Page:Popular Science Monthly Volume 45.djvu/636

616 each would have its own circuit, and, if it broke down, the whole dependent system would be idle until repairs were completed. One of the great aims of the company appears to be to insure the permanence and continuousness of their power service—which is, of course, of the utmost importance to manufacturers.

A remarkable method of construction—not, however, unique—is employed in the generators to secure means for direct coupling to the turbine shafts. These latter are vertical, and come up over one hundred and forty feet out of the wheel pits from the rotating water wheels, which make two hundred and fifty revolutions per minute. In order to obtain direct driving—that is, without the intervention of toothed or friction gearing, or belt or rope driving—the revolving portions of the generator are arranged to rotate in a horizontal instead of, as is usual, a vertical plane.

A dynamo of any type whatever consists, as is well known, essentially of two portions, one of which possesses motion with respect to the other, viz., the armature and the field magnets. Since the field magnets are almost invariably much heavier and much less compact than the armature, the latter is usually chosen as the moving part. In the case under discussion the contrary has been decided on, the armature being fixed and the field magnets rotating. This gives certain advantages in the matter of less complicated electrical connections and of dispensing with the armature's rubbing collectors altogether; it also gives the advantage much more important in this case than with smaller machines—that, since the revolving magnets are arranged on a ring and point inward, the attraction between them and the armature core tends toward neutralization of the strains of centrifugal force. The greatest advantage, however, attained by this method, and again one which is of far greater value in the present case than in ordinary practice, is the high degree of insulation possible with fixed armature coils and connections. The requirements that had to be met in the way of limiting the centrifugal strains were that the product of the sum of the weights of the revolving parts in pounds and the square of their velocities in feet per second should not exceed eleven hundred million. The weight of the moving parts of each dynamo was also limited to eighty thousand pounds, while the weight of the turbine and its shaft amounts to seventy-two thousand pounds.

This whole weight of seventy-six tons acts in one vertical line—i. e., that of the turbine shaft—and revolves two hundred and fifty times per minute. It would have been very difficult to construct thrust bearings to take up the whole of this strain, and a hydraulic balancing piston has been resorted to for supporting it. This device is simply a circular piston fast on the vertical turbine shaft, set in a vertical cylinder. The supporting force consists of