Fig. 711.--Jig for filing brushes to the correct bevel; used with copper brushes to fit them to the commutator.
Sparking.--In all well designed machines there are certain positions upon the commutator for the brushes at which there will be no sparking so long as the commutator is kept clean and in good condition. In other dynamos, badly designed or constructed, sparking occurs at all positions, no matter where the brushes are placed, and in such dynamos it is therefore impossible to prevent this no matter how well they are adjusted.
Fig. 712.--Commutator clamp; a useful device for holding the segments firmly in position in taking out the end rings of the commutator to repair for internal grounds. It is made of 2 × 1/8 inch sheet steel, with a 1/2 inch screw. The illustration clearly shows the adjustable fastening. The notches fit around rivets on one side of each fastening, which can be moved by removing the two cotters. The clamp is made loose or taut by screwing the bolt in the nut.
Ques. What two kinds of sparking may be generally distinguished?
Ans. One kind of sparking is that due to bad adjustment of the brushes, and a second kind, that due to bad condition of the commutator.
Sparks due to bad adjustment of the brushes are generally of a bluish color, small when near the neutral plane, and increasing in violence and brilliancy as the brushes recede from the correct positions upon the commutator.
When sparks are produced by dirty or neglected state of the commutator, they are distinguished by a reddish color and a spluttering or hissing. When due to this last mentioned cause, it is impossible to suppress the sparking until the commutator and brushes have been cleaned. In the former case, the sparks will disappear as soon as the brushes have been rotated into the neutral points.
Another class of sparks appear when there is some more or less developed fault, such as a short circuit, or break in the armature or commutator. These are similar in character to those produced by bad adjustment of the brushes, but are distinguished from the latter by their not decreasing in violence when the brushes are rotated towards the neutral plane.
Having distinguished the classes of sparks which appear at the commutator of a dynamo, it remains to enumerate the causes which produce them. These are:
- Bad adjustment of brushes;
- Bad condition of brushes;
- Bad condition of commutator;
- Overload of dynamo;
- Loose connections, terminals, etc.;
- Breaks in armature circuit;
- Short circuits in armature circuit;
- Short circuits or breaks in field magnet circuit.