The principle advantages of the alternator was that at low speed idling, the alternator still produced enough current to keep the battery charged—something the original DC generators could not do.
Also the main disadvantage with the DC Auto generator was its crude vibrating contact voltage regulator.

 

Not quite strictly true, the one benefit of replacing the old/original DC generators, is that they start to generate at a MUCH lower RPM than its DC counterpart did.

While this may be true if you watch any engine that stumbles for a moment or two below around (usually) 600 RPM the lights will dim, fans slow down, and the volt gauge will drop a bit. Most alternators are matched to start producing a noticeable amount of power when the engine is just slightly below idle speed. Too much below and they won't produce enough power to do any good.

The reason I brought up RPM in the first place is we still don't know exactly what application this is. If it is just a basic retrofit onto an existing engine then even if it doesn't work at idle a simple press on the gas should produce some results.

If it is something entirely different then....