324 ELECTRICAL EQUIPMENT 



speed on his generators and the regulation of his lines much more 

 than if the fluctuations were divided among a greater number of 

 generators and lines. So, for example, in a large system, what 

 would be 50 per cent load thrown on or off one generator if it were 

 feeding a separate customer would, perhaps be only 5 or 10 per cent 

 load on the entire system and neither speed nor voltage would be 

 materially affected. In general, it may, therefore, be said that 

 in many cases it is preferable to operate everything in parallel 

 and to have the governors on as many machines as feasible. This 

 naturally reduces the work of the governors, as a change in load 

 then only requires each governor to work through a small range, 

 allowing a more sensitive adjustment and less speed deviation 

 than would be the case if the system were divided up into sections 

 with different generators supplying individual loads. 



Mechanical Design. Revolving Field Type. Alternating cur- 

 rent generators are almost always of the revolving field type, this 

 construction being preferable as compared with the revolving 

 armature type. Besides relieving the high potential armature 

 winding from strains imposed by a centrifugal force, it gives an 

 increased space for the winding, which is of greatest importance. 

 Only two collector rings are required for handling the field cur- 

 rent, the energy and voltage of which is relatively small com- 

 pared to that which would have to be handled in the case of a 

 revolving armature generator of the same capacity. 



Method of Drive. With regard to the method of drive water - 

 wheel-driven generators are almost always of the direct connected 

 type, only the very smallest sizes being belt or rope driven. 



Horizontal or Vertical. Water-wheel-driven generators may 

 be either of the horizontal or vertical type, the latter being now 

 very extensively used in low-head developments where it becomes 

 desirable to place the generators above the highest flood level. 

 This arrangement requires less excavation, and obviates the neces- 

 sity for special construction to protect from flood water, which 

 would be necessary with horizontal units. In order to obtain 

 commercial speeds for direct connection to horizontal generators 

 it has been necessary for extreme low-head developments to put 

 a number of runners on the same shaft. Recent improvements in 

 the design of single runner turbines for low heads resulting in 

 increased speeds, as well as the comparatively low cost of vertical 

 generators operating at from one-third to one-half the speed of 



