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 I1, the two phase current I, and the resistance of one main R, we have 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 1.5' 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 ; for, as we increase the losses in the four mains 1.5 fold, by superposing a single phase current of half the power, this same total loss would be caused by an increase in the two phase current of a pure two phase system equal to or 23 per cent. more power with the polycyclic system. A three phase single phase system is not so advantageous, and the saving is only about 6 per cent. instead of 23, so that it would seem advisable to convert a three phase current into a two phase current before distribution by means of the well known Scott system. This has been done in Fig. 20. For, since a three phase, three wire system requires a quantity of copper equal to 75 per cent. of that wanted for a single phase plant, all conditions being equal, the saving in material is assuming that the superposed power is equal to half the three phase power. ADVANTAGES OF A POLYCYCLIC SYSTEM. I. The desired frequencies for lighting and power circuits are obtained. 2. The regulation of the lighting pressure is simple. 3. The P.D. for motor circuits is raised, and copper is therefore saved. 4. The power transmitted with a polycyclic system over mains with a given cross-section is greater than that from a pure single phase plant. The advantages which the inventors Arnold- Bragstad-La Cour claim for their system, have been confirmed by a series of tests by Dr. F. Marguerre. * See "Experimentelle Untersuchungen am Polycyklischen Stromverteilungs system Arnold-Bragstad-La Cour." F. Enke, Stuttgart. |