Shaft Root Cause Shaft Failure 5d3s6c

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Root Cause AC Motor Failure Analysis with a Focus on Shaft Failures Austin H. Bonnett, Fellow, IEEE

TABLE I STATOR STRESSES [12]

Abstract—The squirrel-cage induction motor remains the workhorse of the petrochemical industry because of its versatility and ruggedness. However, it has its limitations, which if exceeded will cause premature failure of the stator, rotor, bearings or shaft. This paper is the final abridgement and update of six previous papers for the Petroleum and Chemical Industry Committee of the IEEE Industry Applications Society presented over the last 24 years and includes the final piece dealing with shaft failures. A methodology is provided that will lead operations personnel to the most likely root causes of failure. Check-off sheets are provided to assist in the orderly collection of data to assist in the analysis. As the petrochemical industry evolves from reactive to time based, to preventive, to trending, to diagnostics, and to a predictive maintenance attitude, more and more attention to root cause analysis will be required. This paper will help provide a platform for the establishment of such an evolution. The product scope includes low- and medium-voltage squirrel-cage induction motors in the 1–3000–hp range with anti friction bearings. However, much of this material is applicable to other types and sizes.

TABLE II ROTOR ASSEMBLY STRESSES [14]

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

I. INTRODUCTION METHODOLOGY is provided that will lead operations personnel to the most likely root causes of failure. Check-off sheets are provided to assist in the orderly collection of data to assist in the analysis. This paper is organized as follows. In Section II, a summary of motor stresses is given, followed by root cause methodology in Section III. Section IV contains a methodology checklist, Section V discusses shaft failures, and concluding remarks are given in Section VI. Finally, Appendixes A–D contain photographs of the common failures.

A

II. SUMMARY OF MOTOR STRESSES The majority of all motor failures are caused by a combination of various stresses acting upon the winding, rotor, bearings, and shaft. If these stresses are kept within the design capabilities of the system, premature failure should not occur. However, if any combination of them exceeds the design capacity, then the life Paper PID 00–03, presented at the 1999 IEEE Petroleum and Chemical Industry Technical Conference, San Diego, CA, September 13–15, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Petroleum and Chemical Industry Committee of the IEEE Industry Applications Society. Manuscript submitted for review September 15, 1999 and released for publication 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.

may be drastically diminished and a catastrophic failure could occur. These stresses can be broken down into the groups or classifications 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 various motor components, there are five key areas which should

0093–9994/00$10.00 © 2000 IEEE

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TABLE III BEARING STRESSES [27]

TABLE V MOTOR COMPONENT/STRESSES

TABLE IV SHAFT STRESSES

TABLE VI SUMMARY OF METHODOLOGY FOR ANALYSIS

be considered and related to one another in order to accurately diagnose 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 respect to the stator, rotor, bearings, and shaft. A. Methodology Combining all of these stresses leads to a methodology that falls into two categories. The first deals with failure modes/classes and failure patterns, as shown in Table VI, which can serve as a check-off sheet when conducting an inspection of the failed motor. The second category deals with the motor appearance, application, and maintenance history. This will be covered in Section IV of the paper. B. The Motor and System Although a complete system analysis is beyond the scope of this paper, when conducting a root cause failure analysis, it is important to recognize that the motor is only one component of a system. Many factors affecting the system will also affect the motor and may contribute to the motor failure and vice versa. Fig. 1 shows a typical system. Note it also includes the process requirement.

IV. METHODOLOGY CHECKLIST A. Appearance of Motor and System As the second part of the methodology, this section will serve as a check-off list to be used to gather critical information pertaining to the appearance, application, and maintenance history of the motor and other related equipment Some of these questions overlap. When coupled with the class and pattern of failure, the general motor appearance usually gives a clue as to the possible cause of failure. The following checklist will be useful in evaluating assembly conditions.

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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 ages? • Are there signs of overheating exhibited by insulation, lamination, bars, bearings, lubricant, painted surfaces, etc.? • Has the rotor lamination or shaft rubbed? Record all locations of rotor and stator . • Are the topsticks, coils, or coil bracing loose? • Are the rotor cooling ages 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 the motor is mounted horizontally, where is the failure with respect to the clock? Which phase or phases failed? Which group of coils failed? Was the failure in the first turn or first coil? • 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 balance weights, bolts, rotor teeth, fan blades, etc., or has any occurred between rotating parts that should maintain a clearance? • What is the condition of the coupling device, driven equipment, 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 sketch of the motor and indicate the point where the failure occurred, as well as the relationship of the failures to both the rotating and stationary parts, such as shaft keyway, etc. A picture is worth a thousand words. B. Application Considerations Usually, it is difficult to reconstruct conditions at the time of failure. However, a knowledge of the general operating conditions will be helpful. The following items should be considered. • What are the load characteristics of the driven equipment and the loading at time of failure? • What is the operating sequence during starting? • Does the load cycle or pulsate?

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• What is the voltage during starting and operation? Is there a potential for transients? Was the voltage balanced between phases? • How long does it take for the unit to accelerate to speed? • Have any other motors or equipment failed on this application? • 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 automatic operation? Is it part winding, wye-delta, or variable-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 environmental conditions? What was the environment at time of failure? • What was the ambient temperature, at time of failure, around the motor? Is there any recirculation of air? Is the exchange of cooling air adequate? • Was power supplied by a VFD? What is the distance between the VFD and the motor? • How would you describe the driven load method of coupling and mounting? C. Maintenance History An understanding of the past performance of the motor can give a good indication as to the cause of the problem. Again, a checklist 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 driven equipment have occurred? Was any welding done? • When was the last time any service or maintenance was performed? • What operating levels (temperature, vibration, noise, insulation, resistance, etc.) were observed prior to the failure? • What comments were received from the equipment operator regarding the failure or past failures? • How long was the unit in storage or sitting idle prior to starting? • 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 equipment? • 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 OF MOTOR SHAFT FAILURES A. Introduction In Sections II and III of this paper, the various stresses acting on a motor shaft were covered along with a proposed methodology for determining the root cause failure. This section deals with the various causes of shaft failures, which is a subject not

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

covered in the author’s previous papers for the Petroleum and Chemical industry Committee of the IEEE Industry Applications Society (PCIC). Fig. 2 shows a variety of different rotor shafts used in typical electric motors. B. Motor Shaft Materials For most motor applications, hot rolled carbon steel is a good choice. When higher loads are present, an alloyed steel such as chromium–molybenum (Cr–Mo) is frequently used, and for applications with extreme corrosion or hostile environment a stainless steel material is required. Table VII shows some of the most common steels and their characteristics. C. The Tools of Shaft Failure Analysis The ability to properly characterize the microstructure and the surface topology of a failed shaft are critical steps in analyzing failures. The most common tools available to do this can be categorized as follows: • visual inspection; • optical microscope; • scanning electron microscope; • transmission electron microscope; • metallurgical analysis. This paper assumes that it may be necessary to employ the services of a metallurgical laboratory to obtain some of the required information. However, it is the author’s experience that

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TABLE VII COMMON STEELS AND THEIR CHARACTERISTICS

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

TABLE VIII CAUSES OF SHAFT FAILURES (ADAPTED FROM [18]

Fig. 4.

a significant number of failures can be diagnosed with a fundamental knowledge of motor shaft failure causes and visual inspection.

Axial load. Failure mode: bearing failure.

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

D. Causes of Failure Studies have been conducted to try to quantify the causes of shaft failures. One industry study provided the results for rotating machines shown in Table VIII. There are other informal studies that suggest that fatigue-caused failures are much higher. For motor applications, it climbs into the 90% range when the effects of corrosion and new stress raisers are considered. Hence, the main focus of this paper will be failures associated with fatigue.

Fig. 6. Typical rotor assembly cross section. All of the highlighted areas create stress raisers. Points F, H, I, and J are usually the most vulnerable areas because of the shaft load at these points. A shaft is unlikely to fracture at points A, B, C, D, or E.

E. Stress Systems Acting on Shafts A clear understanding of shaft loading is necessary before causes of shaft failure can be determined. F. Typical Motor Shaft Loading The following three cases (Figs. 3–5), provide the most common types of motor shaft loading that can lead to fatigue-type failures. G. Areas of Highest Concentration Fig. 6 illustrates areas on a normal motor shaft where design stress concentrations (raisers) will exist. Wherever there is a surface discontinuity, such as bearing shoulders, snap ring grooves, keyways, shaft threads, or holes, a stress raiser will exist. 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 on the bearing journal (point H) or in the coupling keyway region (point J). The most common area for shaft damage is on the part

of the shaft from point H–K. Although in most cases where an axial load will result first is in a bearing failure, there are numerous examples where the shaft is damaged before the motor is stopped. H. Shaft Keyways Keyways are commonly used to secure fans, rotor cores, and couplings to the shaft. All of these cause stress raisers. However, the keyway on the take-off end or driven end of the shaft is the one of most concern because it is located in the area where the highest shaft loading occurs. When this loading has a high torsional component, fatigue cracks usually start in the fillets or roots of the keyway. Keyways that end with a sharp step have a higher level of stress concentration than those that use a “sled-runner” type of keyway. In the case of heavy shaft loading, cracks frequently emanate from this sharp step. Fig. 7 illustrates this type of failure. It is important to have an adequate radius on the edges of the keyway.

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

Fig. 8. S–N diagram for 1040 steel.

TABLE IX COMMON CAUSES OF SHAFT FAILURES FOR MOTORS

I. Failure Mode As stated previously, for motor shafts, 90% of all failures can be placed into the fatigue modes shown in Table IX. If the shaft is not designed, manufactured, applied, or used properly, a premature failure can occur with any of the failure modes. The shaft fatigue failures can be classified as bending fatigue, torsional fatigue, and axial fatigue. In the case of axial fatigue for motors, the bearing carrying the load will fatigue ( fatigue) before the shaft does. This is usually evidenced by spalling of the bearing raceways. In the bending mode, almost all failures are considered “rotational” with the stress fluctuating or alternating between tension and compression. This is a cycling condition that is a function of the shaft speed. Torsional fatigue is associated with the amount of shaft torque present and transmitted load. Understanding fatigue strength and endurance limits is important because most shaft failures are related to fatigue associated with cyclic loading. These limits are expressed by an S–N diagram, as shown in Fig. 8. For steel, these plots become horizontal after a certain number of cycles. In this case, a failure will not occur as long as the stress is below 27 klbf/in , no matter how many cycles are applied. However, at 10 cycles, the shaft will fail if the load is increased to 40 bf/in . The horizontal line in Fig. 8 is known as the fatigue or endurance limit. For the types of steels commonly used for motors, good design practice dictates staying well below the limit. Problems arise when the applied load exceeds its limits or 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 material. Usually, as the fatigue cracks progress, they create what is known as ratchet marks.

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 the part. • The crack or cracks propagate until the remaining shaft cross 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 or minimum strength. This is usually at a shaft discontinuity between the edge of the rotor core shaft step and the shaft coupling. K. Appearance of Fatigue Fractures The appearance of the shaft is influenced by various types of cracks, beach marks, conchoidal marks, radial marks, chevron marks, ratchet marks, cup and cone shapes, shear lip, and a whole host of other topologies [17]. Some of the most common ones associated with motor shafts that have failed are due to rotational, bending fatigue. The surface of a fatigue fracture will usually display two distinct regions as shown in Fig. 9. Region A includes the point of origin of the failure and evolves at a relatively slow rate (seconds through years) depending on the running and starting cycle and of course the load. Region B is the instantaneous or rapid growth area (cycles through seconds) and exhibits very little plastic deformation. The shape and spacing

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

M. Corrosion Failures

Fig. 10.

View of slow growth and instantaneous regions.

In corrosion failures, the stress is the environment and the reaction it has on the shaft material. At the core of this problem is an electrochemical reaction that weakens the shaft. Pitting is one of the most common types of corrosion, which is usually confined to a number of small cavities on the shaft surface. Only a small amount of material loss can result in perforation, with a resulting failure in a relatively short period of time without any advanced warning. On occasion, the pitting has caused stress raisers that result in fatigue cracks. N. Residual Stress Failures

Fig. 11.

Initiation sites originated at the root of the keyway.

of the conchoidal mark may assist the “trained eye” to determine how the load is varying or the degree of mechanical unbalance. In Fig. 10, both the slow growth region and instantaneous regions can be seen. This shaft fractured at the snap ring groove which is a high stress raiser area. Note the presence of ratchet marks on the periphery of the shaft; they point to the origin of the cracks. In Fig. 11, the initiation sites originated at the root of the keyway. Both the slow and instantaneous areas are present. L. Surface Finish Effects In most applications, the maximum shaft stress occurs on the surface. Hence, the surface finish can have a significant impact on fatigue life. During the manufacturing process and subsequent handling, repairs must not produce a surface finish coarser than intended by the design. The impact of surface finish and fatigue life in cycles can be seen in Table X.

These stresses are independent of external loading on the shaft. Many manufacturing or repair operations can affect the amount of residual stress, including: • drawing; • bending; • straightening; • machining; • grinding; • surface rolling; • shot blasting or peening; • polishing. All of these operations can produce residual stresses by plastic deformation. In addition to the above mechanical processes, 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 parallel to the load stress and in an opposite direction, it may be beneficial. Proper heat treatment can reduce these stresses if they are of excessive levels. O. Shaft Fretting Shaft fretting can cause serious damage to the shaft and the mating 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 be an exception to this rule and experience little or no fretting The presence of ferric oxide (rust) between the mating surfaces, which is reddish brown in color, is strong confirmation that fretting did occur. The cause of this condition is some amount of movement between the two mating parts. Fatigue cracking may

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Fig. 13.

Failure caused by rotational bending.

Fig. 14.

Shaft fatigue.

Fig. 15.

Failure caused by rotational bending.

Fig. 12. Shaft fatigue.

be initialed by the presence of fretting. Uncorrected shaft vibration can also worsen this condition. P. Surface Coating [17] Metallic coatings to protect or restore a shaft can cause harmful residual stresses which can reduce the fatigue strength of the base metal. In most cases, there are enough safety factors to handle this additional stress. However, if the shaft is being stressed to its design limits, then such processes as electroplating, metal spray, or catalytic deposition could be a source of fatigue failures. During some plating processes, it is possible to introduce hydrogen into the base metal. If it is not removed by the appropriate heat treatment process, severe hydrogen embrittlement may occur, which can greatly reduce the tensile strength of the shaft. The repair of shafts by welding is beyond the scope of this paper. However, caution must be used in this process. The selection of the proper weld material, method of application, stress relieving, surface finish, and diameter transition are all critical to a successful repair. Not all shaft materials are good candidates for welding-type repairs. Q. Miscellaneous Nonfracture-Type Shaft Failures There is a broad category of shaft failures or motor failures that does not result in the shaft breaking. The following is a list of the more common causes (it is acknowledged that fatigue failures that are caught in the early stages would also fit in the nonfracture category): • bending or deflection causing interference with stationary parts; • incorrect shaft size causing interference, run out, or incorrect fits;

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

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Fig. 16.

Fig. 17.

Fig. 18.

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Failure due to impact loading. Fig. 19.

Failed due to rotational bending fatigue.

Fig. 20.

Failed due to rotational bending fatigue.

Fig. 21.

Extreme corrosion wear.

Twist caused by impact loading.

Failure due to reverse torsional loading.

• excessive vibration caused by electrical or mechanical imbalance. Catastrophic bearing failures can cause serious shaft damage, but seldom result in a fracture. R. Prevention Several practices will minimize the probability of a premature shaft failure. The following is a list of some of the more critical steps. 1) Be sure that the application and the possible loading on the motor are well understood and communicated. It is imperative to know if there is an overhung load. The environmental conditions are also critical.

2) The motor manufacturer must be sure that proper materials are selected. For the most part, steel with the properties of hot rolled 1045 steel is adequate. 3) The manufacturing processes are critical. During the processing of the shaft, care must be taken not to introduce stress raisers and to achieve the required shaft finish. 4) The installation phase and operation phases are also critical. Care must be taken not to damage the shaft when coupling it to the driven equipment. For belt-driven

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Fig. 22.

Fig. 23.

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Fig. 24.

Damaged caused by water intrusion.

Fig. 25.

Fretting corrosion caused by loss fit and vibration.

Fig. 26.

Pitting caused by electrical currents.

Smear marks on roller caused by debris.

Metallic contamination in raceway.

loads, the moment principle (force in placement of the pulley.

distance)

VI. SUMMARY AND CONCLUSIONS All too often when a motor fails, the major and sometimes only focus is the repair or replacement and get it “up and running again.” Without diminishing the importance of this goal, time should be spent collecting valuable information that will assist in a root cause analysis. This paper, along with the previous papers will provide the reader with the methodology to conduct an analysis that will properly identify failures and hopefully take the necessary steps to eliminate them. This proposed methodology will yield the best results when the motor under analysis has been “bench marked” along with the system at the time of installation or restart. Also, information collected during a normal or abnormal operating cycle can be of great value. Of course, it may not be practical to have this kind of information on all plant systems; it might be wise to collect

it going forward on critical applications or applications known to have experienced difficulties in the past. Another point to consider is that, when a motor goes down, the normal tendency is to quickly remove the motor and install a new one or get the damaged motor to the repair shop and back into operation. No thought is given to collecting information that may be helpful for an accurate analysis prior to the tear-down. Investing a few extra minutes before shutdown or removal may yield critical information for the eventual analysis. It is recognized that this option is not always available, but when it is utilize it. Appendixes A–D provide the reader with descriptions of photographs of the most common types of motor failures to be used in conjunction with the prescribed methodology of this paper. They will be most useful in identifying the failure mode and pattern.

BONNETT: ROOT CAUSE AC MOTOR FAILURE ANALYSIS

Fig. 27.

Fig. 28.

1445

Fluting caused by internally generated current.

Fig. 30.

Turn-to-turn shorting.

Fig. 31.

Single-phase turn-to-turn shorting.

Fig. 32.

Stator cross section where shorting can occur.

Advanced stages of spalling.

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

APPENDIX A COMMON SHAFT FAILURES Figs. 12 and 13 are of a 1045 carbon steel motor shaft that failed due to rotational bending fatigue. The point of failure was at the shoulder of the customer take-off end Figs. 14 and 15 are of a 1040 carbon steel motor shaft that failed due to rotational bending fatigue. The point of failure was at the bearing journal shoulder. Figs. 16 and 17 are shafts that failed due to high-impact loading. The material is 1045 carbon steel, which has good ductility, which allowed for the severe twisting.

Fig. 18 is a shaft material that is unknown, but possibly 4100 high tensile steel alloy. The failure is a fatigue failure due to reversed torsional loading. Figs. 19 and 20 are of a 1051 carbon steel turbine shaft that failed due to rotational bending fatigue. There were also signs of minor torsional fatigue. Cracks initiated at the toe of a coupling weld. This material has poor weldability characteristics. There were also signs of misalignment. Note the surface pit and grinding marks; both of these conditions can weaken the shaft. Fig. 21 is an example of extreme corrosion, wear; and cracking on a pump shaft; the material is unknown.

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

Fig. 33.

Nonsymmetrical shorting without grounding.

Fig. 34. Nonsymmetrical shorting with grounding.

Fig. 35.

Same stator as Fig. 34 at point of grounding.

APPENDIX B COMMON TYPES OF BEARING FAILURES These include contamination (Figs. 22–24), defective fits/seats (Fig. 25, shaft currents (Figs. 26 and 27), fatigue (Fig. 28), and mechanical failure (Fig. 29). APPENDIX C COMMON TYPES OF STATOR FAILURES In Fig. 30, the pattern is symmetrical; each coil of each phase has been overheated. The failure mode is a multiple turn-to-turn

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 caused by an overload condition. In Fig. 31, the pattern is single phasing; one complete phase has over heated and failed due to turn-to-turn shorting. The cause of failure was single phasing. In Fig. 32 is a cross section of a typical stator slot. It is not uncommon 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 mode is also multiple turn-to-turn shorting. The cause of failure was damaged wire.

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Fig. 35 is the same stator shown in Fig. 34. The actual ground fault can be seen. Note that the turn-to-turn shorting occurred 180 opposite the grounded coil. APPENDIX D COMMON TYPES OF ROTOR FAILURES See Figs. 36–41. REFERENCES

Fig. 39. Typical aluminum squirrel cage without the lamination. Any damage to 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; one coil is grounded and there is multiple turn-to-turn shorting. The cause of failure was damaged cell wall.

[1] R. J. Nailen, “Stop rotor troubles before they start,” Plant Eng., Dec. 1966. [2] G. C. Soukup, “Design of large induction machinery using fabricated aluminum rotor cages,” M.S. thesis, Univ. Wisconsin, Milwaukee, Dec. 1974. [3] R. L. Nailen, “The cause of rotor pullover—And how to cure the problem,” Elect. App., Nov. 1980. [4] E. F. Merrill and C. R. Olson, “Sparking of A-C motor rotors and its effect on division 2 application,” presented at the IEEE PIC, Aug. 24, 1959. [5] J. L. Craggs, “Fabricated aluminum cage construction in large induction motors,” presented at the IEEE PCIC, Sept. 1975, Paper PCIC-75-8. [6] P. G. Cummings, J. R. Dunki-Jacobs, and R. H. Kerr, “Protection of induction motors against unbalanced voltage operation,” presented at the IEEE PCIC, Sept. 1983, PCIC-83-3. [7] R. L. Nailen, “What high torque? Consider the double cage motor,” Power Eng., Apr. 1971. , “New concept in rotor bar shape solves pipeline motor acceler[8] ation problem,” presented at the 1972 IEEE Summer Power Meeting, Paper 72-527-PWR. [9] A. H. Bonnett, “A comparison between insulation systems available for PWM 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 squirrel cage induction motors,” presented at the IEEE PCIC, Sept. 1976, Paper PCIC-76-7. [12] , “Analysis of winding failures in three phase squirrel cage induction 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, Paper PCIC-85-24. [15] , “Analysis of rotor failures in squirrel cage induction motors,” presented at the IEEE PCIC, Sept. 1987, Paper PCIC-87-2. [16] , “The causes and analysis of stator and rotor failures in A.C. machines,” in Proc. Maintenance and Reliability Conf., Knoxville, TN, May 20, 1997, p. 29.01. [17] Metals Handbook—Volume 10: Failure Analysis and Prevention, 8th ed., American Society for Metals, Metals Park, OH, 1966. [18] C. R. Brooks and A. Choudhury, Metallurgical Failure Analysis. New York: McGraw-Hill, 1993. [19] V. J. Colangelo and F. A. Heiser, Analysis of Metallurgical Failures. New York: Wiley, 1974. [20] A. Das, Metallurgy of Failure Analysis. New York: McGraw-Hill, 1996. [21] N. Sachs, “Failure analysis of mechanical components,” Maintenance Technol., Sept. 1993. [22] C. Y. P. Qiao and C. S. Wang, “A taxonomic study of fractograph assisted engineering materials failure analysis,” in Proc. Maintenance and Reliability Conf., Knoxville, TN, May 20–22, 1997, p. 501. [23] O. V. Thorsen and M. Dalva, “A survey of faults on induction motors in offshore oil industry, petrochemical industry, gas terminals, and oil refineries,” 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 Division, 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 in A.C. induction motors,” IEEE Ind. Applicat. Soc. Newslett., Sept./Oct. 1993.

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

Austin H. Bonnett (M’68–SM’90–F’92) was born in Los Angeles, CA, in 1936. He received the B.S. degree in electrical engineering from California State University, Los Angeles, and the Master’s degree in business from the University of Phoenix, Phoenix, AZ. He served in the U.S. Navy from 1955 to 1958 as an Electrician aboard the Icebreaker, Burton Island. He ed U.S. Electrical Motors, a Division of Emerson Electric Company, in 1963 and has held positions in the Service, Manufacturing, Quality Control, 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 Engineering, directing all U.S. Electrical Motors engineering functions for ten years. Presently, he holds the position of Vice President-Technology Emeritus at the Emerson Motor Technology Center, St. Louis, MO. He has published numerous technical papers on rotating machinery. He serves on NEMA, EPRI, and DOE Committees. Mr. Bonnett received the 1994 IEEE Meritorious Award. In 1996, he was selected for the IEEE Industry Applications Society Outstanding Achievement Award. He has also served on various IEEE Committees. He received the 1999 NEMA Kite and Key Award for outstanding service to the electrical industry.