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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 PdMA 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).
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Figure 6:
In-rush test results for NEMA A design motor |
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Figure 7:
In-rush test results for NEMA C design motor |
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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