Current Distribution. 375 



that two positive peaks cannot occur at the same time, as shown in 

 Fig. 13. A similar reduction in the amplitude of the resultant 

 E.M.F. is obtained by making the frequency of one system an even 

 multiple of that of the other, as represented in Fig. 19. 



As regards the modified two phase single phase system, it is 

 always advisable to earth the middle point of the winding of the 

 superposed current, as otherwise — through faulty insulation of the 

 two phase winding — we may obtain too high a P.D. between the 

 lighting mains and earth. 



Pressure Drop in a Polycyclic System. 



Consider a two phase single phase system with an independent 

 primary circuit, as shown in Fig. 18, in which the R.M.S. P.D. of 

 the single phase generator is 



If we assume the same percentage wattloss for both systems, and call 

 the single phase current Ii, the two phase current L, and the 

 resistance of one main R, we have 



4I22R Ii^R 



l-f— X ioo = .ji-j- X 100, 

 2K2I2 Eili 



2I2 II XT„„. T7. _ E 



or — -=^^. Now Ei = — ^, hence 



E2 El a/2' 



li= \/2l2, and Eili = E2l2 = - (2E2I2), 



i.e., the power of the superposed current is equal to half the power 

 of the two phase current. It shows, further, that without altering 

 the efficiency of the two phase system, or the R.M.S. P.D. between 

 any two conductors, we can transmit through the same mains 50 per 

 cent, more power with a polycyclic two phase single phase system 

 than with a pure single phase plant. Again, for the same total 

 power, the amount of copper required for a polycyclic system is only 



I 

 ^' 



or 66.7 per cent, of that wanted by an ordinary single phase circuit. 



If for a transmission of power the R.M.S. P.D. is given, it is 



found that with a two phase single phase polycyclic system we can 



transmit 23 per cent, more power than without a superposed current ; 



