International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD)
Stator Fault Detection in Induction Motor Under Unbalanced Supply Voltage Anju Jacob PG Scholar Dept. of Electrical &ElectronicsEngg. Amal Jyothi College of Engineering Kanjirappally, Kerala
[email protected]
Victor Jose Asst. Prof. Dept. of Electrical & ElectronicsEngg. Amal Jyothi College of Engineering Kanjirappally, Kerala
[email protected]
Abstract—Induction motors are the most commonly used machine in industry because of their robust nature.As any other machine, due to the mechanical and electrical stresses, inductionmotor is also subjected to various types of failures. Among these, after bearing faults, stator faults are found to be the most frequent faults in induction motor. Interturn short circuit(ITSC) faults constitute a large percent of all electrical faults and it is theroot cause of all other stator faults. So continuous monitoring of ITSC fault is necessary as rewinding the motor is usually preferred than replacing it. Several techniques have been developed for the detection of stator short circuit faults. But they have limitations due tothe dependence onload variation, machine inherent asymmetry and unbalanced supply voltage. Amongthese a more reliable and simple method for practical implementation is found to bethe symmetrical component method. Negative sequence current method insensitive tosupply voltage unbalance is discussed here. Simulation study is carried out for differentvoltage unbalance. An experimental setup to model stator fault is developed. Also, ahardware setup is developed for on-line fault detection of induction motor. Keywords—Fault detection; Induction motor; Negative sequence current; Stator short circuit fault; Voltage unbalance
I.
INTRODUCTION
Induction motors, also named asynchronous motor, are the widely used prime mover for various equipments in industrial applications due to their simplicity of construction, reliability,high overload capability, and high efficiency. The range size of induction motors are from fractional HP to over 100,000 horse power. Compared toother motors, induction motors are less expensive, more ruggedand require less maintenance. Therefore they are the preferred choice for industrial motors. Induction motors can also be subjected to various types of failures due toenvironmental factorsor machine internal factors. Motor faults show only minor symptoms but may result in higher energy consumption, lower efficiency and poorer performance. Even small faults results in large losses,
978-1-4799-5202-1/14/$31.00 ©2014 IEEE
Dona Sebastian Asst. Prof. Dept. of Electrical & ElectronicsEngg. Amal Jyothi College of Engineering Kanjirappally, Kerala
[email protected]
high temperature which will deteriorate insulation, increased vibration level which may reduce bearing life. Therefore the diagnosis of induction motor health is necessary andit can prevent maintenance cost. According to published surveys, induction motor failures include bearing related faults (40%), short circuit stator windings (38%), rotor related faults (10%) and other faults such as end ring faults (12%) [1]. Studies show that stator winding faults are the second to bearing faults in frequency of occurrences in induction machines.But bearing faults can be more easily detected by the physical examination. In small motors, they can be detected by the noise produced by the machine whereas in large motors, temperature sensors are provided for online monitoring where a temperature rise indicates the fault. The stator faults start with minor inter-turn short circuits (ITSC) within a coil. These short circuits occur due to hot spots developed in the winding. ITSC leads to severe faults such as phase-to-earth and phase-to-phase short circuits. The large current due to short circuit again increases the winding temperature. This cumulative effect deteriorates the winding insulation and leads to insulation failure. These serious faults whichmay lead to destruction of the stator core can be preventedby the early diagnosis of ITSC faults.If the shorted turns are detected early before the insulation failure, they can be rewound and the machine failure can be prevented. Usually rewinding of motor is faster than replacement of the machine. On-line motor condition monitoring tracks the operatingcharacteristics of the machine so that the current condition of the machine can be estimated. Thevariations and trends of themonitored signal can be used to predict the need formaintenance before a breakdown or serious deterioration occurs [2]. Thus, increasing research has been developed in on-line condition monitoring techniques for electrical equipment, mainly including transformer, generator, and induction motor in power plants. Numerous methods of induction motor fault diagnosis were developed in the last decades [3-8]. Park’s vector approach is one of one of the simplest methods in which by monitoring the deviation of current Park’s vector the motor condition can be predicted and the presence of fault can be
International Conferennce on Magnetics, Machines & Drives (AICERA-2014 iCMMD) detected [3]. However, most of the studiess have been done under the assumption that the machine operates under a symmetrical three phase supply voltage witth no distortion. A simulation study was done on the effect of voltage unbalance on current unbalance using Park's vector approach. It was found that a small percent of voltage unbalaance reflected in a higher percent of current unbalance. So wheen stator current is considered for fault diagnosis, erroneous decision may be made about detection of fault. I.e. under unbalanced u supply voltage a healthy motor may be detected ass faulty due to the large value of current unbalance. mmetry can be used Stator signature analysis and stator asym for stator fault diagnosis of induction motor. The asymmetry of the winding increases when inter turn shhort circuit occurs. Sothe negative sequence components are chhanged when fault occurs. Hencethe variationsin negative sequuence current can be considered as a fault detection techniquue. It is based on themonitoring of current and voltage of the three t phases of the motor. II.
ONLINE DIAGNOSIS OF STATOR SHORTED TURNS BASED ON NEGATIVE SEQUENCE CURRENT
Inter turn short circuit of an inductiion motor causes asymmetry in statorwinding, thus changging the negative sequence component in the line current [9].The negativesequence component of current, Ia2can be eaasily calculated by measuring the phase currents. So neegative sequence component of current can be used for thhe winding health analysis of induction motor.
(1) Where
/
However, unbalanced supply voltages annd inherent motor asymmetries also show a similar effect. So for the fault detection strategy these factors have to be coonsidered. In other words, these are the factors which causeenegativesequence current in healthy motors. When ITSC occuurs this asymmetry will increase. Therefore, the measured negative-sequence n current is the superposition of three com mponents, inherent asymmetries, voltage unbalance and short cirrcuit fault [9]. The negative sequence current due to short cirrcuit fault (Isca2)is obtained by subtracting those due to voltage v unbalance (Iva2)and the machine inherent asymmetry (Iia2). An increase of its valuegives an indication of inter-turn shorrt circuit.
III. STATOR FAULT DETTECTION UNDER UNBALANCED SUPPLY VOLTAGE A lot of studies have been done on induction machines in the last century. However, mosst of the studies have been done under the assumption that thhe machine operates under a balanced three phase supply voltage v with no distortion. Even though induction motors are designed to work efficiently under sinusoidal and symmetriical supply, voltage variation up to10% and frequency variatioon up to 5% are permitted for normal operation of inductioon motor as per IS 325. A substantial deviation from it can be fatal to the machine. Voltage asymmetry can originnate from structural asymmetry and load unbalance. A. Effect of Voltage Unbalance on Induction Motor Three phase induction motors are designed and manufactured to operate under u balanced voltage.When unbalanced voltage is applied to polyphase induction motor, unbalanced currents will flow in the stator windings. A small amount of voltage unbalance may result in large value of current unbalance. This can have h severe effect on the motor and cause motor may overheat.. The presence of unbalaanced voltages on polyphase induction motors introduces a negative sequence voltage which rotates in opposite dirrection to that occurring with balanced voltages. This negativve sequence voltage produces in the air gap a flux rotating in opposite direction to that produced by positive sequencee voltage. As a result a negative torque is developed against thee rotation of the rotor.Thistends to produce high currents[10]]. A small value of negative sequence voltage may generatteconsiderably large currents in windings as compared to thosee present under balanced voltage conditions.The negative sequennce component of torque cannot convey energy to the motoor mechanical load and it is dissipated as losses. As a resultt, the machine torque is reduced and efficiency is decreased. B. Stator Fault Detection Methhod Insensitive to Supply Voltage Unbalance When the induction motoor is supplied with unbalanced voltage, there will be a large peercent of current unbalance for a small percent of voltage unbbalance. So the fault detection based on the stator current mayy not be correct. i.e., the current unbalance in the stator winnding due to supply voltage unbalance may be mistaken ass winding fault in the machine. By separating out the current unbalance u due to supply voltage unbalance from the total cuurrent unbalance, the current unbalance due to winding faullt can be obtained and thus the fault detection technique can bee more accurate.
Fig.2. Positive equivalentt circuit of Induction Motor Fig. 1. Causes of negativee sequence current in IM [9]
International Conferennce on Magnetics, Machines & Drives (AICERA-2014 iCMMD)
IV.
Fig. 3.Negative equivalent circuit of Inducttion Motor
Here the fault detection is based on symmetrical components of voltage and current, moree particularly the negative sequence current [11]. Based on symmetrical component theory unbalanced voltage can bee decomposed into symmetrical components as positive, negativve and zero phase sequences (Vp, Vn and V0 respectively).Vp and Vn produces balanced currents Ip and In respectively. The total current produced by the original unbalanced voltagescan be represented as the combination of the balancced currents Ip and In.Fig.2 and Fig. 3 shows the positive and negative n sequence equivalent circuits of the induction motor respectively.X r m is the magnetizing reactance, Rs and Xs are thhe stator resistance and stator leakage reactance respectively anndR’r and X’r are the rotor resistance and rotor leakage reactannce referred to the stator frame respectively. p sequence In Fig.2 the stator is supplied with positive voltage and the motor operation is similar to that under normal balanced supply. Here the slip of the motor is ‘s’ with respect to the positive sequence field of the stator. In Fig. 3 negative sequence voltage Vn is given to the stator and a the slip of the rotor is (2-s) with respect to the negative sequuence field. Fig. 4 shows the block diagram of thhe fault detection technique which is less sensitive to supply voltage v unbalance. Superposition theorem is applied in this techhnique[11]. VA, VB and VC are the phase voltages and IA, IB annd IC are the phase currents. The total negative sequence currennt (due to voltage unbalance and fault) is calculated from the t phase current values. (2-s) model is supplied with the negative n sequence voltage Vnwhich is calculated from the phhase voltages. The negative sequence current due to supply volttage unbalance can be obtained by solving the (2-s) model suppplied with negative sequence voltage. The total negative sequence current iss denoted as Incal. Inuvand Inf are the negative sequence currennts due to voltage unbalance and fault respectively. All the valuues are in complex form. Applying superposition principle, Inuv is subtracted from Incal to obtain Inf, the negative sequence curreent due to fault. Inf can be taken as a reliable indicator of stator winding w fault.
A three phase squirrel cagee induction motor is modeled in matlabwith the following paraameters: 1HP, 415V, 1480Rpm, 50Hz and 4Pole. Also, Statorr resistance R1= 12.35Ω, Stator inductance (Lls)= 0.04071H, Rotor R resistance (R2) = 6.991Ω, Rotor inductance (Llr)= 0.040071H, Magnetizinginductance = 0.54008H and Moment of inerrtia (J) = 0.010Kgm2. The (2-s) model is obtained from the negative sequence equivalent circuit of the induction motor. Simulation is done for different values of Voltage Unbalaance Factor. According to International Electro technicaal Commission (IEC), Voltage Unbalance Factor (VUF) is givven by %
100
(2)
( and Angle) are the Vp, Vn, Inuv, Incal, and Inf (Magnitude parameters taken up for analysiis. VUF is varied from 0% to 5% and the simulated results are tabbulated in Table I.From the table it can be seen that, the magnitude of Incalincreases as voltage unbalance increases. InBVsimuulated with balanced voltage is taken as reference value. The magnitudes m of Incal, Inf and InBV are plotted against different valuues of VUF in Fig.5. TABLE I. RESULT OF FAULTY INDUCTION MOTOR O SIMULATED UNDER DIFFERENT VOLTAGE UNBA ALANCE FACTOR VUF Vp(V) Vn(V) Incal (A) Inuv (A) Inf (A) (%) 0
0.6
1
1.5
2
2.5
3
3.5
4
4.5
Fig.4. Bock diagram of the fault detection technique less sensitive to supply voltage unbalance [11]
SIMULA ATION RESULTS
5
338.8
0 0.507
0.018
Angle(deg)
0
-96.07
-1.402
-2.535
-0.563
Mag.
336.9
2 2.022
0.046
0.0590
0.0155
Angle(deg.)
0
-165.6
2.37
5.533
-0.106
Mag.
335.6
3 3.357
0.089
0.098
0.0161
Angle(deg.)
0
-171.4
2.284
2.432
2.508
Mag.
333.9
4 4.977
0.142
0.145
0.019
Angle(deg.)
0.001
-174.3
2.251
2.382
0.597
Mag.
332.3
6 6.603
0.195
0.193
0.023
Angle(deg.)
0.002
-175.7
2.235
2.356
0.828
Mag.
330.7
8 8.232
0.248
0.24
0.028
Angle(deg.)
0.002
176.6
2.227
2.341
0.929
Mag.
0.0148
0.018
Mag.
329
9 9.862
0.3012
0.288
0.0346
Angle(deg.)
0.003
-177.1
2.221
2.33
1.082
Mag.
327.4
11.49
0.354
0.336
0.0405
Angle(deg)
0.003
-177.6
2.217
2.323
1.155
Mag.
325.8
13.12
0.407
0.384
0.0465
Angle(deg.)
0.003
177.9
2.214
2.317
0.209
Mag.
324.3
14.62
0.456
0.428
0.0521
Angle(deg.)
0.004
-178.1
2.212
2.312
1.247
Mag.
322.8
16.12
0.504
0.472
0.0577
Angle(deg.)
0.004
-178.3
2.211
2.309
1.278
International Conferennce on Magnetics, Machines & Drives (AICERA-2014 iCMMD)
fo modeling of stator fault Fig.7. Hardware set up for Fig. 5. Comparison of magnitude of Incal,Inf,InBV for different values of VUF
Magnitude of Incalvaries proportionallyywith the voltage unbalance. Hence, Incal value gives erroneouus fault detection. The negative sequence current technique disccussed reduces the sensitivity to supply voltage unbalance, thus introducing significant correction on each value of Incal. This makes fault indicator, Infclose to the reference value InBV. Thus the effect of voltage unbalance on the negative sequencee current for fault detectiontechnique is reduced. Simulated reesults show that in this technique based onsuperposition principle p negative sequence current for fault is less senssitive to voltage unbalance. Therefore the stator windingg fault detection technique is improved.
A. Case I : No Load Conditionn The motor is run at no load condition under different unbalance conditions and the exxperimental result is tabulated in Table II. ZA, ZB, ZC are the exteernal impedances added.V is the voltage measured and I is the measured m current. is the angle by which current lags voltage. The graph of magnitude of Incal, Inf ,InBV for different values of VUF V is plotted in Fig. 8.
VUF (%)
0
V. EXPERIMENTAL SET UP AND HARDWARE The experiment is performed in a 1 HP, 415 V, 1.8 A, 50 Hz, four-pole, squirrel-cage induction motor. m The circuit diagram for the experimental set up is givven below in Fig. 6.The stator currents are stepped down using a current transformer with turns ratio (1/100). Supplyy voltages and the phase currents of the motor under healthy annd faulty condition are measured for different voltage unbalannce factor. Supply voltage unbalance is created by using thhree single phase autotransformers. Stator winding fault is created c by adding external impedances using rheostats in series with star connected stator winding and byy varying the impedance.Hardware set up for modeling of stator fault is shown in Fig. 7. The voltage, current andpoower factorin each phaseis measured using power analyzer.Meaasurement is done for both no load and loaded condition.
1.5
0
0.7
1.5
2.2
3
4.5
Fig. 6. Circuit diagram for experimentall set up
TABLE A II. EXPERIMENTAL RESULT OF FAULTY IM UNDER NO LOAD WITH DIFFEERENT VUF ZA B Vp(V) Vn(V) Incal(A) Inuv(A) R Y ZB V Mag. V V M Mag. Mag. Mag. I I I Ang. Ang. Ang. A Ang. ZC
(Ω)
phi
phi
phi
20
230
230
230
20
1.2
1.2
1.2
20
77
77
80
20
230
230
220
20
1.2
1.2
1.1
20
74
74
85
20
230
230
230
20
1.3
1.3
1.3
10
84
71
79
20
230
230
225
20
1.2
1.3
1.3
10
68
76
82
20
230
230
220
20
1.3
1.3
1.2
10
62
73
84
20
230
230
215
20
1.3
1.3
1.2
10
60
74
86
20
230
230
210
20
1.4
1.4
1.1
10
65
75
95
20
230
230
200
20
1.3
1.3
0.9
10
65
93
75
Inf(A)
(deg)
Mag. Ang. (deg)
0.0209
0
0.0209
1.248
2.146
1.248
3.333
0.031
0.106
0.028
60
-0.31
0.05
-0.22
2 230
0
0.0883
0
0.0883
0
180
2.71
2.15
2.71
2 228.3
1.667
0.1101
0.053
0.089
0
180
0.99
0.05
1.5
2 226.7
3.333
0.1651
0.1064
0.095
0
60
0.63
0.05
1.28
2 225
5
0.194
0.159
0.103
0
60
0.615
0.052
1.583
2 223.3
6.667
0.255
0.213
0.107
0
60
0.477
0.052
1.437
2 220
10
0.304
0.319
0.14
0
60
-0.4
0.052
-1.85
(deg)
(deg)
2 230
0
0
180
2 226.7 0
( (deg)
International Conferennce on Magnetics, Machines & Drives (AICERA-2014 iCMMD)
Fig. 8. Comparison of magnitude of Incal, Inf, InBV for different d VUF under no load condition
Fig. 9. Comparison of magnitude of Incal n , Inf, InBV for different values of VUF underloaded condition
From the graph, it can be seen that the negative sequence current technique using (2-s) model makes the t fault indicator, Infclose to the reference value InBV Thus the impact of voltage unbalance on stator winding fault detection iss reduced. B. Case II : Loaded Condition The motor is loaded and experimental results under loaded condition is tabulated in Table III and the grraph of magnitude of Incal, Inf ,InBV for different values of VUF is plotted in Fig. 9. In loaded condition also experimental results validated simulation results. TABLE III EXPERIMENTAL RESULT OF FAULTY IM UNDER LOADED CONDITION WITH DIFFERENT VUF VUF (%)
0
0
0.7
1.5
2.2
3
4.5
20
R V I phi 230
Y V I phi 230
B V I phi 230
20
1.3
1.3
1.2
20
58
58
80
20
230
230
230
20
1.4
1.4
1.3
10
75
65
55
20
230
230
225
20
1.4
1.4
1.3
10
64
69
73
20
230
230
220
20
1.3
1.3
1.2
10
58
70
73
20
230
230
215
20
1.4
1.3
1.1
10
65
69
69
20
230
230
215
20
1.3
1.3
1.1
10
60
77
72
20
230
230
200
20
1.3
1.3
0.9
10
57
74
73
ZA ZB ZC (Ω)
Inuv(A) Mag. Ang. (deg)
Inf(A) Mag. Ang. (deg)
0.0024
0
0.024
-1..51
2.55
-1.51
0.0041
0
0.041
-2..75
2.388
-2.75
1.667
0.0086
0.0437
0.044
0
60
0.4492
0.294
0.687
226.7
3.33
0.1130
0.087
0.045
0
60
0.4431
0.294
0.698
225
5
0.1157
0.131
0.051
0
60
-0..01
0.293
-0.88
223.3
6.667
0.1175
0.174
0.055
0
60
-0..02
0.294
-1.43
220
10
0.2245
0.262
0.061
0
60
0.0064
0.293
-1.68
Vp(V) Mag. Ang. (deg)
Vn(V) Mag. Ang. (deg)
Incall(A) Maag. An ng. (deeg)
230
0
0
180
230
0
0
180
228.3
Fig. 10. Block diagram for onlinne monitoring of induction motor
Similar graphs are obtainedd for simulation and experiment, thus validating the efficiency of o the technique. There are small differences between the expeerimental and simulated values because of measurement errorrs. The subtraction of two close complex vectors causes sometimes errors.Thisis the difficulty of the discussed technique. Also the non ideality due to machine asymmetry is not connsidered here. The advantage of this method is that thenegativee sequence current can be easily obtained using the simplified (2-s) model.Block diagram for online monitoring of inductionn motor is given in Fig. 10. The stator currents are stepped dow wn using a current transformer with turns ratio 1/100 and intterfaced to a computer by data acquisition card, LabJack U6. VI.
O CONCLUSION
The more reliable method for practical implementation of fault detection is the symmetrrical component method, more precisely the negative sequeence current method. But the negative sequence current is affected by the supply voltage unbalance. So in order to eliiminate the component due to supply voltage unbalance a (22-s) model of induction motor based on negative sequence equivalent e circuit is introduced. The simulated results show that t the method is efficient to reduce thesensitivity of the neegative sequence current to the supply voltage unbalance. Exxperiment is done on a 1 HP squirrel cage induction motor m and the results are experimentally validated. Alsoo ahardware setup is developed for on-line fault detection of induction motor.
International Conference on Magnetics, Machines & Drives (AICERA-2014 iCMMD) REFERENCES [1] Y. Amara and G. Barakat, “Modeling and Diagnostic of Stator Faults in Induction Machines Using Permeance Network Method”, PIERS Proceedings, Marrakesh, Morocco, March 20-23, 2011. [2] J Y. Han and Y. H. Song, “Condition Monitoring Techniques for Electrical Equipment-a Literature Survey”, IEEE Transactions on Power Delivery, vol.18, no. 1 , January 2003. [3] S.M. Shashidhara, Dr.P.S. Raju, “Stator Winding Fault Diagnosis of Three-Phase Induction Motor by Parks Vector Approach”, International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, vol. 2, no. 7, July 2013. [4] S. Nandi, H. A. Toliyat, and X. Li, “Condition Monitoring and Fault Diagnosis of Electrical Motors - A Review”, IEEE Transactions on Energy Conversion, vol. 20, no. 4, pp.719-729, 2005. [5] S. Williamson, K. Mirzoian, “Analysis of Cage Induction Motors With Stator Winding Faults”, IEEE Transactions on Power Apparatus and Systems, vol. 104, no. 7, pp.1838-42, July,1985. [6] J.L. Kohler, J. Sottile, F.C. Trutt, “Condition Monitoring of Stator Windings Ininduction Motors: Part I Experimental Investigation of the Effective Negative Sequence Impedance Detector”, IEEE Transactions on Industry Applications, vol. 38, no. 5, pp.1447-53, 2002. [7] M.A. Cash, T.G. Habetler, G.B. Kliman, “Insulation failure prediction in AC machines using line-neutral voltages”, IEEE Transactions on Industry Applications, vol. 34, no. 6, pp.1234-39, 1998. [8] Bazine I.B.A, Tnani S, “On-line detection of stator and rotor faultsoccurring ininduction machine diagnosis by parameters estimation", IEEETransactions on Industry Applications, 2011. [9] I. Albizu, I. Zamora, A. J. Mazon, A. Tapia, “Techniques for On-Line Diagnosisof Stator Shorted Turns in Induction Motors”, Electric Power Components and Systems, vol. 34, no. 1, pp.97-114, 2006. [10]Yaw-Juen Wan, “Analysis of Effects of Three-Phase Voltage Unbalance on Induction Motors with Emphasis on the Angle of the Complex Voltage Unbalance Factor”, IEEE Transactions On Energy Conversion, vol. 16, no. 3, September 2001. [11] M. Bouzid, G. Champenois, “Accurate Stator Fault detection Insensitive to the unbalanced voltage in Induction Motor”, XXth International Conference on Electrical Machines (ICEM), 2012.
