40— POLYCYCLIC SYSTEMS OF CURRENT 

 DISTRIBUTION. 



By H. Bohle, M.V.D.E., M.I.E.E., Professor of Electro- 

 technics AT South African College. 



When electrical energy has to be transmitted over great distances 

 or distributed over large areas, high pressure alternating current is 

 used in order to limit the expenditure in copper. If the energy is 

 used solely for power purposes, a low frequency current is prefer- 

 able, since the construction of motors and rotary converters is more 

 rational for periodicities below 40 than for those above it. For 

 pure lighting networks, however, the frequency should be 50 or 

 more, as arc lamps do not burn satisfactorily on a lower frequency. 



For mixed lighting and power plants, we usually effect a com- 

 promise, i.e., we choose a frequency of about 50, which is ideal 

 neither for lighting nor for power. 



As regards the number of phases, it is well known that polyphase 

 rotaries and motors are superior in starting and running qualities than 

 .single phase machines, and less expensive to manufacture. The 

 pressure regulation, however, is simpler for single phase than for 

 polyphase currents. One is further justified in allowing a greater 

 pressure drop in power than in lighting circuits, the drop being 

 much less noticeable in the former case than in the latter. Conse- 

 ■quently, if we have a mixed lighting and powder network, the allow- 

 able drop is soon reached, and the expenditure in copper is larger 

 than for a pure power plant of the same total magnitude. 



It is further necessary, on account of the low voltage of the 

 present day incandescent lamp, to employ a pressure below 300 volts 

 for lighting circuits, while it is allowed to run motors from circuits 

 up to 500 volts. It is therefore obvious that the quantity of copper 

 required for a mixed circuit is greater than for a pure power netw'ork 

 ■of the same magnitude. 



The disadvantages of a mixed plant are overcome by having 

 separate plants for lighting and for power purposes ; for instance, by 

 employing two phase or three phase currents of 25 periods for power 

 •circuits, and single phase current of 60 or more for lighting networks. 

 This, however, increases .the capital outlay, and complicates the 

 switching arrangements. 



A more rational solution of this intricate problem is found in a 

 system which transmits at the same time currents of different 

 frequencies and phases through the same network. 



Suppose we have two sine currents of the frequencies fi and fi, 

 produced by sine E.M.F.s of similar periodicities, then the current 

 of frequency fi performs work with the E.M.F. of the same fre- 

 quency, but not with the E.M.F. of frequency fs. If one current, 

 say ii, varies, its E.M.F. will also change, but not the other E.M.F., 

 ;as long as is remains constant, because the two currents are absolutely 



