Ques. How should an armature core be laminated to avoid eddy currents?
Ans. It should be laminated at right angles to its axis.
[Fig. 292] shows the induced eddy currents in a solid armature core, and [fig. 293] shows the manner in which the paths of these currents are interrupted and the losses due to their effect diminished by the use of laminated cores.
Fig. 293.—Armature core with a few laminations showing effect on eddy currents. In practice the core is made up of a great number of thin sheet metal discs, about 18 gauge, which introduces so much resistance between the discs that the formation of eddy currents is almost entirely prevented.
In [fig. 293], only five laminations or plates are indicated, so as to show the sub-division of the eddy currents, but in practical armatures, the number of laminations or punchings ranges from 40 to 66 to an inch, and brings the eddy current loss down to about one per cent. A greater increase in the number of laminations per inch is not economical, however, owing to the difficulties encountered in the punching and handling of extremely thin sheets of iron, and the loss of space between the plates.
Armature cores constructed of the number of plates stated, and forced together by means of screws and heavy hydraulic pressure, contain from 80 to 90 per cent. of iron, and have a magnetic flux carrying capacity only from 5 to 15 per cent. less than when they are made of an equal volume of solid iron.
Magnetic Drag on the Armature.—Whenever a current is induced in an armature coil by moving it in the magnetic field so as to cut lines of force, the direction of the induced current is such as to oppose the motion producing it. Hence, in the operation of a dynamo, considerable driving power is required to overcome this magnetic drag on the armature.
Fig. 294.—Circular concentric magnetic field surrounding a conductor carrying a current. If this conductor be moved across a magnetic field, as between the poles of a magnet, the lines of force will be distorted as in [fig. 295], which will oppose the motion of the conductor.
A conductor carrying a current is surrounded by a circular concentric magnetic field. If now such a conductor, with current flowing toward the observer as in [fig. 294], be placed in a uniform magnetic field, a distortion of the magnetic lines will occur as shown in [fig. 295]. The resulting mechanical actions are easily determined by remembering that the magnetic lines act like elastic cords tending to shorten themselves. There is in fact a tension along the magnetic lines and a pressure at right angles to both, proportional at every point to the square of their density.