Ans. The pressure at the alternator can be doubled, which means, for a given number of lamps, that the current is reduced to half, the permissible drop may be doubled, the resistance of the wires quadrupled, and their cost reduced nearly 75 per cent.
Fig. 2,144.—100,000 volt "Milliken" towers with one circuit strung (Southern Power Co.). These towers are mounted on metal stubs sunk 6 feet in the ground. Where the angle of the line is over 15 degrees, however, these stubs are weighted with rock and concrete, and where an angle of over 30 degrees occurs, two and sometimes three towers are used for making the turn. The weight of the standard "Milliken" tower is 3,080 lbs., and its height from the ground to peak is 51 feet. The towers are spaced to average eight to a mile and a strain tower weighing 4,250 lbs. is used every mile. For particularly long spans a special heavy tower weighing 6,000 lbs. is used. The circuits are transposed every 30 miles. Multiple disc insulators are used, four discs being used to suspend each conductor from standard towers and ten discs to each conductor on strain towers. The standard span is 600 feet, sag 11 ft at 50° Fahr.
Ques. What modification of circuit A (fig. 2,143), should be made to allow for unbalancing in the three wire circuit?
Ans. An auto-transformer or "balance coil" as it is sometimes called should be used as at B.
This is a very desirable method of balancing when the ratio of transformation is not too large.
Ques. For what service would the system shown in fig. 2,143 be suitable?
Ans. For short distance transmission, as for instance, in the case of an isolated plant because of the low pressure at which the current is generated.
The standard voltages of low pressure alternators are 400, 480, and 600 volts.
