'6-A] GENERAL STUDY. J 97 



to be discussed. The sources of these electromotive forces are 

 in principle several rigidly connected single-phase generators, 

 but in practice they are generator coils on a single armature. 

 The secondary coil of a transformer may be considered as a 

 generator coil. The currents from these sources may be utilized 

 separately as single-phase currents (as in lighting), or jointly as 

 polyphase currents (as in an induction motor). 



(0 (<0 () 



FIG. 2. Three-phase connections for generator or receiver circuits, a, Inde- 

 pendent circuits ; see 3a. b, Star- or ^-connected, c, Mesh- or delta- (A) 

 connected, d, T-connected. e, F-connected ; or, open delta. 



2. The load on a polyphase system is balanced when each 

 phase has an equal load with equal power factor. In a balanced 

 polyphase system the flow of energy is uniform, which is a bet- 

 ter and more general definition of such a system; (see Steinmetz, 

 Alternating Current Phenomena). In a single-phase system or 

 unbalanced polyphase system, the flow of energy is pulsating, 

 discussed further in I, Exp. 7-A. The torque is, accordingly, 

 pulsating in all single-phase machinery; whereas it is uniform in 

 polyphase motors and in polyphase generators on balanced load. 

 Furthermore, a polyphase induction motor on account of its 

 rotating field can be given a good starting torque, whereas a 

 single-phase induction motor has none in itself and has only a 

 small starting torque when auxiliary starting devices are used. 

 Polyphase machinery has a greater output than single phase for 

 a given size, or has a smaller size for a given output. These 

 features, together with the copper economy of 3-phase as com- 

 pared with single-phase transmission, all favor the use of poly- 

 phase systems ; see Appendix III. 



