Accordingly, if the temperature of the engine room be noted before applying the thermometer to the machine, it can at once be seen if the latter be working at a safe temperature. In taking the temperature, the bulb of the thermometer should be wrapped in a woolen rag. The screws and nuts securing the different connections and cables should be examined occasionally, as they frequently work loose through vibration.
Fig. 684.--Diagram illustrating forces acting on a dynamo armature. In the figure the normal field magneto-motive force is in the direction of the line 1, 2, produced by the field circuit G, if there were no current in the armature. But as soon as the armature current flows, it produces the opposing force 3, 4, which must be combined with 1, 2 to give the resulting force to produce magnetism and hence voltage. The resultant 1, 5, if 3, 4 be large enough, does not differ much from the original force 1, 2. Or, expressed in a more physical way, the brushes E, F, rest on the commutator and all the turns embraced by twice the angle 6, 3, F, oppose the flow of flux through the armature core as well as all the turns embraced by twice the angle, 7, 3, E. The remaining turns distort the flux, making the pole corners at A and B denser, and at C and D rarer. So that all the effect is to kill an increase of flux, or voltage. This cross magnetism tends also to decrease the flow of flux, for the extra ampere turns required to force the flux through the dense pole tips are greater than the decreased ampere turns relieved by the reduction of flux at the other pole tips; this follows, since iron as it increases in magnetic density requires ampere turns greater in proportion than the increase of flux.
Instructions for Stopping Dynamos.--When shutting down a machine, the load should first be gradually reduced, if possible, by easing down the engine; then when the machine is supplying little or no current, the main switch should be opened. This reduces the sparking at the switch contacts, and prevents the engine racing.
When the voltmeter almost indicates zero, the brushes should be raised from contact with the commutator. This prevents the brushes being damaged in the event of the engine making a backward motion, which it often does, particularly in the case of a gas engine. On no account, however, should the brushes be raised from the commutator while the machine is generating any considerable voltage; for not only is the insulation of the machine liable to be damaged, but in the case of large shunt dynamos, the person lifting the brushes is liable to receive a violent shock.
Ques. What attention should the machine receive after it has been shut down?
Ans. It should be thoroughly cleaned. Any adhering copper dust, dirt, etc., should be removed from the armature by dusting with a stiff brush, and the other portions of the machine should be thoroughly cleaned with linen rags. Waste should not be used, as it is liable to leave threads or fluff on the projecting parts of the machine, and on the windings of the armature, which is difficult to remove.
Ques. What attention should be given to the brushes and brush gear?
Ans. They should be examined and thoroughly cleaned. If necessary the brushes should be refitted and readjusted. All terminals, screws, bolts, etc., should be carefully cleaned and screwed up ready for the next run. The brush holders should receive special attention, as when dirty, they are liable to stick and cause sparking. All dirt and oil should be removed from the springs, contacts, pivots, and other working parts.
It is advisable at stated intervals to entirely remove the brush holders from the rocker arms, and give them a thorough cleaning by taking them to pieces, and cleaning each part separately with emery cloth and benzoline or soda solution.
Another point to which particular attention should be given is the cleaning of the brush rocker. This being composed wholly of metal, and the two sets of positive and negative brushes being only separated from it by a few thin insulating washers, it follows that if any copper dust given off by the brushes be deposited in the neighborhood of these washers, there is considerable liability for a short circuit of the machine to occur by the dust bridging across the insulation.