D. C. COMMUTATING MACHINES 217 This speed curve corresponds to a constant position of brushes midway between the field poles, as generally used in railway motors and other series motors. If the brushes have a constant shift or are shifted proportionally to the load, instead of the saturation curve A in Fig. 121 a curve is to be used correspond- ing to the position of brushes, that is, derived by adding to the abscissas of A the values iq, the demagnetizing effect of arma- ture reaction. 10 60 FIG. 121 100 120 _110 160 ISO -Series motor speed curve. The torque of the series motor is shown also in Fig. 121, derived as proportional to A X i, that is, current X magnetic flux. Compound Motors 76. Compound motors can be built with cumulative com- pounding and with differential compounding. Cumulative compounding is used to a considerable extent, as in elevator motors, etc., to secure economy of current in starting and at high loads at the sacrifice of speed regulation; that is, a compound motor with cumulative series field stands in its speed and torque characteristic intermediate between the shunt motor and the series motor. 218 ELEMENTS OF ELECTRICAL ENGINEERING Differential compounding is used to secure constancy of speed with varying load, but to a small extent only, since the speed regulation of a shunt motor can be made sufficiently close, as was shown in the preceding. Conclusion 77. The preceding discussion of commutating machine types can obviously be only very general, showing the main character- istics of the curves, while the individual curves can be modified to a considerable extent by suitable design of the different parts of the machine when required to derive certain results, as, for instance, to extend the constant-current part of the series gen- erator; or to derive a wide range of voltage at stability, that is, beyond the bend of the saturation curve in the shunt generator; or to utilize the range of the shunt generator load characteristic at the maximum current point for constant-current regulation; or to secure constancy of speed in a shunt motor at varying impressed e.m.f., etc. The use of the commutating machine as direct-current con- verter has been omitted from the preceding discussion. By means of^ one or more alternating-current compensators or autotransformers, connected to the armature by collector rings, the commutating machine can be used to double or halve the voltage, or convert from one side of a three-wire system to the other side and, in general, to supply a three-wire Edison system from a single generator. Since, however, the direct- current converter and three-wire generator exhibit many fea- tures similar to those of the synchronous converter, as regards the absence of armature reaction, the reduced armature heat- ing, etc., they will be discussed as an appendix to the synchro- nous converter.