164 ARMATURE CONSTRUCTION 



terminals. So long as the terminals are not connected to any 

 external closed circuit, the armature circuit remains " open." 



The windings of Class II. find their application frequently in 

 wound rotors for induction motors, and always in rotary converter 

 armatures (which are dealt with in the latter part of this chapter). 

 These windings consist of a continuous path through the conduc- 

 tors, which re-enters on itself, thus constituting a " closed circuit." 

 They are carried out, and follow practically the same laws, as 

 continuous-current windings, the difference being that instead of 

 the winding being connected to the commutator segments at a 

 large number of points, it is tapped at a few suitable points and 

 connected to the slip-rings for the terminals of the phases. 



In general, any of the continuous-current armature windings 

 may be employed for alternating-current work, but the special 

 considerations leading to the use of alternating currents, generally 

 make it necessary to abandon the styles of winding best suited to 

 continuous-current work, and to use windings specially adapted 

 to the conditions of alternating-current practice. 



Attention should be called to the fact that all the re-entrant 

 (or closed circuit) continuous-current windings must necessarily be 

 two-circuit or multiple-circuit windings, while alternating-current 

 armatures may, and generally do, have one-circuit windings, i.e. one 

 circuit per phase. From this it follows that any continuous- 

 current winding may be used for alternating-current work, but 

 an alternating-current winding cannot generally be used for con- 

 tinuous-current work ; in other words, the windings of alternating- 

 current armatures are essentially non-re-entrant (i.e. not closed 

 circuit) windings. Ke-entrant, i.e. closed, windings are, however, 

 the only windings which are applicable to alternating-continuous- 

 current commutating machines. 



We shall consider Classes I. and II. separately, and as practi- 

 cally all alternating-current generator armature windings are of 

 Class I., the greater part of this chapter is given up to the 

 consideration of this class. 



I. OPEN CIRCUIT WINDINGS. 



SINGLE-PHASE WINDINGS. 



We shall first deal with single-phase windings, and this will 

 lead into polyphase windings, as by superimposing a number of 

 single-phase windings on a given armature at regular and suitable 

 intervals we obtain a polyphase winding. In a simple single-phase 



