440 ELECTRICAL MACHINERY 



soon, the short-circuited coil will be burned out. Some- 

 timss the short circuit occurs by a thin copper bridge being 

 spun over the mica insulation between the two copper 

 segments to which the ends of the coil are attached. Or 

 the insulation may have broken down on the armature 

 where the end connections of the coils cross each other. 



8. An open-circuited coil produces very vicious sparking 

 every time the coil passes under a brush. The commutator 

 bars to which the faulty coil is connected soon become 

 very badly burned and, if the machine is allowed to run any 

 considerable time with an open-circuited coil, the bars to 

 which it is attached burn away so badly that it becomes 

 necessary to turn down the whole commutator before 

 sparking can be prevented, even though the faulty coil 

 has been repaired. The method of locating a short-cir- 

 cuited or open-circuited coil will be described in the last 

 section of this Chapter. 



9. This trouble has been discussed on a preceding page. 

 The brushes may be tested for even spacing by measuring 

 accurately the distance around the commutator surface, 

 between brushes. If this proves to be uneven, the spacing 

 may be changed by rocking the brush holder on the brush 

 holder stud. 



10. The air gap may be accurately measured by a 

 tapered steel wedge. This wedge is chalked and then 

 pushed into the air gap as far as it will go in a direction 

 parallel to the shaft. The pole rubs the chalk off and so 

 the distance the wedge was inserted can be easily deter- 

 mined and its width at this point measured by a microm- 

 eter caliper. This distance must be practically the same 

 for all poles; if it is not, a thin piece of sheet iron may be 

 inserted between the poles and yoke, where the air gap is 

 too long. This is generally called " shimming-up the pole." 



11. When discussing the subject of commutation, it 

 was shown that when the coefficient of self-induction of 

 the armature coil was too high or the width of the brush 



