Ans. In the hand regulator through a broken resistance coil or bad contact.
Very frequently the fault occurs in the connecting wires leading from the machine to the hand regulator fixed upon the switchboard, or in the short wires connecting the field coils to the terminals or brushes.
The insulation of a broken wire will sometimes hold the two ends together so as to defy any but the most careful inspection or examination; therefore, in order to avoid loss of time, it is advisable to disconnect the wires if possible, and test each separately for conductivity with a battery and galvanometer connected, as in [fig. 694]. If the fault be not located in the various connections, the magnet coils should be tested with the battery and galvanometer coupled up as in [fig. 706], care being first taken to disconnect the ends of each of the coils. A faulty coil will not show any deflection of the galvanometer.
Fig. 694.--Method of testing dynamo for short circuits. In the figure, one pole of the battery B is placed in contact with the frame of the machine at a point which has previously been well scraped and cleaned; the other pole is connected to one of the galvanometer terminals as shown. The other terminal of the galvanometer is connected to each of the dynamo terminals T T under test in turn. If a deflection of the needle be produced when the galvanometer terminal is in contact with either, the terminals are in contact with the frame, and they should then be removed, and the fault repaired by additional insulation or by reinsulating.
Ques. At what point of a shunt coil does a break usually occur?
Ans. At the point where the wire passes through the flanges of the spool or bobbin.
Ques. How should the coil be repaired?
Ans. In most cases a little of the wood or metal of which the flange is made can be gouged or chipped out, and a new connecting wire soldered on to the broken end of the coil without much difficulty.
If it be necessary to take the magnets apart at any time, care should be taken in putting them together again to wipe all faces perfectly clean, and screw up firmly into contact, and to see that the connections of the coils are made as they were before being taken apart.
If the faulty coil cannot be repaired quickly, and the machine is urgently required, the coil may be cut out of circuit entirely, or short circuited, and the remaining coils coupled up so as to produce the correct polarity in the pole pieces.
