International Journal of Scientific and Research Publications, Volume 2, Issue 11, November 2012 1

ISSN 2250-3153

Condition Monitoring of Induction Motor Ball Bearing

Using Monitoring Techniques
B.Hulugappa* , Tajmul Pashab**, Dr.K.M.Ramakrishnac***
*
Asst.Professor, Mechanical Engineering Department, NIE,Mysore, Karnataka
**
Associate Professor, Mechanical Engineering Department , NIE,Mysore, Karnataka
***
Professor, Mechanical Engineering Department, PESCE, Manday, Karnataka

Abstract- Rolling element bearings are critical components in primarily vibration sensors such as proximity probes. However,
induction motors and monitoring their condition is important to these are delicate and expensive. Various researchers have
avoid failures. Several condition monitoring techniques for the suggested that stator current monitoring can provide the same
bearings are available. Out of these, stator current monitoring is a indications without requiring access to the motor. This technique
relatively new technique. Vibration, stator current, acoustic utilises results of spectral analysis of the stator current or supply
emission and shock pulse methods (SPMs) and FEM, surface current of an induction motor for the diagnosis [1].A detailed
analysis, for the detection of a defect in the Inner race of review of different vibration and acoustic methods, such as
induction motor ball bearing have been compared. The vibration measurements in time and frequency domains, sound
measurements were performed at different loads and different measurement, the SPM and the AE technique for condition
speeds. The defect in the bearing could be detected by all the monitoring of rolling bearings have been presented by Tandon
methods. and Choudhury [3]. Each bearing has a characteristic rotational
frequency. With a defect on particular bearing element, an
increase in vibration energy at this element’s rotational defect
I. INTRODUCTION frequency may occur. These characteristic defect frequencies can
be calculated from kinematics considerations, i.e. geometry of
I nduction motors are widely used in industry and are
considered as critical components for electric utilities and
process industries. In the case of induction motors, rolling
the bearing and its rotational speed. For normal speeds, the
bearing characteristic defect frequencies lie in the low-frequency
element bearings are overwhelmingly used to provide rotor range and are usually less than 500 Hz.The relationship of the
support. Although induction motors are reliable, they are bearing vibration to the stator current spectra can be determined
subjected to some modes of failures .Based on studies according by remembering that any air gap eccentricity produces anomalies
to Motor Reliability Working Group (MRWG) and the in the air gap flux density. Since ball bearings support the rotor,
investigation carried out by Electric Power Research Institute any bearing defect will produce a radial motion between the rotor
(EPRI), a common mode of failure of an induction motor is the and stator of the machine. Riley et al.[4] presented a method for
bearing failure followed by stator winding and rotor bar failures. sensor less on-line vibration monitoring of induction machines.
The bearing failure increases the rotational friction of the rotor. This method assumed a linear relationship between the current
Even under normal operating conditions of balanced load and harmonics and vibration level. Da-Ming Yang and JamesPenman
good alignment, fatigue failure begins with small fissures, [5] addressed the use of stator current and vibration monitoring
located below the surfaces of the raceway and rolling elements, to diagnose bearing condition. In their study it has been reported
which gradually propagate to the surface generating detectable that monitoring of stator current provides an alternative method
vibrations and increasing noise levels. Continued stressing causes for diagnosing bearing condition that is generally less intrusive,
the fragments of the material to break loose producing localized simpler and successful detection of motor bearing condition is
fatigue phenomena known as flaking or spalling. Electric pitting possible using line current sensing.
or cracks due to excessive shock loading are also among the AE is the phenomena of transient elastic wave generation
different types of bearing damage described in the literature [1,2] due to a rapid release of strain energy caused by structural
The widespread application of rolling element bearings alteration in a solid material under mechanical or thermal
in both industry and commercial life require advanced stresses. Generation and propagation of cracks are among the
technologies to efficiently and effectively monitor their health primary sources of AE in metals. AE transducers are designed to
status. There are many condition monitoring methods used for detect the very high frequency (450 kHz) stress waves that are
detection and diagnosis of rolling element bearing defects: generated when cracks extend under load. The most commonly
vibration measurements, temperature measurement, shock pulse measured AE parameters are peak amplitude, counts and events
method (SPM) and acoustic emission (AE). Among these of the signal. Counts involve counting the number of times the
vibration measurements are most widely used. Even though the amplitude exceeds a preset voltage level in a given time and
emphasis is on vibration measurement methods, stator current gives a simple number characteristic of the signal. An event
harmonics measurement is appearing as an alternative to the consists of a group of counts and signifies a transient wave. One
vibration measurement methods. In fact, large electrical machine of the advantages of AE monitoring is that it can detect the
systems are often equipped with mechanical sensors, which are growth of subsurface cracks. Hence, it is an important tool for

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condition monitoring. It has been shown that AE parameters can monitoring methods. Hence, there is a need for a comprehensive
detect defects before they appear in the vibration acceleration [6– study of induction motor rolling element-bearing faults detection
8]. Tandon and Nakra [9] demonstrated the usefulness of some using stator current harmonics measurement in combination with
AE parameters, such as peak amplitude and count, for detection vibration, AE and SPM condition monitoring techniques. So the
of defects in radially loaded ball bearings at low and normal present work was undertaken for the detection and diagnosis of
speeds. Tan [10] has presented the application of AE for the induction motor rolling element-bearing faults have been carried
detection of bearing failures. He has suggested that the out using vibration monitoring, AE and shock pulse along with
measurement of area under the amplitude curve is preferred stator current harmonics measurements. Experimental
method for detection of defects in rolling element bearings. investigation has been carried out to study the changes in these
The shock pulses caused by the impacts in the bearings parameters for bearings in good condition and with simulated
initiate damped oscillations in the transducer at its resonant defects in the outer race of the bearings of an induction motor.
frequency. Measurement of the maximum value of the damped
transient gives an indication of the condition of rolling bearings.
Low-frequency vibrations in the machine, generated by sources II. EXPERIMENTAL SET-UP AND MEASUREMENTS
other than rolling bearings, are electronically filtered out. The 2.1. Test rig
maxim normalized shock value is a measure of the bearing The test rig used in the present research work consists of a
condition. Shock pulse meters are simple to use so that 1.1kW/1440 rpm single-phase induction motor driving the V-belt
semiskilled personnel can operate them. They give a single value drive. The test bearing is the drive end bearing of induction
indicating the condition of the bearing straightaway, without the motor, housed within the drive end cover plate of motor to
need for elaborate data interpretation as required in some other support rotor. The rotor shaft of motor is extended to the left of
methods. drive end cover plate of motor for providing transmission of
The principle is based on the fact that structural power through pulley drive. The test bearing can be radial loaded
resonances are excited in the high-frequency zone due to with V-belt drive with the loading system. Vibration isolation
impulsive loading caused, for example, from spalling of the races rubber sheets were provided under the motor and its supporting
or rolling elements and can be detected by a transducer whose legs to reduce the vibration transmission from ground to the test
resonance frequency is tuned to it. The SPM, which works on bearing. The drive to the induction motor is provided by a.c.
this principle, uses piezoelectric transducer having a resonant power supply. The transducers for measurement of vibration, AE
frequency based at 32 kHz[11,12]. In industries SPM has gained and SPM have been mounted in the zone of maximum load on to
wide acceptance for detecting the rolling element bearing the drive end bearing of induction motor. The Schematic diagram
defects. of the experimental set-up is shown in Fig. 1. The test rig has
The literature indicates that even though the emphasis is been designed to withstand a maximum load of 27 kg based on
on vibration measurement methods for the detection of defects in the rated power of induction motor for 1.5 HP. The test bearing
rolling element bearings in induction motor, stator current of induction motor in this study is normal clearance, deep groove
harmonics measurement can be an effective alternative to the ball bearings SKF 6205. Equal amount of SKF grease (4 g) was
vibration measurement because of its faster diagnosis. Very few applied to the bearing, which was immersed in kerosene and
studies have been carried out on stator current monitoring of an finally cleaned with acetone to remove preserving oil.
induction motor for the detection of defects in these rolling
bearing along with vibration monitoring and other condition

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2.2. Instrumentation respectively.AE measurements were performed by using

Vibrations were measured with the help of a piezoelectric Acoustic Emission Technology Corporation (AET), model AET
accelerometer Bruel and Kjaer (B&K) type 4366having un AC 375 L transducer which are of same frequency of 375 kHz, a
damped natural frequency of 39 kHz. The output of the preamplifier with 60 dB gain (AET 160B)and a filter (AET FL
accelerometer was fed to the B&K charge amplifier 2635 25) with a pass band of 250–500 kHz. The AE transducer was
connected to Ono Sokki CF 3200 portable fast Fourier transform mounted on the test bearing housing with grease as couplant and
analyzer. The schematic diagram of the current sensor(working with the help of cloth adhesive tape. The preamplifier is provided
on Hall effect) in series with motor supply line is shown in Fig. with 712V DC supply. The output from the preamplifier was fed
1. The Hall element located in the air gap of the magnetic circuit to Tektronix TDS 210 digital real time oscilloscope, which can
converts the magnetic field generated by the primary current into also give frequency spectrum.
a proportional Hall voltage. The magnetic field produced by the The Shock Pulse Tester T2000 by SPM Instrument AB,
primary current generates a highly linear magnetic flux in the air Sweden, along with its hand held transducer type SPM 10777
gap of the magnetic circuit, which in turn induces a proportional was used for shock pulse measurements. The hand held probe
Hall voltage in the Hall element. The voltage is then was pressed straight at the zone of maximum load on bearing
electronically amplified resulting in an output voltage that is housing to get maximum normalized shock pulse value, dBM.
highly proportional to the primary current up to the final value of
the measuring range. The current sensor is supplied with 715V 2.3. Measurement conditions
from the power supply unit for 15min before taking the The measurements were carried out from no load to full
measurements. load (27 kg) for the induction motor bearing with an increment of
The current flowing in single-phase induction motor was 5 kg. The motor was run at constant speed of 1440 rpm. Three
sensed by a current sensor of type LEM-HY 25P by connecting it healthy bearings were used to check the repeatability of the
in series with the supply line to the motor. This current sensor measurements. Inner race defect was simulated by a circular
has dynamic performance accuracy of linearity better than hole of diameter varying from 250 to 1500 mm in the outer race
70.2%, response time better than 1 ms and nominal analog output of the same bearing (in steps of 250 mm successively after each
current 25 mA. The output from the current sensor is fed to the measurement) by spark erosion technique.
FFT analyzer with frequency span of500 Hz and 20 kHz for
frequency spectrum analysis and for overall values,

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III. RESULTS AND DISCUSSIONS where fr is the shaft rotational frequency, N is number of balls,
3.1. Vibration velocity bd and pd are the ball diameter and pitch circle diameter
Fig. 2 shows that the overall amplitudes of vibration ,respectively, and b is the contact angle of the ball (with the
velocity of three healthy bearings are very much close to each races). For the shaft rotational frequency fr= 24 Hz and a test
other and their average overall level is also shown. Fig. 3 shows bearing having nine balls of diameter 8.5mm and pitch circle
that overall velocity values follow the same trend as that of the diameter of 38.5mm with contact angle ß=0, the characteristic
good bearing with increase in load. The overall velocity value inner race defect frequency fo is found to be84.15 Hz.
has increased even for a small defect size of 250 mm. The overall Velocity spectrum of one of the healthy bearings in the
velocity significantly increases to 66% in case of a maximum low-frequency range of 500 Hz at 15 kg load is shown in Fig.
defect size of 1500 mm with respect to healthy bearing at 15 kg 4.From the spectrum of velocity, it has been observed that the
load. The spectrum of the vibration velocity signal in the low- peak occurs at fundamental frequency of shaft (i.e. at 24 Hz) and
frequency range was obtained to observe changes at the at twice the supply frequency (i.e. at 100 Hz) in the spectral
characteristic defect frequency of the bearing outer race due to component
defects in it. The characteristic defect

3.2. Stator current signals

Fig. 6 shows the overall value of stator current amplitude
comparison at 15 kg load for 0–20 kHz range of three healthy
bearings and also the average overall stator current of three
healthy bearings. From this chart also it is observed that the
overall amplitudes are very much close tone another for the three
healthy bearings. The overall stator current values were taken for
all the defect sizes in the inner race of the bearing. Fig. 7 shows
that the overall stator current values follow the same trend as that
frequency of the rolling element bearing outer race can be of the good bearing within crease in load. The overall stator
calculated by using the following equation [3,13]: current value has increased slightly even for a small defect size
fo(HZ)=N/2fr[1-bd/pd Cosß) (1) of 250 mm. From 250 to 1250 mm, the amplitude of overall
stator current values increase continuously and the increase is
much more for 1500 mm defect size. As mentioned in the

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literature, a defect in rolling element-bearing causes an increase centre).Eccentricity causes a force on the rotor that tries to pull
in the overall RMS value of stator current for a known frequency the rotor even further from the stator bore centre. In the case of
range. Hence, the results obtained from the stator current static eccentricity this is a steady pull in one direction. This
correlates with the results reported in [4,15]. makes the unbalanced magnetic pull(UMP) difficult to detect
unless specialist experimental equipment is utilized, which is not
possible for motors in service. Dynamic eccentricity produces a
UMP, which acts on the rotor and rotates at rotor rotational
velocity.
Both types of eccentricities cause excessive stressing of
the machine and greatly increase bearing wear due to uneven
magnetic pull produced that leads to variation of the sideband
current magnitudes or predicted current harmonics in relation to
vibration velocity. Hence, any fault condition in the induction
motor causes the magnetic field in the air gap of the machine to
be non-uniform. It has been shown by Schoen [1] that these
vibration frequencies reflect themselves in the current spectrum
as f bng=[fe± mfv] (2)
where fe is the electrical supply frequency, m =1,2,3… is one of
the characteristic vibration frequencies. A current signal of a
single phase of stator current of induction motor and a vibration
signal from a vibration sensor located at the bearing housing of
induction motor for three good bearings were obtained. The
corresponding current spectrum components in relation to
vibrations for the supply frequency fe of 50 Hz and at
characteristic outer race defect vibration frequency of 84.15 Hz
are 34.15 and 134.15 Hz. Fig. 8 shows the acquired current
spectrum for the same low-frequency range of 500 Hz at 15 kg
load as that of vibration velocity spectrum to verify the
relationship between stator current and vibration velocity for
healthy bearing. The spectrum of stator current in Fig. 8 indicates
peak at supply frequency of 50 Hz in the current spectrum,
whereas at twice the supply frequency in the velocity spectrum
(Fig. 4) indicating the UMP even under normal operating
condition. The corresponding current spectrum components in
relation to vibrations were not significant at
[ f e _ f o](i.e. at 34.15 Hz) and [ fe+fo] (i.e. at 134.15 Hz) as
shown in Fig. 8.

Thus, overall stator current has appreciably increased by

39.79% in case of maximum defect size of 1500 mm with respect
to healthy bearing at15 kg load. The relationship of the bearing
vibration to the stator current spectra can be determined by
remembering that any air gap eccentricity produces anomalies in
the air gap flux density [13]. Since ball bearings support the
rotor, any bearing defect will produce a radial motion between
the rotor and stator of the machine. The mechanical displacement
resulting from damaged bearing causes the machine air gap to
vary in a manner that can be described by combinations of
rotating eccentricities moving in both directions [16]. Thus,
bearing fault simulated in the inner race of the bearing may also
cause rotor eccentricity, which is one of the common mechanical
faults in the bearing. The rotor eccentricity in induction motor
takes two forms, i.e. static eccentricity (where the rotor is
displaced from the stator bore centre but still rotating upon its
own axis) and dynamic eccentricity (where the rotor is still
turning upon the stator bore centre but not on its own

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The rest of the peak component other than at 50 Hz

present in the current spectrum occur at multiples of the supply
frequency and these are caused due to saturation, winding
distribution and supply voltage. The stator current spectrum of
motor with the outer race defect of the bearing from 250 to 1500
mm in steps of 250 mm were obtained in the low frequency of
500 Hz for 15 kg load and the plot of 1500 mm is shown in Fig.
9. Predicted current harmonics for outer race of the bearing
relating vibration characteristic defect frequencies with the
supply current frequency are compared with those of healthy
bearing. For minimum defect size in the outer race of the bearing
of motor, there was marginal increase in the amplitude of the
predicted current harmonics component at [ f e - f o]=34:15 Hz
and[ f e + f o]=134:15 Hz. However, significant increase in the
amplitude of predicted current harmonics or vibration sideband
current magnitudes is observed as the defect size increases as
shown in Fig. 9. These results are comparable with results
reported in [5,13,17]for the outer race defect in the bearing.

3.3. Acoustic emission monitoring

In the present work, peak amplitudes of the AE signal of
three healthy bearings from no load to full load were obtained.
Peak amplitudes of the signal at 375 kHz were obtained and are
expressed in dB with 0 dB corresponding to 1 Vrms in the
oscilloscope. The values of AE maximum peak amplitude of
three healthy bearings are shown in Fig. 10. The average value is
31.8 dB. Fig. 11 shows the AE maximum peak amplitude
obtained from no load to full load for all defect sizes in the outer
race of the bearing. It is observed that as the defect size increases
the peak amplitude also increased. It has been observed from Fig.
11; the peak amplitude increased till 10 kg load and then
decreases slightly with increase in load. The range of AE
maximum peak amplitudes from healthy bearing to maximum
defect size are 31.4–67.8 dB at 10 kg load and 31.8–63.2 dB at15
kg load. In general the difference in A maximum peak amplitude
of healthy and smallest defect size is quite significant and makes
it possible to detect the presence of a defect for diagnosis easier
at the early stage in comparison with other condition monitoring
techniques. There is an appreciable increase of 98.74% in case of
maximum defect size with respect to healthy bearing at 15 kg
load. Whereas for 10 kg load, the increase is115.92% in case of
maximum defect size with respect to healthy bearing.

3.4. Shock pulse method (SPM)

To neutralize the effect of rolling velocity on the
measured value, the instrument was fed the shaft diameter of
25mm and rotational speed of 1440 rpm. After this SPM T2000
series calculates the initial shock value dBi,

and displays maximum normalized shock pulse values. Fig. 12

shows the comparison chart for maximum normalized value and
average value of three healthy bearings. The values vary from 14

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to 17 dB and are less than 20, indicating good bearing condition.

Maximum normalized values (dBM) obtained for all defect sizes
of250–1500 mm in the outer race of the bearing from no load to
full load at constant motor speed are given in Fig. 13 in
comparison with the average value of three healthy bearings. It is
seen that the levels for different bearings are more than that of
healthy bearing and for 250–750 mm defect size, values of dBM
are greater than20 and less than 35. For the defect size greater
than 750 mm values of dBM are greater than 35. The dBM
obtained indicates caution zone for the defect size of 250–750
mm and for the defect size above 750 mm dBM values obtained
indicate the damaged bearing condition. Maximum normalized
value as high as 50 was measured for the maximum defect size.

IV. COMPARISON OF TECHNIQUES

The comparative study of different condition monitoring
techniques has been done for minimum and maximum defect size
in the outer race of the bearing. Fig. 14 shows the effectiveness
of each technique in terms of percentage increase with respect to
average value of healthy bearing, for the smallest defect size. As V. CONCLUSIONS
seen in Fig. 14, AE technique is the most effective technique The vibration and stator current signal measurements
followed by SPM. The maximum normalized value of SPM is performed on the bearing of an induction motor are successful in
almost three times less effective as compared to AE technique. detecting simulated defects in the outer race of the bearing.
Overall vibration velocity and stator current come in the third Current harmonics for bearing outer race defect characteristic
and fourth place, respectively. Fig. 15 shows the same order in vibration frequency has shown significant increase in the current
the effectiveness of each technique for maximum defect size. spectrum components for maximum size of defect. The AE and
However, stator current monitoring has the advantage that it SPM measurement performed are very good in detecting the
requires minimum instruments and is sometimes referred to as bearing defect. On comparing the results of good and defective
sensor less technique. bearing, it is observed that AE peak amplitude and shock pulse
maximum normalized value level increase much more than other
techniques as defect size increases. AE monitoring has proved to
be the best technique. Stator current monitoring is perhaps the
most cost-effective technique.

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[8] T. Yoshioka, M. Takeda, Classification of rolling contact fatigue initiation vibration monitoring applications, IEEE Transactions on Industry
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AUTHORS
bearings, NDT International 6 (2) (2010) 92–95. First Author – B.Hulugappa, Asst.Professor, Mechanical
[12] I.E. Morando, Measuring shock pulses is ideal for bearing condition Engineering Department, NIE,Mysore, Karnataka
monitoring, Pulp and Paper 62 (12) (2008) 96–98.
, bhniemech@gmail.com
[13] R.R. Schoen, T.G. Habetler, F. Kamran, R.G. Barthheld, Motor bearing
damage detection using stator current monitoring, IEEETransactions on
Second Author – Tajmul Pasha, Associate Professor,
Industry Applications 31 (6) (2005) 1274–1279. Mechanical Engineering Department , NIE,Mysore, Karnataka,
[14] N. Tandon, B.C. Nakra, Detection of defects in rolling element bearings by tpniemech@gmail.com
vibration monitoring, Journal of Institution of Engineers(India), Mechanical Third Author – Dr.K.M.Ramakrishn, Professor, Mechanical
Engineering Division 73 (2003) 271–282. Engineering Department, PESCE, Manday, Karnataka
[15] M. El Hachemi Benbouzid, A review of induction motor signature analysis
as a medium for fault detection, IEEE Transactions onIndustrial Electronics
47 (5) (2000) 984–993. Correspondence Author – B.Hulugappa.9481439473
[16] C.M. Riley, B.K. Lin, T.G. Habetler, G.B. Kliman, Stator current harmonics
and their casual vibrations: a preliminary investigationof sensor less

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