diameters of conductor. For spans of moderate length, say under 1,000 ft., 1 damper at each end of the span is required.Comparative 24-hr. vibration records are shown in Fig. 17 with supplementary data in Table V, for a damped and an undamped span of 4 sizes of A.C.S.R. at Royse City, including wind direction and velocity recorded at one end of the span. It is evident that the dampers suppressed practically all visible vibra tion; also that the energy input from the wind is directly proportional to the length of the span. In long spans better damping is obtained with 2 dampers at each end, while 3 or more may be required on extremely long spans.A damper cannot act effectively until the conduc tor vibration acquires a certain amplitude. This is manifested by a slight quiver which can be felt but barely seen. The 24-hr. charts, however, do not show this clearly. Recently a series of records from a free and a damped span of 4/0 A.C.S.R. was obtained by engineers of the Idaho Power Company using a carbon pile oscillograph; Fig. 18 shows 2 of their oscillograph records. The ripple in the damped cable is scarcely visible even at the enlarged scale.Many tests on Stockbridge dampers have been made to determine the best dimensions and point of attachment to the conductor. The dampers are not critical either as regards size or spacing-a variation of 25 per cent in any factor will not seriously affect performance. Fig. 19 shows the style of Stockbridge damper used in this investigation. To avoid corona discharge this design embodies cylindrical weights which shield the damper cable, and a clamp having no sharp corners.
ARMOR RODSArmor rods are primarily reinforcement but they also are effective as dampers. Numerous compara tive records have indicated that they reduce vibra tion amplitude by 10 to 20 per cent. Armor rods reduce stresses by distributing the bending from vibration, reinforce the cable at the point where the stresses are greatest, and furnish valuable protection against flashover burns.
CONCLUSIONS
Principal findings of the field tests described in this article may be summarized as follows :1. Theoretical formulas (loc. cit.) expressing relationship between frequency, loop length, tension, and weight of a vibrating conductor are in good agreement With field observations. (loc. cit.) for the frequency of a vibrating cable is substantially correct.
The Relf and Ower formula3. Any suspended cable will vibrate when conditions are favorable.4. Any feasible reduction in tension will not prevent vibration.5. Modifications in shape of cable cross section and stranding have some beneficial effect in reducing amplitude, but the results do not support the idea that a vibrationless cable can be evolved.
IN THE FIELD of education for electrical engineering both the teacher and the practising engineer are vitally concerned;the teacher, because it is his life work, the practitioner because the young graduates are presumably trained to meet the specifications of his profession, to join its ranks, and to work beside him in the ...
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