Now in a three-part commutator the spark occurring as the segments pass under the brushes would very quickly destroy the surface and interfere with the currents in the coil. This difficulty is overcome by blowing out the spark by an air blast given at just the right place and time. The manner in which the blast is delivered is as follows: the segments of the commutator
are separated by gaps of about 5° and in front of each of the leading brushes there projects a nozzle, Fig. 15, which discharges an air blast alternately three times in each revolution. The blast itself is supplied by an ingenious piece of mechanism known as the
Thomson Air Blast.
It consists of an elliptical box II whose sides have perforations II where air can enter while inside of this rotates a steel disk keyed to the armature shaft and having radial slots in which slide three wings RRR of ebonite which as they fly around drives air into the holes JJ leading to the nozzles Fig. 15. The result is that, since the spark is done away with, oil can be supplied to the commutator in limited quantities but still amply sufficient to reduce the wear on the commutator to such an extent that the life of a segment is greatly increased. The air blast is fastened to the dynamo frame just behind the commutator and can be see in Fig. 23.
The Field-Magnets,
as may be seen from Fig. 17, consists of two flanged iron tubes AA whose end consists of a convex segment of a sphere accurately turned to recieve the armature. Coils of wire CC which are in the outside circuit and through which the entire current flows are wound upon the tubes. After the armature is placed between them the two tubes are bolted together by heavy wrought iron bars BB and the whole carried on the frame work PN shown also at PN Fig. 23. Now a little magnetism only remains in the wrought iron bars and iron frame works when the armature first revolves, but the current even though slight, going through the coils makes an electromagnet out of each tube and heavily magnetizes the wrought iron bars and in two or three seconds after the armature first rotates it is entirely surrounded by a heavy magnetic field. One of the good points of these field magnets is that but very little magnetism is lost as compared with most other dynamos and since it takes power to maintain a heavy magnetic field, this dynamo is in this respect very economical.
The Thomson Regulating Gear
Later on we will show that pushing the brushes together or pulling them apart alter the strength of the current, but for the present just accept the fact and we will show how the brushes are varied. It is accomplished by the mechanism shown in Fig. 18. The brushes are fixed to the levers YY and Y2Y2 united by the lever l. The automatic movement is obtained by the electromagnet R while a dashpot J prevents too sudden motion. Suppose the brushes to be in the position shown when the current would get too strong owing to lights being cut out. The electromagnet R getting stronger would raise A and reduce the current taken off until current came to normal. If, instead, some lamps were thrown in the current would become weak and the electromagnet R would become weak, drop A which would increase current and this will continue till current reaches normal.
The foregoing regulating gear is used on small dynamos and old style large ones. On the large new style dynamo a more delicate regulating gear is used, the current which operates it being shown at Fig. 19. Normally the electromagnet R is short circuited by the wire r and only acts when this circuit is broken. At some point in the main circuit is a wall controller or controller magnet shown in Fig. 19, at ST, consisting of two electro magnet, Their yoke supported by a spring and the yoke operating the contact lever S. If the current becomes too strong the controller magnet circuit is broken and all the current of the main circuit goes through the electromagnet R which by its sudden increase of strength quickly raises A and thus alters the brushes. This only exists for a moment until the yoke of the controller magets fall because of their decrease of magnets strength, when current again flows through wire r because when yoke drops contact is made. This decreases the strength of electro magnet R thus dropping A and increasing current. Hence S will again raise and break contact and R again rais A. This is continually repeated.