100 ALTERNATING CURRENTS 



voltage machines having armatures of small diameter. Increased 

 space for copper may be obtained by deepening the slots. If 

 the armature be the rotating member, the deepening of the slots 

 is limited by the contraction of the tooth necks, as shown in Fig. 

 100 (a). No such difficulty is encountered if the armature be 

 stationary, since the tooth necks increase in width with the deep- 

 ening of the slots, Fig. 100 (6). 



FIQ. 100. Effect of slot depth on the width of tooth necks in a rotor and in a 



stator. 



ALTERNATOR WINDINGS 



51. General Principles. The usual direct-current armature 

 generates alternating current, and if provided with properly 

 connected slip-rings, alternating current may be obtained from it. 

 On the other hand, only certain types of alternator windings can 

 be used for direct-current armatures. The ordinary direct- 

 current winding is a closed winding (see Vol. I, page 223), but 

 alternator windings may be either open or closed. 



The general principles which govern direct-current windings 

 hold also for windings of alternators. The span of each coil 

 must be approximately one pole pitch; that is, the two sides of 

 any coil must lie under adjacent poles. The coils must be so 

 connected that their electromotive forces add. 



Alternator windings are divided into several general classes. 

 There are single-layer and two-layer windings, usually made up 

 of former-wound coils. Windings may be either of the lap- 



1RTY OF ELEC 



FACULTY OF APPLIED SCILNCE. 



