Fig. 2,145.—View of a typical isolated plant. The illustration represents an electric lighting plant on a farm showing the lighting of the dwelling, barn, tool house and pump house. The installation consists of a low voltage dynamo with gas engine drive and storage battery together with the necessary auxiliary apparatus.
Ques. In practice are single phase alternators used as indicated in fig. 2,143?
Ans. Alternators are wound for one, two or three phases. Three phase machines are more commonly supplied and in many cases it will pay to install them in preference to single phase, even if they be operated single phase temporarily.
For a given output, three phase machines are smaller than single phase and the single phase load can usually be approximately balanced between the three phases. Moreover, if a three phase machine be installed, polyphase current will be available in case it may be necessary to operate polyphase motors at some future time.
Standard three phase alternators will carry about 70 per cent. of their rated kilowatt output when operated single phase, with the same temperature rise.
Ques. How are three phase alternators used for single phase circuits?
Ans. The single phase circuit is connected to any two of the three phase terminal leads.
Fig. 2,146.—Diagram showing arrangement of single phase system for two wire transmission and three wire distribution, where the transmission distance is considerable. In order to reduce the cost of the transmission line, the current must be transmitted at high pressure; this necessitates the use of a step down transformer at the distributing center as shown in the illustration.