OPERATION AND CARE OF ELECTRIC MACHINERY 439 



removed from the field frame and swung in a lathe for 

 turning the commutator. 



2. If the brushes are in the wrong position they will 

 all spark, but the difficulty is easily overcome by manipu- 

 lation of the brush yoke rocker. On some machines, not 

 equipped with commutating poles, the brush position 

 must continually be changed as the load changes if spark- 

 less commutation is desired. 



3. The brush tension should be between one and one 

 and a half pounds per sq.in. of contact area. If the ten- 

 sion is not enough, the brush will not " follow " the commu- 

 tator and if the tension is too great, the commutator heats 

 because of excessive loss due to friction. Loosen or tighten 

 the springs used for holding the brush on the commutator. 



4. These result through the unskilful fitting of the 

 brushes or by a faulty brush having hard spots in it and 

 so wearing unevenly. This trouble may be detected by a 

 close examination of the wearing surface of the brush, 

 after the brush has been taken from the holder. The 

 surface should be smooth and polished all over; if the 

 wearing surface is polished only in spots, it' indicates a non- 

 homogeneous brush and a new one should be substituted. 



5. A machine not equipped with commutating poles 

 will always spark when overloaded, while a commutating 

 pole machine will not do so unless the overload is very 

 excessive. Inspection of the switchboard ammeter will 

 locate this trouble and if the meter indicates an overload, 

 part of the load should be taken off. 



6. If a machine vibrates to any extent, it is likely to 

 prevent the brushes from making firm contact with the 

 commutator surface and sparking occurs. The machine 

 must be put on a more solid foundation and balanced 

 better to get rid of this trouble. 



7. A short-circuited coil will always produce bad spark- 

 ing at the brush contact every time the short-circuited coil 

 passes under a brush. Unless the trouble is remedied very 



