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538 HANDBOOK OF PROJECTION FOR
not build up its voltage as it should. This may happen, even though the generator operated perfectly up to the time it was last shut down. This may be due to any one of the following causes, or to two or more of them combined :
(A) Slow speed, due to drop in line voltage or to motor trouble. (B) Open shunt field circuit, which may be either a (1) loose or high-resistance connection. (2) To a defective field coil, or (3) to a burned-out or damaged field rheostat coil. (C) An open armature or commutating-field circuit. (D) To incorrect setting of the brushes. See directions for setting them. (E) To reversed series or shut coil connections. (F) Poor contact of brushes with commutator. May be dirty commutator; dirty or burned brushes, or brushes sticking in holders. Examine this possible cause the first thing, should such a condition arise. (G) Loss of residual magnetism.
To remedy, first examine brushes and commutator, making sure that brush contact is good, commutator not dirty and brushes not sticking in holders. Try a temporarilly increased brush tension. Examine all connections and make sure they are in good condition, both mechanically and electrically. Look for a burned out or broken coil in field rheostat. A bell is not a reliable test for field windings, since sometimes the resistance of a field winding is so great that even a magneto bell will not ring through them when they are in perfect condition. An ordinary battery bell is useless for such a test. However, you may try a magneto bell, as it may work, and if it does ring through, then you know the field coil thus tested is alright. I only warn you that failure to ring is not conclusive proof that the coil is defective.
If the brushes and commutator are all right, the field rheostats alright and the field windings OK, then the trouble is probably either in the armature or due to loss of residual magnetism.
If nothing is found wrong with the armature, then it becomes necessary to excite the machine from another power source. Let A be the generator to be excited and B the generator power is to be taken from to do the exciting.
Remove all brushes from generator A and open all its switches. Connect the positive brush holders and the negative brushholders of machines A and B together. UNDERSTAND CLEARLY THAT BOTH THESE MACHINES MUST BE DIRECT CURRENT GENERATORS, hence you cannot use A C power lines for excitation of a D C generator. The connection above described should be made through a