Maintworld Magazine 4/2023

ELECTRIC MOTOR STORAGE
If the windings need to be cleaned and dried, measure the
IR again. If it is greater than 5000 megohms, disregard the PI
(see IEEE 43); otherwise, recalculate the PI.
CARBON BRUSHES
DC machines, wound-rotor motors and some synchronous
machines have carbon brushes. For long-term storage, lift the
brushes away from the commutator/slip rings to prevent a
chemical reaction (sometimes called “photographing”) that
can discolor the underlying commutator or slip ring. When
practical, store the springs in the relaxed state to prevent a
gradual loss of spring pressure.
Putting the motor into service
To ensure proper operation when removing a motor from
storage and putting it into service, perform the following:
• Use compressed air to clean the outside of the motor, and
visually inspect it.
• Assess the condition of the insulation system by measuring
the IR with a megohmmeter.
• Oil-lubricated motors:
• Drain the oil before moving the motor to the
installation site.
• If there is water in the oil, check for and replace any
rusty bearings.
• If sleeve bearings received a protective coating,
disassemble the machine and clean the bearings with
an appropriate solvent before putting the motor into
service.
• Fill the oil reservoir to the correct running level after
installing the motor.
• Grease-lubricated motors:
• Moisture in the grease usually indicates rust-damaged
bearings that need replacement.
• After several years in storage, the grease probably will
be hard and the drainpipe will be plugged; usually it is
best to disassemble the motor, remove the old grease
and repack with fresh, compatible lubricant.
• Run the motor 10-20 minutes without the drain plug to
purge excess grease.
• Vibration and alignment:
• If the storage area has ambient vibration, inspect and
replace damaged bearings before installing the motor.
• After installing and aligning the motor, document the
uncoupled baseline vibration levels; check the levels
again after a week or two of service.
• For motors with rolling element bearings, check for
bearing fault frequencies in the vibration spectra.
• On large machines that are susceptible to shaft sag,
monitor the vibration levels during startup to avoid
catastrophic damage.
High-cost machines obviously justify more precautions than
inexpensive, readily available motors. What is not always apparent
is that some “smaller” motors are equally important to
production and can have enormous consequences if they fail.
TIPS FOR TRACKING IR TEST RESULTS
Attach a card to each motor and record the IR,
temperature and date of each test.
TIPS FOR ROTATING THE SHAFT
Rotating the shaft keyway position in 150-degree
increments every month makes it easy to spot a
neglected motor. If you visualize a clock face, each
increment represents 5 hours: For example, if the
keyway position for September is 12:00, October
will be 5:00, November will be 10:00, and so on.
This puts the rolling elements in a different position
each time and avoids rocking the rotor back and
forth between just two positions (see Figure 1).
TIPS FOR OIL-LUBRICATED MOTORS
Never move a motor with oil in the reservoir. If oil
sloshes over the stand tube, it could contaminate
the windings or even initiate capillary action that
can siphon oil from the chamber. Before putting
the motor into service, always drain the oil and
replace it with compatible lubricant. (Drain it. Move
it. Refill it.)
REFERENCES
IEEE Std. 43-2013: Recommended Practice for Testing Insulation Resistance of Electric Machinery. Institute of Electrical and Electronics Engineers, Inc.
New York, NY, 2013.
4/2023 maintworld 47