Electric transmission of energy and its transformation, subdivision, and distribution; A practical handbook

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1886 edition. Excerpt: ...magnets upon the armature does not depend on the strength of magnetism in the field magnets only, but is the product of that quantity and the current in the armature coils. An increase of pull may therefore be brought about either by making the field stronger, or by increasing the current in the armature, or by both means combined. If we make the field stronger, we not only increase the magnetic pull exerted on the armature, but we also increase the counter-electro-motive force, as will be seen from equation 5), page 81, and thus check, or at least reduce, the flow of current through the armature at the very moment when we want most power. The motor would thus run slower until its reduced counter-electro-motive force again allows a current to pass of sufficient strength for the work imposed on the motor. If, on the other hand, we seek the increase of power by allowing more current to pass through the armature, we do not increase the counter-electro-motive force, but we have a slight increase in the loss of electro-motive force due to the resistance of the armature. To compensate for this slight increase of loss, it is necessary to weaken the field somewhat for heavy currents, and thus bring about the reduction of counter-electro-motive force by an amount corresponding to the increased loss of electromotive force due to the resistance of the armature. If the motor runs without doing external work, Ca is almost zero, and we have the strongest field, _ _ EtW which is entirely due to the shunt coils. Let now a load be thrown on. The immediate effect will be to slightly reduce the speed. The counter-electro-motive force which previously was nearly equal to E„ will thereby become somewhat reduced, thus allowing a considerable current to...