International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
An Experimental Investigation on Broken Rotor Bar in Three
Phase Induction Motor by Vibration Signature Analysis using
MEMS Accelerometer
1
Maruthi.G.S.1, Vishwanath Hegde 2
Department of Electrical & Electronics Engineering,
Smt.L.V.Government Polytechnic, Hassan-573201, Karnataka State, India
2
e-mail: maruthi.s.gujjar@gmail.com
Department of Electrical & Electronics Engineering,
Malnad College of Engineering Hassan-573201, Karnataka State, India
e-mail: hegde_mce@rediffmail.com
This will set up asymmetrical rotor magnetic field and
causing pulsation in speed and torque. As a result high
vibrations, noise and excessive heating sets up in the motor.
This may also results into bearing damage, rotor-stator rub,
thus causing potential damage to rotor core as well as stator
insulation [2-3]. Since the rotor bar damage is an unseen
fault and causes no immediate damage to motor, the
identification, diagnosis and isolation of faults become
challenging task for maintenance engineers. To achieve this,
industry demands simple, cost effective, reliable, portable,
low power consumption, compact size, condition monitoring
device [1].
The motor failure survey related to rotor faults conducted
by IEEE -Industrial Application Society (IAS) reports 8%,
Electrical Power Research Institute (EPRI) reports 9%.
Both survey focused on medium size machines. Allianz
focused on medium to high-voltage large induction
machines, reports 13% failure [4]. The rotor related failures
are more in medium to high voltage motors than small
machines. This is due to the fact that, these categories of
motors are usually employed in high starting torque and
critical applications, where thermal stresses are high due to
abnormal starting current and chances of rotor failure is
more. Hence condition monitoring of them helps to avoid
premature failure. The conventional methods employed by
past researchers in rotor bar damage detection are,
temperature measurement, infrared recognition, axial flux
measurement, measurement of speed, radio frequency (RF)
emission monitoring, and accoustic measurement.The fault
detection based on these methods are invasive, time
consuming, costly and less reliable [4-5].
Based on the stator current as medium, the researchers
[1], [3], [6], [7] and [8] have employed classical side band
harmonic frequency approach to detect the broken rotor
bars. In [7], four, five and six rotor bar damage has been
detected under no load, 65% and 100% load condition.
Abstract— Three phase Squirrel Cage Induction Motors
(SCIM) are continuously subjected to electrical, thermal,
mechanical and environmental stresses in any industrial
application. Such stresses can result in rotor bar damage. Due
to such faults an unsymmetrical magnetic field will be setup
and can lead to reduction in developed torque, increase in
speed fluctuations, noise and vibrations. The present work
utilizes the motor vibrations as a medium to detect and
diagnose the broken rotor bars under no load and on load
condition by using state-of-the-art, MMA6270QT MEMS
accelerometer. Spectral analysis of motor vibration signals by
using FFT analysis yields twice slip frequency components
(1±2ks).fs around fundamental frequency and 2sfs frequency
component in low frequency range which indicates rotor bar
damage fault. A simple method has been proposed to compute
severity of fault by comparing the magnitude average of side
band harmonic frequency with fundamental frequency
component. Experimental results have demonstrated the
effectiveness of the proposed technique.
Keywords – Rotor bar damage; MEMS accelerometer; Condition
monitoring; Vibration analysis; Squirrel cage induction motor.
I. INTRODUCTION
Three phase induction motors employed in mining, iron
and steel, petrochemical, wood chipping plant in paper and
pulp industries, stone crushing plant and coal handling in
thermal plant etc., are subjected to more electrical,
mechanical, thermal and environmental stresses. This may
be due to numerous starts and stops, or operating motor
under various load conditions, which leads to abnormal
current flow in rotor circuit [1]. This will lead to
development of porosity in rotor than in a motor that starts
and runs under steady state. Porosity is one of the defects
that is invisible to the naked eye. It will form the initial stage
of broken bar and commonly found in cast aluminum rotors.
A certain level of porosity can be tolerated, but if the
porosity accumulates in one place then it may lead to
formation of crack and hence bar damage.
357
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
Authors have shown rated speed as 1410 rpm from the
name plate details of motor but during the course of
conducting experiment on faulty motor under 65% and
100% load, 1455 rpm and 1425 rpm respectively have been
recorded, which is practically not possible. Also, practically
a motor with three, four, five and six broken bar will not be
able to drive the load because of decrease in average torque
and reduction in speed. In this regard the paper [7] has to be
critically viewed. In [8], authors have detected mixed fault
diagnosis (rotor bar damage and eccentricity fault) with one
broken bar with eccentricity, two broken bar with
eccentricity and three broken bar with eccentricity. It has
been reported in the paper that speed is kept at 1440 rpm in
all cases. But practically it is impossible with broken rotor
bar motor. This is because of reduction in magnetic flux due
to broken bars, as result there will be reduction in speed.
Also, the authors do not clearly indicate the loading details
and speed of the motor under different fault cases.
The other approaches employed by researchers are,
entropy probability distribution of featured current signal
using Hilbert transform discussed in [9]. Measurement of
zero crossing time (ZCT) instants of stator current was
another approach employed in [10]. Authors have noticed
2sfs frequency components in ZCT signal with rotor fault
and indicated it as fault detector. This method demands
additional hardware circuitry to extract ZCT signals and
digital filtering techniques are needed to make system on
line. Hence the complexity and cost increases. Advanced
signal processing technique like wavelet transform and
Finite element method (FEM) have been employed in [11]
and [12]. The fault analysis based on these methods requires
good knowledge on wavelets and FEM to identify the fault
and its severity, which will be complex from the industrial
point of view. Also, FEM takes more computational time
and computer memory to analyze the motor fault detection.
Hence on line fault detection is difficult. In [13], authors
have proposed a new control strategy to control the speed
and torque in inverter driven induction motor. The
computation of variance under normal and faulty condition
shows that variance value is more under faulty condition
than normal. This method is based on dynamic response of
control strategy and is less sensitive for smaller rotor faults.
A clear distinction between fault cases with less number of
broken bars (e.g., zero versus one broken bar) is therefore
not possible in the above method.
Vibration analysis is another approach employed in [1417] to detect rotor bar damage in induction motors. The
classical side band harmonic frequency analysis has been
carried out on vibration signals. In [14-17] authors have
employed conventional piezo electric accelerometers for
vibration analysis. But these accelerometers are having
limitation of measuring only short duration vibration
measurement. This is due to the fact that sustained
piezoelectric action will cause overheating of crystals and
thus deteriorate their conversion capability [18]. Hence
vibration monitoring using these accelerometers will not be
accurate. In [19], authors have studied the vibrations
developed in induction motors due to rotor imbalance by
changing the external weights to motor shaft using MEMS
accelerometer. Authors have implemented the ZigBee IEEE
802.15.4 protocol for remote sensing of data for condition
monitoring. No studies regarding broken rotor bar detection
have been reported. The severity of broken rotor bar fault
under different load conditions has not been addressed in the
above referred literature.
Recent developments in semiconductor technology have
contributed the state-of-the-art MEMS accelerometers [20].
The MMA6270QT is one among them employed in the
present work. The promising features have drawn the
attention of researchers to employ them in detection and
diagnose faults in motors cost effectively.
The published literature unfolds the fact that MCSA gives
efficient results when motor operates under loaded
condition. But, when motor operates under no load running
condition, the side band frequency component (1 ± 2ks).fs
are very close to the supply frequency component fs, a
natural spectral leakage can hide characteristics frequency
components of the fault. In this case standard MCSA
method fails to detect broken bar faults. Hence, fault
detection based on MCSA is dependent enough on load and
broken bars. The limitations of MCSA and conventional
accelerometers have drawn the attention of researchers for
alternative method. In this direction the present work
proposes extension of vibrations analysis to detect rotor bar
damage in three phase induction motor operating under
variable load conditions using MEMS accelerometer. A
simple and easy method has been proposed to find severity
of fault, which will be based on comparing the magnitude of
side band harmonic frequency with fundamental frequency
component.
II ROTOR BAR DAMAGE ANALYSIS
The classical twice slip side band harmonic frequency
approach employed to detect the broken bar fault [3], [4-13]
is given by
(1)
fbr  1  2ks  f s
where fbr = broken bar frequency in Hz; k = 1, 2, 3...
any integer, s = slip and fs = supply frequency in Hz. The
lower side band(1-2s)fs is fault related, while the upper side
band (1+2s)fs is due to consequent speed oscillations.
In the present work, a simple method to find out
percentage fault severity factor has been proposed in
equation (2). It can be used as diagnostic index for
preventive maintenance.
358
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
%SF 
Magnitude_Average_of (1  2.s ). f s
Magnitude_of ( f s )
X 100
IR = IY = IB = 1.1 Amps; N = 1440 rpm; s = 0.04
(2)
where %SF = percentage severity factor; (1-2s).fs =
magnitude of left hand side, and (1+2s).fs = magnitude of
right hand side harmonic frequency component.
III. EXPERIMENTAL SETUP OF PRESENT WORK
An experimental set up of present work is shown in Fig.1
for investigating rotor bar damage in induction motor under
no load, half load and full load conditions. The test setup
consists of motor with rating 3 phase, 1.5 HP, 415 V, 50 Hz,
1440 rpm, 28 bar rotor, 36 stator slots with mechanical load.
A healthy rotor, one broken bar and two broken bar rotors
are used with common healthy stator, which is as shown in
Fig.2 (a) and (b).
(a)
(b)
Fig.1 Experimental setup of the present work
(c)
Fig.2 (a) Healthy motor rotor
(b) Rotors with one and two broken bar
The instrumentation includes MEMS accelerometer, a high
resolution FFT analyzer and a personal computer connected
to FFT analyzer through RS-232 cable. In the initial stage of
present work, Tektronics100 MHz storage oscilloscope,
which has inbuilt FFT analysis provision is made use for
carrying out FFT operation on accelerometer output signal.
The sampling rate of 2.5 kilo samples/second over span of 1
kHz, 0.5 kHz and 0.25 kHz for 10 seconds have been used to
study high frequency and low frequency range fault signals
by using rectangular window.
(d)
Fig.3 FFT spectrum of healthy motor (a) 1.5 HP, No load condition and
5 HP (b) No load (c) Half load (d) Full load condition
Observations:
When a balanced three phase supply voltage is applied to
healthy induction motor, uniform current flows in all the
phases. This will set up a uniform magnetic field both in
stator and rotor. The interaction of mmf in rotor circuit with
stator magnetic flux will produce steady positive motor
torque, which drives the rotor in forward direction producing
useful mechanical output. A fine radial vibration will be
setup in the stator. The spectral analysis of vibration signal
yields dominant 49.81 Hz frequency component with
magnitude of 68.21 dB and no side band harmonic
II. RESULTS AND ANALYSIS
Case 1: Healthy motor running under no load condition
VRY = VYB = VRB = 415 Volts,
359
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
frequencies as seen from Fig.3 (a). Also, similar kind of
experiments are conducted on healthy motor with 5 HP, 415
V, three phase squirrel cage induction motor under no load,
50% load and 100% load and the corresponding FFT
spectrum are shown in Fig.3 (b),(c) and (d). The percentage
severity factor computed using equation (2) for 5 HP motor
are 15.18 %, 16.59 % and 19.09 %. The 49.81 Hz which is
very close to 50 Hz frequency component is a positive
sequence component which is essential for driving rotor in
forward direction and this component is an indicator of
healthy motor.
(a)
Case 2: Test motor (one and two broken rotor bars) running
under no load condition.
IR = 1.7 Amps, IY = 1.8 Amps, IB = 1.8 Amps
VRY= 410 Volts, VYB = 410 Volts, VRB = 410 Volts
(b)
Fig. 5 FFT spectrum of (a) one broken bar (b) two broken bar – half load
condition
Table -2 Experimental and analytical values of side band harmonic
frequency components –Half load condition
Experimental Values
(a)
Experimental Values
Side band
frequencies
(1-4s)fs
(1-2s) fs
fs
(1+2s) fs
(1+4s) fs
2ksfs
Two Broken Bar
Freq. in
(Hz)
Amplit
ude in
(dB)
Freq.
in (Hz)
Amplitu
de in
(dB)
37.48
43.65
49.91
56.23
62.39
6.165
28.81
38.81
64.01
33.21
32.81
20.41
35.26
42.66
49.81
56.97
64.12
7.152
31.61
37.61
61.21
36.41
35.61
25.61
Side band
frequencies
Freq.
in (Hz)
Amplit
ude in
(dB)
Freq. in
(Hz)
Amplit
ude in
(dB)
One
broken
bar
Two
broken
bar
(1-4s)fs
(1-2s) fs
fs
(1+2s) fs
(1+4s) fs
2ksfs
25.15
37.48
49.81
62.14
74.48
12.82
26.01
33.61
63.61
39.21
26.81
21.61
33.79
41.92
49.81
57.71
65.6
7.89
28.01
37.61
60.41
35.21
38.41
25.61
25.4
37.7
50
62.3
74.6
12.3
34.16
42.08
50
57.92
65.84
7.92
Case 4: Test motor (one and two broken rotor bars)
running under full load condition.
(b)
Fig.4 FFT spectrum of (a) one broken bar (b) two broken bar – No
load condition
Table -1 Experimental and analytical values of side band harmonic
frequency components -No load condition
One Broken Bar
IR = 3.4 Amps, IY = 3.6 Amps, IB = 3.6 Amps
VRY= 391 Volts, VYB = 392 Volts, VRB = 392Volts
Analytical values
of Frequency
(Hz)
One
broken
bar
Two
broken
bar
38.0
44.0
50.0
56.0
62.0
6.0
36.0
43.0
50.0
57.0
64.0
7.0
Two Broken Bar
Analytical
values of
Frequency (Hz)
One Broken Bar
Case-3: Test motor (one and two broken rotor bars) running
under half load condition.
(a)
IR = 2.5 Amps, IY = 2.8 Amps, IB = 2.8 Amps
VRY= 400 Volts, VYB = 399 Volts, VRB = 400 Volts
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 4, April 2013)
which is shown in Fig.5 (a) and (b). Table-2 presents the
magnitude and frequency of side band harmonic frequency
components for one and two broken bar under half load
condition.
Referring to case-4, as the load on the faulty motor
increased to its full load value, the terminal voltage and
speed decreases with increase of slip Nr = 1263 rpm; Slip =
0.158 (one broken rotor), Nr = 1398 rpm; Slip = 0.0920 (two
broken rotor). The magnetic asymmetry in the rotor and
speed oscillation increases, as a result more vibration will
set up in the motor. The magnitudes of side band harmonic
components fb = (1 ± 2ks).fs are increases as compared to no
load and half load condition which is as shown in Fig. 6(a)
and 6(b). Table-3 presents the magnitude and frequency of
(1± 2ks) fs sideband harmonic components under full load
condition. In all the cases, the experimental results are
validated analytically using equation (1) and found results
are closely matching.
(b)
Fig. 6 FFT spectrum of (a) one broken bar (b) two broken bar – full load
condition
Table -3 Experimental and analytical values of side band harmonic
frequency components –full load condition
Experimental Values
Side band
frequencies
(1-4s)fs
(1-2s) fs
fs
(1+2s) fs
(1+4s) fs
2ksfs
One Broken Bar
Two Broken Bar
Analytical values
of Frequency
(Hz)
Freq. in
(Hz)
Amplit
ude in
(dB)
Freq. in
(Hz)
Amplit
ude in
(dB)
One
broken
bar
Two
broken
bar
18.74
34.03
49.81
65.1
80.89
11.84
22.81
32.41
59.21
36.41
26.81
16.41
31.32
40.44
49.57
58.69
67.82
9.124
29.21
42.41
57.61
32.42
36.41
22.81
18.33
34.07
50
65.54
81.28
15.73
31.6
40.8
50
59.2
68.4
6.6
Observations:
Referring to case-2, due to broken bar rotor, an unequal
amount of current flows in rotor will set up an asymmetrical
magnetic field. The interaction between asymmetrical rotor
mmf and stator field produces a pulsating torque. This will
sets up vibrations in the motor. Speed has reduced and slip
increased; Nr = 1407 rpm; Slip = 0.0618 (one broken rotor)
and Nr = 1392 rpm; Slip = 0.0717 (two broken rotor) even
running under no load condition. The spectral analysis of
motor vibration shows frequency component of 49.81Hz
with amplitude 64.01dB amplitude for one broken rotor bar
and 49.81Hz with amplitude 61.21dB amplitude for two
broken rotor bar as shown in Fig.4 (a) and 4(b). The left
hand side (1-2ks) fs and right hand side (1+2ks) fs band
frequency components exists due to rotor asymmetry and
speed fluctuation. Table-1 presents the fault frequency and
its amplitude for one and two broken rotor bar under no load
condition.
Referring to case-3, as the load on test motor increased
to 50 % of its rated value, current increases, speed decreases
and slip increases, Nr = 1314 rpm; Slip = 0.123 (one broken
rotor) Nr = 1381 rpm; Slip = 0.0792 (two broken rotor). The
effect of asymmetrical magnetic field will be more as
compared to no load condition. As a result pulsating torque
and speed oscillations setup in rotor will develop vibrations
and humming noise in motor. The spectral analysis of
vibration signals shows that the magnitude of side band
frequencies fb = (1 ± 2ks) fs increases as compared to case-2,
Fig. 7 Comparison of FFT spectrum one and two broken bar – No load
condition
Fig. 8 Comparison of FFT spectrum one and two broken bar – Full load
Fig.7 and Fig.8 shows the comparison of one broken bar and
two broken bar rotor operating under no load and full load
condition. The magnitude of side band harmonic frequency
component in two broken bar is more than the one broken
bar in both cases. Also, as load increases not only magnitude
increases but frequency of side band harmonic component
shifting away with respect to fundamental has been
observed.
361
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Table -4 Percentage severity factor for present work motor
% Severity factor (%SF)
Healthy
% Load
One
Two
Motor
Broken Bar
Broken Bar
No Load
15.18
56.25
58.83
(0 %)
Half Load
16.59
57.23
60.27
(50 %)
Full Load
19.09
58.11
64.94
(100 %)
It is clear from Table.4, Table.5 and Fig.9, that fault severity
increases with i) increase of load and ii) increase of broken
rotor bar. Based on the above following observations have
been made,
If %SF < 45% then rotor is healthy
else if %SF > 45% and < 58% then
one broken rotor bar fault
else if %SF > 58% and < 65% then
two broken rotor bar fault
else if %SF > 65% then
three or more broken rotor bars
The percentage severity factor (%SF) of broken bar fault has
been computed for present test motor by using equation (2)
is tabulated in Table.4. Fig.9 shows the percentage severity
of fault with no load, half load and full load condition. To
ensure the effectiveness, consistency and repeatability of
proposed method of finding percentage severity factor as in
equation (2), it has been applied to the experimental and
simulation results of different rating of induction motor
under different loading conditions of past researchers and
found the following results as tabulated in Table 5.
IV.
CONCLUSION
In the present work, experimental results and analysis of
broken bar detection based on vibrations of the motor
operating under no load, half load and full load condition by
using MMA6270QT MEMS accelerometer have been
presented. The experimental results show that presence of
twice slip frequency (2sfs) due to pulsation in torque in low
frequency range and side band harmonic frequency
components (1±2ks).fs around fundamental frequency
component can be a clear indicator of rotor bar damage
fault. As the numbers of broken bars are more, the side band
harmonics (1±2ks).fs increases followed with more
vibration, humming noise and reduction in motor torque.
Also, as the load varies on broken bar motor, the amplitude
of side band harmonics (1±2ks).fs increases. As a result the
severity of fault increases leading to premature failure.
Presently research work is in progress to detect other
abnormalities in induction motor.
Fig.9 Percentage severity factor (%SF)
ACKNOWLEDGEMENT
Table -5 Percentage severity factor for past researchers results
% Severity factor (%SF)
Healthy
Two
Three
Four
Five
One
Broken
Motor
Broken
Broken
Broken
Broken
Bar
Bar
Bar
Bar
Bar
Note: A 3 phase 0.5 HP induction motor employed for one
broken bar analysis with 25% load condition [7 ]
34.61
48.33
Expt. result
-
56.45
Simulation result
Authors would like to express gratitude to
Sri.Venkataramana Bhat, Managing Director, NIKHARA
Electrical and Allied Technology, an ISO 9001 Company,
Bangalore, for supplying induction motors with rotor bar
damage.
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