Ques. What may be said with respect to the dimensions of the brushes?
Ans. No general rule can be given for breadth and thickness of brush. The contact face must clearly be wider than the thickness of the insulation between commutator segments, since the period of commutation must last an appreciable interval of time on account of self-induction.
Ques. What should be the minimum width of the brush contact face?
Ans. It may be taken as one and one-half times the thickness of the commutator segments.
Ques. How wide should a carbon brush contact be?
Ans. The brush should be thick enough to cover two and one-half commutator segments. The thickness should in no case be excessive on account of the loss due to heating, which results from the difference of potential at the forward and rear edge of the brush.
Fig. 339.—Contact angle for the different types of brush. At A is shown a brush with tangential contact, and at B, a so called tangent brush; the latter is properly called an inclined brush. Sheet copper brushes are set tangentially as at A, and gauze brushes inclined as at B. Carbon brushes are placed radially as at C when mounted in box holders, and inclined opposite to the direction of rotation when used with reaction holders.
Contact Angle of Brush.—This may be defined as the angle which the brushes make with the commutating plane as shown in [fig. 339]. The several kinds of brush, together with the varied conditions of operation require different contact angles ranging from zero to 90°.
Thus in the figure, a copper strip brush may lie at 90° or tangentially as at A.
Wire or gauze brushes should make a more or less acute angle as at B, in order to present the end and not the side of the brush to the commutator.
Carbon brushes may be placed end on or radially as at C, which is the position almost universally used in the case of traction or other reversing motors.
Sometimes the carbon brush is inclined as at D, in order that the revolving commutator may tend to push the brush against its supports and thus ensure better contact.