ARMATURE TROUBLES 623 to 634

Causes--how avoided--various faults--short circuit in individual coils--location of faulty coil--test for break in armature lead--bar to bar test for open or short circuit in coil or between segments--short circuits between adjacent coils--alternate bar test for short circuits between sections--short circuits between sections through frame or core of armature; between sections through binding wires--partial short circuits in armatures--method of testing for breaks--burning of armature coils--Watson field coils--grounds in armatures--method of locating grounded armature coil--magneto test for grounded armatures--method of binding armature winding--breaks in armature circuit.

CARE OF THE COMMUTATOR AND BRUSHES 635 to 652

Conditions for satisfactory operation--oil for commutator--attention to brushes--Bissell brush gear--two kinds of sparking--commutator clamp--causes of sparking--bad adjustment of brushes--rocking--bad condition of brushes--brushes making bad contact--bad condition of commutator--detection of untrue commutator--high segments--"flats"--causes of flats; remedy--method of repairing broken joint between commutator segment and lug--segments loose or knocked in--how to re-turn a commutator--Bissell commutators--overload of dynamo--method of repairing large hole burned in two adjacent bars of a commutator--operating dynamos with metal brushes--indication of excessive voltage--method of smoothing commutator with a stone--causes of excessive voltage--loose connections, terminals, etc.,--breaks in armature circuit--sandpaper holder for commutator--short circuits, in armature circuits; in field--breaks in field--sandpaper block--short circuits in commutator.

HEATING 653 to 662

Various causes--how detected--procedure--heating, of connections; of brushes, commutator and armature--excessive heating--ventilated commutator--self-oiling bearing--some causes of hot bearing--effect of hot bearings--points relating to hot bearings--operation above rated voltage and below normal speed--forced system of lubrication--heating of field magnets--causes of eddy currents in pole pieces--detection of moisture in field coils--indication of short circuits in field coils.

OPERATION OF MOTORS 663 to 696

Before starting a motor--starting a motor--various starting resistances--starting boxes--speed regulators--Cutler Hammer starter--time required to start motor--how to start--sliding contact starters--series motors on battery circuits--starting a shunt motor--multiple switch starters--effect of reverse voltage--rheostat with no voltage and overload release--failure to start--starting panel--Cutler Hammer starting rheostats--Allen Bradley automatic starter--Monitor starter with relay for push button control--a remote control of shunt motors--regulation of motor speed; various methods--Monitor printing press controller--speed regulation of series motor, by short circuiting sections of the field winding--varying the speed of shunt and compound motors--Cutler Hammer multiple switch starter--regulation by armature resistance--Compound starter--regulation by shunt field resistance--Holzer Cabot instructions for shunt wound motor--Reliance adjustable speed motor--Cutler Hammer reversible starter--combined armature and shunt field control--selection of starters and regulators--Watson commutators--organ blower speed regulator--General Electric controller--speed regulation of traction motors--controller of the Rauch and Lang electric vehicles--two motor regulation--controller connection diagrams--stopping a motor.

CHAPTER XXVI
GALVANOMETERS

If a compass needle be allowed to come to rest in its natural position, and a current of electricity be passed through a wire just over it from north to south, the north seeking end of the needle will be deflected toward the east. If the wire be placed under the needle and the current continued from north to south the needle will be deflected toward the west. Again, if the current be passed from north to south over the needle, and back from south to north under the needle, as shown in fig. 504, the magnetic effect will be doubled, and the needle deflected proportionately. Upon these phenomena depend the working of galvanometers.

Fig. 503.--Effect of neighboring current upon a magnetic needle. Above the needle and parallel to it is a conductor carrying an electric current, the current flowing in the direction indicated by the arrow. This causes the north pole of the needle to turn toward the east. If the conductor be held below the needle, its north pole will turn in the opposite direction or toward the west. These movements are easily determined by Ampere's rule as follows: If a man could swim in the conductor with the current, and turn to face the needle, then the north pole of the needle will be deflected toward his left hand.

Ques. Describe a simple galvanometer.

Ans. It consists essentially of a magnetic needle suspended within a coil of wire, and free to swing over the face of a graduated dial.

Ques. What is a galvanoscope and how does it differ from a galvanometer?

Ans. A galvanoscope, as shown in fig. 504, serves merely to indicate the presence of an electric current without measuring its strength. It is an indicator of currents where the movement of the needle shows the direction of the current, and indicates whether it is a strong or a weak one. When the value of the readings has been determined by experiment or calculation any galvanoscope becomes a galvanometer.

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