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QUALITY ASSURANCE TESTING
The MAT team has a quality assurance
program in place for repaired motors. A PdMA MCE 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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