Hawkins Electrical Guide v. 02 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

5. The spring pressure must be adjustable;

6. The brush holders themselves must be carried on a rocker arm, or rocker ring.

It is desirable that brush holders be capable of individual adjustment, so that each may be set at its own point of minimum sparking. A few forms of brush holder are illustrated in figs. 342 to 345.

The various kinds of brush holder may be divided into four types:

1. Arm or lever type;
2. Spring arm type;
3. Box type;
4. Reaction type.

In the arm or lever type the brush is firmly attached to the extremity of a rigid arm capable of movement about the brush spindle, except in so far as it is restrained by a spring as in [fig. 342].

[Fig. 343] shows a brush holder of the spring arm type. The brush is firmly attached to the extremity of a spring arm, the other end of which is secured to the brush spindle, and when once adjusted is not capable of movement about the brush spindle.

In the box type of brush holder as illustrated in [fig. 344], the brush is free to move up and down in the brush box, so far as it is not restrained by a spring rigidly secured to the arm which carries the brush box at its extremity.

Fig. 346.—Fort Wayne type MPL dynamo; view showing details of armature, commutator and brush rigging of large machine. The laminations of the armature core are punched from thin sheet steel, annealed and japanned. Spacing ribs are built into the core at proper intervals forming air passages for ventilation. In addition, there are recesses in the inside of the flanges which permit the passage of air from the interior around the ends of the core to the openings in the end flanges. The armature coils are constructed of round or bar copper on standard forms. The coils are laid in slots in the surface of the core. The commutator is constructed of bars of hard-drawn copper of uniform size and shape, supported and clamped at either end between beveled rings and securely seated on the commutator drum. The drum is connected by radial arms to the commutator sleeve which is mounted and keyed on the armature hub extension.