Fig. 348.—Western Electric parallel spring brush holder as used on the larger machines.
Ques. How may this be avoided?
Ans. By insulating one end of the spring, and carrying the entire current directly from the brush itself to the main conductors by a flexible copper strip or cable firmly connected to both.
Ques. What may be said with respect to brush construction on machines for electrolytic work?
Ans. The collection of large currents at low voltage, generated by comparatively small machines, requires careful design of brushes and brush holders. The commutator is longer than the commutators on machines of equal capacity at higher voltages, and as a rule the commutator segments are thicker and fewer in number. Each brush set is made up of numerous narrow brushes rather than two abnormally wide ones.
An example of brush and brush gear designed to meet such conditions is shown in [fig. 328].
In large machines for electrolytic work, it is not unusual to find the current divided between two wide commutators, one at each end of the armature, thus giving a longer axial bearing surface for the brushes without inconveniently lengthening the pins upon which the separate brushes are threaded.
Multipolar Brush Gear.—The brush gear which includes the holders and carrier arm or ring, becomes more complicated as the number of poles and magnitude of the current is increased.
In the early days of multipolar machines, schemes of armature winding were devised such that all the necessary cross connections were made inside the machine, and the number of brush holders reduced to two and placed at an angular distance apart depending upon the number of poles. Such windings, though possible, are not used much, chiefly on account of their complexity, which not only increases the danger of error in construction, but also makes repairs costly. In modern multipolar machines, such complicated windings are avoided, and the several sets of brushes are connected together in two groups, positive and negative. These connections are carefully designed as part of the brush gear.
Ques. How are the brushes held in large multipolar dynamos?
Ans. They are held at the proper points of commutation by arms offset from a cast iron rocker ring, which is itself supported by brackets projecting from the magnet yoke as shown in [fig. 346].