Figure 1-12.Gramme-ring armature.
Figure 1-12, view B shows a composite view of a Gramme-ring armature. It illustrates more
graphically the physical relationship of the coils and commutator locations.
The windings of a Gramme-ring armature are placed on an iron ring. A disadvantage of this
arrangement is that the windings located on the inner side of the iron ring cut few lines of flux. Therefore,
they have little, if any, voltage induced in them. For this reason, the Gramme-ring armature is not widely
A drum-type armature is shown in figure 1-13. The armature windings are placed in slots cut in a
drum-shaped iron core. Each winding completely surrounds the core so that the entire length of the
conductor cuts the main magnetic field. Therefore, the total voltage induced in the armature is greater
than in the Gramme-ring. You can see that the drum-type armature is much more efficient than the
Gramme-ring. This accounts for the almost universal use of the drum-type armature in modem dc
Figure 1-13.Drum-type armature.