shaft root cause shaft failure

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 5, SEPTEMBER/OCTOBER 2000 1435

Root Cause AC Motor Failure Analysiswith a Focus on Shaft Failures

Austin H. Bonnett, Fellow, IEEE

Abstract—The squirrel-cage induction motor remains the work-horse of the petrochemical industry because of its versatility andruggedness. However, it has its limitations, which if exceeded willcause premature failure of the stator, rotor, bearings or shaft. Thispaper is the final abridgement and update of six previous papersfor the Petroleum and Chemical Industry Committee of the IEEEIndustry Applications Society presented over the last 24 years andincludes the final piece dealing with shaft failures. A methodologyis provided that will lead operations personnel to the most likelyroot causes of failure. Check-off sheets are provided to assist inthe orderly collection of data to assist in the analysis. As the petro-chemical industry evolves from reactive to time based, to preven-tive, to trending, to diagnostics, and to a predictive maintenanceattitude, more and more attention to root cause analysis will berequired. This paper will help provide a platform for the establish-ment of such an evolution. The product scope includes low- andmedium-voltage squirrel-cage induction motors in the 1–3000–hprange with anti friction bearings. However, much of this materialis applicable to other types and sizes.

Index Terms—AC motors, bearing, failure analysis, failuremethodology, root cause, rotor, stator.

I. INTRODUCTION

A METHODOLOGY is provided that will lead operationspersonnel to the most likely root causes of failure.

Check-off sheets are provided to assist in the orderly collectionof data to assist in the analysis. This paper is organized asfollows. In Section II, a summary of motor stresses is given,followed by root cause methodology in Section III. SectionIV contains a methodology checklist, Section V discussesshaft failures, and concluding remarks are given in Section VI.Finally, Appendixes A–D contain photographs of the commonfailures.

II. SUMMARY OF MOTOR STRESSES

The majority of all motor failures are caused by a combina-tion of various stresses acting upon the winding, rotor, bearings,and shaft.

If these stresses are kept within the design capabilities ofthe system, premature failure should not occur. However, if anycombination of them exceeds the design capacity, then the life

Paper PID 00–03, presented at the 1999 IEEE Petroleum and Chemical In-dustry Technical Conference, San Diego, CA, September 13–15, and approvedfor publication in the IEEE TRANSACTIONS ONINDUSTRYAPPLICATIONSby thePetroleum and Chemical Industry Committee of the IEEE Industry ApplicationsSociety. Manuscript submitted for review September 15, 1999 and released forpublication April 24, 2000.

The author is with U.S. Electrical Motors, Chesterfield, MO 63017 USA(e-mail: [email protected]).

Publisher Item Identifier S 0093-9994(00)07694-5.

TABLE ISTATOR STRESSES[12]

TABLE IIROTOR ASSEMBLY STRESSES[14]

may be drastically diminished and a catastrophic failure couldoccur.

These stresses can be broken down into the groups or classi-fications shown in Tables I–IV.

In summary, these stresses are shown in Table V.

III. ROOT CAUSE METHODOLOGY

Building upon the various stresses as they relate to the var-ious motor components, there are five key areas which should

0093–9994/00$10.00 © 2000 IEEE

1436 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 5, SEPTEMBER/OCTOBER 2000

TABLE IIIBEARING STRESSES[27]

TABLE IVSHAFT STRESSES

be considered and related to one another in order to accuratelydiagnose the cause of failure. They are the following:

1) failure mode;2) failure pattern;3) appearance;4) application;5) maintenance history.

Each of these key areas needs to be considered with respectto the stator, rotor, bearings, and shaft.

A. Methodology

Combining all of these stresses leads to a methodologythat falls into two categories. The first deals with failuremodes/classes and failure patterns, as shown in Table VI, whichcan serve as a check-off sheet when conducting an inspectionof the failed motor. The second category deals with the motorappearance, application, and maintenance history. This will becovered in Section IV of the paper.

B. The Motor and System

Although a complete system analysis is beyond the scope ofthis paper, when conducting a root cause failure analysis, it isimportant to recognize that the motor is only one component ofa system. Many factors affecting the system will also affect themotor and may contribute to the motor failure and vice versa.Fig. 1 shows a typical system. Note it also includes the processrequirement.

TABLE VMOTOR COMPONENT/STRESSES

TABLE VISUMMARY OF METHODOLOGY FORANALYSIS

IV. M ETHODOLOGY CHECKLIST

A. Appearance of Motor and System

As the second part of the methodology, this section will serveas a check-off list to be used to gather critical information per-taining to the appearance, application, and maintenance historyof the motor and other related equipment Some of these ques-tions overlap.

When coupled with the class and pattern of failure, the gen-eral motor appearance usually gives a clue as to the possiblecause of failure. The following checklist will be useful in eval-uating assembly conditions.

BONNETT: ROOT CAUSE AC MOTOR FAILURE ANALYSIS 1437

Fig. 1. Typical motor system including the power supply and driven equipment.

• Does the motor exhibit any foreign material?• Are there any signs of blocked ventilation passages?• Are there signs of overheating exhibited by insulation,

lamination, bars, bearings, lubricant, painted surfaces,etc.?

• Has the rotor lamination or shaft rubbed? Record all loca-tions of rotor and stator contact.

• Are the topsticks, coils, or coil bracing loose?• Are the rotor cooling passages free and clear of clogging

debris?• What is the physical location of the winding failure? Is it

on the connection end or opposite connection end? If themotor is mounted horizontally, where is the failure withrespect to the clock? Which phase or phases failed? Whichgroup of coils failed? Was the failure in the first turn or firstcoil?

• Are the bearings free to rotate and operate as intended?• Is there any sign of moisture present on the stator, rotating

assembly, bearing system, or any other parts?• Are there any signs of movement between rotor and shaft

or bar and lamination?• Is the lubrication system as intended or has there been

lubricant leakage or deterioration?• Are there any signs of stalled or locked rotor?• Was the rotor turning during the failure?• What was the direction of rotation and does it agree with

the fan arrangement?

• Are any mechanical parts missing, such as balanceweights, bolts, rotor teeth, fan blades, etc., or has anycontact occurred between rotating parts that shouldmaintain a clearance?

• What is the condition of the coupling device, driven equip-ment, mounting base, and other related equipment?

• What is the condition of the bearing bore, shaft journal,seals, shaft extension, keyways, and bearing caps.

• Is the motor mounted, aligned, and coupled correctly?• Is the ambient usual or unusual?• Do the stress risers show signs of weakness or cracking?

(The driven end shaft keyway is a weak link.)

When analyzing motor failures, it is helpful to draw a sketchof the motor and indicate the point where the failure occurred,as well as the relationship of the failures to both the rotating andstationary parts, such as shaft keyway, etc. A picture is worth athousand words.

B. Application Considerations

Usually, it is difficult to reconstruct conditions at the time offailure. However, a knowledge of the general operating condi-tions will be helpful. The following items should be considered.

• What are the load characteristics of the driven equipmentand the loading at time of failure?

• What is the operating sequence during starting?• Does the load cycle or pulsate?


• What is the voltage during starting and operation? Is therea potential for transients? Was the voltage balanced be-tween phases?

• How long does it take for the unit to accelerate to speed?• Have any other motors or equipment failed on this appli-

cation?• How many other units are successfully running?• How long has the unit been in service?• Did the unit fail on starting or while operating?• How often is the unit starting, and is this a manual or au-

tomatic operation? Is it part winding, wye-delta, or vari-able-frequency drive (VFD), or across the line?

• What type of protection is provided?• What removed or tripped the unit from the line?• Where is the unit located and what are the normal envi-

ronmental conditions? What was the environment at timeof failure?

• What was the ambient temperature, at time of failure,around the motor? Is there any recirculation of air? Is theexchange of cooling air adequate?

• Was power supplied by a VFD? What is the distance be-tween the VFD and the motor?

• How would you describe the driven load method of cou-pling and mounting?

C. Maintenance History

An understanding of the past performance of the motor cangive a good indication as to the cause of the problem. Again, achecklist may be helpful.

• How long has the motor been in service?• Have any other motor failures been recorded and what

was the nature of the failures? What failures of the drivenequipment have occurred? Was any welding done?

• When was the last time any service or maintenance wasperformed?

• What operating levels (temperature, vibration, noise, insu-lation, resistance, etc.) were observed prior to the failure?

• What comments were received from the equipment oper-ator regarding the failure or past failures?

• How long was the unit in storage or sitting idle prior tostarting?

• What were the storage conditions?• How often is the unit started? Were there shutdowns?• Were correct lubrication procedures utilized?• Have there been any changes made to surrounding equip-

ment?• What procedures were used in adjusting belt tensions?• Are the pulleys positioned on the shaft correctly and as

close to the motor bearing as possible?

V. CAUSE, ANALYSIS, AND PREVENTION OFMOTOR SHAFT

FAILURES

A. Introduction

In Sections II and III of this paper, the various stresses actingon a motor shaft were covered along with a proposed method-ology for determining the root cause failure. This section dealswith the various causes of shaft failures, which is a subject not

Fig. 2. Typical motor shaft configurations. From top to bottom: large motorspider shaft; vertical motor hollow shaft for pumps; totally enclosed fan-cooledshaft; open dripproof shaft; close-coupled shaft for pumps; and splined or gearedtake-off shaft.

covered in the author’s previous papers for the Petroleum andChemical industry Committee of the IEEE Industry Applica-tions Society (PCIC).

Fig. 2 shows a variety of different rotor shafts used in typicalelectric motors.

B. Motor Shaft Materials

For most motor applications, hot rolled carbon steel is a goodchoice. When higher loads are present, an alloyed steel suchas chromium–molybenum (Cr–Mo) is frequently used, and forapplications with extreme corrosion or hostile environment astainless steel material is required. Table VII shows some of themost common steels and their characteristics.

C. The Tools of Shaft Failure Analysis

The ability to properly characterize the microstructure and thesurface topology of a failed shaft are critical steps in analyzingfailures. The most common tools available to do this can becategorized as follows:

• visual inspection;• optical microscope;• scanning electron microscope;• transmission electron microscope;• metallurgical analysis.

This paper assumes that it may be necessary to employ theservices of a metallurgical laboratory to obtain some of the re-quired information. However, it is the author’s experience that


TABLE VIICOMMON STEELS AND THEIR CHARACTERISTICS

TABLE VIIICAUSES OFSHAFT FAILURES (ADAPTED FROM [18]

a significant number of failures can be diagnosed with a funda-mental knowledge of motor shaft failure causes and visual in-spection.

D. Causes of Failure

Studies have been conducted to try to quantify the causes ofshaft failures. One industry study provided the results for ro-tating machines shown in Table VIII.

There are other informal studies that suggest that fa-tigue-caused failures are much higher. For motor applications,it climbs into the 90% range when the effects of corrosion andnew stress raisers are considered. Hence, the main focus of thispaper will be failures associated with fatigue.

E. Stress Systems Acting on Shafts

A clear understanding of shaft loading is necessary beforecauses of shaft failure can be determined.

F. Typical Motor Shaft Loading

The following three cases (Figs. 3–5), provide the mostcommon types of motor shaft loading that can lead to fa-tigue-type failures.

G. Areas of Highest Concentration

Fig. 6 illustrates areas on a normal motor shaft where de-sign stress concentrations (raisers) will exist. Wherever thereis a surface discontinuity, such as bearing shoulders, snap ringgrooves, keyways, shaft threads, or holes, a stress raiser willexist. Shaft damage or corrosion can also create stress raisers.Fatigue cracks and failure will usually occur in these regions.For motors, the two most common places are at the shoulder onthe bearing journal (pointH) or in the coupling keyway region(pointJ). The most common area for shaft damage is on the part

Fig. 3. Overhung load. Failure mode: bending fatigue and shaft rub. The forcemay be in any direction of the 360.

Fig. 4. Axial load. Failure mode: bearing failure.

Fig. 5. Torsional load. Failure mode: torsional failure.

Fig. 6. Typical rotor assembly cross section. All of the highlighted areas createstress raisers. PointsF, H, I, andJ are usually the most vulnerable areas becauseof the shaft load at these points. A shaft is unlikely to fracture at pointsA, B, C,D, or E.

of the shaft from pointH–K. Although in most cases where anaxial load will result first is in a bearing failure, there are nu-merous examples where the shaft is damaged before the motoris stopped.

H. Shaft Keyways

Keyways are commonly used to secure fans, rotor cores, andcouplings to the shaft. All of these cause stress raisers. However,the keyway on the take-off end or driven end of the shaft isthe one of most concern because it is located in the area wherethe highest shaft loading occurs. When this loading has a hightorsional component, fatigue cracks usually start in the fillets orroots of the keyway.

Keyways that end with a sharp step have a higher level ofstress concentration than those that use a “sled-runner” type ofkeyway. In the case of heavy shaft loading, cracks frequentlyemanate from this sharp step. Fig. 7 illustrates this type offailure. It is important to have an adequate radius on the edgesof the keyway.


Fig. 7. Peeling-type cracks in shafts usually originate at the keyway.

TABLE IXCOMMON CAUSES OFSHAFT FAILURES FORMOTORS

I. Failure Mode

As stated previously, for motor shafts, 90% of all failures canbe placed into the fatigue modes shown in Table IX. If the shaftis not designed, manufactured, applied, or used properly, a pre-mature failure can occur with any of the failure modes.

The shaft fatigue failures can be classified as bending fa-tigue, torsional fatigue, and axial fatigue. In the case of axial fa-tigue for motors, the bearing carrying the load will fatigue (con-tact fatigue) before the shaft does. This is usually evidenced byspalling of the bearing raceways. In the bending mode, almostall failures are considered “rotational” with the stress fluctuatingor alternating between tension and compression. This is a cy-cling condition that is a function of the shaft speed. Torsionalfatigue is associated with the amount of shaft torque present andtransmitted load.

Understanding fatigue strength and endurance limits is im-portant because most shaft failures are related to fatigue associ-ated with cyclic loading. These limits are expressed by an S–Ndiagram, as shown in Fig. 8.

For steel, these plots become horizontal after a certain numberof cycles. In this case, a failure will not occur as long as the stressis below 27 klbf/in , no matter how many cycles are applied.However, at 10cycles, the shaft will fail if the load is increasedto 40 bf/in . The horizontal line in Fig. 8 is known as the fatigueor endurance limit. For the types of steels commonly used formotors, good design practice dictates staying well below thelimit. Problems arise when the applied load exceeds its limitsor there is damage to the shaft that causes a stress raiser.

J. Defining the Fatigue Process

Fatigue fractures or damage occur in repeated cyclic stresses,each of which can be below the yield strength of the shaft ma-terial. Usually, as the fatigue cracks progress, they create whatis known as ratchet marks.

Fig. 8. S–N diagram for 1040 steel.

Fig. 9. Surface of a fatigue fracture displaying two distinct regions.

The failure process consists of the following.

• The fatigue leads to an initial crack on the surface of thepart.

• The crack or cracks propagate until the remaining shaftcross section is too weak to carry the load.

• A sudden fracture of the remaining area occurs.Fatigue-type failures usually follow the weak-link theory.

That is, the cracks form at the point of maximum stress orminimum strength. This is usually at a shaft discontinuitybetween the edge of the rotor core shaft step and the shaftcoupling.

K. Appearance of Fatigue Fractures

The appearance of the shaft is influenced by various types ofcracks, beach marks, conchoidal marks, radial marks, chevronmarks, ratchet marks, cup and cone shapes, shear lip, and awhole host of other topologies [17]. Some of the most commonones associated with motor shafts that have failed are due to ro-tational, bending fatigue. The surface of a fatigue fracture willusually display two distinct regions as shown in Fig. 9. RegionA includes the point of origin of the failure and evolves at a rel-atively slow rate (seconds through years) depending on the run-ning and starting cycle and of course the load. RegionB is theinstantaneous or rapid growth area (cycles through seconds) andexhibits very little plastic deformation. The shape and spacing


Fig. 10. View of slow growth and instantaneous regions.

Fig. 11. Initiation sites originated at the root of the keyway.

of the conchoidal mark may assist the “trained eye” to determinehow the load is varying or the degree of mechanical unbalance.

In Fig. 10, both the slow growth region and instantaneousregions can be seen. This shaft fractured at the snap ring groovewhich is a high stress raiser area. Note the presence of ratchetmarks on the periphery of the shaft; they point to the origin ofthe cracks.

In Fig. 11, the initiation sites originated at the root of thekeyway. Both the slow and instantaneous areas are present.

L. Surface Finish Effects

In most applications, the maximum shaft stress occurs on thesurface. Hence, the surface finish can have a significant impacton fatigue life. During the manufacturing process and subse-quent handling, repairs must not produce a surface finish coarserthan intended by the design. The impact of surface finish and fa-tigue life in cycles can be seen in Table X.

TABLE XSURFACE FINISH AND FATIGUE LIFE IN CYCLES (ADAPTED FROM[19])

M. Corrosion Failures

In corrosion failures, the stress is the environment and thereaction it has on the shaft material. At the core of this problemis an electrochemical reaction that weakens the shaft. Pitting isone of the most common types of corrosion, which is usuallyconfined to a number of small cavities on the shaft surface. Onlya small amount of material loss can result in perforation, with aresulting failure in a relatively short period of time without anyadvanced warning. On occasion, the pitting has caused stressraisers that result in fatigue cracks.

N. Residual Stress Failures

These stresses are independent of external loading on theshaft. Many manufacturing or repair operations can affect theamount of residual stress, including:

• drawing;• bending;• straightening;• machining;• grinding;• surface rolling;• shot blasting or peening;• polishing.

All of these operations can produce residual stresses byplastic deformation. In addition to the above mechanical pro-cesses, thermal processes that introduce residual stress include:

• hot rolling;• welding;• torch cutting;• heat treating.

All residual stress may not be detrimental; if the stress is par-allel to the load stress and in an opposite direction, it may be ben-eficial. Proper heat treatment can reduce these stresses if theyare of excessive levels.

O. Shaft Fretting

Shaft fretting can cause serious damage to the shaft and themating part. Typical locations are points on the shaft where a“press” or “slip” fit exists. Keyed hubs, bearings, couplings,shaft sleeves, and splines are examples. Taper fits seem to bean exception to this rule and experience little or no frettingThe presence of ferric oxide (rust) between the mating surfaces,which is reddish brown in color, is strong confirmation that fret-ting did occur. The cause of this condition is some amount ofmovement between the two mating parts. Fatigue cracking may


Fig. 12. Shaft fatigue.

be initialed by the presence of fretting. Uncorrected shaft vibra-tion can also worsen this condition.

P. Surface Coating [17]

Metallic coatings to protect or restore a shaft can causeharmful residual stresses which can reduce the fatigue strengthof the base metal. In most cases, there are enough safetyfactors to handle this additional stress. However, if the shaftis being stressed to its design limits, then such processes aselectroplating, metal spray, or catalytic deposition could be asource of fatigue failures.

During some plating processes, it is possible to introduce hy-drogen into the base metal. If it is not removed by the appro-priate heat treatment process, severe hydrogen embrittlementmay occur, which can greatly reduce the tensile strength of theshaft.

The repair of shafts by welding is beyond the scope of thispaper. However, caution must be used in this process. The se-lection of the proper weld material, method of application, stressrelieving, surface finish, and diameter transition are all criticalto a successful repair. Not all shaft materials are good candidatesfor welding-type repairs.

Q. Miscellaneous Nonfracture-Type Shaft Failures

There is a broad category of shaft failures or motor failuresthat does not result in the shaft breaking. The following is alist of the more common causes (it is acknowledged that fatiguefailures that are caught in the early stages would also fit in thenonfracture category):

• bending or deflection causing interference with stationaryparts;

• incorrect shaft size causing interference, run out, or incor-rect fits;

Fig. 13. Failure caused by rotational bending.

Fig. 14. Shaft fatigue.

Fig. 15. Failure caused by rotational bending.

• residual stress causing a change in shaft geometry;• material problems;• excessive corrosion and wear;


Fig. 16. Failure due to impact loading.

Fig. 17. Twist caused by impact loading.

Fig. 18. Failure due to reverse torsional loading.

• excessive vibration caused by electrical or mechanical im-balance.

Catastrophic bearing failures can cause serious shaft damage,but seldom result in a fracture.

R. Prevention

Several practices will minimize the probability of a prematureshaft failure. The following is a list of some of the more criticalsteps.

1) Be sure that the application and the possible loading onthe motor are well understood and communicated. It isimperative to know if there is an overhung load. The en-vironmental conditions are also critical.

Fig. 19. Failed due to rotational bending fatigue.

Fig. 20. Failed due to rotational bending fatigue.

Fig. 21. Extreme corrosion wear.

2) The motor manufacturer must be sure that proper mate-rials are selected. For the most part, steel with the prop-erties of hot rolled 1045 steel is adequate.

3) The manufacturing processes are critical. During the pro-cessing of the shaft, care must be taken not to introducestress raisers and to achieve the required shaft finish.

4) The installation phase and operation phases are alsocritical. Care must be taken not to damage the shaftwhen coupling it to the driven equipment. For belt-driven


Fig. 22. Smear marks on roller caused by debris.

Fig. 23. Metallic contamination in raceway.

loads, remember themomentprinciple (force distance)in placement of the pulley.

VI. SUMMARY AND CONCLUSIONS

All too often when a motor fails, the major and sometimesonly focus is the repair or replacement and get it “up and runningagain.” Without diminishing the importance of this goal, timeshould be spent collecting valuable information that will assistin a root cause analysis. This paper, along with the previouspapers will provide the reader with the methodology to conductan analysis that will properly identify failures and hopefully takethe necessary steps to eliminate them.

This proposed methodology will yield the best results whenthe motor under analysis has been “bench marked” along withthe system at the time of installation or restart. Also, informationcollected during a normal or abnormal operating cycle can be ofgreat value. Of course, it may not be practical to have this kindof information on all plant systems; it might be wise to collect

Fig. 24. Damaged caused by water intrusion.

Fig. 25. Fretting corrosion caused by loss fit and vibration.

Fig. 26. Pitting caused by electrical currents.

it going forward on critical applications or applications knownto have experienced difficulties in the past. Another point toconsider is that, when a motor goes down, the normal tendencyis to quickly remove the motor and install a new one or get thedamaged motor to the repair shop and back into operation. Nothought is given to collecting information that may be helpfulfor an accurate analysis prior to the tear-down. Investing a fewextra minutes before shutdown or removal may yield criticalinformation for the eventual analysis. It is recognized that thisoption is not always available, but when it is utilize it.

Appendixes A–D provide the reader with descriptions of pho-tographs of the most common types of motor failures to be usedin conjunction with the prescribed methodology of this paper.They will be most useful in identifying the failure mode andpattern.


Fig. 27. Fluting caused by internally generated current.

Fig. 28. Advanced stages of spalling.

Fig. 29. False brinelling and fretting caused by vibration in a nonoperatingcondition.

APPENDIX ACOMMON SHAFT FAILURES

Figs. 12 and 13 are of a 1045 carbon steel motor shaft thatfailed due to rotational bending fatigue. The point of failure wasat the shoulder of the customer take-off end

Figs. 14 and 15 are of a 1040 carbon steel motor shaft thatfailed due to rotational bending fatigue. The point of failure wasat the bearing journal shoulder.

Figs. 16 and 17 are shafts that failed due to high-impactloading. The material is 1045 carbon steel, which has goodductility, which allowed for the severe twisting.

Fig. 30. Turn-to-turn shorting.

Fig. 31. Single-phase turn-to-turn shorting.

Fig. 32. Stator cross section where shorting can occur.

Fig. 18 is a shaft material that is unknown, but possibly 4100high tensile steel alloy. The failure is a fatigue failure due toreversed torsional loading.

Figs. 19 and 20 are of a 1051 carbon steel turbine shaft thatfailed due to rotational bending fatigue. There were also signsof minor torsional fatigue. Cracks initiated at the toe of a cou-pling weld. This material has poor weldability characteristics.There were also signs of misalignment. Note the surface pit andgrinding marks; both of these conditions can weaken the shaft.

Fig. 21 is an example of extreme corrosion, wear; andcracking on a pump shaft; the material is unknown.


Fig. 33. Nonsymmetrical shorting without grounding.

Fig. 34. Nonsymmetrical shorting with grounding.

Fig. 35. Same stator as Fig. 34 at point of grounding.

APPENDIX BCOMMON TYPES OFBEARING FAILURES

These include contamination (Figs. 22–24), defectivefits/seats(Fig. 25,shaft currents(Figs. 26 and 27),fatigue(Fig.28), andmechanicalfailure (Fig. 29).

APPENDIX CCOMMON TYPES OFSTATOR FAILURES

In Fig. 30, the pattern is symmetrical; each coil of each phasehas been overheated. The failure mode is a multiple turn-to-turn

Fig. 36. Typical cast air-ducted rotor; any damage to the fans, end rings, or airducts can cause overheating and damage to the cage.

Fig. 37. Overheated aluminum cast rotor end ring.

Fig. 38. Incomplete rotor bars on aluminum cast rotor.

shorting. The cause of failure was excessive overheating causedby an overload condition.

In Fig. 31, the pattern is single phasing; one complete phasehas over heated and failed due to turn-to-turn shorting. Thecause of failure was single phasing.

In Fig. 32 is a cross section of a typical stator slot. It is notuncommon to have turn-to-turn shorts in this region.

In Fig. 33, the pattern is nonsymmetrical without grounding;several groups of coils have been overheated. The failure modeis also multiple turn-to-turn shorting. The cause of failure wasdamaged wire.


Fig. 39. Typical aluminum squirrel cage without the lamination. Any damageto the cage will affect the motor performance.

Fig. 40. Overheated aluminum fabricated rotor bars.

Fig. 41. Broken and loose aluminum fabricated rotor bar.

In Fig. 34, the pattern is nonsymmetrical with grounding; onecoil is grounded and there is multiple turn-to-turn shorting. Thecause of failure was damaged cell wall.

Fig. 35 is the same stator shown in Fig. 34. The actual groundfault can be seen. Note that the turn-to-turn shorting occurred180 opposite the grounded coil.

APPENDIX DCOMMON TYPES OFROTOR FAILURES

See Figs. 36–41.

REFERENCES

[1] R. J. Nailen, “Stop rotor troubles before they start,”Plant Eng., Dec.1966.

[2] G. C. Soukup, “Design of large induction machinery using fabricatedaluminum rotor cages,” M.S. thesis, Univ. Wisconsin, Milwaukee, Dec.1974.

[3] R. L. Nailen, “The cause of rotor pullover—And how to cure theproblem,”Elect. App., Nov. 1980.

[4] E. F. Merrill and C. R. Olson, “Sparking of A-C motor rotors and itseffect on division 2 application,” presented at the IEEE PIC, Aug. 24,1959.

[5] J. L. Craggs, “Fabricated aluminum cage construction in large inductionmotors,” presented at the IEEE PCIC, Sept. 1975, Paper PCIC-75-8.

[6] P. G. Cummings, J. R. Dunki-Jacobs, and R. H. Kerr, “Protection ofinduction motors against unbalanced voltage operation,” presented atthe IEEE PCIC, Sept. 1983, PCIC-83-3.

[7] R. L. Nailen, “What high torque? Consider the double cage motor,”Power Eng., Apr. 1971.

[8] , “New concept in rotor bar shape solves pipeline motor acceler-ation problem,” presented at the 1972 IEEE Summer Power Meeting,Paper 72-CP527-PWR.

[9] A. H. Bonnett, “A comparison between insulation systems available forPWM inverter FED motors,” presented at the IEEE PCIC, Sept. 1996,Paper PCIC-96-7.

[10] J. F. Calvert, “Forces in turbine generator stator windings,”AIEE Trans.,vol. 50, pp. 178–196, 1931.

[11] A. H. Bonnett, “The cause of winding failures in three phase squirrelcage induction motors,” presented at the IEEE PCIC, Sept. 1976, PaperPCIC-76-7.

[12] , “Analysis of winding failures in three phase squirrel cage induc-tion motors,” presented at the IEEE PCIC, Sept. 1977, Paper PCIC-77-4.

[13][14] A. H. Bonnett and G. C. Soukup, “Rotor failures in squirrel cage

induction motors,” presented at the IEEE PCIC, Sept. 1985, PaperPCIC-85-24.

[15] , “Analysis of rotor failures in squirrel cage induction motors,” pre-sented at the IEEE PCIC, Sept. 1987, Paper PCIC-87-2.

[16] , “The causes and analysis of stator and rotor failures in A.C. ma-chines,” in Proc. Maintenance and Reliability Conf., Knoxville, TN,May 20, 1997, p. 29.01.

[17] Metals Handbook—Volume 10: Failure Analysis and Prevention, 8thed., American Society for Metals, Metals Park, OH, 1966.

[18] C. R. Brooks and A. Choudhury,Metallurgical Failure Analysis. NewYork: McGraw-Hill, 1993.

[19] V. J. Colangelo and F. A. Heiser,Analysis of Metallurgical Fail-ures. New York: Wiley, 1974.

[20] A. Das, Metallurgy of Failure Analysis. New York: McGraw-Hill,1996.

[21] N. Sachs, “Failure analysis of mechanical components,”MaintenanceTechnol., Sept. 1993.

[22] C. Y. P. Qiao and C. S. Wang, “A taxonomic study of fractograph as-sisted engineering materials failure analysis,” inProc. Maintenance andReliability Conf., Knoxville, TN, May 20–22, 1997, p. 501.

[23] O. V. Thorsen and M. Dalva, “A survey of faults on induction motors inoffshore oil industry, petrochemical industry, gas terminals, and oil re-fineries,”IEEE Trans. Ind. Applicat., vol. 31, pp. 1186–1196, Sept./Oct.1995.

[24] J. C. Berren, “Diagnosing faults in rolling element bearings—Part II:Alternative analytical methods,”Vib., vol. 4, no. 2, June 1998.

[25] “Bearing failure analysis and preventive maintenance,” Bearing Divi-sion, NSK Corp., Ann Arbor, MI, ca. 1993.

[26] “Bearing failure and their causes,” SKF Catalog, Gothenburg, Sweden,Form 310M, 10000-11-75GP, 1974.

[27] A. H. Bonnett, “Cause and analysis of anti-friction bearing failures inA.C. induction motors,”IEEE Ind. Applicat. Soc. Newslett., Sept./Oct.1993.


Austin H. Bonnett (M’68–SM’90–F’92) was born inLos Angeles, CA, in 1936. He received the B.S. de-gree in electrical engineering from California StateUniversity, Los Angeles, and the Master’s degree inbusiness from the University of Phoenix, Phoenix,AZ.

He served in the U.S. Navy from 1955 to 1958as an Electrician aboard the Icebreaker, Burton Is-land. He joined U.S. Electrical Motors, a Division ofEmerson Electric Company, in 1963 and has held po-sitions in the Service, Manufacturing, Quality Con-

trol, and Engineering Departments. He was the Plant Manager of the Prescott,AZ, facility for five years prior to holding the position of Vice President of Engi-neering, directing all U.S. Electrical Motors engineering functions for ten years.Presently, he holds the position of Vice President-Technology Emeritus at theEmerson Motor Technology Center, St. Louis, MO. He has published numeroustechnical papers on rotating machinery. He serves on NEMA, EPRI, and DOECommittees.

Mr. Bonnett received the 1994 IEEE Meritorious Award. In 1996, he wasselected for the IEEE Industry Applications Society Outstanding AchievementAward. He has also served on various IEEE Committees. He received the 1999NEMA Kite and Key Award for outstanding service to the electrical industry.

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