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FOR MANAGERS AND OPERATORS 393
not easily seen. See that the brush yoke and housing marks agree, to insure that the brushes are set in same position as when adjusted at the factory. Look all around the commutator at the connections between the armature windings and the commutator bars. Such minute inspection should locate the trouble.
A. C. to D. C. Compensates.— Machine does not start: If the machine does not start when the switch is closed, first examine the fuses and make sure the current is on at the switch terminals. It sometimes happens that a single fuse has blown on a three-phase three-wire outfit, in which case the compensarc will run as a single-phase machine, but if stopped will not start again until the blown fuse has been replaced if a single throw switch be used. However, if a double throw starting switch be used the compensarc will be started up on the unfused side. Therefore, the missing fuse must be detected by the operation of the machine while running. If a fuse is missing it can usually be detected by the unusual noise made by the machine while running, by the motor end heating excessively, and more particularly by change in speed with change in load, and general unsteadiness of the arc. If a fuse be blown it should be replaced immediately, else it may cause the burning out of the motor.
D. C. and A. C. to D. C. Compensates. — Sparking at the brushes: When a vicious sparking develops under the brushes of the compensarc it is an indication that something is radically wrong. The most usual causes are dealt with fully under General Instruction No. 8, Page 372. In addition it may be noted, however, that in removing the brushes from the boxes for cleaning, which should be done once a week, do not take the pig tails loose front the brush holders, and be sure to place the brushes back in the boxes in their original position, for if they are turned around they will not fit the commutator surface. The brushes should have a smooth, unscratched surface, free from any copper deposit.
Open or Short Circuit in Armature: This trouble will most often occur where the armature winding is connected to the commutator, and results generally from a bruise in handling, from some foreign body getting caught in the armature, or from a chip caught when the commutator is being turned or repaired. If an open circuit the trouble is very apparent, since the long heavy spark accompanying it generally eats away the mica between the segments on each side of the break, thus indicating its location. A short circuit in the