Motor-generators vs. synchronous converters; with special reference to operation on long-distance transmission lines

General The particular problem to be considered in this discussion is the following: wanted, direct-current power of approximately constant voltage; for instance, for railways 500 to 700 volts, for an incandescent lighting system, 100 to 275 volts, for an electrolytic process, any direct-current voltage. Given as a source of power, a long-distance alternating-current transmission system, which may be either 25 or 60 cycles, may have a varying voltage at point of delivery, may have the variation occur either on account of the load taken by our particular station or independent thereof and may have a somewhat variable frequency. Problem, what shall be used for making the desired transformation; synchronous converters? motor-generator sets with synchronous motors? or motor-generator sets with induction motors?