SYNCHRONOUS GENERATORS 325 



horizontal generators, have made the construction of vertical 

 units for extreme low heads much simpler than horizontal units. 

 The draft-tube excavation required is, of course, much less and 

 involves less expense. For high-head developments with impulse 

 wheels, horizontal units are of course preferable. 



Stator Frame. The main function of the stationary armature 

 frame is to support the punchings and it should, therefore, be of a 

 rigid construction so as to prevent any sag of the punchings due 

 their weight and an unbalanced magnetic pull. It is usually of a 

 box type construction, and for smaller sizes they are, as a rule, 

 made in one piece, while for larger units they are split so as to 

 facilitate an easy handling and shipping. A number of openings 

 are provided for ventilation, a subject which is treated more in 

 detail in the latter part of this section. 



The core consists of sheet-iron laminations carefully annealed 

 and treated so as to minimize both hysteresis and eddy-current 

 losses. The punchings are stacked together so that the lamina- 

 tions overlap each other. They are held rigidly in place by heavy 

 steel clamping fingers, air circulation being provided for by air 

 ducts formed by spacing blocks inserted at frequent intervals 

 between the laminations. The outer circumference is dove- 

 tailed for fastening to the frame, while the slots for the windings 

 are punched at the inner circumference, the slots generally being 

 of the open type so as to permit the use of form-wound coils, which 

 can easily be removed and replaced in case of damage. With the 

 open slot construction means must be provided to guard against 

 the generation of eddy currents, due to the unequal flux distribu- 

 tion. This is done by subdividing the individual conductors 

 either by using several wires in parallel or, in the case of con- 

 ductors of large cross-section, by using pressed cable; the eddy 

 currents are thus reduced to a negligible quantity. 



Armature Winding. The armature winding is generally of the 

 lap or barrel-wound type, Fig. 189, and the chain winding has 

 been practically abandoned as it requires coils of different shapes, 

 especially with the widely distributed windings which are used in 

 modern machines. 



The coils should be taped and treated with an impregnating 

 compound, the number of layers and dippings being determined 

 by the operating voltage. The materials used should be very 

 carefully selected to avoid deterioration or diminution of the dielec- 



