PRACTICAL OPERATION OF DYNAMOS. 1/9 



sive voltage, or by the short-circuiting of a portion of the field 

 winding. 



3. Moisture in the field coils. See discussion of cause (3) of 

 armature heating. 



V. Heating of bearings. This may be due to : 



1. Conduction of heat to the bearings by another and hotter 

 part of the dynamo. 



2. Any one of the common mechanical defects ; lack of oil ; 

 grit ; roughness of shaft ; too tight a fit between shaft and bear- 

 ing ; crooked shaft ; poor alignment of bearings ; end thrust of 

 collar, or pulley, or shoulder on shaft against the bearing ; or 

 excessive belt tension. 



3. Side pull of field magnet upon the armature when the arma- 

 ture is nearer to one pole-piece than to the other or others. 



VI. Noise. The sound produced by almost any machine is 

 perhaps the best indication to an experienced attendant of its 

 condition. The legitimate sounds of a dynamo are the humming 

 sounds due chiefly to the armature teeth as they pass the tips of 

 the pole pieces, and to the slight movement of the iron parts, 

 especially of the laminations, as they are magnetized and demag- 

 netized by pulsating or alternating currents. The abnormal 

 noises are the following : 



1. The fluttering noise accompanied by more or less violent 

 vibration due to unbalanced armature or pulley. 



2. The noise due to the striking of the armature against the 

 pole pieces, or to the striking of the shaft collar, or pulley, or 

 belt against the bearings. At low speed this is a scraping noise, 

 and at full speed it is like No. i. The striking of the armature 

 against the pole pieces may be detected by the scraping noise at 

 low speed, by the abrasion of the armature surface, or by look- 

 ing through the gap space in front of the various pole pieces. 



3. Rattling noise due to loose parts, such as screws. This 

 noise is most perceptible at moderately low speeds. 



4. Flapping of belt or the pounding of the laced or cemented 

 joint against the pulley. A certain amount of flapping and 



