180 ELEMENTS OF ELECTRICAL ENGINEERING. 



pounding noise is unavoidable and legitimate, but a whip-like 

 crack as the joint passes over the pulley indicates a loose lacing. 



5. A sharp screeching noise, like that produced by rubbing 

 the hand over a varnished table, indicates the slipping of the belt 

 due either to overload or to insufficient belt tension. 



6. A squeaking or hissing noise is produced by the brushes. 

 When this becomes very prominent, a little vaseline is needed. 

 The source of this noise may be determined by pressing the 

 ringer against one brush at a time, or, if there is more than one 

 brush in each set, one brush at a time may be lifted from the 

 commutator. The chattering of the brushes is indicated by 

 noise intermediate between a rattle and a screech. 



VII. Symptoms of trouble peculiar to a generator are those 

 which have to do with its voltage. The generator may not build 

 up at all, it may develop only a weak electromotive force, or it 

 may become reversed. 



I. Failure to build up. This is perhaps the trouble most fre- 

 quently encountered in the starting of a generator. The follow- 

 ing brief statements of the causes of failure to build up are based 

 upon the full discussion in Art. 44. 



(a) Excessive resistance in the field circuit due to a poor con- 

 tact or an actual break. The brush contacts often have an 

 excessively high resistance when a generator is first started, and 

 a momentary pressure of the fingers on the brush or brushes may 

 enable the machine to build up. 



(b) A short circuit in the machine or in the external circuit 

 prevents a shunt generator from building up. 



(c) Residual magnetism too weak. In this case the field must 

 be disconnected from the armature and excited for a moment from 

 an outside source. 



(d) Reversed connections or reversed direction of rotation. 

 (See Art. 44.) 



(e) Brushes not in proper position. In all of the discussion in 

 this treatise the brushes are represented as in their proper posi- 

 tions when they are nearly midway between the field poles. This 



