The brush type DC and Universal motors were developed over 150 years ago and are still good options for consumers desiring a low cost, high power density, electrical motor. One of the most critical parts of this type of motor is the interface between the brushes and the commutator. Poor material selection and improper design considerations can affect both the life and the performance of an electric motor. This post briefly covers the factors which contribute to brush life and what design considerations should be used for selecting the proper brush.

In general, the main environmental factors that contribute to poor brush life include high heat, high vibration and low humidity. Although these factors can not always be avoided, placing motors in these extreme environments will be detrimental to brush life.

Another cause for poor brush life can be attributed to inadequate design considerations for the motor components. The commutator surface finish must be carefully established to provide the proper brush interface. The brush pressure is also of great importance. This pressure is affected directly by the brush springs. Too high of a spring force will cause too much mechanical friction and result in reduced life. A spring force that is too low will result in electrical arching which also results in reduced life. One thing to consider is that newer spring designs exist that maintain a more constant brush pressure even as the brush gets shorter; this is something that may help in applications where prolonging brush life is vital.

Finally, the selection of the brush grade must be considered. This is the most important factor in ensuring good brush life. Most material grades are proprietary to the brush manufacturer and are designed to create a film or “patina” on the commutator surface to reduce friction. The current density (electric current passing through the brush per cross sectional area) must also be properly matched to the correct brush grade.

Even after a brush has been chosen based on proper design considerations and environmental conditions, specific applications often times still require testing; proper test setup is crucial. Testing which overloads the motor as to accelerate the test duration is not advisable. Because specific brushes are designed to perform well at their given current density, lab testing parameters should closely match the parameters seen in the real world application.