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| Figure 1- Gasification process rod mill, 800 HP motor, clutch and gearbox. | Figure 2- Rod mill pinion shaft failure. |
Eastman
Chemical Company started its Motor Analysis Program in late 1998 when we
purchased an online power analyzer. After several months of testing, we found
the analyzer to be limited for rotor/stator testing at our plant, due to the
low-level loads on many of our motors. The power quality testing was also
limited at our facility because we produce most of our own utilities and our
power quality is very good.
Therefore,
in 1999 we started pursuing other options for condition monitoring of motors. We
set up three motors with controlled conditions: shorted wires in turn, offset
rotor 0.01”, shorted stator core steel and an open rotor bar. A vendor
representative was brought in to test these motors offline. All conditions were
correctly identified and a decision was made to purchase the vendor’s
analyzer. We have been using the
analyzer for approximately two years now and are very pleased with the results.
Although most of the analyzers are advertised as rotor bar analysis tools, this
article discusses some of the areas into which we have expanded the use of this
technology at our plant.
Rod
mills are used to crush the coal used in our gasification process. The pinion
gear transmits motor torque through the gear reducer to the rod mill’s gear
reducer (Figure 1). The motor is
connected to the gearbox through a clutch, which uses an inert gas to engage the
clutch disk. The clutch in turn is
coupled to the gearbox, which in turn is coupled to the rod mill. The pinion’s design life is five years minimum. Between
1997 and 1998, five pinions failed (Figure 2) between the two rod mills in our
gasification process.
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| Figure 1- Gasification process rod mill, 800 HP motor, clutch and gearbox. | Figure 2- Rod mill pinion shaft failure. |
A
Root Cause Failure Analysis Team was formed to determine the root cause(s) of
the pinion shaft failures. Several
problems were identified that could be attributed to shorter than design pinion
life. Some of these were:
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Too high of a rod charge level.
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Improper pressure switches that
allowed the mill to be started with inadequate oil pressure.
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An inadequate startup procedure.
The original procedure called for the mill to come to a complete stop before
restarting it. The revised procedure did not include this statement. Operators
had been trained using the revised procedure.
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The inert gas pressure in the
clutch was too high.
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Improper clutch engagement time.
The clutch manufacturer said the clutch should smoke 5-7 seconds to fully
engage. Faster times will overload
the pinion shaft and motor. The clutch is designed to be the wear part of the
system, not the pinion shaft.
During
the root cause investigation, one of the verification steps was to determine the
clutch engagement time. The Motor Analysis Team (MAT) proposed the idea of
monitoring the current on the motor to attempt to determine the clutch
engagement time. The MAT used the current analysis in-rush test function on the
analyzer to capture the motor current data. From this data, we were able to
calculate the engagement time for the clutch. The initial test indicated a very
rapid clutch engagement time, approximately 2.25 seconds, similar to an across
the line start for the motor and clutch (Figure 3).
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Figure 3- In-rush test results on initial clutch engagement test. |
Using
the analyzer to determine the clutch engagement time, while working with
operations, maintenance and plant support engineering; the clutch engagement
time was adjusted back to manufacturer’s specification of between 5 to 7
seconds (Figure 4). The rod mills
have not experienced a shaft breakage now for approximately 2½ years. The
in-rush current test is now used as an annual proactive maintenance procedure to
verify the clutch engagement time is within specification.
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Figure 4- In-rush test results on final clutch engagement test. |
Grinder Motor Failures
We
currently have six grinders in our B-255 Polymer process plant being driven by
50 HP, 480 Volt, 1800-rpm motor. The MAT was requested to evaluate the subject
motors because four of these motors were blowing 100 amp fuses at an excessive
rate of approximately once per 12-hour shift (Figure 5). The CSM’s were
replacing all three 100 amp fuses at the motor starter when one would blow. This
used more fuses, but resulted in fewer failures and less production losses.
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Figure 5- Failed 100 amp fuse graveyard for grinder motors. |
The
MAT evaluated the six motors using the in-rush current analysis test. The
results indicated that four of the motors had a very high startup acceleration
rate compared to the other two motors. The instantaneous amps were 20% greater
and the acceleration time was approximately 2.5 times longer on these four
motors when compared to the other two motors (Figures 6 & 7). When comparing
time versus current for these fuses, the MAT reps found that the motor was
operating at or near the failure point of the fuse during startup. Upon closer
investigation of the motors, it was determined that a NEMA A Design motor was
being used in the four locations that were blowing fuses excessively.
As a result, the MAT recommended replacing the NEMA A motors with a NEMA
C Design motor. However, the motor was replaced with a TECO NEMA B Design, which
has similar starting torque characteristics to a NEMA C Design motor.
The replacement motor was checked on startup and no problems or concerns
were noted.
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| Figure 6- In-rust test results for NEMA A design motor. | Figure 7- In-rush test results for NEMA C design motor. |
VFD Driven Agitator Motor
The
MAT was requested to check horsepower
loading on three crystallizer agitator
drive motors powered by Variable
Frequency Drives (VFD) because operations had
higher than manufacturer specified loading on the motors. Power Analysis Testing
was performed on both the input and output of the inverter (Figure 8). The total
KW on the input side corresponded with the KW displayed on the DCS in the
control room. The total KW on the output side matched the manufacturer’s
estimate for horsepower load in this application.
During
analysis of the test results, we determined that the instrumentation supplying
the DCS was connected to the input side of the VFD creating higher than expected
readings. This can be seen in Figure 9 where the input and output load readings
are compared. When the voltage waveforms of the VFD output were analyzed, we
also discovered one of the VFD output drivers had a distorted waveform (Figure
9). This was due to a failing
output driver on one phase of the VFD. This distortion was not severe enough
to trigger the self-diagnostics of the VFD.
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| Figure 8- Test results on crystallizer motor, test 1-input side, test 2-output side. | Figure 9- Voltage waveforms on output side of VFD feeding crystallizer motor. |
Quality Assurance Testing
The
MAT team has a quality assurance program in place for repaired motors. An
off-line test is performed on all motors over 100 HP to verify the repairs and
to establish a new base line. Quality Assurance testing was recently performed
on a 125HP, 444T frame, 1800-rpm motor. During testing, a 5.1% resistive
unbalance was noted. The motor was
retested using a bridge type ohmmeter to confirm if errors were made during
testing. The off-line results compared very favorably with the bridge results.
The repair data sheet was reviewed and the motor shop had documented the 0%
resistive imbalance. The resistance readings were identical across all phases.
It
was also noted that the readings were off by a factor of 8 when compared to the
readings we had made. The MAT requested that the motor repair shop personnel
come to our facility to verify their resistance readings. Their measurements
with their bridge ohmmeter found a 7% resistive imbalance.
The resistance imbalance results were discussed in a meeting
with the repair shop
, but they did not feel the imbalance was a problem because
the motor had passed the surge comparison and load tests during the repair
process. However, they agreed to perform additional testing for mutual learning.
The motor was torn down and 25% of the nameplate voltage and full nameplate
current were applied to the stator. An
infrared image of the stator was taken after ten to fifteen minutes. As can be
seen in Figure 10, two coils of the stator were approximately 25o
F hotter than the rest of the stator coils. Due to these findings, resistive
imbalance limits have now been added to Eastman Chemical Company’s Motor
Repair Specification.
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Figure 10-Infrared image of 125HP motor with 5.1% resistive imbalanc |
Conclusion
Although
we have found the advertised benefits of the motor analyzers to be very
rewarding, we must be willing to step outside the box to find additional
opportunities to reap the many benefits that can be realized.
Eastman Chemical Company Motor Analysis Team, Eastman Chemical Company, P.O. Box
511, Kingsport, TN 37662, Attn: Motor Analysis Team, B-409
About
the Authors:
Tom
Whittemore, Jr., P.E. is a Senior Mechanical Engineer in the Rotating Equipment
Group at Eastman Chemical Company’s Tennessee Operations. His job functions
include rotating equipment specification, analysis, and troubleshooting in
addition to serving as leader of the Motor Analysis Team.
Danny
Hawkins is a MCA Specialist in the Rotating Equipment Group at Eastman Chemical
Company’s Tennessee Operations. His job functions include online and offline
analysis and troubleshooting of motors. Prior to joining the motor analysis
team, he repaired motors for 24 years in Eastman Chemical Company’s Motor
Repair Shop.
Paul
Aesque is a MCA Specialist in the Rotating Equipment Group at Eastman Chemical
Company’s Tennessee Operations. His job functions include online and offline
analysis and troubleshooting of motors. Prior to joining the motor analysis
team, he repaired motors for 5 years in Eastman Chemical Company’s Motor
Repair Shop.
