Karl Metcalfe, Technical Support at the Association of Electrical and Mechanical Trades (AEMT), outlines key steps that should be taken when installing and commissioning motors which have been in storage or removed for repair.
Careful installation of
motors will help ensure
their efficient and reliable
operation, minimising
the potential for costly downtime or
sub-standard operation. It is therefore
essential that particular care is taken
when installing and starting motors
that have been in storage or that have
been subject to an off-site repair.
STORED MOTORS
A motor which has been in storage
should first be cleaned to bring it back
to the condition it was in when placed
in storage. If there are any signs of
damage, such as broken cooling fins,
they should be investigated to assess
whether any internal damage could
have occurred and, where necessary,
the damage repaired.
In an ideal scenario, motors
should be stored in a vibration-free
environment; however, this is not
always practical. If the motor under
inspection may have been subject
to vibration, there is a potential for
the bearings to have been damaged.
Even minimal ambient vibration
will cause motor bearings to wear
over time and can result in fretting
or false brinelling, which can lead
to motor failure. Therefore, unless
there is complete certainty that the
stored motor was not subjected to
any vibration, the bearings should
be inspected for signs of wear and
replaced if necessary.
Bearing lubrication should also
be considered when taking a motor
from storage. Where the motor
uses sealed bearings, if it has been
in storage for an extended period
of around two years or more, it is
highly likely that the grease inside the
bearing will have separated and will
not work optimally. If the date when
the bearing was fitted is unknown, or
the motor has been in storage for a
long time, then it would be advisable to replace the bearing.
Motors which utilise re-greaseable
bearings should be regreased. And
if there are any signs of water in the
grease purged from the motor, there
will likely be rust damage, and the
bearings should be replaced.
All stored motors will be subject
to some degree of temperature
fluctuation, and this will cause
the seals to expand and contract,
allowing moisture to enter the motor.
Where the motor is oil-lubricated, it
is, therefore, good practice to change
the oil before starting it after a long
period of inactivity. To minimise the
likelihood that the motor’s windings
are contaminated by oil,
it is advisable to drain
the oil before it is moved
from its storage location
and then refill it after it
has been installed.
If good practice was
followed when the motor
was put into storage, an
insulation resistance (IR)
test, which measures
the total resistance between any
two points separated by electrical
insulation, should have been carried out. This test determines how
effective the insulation is in resisting
the flow of electrical current. Before
putting a motor back into service,
repeat this test to ensure that any
decreases in insulation effectiveness
that may have occurred during
storage can be addressed.
REPAIRED MOTORS
If an AEMT member has repaired
the motor, it is likely to have been
done following the international
repair, overhaul and reclamation of
rotating equipment standard, IEC
60034-23:2019, and it is a good idea
to check if this has been followed.
The standard establishes
the benchmarks for
repairing rotating
equipment, maintaining
efficiency levels, high
standards of quality
control and, where
possible, improving
efficiency in associated
pieces of equipment. As
such, bearing checks,
lubrication procedures and insulation
resistance testing will all have been
carried out to the highest standard. However, the following checks
should also be carried out on repaired
motors, as well as stored motors.
Before putting a motor into
service, manual rotation of the shaft
should be carried out if size allows. If
the shaft doesn't spin freely by hand
or doesn't quite sound as it should,
the bearings should be checked for
damage that may have occurred
during transport.
It is essential that the wiring of a
motor is carried out or checked by
a suitably qualified electrician, and
the equipment must be earthed in
accordance with current regulations.
It is also essential that the motor
is securely mounted and accurately
aligned in all three planes – allowing
for shaft end float and thermal
expansion.
Before the motor is fully put
into service, it should be briefly
started. If any vibrations or unusual
noises are experienced, the motor
should immediately be de-powered
to examine the causes. Vibration
or noise caused by magnetic or
electrical issues will typically improve as soon as the power is switched off.
If there is no change when the power
is disconnected, misalignment or
balance issues are more likely to be
the cause.
Assuming no issues
have been identified,
all safety measures
have been put in place,
and the parameters
relating to the motor’s
specification (such as
supply voltage and cable
sizes) have been met,
the motor can be put
into service. After around an hour of
running at normal load, the vibration
levels should be checked. If they
are excessive, the alignment or belt
tensioning should be re-checked. It
is worth noting, however, that there
may be a degree of bearing noise for
the first few hours of operation as the
grease settles, but this should soon
disappear.
Once the motor and load are
running as expected, the full load
voltage and current for all phases
should be recorded, along with the input power under load. If the motor
has the capability, the bearing and
winding temperatures should also be
recorded once they have stabilised.
This information will help
with future maintenance
and assessment of
the motor. Where the
application is critical, and
a predictive maintenance
programme is planned,
the full vibration
signature of the motor
and its load should be
recorded as a baseline.
CONCLUSION
If the correct procedures are
carried out when installing and
commissioning a stored or repaired
motor, it is far more likely to have an
efficient and reliable operational life.
Of course, a sound maintenance
programme is also required, but if
the starting position is strong, the
ongoing maintenance requirements
should also be reduced.