Sparking and bar burning are usually due to one or more of the following causes:

1. Brushes not in the proper position.

2. Incorrect spacing of brushes. This may be checked by marking an adding-machine tape around the commutator.

3. Projecting-bar-edge mica. Mica between bars should be undercut about 0.063 in below the commutating surface, but occasionally slivers of mica are left inadvertently along the bar.

4. Rough or burned commutator. The commutator should be ground according to the manufacturer’s instruction book.

5. Grooved commutator. This may be prevented by properly staggering the brush sets so that the spaces between the brushes of an arm are covered by brushes of the same polarity of other arms.

6. Poor brush contact. This is due to improper fitting of the brushes to the commutator surface. To seat the brushes, sandpaper should be moved between the commutator and the brush face. Emery cloth should not be used because its abrasive is conducting.

7. Worn brushes replaced by others of wrong size or grade.

8. Sticking brushes. These brushes do not move freely in their holders so that they can follow the irregularities of the commutator.

9. Chattering of the brushes. This is usually due to operation at current densities below 35 A/in2 and must be corrected by lifting brushes to raise the density or by using a special grade of brush.

10. Vibration. This may be due to poor line up, inadequate foundations, or poor balance of the rotor.

11. Short-circuited turns on the commutating or compensating fields. These may be obvious on inspection but usually must be found by passing ac current through them for voltage-drop comparisons.

12. Open or very high resistance joints between the commutator neck and the coil leads. In this case, the bar at the bad joint will usually be burned.

13. An open armature coil. A broken coil conductor produces an effect similar to that produced by the poor joints described in the previous item. For emergency operation, the open coil may be opened at both ends, insulated from the circuit, and a jumper placed across the two affected necks. Since some sparking will probably result, operation should be limited.

14. Short-circuited main-field coils. With the resulting unbalanced air-gap fluxes under the poles, large circulating currents must be expected even with good armature cross connections. The offending coil may be found by comparing voltage drops across the individual coils.

15. Reversed main-field coil. This is an extreme case of the one described in the previous item.

16. Overloading.