I'm specifically talking about a motor-generator AC to DC converter. Are there applications where you'd need perfect, ripple-free DC current(and voltage) so you'd use an AC motor coupled to a DC generator?
Yes AC to DC MG-sets still are available ranging from kW to MW.
Prime mover :
- Squirrel cage Induction motor 5 / 7.5HP, 440V AC, 3Phase, 50Hz, 1440rpm.
3/5 kW, 220 or 440v, DC Shunt / Series / Compound generator 1440rpm.
Both AC Motor and DC generator will be horizontal foot mounted type with flexible coupling, continuously rated, self ventilated dynamically balanced and will have E/B class Insulation confirming to IS 4722.
You just need to regulate it with a battery load to make it ripple-free.
The large van de Graaff generators for particle physics are driven by a main motor (at low potential) running off AC. A van de Graaff is itself a DC generator, and it is certainly driven by that big AC motor.
Such generators have circuitry located at the hot end (maybe a million volts away from ground). The high-voltage electrode of the van de Graaff will typically contain a generator or alternator run off the same continuous belt (or chain, if the unit has been converted to pelletron) that delivers the charge. So, there's two DC sources driven by an AC motor.
Half a century ago, AC motor/DC generator sets were a common kind of welding apparatus. In the 1950s, that was how a computer power supply worked. A century ago, they were used to power arc lamps.