A commutator having uneven surface will cause sparking, and should be polished off or turned down.
Fig. 100.
Sometimes the mica segments between commutator bars do not wear as fast as the bars and when this is the case, the brushes will be kept from making good contact when the commutator bars are slightly worn. The remedy is to take the armature into the shop, and groove out the mica between the commutator bars for a depth of about ¹⁄₆₄-inch below the commutator surface.
A greenish flash which appears to run around the commutator, accompanied by scoring or burning of the commutator at two points, indicates that there is an open-circuited coil at the points at which the scoring occurs as in [Fig. 100].
The magnetic field may be weakened by a short circuit in the field coils, as before explained, and this may give rise to sparking.
Short circuits in the armature may give rise to sparking, but will also be made evident by the jerking motion of the car and the blowing out of the fuse.
Failure of Car to Start. The failure of the car to start when the controller is turned on may be due to any of the following causes:
Opening of the circuit breaker at the power house.
Poor contact between the wheels and the rails owing to dirt or to a breaking of the bond wire connections between the rail on which the car is standing and the adjacent track.