Lesson Learned on Electric Motor Shaft
A power station air compressor 750HP motor was sent to the
motor shop for a winding failure, refurbished and returned to the station.
Plant
personnel performed a precision alignment of the motor to the compressor,
yet high vibration was found on initial startup.
Alignment tolerances were reviewed, soft foot conditions
were minimized and motor mounting and support adjustments were made to
ensure no resonant conditions
existed. However, high vibration levels were still found on the motor.
Further investigation of the uncoupled motor vibration and phase identified
a bent motor shaft.
Alignment readings were all within tolerance for the 3600
RPM machine. Soft foot problems were minimal and were removed with step
shimming. Due
to the extremely high vibration readings in the order of 0.8 in/sec velocity
which was predominantly in the horizontal direction, resonance was
suspected.
An investigation of the motor mounting structure revealed
a stiff, good supporting structure. Bump tests on the motor found the natural
frequency well
above and out of range of influence by the motor operating speed. Resonance
was ruled out, so the unit was started and phase data was taken.
Horizontal readings found similar amplitudes of 0.7 to 0.8
in/sec at 180° out-of-phase
between motor bearings. Much lower vertical readings were found on the
outboard (opposite drive end) motor bearing of 0.02 in/sec and a high of
0.26 in/sec at
the inboard (drive end) bearing at 180° out-of-phase. Axial readings
on the motor from one end to the other showed acceptable vibration levels,
but 180° out-of-phase.
Phase readings across the coupling showed mostly in-phase readings with
low, acceptable vibration readings on the compressor.
Misalignment was suspected,
but to rule out motor rotor imbalance and a possible bent motor rotor,
an uncoupled motor run was requested. Vibration levels were low vertically,
but still somewhat
elevated horizontally for an uncoupled motor, in the order of 0.1 in/sec
at running speed. Phase readings were mostly all in-phase except for axial
end-to-end comparison.
The difference in horizontal to vertical readings along with the axial
out-of-phase suggested a possible bent rotor. Dial indicator readings did
find the motor stub
shaft bent by 0.009 inch at the coupling hub mounting location tapering
back to less than 0.002 inch run-out near the shoulder before the inboard
bearing. The effects of the bent shaft were greatly exaggerated in the
coupled condition
resulting
in the coupled amplitude and phase readings mentioned previously.
The motor repair
shop stated that they had balanced the rotor to "as low as we could
get it," however, we had not provided any balance criteria, nor had
we requested any vibration
testing at the motor shop prior to shipment. With the location of
the bent shaft, it is likely that the motor shop would have never
seen the effects of the bent
shaft with the rotor on a balance machine. Shaft run-out tolerances
were never included as part of the repair specifications by our company
and none of the
repair documentation was provided by the motor shop.
Had
we requested shaft run-out checks during repair and provided acceptance
criteria that included a final vibration reading and analysis before the
motor was shipped,
it is likely we may have caught this problem at the motor shop.
Now, we have no idea if the shaft was bent during initial disassembly,
during shipment to
the motor shop, while at the motor shop or damaged upon return
shipment. We had no case for liability against the motor shop, so the cost
of the repairs fell
on us.
In the future, we plan to specify balance acceptance criteria,
disassembly "As-Found" and "As-Left" criteria
and request review of vibration signatures on repaired motors
prior to shipment. We are also developing job plans for alignment work
to include checking shaft
and coupling hub run-outs as part of the initial steps to performing
alignment.
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