POWER May 2011 - 22

phase insulation deterioration as well as a
reversal or open circuit in the connection
of one or more coils or coil groups. This
test has been used for years as a qualitycontrol
measure in motor manufacturing
plants, and development of portable test
instruments now allows this testing to be
conducted as part of troubleshooting a
problem and as a routine PdM tool.
The surge comparison test can also
identify excessive wear by applying a
transient surge at high frequency to two
separate but equal parts of a winding.
The resulting voltage waveforms reflected
from each part are displayed on an oscilloscope
or computer screen. If the windings
are identical, the two waveforms will
be exactly superimposed on each other
and a single trace will appear on the
screen. If, however, one of the two winding
segments contains a short circuit, or
a reversed or open coil, the waveforms
will be visibly different.
Once a problem is detected, the technician
must determine in which of the
segments the problem resides. This can
be done by comparing each segment to
a third segment and noting which combination
produces the same waveform
deflections. Generally, shorted or missing
turns will cause fairly small differences
in waveform amplitude. Misconnections,
such as coil reversal or interphase shorts,
tend to cause large differences or irregularities
in waveform shape. An experienced
test operator can gauge the type
and severity of a fault.
With the surge comparison method it
is often possible to determine the voltage
at which turn-to-turn or phase-to-phase
conduction begins. If this shorting is near
operating voltage, then the motor has a
serious insulation fault and should be replaced
as soon as possible. If shorting is
not detected up to twice operating voltage
plus 1,000 V, the winding is considered
good and the motor can be returned
to service.
Unlike other PdM techniques, surge
comparison produces no numbers that
must be plotted or trended to identify a
problem. Like a high-potential test, this is
simply a pass/fail procedure. Spot checking
(monitoring once a year or less) can
be particularly cost-effective for critical
or expensive electrical motors; however,
it may also be effective for less-critical
or balance-of-plant equipment. Because
long-term surge comparison analysis and
trending can be used to identify improper
motor repair practices, either in-house or
at a repair shop, surge testing can be an
effective quality-assurance test prior to
acceptance of motors that have just been
repaired.
Surge comparison testing is a somewhat
complex and expensive PdM technique.
The test instrument, though very versatile,
is moderately expensive, and it requires a
trained and experienced operator if you
want to obtain the most effective results.
Although often used on stator windings of
either induction or synchronous machines,
the test is equally useful on DC armatures
or synchronous field poles.
Most surge comparison test equipment
is also capable of performing high-potential
tests. The primary short-term economic
benefits from this testing do not come
from reducing the number of electric motor
failures but from identifying problems
early enough that maintenance can be
efficiently planned and scheduled. These
benefits can be substantial. Some companies
pay for the investment in less than
a year by identifying one or two critical
electric motor problems.
Surge comparison testing cannot evaluate
one coil by itself; because it is a comparison
test, it requires careful repetition
to determine the location and severity of
an observed fault.
Motor-Current Signature Analysis
Motor-current signature analysis (MCSA)
is a nonintrusive method for detecting
mechanical and electrical problems in
motor-driven rotating equipment. The system
was developed by Oak Ridge National
Laboratory as part of a study on the effects
of aging and service degradation of
nuclear plant components.
The basis for MCSA is the recognition
that an electric motor driving a mechanical
load acts as an efficient, continuously
available transducer (the motor can be either
AC or DC). The motor senses mechanical-load
variations and converts them into
electric current variations that are transmitted
along the motor power cables.
These current variations, though very
small in relation to the average current
drawn by the motor, can be monitored and
recorded at a convenient location away
from the operating equipment. Analysis
of these variations can provide an indication
of machine condition, which may
be trended over time to provide an early
warning of machine deterioration or process
alteration.
Although MCSA was developed for the
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