detection of stator-slot magnetic wedge failures for induction motors without disassembly

2410 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 4, JULY/AUGUST 2014

Detection of Stator-Slot Magnetic Wedge Failuresfor Induction Motors Without Disassembly

Kun Wang Lee, Student Member, IEEE, Jongman Hong, Student Member, IEEE,Doosoo Hyun, Student Member, IEEE, Sang Bin Lee, Senior Member, IEEE,

Ernesto J. Wiedenbrug, Senior Member, IEEE, Mike Teska, and Chaewoong Lim

Abstract—The recent trend in large ac machines is to employmagnetic stator-slot wedges for improving the motor efficiency,power factor, and power density. The mechanical strength ofmagnetic wedges is weak compared to the epoxy glass wedges,and many cases of loose and missing wedges have recently beenincreasingly reported. Magnetic wedge failure deteriorates theperformance and reliability of the motor, but there is no methodavailable for testing the wedge quality other than visual inspectionafter rotor removal. Monitoring of the overall wedge conditionwithout motor disassembly can help reduce the cost of mainte-nance and risk of degradation in performance. In this paper, anew offline standstill test method for detecting magnetic wedgeproblems for ac machines without motor disassembly is proposed.An experimental study on 380-V 5.5-kW and 6.6-kV 3.4-MWmotors with magnetic wedges is performed to verify the effec-tiveness of the new test method. It is shown that the new methodcan provide reliable monitoring of magnetic wedge problems overtime, independent of other faults or motor design.

Index Terms—AC machines, fault diagnosis, inverters, magneticwedges, nondestructive testing, stators.

I. INTRODUCTION

MAGNETIC stator-slot wedges have become prevalentin large ac machines for improving the efficiency and

power factor despite the higher cost compared to the epoxy

Manuscript received September 13, 2013; revised November 1, 2013;accepted November 4, 2013. Date of publication November 13, 2013; dateof current version July 15, 2014. Paper 2013-EMC-679.R1, presented at the9th IEEE International Symposium on Diagnostics for Electrical MachinesPower Electronics and Drives, Valencia, Spain, August 27–30, and approvedfor publication in the IEEE Transactions on Industry Applications by theElectric Machines Comittee of the IEEE Industry Applications Society. Thiswork was supported in part by the Human Resources Development program(20134030200340) of the Korea Institute of Energy Technology Evaluationand Planning (KETEP) grant funded by the Korea government Ministry ofTrade, Industry and Energy, and in part by the Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded by theMinistry of Education, Science and Technology (NRF-2013R1A1A2010370).

K. W. Lee, D. Hyun, and S. B. Lee are with the Department ofElectrical Engineering, Korea University, Seoul 136-713, Korea (e-mail:[email protected]; [email protected]; [email protected]).

J. Hong is with the Agency for Defense Development, Daejeon 305-600,Korea (e-mail: [email protected]).

E. J. Wiedenbrug, deceased, was with the Condition MonitoringCenter, SKF Corporation, Fort Collins, CO 80525 USA.

M. Teska is with the Condition Monitoring Center, SKF Corporation, FortCollins, CO 80525 USA (e-mail: [email protected]).

C. Lim is with Hansung Electric Industrial Company, Dangjin 343-844,Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2013.2290869

glass wedges [1]–[5]. The overall trend is to use magneticwedges for high-output motors above 500 kW and for slow-speed motors with four or more poles. They are typically usedfor large ac machines with form wound stators and open-stator-slot design but have also been considered for performanceimprovement of small induction and permanent-magnet syn-chronous motors [5]–[7].

Most magnetic slot wedges are made from iron powder(70%–75%), glass fabric, and epoxy resin binders and thereforehave larger relative permeability (μr ≤ 10) compared to epoxywedges. The increased μr in the slot section makes the fluxconcentrated on the tooth surface, more uniformly spread overthe tooth and wedge surface, and the behavior similar to that ofsemiclosed-slot design [1]–[3]. This reduces the fluctuation offlux on the rotor core surface and decreases the effective air gap.The smoothened air-gap flux distribution results in significantreduction in the surface core losses and also helps reducethe acoustic noise and improve the torque characteristics. Thedecrease in effective air gap and closing of the slot results inincreased magnetizing and leakage inductances and decreasein the magnetizing current. The positive effect of reduced airgap is improvement in power factor and reduction in copperlosses. Reduction of core and copper losses contributes to im-proved machine efficiency, reduced operating temperature, andincreased power density of the motor. With magnetic wedges,there can be cost and energy savings due to improved efficiencyand reduction in material content and frame size, although thecost is increased. The price to pay for the advantages gainedwith magnetic wedges is the reduction in starting and pullouttorque, increased maintenance requirements, and reliabilityrisks [1]–[5].

Since the iron content in the magnetic wedge makes themechanical strength weak, they are susceptible to failure. Anexample of a stator with loose or missing magnetic wedgesegments that was recently found on a 6.6-kV 7.7-MW boilerfeed pump induction motor at a power plant is shown in Fig. 1.Many cases of wedge failures in medium–high-voltage motorsabove 3.3 kV, where up to 50% of the wedges were lostwithin three years of service, are reported in [8]–[12]. For thecase presented in [9], the decision was made to replace themagnetic wedges with nonmagnetic epoxy wedges for 6.6-kV2.85-MW induction motor units with reoccurring wedge prob-lems, since the cost of lost production and repair outweighedthe efficiency and performance benefits. The examples pre-sented in this section and in [8]–[12] show that missing mag-netic wedge problems are very common and can have seriousconsequences.

0093-9994 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

LEE et al.: DETECTION OF MAGNETIC WEDGE FAILURES FOR INDUCTION MOTORS WITHOUT DISASSEMBLY 2411

Fig. 1. Example of stator-slot magnetic wedge damage on a 6.6-kV, 7.7-MWfour-pole induction motor. (a) Magnetic wedges randomly missing or loose.(b) Wear band mark on rotor surface due to magnetic wedge protrusion intoair gap.

II. DETECTION OF MAGNETIC WEDGE FAILURE

The main degradation mechanism behind magnetic wedgefailure is wear, breakage, and/or disintegration due tomechanical stress [8]–[12]. In addition to the stress caused bythe vibration of the rotor and stator winding, electromagneticforce is induced in the wedge itself as it is an active componentin the magnetic circuit. Therefore, a “wet” process such asepoxy gluing or vacuum pressure impregnation is used in thewedging process to prevent wedge vibration. However, repeatedelectromechanical and thermal stress on the magnetic wedgeand bonding epoxy makes the wedges loose [12]. Once thewedges are free to move and begin to vibrate, the wedgeand core material are eroded, and the wedge eventually fallsout, breaks, and disintegrates into small pieces. Evidence ofmagnetic wedge protrusion into the air gap and disintegrationcan be clearly observed in the wear band of the rotor surfacein Fig. 1(b). It is shown in [9] and [12] that wedge segmentsfall out randomly along the circumference of the bore and aremore likely to fail in the center of the slot in the axial direction.This is because wedge wear occurs during the wedge insertionprocess due to nonuniform lamination surface, and the wedgesegments in the center travel the most. It has been observedthat magnetic wedge failures are more likely to occur for thehigh-stress applications with frequent starts, long start-up time,thermal overload, and large load oscillations [8]–[12].

Loose or missing magnetic wedges usually do not causeserious motor failure leading to a forced outage, since thewedges disintegrate into small pieces. However, inspection ofthe magnetic wedge tightness is a standard procedure of thestator inspection in most motor repair facilities, and partialor full rewedging is performed if loose wedges are presentsince they can fall out and degrade motor performance. Miss-ing stator-slot wedges can degrade the efficiency and causelocalized temperature rise in the slot teeth portion where thewedges are lost. If a considerable portion of magnetic wedges ismissing in the slot, this can result in stator winding movement,particularly during the start-up transient [12]. Localized heatingand winding movement are known to cause degradation ofthe stator winding insulation. The iron debris produced as a

result of wedge wear contaminates the stator end winding andcan cause partial discharge [10] and increase surface leakagecurrent for medium–high-voltage motors.

Therefore, it is important to monitor the magnetic wedgecondition regularly to minimize the risk of degradation inmotor performance or reliability. However, there is currently noknown field measurement technique for assessing the qualityof magnetic wedges without disassembling the motor. Visualinspection is the only reliable method used in the field formagnetic wedge inspection [11], [12]. Inspection of the exitingexhaust duct for iron/glass fabric debris [12] and borescopeinspection of the rotor surface for wear bands [11] have beenattempted for detecting missing wedges without disassemblingthe motor but are not applicable to all motors.

The first attempt to detect missing wedges electrically with-out motor disassembly was made in [13] and [14] for inverter-fed machines. The current response to a high-frequencyvoltage pulse (tens of microseconds duration) is used to mea-sure the transient leakage inductance, and the asymmetry in theinductance between phases is used as an indicator of missingwedges. It was shown that missing wedges can be detectedwith high sensitivity; however, it is possible that progressingwedge failures cannot be reliably monitored since the randomdistribution of missing wedges (Fig. 1) tends to cancel outthe asymmetry. The ability to monitor the progress of missingwedges with time is an important requirement considering thatit is not cost effective to replace a small portion of missingwedges as the degradation in performance and reliability risksare not significant. Rotor cage problems produce an asym-metry in the inductance pattern identical to that of missingwedges; therefore, testing must be performed at two differentrotor positions to distinguish the two problems, if the testis performed offline. There are also problems introduced bytesting with high-frequency pulses. It has been observed in [15]and [16] that testing of closed-rotor-slot induction motors withhigh-frequency signals can be influenced by the residual fluxin the rotor slot bridge distorting the test results. It is alsopossible that stator core interlaminar insulation problems notobserved with low-frequency excitation can interfere in case ofhigh-frequency excitation [17]. The application of online statorcurrent spectrum analysis was studied in [18]; however, it hasthe same potential problem of being insensitive to progressingwedge failures as the random missing wedges tend to cancel outthe asymmetry.

It can be seen that loose or missing magnetic wedges area common problem that can degrade motor performance andreliability. Although cases of magnetic wedge problems haverecently increased, there is currently no practical test methodavailable for monitoring the overall magnetic wedge conditionother than visual inspection. There has not been much researcheffort on the detection of this issue, and research is currentlyin its initial stage [11]–[14]. Given the importance of testingmagnetic wedge condition and the need for a reliable testmethod, the objective of this paper is to develop a new testmethod for magnetic wedge quality assessment. The require-ment of the new method is the ability to monitor deteriorationof wedge condition over time without motor disassembly andindependent of other faults or motor design.


Fig. 2. Main concept: Portable low-power three-phase inverter used for testingmagnetic wedges with pulsating field excitation at multiple circumferentialpositions θ at motor standstill.

III. PROPOSED METHOD FOR DETECTION

OF MISSING WEDGES

A. Main Concept

The main concept of the new method is to use a portablepower-converter-type equipment to test the motor for missingmagnetic wedges when the motor is at standstill. Testing can beperformed at the motor control center (MCC) or at the motorterminals whenever the motor is shut down, as shown in Fig. 2.The test equipment is a low-power portable three-phase invertercapable of injecting test signals for extracting parameters thatare sensitive to missing wedges. Under the expectation that thestator leakage inductance will decrease with missing wedges,the motor is excited with pulsating magnetic fields at multiplecircumferential locations, as shown in Fig. 2. It is shown inSection III-B that the change in the stator leakage inductancecan be monitored with high sensitivity under pulsating fieldexcitation. The rating of the test equipment is much lowerthan that of the motor since small test signals are sufficientfor extracting information on wedge condition, as will bedemonstrated in Section V. The rotor does not rotate sincethe induced torque is zero with pulsating field excitation, andtherefore, the proposed test method can provide noninvasiveand remote testing whenever the motor is at standstill. Thevariation in the pattern and value of the equivalent resistanceand inductance parameters of the motor as a function of pulsat-ing field electrical angle θ is used as an indicator for detectingmissing magnetic wedges.

If the electrical angle of the pulsating field vector is θ, thefield pulsates between θ and θ + 180◦ electrical degrees, asshown in Fig. 2. The pulsating vector at θ can be produced withsine-triangle pulsewidth-modulation excitation with the voltagereferences

v∗as(θ, ωt) =V cos(θ) · sin(ωt) (1)

v∗bs(θ, ωt) =V cos(θ − 2π/3) · sin(ωt) (2)

v∗cs(θ, ωt) =V cos(θ + 2π/3) · sin(ωt) (3)

where V is the excitation voltage amplitude and ω is theexcitation frequency. This results in current flow in the statorand rotor conductors, and the voltage and current vectors in thedirection of θ, vθ, and iθ can be calculated from

v∗θ(ωt)= k · [cos(θ) cos(θ−2π/3) cos(θ+2π/3)] · v∗abcs (4)

Fig. 3. Electrical equivalent circuit model of induction machine with pulsatingmagnetic field excitation at motor standstill (s = 1).

iθ(ωt)= k ·[cos(θ) cos(θ − 2π/3) cos(θ + 2π/3)] · iabcs (5)

where vabcs and iabcs represent the stator voltage and currentmatrices and k is 2/3. The equivalent input impedance of themotor for excitation in the θ direction Zeq(θ) can be calculatedfrom the phasors of the voltage and current vectors in the θdirection Vθ and Iθ as

Zeq(θ) = �Vθ/�Iθ = Req(θ) + jXeq(θ). (6)

The electrical equivalent circuit of an induction motor ex-cited with a pulsating field can be derived from the doublerevolving field theory, where the field is resolved into two fieldsof equal magnitude rotating in opposite directions [19]. Thiscircuit can be simplified since the slip s is equal to one whenthe motor is at standstill, as shown in Fig. 3. If the excitationfrequency is not too low (e.g., 50 Hz), Zeq can be approximatedas the following equation since ωLm � Rr + jωLlr at motorstandstill (s = 1):

Zeq(θ) ≈ (Rs(θ) +Rr(θ)) + j (Xls(θ) +Xlr(θ)) . (7)

If magnetic wedges are missing, the pattern and values ofReq(θ) and Xeq(θ) as functions of θ are expected to change.The expected changes in pattern and values are analyzed inSection III-B for deriving a sensitive indicator for detectingmissing wedges. The behaviors of Req(θ) and Xeq(θ) withother types of motor faults are also studied for distinguishingthe faults.

B. Analysis of Equivalent Circuit Parameter Variation WithMissing Magnetic Wedges

If the stator winding is excited with a 50-Hz pulsating fieldat motor standstill, the flux cannot penetrate into the rotor yokedue to eddy current flux rejection by the rotor cage. As a result,the flux is mainly distributed near the rotor surface, and the Xeq

component mainly consists of leakage inductance, as shownin (7). This can be seen in the result of a 2-D finite-element(FE) simulation in Fig. 4, where a four-pole motor was excitedwith a 50-Hz pulsating field in the direction of θ = 0◦ at 10%rated current. The magnetic permeability μr of the magneticwedge was assumed to be ten, which is typical of the wedgesused. It can be clearly observed in Fig. 4 (upper figure) thatthe flux is concentrated on the stator and rotor surface and themagnetic wedge; therefore, Xeq mainly consists of zigzag andslot leakage components under pulsating field excitation.

The simulation was repeated for the case where the magneticwedge of the slot in the θ = 90◦ direction (mechanical angleθm = 45◦) is missing, and the results are shown in the lower


Fig. 4. Flux distribution in a four-pole motor at standstill with 50-Hz pulsatingfield excitation at θ = 0◦ (10% of Irated) for the cases of (upper) no missingwedge and (lower) wedges missing in the slot at θm = 45◦ location.

figure of Fig. 4. It can be seen that the difference in the fluxstrength and distribution is noticeable for the two cases underidentical excitation conditions (10% Irated), particularly in thevicinity of the slot with the missing wedge. For the case ofθ = 0◦ excitation shown in Fig. 4, the missing wedge increasesthe equivalent reluctance in the magnetic circuit, which resultsin decrease in the equivalent inductance. It can be clearlyobserved in Fig. 4 that the magnetic wedge is the main pathfor the magnetic flux under the given excitation condition.Fifty-hertz pulsating field excitation is used in the proposedmethod because the variation in the value of Xeq obtained underthis condition is sensitive to missing wedges. The resistancecomponents Rs and Rr are independent of missing wedges andonly vary with temperature.

If the pulsating field excitation is in the direction of themissing wedge (θ = 90◦), the value of Xeq is not influencedsince the magnetic wedge is not a part of the magnetic circuit, asshown in Fig. 5. Therefore, if Xeq is measured under pulsatingfield excitation for multiple circumferential positions betweenθ = 0◦ and 180◦, the expected pattern of Xeq(θ) for a numberof cases is shown in Fig. 6. For a healthy stator where no wedgesare missing, Xeq(θ) is constant and independent of θ since themotor is symmetric. If the wedge is missing at the θm = 45◦

location, Xeq decreases at θ = 0◦ and 180◦ and does not changeat θ = 90◦, resulting in the sinusoidal pattern of Fig. 6. Ifwedges are missing at the slots located at θm = 45◦ and 135◦,the values of Xeq at θ = 0◦ and 180◦ decrease by a largeramount since the degree of increase in magnetic reluctance islarger. For the case where the wedge is missing at the θm = 0◦

(or 90◦) location, Xeq decreases at θ = 90◦ and does not change

Fig. 5. Flux distribution in four-pole motor at standstill with 50-Hz pulsatingfield excitation at θ = 90◦ (10% Irated) with missing wedge at θm = 45◦.

Fig. 6. Pattern of Xeq(θ) with magnetic wedges missing in zero, one, andtwo stator slots (θm = 0◦, θm = 45◦, θm = 45◦ and 135◦, and θm = 0◦

and 45◦).

at θ = 0◦ and 180◦ (Fig. 6). If the magnetic wedges of two slotsat the θm = 0◦ and 45◦ locations are missing, it can be predictedthat Xeq(θ) would be constant at a lower value of Xeq since theasymmetry is canceled (Fig. 6).

It can be seen that the influence of missing wedges at slots180◦ (electrical) apart (90◦ mechanical in this case) is additive,whereas missing wedges at slots 90◦ (electrical) apart (45◦

mechanical) tend to cancel out the asymmetry. The values ofXeq decrease with increase in the severity of wedge failure re-gardless of the location of the missing wedges. Magnetic wedgefailure can therefore be detected by observing the change in thepattern and/or magnitude of Xeq(θ) over time. Xeq can serveas a reliable fault indicator provided that consistent excitationfrequency and voltage level are used for Xeq measurementsince it is independent of temperature, saturation, or skin effectunder identical excitation conditions.

IV. IMPLEMENTATION AND PRACTICAL ISSUES

A. Fault Indicator

When the measurements of Xeq are obtained as a function ofθ for a healthy motor Xeq0(θ), it is not constant or independentof θ due to the inherent asymmetry in the motor. Experimentalmeasurements of Xeq0(θ) were obtained on the test motor inSection V under 50-Hz excitation with the rotor turned to differ-ent angles (0◦, 22.5◦, 45◦, and 67.5◦) with respect to a referenceposition. The measurements of Xeq0(θ) plotted in Fig. 7 showthat it does not change significantly with rotor position. Thisimplies that the fluctuations in Xeq0(θ) are due to the nonideal


Fig. 7. Inherent asymmetry in Xeq(θ) measurements for healthy motor withno missing magnetic wedges with rotor positioned at 0◦, 22.5◦, 45◦, and 67.5◦

with respect to a reference position.

inherent asymmetry in the stator winding, magnetic wedge, orcore and that it is independent of the rotor. The fluctuating (ac)part of Xeq0(θ), Xeq0,ac(θ), can be separated from the average(dc part) Xeq0,dc(θ), as shown in (8), and stored to compensatefor inherent stator asymmetries

Xeq0,ac(θ) = Xeq0(θ)−Xeq0,dc(θ). (8)

When determining the severity of magnetic wedge failure, it isdesirable to have a fault indicator that can be used for all ma-chines. Since the value of Xeq differs significantly from motorto motor, the normalized (and compensated) Xeq, Xeq,norm(θ),shown in the following equation is suggested as a faultindicator:

Xeq,norm(θ) = (Xeq(θ)−Xeq0,ac(θ)) /Xeq0,dc. (9)

If Xeq,norm(θ) is monitored over time, the degree of asymmetryand decrease in Xeq(θ) due to missing wedges can be observed.

The circumferential distribution of missing wedge segmentsis typically random, as shown in the example in Fig. 1, andthe cases shown in [8]–[12]. If a small portion of the wedge ismissing, corrective action will not be taken even if this can bedetected, considering that it is not cost effective as the risk ofmotor performance degradation or failure is minimal. However,if the percentage of missing wedges increases significantly, asin the cases reported in [8]–[12], the wedges must be repairedto prevent performance degradation or failure. As the numberof missing wedges increases, it is unlikely that they will allfall out from the same slot or from slots located 180 electricaldegrees apart, and the “asymmetry” in the Xeq,norm(θ) patternwould not be observable. Therefore, the percent deviation inthe average of (9) from unity %ΔXeq,norm,avg is proposedas the quantitative fault indicator since Xeq,norm decreasesregardless of the distribution of missing wedges. The increasein %ΔXeq,norm,avg can be observed over time for monitoringmissing magnetic wedges, and the pattern of Xeq,norm(θ) canbe used as a reference for observing the asymmetry

%ΔXeq,norm,avg = (1− avg (Xeq,norm(θ))) ∗ 100%. (10)

The correlation between the change in %ΔXeq,norm,avg andthe severity of missing wedges is expected to depend on themotor stator slot and wedge design/material, as in all condition

Fig. 8. Pattern of Req(θ) and Xeq(θ) for motor with stator interlaminar core,air-gap eccentricity, rotor cage, and magnetic wedge failures.

monitoring methods. The fault threshold can be determinedfrom a 2-D FE analysis if the motor design is available.

B. Influence of Other Types of Faults and Motor Design

For every test method, it is very important to identify otherconditions or factors that can cause false indications. Theproposed test method is not influenced by source anomaliesor problems in the bearing, shaft, coupling, or load since it isa standstill offline test. If Xeq measurements are made underidentical excitation conditions, it will not be influenced bytemperature or saturation; however, it can be influenced byother types of faults [17], [20], [21]. The pattern of normalizedReq(θ) and Xeq(θ) for stator core interlaminar insulation fail-ure, rotor cage failure, and air-gap eccentricity measured under50-Hz excitation is summarized in Fig. 8.

Stator core interlaminar insulation failure causes increase inthe local lamination eddy current and core loss since it is equiv-alent to having thicker insulation. Although this can result inchange in Req(θ) and Xeq(θ) parameters under high-frequencyexcitation, the influence is negligible under low-frequencyexcitation, as verified in [17]. The proposed method is notinfluenced by interlaminar core insulation faults since 50-Hzpulsating field has very little impact on eddy current core loss.Air-gap eccentricity causes variation in the air-gap distributionand decrease in the minimum air gap. This results in a smallamount of increase in the zigzag leakage inductance, where theincrease in Xeq(θ) is uniform and independent of θ, as shownin Fig. 8 (Req(θ) is not influenced by air-gap eccentricity)[19]. Since Xeq increases with air-gap eccentricity, it can beeasily distinguished from magnetic wedge failures. Rotor cagedamage results in increase in the rotor resistance and leakageinductance when the positions of the broken bar and pulsatingfield θ are 90 electrical degrees apart. The change in the patternof the Req(θ) and Xeq(θ) measurements with a broken rotorbar is shown in Fig. 8 [20]. Since missing wedges result ina decrease only in the average of Xeq, the two faults can beeasily distinguished. Rotor faults can also be distinguished fromother faults since the Req(θ) and Xeq(θ) patterns shift in thex-axis direction depending on the rotor position, whereas allother faults are independent of rotor position.

It is reported in [15] and [16] that testing of closed-rotor-slot induction motors under low flux excitation can produceerroneous results because the residual flux in the bridge ofthe closed slot influences the test results. This occurs with


high-frequency excitation in the kilohertz ∼ 10 s of kilohertzrange because the excitation flux is too low to overcome theresidual flux. However, this is not a concern for the proposedmethod since 50-Hz excitation is two to three orders of mag-nitude smaller than the frequencies of concern (excitation fluxis two to three orders of magnitude larger for the same exci-tation voltage). Therefore, the proposed method can be usedfor all motors whether they are of closed- or open-rotor-slotdesign.

V. EXPERIMENTAL STUDY

A. Experimental Test Setup

To verify the validity of the proposed test method formagnetic wedge failure detection, an experimental study wasperformed on 380-V, 5.5-kW (four-pole 36-stator-slot) and6.6-kV, 3.4-MW (six-pole 72-stator-slot) induction motors withmagnetic wedges. An insulated-gate bipolar transistor inverterwas built and controlled with a commercial DSP for exciting themotor with variable voltage and frequency pulsating magneticfields. The excitation voltage and frequency in (1)–(3) wereset after comparing the Xeq measurements obtained under anumber of excitation conditions. The consistency of the testresults improved with higher voltage, and the voltage was setat the minimum voltage that provides consistent results. Theexcitation voltages were 18 and 104 V for the 380-V and6.6-kV units, which are lower than 5% and 2% of the ratedvoltage, respectively. The method was tested at the minimumvoltage to verify that a portable inverter rated at much lowerpower than that of the motor can be used. The sensitivity of de-tection was improved as the excitation frequency was increasedsince flux penetration into the rotor decreases. Fifty hertz waschosen because the sensitivity did not improve noticeably above50 Hz and because there is the risk of interference with coreinterlaminar fault problems and also with slot bridge saturationfor closed-slot rotor machines when high-frequency excitationis used.

B. Experimental Results: 380-V, 5.5-kW Laboratory Motor

The 380-V test motor is a low-voltage random wound motorthat employs the semiclosed-stator-slot design with nonmag-netic wedges. To test the proposed method under controlledconditions where the magnetic wedges can be inserted andremoved, 3-mm-thick magnetic wedges were precision ma-chined to fit the slot opening, as shown in Fig. 9(a). Thesame magnetic material sheets used for high-voltage motorswere used for fabrication of the wedges. The stator of the testmotor was rewound with windings of smaller diameter to makespace for the wedges, and cable ties were placed between themagnetic wedge and stator winding, as shown in Fig. 9(b), tofix the magnetic wedges in place to prevent them from moving.Examples of slots with and without the magnetic wedges areshown in Fig. 9(b).

The values of Req and Xeq were measured under 18-V,50-Hz excitation with the pulsating field electrical angle θvaried between 0◦ and 180◦ in 10◦ intervals. The Req(θ) and

Fig. 9. (a) Precision-machined magnetic wedge sample for 380-Vsemiclosed-slot test machine. (b) Stator-slot opening with and without magneticwedges (cable tie used to fix magnetic wedge).

Fig. 10. Experimental measurements of Req(θ) and Xeq(θ) for 380-V motorstator with 0, 1 (θm = 45◦), 2 (θm = 45◦ and 135◦), 3 (θm = 45◦, 135◦,and 225◦), 4 (θm = 45◦, 135◦, 225◦, and 315◦), and 36 missing wedges.

Xeq(θ) measurements are shown in Fig. 10 for the cases listedin the following:

1) zero missing wedge: healthy;2) one missing wedge: θm = 45◦;3) two missing wedges: θm = 45◦ and 135◦;4) three missing wedges: θm = 45◦, 135◦, and 225◦;5) four missing wedges: θm = 45◦, 135◦, 225◦, and 315◦;6) 36 missing wedges: all wedges removed.

The results in Fig. 10 are the values of Req(θ) and Xeq(θ)obtained without any processing for compensation of inherentasymmetry described in (8)–(10). It can be seen that the changein Req(θ) is negligible and that Xeq(θ) decreases with increasein missing wedges. The change in the pattern of Xeq(θ) withmissing wedges can be observed more clearly with the inherent


Fig. 11. Experimental measurements of Xeq,norm(θ) for 380-V motor statorwith 0, 1 (θm = 45◦), 2 (θm = 45◦ and 135◦), 3 (θm = 45◦, 135◦, and225◦), 4 (θm = 45◦, 135◦, 225◦, and 315◦), and 36 missing wedges.

asymmetry removed using (9). In addition, the decrease inXeq(θ) with missing wedges is more evident, if it is normalizedwith respect to Xeq0,dc(θ) using (9). The normalized Xeq(θ),Xeq,norm(θ), is plotted in Fig. 11 for the same cases shown inFig. 10. The increase in asymmetry and decrease in magnitudeof Xeq(θ) due to missing wedges can be observed with moreclarity, as expected. Since the missing wedges are 180 electrical(90 mechanical) degrees apart, the influence is additive. Thepatterns of Xeq,norm(θ) for the cases with one, two, three, andfour missing wedges are in phase, and the degree of asymmetryincreases as the number of missing wedges increases. There-fore, the decrease in magnitude and asymmetry in the patternof Xeq(θ) can serve as a good indicator for detecting missingwedges. The results with all 36 magnetic wedges removedshow how much the expected decrease in Xeq is for the worstcase. If this information is available through FE analysis ortesting, it could be used to determine the fault threshold, sincethe correlation between the decrease in Xeq,norm,avg and faultseverity can be found.

The values of Xeq,norm(θ) calculated from (9) for the addi-tional cases listed in the following are shown in Fig. 12:

1) zero missing wedge: healthy;2) one missing wedge: θm = −5◦ and θm = 45◦;3) two missing wedges: θm = −5◦ and 45◦;4) three missing wedges: θm = 45◦, 105◦, and 165◦.

The wedge fault conditions considered in Fig. 12 are caseswhere the missing wedge distribution in the slots is such thatthe asymmetry in Xeq is canceled, as shown in the example inFig. 6 (wedges missing in the θm = 0◦ and 45◦ slots). To testthe case where wedges are missing in two slots 90 electricaldegrees apart, wedges were removed from slots located atθm = −5◦ and 45◦. Wedges at θm = 0◦ could not be removedas the test motor is a 36-slot machine with slots in 10◦ intervals.It can be seen in Fig. 12 that the pattern of Xeq,norm(θ) isroughly out of phase for the cases where the wedges are missingat the θm = −5◦ and 45◦ slots. The Xeq,norm(θ) pattern isalmost constant and independent of θ for the case where bothwedges are missing since the asymmetry is canceled. Similarly,if wedges are missing in three slots 120 electrical degrees apart(θm = 45◦, 105◦, and 165◦), the asymmetry is canceled, as

Fig. 12. Experimental measurements of Xeq,norm(θ) for 380-V motor statorwith 0, 1 (θm = −5◦ and θm = 45◦), 2 (θm = −5◦ and 45◦), and 3 (θm =45◦, 105◦, and 165◦) missing wedges.

Fig. 13. Experimental measurements of %ΔXeq,norm,avg (380-V motor)as a function of the percentage of missing wedges for all cases considered inFigs. 11 and 12.

shown in Fig. 12. The results in Fig. 12 clearly show that onecannot rely solely on monitoring the asymmetry of Xeq forprogressing wedge failures since it can be misleading in casethe multiple missing wedges cancel the asymmetry.

To observe the amount of decrease in Xeq,norm with the per-centage of missing wedges, the percent decrease in the averageof Xeq,norm(θ), %ΔXeq,norm,avg, for all the cases consideredin Figs. 11 and 12 was calculated using (10) and shown inFig. 13. It can be seen that the fault indicator %ΔXeq,norm,avg

increases with the percent of missing wedges. It was observedthat the percent increase in Xeq,norm,avg is different for thesame number of missing wedges, depending on whether theslot with the missing wedge contains windings from the sameor different phases (the test motor stator is a 7/9 fractional-pitchdouble-layer lap winding).

C. Experimental Results: 6.6-kV 3.4-MW Field Motor

The 6.6-kV, 3.4-MW test motor (six poles, 345 A) is aforced draft fan induction motor used at a power plant and wasbrought into a motor repair shop for stator rewind. This high-voltage motor has form wound stator bars with open-stator-slotdesign and employs magnetic wedges, as shown in Fig. 14(a).The authors were given a 4-h window for testing the motor atthe repair shop before the stator rewind. One segment of the


Fig. 14. 6.6-kV, 3.4-MW test motor with magnetic wedges. (a) One missingwedge segment. (b) Wear band on rotor surface due to missing wedge.

magnetic wedge in one of the slots was missing when the rotorwas removed, as shown in Fig. 14(a). The wear band on therotor surface at the same axial location as the missing wedge[Fig. 14(b)] indicates that the magnetic wedge fell out into theair gap during motor operation. The missing wedge segmentcould not be found in the motor as it has disintegrated.

Testing was performed with all wedges removed in zero tofour slots, as shown in Fig. 15. Wedges were removed in slots60◦ apart (180 electrical degrees) for the six-pole motor so thatthe influence is additive. The test conditions are listed as fol-lows, where the mechanical angle θw is measured with respectto the missing wedge segment location, as shown in Fig. 14(a):

1) no missing wedge: healthy;2) one slot with missing wedge: θw = 0◦;3) two slots with missing wedges: θw = 0◦ and 60◦;4) three slots with missing wedges: θw = 0◦, 60◦, and 120◦;5) four slots with missing wedges: θw = 0◦, 60◦, 120◦,

and 180◦.

The reference (or healthy) case is the condition shown inFig. 14(a), where there was one wedge segment missing atthe θw = 0◦ slot. The test conditions listed previously do notrepresent how the wedges randomly fall out in actual cases ob-served in the field, as shown in Fig. 1 and [8]–[12]. Testing wasperformed in this manner considering the short time windowgiven for testing, since the purpose was to verify the validityof the proposed method. Testing was performed with the rotorremoved, since insertion and removal of the rotor are time con-suming and labor intensive, given the short time window. The

Fig. 15. 6.6-kV, 3.4-MW test motor with all magnetic wedge segmentsintentionally removed in one slot for testing purposes.

Fig. 16. Experimental measurements of (a) Xeq(θ). (b) Xeq,norm(θ) for6.6-kV motor stator with zero, one (θw = 0◦), two (θw = 0◦ and 60◦), three(θw = 0◦, 60◦, and 120◦), and four (θw = 0◦, 60◦, 120◦, and 180◦) missingwedges.

result obtained with the rotor removed is expected to be moresensitive compared to when the rotor is inserted. This is becausethe alternate flux path through the rotor, shown in Fig. 4, isnot present when the stator-slot wedge is missing, and this isexpected to make the “change” in the leakage inductance larger.

The values of Xeq were measured under 104-V, 50-Hz ex-citation under identical pulsating field excitation as the 380-Vmotor. The measurements of Xeq(θ) and Xeq,norm(θ) obtainedfor the five cases listed previously for the 6.6-kV motor areshown in Fig. 16. The results are similar to that in Figs. 10and 11, where the missing wedges are 180 electrical degreesapart and the influence is additive. The patterns of Xeq(θ) andXeq,norm(θ) for the cases with one, two, three, and four missingwedges are in phase, and the degree of asymmetry increasesas the number of missing wedges increases. The values of%ΔXeq,norm,avg calculated for all cases, and plotted in Fig. 17,show that the fault indicator %ΔXeq,norm,avg increases withthe percentage of missing wedges. The results presented inFigs. 10–13, 16, and 17 verify that magnetic wedge failurecan be monitored with the proposed method and fault indicatorwithout disassembling the machine.

VI. CONCLUSION

An offline test method for detecting magnetic wedge fail-ures without disassembling the motor has been proposed in


Fig. 17. Experimental measurements of %ΔXeq,norm,avg (6.6-kV motor)as a function of the percentage of missing wedges for the cases considered inFig. 16.

this paper. The main concept is to use a low-power portableinverter for injecting test signals into the motor from theMCC for extracting parameters indicative of missing wedgeswhen the motor is at standstill. The pattern and decrease inthe equivalent inductance measured as functions of pulsatingfield angle have been identified as sensitive indicators of mag-netic wedge failures since the stator leakage inductance de-creases with missing wedges. An experimental study on 380-V,5.5-kW, and 6.6-kV, 3.4-MW motors with magnetic wedgesverified that the proposed fault indicator can provide reliabledetection of missing wedges over time without motor disassem-bly. It was also shown that other types of faults such as statorcore insulation, eccentricity, or rotor cage faults do not interferewith the proposed method.

The proposed technique is meaningful considering that theonly practical means of detecting wedge problems is withvisual inspection after motor disassembly. It was shown thatthe proposed method can be developed as small stand-aloneoffline wedge condition test equipment as the required excita-tion power level is low. For inverter-fed motors with magneticwedges, the test algorithm can be implemented in the inverter asa built-in diagnostics feature without additional H/W for testingwedge automatically whenever the motor is stopped. Sincemissing magnetic wedges do not cause significant performancedegradation or reliability risks unless it is severe, the proposedmethod can be used for regular monitoring of missing wedgesfor scheduling maintenance in an efficient manner.

ACKNOWLEDGMENT

The authors would like to thank Hansung Electric IndustrialCompany for sharing their experience on the inspection andrepair of magnetic wedges, photographs of magnetic wedgefailures, providing the custom-made magnetic wedges andrewind for the test motor, and the support in testing the 6.6-kVmotor with magnetic wedge removal.

REFERENCES

[1] B. J. Chalmers and J. Richardson, “Performance of some magnetic slotwedges in an open slot induction motors,” Proc. Inst. Elect. Eng., vol. 114,no. 2, pp. 258–260, Feb. 1967.

[2] H. de Swardt, Changing the Wedge Material in an Electric Motor(rev. 03). Cleveland,OH,USA:Marthinsen&Coutts, Jun. 2007. [Online].

Available: http://www.rmwg.co.za/Presentations/Magnetic_Wedges/Magnetic_Wedges_Rev_03.pdf

[3] S. Wang, Z. Zhao, L. Yuan, and B. Wang, “Investigation and analysis ofthe influence of magnetic wedges on high voltage motors performance,”in Proc. IEEE VPPC, Sep. 2008, pp. 1–6.

[4] R. Curiac and H. Li, “Improvements in energy efficiency of inductionmotors by the use of magnetic wedges,” in Conf. Rec. IEEE IAS Annu.Meeting PCIC, Sep. 2011, pp. 1–6.

[5] Z. Milojkovic et al., “Application of magnetic wedges for stator slots ofhydrogenerators,” in Proc. CIGRE Session, Aug. 2010.

[6] A. Kaga, Y. Anazawa, H. Akagami, S. Watabe, and M. Makino, “Aresearch of efficiency improvement by means of wedging with soft ferritein small induction motors,” IEEE Trans. Magn., vol. MAG-18, no. 6,pp. 1547–1549, Nov. 1982.

[7] F. Caricchi, F. G. Capponi, F. Crescimbini, and L. Solero, “Experimentalstudy on reducing cogging torque and no load power loss in axial fluxpermanent magnet machines with slotted winding,” IEEE Trans. Ind.Appl., vol. 40, no. 4, pp. 1066–1075, Jul./Aug. 2004.

[8] H. de Swardt, Electric Motor Failure Prevention: Wedge Failures(rev. 08). Cleveland, OH, USA: Marthinsen & Coutts, Oct. 2007.

[9] R. Scollay and W. Stewart, “The real cost of magnetic wedges in improvedperformance of induction motors,” in Proc. CWIEME, 2007.

[10] M. Davis, “Problems and solutions with magnetic stator wedges,” in Proc.IRMC, 2007, pp. 1–5.

[11] R. Hanna and D. W. Schmitt, “Failure analysis of induction motors:Magnetic wedges in compression stations,” IEEE Ind. Appl. Mag., vol. 18,no. 4, pp. 40–46, Jul./Aug. 2012.

[12] R. A. Hanna, W. Hiscock, and P. Klinowski, “Failure analysis of threeslow-speed induction motors for reciprocating load application,” IEEETrans. Ind. Appl., vol. 43, no. 2, pp. 429–435, Mar./Apr. 2007.

[13] G. Stojicic et al., “A method to detect missing magnetic slot wedges inAC machines without disassembling,” in Proc. IEEE IECON, Nov. 2011,pp. 1698–1703.

[14] G. Stojicic, M. Vasak, N. Peric, G. Joksimovic, and T. M. Wolbank,“Detection of partially fallen-out magnetic slot wedges in inverter-fedAC machines under various load conditions,” in Proc. IEEE ECCE, 2012,pp. 4015–4020.

[15] T. Kang et al., “The influence of the rotor on surge PD testing of lowvoltage AC motor stator windings,” IEEE Trans. Dielectr. Electr. Insul.,vol. 20, no. 3, pp. 762–769, Jun. 2013.

[16] Testing the Rotor of a Induction Motor by Measuring the Inductance asa Function of Shaft Position, GET-8065 rev. 1, GE Ind. Control Syst.,Fort Wayne, IN, USA, Sep. 1997.

[17] K. Lee, J. Hong, K. Lee, S. B. Lee, and E. J. Wiedenbrug, “A stator corequality assessment technique for inverter-fed induction machines,” IEEETrans. Ind. Appl., vol. 46, no. 1, pp. 213–221, Jan./Feb. 2010.

[18] M. Orman, A. Nowak, J. R. Ottewill, and C. T. Pinto, “A novel non-invasive method for detecting missing wedges in an induction machine,”in Proc. IEEE SDEMPED, 2013, pp. 200–206.

[19] S. J. Chapman, Electric Machinery Fundamentals, 5th ed. New York,NY, USA: McGraw-Hill, 2011.

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Kun Wang Lee (S’12) received the B.S. degree inelectrical engineering from Korea University, Seoul,Korea, in 2012, where he is currently working towardthe M.S. degree.

His research interests include electric machinesand power electronics.

Mr. Lee was the recipient of the 2013 SDEMPEDPrize Paper Award from the Technical Committee onDiagnostics of the IEEE Power Electronics Society.


Jongman Hong (S’09) received the B.S. and Ph.D.degrees in electrical engineering from Korea Univer-sity, Seoul, Korea, in 2008 and 2013, respectively.

In 2009, he was with the SKF Condition Monitor-ing Center, Fort Collins, CO, USA, where he workedon the development of condition monitoring tools forelectric machines as a summer intern. In 2011, hewas an intern with the Austrian Institute of Technol-ogy, Vienna, Austria, where he worked on conditionmonitoring of permanent-magnet synchronous ma-chines. He is currently a Senior Researcher with the

Agency for Defense Development, Daejeon, Korea, where he is involved withsurface-to-surface missile systems.

Dr. Hong was the recipient of four paper awards from the Electric MachinesCommittee of the IEEE Industry Applications Society and the TechnicalCommittee on Diagnostics of the IEEE Power Electronics Society.

Doosoo Hyun (S’09) received the B.S. and M.S.degrees in electrical engineering from Korea Univer-sity, Seoul, Korea, in 2009 and 2011, respectively,where he is currently working toward the Ph.D.degree.

In 2009, he was with the SKF Condition Mon-itoring Center, Fort Collins, CO, USA, where heworked on the development of condition monitoringtools for electric machines as a summer intern. In2011, he was an intern with the Austrian Instituteof Technology, Vienna, Austria, where he worked

on condition monitoring of permanent-magnet synchronous machines. Hisresearch interests include condition monitoring, diagnostics, and analysis ofelectric machinery

Mr. Hyun was the recipient of the 2013 IEEE International Symposiumon Diagnostics for Electric Machines, Power Electronics and Drives PrizePaper Award from the Technical Committee on Diagnostics of the IEEE PowerElectronics Society.

Sang Bin Lee (S’95–M’01–SM’07) received theB.S. and M.S. degrees in electrical engineering fromKorea University, Seoul, Korea in 1995 and 1997,respectively, and the Ph.D. degree in electrical en-gineering from the Georgia Institute of Technology,Atlanta, GA, USA, in 2001.

From 2001 to 2004, he was with the GeneralElectric Global Research Center (GE GRC), Sch-enectady, NY, USA. At GE GRC, he developed aninterlaminar core fault detector for generator statorcores and worked on insulation quality assessment

for electric machines. From 2010 to 2011, he was a Research Scientist withthe Austrian Institute of Technology, Vienna, Austria, where he worked oncondition monitoring of permanent-magnet synchronous machines. Since 2004,he has been a Professor of electrical engineering with Korea University. Hiscurrent research interests are in protection, monitoring, and diagnostics, andanalysis of electric machines and drives.

Dr. Lee was the recipient of nine prize paper awards from the IEEE PowerEngineering Society, the Electric Machines Committee of the IEEE IndustryApplications Society (IAS), and the Technical Committee on Diagnostics ofthe IEEE Power Electronics Society. He serves as a 2014-2015 DistinguishedLecturer for the IEEE IAS and as an Associate Editor for the IEEE TRANS-ACTIONS ON INDUSTRY APPLICATIONS for the IEEE IAS Electric MachinesCommittee.

Ernesto J. Wiedenbrug (S’94–M’00–SM’01), de-ceased, was born in Buenos Aires, Argentina. Hereceived the Dipl. Ing. degree from RWTH AachenUniversity, Aachen, Germany, in 1995 and the Ph.D.degree in electrical engineering from Oregon StateUniversity, Corvallis, OR, USA, in 1998.

In 1984, he was with Siemens S.A., Buenos Aires,and from 1994 to 1995, he was a Power Engi-neer with ArcelorMittal South Africa (formerly IscorLtd.). During his doctoral degree, he had a fellowshipwith Volkswagen AG Germany and was the General

Manager with the Motor Systems Resource Facility, an Electric Power ResearchInstitute-funded center, Corvallis, OR, USA. In 2006, he was an AdjunctProfessor of electrical machines with Colorado State University, Fort Collins,CO, USA. In 2013, he was a Visiting Researcher with Korea University, Seoul,Korea. In 1998, he joined Baker Instrument Company, an SKF Group Company,Fort Collins, CO, USA, where he was the Technology Manager. His main areaof interest was predictive and preventive maintenance of electrical motors.

Dr. Wiedenbrug was a recipient of three paper awards from the ElectricMachines Committee of the IEEE Industry Applications Society and theTechnical Committee on Diagnostics of the IEEE Power Electronics Society.

Mike Teska received the B.S. degree in electri-cal engineering from the University of Michigan,Ann Arbor, MI, USA.

He has held a variety of positions with HewlettPackard and Agilent Technologies, including DesignEngineer, Project Manager, Business Team Leader,Process Improvement and Standardization Lead, andCustom Systems Engineering Manager. He also hasexperience in aerospace electronics design and de-velopment with Ball Aerospace and TechnologiesCompany. In 2008, he became the Research and

Development Department Manager of Baker Instrument Company, an SKFGroup Company, Fort Collins, CO, USA.

Chaewoong Lim received the B.S degree in mecha-tronics engineering from Korea Polytechnic Univer-sity, Siheung, Korea, in 2008.

He has worked in the motor repair industry for thelast 20+ years and has many years of experiencein design and fabrication of custom-built ac and dcmotors. From 1993 to 2007, he was with the KoreanHeavy Electric Company, Siheung, where he workedon repair, inspection, and testing of electric ma-chines. Since 2008, he has been the Sales Managerof Hansung Electric Industrial Company, Dangjin,

Korea.

detection of stator-slot magnetic wedge failures for induction motors without disassembly

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