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How to solve spark and short brush life problem?

Q: We have tandem-operated DC motor application for Rubber mixing (Banbury). We have two DC motors, each one 1,500 KW and 2,250 Amper Armature current. One of them is the master and other is the slave DC motor. They are sharing the load as equal. We have spark and short brush life (three months) problem. Due to the sparking problem, we have been having a problem on the commutator surface. Due to sparking and wearing of the commutator surface, we are grinding the commutator surface in a four-month period. We use original Carbon brushes, and the current density is about 10 Amper/cm2. The motors are running less than 80% of the rated currents on average. Current limit was set at 95% of the motor current in the drive side. The motors are totaly enclosed and are Water to Air cooling type. Commutator surface temperature is around 60 degrees Celcius. What is your recommendation to solve this chronic trouble?

Bayram Etkar, Utility Leader
BRISA Tyre Man. Co.

A:

The reported average operating load results in a current density of 8 amps/cm2, which should be well within the capacity of the carbon brushes on a properly designed DC motor. The comfortable commutator temperature of 60 degrees C also is confirmation that the wear problem is not due to overload.
 
The arcing is indication of current passing across a high-resistance contact. In most cases, this involves interruption of consistent contact of the brushes with the commutator. The arcing then causes rapid wear in the form of electrical erosion of the carbon brush and the commutator surface. The level of arcing can be exaggerated further if the spring pressure on the brushes is inadequate. For most DC motor applications, 280 grams/cm2 is the recommended minimum level for proper brush contact.
 
There are a variety of mechanical faults that can cause the interruption of brush contact, thereby leading to excess brush and commutator wear. In the case of this application, it is very common when fresh material is introduced into the banbury mixer, to have a period of very high amp loads and high vibration. The vibration is transferred back to the brushes, resulting in brush bounce and accelerated electrical wear. Following softening of the material mix, the motor amp loads and the vibration return to normal lower levels.
 
Deficiencies in the electrical adjustment of the motor can also lead to excess arcing. For example, neutral position, interpole strength and brush spacing could be involved in the problem if the arcing is only at the edge the brush and on the edge of commutator bars.

Jeff D. Koenitzer, P.E.
Vice President – Engineering
Helwig Carbon Products Inc.