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308 RICHARDSON'S BLUEBOOK OF PROJECTION
Careful Attention Essential
(103) A motor generator must have careful, intelligent attention in order to work with reasonable efficiency. Many motor generator sets that should work at sixtylive to seventy per cent in efficiency are, because of careless or unintelligent care, working at fifty or fifty-five. Many drop even below that.
(104) To work at maximum efficiency a motor or generator or a motor generator set must (a) be kept clean throughout, (b) It must have the exact size and kind of brushes indicated for the service, (c) Its brushes must make good electrical contact with the commutator over their entire contact surfaces, (d) Brushes must contact the commutator with between 1.25 and 1.50 pounds of pressure per square inch of contact surface. Too little pressure gives poor contact; too much pressure induces sparking and tends to wear grooves in the commutator surface, (e) The brushes must fit in -their holders as snugly as possible without binding or sticking. A brush that is loose in its holder will "chatter" and move about, making poor electrical contact ; one that is too tight may stick in its holder and set up very bad sparking, (f ) The commutator surface must be kept clean. It should present a smooth, glazed surface, brown in appearance, (g) The mica insulation between the commutator bars must be kept undercut (its surface below the surface of the commutator) from l/32nd to 3/64th of an inch. The mica must never be allowed to be even with or project above the surface of the commutator, (h) The commutator bar connections (soldered) must be perfect, (i) There must be no sparking, or at best a negligible amount, (j) The commutator bars must not be permitted to wear down unevenly, or to become so thin that they are unable to carry the current without heating, (k) All bearings must be lubricated with oil or grease of the grade recommended by the manufacturer, (m) All wires, switches, etc., must be large enough to carry the load without undue resistance, (n) Allow a slight end-play of the armature in sleeve-bearing horizontal type generators as this aids to prevent commutator grooving.