ARCING ON THE COLLECTOR SURFACE.

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In order for a any motor to perform satisfactorily the commutator or slipring must be perfectly round. This means that you must never try to clean it by randomly rubbing it with emery paper, glass paper or sand paper. The reason for this is, it will produce flat spots around the surface of the collector. If the collector is not truly round then the carbon brushes will not be able to follow the contours of the collector instead the brushes will bounce causing arcing. Also never use emery paper on commutators as small particles of emery will become imbedded in between the segments of the commutator. Particles of emery may be electrically conductive and will cause short circuits. 

If you must clean the surface of the collector only use very fine glass paper cut into narrow strips. Each strip must be cut to the same size as the width of the collector. Place the armature carefully in a vice with the commutator overhanging one end of the vice. Hold one end of the strip of glass paper with the left hand and the other end with the right hand then carefully wrap it half way around the top of the commutator and apply an up and down action with each hand to allow the sand paper strip to lightly clean the surface of the commutator. Do not overdo this action use only light pressure and stop now and again to turn the armature in the vice until you have cleaned the whole surface of the commutator. 

Ideally if you have access to a lathe use it to rotate the commutator while holding the strip of sand paper around the commutator. After this operation has been carried out you must carefully clean out all the foreign particles caught between the copper segments. This may be carried as follows. Obtain a saw blade from a junior type hacksaw these blades are thin and about 6 inches long. Break off approx. 1 inch of the blade from one end. Wrap a length of adhesive tape around most of the other end to make a handle. Then holding the broken end of the blade against a grind stone carefully grind the sides of the blade to the thickness of the blade. Do this in such a way so as to leave the teeth still showing but thinner until you can fit the blade in between the copper segments When you finish this operation place the thin blade end in between each segment in turn and draw the blade backwards and forwards so as to scrape out any particles in the gap.

When this operation is finished you may notice that this action has slightly raised the edges of some of the copper segments slightly. Obtain a smooth very thin small triangle shaped file and carefully run one of it's edges along between the segments to remove any high points of copper. i.e. this will create a light bevel on the edge of each segment. Note you can if you wish obtain a tool that will do this for you. With regards to the surface finish of any commutator or slipring you do not require a very smooth finish. With very smooth finishes there is the danger of aero dynamic problems. This is where an armature that turns at very high speed will draw air in under the brush. This cushion of air will cause the brush to ride above the collector and produce an arc. 

The next operation will require an Ohm meter. You will need to carry out a resistance check between each pair of copper segments. The reading on the ohm meter must be very low and all readings must be approx. equal. If you find any resistance reading suddenly doubles or reads very high than you have an open circuit in the windings and you will require a new armature. Next you must check the brushes in their brush boxes to ensure they are able to slide up and down freely. You must also check to ensure that the sides of the brushes are not too badly worn and or the insides of the brush boxes are not too badly worn. If they are this will cause brush instability. On large powerful motors Check all brush leads to ensure they have not become to stiff and corroded. Stiff flexibles interfere with and reduce spring pressure on the brushes.

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Date 20-03-2001