122 



ARMATURE CONSTRUCTION 



commutator. In these cases, where practicable, it is preferred to 

 mount the commutator spider upon an extension of the armature 

 spider, rather than upon the shaft. 



It is sometimes customary to construct the end V clamping 

 ring in several sections, each of which is separately bolted on to 

 the spider, as this arrangement enables any single faulty bar to be 

 repaired and replaced without disturbing the rest of the commu- 

 tator. This is only to be recommended for large commutators, but 

 even in these cases it is not now often used. 



Fig. 139 illustrates a commutator of this type by Messrs 

 Siemens & Halske. 



FIG. 139. Commutator for 110 k.w. 500 volt. Dynamo. Siemens & Halske. 



For small commutators the construction shown in Fig. 140 is 

 suitable. This construction is arranged to provide for good 

 ventilation on the inside of the commutator, and to this end the 

 internal surface is left exposed and the end plates left as open as 

 practicable. 



In moderate and large-sized commutators it is standard practice 

 to employ rolled or drawn copper for the segments, with a radial 

 lug of copper strip fixed at the back end of each segment for con- 

 necting it on to the ends of the armature coils. 



The lug is commonly fixed as shown in Fig. 141 ; the segment 

 is drilled with three or more holes A, and one side milled away to 

 a depth equal to the thickness of the strip, which is held on by 

 rivets in the holes A and then brazed or sweated un. 



