For instance, if four customers having 20 lamps each were supplied from a single transformer, the average load would be about 8 lamps, and at most not over 10 or 15 lamps, and a transformer carrying 30 to 35 lamps at over load would probably be sufficient. A 1,500 watt transformer would therefore be larger than necessary. At 3 per cent. core loss, this gives a constant loss of 45 watts, while the average load of 8 lamps for 3 hours per day gives a useful output of 60 watts, or an all year efficiency of nearly 60 per cent., while a 1,000 watt transformer would give an all year efficiency of 67 per cent.
For long distance transmission lines, the voltage at the alternator is increased by passing the current through a step up transformer, thus transmitting it at very high pressure, and reducing the voltage at the points of distribution by step down transformers as in fig. 2,141.
Fig. 2,141.—Diagram illustrating the use of step up and step down transformers on long distance transmission lines. The saving in copper is considerable by employing extra high voltages on lines of moderate or great length as indicated by the relative sizes of wire.
Ques. In practice, would such a system as shown in fig. 2,141 be used?
Ans. If the greatest economy in copper were aimed at, a three phase system would be used.
The purpose of fig. 2,141 is to show the importance of the transformer in giving a flexibility of voltage, by which the cost of the line is reduced to a minimum.
Ques. Does the saving indicated in fig. 2,141 represent a net gain?
Ans. No. The reduction in cost of the transmission is partly offset by the cost of the transformers as well as by transformer losses and the higher insulation requirements.