A Summary of AC Induction Motor Monitoring

A Summary of AC Induction
Motor Monitoring
By Dr. Bob Jones

 Purpose
This paper summarizes the methods of
analyzing an AC motor’s electrical and
mechanical condition based on both
electrically induced mechanical vibration and
electrical signals detected with a clamp on
ammeter. It will also provide the Microlog user
with suggestions and hints on analyzing motor
problems. The author has applied these
techniques to motors from 5 to 700 HP.

Mechanical Vibration
Using a standard accelerometer placed on the
bearing cap, several unique mechanical
vibration signals will be generated by electrical
faults in the motor circuits. One of the more
common is a signal at twice line frequency. If
the line frequency is 60 Hz, this signal will be
at 120 Hz or 7200 CPM. If the line frequency
is 50 Hz, the signal will be seen at 100 Hz or
6000 CPM. Care must be taken when testing
two pole motors (3600 RPM or 3000 RPM) that
the signal is not twice rotating speed instead of
twice line frequency. Verify the frequency by
placing the cursor on the signal and on the
Microlog pressing the 1x RPM button or Set
Speed with the PRISM software.
This two times line frequency signal will be
created by any of the following faults either
singly or in combination:

An Uneven Air Gap
Between The Rotor and
Stator
As the poles of the motor pass the narrow gap,
the magnetic pull is greater versus 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 will result in a velocity spectrum signal
at 2x line frequency for any size or speed
motor.
The cause of this uneven air gap is often a
‘soft foot’ caused by an uneven base plate. 

 

As Figure 1. The SKF Condition Monitoring CMVA55
Microlog.
the motor is mounted to the base, the motor
housing and stator are distorted, resulting in
the uneven air gap between the stator and the
rotor. Some empirical data seems to indicate
that the twice line frequency signal will appear
when the gap clearance exceeds 10%
variance. Soft foot can be confirmed by
loosening and tightening one bolt at a time with
the motor running while observing the
spectrum on the Microlog. When the soft foot
is loosened, the velocity signal at 2x line
frequency will decrease, then increase as the
nut is tightened. At the next shutdown, this
foot should be shimmed to the same plane as
the others.

Damage to the Stator
Windings or Insulation
There are numerous causes to stator damage:
manufacturing, environment, or flaws in the
insulation. Any damage to the stator will again
create an uneven magnetic field around the
rotor. This uneven field will in turn generate an
uneven pull on the rotor regardless of the A Summary of
AC Induction
Motor
Monitoring
motor speed and cause a mechanical
vibration at twice line frequency.
It is often possible to locate the area of
damage with either a infrared or thermal
detector. Often there will be an area on
the motor housing where the surface
temperature will be 20–30 degrees
hotter.
A damaged stator will also generate a
mechanical vibration signal at a
frequency equal to the number of rotor
bars times the rotation speed. Again, in
the area of stator damage, the magnetic
field will be weakened and therefore
stronger 180 degrees away. As each
rotor bar passes this area of higher
strength, the bar will mechanically pulled
in that direction.
Typically induction motors will have
between 45-55 bars in the rotor but this
can vary greatly depending on the
manufacturer. For this reason, it is very
2

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.

Figure 3. Motor in lab with four cut rotor bars and broken end ring. Log 0.0908/8.777 x 20 = 39.7 dB.

 
important when troubleshooting a motor
vibration, to set the Fmax at least 100
times rotation speed. Please note this
Fmax is for TROUBLESHOOTING only.
Since the number of rotor bars can vary
greatly, it is most important to establish a
procedure that anytime a motor is down
for repair, the actual number of rotor bars
are counted and recorded for future
reference. It is also important to record
the full bearing model number 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 velocity
spectrum in the analyzer mode. The
moment the power is removed, the
distorted magnetic field is instantly
collapsed and the twice line signal will
disappear. If the signal does not
disappear but slowly degrades, then the
user knows there is some type of
mechanical problem. When setting up
the analyzer, use 100 lines, 0 averages
and an Fmax of 2000 Hz to provide a fast
cycle time.
If you are using a CMVA55, and there is
damage in the stator, then the signal will
also be seen in any enveloped
acceleration spectrums and will most
certainly generate harmonics of the
fundamental.
There is no agreed upon amplitude of
concern if the twice line frequency signal
is present in the velocity spectrum. It is
generally agreed that it is not desirable to
have any signal at 2x line frequency,
however it is often seen. Generally
accepted limits are between 0.04-0.06
IPS at 2x line frequency. One case
using enveloped acceleration, where the
2x line frequency was trended over six
months, showed an increase from 0.4
Env G’s to 1.6 Env G’s, when the motor 

Figure 4. Twice line frequency with harmonics using Env G’s.

Figure 5. Motor in lab with no damage. Log 0.003079/1.704 x 20 = 54.8 dB.
failed. However after the motor was
repaired, the amplitude started at 0.8
Env G’s and has remained fairly level to
the present.
The first occasion was most probably a
damaged stator with soft foot. After
repair the soft foot is still present, though
somewhat different because of a
different torque on the mounting bolts.

Sidebands
As in most vibration signals, the
presence of sidebands around
fundamental frequencies is a measure of
increasing severity as the sidebands
increase in number and amplitude.
Some of the sideband energy that may
be seen will be pole pass frequency,
(number of poles times slip) and slip,
(nominal speed minus actual speed). At
the rotor bar pass frequency (number of
rotor bars times actual motor speed) it is
possible to see sidebands of 2x line
frequency. In troubleshooting, the user
may find it necessary to increase the
resolution to either 1600 or 3200 lines of
resolution to be able separate these
sidebands. By starting at 400 lines and
zooming with the Microlog in the
analyzer mode, the existence of this
energy can be verified.
Analysis of AC Motor
Current
The technique of evaluating the motor
condition by performing an FFT of the
motor current has been verified many
times over the past 6 years. And,
although it is often referred to as a
method to detect broken rotor bars, the
fact is that it detects abnormal high
resistance in the rotor circuit. In other
words, bad solder joints, loose
connections and damaged rotor bars.
The users of the new CMVA55 will note
that all the mathematical functions are
performed automatically by the Motor
Current Monitoring Wizard™ which
quickly provides the user with the
information he needs. In all the cases,
the motor must be at 70-75% load.
For users of other Microlog models, the
following is a quick review of the
methodology. From either the route
mode or analyzer mode, the data is
collected using the point setup outlined in
the instruction manual or user notes. If
there is a fault in the rotor circuit, then
the spectrum will have two prominent
features when displayed with the ‘Y’ axis
as a logarithmic function. At 60 Hz, line
frequency, there will be a large spike. To
the left at a distance equal to the rotor
slip times the number of poles will be
another spike of energy. These spikes
can be labeled ‘A’ and ‘B’. Note that the
amplitudes will have to be obtained from
the software display because it is
necessary to use amplitudes to four
decimal places.

Figure 6. Enveloped AC motor current, slip frequency of 0.8125 generated by 5 broken rotor bars and a damaged
end ring. Ratio of pole pass frequency amplitude to overall amplitude is 63%.
frequency is seen, check connections, SCR’s,
control cards, and fuses.

Enveloped AC Motor
Current
When the motor current from a motor with a
damaged rotor circuit is enveloped, the
resulting spectrum will show energy at the
actual pole pass frequency. For example, at
0.8 Hz, not as a sideband of the 60.0 Hz signal
or 59.2 Hz. Initial research has shown there is
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
will be a very low amplitude signal and will not
be seen in an enveloped spectrum. So, the
frequency will have to be calculated to locate
it. Initial data has shown a good motor will
have a ratio of 5% or less but as damage
increases, this percentage will increase. See
the example with broken rotor bars (Figure 6).
Also harmonics of slip frequency are additional
indicators of damage. Initial testing has shown
this to be a very sensitive method and will
detect very early degradation in the rotor
circuit.
Technology Facilitates
Induction Motor Analysis
By utilizing velocity and enveloped
acceleration in conjunction with motor current
analyses, users can dramatically increase their
success in trending, analyzing, and evaluating
the condition of AC induction motors. Thanks
to data collectors like the Microlog CMVA55,
plant maintenance and reliability personnel can
easily and successfully detect electrical and
mechanical faults that lead to unexpected
downtime.
To determine the condition, perform the
following calculation:
Log (A/B) times (20) = amplitude in dB.
54-60 dB = Excellent
48-54 dB = Good
42-48 dB = Moderate
36-42 dB = Cracked rotor bars
or other source of
high resistance.
30-36 dB = Multiple sources of
high resistance.
< 30 dB = Severe damage
Note that this 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 and if these amplitude are used,
the results will not be accurate. The examples
in Figures 5 and 6 provide illustrations from
both good and bad motor circuits.
Observations of Other
Motor Problems
High efficiency induction motors obtain their
higher efficiency, and use less electricity, by
two methods- a smaller air gap and thinner
insulation on the windings. If the owner installs
these motors on the same transformer circuit
that has DC motors installed, it is possible for
the DC motor silicon control rectifiers (SCRs)
to back feed onto the AC circuit and induce
high voltage spikes into the motors. The
reduced insulation will rapidly deteriorate and
lead to a reduced motor life. Field results have
shown as much as a 50% reduction in the life
of the motor due to such an occurrence.
DC motor problems will be seen at the SCR
firing frequency, 6 times line frequency. If this