The Diseases Of Electrical Machinery

ELECTRICAL MA CH1NER Y. 15 to have at least two, and frequently more than two, brushes in each row. A row of narrow brushes will make better contact than one wide brush. In the case of carbon brushes, as many narrow brushes as possible should be used side by side in each row. A single wide brush would be far too stiff and immovable, and the brush holder would be too heavy to follow small inequalities in the commutator. In general, a brush- width * of 1 inch should not be exceeded ; carbon brushes of above i|th inch in width, should be al- lowed in exceptional cases only. It is not only the width, but also the breadth f of the brushes which is of importance. In this respect the breadth, as used originally by the builder, should be followed. He alone is in a position to prescribe the most suitable breadth for the brushes, since this dimension is of great influence upon the commutation. It is therefore a mistake to oi'der carbon brushes later from another maker, who might supply brushes of a wrong breadth. Carbon Brushes.—The quality of the brushes also plays an important part. The resistance of the different qualities is very different. It is not desirable for every continuous-current machine that the brushes should have an especially high or low resistance. * The term “width” means here the width in a direction parallel to the axis of the machine. f The term “ breadth ” is here used for the breadth in the direction around the commutator. The breadth of a carbon brush is usually at least twice the breadth of one of the segments of the commutator, and frequently as great as that of three segments.