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.
=Ques. What provision is made for shifting the ring to adjust the lead?=
=Ans.= The ring is rotated by means of a worm gear and hand wheel.
The armature of a dynamo has been defined as: _a collection of coils of wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field._
From the mechanical point of view the armature may be said to be made up of the following parts:
1. Shaft; 2. Core; 3. Spider (in large machines); 4. Winding; 5. Commutator. (broadly speaking).
Of the two types of armature, ring and drum, the latter is almost universally used, hence the examples of construction which follow will be confined chiefly to this type.