Maintenance Technology May 2017 - 34

MOTORS

Understand
Motor/System
Baselines
WANT TO GET the most from your electric motors? Think of
St. Louis-based EASA (Electrical Apparatus Service Association,
easa.com) as a treasure trove of practical information and its
members as a "go to" source for help with specific applications.
Consider this insight on motor/system baselines.
- Jane Alexander, Managing Editor
According to EASA's technical experts,
changes in motor/system vibration readings provide the best early
warning of developing problems in a
motor or system component. Other
parameters to monitor may include
operating temperature of critical components, mechanical tolerances, and
overall system performance, including
outputs such as flow rate, tonnage, and
volume.
Motor-specific baselines incorporate records of electrical, mechanical,
and vibration tests performed when
units are placed in operation or before
they're put in storage. Ideally, baselines would be obtained for all new,
repaired, and in situ motors, but this
may not be practical for some applications. These baselines typically include
some or all of the following:
Load current, speed, and
terminal voltage
Changes in these parameters usually
indicate that a vital system component is
damaged or about to fail. Other electrical
tests may include insulation resistance, leadto-lead resistance at a known temperature,
no-load current, no-load voltage, and
starting characteristics.
QUICK TIP: Some changes in the current
and speed may be normal, depending
on the type of load.

Motor current signature
analysis (MCSA)
This test diagnoses squirrel cage rotor
problems, e.g., broken bars or an uneven
air gap. It's more accurate if a baseline is
established early in the motor's life.
Mechanical tests
These normally consist of measuring shaft
runout (TIR) and checking for a soft foot.
Vibration
Although overall vibration readings can
be used as baseline data, Fast Fourier
Transform (FFT) spectra in all three planes
at each bearing housing are preferred (see
"Vibration Analysis" on page 22). Shaft
proximity probes can be used to determine
sleeve bearing motor baselines.
Infrared thermography
This tool can detect changes in the operating
temperature of critical motor components,
especially bearings.
New-motor baselines
Comparing factory terminal winding
resistance and no-load amps with data taken
under load can be useful when monitoring
the condition of a new motor or troubleshooting system problems. Factory baselines
are often available from the manufacturer
or its website. The accuracy of factory data
depends on how it was obtained, but it's
usually sufficient for field use.
Baseline data for a newly installed motor

could reveal an error, e.g., misconnection
for an incorrect voltage, and prevent a
premature motor failure. Rather than simply
"bumping" a motor for rotation before coupling it to the load, operate it long enough to
measure the line current for all three phases,
as well as the voltage and vibration levels.
QUICK TIP: Comparing the baselines
of a failed motor and its replacement
could reveal application- or processrelated weaknesses in the system.

Repaired motor baselines.
Service centers usually provide no-load and/
or full-load (when stipulated) test data for
repaired motors, including voltage, current,
and vibration spectra. Comparing these
results with historical baselines and those
obtained on site when the motor is returned
to service may confirm the quality of the
repair or possibly reveal underlying system
problems. For example, increased vibration
levels in on-site tests might indicate a
deteriorating motor base or a problem
with the driven equipment rather than a
balancing issue with the motor.
With newly repaired motors that have
been in operation for many years, baseline
comparisons are invaluable in root-cause
failure analysis and may even expose
consequential damage from certain kinds
of failures, e.g., a broken shaft. To correctly
identify cause and effect and prevent recurrences, always investigate equipment failure
at the system level. MT

For details on using motor/system baselines, as well as expert advice on a wide range of other motor-related issues, download
Getting the Most from Your Electric Motors at easa.com/resources/booklet/getting-most-your-electric-motors, or contact a local
EASA service center (see easa.com/find).

34 |

MAINTENANCE TECHNOLOGY

MAY 2017


http://www.easa.com http://www.easa.com/resources/booklet/getting-most-your-electric-motors http://www.easa.com/find

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