Proper maintenance and upkeep in DC motor units

When cared for correctly, direct current (DC) drives, motors, and their controls can offer excellent reliability and a long service life. Other benefits, when motors are properly maintained, include relatively low operational costs, low overall weight, and simple operation.

Proper care for DC motor units – used widely in aerospace – primarily consists of regular, thorough inspections with a focus on: long commutator service life, minimal arcing and destructive sparking, and increased carbon brush life. Regular inspections prioritizing these goals and adequate maintenance work help ensure DC motor units perform reliably.
 

Regular inspections

Appropriate inspections are at the heart of successful DC unit upkeep. Inspections give operators visibility into motor unit condition, encourage timely repair and preventative maintenance work, and provide a regular source of information for reference throughout service life.

The recommended frequency is based on motor unit usage, the individual unit’s criticality, or other conditions that may affect motor service life. Though this timeframe can vary from application to application, the frequency of inspection should adhere to regular inspection periods to monitor performance. Whether your application calls for monthly, quarterly, annual, or otherwise timed inspections, setting and following an inspection schedule is critical to reaping the benefits of inspection.

Successful inspections include visual assessment of the motor unit with detailed notes for future reference. Operators concerned with worsening commutator conditions may also photographically record their inspections for comparison between inspections.

Two types of inspections can be very valuable, an operational inspection and a static inspection. Operation inspection reviews commutation (sparking levels), air temperature, and any unusual vibrations. Static inspection focuses on cleanliness, brush wear, brush wearing signals, signs of overheating, commutator film evaluation, and spring condition.
 

Assessing commutator condition

Maintaining the condition of the commutator is the most important aspect of DC motor unit upkeep because it is the most expensive component to replace. Creation and maintenance of an acceptable brush film, and avoidance of metal transfer, burning, and other destructive conditions are key to ensuring the commutator’s condition.

Brush films are formed by the gradual, expected degradation of carbon brushes and the mixing of the resulting carbon dust with humidity and small amounts of copper from the commutator. An ideal brush film allows the commutator to spin smoothly without being damaged by the brushes, while still effectively transferring power to the motor. Though it is commonly believed that normal commutators’ brush film must be chocolate brown in color with a medium polish, a number of acceptable commutator conditions are concerning but not critical.

The most common type of commutator film type is not a uniform film but rather a blotchy, non-uniform finish. Accumulated tolerances in the motor unit such as commutator roundness, brush contact pressure, unequal magnetic fields, and chemical vapors are all possible factors in the development of the film appearance. This type of film is a completely acceptable filming pattern.

Commutators with uniform films of any color are also acceptable for use. Film color is a result of brush composition, impregnations, and processing, so motors with different brush grades will display different film shades.

Slot bar filming, a repeated pattern of dark and light films related to the number of armature coils per slot is the final type of fully acceptable film pattern. Slot bar filming results from machine design and is not usually impacted by brush grade.

Streaked films or films with bright spots are concerning, but not unacceptable by themselves. Streaking of only the film is not detrimental to the commutator. However, if metal transfer develops, streaked filming can progress into threading, an unacceptable commutator condition. Similarly, bright spots on an otherwise dark commutator film could indicate that the machine is subject to frequent overload cycles. If the bright spots disturb only the film, the machine can operate under this condition for long periods of time. However, if severe metal transfer begins, the bright spots could become bar burning or film stripping, both unacceptable commutator conditions. Commutators with concerning film patterns should be observed and maintained to avoid progression into severe issues.

Unacceptable conditions – bar burning, slot bar burning, pitch bar burning, threading, grooving, and copper drag – lead to severe damage of the materials within the commutator itself. While minimal damage to commutators is expected over a machine’s service life, regular inspection for these dangerous conditions, profiling commutators to measure roundness, performing touch-up machining, and changing brush grade to eliminate negative impacts will help to detect and repair damage, avoid catastrophic failures, and improve overall motor unit service life.
 

Minimizing issues

Keeping motors clean and clear of brush-wear carbon dust minimizes grounding, arcing, and destructive sparking. Some of this dust goes into the formation of the film on the commutator; but additional dust may become loose and settle within the motor. This, combined with other particulates from the air, can cause grounding. When this happens and the insulation of the motor fails, motors can experience catastrophic failure. Particulates can also make it difficult for motor parts to move freely as they need to. Finally, particulate buildup can inhibit normal dissipation of heat through the outside surfaces of the motor unit frame and make the motor operate at a higher temperature than is ideal.

Black commutators are common in DC motor units. Nondestructive sparking is tolerable and pinpoint sparking that does not cause brush or commutator deterioration is also acceptable. Arcing and destructive sparking, however, can electrically erode both the brush and commutator surfaces. If not corrected, this can eventually lead to equipment failure. Arcing and destructive sparking can be identified by the previously mentioned unsatisfactory commutator conditions in which burning occurs, such as slot bar or pitch bar burning.

Slot bar burning, for example, occurs when sparking levels become high enough to remove metal from the trailing edges of the commutator bars, causing them to appear burned or etched when the brush film is removed. Worsening conditions may cause burning to become evident, even without film removal. Destructive sparking that causes this condition may be caused by the use of a carbon brush grade with insufficient commutating ability, improper electrical adjustment of the motor unit, or exceeding load design limits.
 

Selecting brush grades

Carbon brushes are the least expensive component of a DC motor unit, but the most frequently replaced. Depending on the motor unit and service conditions, carbon brushes are likely to last from 3 months to 3 years. Brush life is understandably less important than commutator life, and brush grades are chosen with this in mind – carefully selected to suit the motors’ specific application.

Morgan offers a wide range of brush grades, each containing a different material composition and impregnation designed to optimize service life of the brushes and commutators. Primarily made from graphite, some grades include copper or other metals as abrasive or cleaning materials in service conditions where excessive brush film buildup is likely. Similarly, impregnations, or chemical compounds that are introduced into the graphite during production, help optimize brush film and protect motor unit components. Selecting the appropriate brush grade helps to avoid commutator damage, arcing, and destructive sparking.

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 each installation, leading to extended brush life and service life for commutators and brush holders.

With proper inspection and maintenance practices, commutator life expectancy ranges from 10 years to 20+ years. These upkeep practices help motor operators get the best value, performance, and reliability from commutators, carbon brushes, and motor units in general, providing cost savings and lengthened service life.

Morgan Advanced Materials
www.morganadvancedmaterials.com

About the author: Roly Roberge is design and application engineer manager for Morgan Advanced Materials. He can be reached at roly.roberge@morganplc.com.