126 ARMATURE CONSTRUCTION 



COMMUTATORS FOR HIGH SPEEDS. 



In ordinary dynamos the linear speed at the periphery of the 

 commutator does not exceed 20 metres per second, and is prefer- 

 ably less than 14 metres per second; but with high speed 

 dynamos, such as are used for direct coupling to steam turbines, 

 the peripheral speed may necessarily be as high as 35 or 40 

 metres per second. 



To keep down the peripheral speed it is necessary to employ 

 small diameters for commutators, but to get a sufficient number of 

 segments in the periphery and also to obtain a well-proportioned 

 commutator with sufficient collecting and radiating surface, such 

 diameters have to be employed, with high speed dynamos, as lead 

 to peripheral speeds as high as those just mentioned. 



The characteristics of high speed commutators are the small 

 diameters and great axial lengths. The high centrifugal forces on 

 the segments, incident to the high speed, render inadequate the 

 ordinary constructions as outlined in the previous part of this 

 chapter. Consequently, special constructions have been necessary, 

 of which that shown in Fig. 148 is typical, and the general lines 

 on which this is built are now becoming standard for high speeds. 



Instead of the ordinary V clamping rings, the segments are 

 retained by stout steel rings shrunk on the outside of the 

 commutator. 



The process of building such a commutator is as follows: 

 The segments are . first assembled with the mica pieces between 

 them, in the manner described earlier in this chapter, and held 

 together with a few turns of wire or rope. The steel rings are 

 then shrunk on over the segments, with a layer of mica to 

 insulate them from the segments. The assembly is now put on a 

 boring mill and the interior machined, the ends being bored taper, 

 as seen in Fig. 148. 



The commutator shown is built up on a sleeve ; but it is 

 common practice, especially with smaller commutators, to bed 

 the commutator direct on the shaft, a taper being turned on the 

 shaft at the end nearest the armature, to fit the taper to which 

 the interior of the commutator is bored. In the latter case the 

 shaft, or, in Fig. 148, the metal sleeve, is covered with a sleeve 

 of mica. 



The commutator is held up against the taper at the armature 

 end by means of a double taper collar fitting the taper bore on 

 the inside of the commutator at the other end. These collars are 



