SENSORLESS BROKEN BAR DETECTION IN INDUCTION MACHINES

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1 SENSORLESS BROKEN BAR DETECTION IN INDUCTION MACHINES Hakan Ç ALIŞ 1 Abdülkadir Ç AKIR 1 Mülüm ARKAN 2 1 e-pota: hcali@tef.du.edu.tr 1 e-pota: cakir@tef.du.edu.tr 2 e-pota: markan@inonu.edu.tr 1 Department of Electronic Computer Education The Faculty of Technical Education Suleyman Demirel Univerity, 32260, Çünür- Iparta 2 Department of Electrical- Electronic Engineering The Faculty of Engineering, Inonu Univerity Malatya Key Word: Broken bar, Fault detection, Induction machine ABSTRACT Thi tudy decribe broken bar detection in induction motor before actual breakdown occur. It i baed on fluctuation of tator current Zero-Croing Time (ZCT). Intead of ampling motor current with an high reolution A/D converter, zero croing intant are recorded a waveform cro zero. Fluctuation in the interval between ucceive zero-croing of the three phae current waveform are analyed uing Fat Fourier Tranform (FFT). Rotor bar fault are identifiable from amplitude change of 2 f pectral component where i the motor lip. I. INTRODUCTION There are two kind of induction rotating motor, the quirrel-cage motor and the wounded rotor motor. Since the quirrel-cage induction motor i low-priced, robut and rugged, imple and eay to maintain, it ha become the mot commonly ued electrical rotating machine in indutry. It i becoming increaingly important to ue condition monitoring technique to give early warning of imminent failure. Detection of broken rotor bar at particularly an early tage, i rather difficult than tator fault. On the one hand, rotor bar fault are uually aociated with the high temperature attained in the rotor, and the high mechanical loading on the end ring of the cage, particularly during tarting time. On the other hand, fault may occur during manufacture proce, either through defective cating in the cae of die cat rotor, or through poor jointing in the cae of brazed or welded end ring. Such a defect in rotor will reult in a high reitance which will overheat at that area, and the high temperature will impair the trength of the cage. Cracking or mall hole may then occur in the rotor bar. Rotor bar fault are more likely to take place at the cage end ring where the rotor bar are not fully upported by the laminated rotor core. It i known that the early indication of induction motor rotor bar fault are pulation in the peed, motor current and tray leakage flux of the machine. A continual attempt to develop new approach for cot-effective, enitive, reliable and on-line rotor bar failure prediction i alway highly encouraged by indutry. Rotor aymmetry can be detected by monitoring the fluctuation in rotor peed [1][9]. However, it i difficult to determine the degree of aymmetry. In addition, an extra enor (e.g. tachometer) i needed for the monitoring. The diadvantage of thi method i that the enitivity of the meaurement depend largely on the inertia of the load, and the amplitude of the peed fluctuation will be much maller when a high-inertia load i applied to the machine. Vibration analyi, thermal analyi and current pectrum analyi have been applied to monitor rotor bar fault [1]. Mot recent effort eem to be focuing on current pectrum analyi a the current ignal i eaily acceible for all induction motor. Practice and theory ha demontrated [2][3] that when a induction motor i upplied by a balanced main upply of frequency f and running at a lip of, there will be a tator frequency component ( 1 2) f which i induced due to rotor light unbalance caued by ome mall hole in the rotor bar. The magnitude of thi induced e.m.f. i generally very mall compared to the component of the main, and therefore it i very difficult to detect. It i only poible to detect on full-load, i.e., running with maximum lip. Alo, when the load change during the ampling period the ( 1 2) f component may mear out. Hu [6] propoed to ue air-gap torque to monitor defect uch a cracked rotor bar and horted tator coil. Elkaabgy et al [8] ued an external earch coil mounted on the motor frame to analye the induced voltage waveform. Some of thee technique will have limited uage, ince they either need ophiticated intallation with expenive enor or complicated mathematical modelling and high computational power. In indutry however, low cot and PDF created with FinePrint pdffactory trial verion

2 high enitivity of a motor diagnotic ytem i highly demanded. In thi tudy, Zero-Croing Time (ZCT) method, invented firtly by Wang in Suex Univerity, England, to detect broken rotor bar in an induction machine i ued [10]. The method i utilied to extract the new characteritic rotor failure component. The ZCT method avoid the ue of traditional tator current pectrum for rotor defect detection o that the annoying proximate relation between main frequency component f and failure component ( 1 2) f i no longer a problem. The detail of the ZCT method for rotor fault detection will be decribed in following ection. It i hown that the ZCT method i both enitive and low cot. Again, the principle of ZCT method can be applied in the variable peed drive or oft tarter by oftware modification only. In addition, an inexpenive and independent tandalone circuitry can be employed.. II. OBTAINING THE ZCT SIGNAL The ZCT ignal conit of a erie of data value, obtained at each zero croing time of the 3-phae current. The value of data are defined a the time difference between two adjacent zero-croing time ( Tn Tn 1 ) minu the time interval for 60 degree T60 of the main upply a hown in figure 1 [10, 11]. Current ZCT n+1 = T n+1 - T n - T60 Tn+3 - Tn = T180 Figure 1 Decription of ZCT ignal In ideal condition (perfectly balanced upply and completely ymmetrical tator winding) the value of ZCT ignal i zero. In practice, it i almot impoible to aume an ideal upply and a complete ymmetrical induction motor, which inevitably reult in correponding fluctuation in the 3-phae current zerocroing time [10,11]. In fact, we have found that the ZCT ignal carrie rich information that reflect both t induction motor internal condition and external influence uch a upply unbalance. III. SIMULATION OF BROKEN BAR FAULTS USING MATLAB/SIMULINK MODEL In thi tudy, 3Hp, 4 pole quirrel cage induction motor i ued for imulation. Block diagram of an induction motor Simulink model i hown a in figure 2. Rotor bar fault i imulated by externally adding three tar connected rotor reitor. The each value of rotor phae reitance, between 0,10Ω and 0,16Ω. i changed in 0,02Ω tep. In addition, zero croing detection circuit, power pectrum etimation of ZCT and motor current ignal are included with diplaying and toring in a data file block. In thi model, for the each value of rotor reitance amplitude change in pectrum of tator current and ZCT ignal are determined. The imulation reult for unloaded motor are hown a in Table 1. Loaded condition uch a 7.5 Nm 15 Nm (half load) and full load (30Nm) are diplayed in Table 2, Table 3 and Table 4 equentially. For unloaded motor, broken bar detection from amplitude change, in tator current (at f ± f component) and ZCT pectrum (at 2 f ) can not give good reult. (ee Table 1) For full load condition wherea amplitude change (at ± f) in motor current pectrum are very mall, magnitude change of 2 f component in ZCT pectrum are more evident even for mall rotor bar fault condition (in a 0,02Ω increment). With development of fault, thi increment reache to value of A hown in figure 3a-b increment on f ± f component eem ditinct due to caling vertical axi in db unit. In practice, the amplitude of thee ideband are too mall to detect compare to large amplitude of upply frequency (ee figure 3a-b). On the other hand, the pectral change at 2f in ZCT pectrum in cae of rotor bar fault i around 50dB. Thi variation i till in detectable level for variou load condition. (See figure 4a-b) f PDF created with FinePrint pdffactory trial verion

3 Figure 2 Simulink model of enorle broken bar detection for induction motor Table 1 Simulation reult for unloaded induction motor Rotor reitance change (Ω ) component 2f 0, , , , f 9,824 9,824 9,824 9,824 (1-)f 9,824 9,824 9,824 9,824 (1+)f 9,824 9,824 9,824 9,824 2f 1, , , , f 6, , , , (1-)2f 6, , , , (1+)2f 6, , , , Table 2 Simulation reult for loaded induction motor (7,5 Nm) Rotor reitance change (Ω ) component 2f 0, , , , f 10,48 10,48 10,48 10,48 (1-)f 10,48 10,48 10,48 10,48 (1+)f 5,214 5,214 5,214 5,214 2f 6, , , , f 2, , , , (1-)2f 2, , , , (1+)2f 2, , , , Table 3 Simulation reult for half loaded induction motor (15 Nm) Rotor reitance change (Ω ) component 2f 0, , , , f 12,36 12,36 12,36 12,36 (1-)f 0, , , ,02041 (1+)f 0, , , , f 2, , , , f 1, , , , (1-)2f 1, , , , (1+)2f 4, , , , PDF created with FinePrint pdffactory trial verion

4 Table 4 Simulation reult for fully loaded induction motor (30 Nm) Rotor reitance change (Ω ) component 2f 0, , , , f 18,32 18,32 18,32 18,32 (1-)f 0, , , ,01563 (1+)f 0, , , , f 9, , , , f 1, , , , (1-)2f 3, , , , (1+)2f 2, , , , (a) Figure 3 Motor current pectrum a-) For ideal rotor (0,1Ω) b-) For faulty rotor bar (0,06Ω increment in rotor reitance) (a) Figure 4 ZCT Spectrum a) For ideal rotor (0,1Ω) b-) For faulty rotor bar (0,06Ω increment in rotor reitance) The effect of upply voltage unbalance on rotor bar fault detection characteritic frequency component (2 f ) i examined a hown in Table 5. Separation of thee two fault, imultaneouly occurred i realized the monitoring of 2 f component in ZCT pectrum. component 2 f can till be ued when motor upplied from unbalanced voltage ource. 2 f Frequency component appear only when upply i unbalance. Motor current and ZCT pectrum are hown in Figure 5-6 repectively for mixed fault. Table 5 Simulation reult for fully loaded motor upplied by unbalanced voltage ource (216V 1.8 % decrement in voltage level) for variou broken bar level Rotor reitance change (Ω ) component 2f 0, , , , f 9,576 9,576 9,576 9,576 (1-)f 9,576 9,576 9,576 9,576 (1+)f 4,764 4,764 4,764 4,764 2f 5, , , , f 0, , , , (1-)2f 0, , , , (1+)2f 0, , , , PDF created with FinePrint pdffactory trial verion

5 (a) Figure 5 Motor current pectrum of faulty rotor bar (0,06Ω increment in rotor reitance) a) Balanced upply b) Unbalanced upply (a) Figure 6 ZCT pectrum of faulty rotor bar (0,06 Ω increment in rotor reitance) a) Balanced upply b) Unbalanced upply IV. CONCLUSIONS Broken bar detection in three phae induction motor i implemented with ZCT method intead of uing traditional upply current monitoring in frequency domain. Motor model i imulated in Matlab/Simulink environment and broken bar fault are imulated by adding extra rotor reitance. The amplitude change in 2 f characteritic frequency component of ZCT ignal i monitored clearly. The main advantage of ZCT method i not requiring fat ampling rate and high reolution A/D converter. It may be neceary to include additional ignal uch a motor vibration or temperature to extend reliability of fault detection ytem for critical machine. In the next tage of work i going to be implementation of the ZCT method a online detection uing Goertzel algorithm. REFERENCES 1. Va, P., Parameter Etimation, Condition Monitoring, and Diagnoi of Electrical Machine, Claredon Pre, OXFORD, UK, Ho, S. L. (1991). Condition Monitoring of Induction Motor, Fifth International Conference on Electrical Machine and Drive, London, U.K. pp Kliman, G. B., Stein, J. and Endicott, R. D.,(1988), Non-invaive Detection of Broken Rotor Bar in Operating Induction Motor, IEEE Tranaction on Indutry Application, Vol. 3, No. 4, pp Ho, S. L., Chan, W. L. and Leung H. W. (1993). Application of Statitical Signal Proceing for Condition Monitoring of Rotor Fault in Induction Motor, Sixth International Conference on Electrical Machine and Drive, Oxford, U.K. pp Jiang, J. G., Zhang, Z. P., Su, P. S. and Wang, X. H.,(1992) Time Frequency Spectrum (TFS) of Line Current During Starting Proce --- A Tool for Diagnoing Failure of Induction Motor International Conference on Electrical Machine. Vol 3, UMIST, Mancheter, U.K. pp Hu, J. S. (1994). Monitoring of Defect in Induction Motor Through Air-gap Torque Obervation, IEEE IAS Annual Meeting. 7. Waton, J. F., Elder, S. (1992). Tranient Analyi of the Line Current a a Fault Detection Technique for 3-phae Induction Motor, International Conference on Electrical Machine. Vol 3, UMIST, Mancheter, U.K. pp Elkaabgy, N. M., Eatham, A. R. and Dawon, G. E. (1992) Detection of Broken Bar in the Cage Rotor on an Induction Machine, IEEE Tranaction on Indutry Application, Vol. 28, No.1, pp Tavner P. J., Penman J. (1987) Condition Monitoring of Electrical Machine, Reearch Studie Pre Ltd., U.K. PDF created with FinePrint pdffactory trial verion

6 10. Wang, Y., The ZCT Method of Induction Motor Failure Prediction and Speed Monitoring, Phd Thei, Univerity of Suex, England, Gürdal, O., Ceur O., A New Senorle Speed Detection Method By Fluctuation of Zero Croing Time Signal in Induction Motor, Journal of Polytechnic vol:2 no:1, pp , Ankara Boyle, C. and et al. : Online Current Monitoring to Detect Mialignment and Dynamic Eccentricity in Three Phae Induction Motor Drive, Proc. 29 th Power Engineering Conf., Vol 1, pp 5-8,1994, Ireland. 13. Bruel & Kjaer : Vibration Diagnotic for Indutrial Electric Motor Drive, Application Note. 14. Dorrell, DG. and et al. : Analyi of Airgap Flux, Current, and Vibration Signal a a Function of the Combination of Static and Dynamic Air-gap Eccentricity in 3-Phae Induction Motor, IEEE Tran. On Indutry Application Vol 33, No 2, January/February Farag, S.F., et al : An Integrated On-Line Motor Protection Sytem, IEEE IAS Annual Meeting, October Kliman, GB, et al.: Method of Motor Current Signature Analyi, Electrical Machine & Power Sytem, Vol 20, (5), pp463-74, Kliman, GB, et al.: Induction Motor Fault Detection via Paive Current Monitoring, Electrical Machine & Power Sytem, Vol 20, (5), pp463-74, Leith, D. and et al, Real Time Expert Sytem for Identifying Rotor Fault and Mechanical Influence in Induction Motor Phae Current, IEE 5th ICEM, pp46-50, September Penman J, and Tavner P.J.,: Condition Monitoring of Electrical Machine, Wiley & Son, New York, Rao, B.K.N.: Handbook of condition monitoring, Elevier Science Ltd, Schoen, R.R., et al : An Unupervied On- Line Sytem for Induction motor Fault Detection Uing Stator Current Monitoring, IEEE IAS Annual Meeting, October Va, P., Parameter Etimation, Condition Monitoring, and Diagnoi of Electrical Machine, New York, Oxford Univerity, Yang, SJ. Low Noie Electrical Motor, Clarendon Pre, UK, Ç alış, H., Unworth, P., : Fault Diagnoi in Three Phae Induction Machine by Motor Current Signal Analyi, The 1999 IEEE International Sympoium on Diagnotic for Electrical Machine, Power Electronic and Drive, Univerity of Oviedo, Spain, September Ç alış, H., Ç akır, A.: State of Art of Fault Diagnoi Technologie, IMS2001, International Intelligent Manufacturing Sytem, Sakarya Univerity, Sakarya, Augut PDF created with FinePrint pdffactory trial verion