Efficient Plant January 2018 - 43

feature | solution focus
Significant changes in test results between
the time this 'practically new' trolley motor
was first put into service and after six
months of operation raised red flags with
Gerdau's PdM team in Midlothian, TX.

WHAT THEY LEARNED

This is a standard approach at the site. It's
important to test new motors at the point of
installation. If a "new" unit is being installed
after having been stored for a significant
period of time, testing can confirm that it will
operate as promised. Testing at the point of installation might help to uncover problems that
can be resolved within the warranty period.
From a long-term standpoint, motor testing
will also establish baseline data that may come
in handy for future maintenance review and
decision making.
They continued the testing instead of immediately replacing the motor. They didn't
waste time trying to replace a motor that was
in perfectly good condition. They knew how
much time and how many resources would
be required to replace the motor, including
an overhead crane, multiple technicians, and
additional man-hours. By getting to the root
of the problem, they saved time and money.
They used a reliable testing instrument.
The AT5 motor tester has proven to be an
invaluable tool for motor diagnostics. Lightweight and portable, it provides fast, accurate
results. These features and capabilities allowed
Gerdau's PdM team to find and resolve the
real problem quickly, and then move on to
other projects. EP

Gerdau's PdM team did everything right:
They tested the new motor at installation.

For more information, visit alltestpro.com.

(MCA) was performed on the 20-hp trolley
motor. Within only a few minutes, the test
results showed a disconcerting insulation-to-ground readout of 10.1 megohm. This
"practically new" motor had been installed
onto the crane and tested in Oct. 2016. That
first test had shown a reading of greater than
999 megohm. This marked difference in test
results for a motor that had only been in
operation six months raised red flags for the
PdM team. The motor should not have deteriorated so quickly.

THE REAL PROBLEM
The PdM team scheduled a retest of the
trolley motor the following week. At that
time, however, they planned to connect the
AT5 directly to the motor. Within minutes of
performing the second test, the AT5 reported an insulation readout of greater than 999
megohm.
At this point, the team realized the issue
was not with the motor. It lay in the festoon
cabling from the motor-control cabinet up to
the trolley motor. The cables from the drive to
the festoon-trolley junction box were checked
for damage and then promptly replaced.

JANUARY 2018

+
WHAT IS MOTOR
CIRCUIT ANALYSIS?
A low voltage, non-destructive
AC signal is sent through the
motor windings to measure
the response to these signals.
The phase angle (Fi), current/
frequency response (I/F),
impedance (Z), resistance (R),
and inductance (L) of each
winding are measured and
analyzed. Winding faults are
indicated by variances in the
response to the applied signal
through the windings. These
variances cause imbalance in
the measured response to the
applied signal. Therefore, when
testing three-phase equipment
such as motors, generators, or
transformers, the response of
each phase is compared with
the other two. Phase resistance
is evaluated to determine
connection issues: inductance
is evaluated for possible
rotor problems; impedance
and inductance matching
are used to detect winding
contamination or over-heating;
insulation-to-ground tests
detect conductor-to-ground
issues.
EFFICIENTPLANTMAG.COM |

http://www.alltestpro.com http://www.EFFICIENTPLANTMAG.COM

Table of Contents for the Digital Edition of Efficient Plant January 2018