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MANAGERS AND PROJECTIONISTS 509
the shaft be swung on its centers in the lathe the commutator will show eccentric. A competent machinist will, of course, know what precautions to take in a case of this kind.
(h) Brushes having too high resistance, the evidence of which is that they get very hot and slowly crumble away at the end next the commutator. The remedy is to get good brushes.
(i) Low bearings. In some types of machines low bearings will throw armature out of center sufficiently to distort the magnetic field, which will cause sparking. The evidence of this fault is that the air gap between the armature and the pole piece is smaller at the bottom than at the top. The only remedy is to replace the worn bearings with new ones.
(j) Short-circuited armature coil. This trouble may cause the voltmeter to fluctuate badly and the shorted coil to heat very quickly. The coil may be shorted within itself, or there may be a connection between two adjoining commutator segments. Remedy: Locate and remove the short.
(k) A reversed armature coil. This may be located by holding a compass over each coil of the armature in turn, and sending a few amperes of direct current through the coil, with the brushes raised and resistance in series to limit current flow, or current from a battery may be used. The coil which causes the compass to turn in the opposite direction from its mates is the guilty party. The remedy is, reverse the connection or direction of the windings of the defective coil.
(1) A bent armature shaft. This of, course, will cause the whole armature to wobble. The only practical remedy is a new shaft.
(m) Overload. The most prominent symptom of overload is the armature heating all over. Sparking may be lessened but not entirely stopped by moving the brushes ahead or back. By "ahead" we mean in the direction in which the armature is revolving. The remedy is obvious. Get a machine of larger capacity, or reduce the load on the one you have.
(n) High speed sparking is caused by the brushes not being able to make proper connection with the commutator by reason of excessive armature speed.
(o) A weak field. This may be detected in a generator by the inability to pick up readily, and by failure to maintain normal voltage. In a motor the starting power is decreased, but the speed and current are increased. A weak