Fig. 327.—Sectional view of a General Electric commutator. The segments are of rolled or forged copper and are separated by soft mica insulating sheets. This mica must wear down evenly with the copper, hence its consistency is important. The segments are wedge shaped so that when drawn radially inward they support each other like the stones of an arch. They are drawn together by hollow cone collars which bear upon lugs projecting from the ends of the segments. These lugs are turned to form a smooth cone after the segments are assembled. The collars are insulated with mica from the segments and they are held in place by nuts upon the commutator shell or by bolts passing from end to end under the segments. The segments are also provided with lugs for connection to the windings.
Points Relating to Commutators.—1. The number of commutator segments depends on the scheme of winding and on the number of sections in the armature winding.
2. Increasing the number of bars diminishes the tendency to spark, and lessens the fluctuations of the current.
There are two practical reasons for not using a very great number of segments: it increases the cost, and in small machines the segments would be so thin that a brush of the proper thickness to collect the current would lap over, or bridge several segments.
Types of Commutator.—Commutators are made in various forms, but they may be grouped into two general types:
1. Commutators for closed coil armatures;
These consist of a large number of segments or bars, insulated from each other and varying in number according to the scheme of armature winding, and on the number of sections into which that winding is grouped.
Fig. 328.—A large current low voltage bipolar dynamo built for electrolytic work and here shown to illustrate the large size commutator and brushes necessary to collect the large current. Carbon brushes would not be suitable for this class of machine because even with copper brushes, whose conductivity is much higher than carbon, the commutator must be of considerable size to give the required brush contact area. The contrast between the axial lengths of the armature and the commutator is very marked. The rocker construction is of the ordinary type, and heavy flexible cables conduct the current from the brush holders to the fixed terminals. The machine here illustrated gives 310 amperes at 7 volts when running at a speed of 1400 R. P. M., corresponding to an output of 2.17 kilowatts.
2. Commutators for open coil armatures;