GENERATING ELECTRICITY COMMERCIALLY 195 



MAIN CIKCtm 



factor is so large that the varying demands of customers tend to 

 average up and keep the load either constant or very nearly so. 



236. Compound Machines. A compound dynamo (Fig. 

 87) is one having two series of windings; one series winding, 

 around the part through which the main 

 current flows, and a shunt winding 

 through which a fraction of the main 

 current flows. Compound machines 

 are used in railway power plants, be- 

 cause of the violent fluctuations of load, 

 and in small lighting plants of low 

 diversity factor, where the consumers' 

 demands fluctuate widely. 



FIG. 87. Compound- 

 Wound Dynamo. 



237. Direct Connected Machine. 

 A direct connected dynamo is one 



which is driven by an engine without the use of a belt ; that 

 is, the armature shaft is connected to the engine shaft by 

 means of a flexible device; or the engine shaft is made extra 

 long with a bearing, and the armature is mounted on the 

 shaft. This device saves space, is quiet in operation, and 

 increases efficiency, since there is no loss due to transmission 

 of power by belts. 



238. Direct and Alternating Dynamos. While dynamos 

 vary in the manner of winding the fields and armatures as 

 described above, the most important difference between 

 the different types is in the kind of current generated. This 

 classification divides dynamos into the two types of direct 

 and alternating. An alternating current dynamo is similar 

 in its action to a direct current dynamo, except that in the 



