Brush life in DC motors

Do you want to reduce downtime and achieve longer, trouble free motor life? In your attempts, have you been stymied by DC motor brush and commutator problems? Would you like to know what is considered normal brush life? Are you looking for ways to get longer brush life? If your answers are yes, then this is just the article for you.

To ensure good brush life, we need to know the conditions that contribute to brush wear, the tradeoffs when we alter these conditions and how much wear is considered normal for the application. We can then look at, evaluate and change conditions to achieve the best compromise among brush life and the costs associated with things such as commutator wear, clean air duct work, production load changes, humidity control and the like.

Why brushes wear

DC brush wear results from mechanical friction and electrical erosion. Friction produces carbon dust. Electrical erosion vaporizes carbon leaving little physical residue.

Friction

“Identifying the variables and determining brush life gets to be quite a job, especially since some variables constantly change.”

- Robert J. Hamilton, senior product specialist, Reliance Electric

Carbon rubbing on bare copper has a high coefficient of friction. A standard, non-plated commutator with a good film achieves the lowest coefficient of friction. With good film, the coefficient of friction can be reduced to 10 % of the original bare copper value.

The coefficient of friction between the filmed commutator and brush decreases, up to a point, with increases in commutator temperature. For example, a given brush might have a coefficient of friction of 0.15 while running on a commutator with a surface temperature of 140 °F. When the surface temperature reaches 220 °F, the coefficient of friction could be 0.08. Higher temperatures result in an increase in the coefficient of friction.

Standard brushes on warm commutators at medium speeds have a coefficient of friction of 0.13 to 0.19 — a low coefficient of friction.

Some brushes with low coefficients of friction are not as hard as brushes with higher coefficients. A hard brush with a medium coefficient could provide longer life but could be noisier in operation. There are many exceptions however. There are a number of hard grades with low coefficients of friction.

Mechanical problems such as high mica, high brush spring pressure, a feather edge on a copper bar or other imperfections on the commutator surface also cause friction. Brush wear on an unpowered motor in a tandem motor-motor set or on an unloaded generator in a motor-generator set comes from friction. Friction is a function of the atmosphere, temperature, current loading and the mechanical characteristics of the motor.

Erosion

Erosion is the result of improper commutator film or a wear condition such as threading. Other motor set up conditions or mechanical problems such as the brush neutral setting, interpole strength, low brush spring pressure, poor brush seating, high mica, and commutator eccentricity can also cause sparking and erosion. Sparking increases with current loading and motor speed. Brush life decreases with increased sparking.

The condition of the commutator film directly affects friction and erosion and brush life. The commutator must have good film to get good brush life.

What is good commutator film

The proper current passing between the carbon and copper in the presence of water vapor forms a microscopic layer of copper carbon composite or film. This film color is chocolate brown or burnished bronze to dark brown or black and uniform. It is not bright copper or burnt black copper. Consult a commutator color and appearance picture chart to determine the condition of your commutator. There is a condition known as false filming in which brush graphite deposits cook on the commutator resulting in an appearance similar to dark film. Oil can also leave a coating that resembles film. If your film can be easily wiped away, it's not the desired good commutator film!

Commutator filming is a continuous process of forming and stripping away. A good film is only 0.000,000,2 inch (200 nanoinch) thick. Thus, the conditions required to build good film must always be present. Changes in current or humidity affect the commutator film.

Requirements for good film

Certain conditions develop and maintain good film. A majority of your operating time must be within the designed brush current density range. For some grades of brush on warm commutators, this range is generally 55 to 85 amps per square inch. If the current density exceeds this value for long periods, the commutator runs hot, blackens and brush life is reduced. If the current density is too low, the film will be striped from the commutator and begin to thread. If allowed to continue, sparking and threading increase, brushes wear rapidly and the commutator requires resurfacing.

Often, motors run continuously at light loads so brush current density is always below the minimum. Changing to a brush grade that films at lower current densities may solve the light loading problem.

Many times the best solution is to remove one or more rows of brushes to bring the current density back into the acceptable range. When removing brushes from staggered brush sets, the remaining brushes on each stud must cover the same commutator surface arc as was covered prior to brush removal. This means removing the brushes from the center stager set on each stud first. When using wear indicator brushes, be sure that some remain. Usually this is not a problem since the wear indicator brushes are most often the inboard, middle and outboard brushes on the stud. Usually the manufacturer recommends the order in which the brushes are to be removed. Some system must be in place to ensure that the removed brushes are replaced if the load returns to near nameplate load.