shorted turns on the cylindrical the work (such as pole-drop tests) for detecting rotors of high-speed steam-turbine performed during unit shutdowns generators [1]–[3]. This technology due to restricted resources. In addi- shorted turns has rarely been applied to salient pole tion, many companies are moving to windings, possibly because the predictive maintenance to plan any in the rotor is salient pole rotors are very different repair work based on online condi- from cylindrical pole rotors, and tion monitoring. Thus, there is a the pole-drop interpretation of the flux patterns is need for an online tool that can test. not obvious. In 2003, EPRI and the replace the pole-drop test. For the New York Power Authority initiated a research project to determine whether a variant of flux monitoring could be applied to salient pole rotors so that an online monitor could produce a reliable indication of rotor Raw Flux shorted turns [4]. 500 Rotor flux monitoring involves measuring the magnetic flux in the generator or motor air gap to 1,000 determine whether field winding shorts have occurred in the rotor poles. The radial magnetic flux from the 0 rotor is detected by means of a coil or probe consisting 024680246802 of several dozen turns that is glued to a stator core -500 tooth (Figure 4). As each rotor pole sweeps by the flux probe, a voltage is induced in the coil, which is pro- -1,000 portional to the flux from the pole that is passing the Integrated Flux coil. The voltage is measured by electronic instru- ments. In a salient pole machine, the radial magnetic Raw Data flux profile across each rotor pole depends on the IEEE Industry Appl i c a t ons M g z n e • r|Apr 2013 www. ieee .org/ ia s Sum 1 MW and Mvar loading of the machine. After correc- 6 tions for variations in the air gap, any change in the flux profile within a pole at a given load must be due The shape of the signal at 125 MW, 0 Mvar. to shorted turns.

64 2 1 6362 3 6160 4 3,400 59 5 58 2 1 64 6362 6 3,200 4 3 6160 7 57 5 3,400 59 8 3,000 56 6 58 9 55 7 3,200 57 10 2,800 54 8 3,000 56 11 53 9 55 12 2,600 10 2,800 54 52 53 13 51 11 2,400 12 2,600 52 50 14 13 51 15 2,200 49 2,400 14 50 16 2,000 48 15 2,200 49 17 47 16 2,000 48 18 46 17 47 19 45 18 46 20 44 21 19 45 43 20 22 42 44 23 41 21 43 24 40 22 42 25 39 23 41 26 38 40 27 24 28 3637 25 39 29 35 26 38 3031 32 3334 27 28 3637 7 2930 34 35 31 32 33 A polar flux diagram of a hydrogenerator operating at full power. The machine has 64 poles and no known shorts. The 8 radial scale is the accentuated average flux, and the cir- Polar diagram of a 64-pole rotor with shorts detected at 67 cumferential scale is the rotor pole number. poles 8 and 48. 68 IEEE Industry Applications Magazine • Mar|Apr 2013 • www.ieee.org/ias Flux plotrecordedin2006froma22-polemotor. pole toanoppositepolaritypole. autocorrelation, andcomparingtheintegralfromeach involves integrating the data from each pole, applying tivity toapolewithshortedturns.Thealgorithm the rotorshaft. brated thepolelocationfromastartmarkedon ing thefaultaslongonehascali- analyzed tolocatethepolescontain- recorded waveformdatacanthenbe associated withthatpole. thus thesignalfromfluxprobe tive ampereturnsofthatpoleand turn shortinapolereducestheeffec- age fluxacrossonerotorpole.Any of the waveform represents the aver can thenberecorded,andeachpeak netic fluxfromthepole.Thevoltage induced voltagecausedbythemag- the coil, there will be a peak in the A polar plot recorded in 2007 of flux from a six-pole motor. A polarplotrecorded in2007offluxfromasix-pole motor. An algorithm was developed to maximize the sensi- As eachpoleintherotor passes 17 16 18 5 15 19 14 20 13 4 6 21 12 22 105.0 105.0 95.0 95.0 Rotation Rotation 11 1 10 2 3 1 3 9 8 4 7 5 6 2 The Upper AlertLevel Lower AlertLevel 100% Level Pole FluxDensity Upper AlertLevel Lower AlertLevel 100% Level Pole FluxDensity - use of A likelycauseof contamintion. rotor-winding rotor-winding

tions is applictions is ir shorts inair tion separtion 11 9

Polar plot recorded in 2011 of the same motor as in Figure 11. Polar plotrecorded in2011ofthesamemotoras inFigure11. Flux plot recorded in 2011 from the same motor as in Figure 9. Flux plotrecordedin2011fromthesamemotorasFigure9. place, usingaClassFepoxyglue. it cangenerallybeinstalledwiththerotorstillin machines with small air gaps. In salient pole machines, profile oftheinstalledprobeenablesitsuseon designed for installation on a stator tooth. printed onaflexiblebasematerial.Theprobeis probe iscomprisedofanumberprintedcircuitlayers 17 5 16 18 15 19 14 20 4 6 13 21 12 22 105.0 105.0 95.0 95.0 tages ofexistingdesigns.Thenew designed toovercomethedisadvan- in hydrogeneratorsandmotors. turbine generatorscouldnotbeused ferent, fluxprobesusedinhigh-speed and constructionoftherotorisdif- salient polemotorsismuchsmaller stator androtor. Sincetheairgapin and installedintheairgapbetween designed intheshapeofacylinder high-speed machinesareusually Flux probesusedinsynchronous Flux ProbeDesign Rotation Rotation 11 A new type of probe [5] has been 1 10 2 3 1 3 9 8 4 7 5 6 2 Upper AlertLevel Lower AlertLevel 100% Level Pole FluxDensity Upper AlertLevel Lower AlertLevel 100% Level Pole FluxDensity The very low 12 10

Portable Flux Monitoring Instrument Figures 9 and 10 show the resultant and Software flux plot from a 22-pole motor with a The signals from the flux probe cannot For larger strip-on-edge rotor pole design. Fig- directly indicate shorted turns since machines, the ures 11 and 12 show the flux data other factors can affect the flux, e.g., the from a six-pole motor. There are minor distance between the pole and the sta- strip-on-edge differences in the magnetic flux from tor. Thus, a new portable instrument each pole caused by different pole was developed that digitizes the voltage design is favored mounting (and thus slightly different signal from the flux probe with high air gaps). However, none of the poles resolution. Besides an input for the flux since it can be has a significant enough reduction in probe, the device also has an input for a flux to indicate that shorted turns on a shaft synchronization sensor (or key made to better pole are indicated. Figures 9–12 also phasor), which enables any field pole withstand indicate that the flux measurements with a short to be identified. The are repeatable over time. These instrument has a custom-designed rotational machines do not have increased bear- field-programmable gate array and is ing vibration, which supports the capable of fast data acquisition at a very forces. belief that there are no shorted turns. high sampling rate. This sampling rate To date, none of the motors monitored enabled the use of new algorithms for shows evidence of shorted turns. detection of shorted turns, even if the air gap is changing due to rotor imbalance, stator core movement, etc. Conclusions Gradual aging of the turn insulation in salient pole rotor Test Results windings can eventually lead to shorted turns. These shorted turns may create the high vibration of the rotor, Hydrogenerator Tests and, in severe cases where there are few poles and a num- The initial experiments with the flux monitoring sys- ber of shorted turns, the probability of a motor shutdown tem were carried out on many dozens of hydrogenera- due to high vibration increases. tors, which generally have a large number of salient The most widely applied method for detecting poles. An example is shown below. The voltage wave- shorted turns in the rotor is the pole-drop test. Since this form from a flux probe is shown in Figure 5. The gen- test is done with the rotor not spinning, the test may not erator was operated at no-load conditions, 0 MW and 0 find all the shorts that can occur in service. Also, the IEEE Industry Appl i c a t ons M g z n e • r|Apr 2013 www. ieee .org/ ia s Mvar, resulting in a symmetrical shape of both the raw pole-drop test is usually performed infrequently, and signal measured (blue line) and integrated values of raw thus the need for rotor refurbishment is often a surprise, voltage signal (red line). Since the shape of the signal which can extend the turnaround duration. changes with the load of the machine (Figure 6), raw Research has led to a new online test that can detect the data should be integrated for further processing. shorted turns during normal service, by measuring the Given that change in the flux profile within a pole magnetic flux from each rotor pole using a stator-based at a steady load must be due to shorted turns, compari- magnetic flux sensor. Initial results with the new technol- son of one pole to another can be used as an indication ogy were presented. of shorted turns. The transformed flux data can then be shown as a polar plot (Figure 7). Minor differences References between poles and rotor or stator eccentricity will [1] G. C. Stone, E. A. Boulter, I. Culbert, and H. Dhirani, Electrical Insula- result in a less than perfect circle. tion for Rotating Machines. New York: Wiley, 2004. [2] D. R. Albright, “Interturn short-circuit detector for turbine-generator Figure 8 is the result from the same 64-pole rotor with rotor windings,” IEEE Trans. Power Apparatus Syst., vol. PAS-90, no. 2, temporarily applied shorts on poles 8 and 48. The poles pp. 478–483, Mar./Apr. 1971. with shorted turns are clearly determined by the lower [3] S. R. Campbell, J. Kapler, M. Sasic, and G. C. Stone, “Detection of flux magnitude at these poles. rotor winding shorted turns in turbine generators and hydrogenerators,” in Proc. CIGRE, paper A1-206, Aug. 2010. In many of the tests on hydrogenerators, the pole-drop [4] S. R. Campbell, “Instrument development for continuous flux monitor- test did not find the shorts found with the online flux ing in hydrogenerators,” EPRI, Knoxville, TN, Tech. Rep. 1016148, measurement and vice versa. Only after installations on 2008. many dozens of hydrogenerators did we become convinced [5] U.S. Bureau of Reclamation, “Flexible printed circuit magnetic flux that the cause of the bad correlations with the pole-drop probe,” U.S. Patent 6 466 009 B1, Oct. 15, 2002. test was that the pole-drop test itself was not a good indi- cator of what shorts were present in service. Presumably, Mladen Sasic ([email protected]) and Greg C. Stone are with shorts that are present when the rotor is rotating may Iris Power LP in Mississauga, Ontario, Canada. Jan Stein is clear when it is shut down and vice versa. with the EPRI in Palo Alto, California. Chris Stinson is with Air Liquide in Houston, Texas. Sasic and Stein are Senior Mem- Motor Test Results bers of the IEEE. Stone is a Fellow of the IEEE. This article first In 2004, flux probes were installed on seven stator cores appeared as “Using Magnetic Flux Monitoring to Detect Synchro- of salient pole motors used to drive compressors for nous Machine Rotor Winding Shorts” at the 2011 IEEE IAS air separation processes. These motors had 6–22 poles. Annual Meeting. 69