Abstract -- A high precision search coil is proposed for monitoring of the broken bars in the squirrel cage induction machines. The search are implemented in finite element model witch describe the real behavior of the machine. The external search coils are fixed on Ox and Oy axis for analyzing the leakage flux in the outer region. The method offer a number of advantages including compact size of the sensor, great accuracy, high bandwidth, low cost, and it is applicable even for already fabricated electric machine. The performance of the search coils is evaluated by some default degrees. The Ox search coils voltage spectral analysis has also been performed. All tests were performed for the proposed FE model of 5,5kW, 400 V, 1455 r/min induction motor, for the case of a healthy rotor and for the cases with different deliberately damaged rotor.

Index Terms-- Broken bars, Faults diagnosis, Finite Element Method, Induced voltage, Induction machine, Leakage flux, Search coil,:

I. INTRODUCTION

Condition monitoring, fault diagnosis, and prognosis are significant for medium and high power induction machines. Various approaches for condition monitoring and fault diagnosis have been proposed for different types of electrical machines [9-10]. However the offline machine fault detection and diagnostic methods do not allow for frequent testing and are financially impractical, many online methods have been proposed by researchers to reduce maintenance costs and provide more reliable diagnosis. One cost-effective way is based on stator current spectrum, usually called motor current signature analysis (MCSA). Specific harmonics in the motor current spectrum can be detected as a signature of a specific type of fault. The limitations of these frequency analysis based algorithms are relatively time consuming, and it can be difficult to determine the source of specific harmonics. For a brushless permanent magnet machine, additional harmonic frequencies due to partial demagnetization are the same as dynamic eccentricity signature frequencies [6], and they cannot be distinguished. In reality, not only partial demagnetization, but also other asymmetric problems such load imbalance, misalignment, or oscillating load can produce the same harmonics.

Financial support should be acknowledged here. Example: This work

was supported in part by the U.S. Department of Commerce under Grant BS123.

The paper title should be in uppercase and lowercase letters, not all uppercase.

The name and affiliation (including city and country) of each author must appear on the paper. Full names of authors are preferred in the author line, but are not required. Initials are used in the affiliation footnotes (see below). Put a space between authors' initials. Do not use all uppercase for authors' surnames.

Examples of affiliation footnotes: J. W. Hagge is with Nebraska Public Power, District Hastings, NE

68902 USA (e-mail: j.hagge@ieee.org). L. L. Grigsby is with the Department of Electrical Engineering,

Auburn University, Auburn, AL 36849 USA (e-mail: l.grigsby@ieee.org).

In this paper, an alternative for the rotor broken bars detection method using external search coils is proposed. These coils are wound around armature core and the detection is based on the analysis of the induced voltage. As a matter of fact, search coils are not a new concept at all for electric machine fault detection. Various works [1], [2] , [5] have been developed a similar approach using a search coil to measure axial leakage flux signal of an induction machine to detect some common faults in induction machines, such as broken rotor bars, wound rotor short circuit, inter-turn short circuit and mechanical faults, etc. In order to evaluate the validity of the presented method, simulation has been conducted for an induction machine. Broken bars under full load conditions have been modeled by Finite Element Analysis (EFA) and the search coils induced voltage has been analyzed. The most useful results have been taken at position around the middle of a stator joke where the leakage flux is concentric [ ].

Fig. 1. Leakage flux lines and external search coil

II. MODELING OF INDUCTION MACHINE USING FINITE ELEMENT METHOD (FEM)

The basis of any reliable fault diagnosis method is the analysis of behaviors and conditions of the machine. A real and proper modeling is the first step in this process. Winding function method (WFM) has been used to model induction machine under fault [8], and then winding function method (WFM) and finite-element method (FEM) have been introduced as the most powerful modeling methods.

WFM was first used for analysis of the induction motor transient mode under internal faults [8]. Then, dynamics of a faulty induction motor and harmonics of the stator current over different stator winding fault, broken rotor bars and eccentricities have been treated. Recent works based on the WFM has been used for faulty induction motor modeling in which the air gap is considered to be symmetrical. FEM allows to calculate the magnetic field distribution within induction motor using its geometry and magnetic parameters. Having this field distribution, other

Rotor Fault Diagnosis Using External Search Coils Voltage Analysis

quantities such as induced voltage waveform, winding inductances, currents, air gap magnetic flux density, torque and speed are easily extracted [7-8]. TSFE method has been used to study the air gap eccentricity and in order to decrease the noise due to the eccentricity, parallel paths within the stator windings are employed. Time stepping finite elements analysis (TSFE) is used to calculate the unbalanced magnetic pull (UMP) created by any fault in the induction motors [11].

A. Geometry and meshing

In this work a CAD software package is used to simulate the induction machine and the magnetic transitory formulation is included, which solves the problem in discrete time points. The geometry of the materials and the development of the winding were obtained by fragmenting a real motor, in which field test were performed. Fig. 2 shows the machine geometry entirely, meshing, in which stator and rotor core regions, squirrels cage bars and the two search coils are shown.

Fig. 2: Induction machine Geometry and meshing

Fig. 3 shows electrical circuit used in the healthy motor simulations. This circuit is divided in three parts: external sources, stator circuit and the squirrel cage. To make the different simulations of the broken bars , the faults have been introduced by affecting a high resistivity to the bars.

LA1 LA2 LA3 LA4 LA5 LA6 LA7 LA8

LA16LA15LA14LA13LA12LA11LA10LA9

LB9 LB10 LB11 LB12 LB13 LB14 LB15 LB16

LB8LB7LB6LB5LB4LB3LB2LB1

LC9 LC10 LC11 LC12 LC13 LC14 LC15 LC16

LC8LC7LC6LC5LC4LC3LC2LC1

+

V1

+

V2

+

V3

Fig. 3: Electrical Circuit coupling

The study into magneto-transient is appropriate particularly well for our need. The coupling of our magnetic diagram to an electrical circuit and the presence of a tread in the air-gap make it possible to follow the dynamic behavior of the machine. The proposed cad software solves

the following equation:

1( )

( )e

d Arot J rot H

dt rot A

(1)

Where: A : Magnetic potential (Weber/m) J : Density of current (A/m) : Magnetic permeability (H/m) H : Magnetic field (A/m) e: Electric conductivity (1/m) t : Time(s) The numerical simulations in this paper refer to

following induction motor specifications:

TABLE I INDUCTION MOTOR SPECIFICATIONS

Rated power 5,5 KW Rated Voltage 400 V Rated line current 12.45A Rated speed 1455 tr/mn Coupling Star Poles 4

The motor has been tested under full load condition

for healthy condition and with four different defectives cases. In the first case one of the bars was brooked, representing a first stage of the rotor fault. Later, the tests were performed with tow broken bars and three broken bars. Finally four bars were taken out. This represented completely damaged rotor. In this digest the voltage induced in the search coils is shown for the all cases:

Case A: Healthy Machine Case B: Machine With 1 broken bar Case C: Machine With 2 broken bars Case D: Machine With 3 broken bars Case E: Machine With 4 broken bars

B. Results of flux distribution

By the computation of the electromagnetic field using the proposed finite element CAD package, the machine inductances, back EMFs and leakage flux can be obtained. The flux density distribution in the outer region for the healthy and three broken bars cases are shown in the Figures 3 and 4.

Fig. 4: Outer region Flux density for healthy case

Fig. 5: Outer region Flux density for 3 broken bars

The numerical results corresponding to a magnetic flux analysis shows the unsymmetrical distribution of the leakage flux in the outer region for the case of 3 broken bars.

III. SEARCH COILS INDUCED VOLTAGE ANALYSIS

External coils voltage analysis can be used to detect various rotor faults. The goal of performed simulation was to detect broken rotor bars and in the case of a greater rotor fault to find out the amount of damage. The analysis can be performed in time as well in a frequency domain. The asymmetry caused by a rotor fault will induce voltage in a search coil with additional harmonics at frequencies given by [1]:

. 1 .scoil sf

f k s s fp

(2)

Where fs : supply voltage frequency, s : slip, p : number of pole pairs. from the equation (2), a three components can be induced by the broken bars as flowing:

. 1sbcf

f k sp

(3)

. 1 .sb sf

f k s s fp

(4)

. 1 .sb sf

f k s s fp

(5)

A. Induced voltages waveforms

In this digest the induced voltages in Ox and Oy search coils are shown for the different simulation cases (Fig. 6), healthy and defective rotors at nominal load.

Fig. 6a: Search coils induced voltages for healthy case

Fig. 6b: Search coils induced voltages for one broken bar

Fig. 6c: Search coils induced voltages for two broken bars

Fig. 6d: Search coils induced voltages for three broken bars

Fig. 6e: Search coils induced voltages for four broken bars

As can be seen in figures, in the case of broken rotor

bars the induced voltages in Ox and Oy search coils are distorted compared to the case of a healthy rotor. This distortion became more and more important when the number of broken bars increases and if the graph is zoomed the distortion of the voltage is even more obvious.

B. Induced Voltage Harmonics analysis

In order to confirm this fact that the induced voltages include sufficient information about the motor condition, Fast Fourier Transform of the induced voltages relevant to one of the coils is calculated. Fast Fourier Transformer (FFT) of the output voltage relevant to one of these coils is considered to show the capability of the proposed sensor for fault diagnosis purpose. In Figs. 7 the FFTs of Ox search induced voltage coil are presented for normal condition and different faulty cases, respectively. In these figures the frequency bands, in which more considerable variation occur are reported.

Fig. 7a: FFT of induced voltage for healthy case

Fig. 7b: FFT of induced voltage for one broken bar

Fig. 7c: FFT of induced voltage for 2 broken bars

Fig. 7d: FFT of induced voltage for 3 broken bars

Fig. 7e: FFT of induced voltage for 4 broken bars

As shown in Fig. 7b, 7c, 7d and 7e the main faults

frequency components are shown near 25Hz, 50Hz, 75Hz, 100Hz....etc in the faulty condition. The spectral components rated to broken bars at the selected frequency band shown in Fig. 7 does not appear in the normal condition. Consequently, the capability of the proposed search coils for monitoring of the rotor bars is confirmed by the extracted frequency components. The Table II shows the amplitude evolution of different components for the studied cases.

TABLE II AMPLITUDES OF FREQUENCY COMPONENTS FOR DIFFERENT CASES

fb+ (Hz) 25,9 73,9 98,9 125,8 173,8 Case A - - - - -

Case B 4,1mV 2,6 mV 1,6 mV 2,9 mV 1,5 mV Case C 7,7 mV 4,1 mV 3,4 mV 5,6 mV 2,8 mV Case D 12,0 mV 5,1 mV 5,6 mV 8,2 mV 3,4 mV Case E 16,0 mV 5,7 mV 7,7 mV 11,0 mV 5,1 mV

The figure 8 shows the plot of the frequency components amplitudes for different simulated cases. A comparison of curves in Fig. 8 indicates that there is no components due to normal condition because of the uniformed flux distribution in outer region.

Fig. 8: Variation of the frequency components

As can be seen in figure, in the cases of broken rotor bars

the amplitude of the additional components is more important than in the case of a healthy rotor. This amplitude became more and more significant when the number of broken bars increases.

IV. CONCLUSION

This paper presented the modeling method for induction motor under broken bars condition using FE techniques. The proposed detection method was based on the external search coils and the induced voltages are used to diagnosis of the broken bars faults. It clearly observed the distortion of the induced voltages for the defective cases and was indicated that the amplitude of simulated harmonic components obtained by FFT due to the faults has considerable differences with the healthy case, and this was justified by the non unsymmetrical distribution of leakage flux.

The search coils voltage recording can easily be performed in real-time for on line monitoring during the normal motor operation, in a very short time. The result could be very useful especially when the signal of a healthy motor is known. In those situations short comparison of that signal and signal taken after some period of usage could make easier to early detect motor faults.

V. REFERENCES

[1] A. Miletic, " Experimental Research on Rotor Fault Diagnosis Using External Coil Voltage Analysis and Shaft Voltage Signal Analysis," Symposium on Diagnostics for Electric Machines, Power Electronics and Drives SDEMPED 2005, Vienna, Austria, 2005.

[2] Don-Ha Hwang, Jung-Hwan Chang, Dong-Sik Kang, Jin-Hee Lee, and Kyeong-Ho Choi, " A Method for Dynamic Simulation and Detection of Air-gap Eccentricity in Induction Motors by Measuring Flux Density," 12th Biennial IEEE Conference on Electromagnetic Field Computation, 2006.

[3] Yao Da, Xiaodong Shi, and Mahesh Krishnamurthy. A New Approach to Fault Diagnostics for Permanent Magnet Synchronous Machines Using Electromagnetic Signature Analysis. IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol. 28, No. 8, 2013, pp.4104-4112.

[4] Don-Ha Hwang, Ki-Chang Lee, Joo-Hoon Lee, Dong-Sik Kang, Jin-Hee Lee, Kyeong-Ho Choi, " Analysis of a Three Phase Induction Motor under Eccentricity Condition," Industrial Electronics Society, 31st Annual Conference of IEEE , IECON 2005.

[5] E. E. Reber, R. L. Mitchell, and C. J. Carter, " Application of Rogowski Search Coil for Stator Fault Diagnosis in Electrical Machines," IEEE SENSORS JOURNAL, VOL. 14, NO. 2, pp.311-312. 2014.

[6] Pedro Vicente Jover Rodrguez, Anouar Belahcen, Antero Arkkio, Antti Laiho, Jos A. Antonino-Daviu ," Air-gap force distribution and vibration pattern of Induction motors under dynamic eccentricity". Electr Eng , 2008, N. 90:pp. 209-218.

[7] S. Palko, Structural Optimization of an Inductive Motor using Genetic Algorithm and a Finite Element Method, Thesis, Acta Polytechnica Scandinavia, Helsinki, 1996.

[8] T. Ilamparithi, T. ; Nandi, S., Comparison of Results for Eccentric Cage Induction Motor Using Finite Element Method and Modified Winding Function Approach. Conference on Power Electronics, Drives and Energy Systems (PEDES) 2010, pp.1-7.

[9] A. Bellini, F. Filippetti, C. Tasoni, and G.-A. Capolino, Advances in diagnostic techniques for induction motor, IEEE Trans. Ind. Electron., vol. 55, no. 12, pp. 41094126, Dec. 2008.

[10] S. Nandi and H. A. Toliyat, Condition monitoring and fault diagnosis of electrical machinesA review, in Proc. Conf. Rec. IEEE Ind. Appl. Conf., 34th IAS Annu. Meeting, Oct. 1999, pp. 197204.

[11] JawadFaiz, Bashir Mahdi Ebrahimi, Bilal Akin, and Hamid A. Toliyat. Finite-Element Transient Analysis of Induction Motors Under Mixed Eccentricity Fault. IEEE TRANSACTIONS ON MAGNETICS, Vol. 44, No. 1, 2008, pp.66-74.

VI. BIOGRAPHIES

A technical biography for each author must be included. It should begin with the authors name (as it appears in the byline). Please do try to finish the two last columns on the last page at the same height. The following is an example of the text of a technical biography:

Nikola Tesla was born in Smiljan in the Austro-Hungarian Empire, on July 9, 1856. He graduated from the Austrian Polytechnic School, Graz, and studied at the University of Prague.

His employment experience included the American Telephone Company, Budapest, the Edison Machine Works, Westinghouse Electric Company, and Nikola Tesla Laboratories. His special fields of interest included high frequency.

Tesla received honorary degrees from institutions of higher learning including Columbia University, Yale University, University of Belgrade, and the University of Zagreb. He received the Elliott Cresson Medal of the Franklin Institute and the Edison Medal of the IEEE. In 1956, the term "Tesla" (T) was adopted as the unit of magnetic flux density in the MKSA system. In 1975, the Power Engineering Society established the Nikola Tesla Award in his honor. Tesla died on January 7, 1943.