Ans. One built up of layers of iron sheets, stamped from sheet metal and insulated, as shown in fig. 234.
Ques. What may be said of this construction?
Ans. It is a most approved method, and one frequently employed in the construction of cores and pole pieces.
Fig. 234 shows a combined core and pole piece made entirely of sheet iron punchings assembled and riveted together, and fig. 235, a core to be used with separate pole piece. It should be noted that in both cases there is a longitudinal slot extending from the end into the core. This was first suggested by Lundell, the object being to prevent, as far as possible, the distortion of the magnetic field due to armature reaction especially on heavy overloads.
Ques. What mode of construction is adopted to reduce the reluctance of the magnetic circuit when laminated poles are used?
Ans. They are cast welded into the frame.
The frame end of the core as shown in the illustrations has irregularities in the heights of the different sheets, as well as grooved undercut surfaces, in order to enable the molten metal of the frame to key well into the laminations of the core, making a good joint, both mechanically and electrically. By this construction, the continuity of the magnetic circuit is practically unbroken save for the air gap between the pole piece and armature.