Breaks in Armature Circuit.--If there be a broken circuit in the armature, as sometimes happens through a fracture of the armature connections, etc., there will be serious flashing or sparking at the brushes, which cannot be suppressed by adjusting the rocker. As a rule it results in the production of "flats" upon one or more bars of the commutator.
Fig. 725.--Sandpaper holder for commutator. The sandpaper is made fast on top by a clamp and screw. The two face blocks are pivoted and adjust themselves to the commutator, and will fit any size of commutator. If it have four brushes, the lower block will go in between the brush-holders.
Ques. How may such sparking be reduced without stopping the machine?
Ans. By placing one of the brushes of each set a little in advance of the others, so as to bridge the gap.
Short Circuits in Armature Circuit.--This fault is indicated by sparking at the commutator, and in bad cases by an excessive heating of the armature, dimming of the light and slipping of the belt, and in the case of a drum armature, by a sudden cessation of the current.
Fig. 726.--Sandpaper block. It is made to fit the surface of the commutator. At S is a saw cut into which the ends are pushed after being wrapped around the block. The latter should be cut down on the dotted lines to form a handle.
Short Circuits or Breaks in Field Magnet Circuit.--Either of these faults is liable to give rise to sparking at the commutator. If one of the coils be short circuited, the fact will be indicated by the faulty coil remaining cool while the perfect coil is overheated. The fault may arise through some of the connections to the coils making contact with the frame of the machine or with each other. To ascertain this, examine all the connections, and test with a battery and galvanometer. A total break in one or more of the field coils may readily be detected by means of the battery and galvanometer.
A partial break is not, however, so readily discovered, for the reason that the coil wires may be in sufficiently close contact to give a deflection of the galvanometer needle. The only methods of detecting this fault is by measuring the resistance of the coils with an ohmmeter or Wheatstone bridge, or by placing an ammeter in circuit with each coil in turn, and comparing the amount of current flowing in each. If the partial break be not accessible, the only way to remedy the fault is to rewind the coil, and the same applies to a break in the interior of the coil.