In the construction of commutators, the conditions of operation require that there be:
1. Adequate insulation;
It is necessary to have good insulation between each segment, and a specially good insulation between the segments and the hub or sleeve on which they are mounted; also between the segments and end clamps. The insulating material must not absorb moisture, hence asbestos, plaster, or vulcanized fibre are not used. The end insulating rings are usually built up of mica and shellac, moulded while hot under pressure to the correct shape.
Fig. 326.—Front view of Western Electric Commutator for bar wound armature. This commutator is made of hard-drawn copper and insulated throughout, ventilating spaces being provided near the shaft.
2. Rigidity against centrifugal force;
Since the segments are subject to centrifugal force, they must be securely clamped in place. Screws cannot be used, for that would destroy the insulation. They are therefore held in place by insulated clamps as shown in [fig. 324]. These clamps should be strong and capable of holding the segments firmly in position, for if a segment should rise out of its place through centrifugal force, it would disturb the action of the brush and cause sparking.
3. Provision for wear.
The segments should be of considerable radial depth, so that the commutator may be turned down from time to time to preserve its circular form.
