Vibration Monitoring and
Current Analysis of AC Motors
Using Motor Current and Vibration Analysis to
Detect AC Motor Problems

Summary

Jason Mais
22 pages
November 2002
SKF Reliability Systems
@ptitudeXchange
4141 Ruffin Road
San Diego, CA 92123
United States
tel. +1 858 244 2540
fax +1 858 244 2555
email:
Internet: www.aptitudexchange.com

Use of this document is governed by the terms
and conditions contained in @ptitudeXchange.

This article contains an extensive summary, and several
examples, of analyzing AC motors using vibrational data and
motor current analysis. The first example references a large
pulper motor in a paper mill operation, in which a shortened
stator was causing the motor to fail. The second example is in
reference to an electric motor manufacturer in Israel that uses
motor current analysis and vibration analysis to test electric
motors in field applications. The third example considers the

monitoring of electric motors used in pumping stations. The
fourth example deals with a company using motor current
analysis as a quality control, or testing process to establish the
health of the motors being manufactured. In the fifth case, a few
more examples of pole pass frequencies are shown. Finally, a
case of motor excentricity is provided.


Vibration Monitoring and Current Analysis of AC Motors

Introduction
Apart from vibration analysis, motor current
signature analysis (MCSA) is a powerful
monitoring tool for electric-motor-driven
equipment. It provides a non-intrusive means
for detecting the presence of mechanical and
electrical abnormalities in motor-end driven
equipment, including altered conditions in the
process that may be downstream of the motor
driven equipment. MCSA is based upon the
recognition that a conventional electric motor
powering a machine also inevitably acts as a
transducer of variations in the driven
mechanical load, as the latter are converted
into electric current variations that are
transmitted along motor power cables. These
current variations, though very small in
relation to the average current drawn by the
motor, can be extracted reliably and nonintrusively at a location remote, and processed
to provide indicators of condition (signatures).

These signatures may be trended over time to
give early warning of performance
degradation or process alteration.
Although MCSA technology was developed
for the specific task of determining the effects
of aging and service wear on motor-operated
values used in nuclear power plant safety
systems, it is recognized as applicable to a
much broader range of machinery. MCSA is
used to analyze pumps of various design,
blowers, compressors, and air-conditioning
systems powered by AC and DC motors.

minimal. The resulting raw current signal is
amplified, filtered, and further processed to
provide a sensitive and selective means for
extracting motor current noise information
that reflects instantaneous load variation
within the drive train and the ultimate load.

Figure 1. SKF Microlog and AC/DC Current Clamp.

This article provides an extensive summary of
several case studies that relate to the use of
motor current signature analysis and vibration
analysis. The reader is also referred to the
article entitled An Introduction into Motor
Current Spectrum Analysis (MCSA), JM02010
on the @ptitudeXchange website.


Summary of AC Motor Monitoring

The use of devices, such as a data analyzer
Mechanical Vibration
(the instrument on the left in Figure 1), and
Using a standard accelerometer placed on the
current clamp (the instrument on the right in
bearing cap detects several unique mechanical
Figure 1) are common to collect and analyze
vibration signals generated by electrical faults
vibration and motor current data from an AC
in the motor circuits. One of the more
motor. These types of devices obtain data in a
common faults produced is a signal at twice
non-invasive manner, which is important to
line frequency (2FL or 2 F.L.). If the line
the operating process. The use of a single
frequency
is 60 Hz, this signal is at 120 Hz or
split-jaw current probe placed on one power
7200 CPM. If the line frequency is 50 Hz, the
lead is sufficient to obtain data. Since there is
signal is at 100 Hz or 6000 CPM. Care must
no electrical connection being made or
be taken when testing 2 pole motors (3600
broken, the possibility of shock hazard is
© 2002 SKF Reliability Systems All Rights Reserved
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Vibration Monitoring and Current Analysis of AC Motors
RPM or 3000 RPM) to ensure the signal is not
twice rotation speed of the shaft, rather than
twice line frequency.
This two times line frequency signal is created
by any of the following faults:
1. Uneven air gap between rotor / stator
As the motor poles pass the narrow gap, the
magnetic pull is greater v. 180 degrees on the
opposite side where the gap is the widest. The
number of poles (motor speed) does not
change the results: an uneven air gap results in
a signal at twice line frequency.
The cause of this uneven air gap is often due
to an uneven base plate under the machine’s
foundation, and is referred to as a soft foot.
Some empirical data seems to indicate that the
twice line frequency signal appears when the
gap clearance exceeds 10% variance.
Loosening and tightening one bolt at a time on
the foundation, with the motor running, while
observing the spectrum on the data analyzer,
can confirm soft foot. When the soft foot is
loosened, the signal decreases, and then
increases as the nut is tightened. If soft foot
occurs the foot should be shimmed during the
next shutdown to the same plane as the others.
2. Damage stator windings or insulation
There are numerous causes of damage to the
stator: poor manufacturing, poor environment,

flaws in the insulation, etc. Any damage to the
stator again creates an uneven magnetic field
around the rotor. This uneven field generates
an uneven pull on the rotor, regardless of
motor speed, and causes a mechanical
vibration at twice line frequency.
It is often possible to locate the area of
damage with either an infrared or thermal
detector (refer to JM02008 - Thermography
on the @ptitudeXchange website). Usually
there is an area on the motor housing where

the surface temperature is elevated 20 to 30
degrees.
A damaged stator also generates a mechanical
vibration signal at a frequency equal to the
number of rotor bars, multiplied by rotation
speed. Again, in the area of stator damage, the
magnetic field is weakened, and stronger 180
degrees away. As each rotor bar passes this
area of higher strength, the bar is
mechanically pulled in that direction.
Typically, rotor bars have between 45-55 bars
in the rotor, but this can vary depending on the
manufacturer. For this reason, it is very
important to set the Fmax at least 100 times
rotation speed when troubleshooting motor
vibration. Please note that this Fmax value is
for troubleshooting purposes only.
Since the number of rotor bars can vary

greatly, it is most important to establish a
procedure that states that at anytime a motor is
down for repair, a count of the actual number
of rotor bars should be made and recorded. It
is also important to record the bearing
nomenclature so that the bearing frequencies
can be accurately determined when analyzing
for bearing degradation.
The user can verify that the vibration is
electrically induced by shutting off the motor
while observing the frequency spectrum in the
analyzer mode. The moment the power is
removed, the distorted magnetic field is
instantly collapsed and the twice line signal
disappears. If the signal does not disappear,
but rather slowly degrades, then the user
knows there is some type of mechanical
problem. When setting up an analyzer, use
100 lines, 0 averages, and an Fmax of 2000 Hz
to provide a fast cycle time.
If the data analyzer has enveloping circuits,
and these two conditions are met, then the
signal is also seen in any enveloped
acceleration spectrums and will most certainly

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors
generate harmonics of the fundamental
frequency.
There is not an agreed upon amplitude of
concern if the twice line frequency signal is
present. It is generally agreed that the
presence of 2x line frequency is not desirable.
Generally accepted limits are between 0.04 0.06 IPS in the velocity spectrum at twice line
frequency. As an example of the amplitude
levels of 2FL, a trend of an enveloped
acceleration reading, in which twice line
frequency was taken over a six months period,
shows an increase from 0.4 gE to 1.6 gE.
When the motor reached a vibration level of
1.6 gE, the motor failed. However, after the
motor was repaired, the trend was
reestablished, and the amplitude of that trend
consistently remained at 0.8 gE. It is suspected
that the first failure was due to a damaged
stator. In addition to the stator problem, soft
foot concerning the foundation was also
present. After repairing the motor, the soft
foot contribution is still present, although it
exhibits itself differently. This is most likely
due to torque on the mounting bolts in
addition to machine placement and
configuration.
Sidebands
As with most vibration signals, the presence
of sidebands around fundamental frequencies

is a measure of an increase in severity. As the
sidebands increase in number and amplitude,
so does the severity of the problem. Some of
the sideband energy is pole pass frequency
and slip.
Pole Pass Frequency =(number of poles)(slip)
Slip = (nominal speed - actual speed)

may find it necessary to increase resolution to
either 1600 or 3200 lines to separate these
sidebands and verify the existence of this
energy.
Analysis of AC Motor Current
The effectiveness of evaluating motor
condition by performing an FFT of the motor
current is verified many times over in the
analysis of motors. And, although it is often
referred to as a method of detecting broken
rotor bars, it is actually detecting abnormally
high resistance in the rotor circuit (bad solder
joints, loose connections, and damaged rotor
bars).
At this point, a review of basic spectrum
components is necessary to ensure a clear
understanding of vibration analysis. If there is
a fault in the rotor circuit, then the spectrum
has two prominent features when displayed.
Using a logarithmic scale for clarity
concerning amplitude peaks with respect to
the Y-axis, the display at 60 Hz or line

frequency contains a large spike. To the left,
at a distance equal to the rotor slip, times the
number of poles, pole pass frequency is
another spike of energy. These spikes can be
labeled A for line frequency, and B for pole
pass frequency. Note that the amplitudes of
peaks A and B have to be obtained using
cursor overlays, as it is necessary to use
amplitudes to four decimal places, and most
data analyzers only read the value to three
places. To determine the health of the
machine, perform the following calculation:
Log (A/B) times (20) = amplitude (dB)
Examples are included in Figures 2-4.

Around the rotor bar pass frequency it is
possible to see sidebands of twice line
frequency. In troubleshooting, the data analyst

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors
Category
Number

dB


1

>60

A/B

Rotor Condition

Corrective
Action

Excellent

None

Good

None

Moderate

Trend

Rotor Bar May be Cracked or High
Resistance Joints

Increase
Monitoring
Time


Two Rotor Bars Cracked and High
Resistance Joints

Vibration
Testing

>1000

2

54-60

5001000

3

48-54

250500

4

42-48

125250

5

36-42


63125

6

30-36

32-63

Multiple Rotor Bars Broken, Slip Ring
and Joint Problems

Overhaul

7

<30

<32

Severe Problems Throughout

Overhaul /
Replace

Table 1. Motor Current Analysis Severity and Recommended Corrective Action Chart.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 2. This is the spectrum from a damaged compressor motor that had 5 broken rotor bars, and a damaged end
ring. Log 0.018572/1.0571 X 20 = 35.1 dB. Refer to Table 1: category 6 severity.

Figure 3. Motor in lab with four cut rotor bars and a broken end ring. Log 0.0908/8.777 X 20 = 39.7 dB. Refer to
Table 1: dB severity level.

Figure 4. Motor in lab with no damage. Log 0.003079/1.704 X 20 = 54.9 dB. Refer to Table 1: dB severity level.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors
Note that the chart applies to rotor circuit
damage, and that the motor must be under at
least 75% load. The amplitude of the pole pass
frequency is not linear with respect to reduced
loads. If the amplitudes that correspond to
reduced loads are used, the results will not be
accurate. The examples in the following
section help illustration good and bad motor
circuit spectra.
Observations of Other Motor Problems
High efficiency motors obtain their efficiency
and use less electricity by employing two
methods: use of a smaller air gap, and a layer

of thinner insulation on the windings. If the
owner installs these motors on the same
transformer circuit as a DC motor, it is
possible for the DC motor’s Silicon Circuit
Rectifiers (SCR) to provide feedback onto the
AC circuit and induce high voltage spikes into
the motors. The reduced insulation rapidly
deteriorates and leads to a reduced motor life.
Field results have shown as much as a 50%
reduction in the life of the motor due to this
type of problem. DC motor problems can be
seen at the SCR firing frequency.
SCR Frequency = (6)(Line Frequency)
Check connections, SCRs, control cards, and
fuses if you experience this frequency.

Enveloped AC Motor Current
The principal for an enveloped AC motor
current operation is as follows. When the
motor current with a damaged rotor circuit is
enveloped, the resulting spectrum shows
energy at the actual pole pass frequency (e.g.
at 0.8 Hz), not as a sideband of the 60 Hz
signal or 59.2 Hz. Initial investigation shows a
relationship between the pole pass frequency
amplitude as a ratio to the overall amplitude
of an FFT spectrum taken with an Fmax of 25
Hz. Typically, in a good motor this is a very
low amplitude signal and is not seen in an
enveloped spectrum. So, the frequency must

be calculated to locate it.
Initial data shows that a good motor has a ratio
of 5% or less. but as damage increases the
percentage also increases. Additionally,
harmonics of pole pass frequency are an
indication of damage. Initial testing shows that
this method is sensitive to the condition of the
motor and will detects very early stages of
degradation in the rotor circuit.
Figure 5 shows an example of an enveloped
current spectrum with no sidebands around the
2x line frequency. Zooming into the 25 Hz
area, Figure 6 shows clearly the pole pass
frequency. The ratio between this pole pass
frequency peak and the overall amplitude is
63%, indicating a possible problem.

Figure 5. Twice line frequency (119.792 Hz) with harmonics (239.583 Hz, #59.375 Hz,…) using Enveloped Gs (gE).
Running speed is 4792.4 RPM.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 6. Enveloped AC motor current, pole pass frequency of 0.8125 Hz generated by 5 broken rotor bars and a
damaged end ring. Ratio of pole pass frequency amplitude (0.81) to overall amplitude (1.29) is 63%, indicating
possible damage to the motor.


Case 1: Pulper Motor with
Shorted Stator
Background
This case study was developed from the
analysis of a large pulper motor in a paper mill
in the southern United States. It was common
practice for the paper mill to replace the
pulper motor routinely during the plant
shutdown over the winter holiday. This
shutdown occurred during the end of
December, and into the beginning of January.
The paper manufacturing facility uses
predictive maintenance technologies, such as
vibration analysis and motor current analysis,
to help assess machinery health. In this case,
after the winter holiday, the newly installed
pulper motor began operation. Based upon
data obtained shortly after the refurbished
motor was replaced while the mill was
operating at full capacity, the pulper motor
began to exhibit an abnormally high level of
vibration. In an instance when machinery is
not monitored using predictive maintenance
technology, a refurbished motor would
probably not be considered as a “bad actor” or
a poorly performing machine in the plant. The
results of this case study point directly to the
refurbished motor as the problem. The
following measurements are related to a

specific instance in which MCSA and

vibration data were used to determine a
problem affecting a pulper motor in the plant.
Vibration data was obtained on the motor in
two specific areas. These areas exhibited the
greatest response to the condition of the
motor. The two areas, A and B, are diagramed
in the illustration below.

Figure 7. An illustration of the electric motor on which
analysis data was collected. The data was collected in
areas A an B disignated in the illustration.

Trending
One key elements in assessing mechanical and
electrical health of a machine is to acquire
data related to the machine on a regular basis
or interval. Once several data intervals have
been collected a trend can be established. In
Figure 8, the trend is shown. The overall trend
of the vibration data is increasing. This
increasing level indicates that a possible motor
problem.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 8. This is the trend data of the large pulper motor over a six-month period. The data is taken in Velocity
(IPS) and is plotted versus Time. An increase from ~0.05IPS to < 0.3 IPS can be seen in the trend. This increase
shows a higher level of vibration of the pulper motor, which usually indicates a problem with the system.

Specific Measurements
After several months of data collection, a
variation in the trend was apparent. In Figure
9, a spectral peak at 7200 RPM or 120 Hz can
be seen. This peak is twice the line frequency
of the motor, or twice the frequency at which
the power supply is operating (60 Hz in the
USA).
Line Frequency (L.F.):
L.F. = Power Line Frequency
L.F. = 60 Hz (US) or 50 Hz (Europe)
2 x L.F. = 2(60 Hz) = 120 Hz or 7200 RPM

When a stationary problem such as a
shortened stator occurs, it produces a peak in
the spectrum at 2x life frequency. In this case,
once the machine was taken out of service and
evaluated visually, it was determined that
indeed there was a shorted stator problem. The
two times line frequency problem will be
explained in detail in the following section.
It is important to point out that electrical or
magnetic defects quickly disappear once the
power supply is disconnected. Additionally,

most electrical problems only occur when the
motor is loaded. This can help differentiate
mechanical problems from electrical problems
in spectrum analysis.

or,
2 x L.F. = 2(50 Hz) = 100 Hz or 6000 RPM

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 9. Individual Vibration Spectrum of the failing pulper motor. The failure was due to a shortened stator
indicated by a peak at 7200 RPM. Running speed of the motor happens to be 700 RPM - indicated by the vertical
line on the left-hand side of the spectrum.

2x Line Frequency
One of the more common problems exhibited
at 2x line frequency occurs from a shortened
stator inside the motor. In Figure 9, the 2x line
frequency displayed is produced by such a
problem. This shortened stator creates an
uneven air gap between the motor and the
stator. When a motor rotates at 3600 RPM, the
magnetic pull towards the closest pin rises and
falls from 0 to maximum twice during the
displacement of the rotor with respect to the

stator. Since this occurs twice per rotation it
produces 2x line frequency, or:
2FL = 2 x 60Hz = 120Hz (U.S.A.)
2FL = 2 x 3600CPM = 7200CPM
If an uneven air gap is left undetected or unrepaired, the motor will fail.

rotors, arching between loose rotor bars and
end rings, and phase problems due to loose or
broken connectors. All of these problems will
exhibit various levels of 2x line frequency. As
stated before, any peak at 2x line frequency is
suggested as being a poor condition for the
motor.
Conclusion
Based upon the increase in the trend in Figure
8, and the specific measurement from that
increase, the reconditioned motor was
disassembled. When the motor was examined,
it was determined that the stator in the motor
was damaged. The motor was replaced and
operations continued as schedule with very
little interruption to the process. Figure 10
shows measurements taken after the stator
problem was resolved. Operating levels of the
motor returned to normal.

Other typical problems that exhibit 2x line
frequency are eccentric rotors, loose or open
© 2002 SKF Reliability Systems All Rights Reserved


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Vibration Monitoring and Current Analysis of AC Motors

Figure 10. Overall trend of the pulper motor after the shortened stator problem was resolved. The overall values
returned to normal operating levels. Indicated by the marker towards the right-hand side of the trend.

Case 2: Broken Rotor Bar

where:

Background

P = Number of Poles

This case study relates to the testing of electric
motors in air fans. The testing is conducted on
50 Hz motors containing four (4) poles and
running at a speed of 1483 RPM – 1490 RPM
(average 1487 RPM). In addition to vibration
data, Motor Current Signature Analysis
(MCSA) is also used as an additional test to
validate the motors and aid in the confirmation
of vibration data.

L.F. (FL) = Power Line Frequency

Specific Measurements
An example of test results from a good motor

and a bad motor are contained in Figures 11
and 12. An evaluation of peaks to either side
of the center frequency, helps determine the
state of the motor. When considering sideband
data, the following calculations are used.

Sync = Synchronous Rotation
Sync = 60(L.F.)/(P/2)
Sact. = Actual Rotation

With 4 poles, we can calculate the following:
Sync = 60(50 Hz)/(4/2) = 1500 Hz
Sact. = 1487 RPM (mean RPM Figures 6 & 7)
Slip = 1500 RPM – 1487 RPM = 13 RPM
Slip
sidebands = 2 x
xL.F .
Sync
13RPM
x50 Hz
sidebands = 2 x
1500 RPM

 Sync − Sact 
sidebands = 2 x 
 xL.F .
 Sync 
Slip
sidebands = 2 x
xL.F .

Sync

© 2002 SKF Reliability Systems All Rights Reserved

sidebands = 0.866 Hz
Slip = Sync – Sact.

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Vibration Monitoring and Current Analysis of AC Motors
Motor Current Analysis

With respect to MCSA, the most common
spectra contain the center frequency, which is
the running speed of the motor, and if there is
a component failure, the frequency of that
component. In the air fan example, the
spectrum exhibits sidebands of the pole pass
frequency. In Figures 11 and 12 there are

several comparisons to be made. Figure 11 is
the motor current signature of another air fan
motor in good condition. Figure 12 is the same
type of measurement except it contains
sidebands in the spectrum that are spaced, at
about 0.9 Hz to the left and right of the center
frequency (50 Hz). The sidebands have peak
amplitudes of approximately 1.5% (36 dB).


Figure 11. Use of MCSA to diagnose rotor bar failure. The 50 Hz (EU) peak, line frequency, is the center frequency
and is related to the running speed of the motor (50 Hz or 3000 RPM). There are no sidebands, or high spikes to the
sides of the center frequency. The ratio between the little spiked and the center frequency peak is 60 dB. The lack of
high sidebands indicates no anomalies.

Figure 12. Use of MCSA to diagnose rotor bar failure. The 50 Hz (Europe) peak, line frequency, is the center
frequency on the spectrum. There are sidebands, or high spikes to the sides of the center frequency, which indicate a
problem in the motor. In this instance, they are spaced at about 0.9 Hz from the center frequency, as calculated in
the Specific Measurement Section. These spikes are related to pole pass frequency and indicate a problem with the
rotor bars of the motor. The ratio of the center frequency amplitude compared to the sideband amplitude determines
the severity of the problem, about 36 dB (1.5%).

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors
Vibration Data Analysis

Vibration analysis technology is used in
addition to Motor Current Signature Analysis
(MCSA) to detect damage in motors. When
using vibration data, the analyst must keep in
mind that this type of technology can detect a
facet of other types of problems. Therefore, it
can be more difficult to use, as compared to
MCSA. In vibration signatures, rotor bar
defects are


exhibited in the spectrum in what are termed
haystacks. The center frequency of the
haystack corresponds to the multiples of
running speed of the motor, with sidebands
spaced at pole pass frequency. In this case,
the calculation arrived at a value of 0.866 Hz.
In Figure 13, vibration data collected using an
accelerometer shows haystacks in the scenario
of a failing rotor bar.

Figure 13. Signature analysis using vibration data to detect problems in the motor. The haystacks are the key
component of this spectrum. The third series of haystacks are the set that are most interesting from an analysis
standpoint because they contain the greatest amount of energy. The left most vertical marker is running speed at
approximately 25 Hz (1500 RPM) The additional markers to the right of the running speed are harmonics of
running speed. The peak at 92 Hz was explained as a typical bearing frequency and not found suspicious

© 2002 SKF Reliability Systems All Rights Reserved

13


Vibration Monitoring and Current Analysis of AC Motors

Figure 14. Signature analysis using vibration data to detect problems in the motor. This illustration is a close up of
the third set of haystacks from Figure 13. This close up is used to illustrate the spacing of the sidebands (0.866 Hz)
around the center frequency (3x running speed ~73.42 Hz).

Visual Inspection of Rotor Bar Damage

Based upon the spectra data, the motor was

inspected for defects. The following pictures
show the rotor bar damage.

Figure 16. Broken rotor bars causing inconsistent
output of electrical energy from the motor.

Case 3: Monitoring Pump Motors
Figure 15. Cracked rotor bars causing inconsistent
output of electrical energy from the motor.

Three stations of a well-known pipeline
company in the Gulf of Mexico are using
remote monitoring to track the health of their
pumping stations that control the flow of their
product. The systems pump a combined
220,000 barrels per day using multiple pumps
driven by 1,000 to 1,500 HP electric motors.
Some of the stations do not have spares, as all
resources are in use. Under these conditions,

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors
any loss of pumping capacity would quickly
impair their ability to meet production goals.
The logistics problem of traveling several
hundred miles to each station to collect

vibration data is readily apparent; thus, a new
solution was needed. Each pumping station
was provided with a vibration data analyzer.
Prior to delivery of the units, the central office
programmed the station’s machines into
SKF’s vibration analysis software program
PRISM 4, and downloaded the information to
the data collector. Using a dedicated phone
line, the internal modem is connected to the
central office to transfer the data to New
Orleans. After the download is completed, the
data registers are cleared and the unit is ready
to be used again, either on the same machine
or any others that have been loaded. Any
parameters can easily be changed or added
from the central office through the modem
setup.
Using the various alarms available, the central
office engineer filtered out units that are
operating normally and focused on equipment
displaying abnormal conditions. With
advanced knowledge of pending problems, the
user scheduled downtime with operations and
prevented surprise shutdowns and failures.

The value of this type of program was
immediately apparent when a system analysis
was conducted and the spectrum in Figure 17
was collected.
This spectrum is the horizontal velocity

measurement taken on the inboard bearing of
the motor. The signal at 98,400 CPM is
interpreted as rotor bar pass frequency. The
orders indicate the 56 rotor bars in this motor.
Note the twice line frequency sidebands of
7200 CPM. The rotor bar pass frequency is
generated when there is damage in the stator
of the motor. This damage reduces the
strength of the magnetic field in that area, and
leaves a stronger field on the other side of the
stator, 180 degrees away. As each of the 56
rotor bars pass this stronger field, it is
mechanically pulled in that direction,
generating a frequency of 56 times the rotating
speed, in this case: 1746 RPM.
Although the amplitude is very low, this is an
abnormal condition. If the damaged area
continued to spread, the amplitude would
probably increase and could be trended to
provide advance knowledge that the problem
needs to be corrected.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 17. Horizontal Velocity spectrum of the inboard bearing of a pumping motor. The motor is running at 1746

rpm and indicates Rotor Bars Passing Frequency at 98,400 CPM with sidebands at 7200 CPM. This frequency
indicates damage in the stator of the motor. If left undetected, a damaged stator can cause the motor to fail.

Case 4: Quantified Motor Test

Specific Measurements

Background

The first step is to perform the necessary
calculation to determine the frequency at
which peaks in the spectrum will appear. First
calculate the pole pass frequency. Finally,
divide the pole pass frequency (Fp) amplitude
by the overall amplitude.

A major AC motor manufacturer that supplies
quiet motors for the U.S. Navy nuclear
submarine fleet, requested SKF assistance in
the detection of occlusions in cast rotor bars
on Navy motors. An occlusion, or bubble cast
into the metal, has a similar effect on the
motor current field as a cracked or broken
rotor bar.
It was determined that zooming in on a motor
current analysis spectrum and looking for
sidebands could not detect variations in the
rotor bar condition. Drilling a hole 25%
through one rotor bar of a 250 HP AC motor,
and comparing the FFT signatures of the

damaged and undamaged motors was
determined to be the most useful method of
testing.

Figures 18 and 19 show the results of
enveloped spectra of a healthy and damaged
motor. Note that the pole pass frequency is
clearly above the noise floor, and that there
are harmonics. The change in pole pass
frequency is attributed to the rotor damage. In
case of damage, the pole pass frequency
amplitude is 0.019, divided by the overall
amplitude of 0.053, which equals 37%.
Additional tests were conducted by drilling
deeper through the rotor bar. This resulted in a
percent increase to 59%.
.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 18.Undamaged Motor; Low Amplitude Levels.

Figure 19. Rotor bar drilled for 25%; the ratio pole pass frequency and overall amplitude is 37%.

DC Motors


The most common failure mode of a DC
motor is a breakdown in the winding
insulation. Some testing was done to
determine if enveloped armature current
would show any changes based on inflicted
damage to the windings.

Using a DC motor under NO LOAD
conditions resulted in the spectrum in Figure
20. Scraping off some insulation from
adjacent windings damaged the motor
windings. Note the differences in amplitude
with the spectrum of Figure 21. With a small
amount of damage, the spectrum and
amplitudes changed. Unfortunately, the
change in SCR frequency is not shown in the
spectrum.

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 20. DC Motor, No Damage.

Figure 21. DC Motor, No Load, Damaged Windings.


© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Case 5: Enveloped Current
Again, this example shows the use of motor
current analysis. In addition to the frequency
spectrum, the time waveform is shown for
comparison. First, the overall current

spectrum, with the line frequency and
sidebands, is shown in Figure 22. The signal is
then enveloped in small bands, showing use
the pole pass frequency in more detail,
(Figures 23-26).

Figure 22. Use of MCSA to diagnose rotor bar failure. The 50Hz (Europe) peak, line frequency, is the center
frequency on the spectrum. There are sidebands, or high spikes to the sides of the center frequency, which indicate a
problem in the motor. In this instance, they are spaced at 3.4 Hz from the center frequency, as shown in the markers
box in the upper right-hand corner of the figure.

Figure 23. Time Waveform of the current in Figure 21. The Time spacing indicates an event occurring every 0.0199
seconds. If the conversion from Time to Frequency is used; T = 1/F then the frequency is 50.3 Hz, corresponding to
the center frequency of MCSA spectrum, which is Line Frequency (FL).

© 2002 SKF Reliability Systems All Rights Reserved


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Vibration Monitoring and Current Analysis of AC Motors

Figure 24. Enveloped current reading using Filter 2, 50Hz-1000Hz, showing the pole pass frequency (Fp) at 3.4 Hz.
The pole pass frequency related to Figure 21, where it is spacing between the center frequency and the sidebands.

Figure 25. Time waveform of enveloped current. Spacing of peaks in time waveform are 0.2979 seconds. The
frequency relationship is 3.4 Hz (F = 1/T).

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Figure 26. Enveloped current reading using Filter 1, 5Hz-100Hz, showing the pole pass frequency (Fp) at 3.4 Hz.
The pole pass frequency related to Figure 21, the spacing between the center frequency and the sidebands.

Case 6: Motor Eccentricity
The following example shows the use of
motor current analysis in a motor eccentricity
(Figure 27). As explain earlier, motor
eccentricity is exhibited as 2x line

frequency. The current was measured at the
non-drive end of the motor. The necessary
action was better rotor alignment.


Figure 27. Motor eccentricity problem shows itself at 2x line frequency (100Hz (Europe))

© 2002 SKF Reliability Systems All Rights Reserved

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Vibration Monitoring and Current Analysis of AC Motors

Conclusion
The use of vibration monitoring and motor
current signature analysis can help determine
problems within an AC motor that can either
be repaired or scheduled for planned
maintenance downtime. Trending to track
changes in the overall level of vibration helps
determine when repairs need to take place.
Being aware of the types of peaks in the
spectrum that relate to the defect frequencies
found in a plant’s machinery is also a key
component needed to help determine the
electrical problems of the machine.

considered as a viable addition to any type of
maintenance assessment program. In the case
study examples, it was shown that the use of
both vibration analysis and MCSA were
necessary to detect defects in a motor. In most
cases, it is advantageous to use both types of

analysis methods. The key with MCSA is to
trend the readings over a given time period
and become familiar with analyzing the
collected data.

Many types of preventive maintenance
techniques should be used to help reveal the
root cause of the problem. Non-intrusive
technology, such as MCSA should be

The author would like to thank André
Smulders, Bob Jones, and Eytan Dor for
providing the various case examples in this
article.

Acknowledgements

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