Maintenance Technology April 2017 - 30

PREDICTIVE STRATEGIES

imbalance, and experiences numerous
starts under a heavy load, its life will be
short. Given these conditions, a motor
that should last, say 20 years, will
probably fail in two or three. While
correcting some of those issues could
prolong the life of the unit, keeping
tabs on the health of its insulation
could provide greater payback.
Preventive maintenance (PM)
efforts are clearly important when
it comes to a plant's motor fleet. For
example, in facilities where contamination is an issue, PM routines to reduce
its effect on motor life are a must.
Whenever possible, however, a PdM
strategy that leverages as many proven
predictive tools as possible should
replace preventive activities. After
all, to develop a complete picture of a
patient's health, a physician will typically perform a battery of tests. Your
site's motors deserve similar treatment.
Fortunately, state-of-the-art
equipment and methodologies are
available to identify early issues before
they lead to catastrophic failure(s).
Routine testing and trending will
detect weaknesses long before they can
propagate into an insulation failure. To
design this type of PdM program, site
personnel need to identify the motors
that are most critical to the operation and, in turn, those that are the
most problematic. This information
will indicate which tests need to be
performed and how often.
Many independent testing
organizations have detailed specific
test parameters, proven to provide
sufficient data for the technician to
evaluate the immediate health of the
insulation. To ensure the capture of
accurate data, it's important that those
guidelines be strictly followed. For
more information on testing parameters and requirements, refer to IEEE
(Institute of Electrical and Electronic
Engineers, ieee.org), IEC (International Electrotechnical Commission,
iec.ch), EPRI (Electric Power Research
Institute, epri.com), and EASA
(Electrical Apparatus Service Association, easa.com).
Low-voltage testing, comprised

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MAINTENANCE TECHNOLOGY

of capacitance, inductance, and
resistance tests, provides some useful
information, but will not provide early
warnings regarding turn insulation. A
complete set of tests that will provide
the predictive information you need
include:

Predictive
Motor-Testing
Tools
These tools will tell you whether your
electric motors need maintenance:
■ Winding-resistance test: Verifies
all three phases are similar; all
internal connections are tight.
■ Megohm test: Measures insulation resistance.
■ High-potential test: Reveals
aging and physically damaged
insulation by applying a stress
to cracks.
■ Surge test: Locates weak turn
insulation before it has a chance
to become a hard-welded fault.

■ Winding-resistance test: This test
will verify that all three phases are
similar and all internal connections are tight. It uses a Kelvin
bridge and injects about 12 VDC
at approximately 7 A into the
windings. One poor connection
will lead to unbalanced current and
uneven heating.
■ Megohm test: After passing the
winding-resistance test, a megohm
test is run to measure insulation
resistance. The test uses a lowcurrent DC voltage that depends on
the nameplate voltage of the motor.
A polarization index test (PI), which
is an extended megohm test, may
provide important information
about the insulation if it is old,
cracked, or brittle.
■ High-potential test: If the windingresistance and megohm tests
are acceptable, a high-potential
(HiPot), or step-voltage, test may be
performed. This test uses increased

voltage to create electrical stresses
on internal insulation cracks. This
can reveal aging or physically
damaged insulation. The HiPot test
is usually conducted at an elevated
level that is at least twice the line
voltage plus 1,000 V. EASA and
other testing organizations recommend even higher test-voltage
levels. The HiPot test may not be
appropriate for in-service motors
that display low megohms during
the megohm test.
■ Surge test: Once the above tests
are satisfactorily completed, a surge
test is performed. Since more than
80% of all winding failures begin as
a turn-to-turn weakness that can
only be detected by a surge test, it
is the most important static-testing
method. Surge testing applies
pulses through a large capacitor at
ever-increasing voltage levels and
monitors the reaction as the voltage
is discharged into a motor's windings. The purpose is to re-create the
spiking that occurs at startup. Doing
so requires the capacitor to "fire off "
each pulse at a very fast rise time.
The intention is to locate weak turn
insulation before it has a chance to
become a hard-welded fault that
leads to a quick failure.

Value proposition
Adding electrical testing of motors to
your site's PdM toolkit puts personnel
in the reliability driver's seat with
regard to these units. The value
proposition is clear: Routine testing
and trending provides sufficient data to
make a diagnosis regarding the ability
of a motor to remain in operation or to
determine if it needs attention. Being
in control, i.e., being able to specify
when a motor is pulled from service
and sent out for repairs, is the essence
of predictive maintenance-and an
enormous benefit. MT
Information in this article was supplied
by Timothy M. Thomas, senior electrical
engineer, Hibbs Electromechanical Inc.,
Madisonville, KY (hibbsinc.com). Email
him at TThomas@HibbsInc.com.
APRIL 2017
http://www.ieee.org http://www.iec.ch http://www.epri.com http://www.easa.com http://www.hibbsinc.com
Table of Contents for the Digital Edition of Maintenance Technology April 2017
