SYNCHRONOUS GENERATORS 331 



wound coils with the exception that the insulation between turns 

 consists of varnish, paper, asbestos, etc. It is evident that this 

 type of coil will not only stand much greater vertical forces, but 

 also, on account of the high moment of inertia of this flat strip, it 

 is better able to resist the horizontal component of the centrifugal 

 force. In extreme cases it is necessary, however, even with strip 

 winding to place brackets between the field coils to overcome this 

 tendency toward lateral distortions of the coil, as shown in Fig. 

 192. Means should also be provided to thoroughly fasten the 

 connections between the coils, and prevent them from working 

 loose, due to the strains imposed by the centrifugal force. 



The bare outside edge of the copper strip is exposed to the 

 direct fanning action of the rotor, and since the temperature drop 

 in the copper itself is negligible, that is, for the widths of strip 

 ordinarily used, the heating of the coils is due almost entirely to 

 surface drop. As a result, a much higher current density can be 

 used than would be permissible with the wire-wound field. 



The exciter current is conveyed to the revolving field through 

 two collector rings mounted on the shaft of the machine. 



According to the A.I.E.E. rules field windings for A.C. gen- 

 erators must withstand a one-minute test voltage of a value ten 

 times that of the exciter voltage; but in no case less than 1500 

 volts nor more than 3500 volts. 



Shaft. Shafts are, as a rule, furnished with water-wheel- 

 driven generators and provided for couplings to be connected 

 to the water-wheel shaft. Occasionally one single piece shaft is 

 used for mounting both the water-wheel runner and the generator 

 field. 



Provision is often made for moving the frame along the shaft 

 for convenience in repairing the windings. With the construction 

 shown in Fig. 193, this, of course, means an extra long and con- 

 sequently larger and more expensive shaft, and in many cases the 

 advantages are hardly worth the extra cost. With the base con- 

 struction shown in Fig. 194 a movement of the armature frame is 

 obtained without the additional expense of a heavier shaft and 

 sometimes also larger bearings. 



Bearings. The bearings of horizontal units are ordinarily of 

 the self-aligning pedestal type arranged for oil ring lubrication. 

 In large bearings, particularly for high-speed service, it often 

 becomes necessary to provide artificial water cooling for carrying 



