~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.
[Illustration: 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.]
~Ques. What form of single phase system should be used where the transmission distance is considerable?~
Ans. The current should be transmitted at high pressure, a step down transformer being placed at each distribution center to reduce the pressure to the proper voltage to suit the service requirements as shown in fig. 2,146.
Thus, if 110 volt lamps be used on the three wire circuit, the pressure between the two outer wires would be 220 volts. A transformation ratio of say 10:1 would give 2,220 volts for the primary circuit. The current required for the primary with this ratio being only .1 that used in the secondary, a considerable saving is effected in the cost of the transmission line as must be evident.
With the high pressure alternator only one transformation of the current is needed, as shown at the distribution end.
In place of the high pressure alternator, a low pressure alternator could be used in connection with a step up transformer as shown in fig. 2,147, but there would be an extra loss due to the additional transformer, rendering the system less efficient than the one shown in fig. 2,146. Such an arrangement as shown in the fig. 2,147 might be justified in the case of a station having a low pressure alternator already in use and it should be desired to transmit a portion of the energy a considerable distance.