. A text-book of electrical engineering;. e number of pairs of poles, which is desirable forthe sake of symmetry and sparkless commutation, there can be no difficultyin determining which segments should be interconnected. It is by no meansnecessary that all the segments should be so connected, since a smallfraction of the total number of possible connections is generally found tosuffice. CHAPTER VII 51. The excitation of dynamos.—52. The field magnets.—53. Position of the brushes.—54. Armature reaction.—55. Sparkless commutation.—56. Three-wire dynamos. 51. The Excitation of Dynamos. The earli
. A text-book of electrical engineering;. e number of pairs of poles, which is desirable forthe sake of symmetry and sparkless commutation, there can be no difficultyin determining which segments should be interconnected. It is by no meansnecessary that all the segments should be so connected, since a smallfraction of the total number of possible connections is generally found tosuffice. CHAPTER VII 51. The excitation of dynamos.—52. The field magnets.—53. Position of the brushes.—54. Armature reaction.—55. Sparkless commutation.—56. Three-wire dynamos. 51. The Excitation of Dynamos. The earliest machines in which electromotive force was induced by themotion of a wire in a magnetic field were provided with permanent magnetsof hardened steel. They were built up of laminations which gave muchstronger magnets than could be obtained from massive steel. Even thesecompound magnets, liowever, gave a relatively small magnetic flux, sothat the electromotive force and current of a machine of reasonable di-mensions were Fig. 122 It was a great advance, therefore, when Dr Wilde of Manchester passedthe current obtained from the two-part commutator of a small permanent-rmagnet dynamo around the wrought-iron poles of a second dynamo. Sincewrought-iron has a high permeability, it follows that the magnetic flux inthis latter machine, and therefore also the electromotive force induced, werevery large. An arrangement similar to that of Wilde is shown in Fig. 122,in which N^ and S^ represent the steel magnet of the exciting machine. Thecurrent taken from this machine is passed through the field coils of a largerdynamo with magnets of wrought-iron. The coils must be wound in sucha way that one pole is north and the other south. When, for example, thebottom end of the left-hand coil lies in front, the bottom end of the right-hand coil, to which it is connected, must be behind. The substitution ofelectromagnets for permanent magnets had been proposed and carried out 140
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