POWER July 2020 - 33

ELECTRICAL MAINTENANCE
the megger test results be at an acceptable
level, a dielectric withstand test
(also known as a high-potential or hipot
test) can be performed to ensure final
acceptance of ground wall insulation. If
the megohm reading does not increase
to acceptable levels, further cleaning,
inspection, and additional ground wall
testing may be required.
Potential Issues and Repairs
If the stator ground wall insulation fails
testing and there is no visual evidence of
a failed coil, the fault can be isolated by
systematically opening the connection
and retesting until the fault is located.
The technician will split the winding in
order to determine which of the winding
halves the failed coil is located within.
The investigator will then continue to
halve the coils on the failed side until the
failure is located.
At that point, a decision must be made
to either cancel the coil (Figure 3) or rewind
the stator. Coil canceling requires
the end turns of the failed coil to be cut
and bypassed. There is no firm rule to
determine the number of coils that can
be removed from the circuit, and engineering
judgment should be used to determine
the performance effects of coil
losses. Coil canceling is only a temporary
repair option until downtime and funding
can be allocated to a rewind.
The rotor circuit analysis should include
the same stator tests along with
a voltage drop test. This test is used to
evaluate the turn-to-turn insulation in
the rotating fields. To perform the test,
120 VAC is applied to the circuit and the
voltage drop is measured across each
field pole. The resulting readings should
all fall within +/-10% of the average.
Any reading outside of this range indicates
contamination or insulation failure,
resulting in turn-to-turn shorting.
The bad poles can then be removed for
repair or rewinding.
Collector Ring Assembly
Considerations
Monitoring brush wear and collector ring
condition is also critical to long-lasting
performance. The rings should be concentric
with the shaft to avoid lifting of
the brushes. When a brush lifts off the
collector rings an arc will occur and create
a pit in the surface of the ring. This
will cause premature wear of the brushes
and incur more material cost along
with more downtime cost.
Collector ring resurfacing can be performed
in a repair partner's machine shop
or onsite. The onsite repair of collector
rings can only be done if the location can
be made safe enough for a machinist to
work. This will require custom temporary
machine guarding to be fabricated and installed.
The rings should then be turned
concentric with the generator shaft and
stoned to an appropriate surface finish of
32 Ra (roughness average).
Spring tension must also be kept,
ensuring brush lifting does not happen.
Older styles of brush holders use arms
with a tensioner coil attached with adjustment
slots for increasing tension
as brush wear occurs. This requires operators
to adjust tension as brush life is
reduced. If not done at the correct frequency,
brush lifting can occur-resulting
in arc damage to collector rings.
Most brush manufacturers offer to
fabricate new brush holders with a
3. Coil canceling requires the end turns of
the failed coil to be cut and bypassed. Courtesy:
Jenkins Electric
constant force coil spring that eliminates
this issue. The new design allows
for the contact pressure between
the brush and the collector ring to remain
constant, even with an eccentric
ring. This allows for more consistent
predictable wear, and removes the
downtime of the operator adjusting the
spring tension. This new style of holder
can be designed as a direct replacement
with no modification required,
providing consistent predictable wear
that eliminates unnecessary stopping
for spring tension adjustment.
Repair and Testing Documentation
Any time work is performed on a generator,
the test data and work performed
should be documented for historical
purposes. Future repairs can be compared
against this data for trending. For
repair partners, having the data will give
them a guide of what to look for and
will expedite the evaluation process. If
they know what the symptoms and the
solutions in the past have been, it will
point them in the right direction to get
answers quickly, minimizing downtime
expenditures.
All the tests and observations can
and should be integrated into a preventative
maintenance program. Repair
partners
can
help
design and
2. The presence of dirt and oil can make it difficult to thoroughly inspect insulation. Courtesy:
Jenkins Electric
July 2020 | POWER
www.powermag.com
implement this program, and working
together with owners, can lower operating
and maintenance costs, and
keep critical equipment running reliably.
Planning for repair downtime and
periodic upgrades is made easier when
owners and repair partners have a cohesive
plan, historical data, and best
practices in mind, while working together
to keep t he lights on for millions
around the world. ■
-Ryan Ossmann is the repair operations
manager at Jenkins Electric. He is a
trained machinist with more than 22 years
of experience in the motor repair industry.
33
http://www.powermag.com

POWER July 2020

Table of Contents for the Digital Edition of POWER July 2020

Contents
POWER July 2020 - Cover1
POWER July 2020 - Cover2
POWER July 2020 - Contents
POWER July 2020 - 2
POWER July 2020 - 3
POWER July 2020 - 4
POWER July 2020 - 5
POWER July 2020 - 6
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POWER July 2020 - Cover3
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