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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: [email protected]). L. L. Grigsby is with the
Department of Electrical Engineering,
Auburn University, Auburn, AL 36849 USA (e-mail:
[email protected]).
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
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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
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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
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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
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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
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[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.