36 ALTERNATING CURRENTS 



17. Comparison of Single-, Two=, and Three- 

 phase Systems 



The relative advantages and disadvantages of the single-, two-, 

 and three-phase systems of power generation and transmission may 

 be briefly summarized as follows. The single-phase system is the 

 simplest, since it requires only two conductors, with a correspondingly 

 simple arrangement of switch-gear. Where the number of conductors 

 is a matter of importance as in connection with electric tramways 

 and railways the single-phase system possesses an important advan- 

 tage over its polyphase rivals. On the other hand, polyphase generators 

 are, for a given speed and power, lighter and cheaper than single- 

 phase ones ; induction motors and rotary converters of the polyphase 

 type are also superior to single-phase machines.* The cost of the 

 conductors to transmit a given amount of power with a given strain 

 on the insulation is least in the case of a three-phase system, and it 

 is for this reason that the three-phase system has been much more 

 generally adopted than the two-phase one. The latter has been 

 mostly used to replace existing old single-phase systems, in which 

 case the single-phase concentric mains could be utilized; a three- 

 phase system, requiring three-core or triple concentric mains, would 

 have involved the scrapping of the existing system of single-phase 

 mains. 



18. Simple Alternating Wave of Magnetic 



Flux 



In 15 we explained how a rotating field at a given point of 

 space the common centre of suitably arranged coils may be pro- 

 duced by supplying the coils with polyphase currents. We now 

 proceed to the study of the waves of magnetic flux which are pro- 

 duced in alternating-current machines. 



Numerous types of alternating-current machinery consist essen- 

 tially of two coaxial iron cylinders separated from each other by a 

 narrow gap the air-gap as shown in Fig. 27. One of the cylindrical 

 cores generally the outer is stationary, and forms the stator of the 

 machine ; while the other is maintained in rotation, and forms the 

 rotor. The surfaces of the cores may be either continuous, as in 

 Fig. 27, or more or less discontinuous. For the sake of simplicity, 

 we shall assume that each surface is continuous. By means of 

 suitable windings, embedded in the cores, a system of magnetomotive 



* The performance of a single-phase rotary converter is so poor that this type of 

 machine is never used in modern practice. 



