438 ELECTRICAL APPARATUS H 51 *! j i; reaction is negative, or motor reaction, and the magnetic flux produced by it at the commutator brushes thus a commutating flux. If p < 1, or subnormal direct voltage, the armature reaction is positive, that is, the same as in a direct-current gen- erator, but less in intensity, and thus the magnetic flux of arma- ture reaction tends to impair commutation. In a direct-current generator, by shifting the brushes to the edge of the field poles, the field flux is used as reversing flux to give commutation. In this converter, however, decrease of direct voltage is produced by lowering the outside sections of the field poles, and the edge of the field may not have a sufficient flux density to give commuta- 3' ft I 2 3 ft I 2' 3' BL FIG. 210.—Three-section pole for variable-ratio converter. tion, with a considerable decrease of voltage below normal, and thus a separate commutating pole is required. Preferably this type of converter should be used only for raising the voltage, for lowering the voltage the other type, which operates by a shift of the resultant flux, and so gives a component of the main field flux as commutating flux, should be used, or a combination of both types. With a polar construction consisting of three sections, this can be done by having the middle section at low, the outside sections at high excitation for maximum voltage, and, to de- crease the voltage, raise the excitation of the center section, but instead of lowering both outside sections, leave the section in the direction of the armature rotation unchanged, while lowering the other outside section twice as much, and thus produce, in addition to the change of wave shape, a shift of the flux, as represented by the scheme Fig. 210. Pole section . Max. voltage 2 0 Magnetic Density 3 I' Min. voltage... —} 0 +(B +