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484 HANDBOOK OF PROJECTION FOR
to excite itself. If it does not pick up it is evident that something is wrong with the shunt winding or connections.
If a new machine refuses to build up voltage, and the connections apparently are correct, reverse the field connections, i. e., interchange the field wires which are connected to the positive and negative terminals of the generator. If this interchange of connections does no good, re-establish the original connections and locate the fault as previously advised.
BRUSHES.— See General Instruction No. 7. All brush faces resting on the commutator should be fitted to the commutator so that they make good contact over the entire area. This can be most easily accomplished after the brushholders have been adjusted and the brushes inserted. Lift one set of brushes so that they will not be forced against the commutator. Place a piece of No. l/2 sandpaper against the commutator with the sanded side toward the brushes. Replace one brush in its holder and allow the spring to force it against the sandpaper. Draw the sandpaper in the direction of rotation under the brush, releasing the pressure as the paper is drawn back, being careful to keep the ends of the paper as close to the commutator surface as possible, thus avoid rounding the ends of the brush. After the first brush is properly ground, it should be lifted sufficiently to prevent its being forced against the commutator, after which the remaining brushes of the set may be similarly ground, one at a time.
By this means a satisfactory contact is quickly secured, each set of brushes being similarly treated in turn. If the brushes are copper plated, their edges should be slightly beveled, so that the copper does not come in contact with the commutator.
INSPECTION.— Make frequent inspection to see that (a) brushes are not sticking in holders ; (b) pig-tail shunts are properly attached to brushes and holders; (c) tension is readjusted as the brush wears; (d) copper plating is cut back so it does not make contact with commutator; (e) wornout brushes are replaced before they reach their wearing limit and break contact with the commutator; (f) any free copper picked up by the face of the brushes is removed.
COMMUTATOR.— See General Instruction No. 7.
HEATING FIELD COILS.— Heating of field coils may result from any of the following causes : (a) too low speed ;