Fault Analysis of Current-controlled Pwm-myerter Fed

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Proceedings of the 7th lnlemational Conference onProperties an d Applications of Dielectric MaterialsJune 1-5 2003 Nagoya S17-2

FAULT ANALYSIS OF CURRENT-CONTROLLED PWM-MYERTER FEDINDUCTION-MOTOR DRIVESE. A. EBRA HIM and N. AMMADPower Electronics and Energy Conversion Department,Electronics R esearch Institute, 12262 D okki, Cairo, EgyptAutomotive and Tractor Department, Helwan University, Egypt*E-mail: essamudin@,yahoo.com or essamudin@,valla.com

Abstract: In this paper, the fault-tolerance capability ofIM-drive is studied. The discussion on the fault-tolerance of IM drives in the literature has mostly beenon the conceptual level without any detailed analysis.Most of studies are only achieved experimentally. Thispaper provides an analytical tool to quickly analyze andpredict the performance under fault conditions. Also,most of the presented results were machine specific andnot general enough to be applicable as an evaluationtool. So, this paper will present a generalized methodfor predicting the post-fault performance of IM-drivesafter identifying the various faults that can occur. Thefault analysis for IM in the motoring mode will bepresented in this paper. The paper includes an analysisfor different classifications of drive faults. The faults inan IM-drive -that will be studied- can be broadlyclassified as: machine fault, (i.e., one of stator windingsis open or short, multiple phase open or short, bearings,and rotor bar is broken) and inverter-converter faults(i.e., phase switch open or short, multiple phase fault,and DC-link voltage drop). Briefly, a general-purposesoftware package for variety of IM-drive faults -isintroduced. This package is very important in IM-faultdiagnosis and detection using artificial intelligenttechniques, wavelet and signal processing.INTRODUCTIONInduction motors (IMs) are widely used in manyindustrial processes because they are coast effective andmechanically robust. A modem IM-drive hardwareincludes the motor, the converter-inverter combination,dc-link components, transducers for feedback signalsand host processor for software control. Among theseelectro-mechanical components that are especiallysubject to wear, ageing, destroyed, damaged or their di-electric are failed. Therefore, an early fault detectionand diagnosis of such components is of high interest forhigh quality machines and drives to avoid failures. Thearea of system maintenance cannot realize its fullpotential if it is only limited to preventive approaches.Rather, the early diagnosis of a developing fault isnecessary to allow maintenance personnel to schedulerepairs prior to an actual failure. During the last decade,there has been much interest in early fault detection anddiagnosis techniques for use in Condition -Based

Maintenance (CBM ). The key for the success of CBM iseffective condition assessment or at least faultdiagnosis.In [I], the authors suggested online testing of motors,diagnostics, and motor monitoring that have shownlarge scientific development in the past few years,outgrowing the past stage of r m s current and voltagemeasurements. This stems from current and voltagesignature analysis which has progressed from thelaboratory and the work of dedicated specialist expertsto become the base for modem instrumentationavailable to plant operatives. In addition lo line-fedmotors, a clear view of variable-frequency driveapplications is now possible without requiringconnection directly to the motor terminals. The use ofthe motor itself as a transparent sensor, making the shafttorque signatures available at the motor control cabinet,allows for advanced diagnostics of the voltage supply,motor, and load system.Also, in [2], a laboratory tests for induction motor sshort-circuit current were being presented andperformed on two different induction motors with lowpower rating.In [3,4], the papers for the same authors and divided totwo parts: part 1 is a paper one that addressed the levelof tum-to-tun insulation deterioration that can beresolved using an online monitoring technique basedupon an effective negative-sequence impedancedetector. The detection of turn-to-tum defects isespecially important because they are believed torepresent the beginning stage of most motor windingfailure. The second part is paper two that presented anexperimental investigation of voltage mismatchdetectors for condition monitoring of stator windings ininduction motors.J. H. Dymond et al. In [SI, ooked at tracking as onefailure mechanism for stator winding failure in electricmachines and described a series of tests on standardinsulation materials aimed at comparing the anti-tracking capability of the materials. The tests showedthat combinations of insulation materials can reduce theanti-tracking capacity of a rather robust insulationsystem and predispose itt o failure.Early detection and diagnosis of incipient faults isdesirable for online condition assessment, productquality assurance and improved operational efficiency

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of induction motors running off power supply mains.So, in [6], a model-based fault diagnosis system isdeveloped for induction motors, using recurrentdynamic neural networks for transient responseprediction and multi-resolution signal processing fornon-stationary signal feature extraction.Another fault for IM is a bar broken of squirrel-cagerotor. Alberto B. t al., reported in [7], he comparisonand performance evaluation of different diagnosticprocedures that use input electric signals to detect andquantify rotor breakage in induction machines suppliedby the mains.Mechanical faults as bearings and slip rings damagewere studied in [S-IO]. Most of these studiesexperimentally introduced without analytical study orstudying induction motor as fed directly from the supplywithout any controller o r drive.The fault analysis for IM in the motoring mode will bepresented in this paper. The paper includes an analysisfor different classifications of drive faults. The faults inan IM-drive -that w ill be studied- can be broadlyclassified as: machine fault, (i.e., one of stator windingsis open or short, multiple phase o pen or short, bearings,and rotor bar, is broken) and inv erter-converter fau lts(i.e., phase switch open or short, multiple phase fault,and DC-link voltage drop). Briefly, a general-purposes o h a r e package for variety of IM-drive faults -isintroduced. This package is very important in IM-faultdiagnosis and detection using artificial intelligenttechniques, w avelet an d signal processing.The paper consists of the following sections: first is anintroduction, second is an overall IM-drive system, thirdsection includes machine-terminal equations, the fourthdescribes the machine control m odel, the fifth part is ananalytical study for most expected IM-faults, six sectionis the simulation results and finally the last hvo sectionsare the conclusion and references.THE OV ERALL I M - DRIVE SYSTEMThe overall proposed IM-drive system is shown infigure ( I ) . The system consists of the PI-controller withfield-oriented vector control, predictive currentcontroller inverter technique and the motor. The motor-stator windings are connected in star form with neutralas shown in figure (2).

Figure (1 ) Overall IM-drive system

3 ilM.sirtor 4ndi.dFigure (2 ) Converter-inverter IM combination

L - - - - - J

MACHINE-TERMINAL EQUATIONSThe study of three -phase systems is usually carried outby using a 123-dqo matrix transfonnation. The dqovariables are determined from 123 variables by usingthe transformation equation:Y:123 (1)

Y h 3 =[Y:I Y h &I T ( 2 )Where

(3)

In (4), A-' =A' and the vectors y: ,23(y:dqo) an beeither the stator voltage vector V : ~ ~ , ( V : ~ ~ ~ )r the statorcurrent vector i:123(i~dqb)The three-phase machine voltages for balancedoperation (as shown in figure 2) are given by:': I = g v : d (5 )

(7)Such expressions have been derived by neglecting thevoltage drop between the neutral of the machine andmiddle point 0, f which the mean value is zero, andalso by using ( I ) with v: = 0 .M ACH I NE CURRENT- CONTROLLER M ODELThe model used to design the current controller for athree-phase machine is obtained from the machinemodel written in dq co-ordinates. It is given by:vZd = rs , +a/,- eid (8)di:ddt

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where, rs,= rs + r,C /If, = 1 I : / l * l , and r3 is thestator resistance, r, is the rotor resistance, I s is thestator inductance, I, is the rotor inductance, and I, isthe mutual inductance. Terms e; and e:q are counterEMFs given by:

=-c;/;/~w,~;~ + ( r , / I r % : d l (10)e;, = +()@r+:d - ( rr / LM:~ I (11)where +:d,+:q and w, are the rotor flux componentsand the machine speed respectively.DIFFERENT FAULTS MODELINGThe common fault in IM is one of stator coil is openedor shorted, so, we will study the machine parametersand performance under different faults.ONE OF STATOR COILS IS OPENEDCase ( I ) : Winding 1 isopenedUsing the transformation equation and the conditioniii 0 , the following relation can be derived for thesystem when winding 1 is opened:vf* = m ( - v : d + &v:q +&io) (12)

(13)

i:? =-gi:dJ x i ;

Case (3): Winding 3 is opened

i: = + (24)

i,:* = JZ,;i;b = Jl/Zifd +mi:qO N E S T A T O R C O I L I S S H O R TA stator phase coil may be completely or partiallyshorted due to insulation failure. In case of short circuit,a high current limited only by the contact and windingresistances will flow through the faulted phase duringthe designated conduction period. Eventually, the sw itchof the inverter fails or a fuse in series with thewindinglswitch blows out and the faulted-phase stopsproducing torque. During the time when the fault occursand the fuse blows out, the speed of the machine mayincrease or decrease drastically depending on thetimming of the fault occurrence. Once, the transient isover, the remaining healthy phases maintain the theconstant speed again by increasing their torquecontribution to the system. The complete shortsimulation of a phase can be simulated by replacing thatparticular phase by a short circuit resistance value.C O N V E R T E R S W I T C H O P E N

When a switch in the converter is open-circuited, itstops supplying excitation current to the correspondingphase winding. This case is similar to the stator-phasewinding open case and can br simulated by stoppinggate signals to that phase at the instant of fault.C O N V E R T E R S W I T C H S H O R TWhen a switch in the inverter is short-circuited, thecorresponding phase winding receives continuousexcitation irrespective of the rotor position andcontroller logic. This faulty excitation results unlimitedcurrent through the phase supported only by the smallwinding resistance and back-emf of the machine.Eventually, the switch or the fuse blows out and thefaulted phase stops producing further torque. In aninverter with bridge topology of two switches per phase,both the switches of the phase are to be shorted for theshort circuit fault. So, the scenario of faults depends onthe inverter topolgy used, since a wide variety ofinverter topologies are available for IM.D C V O L T A GE D R O PWhen the supply DC voltage drops, the speed decreasesmomentarily and then returns to the previuos valuedrawing more line current from the source. The linecurrent increases to compensate for the decreasedsupply voltage, and hence, to satisfy the, input-outputpower conservation relationship bt a constant speed.However, speed regulation is only possible up to acertain maximum amount o f voltage drop. If the supplyvoltage drops below the maximum limit, the phasecurrent reaches its upper limit and the speed settles at avalue lower than the commanded speed.

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SIMULATION RESULTSThe performance charactentics of IM under differentmachinelinverter-converter faults are presented hereshowing the waveform of speed, electrical internaltorque and current in the healthy/faulty phases. Themain simulation program was written in Borland Ctt-and this software package is a main general purposepackage for testing any induction rno1or.h this paper,we test this package for simulation on only 1-HPlaboratory induction motor. The machine analyzed hasthe following data (table 1)

Rated CurrentStator resistanceRotor resistance

Table 1: Parameten of the test machine

2.3 A10.5 n

11.08 n

Parameter Value unitRated Line VoltageRated PowerRated frequencyNumber of poles pulesRated soeed 910 r.u.m

Magnetizing reactance 126.3Moment of rotor inertia

The selected-motor reference speed is 50 ele. rad./ sec.Results for the faults of stator-phase winding open andshort are given here.STATOR-PHASE WINDING O P E NFigure (3) shows the simulation results for the statorhealthy and faulty phase currents when stator windingl(a) is open. The falut occurs at 2.0 seconds duringsteady-state operation. In this figure, the current isnormally before the fault, then the phase 1 currentreaches to zero. Figures ( 4 3 describe the other twophases currents before and after the fault occuredrespectively. We notice that the two currents aresuddenly increased after the fault occurred and becomegreater than the normally rated currents. Figure (6)shows the actual and reference speed curves before andafter the fault occurance. Through the faull, the actualspeed is suddenly decreased to zero or the mtor isoperated in a regeneartive modeand then alternativelyincreased and oscillated around the reference speedvalue. Also, figure (7) describes the electrical internaltorque for healthy and faulty test motor. We notice that&e instantenous-electrical -internal torque is with in thenormal rated range before the fault and suddenlydecreased to zero then increased with oscillated value.

0

0 i *,Figure (3) Stator-phase l,a current

Figure (4) Stator phase-2,b current

I*,Figure ( 5 ) Stator phase-3,c current

Figure (6) Rotor speed (elec. radlsec)

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Figure (7 ) Electrical internal torque (7V.m)STATOR-PHASE WINDING SHORT

Figure (IO) tator 3,c phase current

The studoed case, puposes that the faluty phase is phae1,a. Figure (8) describes the stator healthy/faulty phasecurrent. We observe that the current is in normal ratedvalue before the fault and suddenly increased to a hugevalue. Dependently, other two phases currents aredecreased as shown in figures (9,lO) respectively.Because the motor operates at very low speed (i.e., 50elec. radkec ), through the fault, we notice a speed dip ate 2 . 0 sec with a flicker value and noise (figure 11).Figure (12) describes the electrical internal torque.1

IllIIl Figure (1 1) Rotor speed

L t aFigure (8) Stator I,a phase current

' ' ; ' (0

1Figure (12) Motor electrical internal torque

OTHER FAULTSFurther analysis showed that the machine was able tomaintain constant speed up to a DC voltage drop of40%, although with higher speed and torque ripples. ADC voltage drop of 60% reduced speed by 40%. Th espeed dropped 80% due to a DC voltage reduction.Figures (13,14) how the speed and torque respectively,when a DC link voltage was reduced from (540V o 200

2 2

Figure (9) Stator 2,b phase current

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V) . The speed and torque after reduction are reduced tozero.

Figure (13) Rotor speed

-1 IFigure (14) Mo tor electrical internal torque

CONCLUSIONThe fault simulation methodolgy presented here makesit possible to investigate speed-ripple, torquefluctuation, dynamic response, drive stresses etc. undervarious fault conditions in an induction motor. Theprocedure is general enough to be applied to any type ofinduction motor. The proposed ready-made softwarepackage is reliable and easy to be used in simulation.Also, this package is very important in IM-faultdiagnosis and detection using artificial intelligenttechniques, wavelet and signal processing. In general,we concluded that the IM is capable of operating undervarious fault conditions, although the performancedeteriorates.

REFE E CE[ I] E. J. Wiedenbrug, A. Ramme, E. Matheson, A. V.Jouanne, and A. K. Wallace ,* Modem online

testing of induction motors for predictivemaintenance and monitoring", IEEE Transactionson Industry Applications, Vol. 38 , No. 5, SepdOct.2002.[2 ] Z . Maljkovic, M. Cettolo, and M. Pavlica, "Theimpact of the induction motor on short-circuitcurrent", IEEE Industry Applications Magazine,July-Aug. 2001.[3 ] J. L. Kohler, J. Sottile and F. C. Trutt, "Conditionmonitoring of stator windings in induction m otors:Part I- Experimental investigation of the effectivenegative-sequence impedance detector " , IEEETransactions on Industry Applications, Vol. 38,No . 5, Sept./Oct. 2002.[4] J. Sottile, F. C. Trutt , and I. L. Kohler, "Conditionmonitoring of stator windings in induction motors:Part 11- Experimental investigation of voltagemismatch detectors ", IEEE Transactions onIndustry Applications, Vol. 38 , No . 5, Sept./Oct.2002.

[SI 1. H. Dymond, Nick Stranges, K. Younsi, and J. E.Hayward, "Stator winding failures: contamination,surface discharge, tracking", IEEE Transactions onIndustry Applications, Vol. 38, No. 2, Mar./Apr.2002.

[ 6 ] K. Kim and A. G. Parlos," Induction motor faultdiagnosis based on wavelet signal processing",IEEE/ ASME Transactions on Mechatronics, Vol.7, No. 2, June 2002.[7] A. Bellini, F. Filippetti, G . Franceschini, C. Tassoni,an d G . K h a n , " Quantitative evaluation ofinduction motor broken bars by mean s of electricalsignature analysis", IEEE Transactions on IndustryApplications, V ol. 37 , No . 5 , Sept./Oct. 2001.[8] W. R. Finley and M. M. Hodowanec, "Sleeve versusanti-friction bearings: selection of the optimalbearing for induction motors", IEEE Transactionson Industry Applications, Vol. 38, N o. 4,July/Aug. 2002.[9 ] H. E. Boyanton and G . Hodges, "Bearing fluting:the results of a long-term investigation into bearingfluting on AC motors, DC motors, and rolls onpaper machines", IEEE Industry 'ApplicationsMagazine, Sept/ Oct. 2002.[IO] J. W. Sm ith, "Tolerance rings: presenting examplesof motor-overload- protection applications fromthe appliance and automotive industries", IEEEIndustry Applications Magazine, Sept/ Oct. 2002.

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Fault Analysis of Current-controlled Pwm-myerter Fed

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