Detection of Simultaneous Unbalanced Under- Voltage and

2013 IEEE 1st International Conference on Condition Assessment Techniques in Electrical Systems

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Detection of Simultaneous Unbalanced Under- Voltage and Broken Rotor fault in Induction Motor

Sridhar. S Dept. of Electrical and Electronics Engineering

RNS Institute of Technology Bangalore, INDIA

[email protected]

K. Uma Rao Dept. of Electrical and Electronics Engineering

R. V. College of Engineering Bangalore, INDIA

[email protected]

Abstract— In recent years voltage unbalance and fault analysis in induction motor independently has been researched by engineers dealing with power quality issues. The exposure of the motor to unbalanced supply, combined with broken rotor bar fault significantly increases the temperature and reduces the efficiency and life of the motor. In this paper the effect of unbalance on broken rotor bar fault of the squirrel cage induction motor is analyzed. The performance of the induction motor is studied using symmetrical component analysis for the extracted stator current. The electromagnetic torque is analyzed, and is found to give sufficient information about the fault and the unbalance condition. The simulation of the proposed methodology is done using MATLAB SIMULINK.

Keywords—Induction motor, symmetrical components, unbalanced condition, broken rotor bar fault

I. INTRODUCTION Induction motor is widely used because of its ruggedness,

low cost, simplicity, high torque, low inertia and high reliability. Hence, induction motors play a pivotal role in industry and there is a strong demand for their reliable and safe operation. The modern industry has widely used condition based and reliability based maintenance strategies to reduce unexpected failures and downtime. There are many published techniques [1-3] and many commercially available tools to monitor induction motors to ensure high degree of reliability. In spite of these condition monitoring tools, many companies are still facings unexpected system failures and reduced motor lifetime. Environmental, duty, and installation issues may combine to accelerate motor failure far sooner than the designed motor lifetimes. The major faults of electrical machines can broadly be classified as follows [4]:

1. Stator faults (38%) 2. Rotor faults (10%) 3. Bearing balls faults (40%) 4. Other faults (12%)

Mechanical faults contribute for more than 95% of the overall failures. Fault in electrical machines produces one or more of the following.

Unbalanced air gap flux and line currents Increased torque pulsation Increased losses

Excessive heating Decreased average torque Reduced efficiency

As rotor bar fault is an ‘incipient fault’ as it represents a

small and often slowly developing continuous fault. Its effects on the system in the beginning are almost unnoticeable. The resistance of the rotor becomes a time varying parameter, and hence the impedance of the rotor is no longer balanced upon the occurrence of the fault. Several diagnostic methods have been applied for the identification of rotor bar fault in the induction motor. Motor Current Signature Analysis (MCSA) is one of the powerful technique for the diagnosis of the motor faults.

Many researches are normally focused on the unsymmetrical supply voltage, load oscillation and monitoring techniques to ensure a high degree of safety of induction motor. Detection of motor fault under unbalance supply voltage in the early stage is needed to avoid the reduction in efficiency, heating and consequently, the damage to the induction motor [5]. There are many reasons for unbalance in the supply, such as generators terminal voltages, load currents, faults, power factor correction equipment and voltage regulators in the utility distribution lines [6]. According to National Electrical Manufacturers Association (NEMA) the unbalanced voltage percentage is given by (1):

Uunb = (maximum voltage – rated voltage) / rated voltage (1)

The general rule of thumb is that for every 1% voltage unbalance, a 7% current unbalance is expected which is equivalent to the negative sequence voltage that causes negative braking torque. Unbalanced currents in stator winding occur due to the unequal line voltage of induction motor. The allowable variation in voltage is ±10% when the frequency is at its rated value and ±5% in frequency when the voltage is at its rated value [7]. These allowable may vary between utilities across different countries.

The main aim of this paper is to evaluate the performance

of induction motor subjected to broken rotor bar fault and unbalanced under voltage supply simultaneously. The fault is


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simulated using MATLAB SIMULINK software. An induction motor of 1.1KW, 380V, 50Hz, 4 pole was selected to perform the simulation. The rotor bar is broken at 0.3 sec after the start up in the steady state region and an unbalanced under voltage of 7% in magnitude of the supply voltage was introduced in one phase at 0.3 sec for a period of 0.2 sec (i.e upto 0.5 sec). Symmetrical component analysis is performed for the stator currents, there by extracting the magnitude of negative sequence component. The electromechanical torque and the negative sequence component were simultaneously analyzed to capture the pattern in them, to enable detection of both broken rotor bar fault and unbalanced under voltage simultaneously.

II. ROTOR FAULTS Rotor failure account for 8-10% of the total induction

motor failure. Rotor faults are of the following types [8-10];

Broken or cracked (high resistance) rotor bars or end rings.

Bad or high resistance connection between rotor bars and end rings.

Short circuit rotor laminations.

A. Broken rotor bar fault The main reasons for a broken rotor bar in squirrel cage

induction motors are [11]

Residual stress due to manufacturing problems.

Thermal stress due to thermal overload and unbalance, hot spots or excessive losses, sparking (mainly in fabricated rotors).

Magnetic stress caused by electromagnetic forces, unbalanced magnetic pull, electromagnetic noise and vibration.

Dynamic stresses arising from shaft torque, centrifugal forces and cyclic stresses.

Environmental stresses caused by contamination or abrasion of rotor material due to chemicals or moisture.

Mechanical stresses due to loose laminations, fatigue parts or bearing faults.

Broken rotor bar model should reflect the amount of severity of the fault and their locations. However, this process is very complicated [12]. Rotor bar fault are simulated by increasing one of the rotor phase resistance [12-14] while neglecting the inductance involved. This is because the change in the inductance is not significant, if only very few rotor bars are assumed to be broken. The incremental resistance corresponding to the broken bar to be considered in the external rotor circuit is computed as follows [15].

∆Rinc = bs R

NN

nNn

3/

)2()3/(

2

(2)

Where ‘n’ is the number of contiguous broken bars, N is the number of rotor bars, Ns is the turn number of stator phase

winding and Rb is the rotor bar resistance. The unbalance created due to rotor broken bar produces positive and negative sequence currents in the rotor. The positive sequence rotor current results in positive sequence emf and a positive sequence stator current in the stator with angular frequency

ωr + sω = ω (1 - s) + sω = ω (3)

The negative sequence rotor current results in emfs and currents of frequency in the stator

ωr - sω = ω (1 - s) - sω = (1- 2s)ω (4)

where ωr is the angular frequency in rad/ sec of the rotation of the rotor shaft and ‘s’ is the slip of the induction motor. The rotor asymmetry induces the current in the stator with frequency (1-2s)f. Due to the interaction of (1-2s)f component of flux, an oscillation is seen in the torque at 2sf frequency as in equation (5)[16]. Where ‘f’ is the supply frequency.

T(t) = 23

pψInrsin(2sωt- (Φψ – Φnr)) (5)

Where ‘p’ is the number of pole pairs, Inr is the amplitude of the stator current component originating from the negative sequence rotor current, Φψ and Φnr (rad) the displacement referring to the fundamental component of current and ψ (wb) is the amplitude of the magnetic flux in induction motor.

III. PERFORMANCE OF INDUCTION MOTOR UNDER UNBALANCE SUPPLY VOLTAGE

An unbalanced voltage applied across the stator of a three phase symmetrically wound induction motor can be resolved into positive, negative and zero sequence components. The effect of applying unbalanced voltage can be obtained by superposing the effects due to positive, negative and zero sequence components of voltages.

Positive sequence voltages induce positive sequence rotor currents which circulate at ‘s’ times the supply frequency (where ‘s’ is the slip of the induction motor) and produces a positive torque. The negative sequence voltage induce negative sequence rotor current at (2-s) times the supply frequency in the reverse direction and hence produce negative torque. Torques produced by positive and negative sequence component is

Tp = s

rpr

sRI

23 and Tn =

s

rnr

sRI)2(

3 2

Where Tp = positive sequence torque, Tn = negative sequence torque, Ipr = positive sequence rotor current, Inr = negative sequence rotor current, Rr = rotor resistance referred to stator, ωs = synchronous speed. The net output torque is given as

T = Tp + Tn =

sI

sIR nrpr

s

r

23 22

(6)

Though the resultant torque is due to both positive and negative sequence component, the variation in the torque is mainly because the variation in the negative sequence component.


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Since the negative sequence varies predominantly under broken rotor bar fault and unbalanced under voltage supply condition, the stator current analysis is done for the negative sequence component which gives a clear indication of both unbalance and broken rotor bar fault. Since zero sequence voltages do not produce any net torque, they are not considered for the analysis in this paper. The nature of variation in the torque is investigated for the following cases.

Healthy machine with balanced supply. Healthy machine with unbalanced supply. Faulty machine with balanced supply. Faulty machine with unbalanced supply.

Faulty machine, in this paper is referred to broken rotor bar fault only. The MATLAB simulation model used for this work is presented in Fig. 1

Fig. 1. MATLAB SIMULINK model for the proposed techniques

IV. RESULTS AND DISCUSSION

case 1:Healthy machine with balanced supply

Fig. 2. Stator current waveforms of healthy motor with balanced supply

Fig 2 to Fig 4 corresponds to the waveforms of stator currents, negative sequence component and electromechanical torque of a healthy motor with balanced supply. It is observed that the current in all the three phases of the induction motor attain steady state after 0.2 sec. The magnitude of the negative sequence current and electromechanical torque in the study

state region are zero and one per unit respectively, which indicates that it is a healthy machine with balanced supply.

Fig. 3. Magnitude of negative sequence stator current of healthy motor with balanced supply.

Fig. 4. Electromechanical torque of healthy motor with balanced supply.

case 2:Healthy machine with unbalanced supply

Fig. 5. Stator current waveforms of healthy motor with unbalanced supply from 0.3 sec to 0.5sec


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Fig. 6. Magnitude of negative sequence stator current of healthy motor with unbalanced supply from 0.3 sec to 0.5sec.

Fig. 7. Electromechanical torque of healthy motor with unbalanced supply from 0.3 sec to 0.5sec.

Fig. 8. FFT of stator current.

Fig 5 to Fig 8 corresponds to the waveforms of stator currents, negative sequence component, electromechanical torque and FFT of the stator current of a healthy motor with unbalanced supply. The magnitude of the negative sequence stator current is increased because of unbalance. It is observed that the

frequency of variation in the magnitude of the torque is twice the supply frequency with a peak magnitude of 1.4A as seen in Fig.7. The FFT of the stator current in Fig. 8 indicate the presence of additional frequency component at 5 Hz along with the fundamental due to unbalance.

case 3: Faulty machine with balanced supply

Fig. 9. Stator current waveforms of IM with balanced supply and fault from 0.3 sec onwards.

Fig. 10. Magnitude of negative sequence component of stator current with balanced supply and fault from 0.3 sec onwards.

Fig. 11. Electromechanical torque of induction motor with balanced supply and fault from 0.3 sec onwards.


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Fig. 12. FFT of stator current

Fig 9 to Fig 12 corresponds to the waveforms of induction motor with balanced supply and fault being present from 0.3sec onwards. The variation in the negative sequence components of the stator current appeared due to the reaction to the rotor asymmetry. The effect of the newly induced stator currents, which are the reaction of the induction motor to the oscillating torque is also seen. The variations in the magnitude of negative sequence and torque from 0.3 to 0.5 sec, indicating the presence of unbalance. As seen from the FFT waveform of the stator current, an additional frequency component is introduced at 35Hz due to presence of unsymmetrical rotor.

case 4: Faulty machine with unbalanced supply

Fig. 13. Stator current waveforms of IM with unbalanced supply from 0.3 sec to 0.5sec and fault after 0.3 sec.

Fig. 14. Magnitude of negative sequence component of stator current with unbalanced supply from 0.3 sec to 0.5sec and fault after 0.3 sec

Fig. 15. Electromechanical torque of induction motor with unbalanced supply from 0.3 sec to 0.5sec and fault after 0.3 sec

Fig. 16. FFT of stator current

Fig 13 to Fig 16 corresponds to the waveforms of induction motor with simultaneous unbalanced supply from 0.3 to 0.5 sec and fault being present from 0.3 sec onwards. The variation in the magnitude of negative sequence and torque waveforms from 0.3 to 0.5 sec, indicating the presence of both unbalance and fault. The high frequency oscillation in the magnitude is observed because of the difference in the frequency of the asymmetrical rotor induced stator current and the frequency of the unbalanced supply. As seen from the FFT waveform of the stator current, an additional frequency component is introduced at 35Hz, 68.5Hz, 78.1Hz and 132Hz due to presence of both unsymmetrical rotor and unbalanced supply. It is observed that the peak value of the torque is 2.34 (pu). Magnitude of torque, positive sequence, negative sequence and Total Harmonic Distortion (THD)[18] for a healthy and faulty motor are presented in Table 1 and 2 respectively.

TABLE I. TORQUE, SEQUENCE COMPONENTS AND THD OF A HEALTHY INDUCTION MOTOR

Torque

(pu)

Magnitude of positive

sequence component

(pu)

Magnitude of negative sequence

component

(pu)

THD

(%)

Balanced condition 1.05 1.4 0 0.01

Unbalanced condition 1.4 1.47 0.436 0.84


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TABLE II. TORQUE, SEQUENCE COMPONENTS AND THD OF A BROKEN ROTOR BAR INDUCTION MOTOR

Torque

(pu)

Magnitude of positive

sequence component

(pu)

Magnitude of negative sequence

component

(pu)

THD (%)

Balanced condition 2.14 2.25 0.187 0.87

Unbalanced condition 2.34 2.405 0.493 3.19

. During the fault and unbalance condition, the magnitude of the torque and sequence component are not steady and vary at a certain rate. Hence the peak values of the components have been presented. It is observed that the torque, sequence components and THD value are highest in the presence of simultaneous under voltage unbalance and broken rotor fault.

V. CONCLUSION In this paper an attempt has been made to study the nature of variation of stator current, sequence components of the stator current and torque during under voltage unbalance, rotor bar fault and simultaneous under voltage unbalance and broken rotor bar fault. The observations of the simulation results indicate that the stator current is sufficient to give information about the nature of the fault and the health of the induction motor. When a healthy machine is subjected to unbalance under voltage supply, the magnitudes of the three phase stator currents differ, providing the signature for the fault. In the presence of a rotor bar fault, in an otherwise healthy machine, it has been observed that the stator current has other frequency components lower than the supply frequency, superimposed on the fundamental. When both unbalanced under voltage and rotor bar fault exist, the stator current has multiple frequencies arising due to asymmetrical rotor and the presence of negative sequence currents, due to voltage unbalance. Hence, we can conclude that the pattern of the stator current can be used to make an inference on the nature of the supply and the health of the rotor bars. If the stator current can be captured and displayed, the waveform observation will alert the operator on the onset of asymmetry in the rotor due to broken rotor fault. Therefore, this simple method can be used for reliability centered maintenance, where an incipient fault can be detected at the early stages before the degradation is large enough to trigger, protection relays. This would lengthen the life of the machines and improve the efficiency of their performance.

REFERENCES [1] H. Toliyat and T. Lipo, “Transient analysis of cage induction machines

under stator, rotor bar and end ring faults”, IEEE Trans. Energy Conversion, vol. 10, pp.241- 247, June 1995.

[2] M. Elkasabgy, A. Eastam and G. Dawson, “Detection of broken bars in the cage rotor of an induction machine”, IEEE Trans. Ind. Applications, vol. 28, pp. 165-171, Jan- Feb 1992.

[3] M. E. Benbouzid, “Induction motor asymmetrical fault detection using advanced signal processing techniques”, IEEE Trans. Energy Conversion, vol 14, pp.147-152, June 1999.

[4] S. Nanadi and H. A. Toliyat, “Condition monitoring and fault diagnosis of electrical machines- A review,” IEEE Ind. Appl. Society annual meeting, 1999.

[5] E. Muljadi, C.P. Butterfield, T. Batan and D. Yildirim “Un-derstanding the unbalanced voltage problem in wind tur-bine generation”, IEEE Industry

Applications, vol.2, pp. 1359 – 1365,1999. [6] K. Gary, T. Surles, M. Masri and R. L. Therkelsen “Power quality

guidelines for energy efficient device application”, California Energy Commission, pp. 1-13, 2003.

[7] Engineering Data, Integral AC Motor Selection and Application Guide for Fans, Technical report, Twin City fan Companies Ltd. November 1999. [8] M. Haji, H. A. Toliyat, “Pattern recognition- a technique for induction

machine rotor fault detection ‘Eccentricity and broken bar fault’”, IEEE Industry Applications conference, 2001,vol 3, pp.1572-1578.

[9] A. Yazidi, H. Henao and G. A. Capolino, “Broken rotor bars fault detection in squirrel cage induction machines”, IEEE International Conference on Electrical Machines and Drives 2005, pp. 741- 747, 15th May 2005.

[10] Arabac. H, Bilgin. O., “The detection of broken rotor bars in squirrel cage induction motors based on neural network approach”, 4th International Electrical-Electronics Engineering symposium, Turkey, pp 148-151, 2005.

[11] K. Abbaszadeh, J. Milimonfared, M. Haji, H. A. Toliyat, “Broken bar detection in induction motor via wavelet transform”, IECON’ 01, The 27th annual conference of IEEE Industrisl Electronics society, pp. 95- 99, 2001.

[12] Z. Cai, A. Gao, J. Jiang, “Modelling for interior faults of induction motors and its simulation on EMTDC”, International Conference on Power System Transients, IPST, New Orleans, pp. 1-5, 2003.

[13] D. Kostic-Perovic, “Analysis of a SVAF rotor fault index”, SDEMPED’01, IEEE International Symposium on Diagnostics for Electrical Machines, Power Electronics and Drives, Grado (GO), Italy, pp. 513- 516, September 2001.

[14] A. Bellini, F. Filippetti, G. Franceschini, C. Tassoni and G. B. Kliman, “Quantitative evaluation of induction motor broken bars by means of electrical signature analysis”, IEEE transaction on Industry Applications, vol. 37, no. 5, pp. 1248- 1255, 2001.

[15] D.Kostic-Perovic, On-line diagnosis of faults in induction motor and pump, D.Phil. Thesis, University of Sussex, Brighton, UK, 2001.

[16] Somaya A. M. Shehata, Hamdy S. El-Goharey, Mostafa I. Marei, Ahmed K. Ibrahim, “Detection of induction motors rotor/stator faults using electrical signatures analysis,” International Conference on Renewable Energies and Power Quality (ICREPQ’13) Bilbao (Spain), 20th to 22th March, paper ID 318, 2013.

[17] Seyed Abbas Taher and Majid MaleKpour, “A Novel technique for rotor bar failure detection in single- cage induction motor using FEM and MATLAB/ SIMULINK”, Hindawi Publishing Corporation, Mathematical Problems in Engineering, Volume 2011, Article ID 620689, pp. 1-14.

[18] Math H. Bollen, “Understanding Power Quality Problems: Voltage Sags and Interruptions”, Wiley-IEEE Press, vol.5, 2000.

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Detection of Simultaneous Unbalanced Under- Voltage and