DIRECT-CURRENT MACHINERY 



155 



98. Double Windings. If space is left between adjacent 

 coils of a multiple winding, a second winding may be placed on 

 the same core. The second winding may be entirely separate 

 from the first, that is, each of the two windings closed upon itself; 

 or after passing through the first winding the circuit enters the 

 second and after passing through the second reenters the first. 

 In the first case the winding is duplex doubly reentrant and in the 

 second case duplex singly reentrant. Duplex multiple windings 

 have twice as many circuits in multiple between terminals as there 

 are poles. Such windings are suitable for large low-voltage ma- 

 chines used in electrolytic work. The brushes must be wide 

 enough to collect current from both sections of the winding at the 

 same time. Fig. 118 shows a duplex doubly reentrant winding 

 for a six-pole machine with 72 conductors and 36 slots. 



Similarly the series winding may be made double by placing a 

 second winding in alternate slots and connecting it to alternate 

 commutator bars. The second winding is in multiple with the 

 first and there are four paths in multiple between terminals. 



99. Commutator. The commutator is one of the most im- 

 portant parts of a direct-current machine. It consists of a number 



.. 



I /Micanite 

 I/ "V'Ring 



FIG. 119. Commutator. 



of bars of hard-drawn copper, insulated from one another by thin 

 sheets of mica or other insulating material, and built up into the 

 form of a cylinder. (Fig. 119.) The bars are held together by a 

 cast-iron spider from which they are insulated by micanite " V " 

 rings. The terminals of the coils forming the armature winding are 

 connected to the bars either directly by soldering them into slots in 



