OF ELECTRICAL LABORATOKY, 



FACULTY OP APPLIED SCIENCE. 



OPEN-CIRCUITS, SHORT-CIRCUITS, ETC. 9 



dropped over a portion of the armature coil after the 

 coils have been placed in their slots. The yoke and 

 armature form an alternating current transformer, 

 with the yoke winding as primary and the armature 

 coil as secondary. If there is a short circuited turn, 

 layer, or coil in the armature, the magnetizing current 

 in the yoke winding rises. If the current is maintained 

 a short time, the insulation on the short circuited section 

 will warm up appreciably, or burn sufficiently to indi- 

 cate the defective coil. 



On larger size alternator armatures, tests may be 

 made for short circuits by passing alternating current 

 through the armature coil itself. In this case it is 

 usually necessary to increase the reactance of the coil 

 by placing a magnetic bridge over its armature slots 

 after it has been assembled in the core. 



The above tests may be made with the apparatus 

 at rest, and these faults can be more readily corrected 

 when apparatus is being wound, with a resulting saving 

 in time and cost. However, the test may be equally 

 well employed after the apparatus is completed. 



Practically all direct current machines are delivered 

 to the Testing Department unassembled, with few 

 exceptions. 



To assemble a machine, the field spools are first 

 mounted upon the frame and connected up in accordance 

 with the connection print which is furnished for each 

 machine. As soon as this is done, the windings are 

 tested electrically for resistance and insulation; the 

 polarity of the poles is also tested by exciting the field 

 coils. These tests check the assembly of spools and their 

 position upon the frame. 



In testing field coils for polarity, all field windings 

 must be tested separately to ascertain if the series and 

 shunt commutating pole windings are wound and 

 assembled correctly. Polarity may be tested by means 

 of a compass, in which case the compass must not be 

 carried too near to the pole, otherwise it may be de- 

 magnetized or even reversed. To test for opposite 

 polarity of alternate poles, two pole tips may be bridged 

 with a piece of soft iron. If the polarity of the poles 



