A long service life along with superior reliability for direct current (DC) drives and motors is ensured if they are maintained correctly. Additional benefits as a result of proper maintenance are; low overall weight, low operational cost, and simple operation. Mining, aerospace, and power generation are some of many industrial sectors where these motors are used.
Proper maintenance of DC motors involves thorough inspections, focusing on minimal destructive sparking and arcing; prolonged commutator service life; and prolonged carbon brush life. DC motor units provide long-term reliable performance if periodic inspections prioritize the three objectives along, with sufficient maintenance.
For successful upkeep of DC units, appropriate inspections are essential. Inspections provide operators with visibility of the motor unit condition, provide a constant source of information for reference, and encourage quick repair and preventative maintenance work throughout the lifetime of the unit.
The motor units usage condition affects the recommended frequency of inspections. Regular inspections for monitoring the motor unit’s performance are necessary, though the time frame for such inspections can vary. Setting the inspection schedule, whether it is monthly, quarterly, annually or in any other frequency, and following it, is vital to obtain the full benefits of inspections. The inspection schedule can be adjusted depending on the motor unit’s role in the process environment, the unit’s usage, and other process conditions.
Visual assessments of the motor unit, and recording the data as detailed notes for further reference is vital for successful inspections. Photographic recordings of inspections for comparison purposes are helpful in some cases, including recording the condition of commutator.
Operational and static inspections are both valuable. Operational inspections include observing unusual vibrations, air temperature, and sparking levels, and static inspections concentrate on the wearing effect on brushes, overheating signs, brush wearing signs, cleanliness, the condition of spring, and evaluation of commutator film.
Assessing Commutator Condition
The most vital aspect of DC motor upkeep is maintaining the condition of the commutator. The commutator condition relies on the generation and maintenance of acceptable brush film, and avoiding metal transfer, and burning and other damaging effects.
Brush films are created by the steady degradation of carbon brushes, and the mixing of carbon dust with copper traces in the commutator with humidity present in the atmosphere. Ideally a brush film will allow smooth spinning of the commutator, without any damage from the brushes, while ensuring the transfer of power to the motor. There are several acceptable commutator conditions, as there are those that cause concern but are not critical, though it is generally believed that the brush film is normal if it is in “chocolate brown in color with a medium polish.”
The common type of brush film is non-uniform, with a blotchy finish. The film appearance depends on many factors, such as commutator roundness, brush contact pressure, chemical vapors, and unequal magnetic fields. This common type of film is an acceptable filming pattern. Also acceptable are commutators with uniform films of any color. Film color results from brush composition, processing and impregnations. Various film shades are a result of motors using various grades of brushes.
The final acceptable film pattern is slot bar filming, where a repeated pattern of light and dark films, depending on the number of armature coils per slot, is present. Slot bar films depend on machine design, and are generally not affected by brush grade.
Although films with streaks or bright spots are concerning, they are acceptable as film streaks do not damage the commutator. However when metal transfer occurs film streaks can lead to threading, which is not an acceptable form of commutator condition. A dark commutator film with bright spots may give an indication that the motor is experiencing frequent overload cycles. The motor can be operated for prolonged periods of time when the spots disturb only the film. When acute metal transfer occurs, the spots can result in film stripping or bar burning, which are not acceptable commutator conditions. Commutators with concern-causing film patterns should be monitored and maintained regularly to avoid other problems.
Preserving a commutator’s service life is possible if immediate maintenance is carried out when unacceptable commutator states become evident during inspections. The unacceptable states include slot bar burning, bar burning, pitch bar burning, copper drag, grooving and threading, all of which result in acute damage to the materials in the commutator. A commutator is expected to have minimal damage over a motor’s prolonged service life. However performing touch-up machining, and changing brush grade to eliminate negative impacts will help to detect and repair damage, avoiding catastrophic failures and improving overall motor unit service life. The commutator should get maximum priority during maintenance work and inspection, as it is the most expensive part of a DC motor unit to replace.
Minimizing Grounding, Arcing and Destructive sparking
Minimizing grounding, destructive sparking and arcing by keeping the motors clean, and avoiding a collection of carbon dust, created when brushes get worn out, is another priority for DC motor maintenance. Some of the dust particles go into the film formation, and additional dust can settle in the motor unit. When combined with other particles existing in the atmosphere this can cause grounding effect. Grounding, combined with insulation failure, will cause catastrophic failure in motors. The particulates also restrict the free movement of the motor parts, and inhibit normal heat dissipation through exterior surface of the motor unit frame, resulting in the motor operating at a higher temperature condition than what is considered ideal.
In order to prevent such issues, it is necessary for motor operators to undertake heavy duty cleaning on an annual basis, with a few light-duty cleans of the DC motor unit. These light duty cleanings are ideally performed on a quarterly basis, but the frequency is based on the inspection observations, as operating conditions vary between installations.
Care should be taken when cleaning, as sweeping particulates further into the motor unit can worsen the condition. Proper cleaning involves vacuuming dry particulates with a soft bristle brush to loosen the accumulated dust. A cloth, moistened with solvent, helps to remove oily dirt. Excess solvent during cleaning can push the conductive particulates deep into the motor unit through cracks present in the insulation, causing more damage. Components such as varnish-insulated coils and commutators cannot be cleaned with solvent, though individual springs and brush holders can.
Carbon films can be removed from commutators to reduce the risk of excessive buildup. The commutators can be inspected under the film with the help of a medium, soft, white abrasive stone, “seater stone,” which is available with from suppliers of motor repair parts. Air curing technique is utilized to clean the undercut between the commutator bars. Clean, pressurized air is blown onto the commutator when it is spinning. If the dirt is not removed by the first curing process a second round of air curing is carried out after mechanical removal of the particulates. Care should be taken to ensure operator’s safety when performing the various cleaning processes.
Black commutations are not unusual in DC motor units. Non-destructive sparking is acceptable, and pinpoint sparking not causing commutator or brush deterioration is also tolerable. Destructive sparking and arcing can cause electrical erosion of commutator and brush surfaces, and if left uncorrected can lead to the failure of the equipment itself. Slot bar or pitch bar burning and other unacceptable commutator states are indications of destructive sparking and arcing.
Slot bar burning takes place if sparking levels are so high that metal can be removed from the trailing edges of commutator bars, making them look etched or burned after the removal of the brush film. Even if the film is not removed, worsening conditions can make the burning evident. This is due to destructive sparking, caused by using a carbon brush grade with inadequate commutating ability, excessive load conditions, or inappropriate electrical adjustment of the motor unit. Avoiding the conditions that cause destructive sparking and arcing can aid in preventing commutator damage and motor failure.
Selecting the Appropriate Brush Grade
Monitoring the carbon brush condition also helps to prolong the service of the motor unit. Carbon brushes must be replaced often, but they are the least expensive part of a motor unit. Generally carbon brushes last for a period of three months to three years, based on the motor unit and the service conditions. Brush life is not as important as commutator life, and this fact should be kept in mind when selecting brush grades. The grades should be chosen according to motor’ specific application.
Currently, Morgan Advanced Materials provides the widest range of brush grades. A unique material composition and impregnation is present in each of the grades to ensure the optimization of the service life of commutators and brushes in a particular service condition. Usually graphite is used to make the brushes, but some brush grades use metals, such as copper, as cleaning or abrasive materials for those service conditions where there is a likelihood of excessive film buildup. Likewise, impregnations or chemicals are added to graphite during the production stage to optimize brush film and protect motor parts. To avoid commutator damage, destructive sparking or arcing, it is vital that an appropriate brush grade is selected.
DC motor operators can identify the suitable brush grade for a particular motor installation by monitoring temperature, humidity and other environmental factors, and by consulting experts; including Morgan’s sales personnel or application engineers. This can lead to a longer brush and service life of brush holders and commutators.
The service life of commutators ranges from 10 years to over 20 years when the correct maintenance and inspection procedures are followed. These procedures can aid operators of motor units to gain the best value, reliability and performance from carbon brushes and commutators, as well as from the motor units, which can result in prolonged service life and cost-savings.
This information has been sourced, reviewed and adapted from materials provided by Morgan Advanced Materials.
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