DC Motor Brushes - What Causes Them to Wear? DC motor brush wear is the result of many different factors.   As you might suspect, the primary suspects are mechanical and electrical in nature. The mechanical action of a copper commutator turning beneath the carbon brush causes friction that eventually wears the carbon brush into dust. Electrical currents flowing between the carbon brush and the commutator bars eventually cause the brush to vaporize leaving little if any residue.

Friction The process of commutation involves a carbon brush rubbing on a copper surface. This process has a high coefficient of friction. The coefficient of friction can be lowered significantly when the commutator has a "good film." Since increased friction leads to increased brush wear it is important to keep the friction between the brush and the copper commutator to as low of a value as possible. Some of the conditions that influence increased friction are temperature, brush hardness, mechanical problems, and localized heating of the commutator bars.

	Temperature It is intuitive to think that as temperature increases friction will also increase. However, this is not the case, there is an optimal temperature where friction is at its lowest. For instance, a particular brush may have a coefficient of friction of 0.09 on a commutator at 145° F and a coefficient of friction of 0.17 at 225° F, yet temperatures below 145° F and above 225° F may result in a greater coefficient of friction.
	Brush Hardness In general, harder brushes have lower coefficients of friction, resulting in longer brush life. However, harder brushes are usually noisier. Therefore, it is sometimes necessary to substitute brush life for quieter operation.
	Mechanical Problems Imperfections on the commutator surface, excessive brush spring force, and high mica, just to name a few, will also increase friction and brush wear.
	Localized Heating of Commutator Bars Whenever a brush type motor is stalled the commutator bars directly underneath the brushes are conducting the full locked rotor current of the motor. This will result in heat that can harden the bars. Over time, these bars will not wear at the same rate as the rest of the commutator and become high spots that will increase brush wear.
	Arcing and Sparking Excessive arcing and sparking, also known as erosion, can contribute to excessive brush wear. Sparking is typically caused by motor setup problems such as over/under bucking, low brush spring force, improper brush seating, commutator surface imperfections, and other similar conditions.

What is Commutator Film? Commutator film is a microscopic layer of copper carbon composite. Under normal atmospheric conditions, as current flows from the carbon brush through the copper commutator a film is deposited on the commutator surface. This film acts as a lubricant between the brush and the commutator surface, thus preventing excessive or premature brush wear. The process of depositing film on the commutator is an on going one, film is constantly being worn away and new film is constantly being deposited. Changes in atmospheric conditions such as humidity or the presence of other gases can affect the quality of the commutator film.

Good Film vs. Bad Film The condition of the film can be identified by its color. Good film ranges from brown, like your favorite chocolate bar, to black and is uniform in color. A bright copper color or burnt copper color does NOT identify good commutator film. Commutator film should not be easily wiped away.

The Right Brush for the Application Each brush is designed for a specific current density. Using the correct brush for your application will insure good commutator film. Brush current density is typically rated in amps per square inch and is calculated by dividing motor amps by the total surface area of the all brushes in the motor. If a motor typically runs at a current greater than the rated brush current density then the commutator will become hot and eventually will burn taking on a burnt copper color. Running a motor at a current less than the rated current density will wipe the film from the commutator and lead to threading. Choosing the right brush for your application is extremely important. Keep in mind that there is no perfect brush for all operating conditions, compromises must be made and sometime a brush that corrects one problem can lead to problems in other areas.

What is the Brush Surface Telling You?

Normal Wear Bright shiny consistent or uniform porous wear patterns indicated good brush wear
Normal brush wear for bi-directional motor
Uniform commutator film indicates normal brush wear
Brush Problems Streaky or grooved wear pattern indicates that the motor is under loaded; the brush is the wrong material, or the presence of corrosive gas, dust, or high humidity
Burning caused by arcing and sparking indicates incorrect brush material, low spring pressure, or excessive current
Commutator burning caused by arcing and sparking
Brush damage caused by over voltage condition
Copper in the brush surface indicates low current
Chipped edges or bright surfaces indicate low spring pressure, low current, low commutator temperature, or excessive friction caused by a deformed commutator
Excessive vibration caused by deformed commutator, low commutator temperature, or low spring pressure
Individual bars burned on the commutator indicate the position where the motor stalled