326 



ELECTRICAL EQUIPMENT 



trie strength, this being especially important for high potentials. 

 After being tested the coils are inserted in the armature slots in an 



armor of horn fiber or mica, 

 and retaining wedges of wood 

 are dovetailed into the sides 

 of the slots near the top. Ac- 

 cording to the A.I.E.E. rules 

 the insulation should be such 

 that the winding will with- 

 stand a test voltage for one 

 minute continuously of twice 

 the normal voltage plus 1000 

 volts. The frequency of the 

 testing circuit shall not be less 

 than the rated frequency of 

 the generator. 



Where heavy windings pro- 

 ject beyond the laminations, 

 an additional support is pro- 

 vided by means of an insu- 

 lated metal ring or brackets 

 to which the outer ends of 



the coils are fastened, thereby protecting them from mechanical 

 displacement or distortion due to magnetic disturbances caused 

 by violent fluctuations or short circuits. This bracing of the 

 armature winding is particularly necessary with single-phase 

 generators where the severe mechanical strains are imposed on 

 the armature windings by the pulsating flux. 



Flexible terminal leads provided with suitable connection joints 

 should be brought through the frame near the bottom. With 

 three-phase machines it is in many cases necessary to bring out the 

 neutral lead, as the machine may have to be operated on the four-, 

 wire principle or it may be desired to ground the neutral. 

 ! Field Spider. The rotating field generally consists of pole 

 pieces mounted on a cast-iron or steel ring connected to the hub 

 by means of arms of ample cross-section. For smaller and 

 medium-size machines the field centers may, however, consist of 

 built-up punchings to which the pole pieces are dovetailed. Where 

 shipping conditions permit, the field spider and the rim may be 

 cast in one piece, otherwise it must be split into sections. When 



FIG. 189. Lap or Barrel-type 

 Armature Winding. 



