Current Distribution. 373 



In Fig. 6, A represents the primary, B the sub-station. In A, 

 two kinds of currents are generated and transformed, from B, two 

 networks are supplied, the transmission of both currents taking place 

 through the same mains. At the primary station, we have three single 

 phase transformers, with two primary windings — one for the principal 

 three phase, the other for the superposed single phase current — and 

 one secondary winding carrying both currents. The coils of the 

 three phase primary circuit are joined in star form, those of the 

 single phase circuit in series. In this way, we induce both kinds of 

 currents in the secondary winding,* and the superposed current is 

 carried along with the three phase currents, its return circuit being 

 formed by the neutral wire. 



The receiving, or sub-station, may be constructed in a similar 

 manner, but with one primary and two secondaries. Instead of 

 single phase transformers, we may employ one three phase trans- 

 former with a common magnetic return, such as is shown in Fig. 7. 

 The magnetic return is required for the fluxes produced by the super- 

 posed single phase currents, which pass through the other three 

 columns in the same direction. The transformer serves at the same 

 time for transforming both kinds of currents, the original three phase 

 current being used for motor, the superposed single phase for lighting 

 circuits. 



It is not absolutely necessary to have a special return for the 

 superposed current, as is seen from Fig. 8. The superposed current 

 enters here by the neutral points, and takes its course as follows : — 

 Generator d, transformer T3', busbars BB, transformer T,", 

 generator d. The transformers transform not only the original, but 

 also the superposed single phase current, and the two sets of mains, 

 Si and S2, serve as going and returning conductors respectively for 

 the superposed current. The separation of the two currents is also 

 illustrated in the same figure; it is effected by means of single phase 

 transformers possessing one primary and two secondary windings. 



Fig. 9 shows how four phase and a superposed single phase 

 currents are transmitted over the same lines and separated at the 

 other end. The figure is self-explanatory. 



Simultaneous Production and Transformation of Two 

 Currents of Different Frequencies. 



It is possible not only to set up independent E.M.F.s. of various 

 periodicities in transformers, but we can also produce them in the 

 same generator, for which purpose v/e require a machine with tv. o 

 independent field systems. 



Suppose we require three phase currents of 25 and single phase 

 current of 75 periods per second, and that the combination generator 

 is coupled to a prime mover running at 187.5 revolutions per minute, 

 the number of poles of the field system producing three phase 

 currents must then be equal to 16, that setting up single phase 

 currents 48. If the poles are arranged as shown in Fig. ro, the 



