If the projecting tips of the pole pieces, or horns as they are called, be widely separated, as in fig. 222, they are not always good, even though thin. It is better that they should be extended as in fig. 223 so that they may be saturated by the leakage field or else cut off as in fig. 224.
An extreme design, suggested by Dobrowolski, as shown in fig. 225, surrounds the armature with iron.
Another scheme, proposed by Gravier, employed the unsymmetrical form shown in fig. 226. In this pole piece the forward horn is elongated. The action due to this arrangement is such that when the machine is working at small loads, the field in the gap is nearly uniform, but at heavy loads with distorting reactions which have a tendency to drive the flux into the forward horn, the small section of the latter causes it to become saturated, thus reducing the distortion to a minimum.
Eddy Currents; Laminated Fields.—The field magnet cores and pole pieces, as well as the armature of a dynamo are subject to eddy currents, that is, induced electric currents occurring when a solid metallic mass is rotated in a magnetic field. These currents consume a large amount of energy and often occasion harmful rise in temperature. This loss may be almost entirely avoided by laminating the pole piece, or both pole piece and core; in the latter case, both form one part without any joint.
Ques. What is a laminated pole?