198 ELEMENTS OF ELECTRICAL ENGINEERING. 



be commercially economical in spite of the great loss of power in 

 the feeder, if the demand for power at the distant section of the 

 railway lasts but two or three hours each day, otherwise the use 

 of the booster should be looked upon as a temporary expedient, 

 inasmuch as a continuous loss of power in the feeder might war- 

 rant either the use of a much larger feeder or the installation of 

 the alternating-current long-distance transmission system which 

 is described in a subsequent volume of this text. 



Modifications of the connections shown in Figs. 112, nj and 

 119. It is sometimes considered desirable to leave the series 

 field windings of all the compound generators which operate in 

 parallel as a set, always in circuit, whether all of the generators 

 are delivering current or not. One reason for this may be best 

 explained by an example. 



Given two similar compound generators arranged for parallel 

 operation. Suppose that it is desired to provide a ten per cent, 

 increase of voltage at the station at full-load to compensate for a 

 ten per cent, line drop at full-load so as to provide for a constant 

 voltage at the end of the line, that is, at the lamps. Now if the 

 station output is half full-load value (equal to full-load on one 

 machine), it is not desirable to have the voltage more than five 

 per cent, in excess of the zero-load value. But if this half of full 

 station load is carried by one generator and if the series field coil 

 of the other is disconnected, then the full current output will flow 

 through the series field coil of the generator which is in action, 

 and this generator will give its normal full-load voltage which is 

 ten per cent, in excess of the zero-load voltage. If, on the other 

 hand, both series field coils are left connected, then only one half 

 of the current output of the station (and of the one generator) 

 flows through each series field coil and the voltage does not rise to 

 no per cent, of the zero-load value, nor indeed to 105 per cent, 

 for that matter. See problems on Chapter VII. 



Another advantage of leaving all the series field coils perma- 

 nently in circuit is that the field excitation due to these coils always 

 causes each generator to build up in the right direction at start- 



