CARE AND MANAGEMENT OF THE DYNAMO.

having raised the brushes from contact with the commutator by means of the hold-off catches, the dynamo may be started and allowed to run light.

Whilst thus running, the bearings should be tested from time to time to ascertain if they heat unduly, and an opportunity is also afforded, while the dynamo is thus running, for cleaning the commutator, if this is dirty, with finest emery cloth, afterwards wiping clean with a linen rag. The connections of the machine and external circuits should be verified, and all terminals, &c., cleaned and examined. If found correct, the brushes should be let down on to the commutator, and their tips adjusted by rotating the rocker into the neutral points.

The tips of the brushes carried by one arm of the rocker will, in bi-polar dynamos with vertical field magnets, bear exactly upon the top or highest point of the commutator, while the tips of those carried by the other arm will bear exactly upon the bottom or lowest point of the commutator. In other types of machines, the positions for the brushes will vary according to the class or form of the field magnet and the system of armature winding. If the machine is compound or shunt wound, all switches controlling the external circuits should be opened, as the machine excites best when this is the case; and when the machine is provided with a rheostat or hand regulator and resistance coils, these latter should all be cut out of circuit, or short circuited, until the machine excites, when they can be gradually cut in as the voltage rises.

CARE AND MANAGEMENT OF THE DYNAMO.

When the machine is giving the correct voltage, as indicated by the voltmeter or pilot lamp, the machine may be switched into connection with the external or working circuits. When the machine is series wound, it is absolutely necessary to have the external circuit closed, otherwise a closed circuit will not be formed through the field magnet windings, and the machine will not excite.

Attention to Dynamo after it is started.-When the machine is started and at work, it will need a certain amount of attention to keep it running in a satisfactory and efficient manner. The first point to which attention should be paid is the adjustment of the "lead" of the brushes. If this is neglected, the machine will probably spark badly, and the commutator and brushes will constantly require filing and trimming. The "lead" is the term applied to the slight forward movement which it is found necessary to give to the brushes of most dynamos in order to avoid sparking with an increase of load.

This lead is all good dynamos is very small, and varies with the load and class of machine. The best lead to give to the brushes can in all cases be found by rotating the rocker and brushes in either direction to the right or left of the neutral points, until sparking commences increasing with the movement. The position midway between these two points is the correct position for the brushes, for at this position the least sparking occurs, and it is at this position that the brushes should be fixed by clamping the rocker.

CARE AND MANAGEMENT OF THE DYNAMO.

In series dynamos giving a constant current, such as for arc lamps in series, the brushes require practically no lead. In shunt and compound dynamos the lead varies with the load, and therefore the brushes must be rotated in the direction of rotation of the armature with an increase of load, and in the opposite direction with a decrease of load.

In cases where the dynamos are subjected to a rapidly varying or fluctuating load, it is of course not possible to constantly shift the brushes as the load varies, therefore the brushes should be fixed in the positions where the least sparking occurs at the moment of adjustment. If at any time very violent sparking occurs, which cannot be reduced or suppressed by varying the position of the brushes by rotat ing the rocker, the machine should be shut down at once, otherwise the commutator and brushes are liable to be destroyed, or the armature burnt up. This especially refers to high tension machines.

As soon as any abnormal sparking is seen at the com mutators of such machines, their speed should be at once reduced, and the commutator cleaned up, and the brushes readjusted. Another very important point to be looked to is the lubrication of the machine. The lubricators should be inspected from time to time, to see that they feed the lubricant properly, and that none of the waste oil passages are clogged. The oil should on no account be allowed to get on to the commutator or brushes, or into the windings of the armature, as it is liable to cause sparking at the brushes, and to destroy the insulation of the armature.

CARE AND MANAGEMENT OF THE DYNAMO,

In filling the lubricators, oil cans made of some nonmagnetic material such as copper, brass or zinc, should always be used. If iron cans are used, they are liable to be attracted by the field magnets, and thus possibly catch in the armature, and destroy the insulation of the latter. The bearings, and also the field magnet coils, should be tested at intervals, to see that they do not become unduly heated. When testing the temperature of low tension machines, the hand may be used as a guide for judging when the machine is running at a safe temperature.

If the heat of any portion can be easily borne by the naked hand, it may be taken that the temperatnre of the machine is within safe limits. In the case of high tension machines, however, the naked hand cannot be brought with safety into contact with any portion of the machine, and therefore the only way to ascertain if the windings or other electrical parts are at a safe temperature is to apply a ther

mometer.

It may be taken as a safe rule that no part of a working dynamo should have a temperature of more than 80° Fahr. above that of the surrounding air. Hence, if the temperature of the engine-room is noted before applying the thermometer to the machine, it can at once be seen if the latter is 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 louse through the vibration.

CARE AND MANAGEMENT OF THE DYNAMO. Attention to Brushes and Commutator.-The brushes and commutator are the most troublesome parts of a dynamo and require most attention. To keep them in a satisfactory working condition, the main thing to be guarded against is the production of sparking at the brushes. If care be taken in the first instance to properly adjust the brushes to their setting marks, and their pressure upon the commutator, and afterwards to attend to the lead as the load varies, so that little or no sparking occurs, and to keep the brushes and commutator free from dirt, grit, &c., and excessive oil, the surface of the commutator will assume a dark burnished appearance, and all wear will practically cease.

Under these circumstances the commutator will run cool, and free from sparking, and will give very little trouble. In order to maintain these conditions it will only be necessary to see that the brushes are properly trimmed and fed forward to their setting marks, as described above, as they wear away, and that the commutator is occasionally polished with the finest emery cloth. If, on the other hand, the pressure of the brushes upon the commutator is too great, or their adjustment is faulty, or the commutator is allowed to get into a dirty condition, sparking will inevitably result, and, if not at once attended to and remedied, the brushes will quickly wear away, and the surface of the commutator will be destroyed.

If this condition of things is allowed to continue, matters will rapidly get worse. In the earlier stages, the surface of the commutator will become roughened or scored, resulting in jumping of the brushes, and increased sparking; in the