OPERATION AND CARE OF ELECTRIC MACHINERY 437 



A. Sparking at 

 the com- 

 mutator 



1. Rough surface 



2. Brushes in the wrong position 



3. Insufficient brush tension 



4. Poorly fitted brushes 



5. Armature overloaded 



6. Vibration of the machine 



7. Short-circuited coil 



8. Open-circuited coil 



9. Unequal brush spacing 



10. Unequal air gaps 



11. Poor design 



The motor is then run at a low speed and the surface 

 first smoothed by coarse sandpaper held on the commuta- 

 tor surface by a block of wood, the face of which is perfectly 

 flat. The sandpaper is moved slowly back and forth across 

 the commutator as it revolves and this process is continued 

 until all rough spots have been removed. Then finer sand- 

 paper may be used in a similar fashion to polish the surface 

 and, finally a commutator stone should be used, or else 

 fine sandpaper may be used with a small quantity of machine 



