Maintenance Technology April 2017 - 29

PREDICTIVE STRATEGIES
Left. A 700-hp, high-voltage stator is being tested after undergoing rewind and the
vacuum-pressure-impregnation process.

affects the early turns. This startup
current causes the motor's windings
to flex or breathe and allows the
copper magnet wire to rub and abrade.
Because there are only about 1 1/2 mils
of insulation baked on the magnet
wire, over time it will deteriorate,
resulting in arcing during starting and
stopping. The appearance of arc is a
sign that the insulation is basically at
the end of its life. The wearing away of
that thin insulation film, in turn, is the
beginning of the end for many motors.
Once arcing has begun, it occurs
during every startup, and often during
shutdown. It may continue for weeks
or even months before it creates a
failure. Eventually, though, it will
create a carbon path and short out a
portion of the windings. These shorted
turns will then act as the secondary
side of a transformer with voltage and
current being induced by the rest of
the circuit.
Keep in mind that the ratio of good
turns to shorted turns will dictate
how severe and how quickly a failure
will occur. When shorted turns occur,
however, a motor will fail within
minutes. Thus, it's imperative to find
weak turn insulation before it becomes
a hard-welded fault. If not detected in
time, a few weak turns will carry an
exponential amount of current that is
induced by the transformer effect and
quickly burns through the slot-cell
liner to ground in the laminations.
The result is often a damaged core
with a large hole that will always
make the rewound motor less efficient
and run hotter.
Finding the weak turn insulation
and taking the motor out of service
before a short occurs provides two
valuable benefits:
■ Plant-floor personnel are in control
of the motor. They decide when a
unit is to be removed from service,
which minimizes or eliminates
unscheduled downtime, emergency
repairs, and lost production.
■ Since the motor in question still has
a good core, a reputable repair shop
APRIL 2017

can rewind it using better materials
and parts and tighter balance
specifications than when it was first
installed.
In practical terms, the site gets a
"new" motor back from the shop (not
a patched-up one).

The predictive route
A strong PdM strategy can allow
personnel to make realistic predictions
regarding the useful life expectancy of
their motors. The unfortunate fact is
that a motor begins (and continues)
to weaken and deteriorate from the
moment of its very first startup. If it
operates in high ambient temperatures,
with some misalignment and voltage

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MAINTENANCETECHNOLOGY.COM | 29


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Table of Contents for the Digital Edition of Maintenance Technology April 2017

Maintenance Technology April 2017 - 1
Maintenance Technology April 2017 - Cover1
Maintenance Technology April 2017 - Cover2
Maintenance Technology April 2017 - 1
Maintenance Technology April 2017 - 2
Maintenance Technology April 2017 - 3
Maintenance Technology April 2017 - 4
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Maintenance Technology April 2017 - Cover3
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