. A text-book of electrical engineering;. the right of the origin. The action of the shunt dynamo will probably be made much clearerif we evolve the external or dynamic characteristic from the internal orstatic characteristic. We shall neglect the effect of armature reaction. Weplot, in the first place, the magnetising current as abscissae and the electro-motive force E as ordinates. This is the internal characteristic, and isshown in Fig. 160. Since, in our present experiment, we vary only theexternal current and leave the resistance of the field circuit unaltered, theterminal pressure must b
. A text-book of electrical engineering;. the right of the origin. The action of the shunt dynamo will probably be made much clearerif we evolve the external or dynamic characteristic from the internal orstatic characteristic. We shall neglect the effect of armature reaction. Weplot, in the first place, the magnetising current as abscissae and the electro-motive force E as ordinates. This is the internal characteristic, and isshown in Fig. 160. Since, in our present experiment, we vary only theexternal current and leave the resistance of the field circuit unaltered, theterminal pressure must be proportional to the magnetising current, inaccordance with the equation ,. For the curve of terminal pressure V we therefore get the straight linein Fig. 160. Now, the difference between the ordinates of the two curves Eand V must be equal to the pressure drop I^.Ra^ [I + I^-Ra in the ar-mature, and is therefore proportional to the armature current. When thereis no load on the dynamo, the only current through the armature is the field. 59- Characteristics of Shunt Dynamo 177 current, which is then equal to OC, and the pressure drop in the armature is then OC. R^. From this it follows that the difference between the ordinates of the two straight lines OA and OB is always equal to the product I,.Ra. The difference between the ordinates of the curve E and the straight hne OA is therefore equal to the product I. Ra, and is consequently proportional to the external current. By plotting the values of this current as ordinates to a different scale, the curve I in Fig. i6o is obtained. As the curve shows, there is one special load at which the current is a maximum. Since the abscissae I^ in Fig. i6o are pro-portional to the terminal pressure, the curveI in Fig. 160 is identical, except as regardsscale, with the curve V in Fig. 159. It is,however, turned through 90°, so that the left-hand part of the curve / inFig. 160 corresponds to the lower curve in Fig. 159, where, owing to thela
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