Page:Electricity (1912) Kapp.djvu/251

Rh current. The ratio between the two e.m.f.'s varies a little with the constructive details, but may be taken as about 2 to 3, so that if 500 volts is required at the D.C. side, an A.C. of about 330 volts must be supplied to the slip rings. Obviously this is far too low a pressure for economic transmission of electric power from the central station. To carry the thousands of kw. required for the supply of a town from the central station to the sub-stations by means of cables of moderate size and cost, we require pressures of 10,000 volts or more, so that the use of a transformer, T, becomes necessary. The use of this transformer has the incidental advantage that we obtain an easy and inexpensive way of subdividing the pressure between the outer wire if the distribution on the D.C. side is to be made on the three-wire system. We need only attach the zero, or.middle wire o, to the electrical centre of the secondary winding of the transformer as shown.

The efficiency of a converter set, including the transformer, is sensibly higher than that of a motor generator set. It generally reaches 92 per cent. In another respect the converter has also an advantage over the