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Carbon Brushes (Aug 2013): Current Density

August 28, 2013
Current density is a measure of the density of electrical current. It is an important consideration in the design of an electrical system. Most electrical conductors (i.e. carbon brushes) have a finite, positive resistance, making them dissipate power in the form of heat. The current density must be kept sufficiently low to prevent the conductor from melting.
A majority of the operating time of a brush must be within the designed current density range. If the current density exceeds this for long periods, the commutator will run hot, blacken and brush life will be reduced. If the current density is too low, the film will be striped from the commutator and the commutator will begin to thread. If allowed to continue, sparking and threading will increase, brushes will wear rapidly and the commutator will require resurfacing.
Often motors are run continuously at light loads where brush current density is always below the minimum. In such cases, a change in brush grade to something that will film at lower current densities, may solve the light loading problem. Many times the best solution is to remove a row or more of brushes to bring the current density back into the acceptable range. The motor manufacturer can advise the order in which the brushes are to be removed. Some system must be in place to insure that the removed brushes are put back in place if the load conditions increase to near motor nameplated load.

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