PARTS. OF A DYNAMO-ELECTRIC MACHINE 49 



the machine is placed. As the load of 40 kw. increases 

 the armature temperature only 30 C., the machine can 

 evidently carry more load with safety, and as 50 kw. 

 load overheated it, the safe capacity must be somewhere 

 between 40 kw. and 50 kw. 



One way of keeping down the temperature rise of a 

 machine is by continually forcing cool air through it, thus 

 carrying off the heat; or we may say that the temperature 

 of the machine may be kept down by properly ventilating 

 it. Heat is generated in the armature core by the hysteresis 

 and eddy currents and it is necessary to carry off this heat 

 to keep down the core temperature. For this purpose 

 ventilating ducts are built into the armature core. As the 

 laminations are being assembled, spacers are introduced 

 about every three inches of core. These spacers are placed 

 radially so that air passages, about J" wide, are formed 

 through which air can circulate from the inside periphery 

 of the core to the outside. As the armature revolves, 

 cool air is drawn in through the ends of the core by the 

 spider and is thrown by centrifugal force through these 

 ventilating ducts out nihilist the pole faces, etc. Fig. 24 

 shows how these ducts are placed in an armature core, 

 and the paths taken by the air as it passes through them 

 and Fig. 25 shows a partially assembled core, in which 

 the air ducts may be seen; on the top of the core may 

 be seen one of the spacing laminations which has been 

 specially formed with radial ridges in it about J" high. 

 These ridges serve to form the air ducts as the core is 

 built up. 



15. Commutator. The e.m.f. generated by a coil of 

 wire as it rota Irs in n magnetic field is an alternating one, 

 i.e., the e.m.f. is first in one direction and then in the 

 opposite direction, a complete reversal taking place as a coil 

 side moves by one pair of poles. Now even if an armature 

 winding is made up of a great many coils, connected together 

 in any fashion whatever, the e.m.f. generated in this winding 



