Page 10 - North American Clean Energy November December 2015

Proper Maintenance and Upkeep in DC Motor Units

.continued from page 8.

When inspection reveals unacceptable commutator conditions, immediate maintenance as a “seater stone”. he undercut area between individual commutator bars can be cleaned
should be undertaken to preserve the commutator’s service life. hese unacceptable through a process called air curing. his involves clean, pressurized air being blown onto

conditions include: bar burning, slot bar burning, pitch bar burning, threading,
the commutator as it spins. If dirt is not freed through this process, it may be necessary
grooving, and copper drag. All of these lead to severe damage of the materials within the to mechanically dislodge the particulates and then perform a second round of air curing.
commutator itself. While minimal damage to commutators is expected over a machine’s Precautions should be taken during any cleaning processes to ensure operator safety.
Black commutations are common in DC motor units. Nondestructive sparking is
long service life, regular inspection for these dangerous conditions will assist in avoiding
catastrophic failures and improving overall motor unit service life. he commutator is the tolerable and pinpoint sparking not causing brush or commutator deterioration is also
most expensive component of a DC motor unit to replace, so these parts should be the acceptable. Arcing and destructive sparking, however, can electrically erode both the brush
main priority of inspection and maintenance work.
and commutator surfaces. If not corrected, this eventually leads to equipment failure.

Arcing and destructive sparking can be identiied by unsatisfactory commutator conditions
Minimizing grounding, arcing, and destructive sparking
in which burning occurs, such as slot bar or pitch bar burning.
After commutator condition, the next most important priority for DC motor upkeep is Slot bar burning, occurs when sparking levels become high enough to remove metal
minimizing grounding, arcing, and destructive sparking by keeping motors clean and from the trailing edges of the commutator bars, causing them to appear burned or etched

clear of the loose carbon dust created as brushes wear. Some of this dust goes into the when the brush ilm is removed. Worsening conditions may cause burning to become
formation of the ilm on the commutator, however, additional dust may become loose and evident, even without ilm removal. Destructive sparking resulting in this condition may be
settle within the motor. his, combined with other particulates from the air, can cause caused by the use of a carbon brush grade with insuicient commutating ability, improper
grounding to occur. When this happens, motors can experience catastrophic failure. In electrical adjustment of the motor unit, or exceeding load design limits. Improving the

addition to the danger from undesired grounding, particulates can also make it diicult conditions causing arcing and destructive sparking will help ensure motor failure and
for motor parts to move freely. Particulate buildup can inhibit the normal dissipation of commutator damage is avoided.
heat through the outside surfaces of the motor unit frame causing the motor operate at a
higher than ideal temperature.
Selecting the appropriate brush grade

To avoid issues caused by dirty motor units, motor operators should perform
he third priority when trying to prolong service life is monitoring carbon brush condition.
regular cleaning. he frequency of these cleans can be determined based on inspection While carbon brushes are the least expensive component of a DC motor unit, they must
observations, since conditions will vary between installations, but performing quarterly be replaced most frequently. Depending on the motor unit and service conditions, carbon
light-duty and annual heavy-duty cleans is good practice.
brushes are likely to last from three months to three years. Brush life is less important

When cleaning, operators should be careful not to worsen conditions by driving than commutator life and brush grades are chosen to suit the motors’ speciic application
particulates deeper into motor units. Proper cleaning consists of vacuuming dry with this in mind.
particulates with a soft bristle brush attachment to loosen large accumulations. Oily dirt
should be removed with a cloth dampened with solvent. Soaking or looding a motor
A wide range of brush grades are available. Each grade contains a diferent material

unit with solvent can draw conductive particulates deeper into the unit through cracks
composition and impregnation designed to optimize service life of the brushes and
in insulation, causing more damage. Individual motor parts such as brush holders and commutators in a given service condition. While the brushes are primarily made from
springs can be solvent cleaned outside of the motor safely, but parts like commutators and graphite, some grades include copper or other metals as abrasive or cleaning materials
varnish-insulated coils cannot be.
for conditions where excessive brush ilm buildup is likely. Similarly, impregnations, or

Carbon ilms may be removed from commutators to avoid excessive buildup or inspect chemical compounds which are introduced into the graphite during production, help to
commutator condition under the ilm using a medium, soft, white abrasive stone known
optimize brush ilm and therefore protect motor unit components.
Monitoring environmental conditions
such as humidity and temperature,

as well as consulting with an expert
can help DC motor unit operators
to identify the ideal brush grade for A highly secure safety system

each motor installation. his leads to
extended brush life and service life for
commutators and brush holders.
With proper inspection and

maintenance practices, commutator
life expectancy ranges from 10 to more
than 20 years. hese upkeep practices
help motor operators get the best

value, performance, and reliability from
commutators, carbon brushes, and the
motor units in general, resulting in cost
savings and lengthened service life.

Roly Roberge is the design and application
engineer manager at Morgan Advanced
Materials

Morgan Advanced Materials
Greeneye Smart PV Switch
www.morganadvancedmaterials.com

• Failsafe safety system
• Individual shutdown feature per PV module
• Protection for iireiighters or PV installers and electricians

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10 nacleanenergy.com
NOVEMBER/DECEMBER 2015