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GENERATORS AND MOTORS 157
(95) A well designed, well constructed motor
generator set should withstand an overload beyond its nameplate rating, which for projection work already presupposes a temporary overload. It should be able to withstand indefinitely a ten percent continuous overload beyond nameplate rating. Imposing a continuous overload in any amount, however, is very bad practice, as it makes for inefficient operation and rapid deterioration.
Weak Field
(96) If your generator does not pick up its load
readily or will not retain its normal voltage while in operation, it is evidence that its field is weak. This fault in a motor will decrease its starting power but increase its speed and current consumption. (97) A weak field may have its seat either in a loose connection somewhere in the field circuit, or it may be caused by a short-circuiting of the current through field windings induced by weakened insulation caused by abnormal heat or otherwise; or it may be caused by a metallic short in the field coil. Test across each coil with a voltmeter. The defective coil is the one showing the least drop. If all readings are identical, the trouble, you may be sure, is due to a loose joint in the magnetic circuit.
Undue Vibration
(98) Too much vibration in a generator may cause brush sparking. Immediately after installation of a new motor or generator, place your hand on its frame. If serious vibration is detected, refuse the machine. Modern motors and generators can be so balanced that there need be no discernible vibration.
Ring of Fire
(99) Should a ring of fire appear on the commutator it will be due to one of two things : it is either a currentcarrying material that connects two or more commutator bars or it is an open circuit in the armature. If it is the first, the "ring" will be bright and pronounced. If it is