Cyclopedia of automobile engineering Volume 2 ; a general reference work

 
9781130260205: Cyclopedia of automobile engineering Volume 2 ; a general reference work

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1909 Excerpt: ...of the flux in it; but this would have small effect upon the e. m. f. if the line of commutation did not also change with the armature current. This change necessitates, the moving of the brushes; and then the armature not only produces a crossmagnetizing effect, but also has a demagnetizing tendency. It is this latter which diminishes the generated e. m. f. In order to study the result, let us suppose the simple bipolar dynamo of Fig. 45 to be arranged so that current may be passed through its field windings, its armature when rotating, or through both. When the field alone is excited, the flux distribution will be substantially as indicated in Fig. 55--that is, quite uniform in the Magnetization Duo to Armature Current. Fig. 55. Flux Due to Field Alone. ooooo ooooo pole-pieces, air-gaps, and armature. If, now, the field-exciting circuit be opened, and if a current be supplied to the rotating armature equal to its rated load current, the flux distribution shown in Fig. 5(5 will exist. By combining these two conditions of the flux, there is produced the distortion shown in Fig. 57, which is the resultant flux distribution in the generating portion of the machine. The magnetism is distorted in the direction of rotation, as if the armature tended to draw after it the flux issuing from the field-poles. But this is not the physical fact, because we find that in electric motors the flux is distorted in a direction opposite to that of the rotation of the armature. In fact, the flux-distribution is the same whether the armature turns one way or the other, or not at all. On account of this flux distortion, brushes must be set, not midway between the field-poles, but, in the case of generators, somewhat ahead of this line of symmetry. Consequently the armature m. m....

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