Figs. 216 to 221, show a series of cross sections, all of the same area. The number marked on each section indicates the length of the boundary line, that of the circle being taken for convenience as 100.
Ques. What are the pole pieces?
Ans. These are the end portions of the field magnets, joined to, or cast together with the core and placed adjacent to the armature.
The faces of the pole pieces are of circular shape, thus forming the sides of the so-called armature chamber within which the armature rotates.
Ques. Why are the pole faces made larger than the coils?
Ans. In order to reduce the reluctance of the air gap between the face and the armature, thus enabling fewer magnetizing coils to be used.
It is important that the field should be magnetically rigid, that is, not easily distorted. This stiffness of field can be partially secured by judicious shaping of the pole pieces. A few forms of pole piece are shown in figs. 222 to 231.