Page 8 - North American Clean Energy November December 2015

Proper Maintenance and Upkeep

in DC Motor Units

by Roly Roberge

When cared for correctly, direct current (DC) drives, motors, and their controls can ofer excellent reliability over a long
service life. When properly maintained, they have a number of beneits in addition to long service life including; relatively low

operational costs, a low overall weight, and simple operation.

PROPER CARE FOR DC MOTOR UNITS CONSISTS OF REGULAR, thorough inspections focusing Brush ilms are formed by the gradual, but expected, degradation of carbon brushes and
on long commutator service life, minimal arcing and destructive sparking, and increased mixing of the resulting carbon dust with humidity and small amounts of copper from the

carbon brush life. Regular inspections prioritizing these three goals, and adequate commutator. An ideal brush ilm allows the commutator to continue spinning smoothly,
maintenance work, will ensure that DC motor units perform reliably in the long term.
without being damaged by the brushes, while still efectively transferring power to the
motor. hough it’s commonly believed that commutators’ brush ilm must be “chocolate
Perform regular inspections
brown in color with a medium polish” in order to be considered normal, there are a number

Appropriate inspections are at the heart of successful DC unit upkeep. Inspections give of acceptable commutator conditions, as well as several conditions which are concerning,
operators visibility of motor unit condition, encourage timely repair and preventative but not critical.
maintenance work, and provide a regular source of information for reference throughout he most common type of commutator ilm type is not a uniform ilm but rather a
the motor service life.
blotchy, non-uniform inish. Accumulated tolerances in the motor unit such as commutator

he recommended frequency of motor unit inspection depends on its use conditions. roundness, brush contact pressure, unequal magnetic ields, and chemical vapors are all
hough frequency can vary from application to application, the priority is to maintain possible factors in the development of the ilm appearance. his is the most common and
regular inspection periods to monitor performance. Whether an application calls for completely acceptable ilming pattern.
monthly, quarterly, or annual inspections, setting and following that schedule closely is Commutators with uniform ilms of any color are also acceptable for use. Film color is a

critical to reaping its beneits. he schedule can be adjusted based on motor unit usage, the result of brush composition, impregnations, and processing, so motors utilizing diferent
individual unit’s criticality, or other conditions afecting motor service life.
brush grades will display diferent ilm shades.
Successful inspections include visual assessment of the motor unit with detailed data Slot bar ilming, where a repeated pattern of dark and light ilms occur, is the inal type
recorded for future reference. Operators concerned with worsening commutator conditions of fully acceptable ilm pattern. Slot bar ilming results from machine design and is not

may also photograph their inspections for accurate comparison between inspections.
usually impacted by brush grade.
Two types of inspections can be very valuable, operational and static. An operational Streaked ilms or ilms with bright spots are concerning, but not unacceptable. Streaking
inspection reviews commutation (sparking levels), air temperature, and any unusual of the ilm is not detrimental to the commutator, however, if metal transfer develops,
vibrations. A static inspection focuses on cleanliness, brush wear, brush wearing signals, streaked ilming can progress into threading; an unacceptable commutator condition.

signs of overheating, commutator ilm evaluation, and spring condition.
Similarly, bright spots on an otherwise dark commutator ilm indicate that the machine
is likely subject to frequent overload cycles. If the bright spots disturb only the ilm, the
Assessing commutator condition
machine can operate under this condition for long periods of time. However, if severe
Maintaining the condition of the commutator is the most important aspect of DC motor metal transfer begins, the bright spots could become bar burning or ilm stripping, both of

unit upkeep. he creation and maintenance of an acceptable brush ilm, as well as the which are unacceptable. Commutators with concerning ilm patterns should be observed
avoidance of metal transfer, burning, and other destructive conditions, are key to ensuring and regularly maintained to avoid progression into severe issues.
the commutator’s condition.
Continued on page 10.

8 nacleanenergy.com
NOVEMBER/DECEMBER 2015