IX. Inverted Converters 100. . Converters may be used to change either from alter- nating to direct current or as inverted converters from direct to alternating current. While the former use is by far the more 256 ELEMENTS OF ELECTRICAL ENGINEERING frequent, sometimes inverted converters are desirable. Thus in low-tension direct-current systems outlying districts have been supplied by converting from direct to alternating, transmitting as alternating, and then reconverting to direct current. Or in a station containing direct-current generators for short-distance supply and alternators for long-distance supply, the converter may be used as the connecting link to shift the load from the direct to the alternating generators, or inversely, and thus be operated either way according to the distribution of load on the system. Or inverted operation may be used in emergencies to produce alternating current. When converting from alternating to direct current, the speed of the converter is rigidly fixed by the frequency, and cannot be varied by its field excitation, the variation of the latter merely changing the phase relation of the alternating current. When converting, however, from direct to alternating current as the only source of alternating current, that is, not running in multiple with engine- or turbine-driven alternating-current generators, the speed of the converter as direct-current motor depends upon the field strength; thus it increases with decreasing and decreases with increasing field strength. As alternating-current generator, however, the field strength depends upon the intensity and phase relation of the alternating current, lagging current reducing the field strength and thus increasing speed and frequency, and leading current increasing the field strength and thus decreasing speed and frequency. Thus, if a load of lagging current is put on an inverted con- verter, as, for instance, by starting an induction motor or another converter thereby from the alternating side, the demagnetizing effect of the alternating current reduces the field strength and causes the converter to increase in speed and frequency. An in- crease of frequency, however, may increase the lag of the current, and thus its demagnetizing effect, and thereby still further in- crease the speed, so that the acceleration may become so rapid as to be beyond control by the field rheostat and endanger the machine. Hence inverted converters have to be carefully watched, especially when starting other converters from them, and some absolutely positive device is necessary to cut the in- verted converter off the circuit entirely as soon as its speed ex- ceeds the danger limit. The relatively safest arrangement is separate excitation of the inverted converter by an exciter SYNCHRONOUS CONVERTERS 257 mechanically driven thereby, since an increase of speed in- creases the exciter voltage at a still higher rate, and thereby the excitation of the converter, and thus tends to check its speed. This danger of racing does not exist if the inverted converter operates in parallel with alternating generators, provided that the latter and their prime movers are of such size that they cannot be carried away in speed by the converter. In an in- verted converter running in parallel with alternators the speed is not changed by the field excitation, but a change of the latter merely changes the phase relation of the alternating current supplied by the converter; that is, the converter receives power from the direct-current system, and supplies power into the alter- nating-current system but at the same time receives wattless current from the alternating system, lagging at under-excitation, leading at over-excitation, and can in the same way as an ordinary converter or synchronous motor be used to compensate for watt- less currents in other parts of the alternating system, or to regu- late the voltage by phase control.