SOLUTION

Commutators and brush assemblies are a source of trouble in dc rotating machines. The continuous sliding of the brushes against the commutator wears the brushes down and tends to push them out of alignment causing bad contact between the commutator and brushes. When something does go wrong in commutation, it may be accompanied by excessive sparking, which aggravates the original trouble at once. When corm commutation is taking place, there will be very little sparking.

For satisfactory commutation of dc machines, continuous contact must be maintained between commutator and brushes. The commutator must be mechanically true, the unit in good balance, and the brushes should be in good shape and well adjusted. 

Excessive sparking can be caused by a number of external factors, such as shorted or open armature windings, overload, and improper field strength.

There can be many reasons For sparking at the brushes in a DC motor.

1. Brush holder is not equally spaced:-  this condition appears as the unequal sparking on different holders.
2. Holder is too far from commutator surface– If the holder is too far from the commutator surface it may fail to support the brush properly
3. Overload:- Overload. Overloading of the starter will cause high currents in the brushes, resulting in overheating and burning. Overloading is usually encountered when the starter is run for Periods long than 30 seconds
4. Dirty commutator: Oils, greases, and water will foul the commutator and cause excessive sparking between the commutator and the brushes, thus causing high temperatures

When there is excessive sparking at the commutator then good commutation cannot be obtained, the commutator and brush assembly must be checked, and any defect corrected as soon as possible. The inspection procedure and the steps taken to eliminate troubles are as follows:

1. Watch the machine operating to see if you can spot any arcing or excessive sparking elsewhere that might indicate a loose connection.
2. Check the brush positions to be sure that the brushes are commutating at the proper point (neutral plane).
3. Inspect all connections and make sure that none are loose.
4. Check the relative position of the brushes on the commutator. If they are unequally spaced, look for a bent brush holder.
5. Check the condition of the brushes. If they are badly worn, they should be replaced. When removing a brush, first lift the spring lever to release the pressure, then remove the brush. Insert a new brush, making sure that it can move freely in the holder.
6. Check the commutator for dirt, pitting, irregularities, etc. Dirt can be removed with a piece of light canvas. Fine sandpaper will remove slight roughness. Never use emery cloth on a commutator.
7. Badly worn commutator should be skimmed on a lathe. This refers to machining away the irregularities of the surface.