Motion picture handbook; a guide for managers and operators of motion picture theatres ([c1916])

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FOR MANAGERS AND OPERATORS 379 be bad it will cause the brushes to move in and out of their holders perceptibly when the armature is revolved slowly. The only remedy is to turn the commutator down, and this can only be sucessfully done in a machine shop where work of this character is understood. (h) Brushes having too high resistance, the evidence of which is that they get very hot and slowly crumble away at the end next to 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, and this will cause sparking. The evidence of this fault is that the air gap between the armature and the pole piece will be smaller at the botttom than at the top. The only remedy it to replace the worn bearings with new ones. (j) A short-circuited armature coil. This trouble will 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 direct current through the coil, with the brushes raised and resistance in series; 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 aihead or 'back. By "ahead" I mean in the direction in which the armature is revolving. The remedy is obvious. Get a machine of larger capacity, or cut down 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 its inability to pick up readily, and by failure to maintain normal voltage. On a motor the starting power is decreased,