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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?

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    \$\begingroup\$ I don't think I'd ever call the output of a DC generator "ripple-free"... \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 12 '16 at 0:10
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    \$\begingroup\$ AC to DC conversions are much more easily and efficiently done electronically these days. \$\endgroup\$ – Peter Bennett Sep 12 '16 at 0:23
  • \$\begingroup\$ What you are describing is Ward Leonard system. Nowdays, you won't find it in pratical use. You can make perfect "ripple-free" Ac to DC converter, it just matters size and price. \$\endgroup\$ – Marko Buršič Sep 12 '16 at 6:25
  • \$\begingroup\$ @Marko Buršič That is not correct. A Ward-Leonard system is a variable-speed drive system in which a DC generator armature is directly connected to the armature of a DC motor. The field currents of both machines are adjusted to control the speed of the motor. Some such systems may still be in operation, but it is unlikely that any new systems are being built. DC power supply systems using an AC motor and DC generator are not Ward-Leonard systems. \$\endgroup\$ – Charles Cowie Sep 12 '16 at 10:51
  • \$\begingroup\$ "Available" is probably not relevant. Motors are still available, generators are still available, and shaft couplers are easy to machine. So on one level, the answer must be yes. \$\endgroup\$ – Brian Drummond Sep 12 '16 at 11:58
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Yes AC to DC MG-sets still are available ranging from kW to MW.

example

enter image description here

Prime mover :

  • Squirrel cage Induction motor 5 / 7.5HP, 440V AC, 3Phase, 50Hz, 1440rpm.

DC Generator:

  • 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.

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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.

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  • \$\begingroup\$ Thanks but I am not interested in a Van de Graaf generator. I am talking about a machine with an AC motor whose shaft is directly coupled to the armature of a DC generator. That is, it converts AC to DC electromechanically rather than electronically. \$\endgroup\$ – Mr X Sep 12 '16 at 17:14
  • \$\begingroup\$ Yep. A van de Graaff is a mechanically coupled generator. Belt drive (the original) or chain drive. You need a LONG belt or chain, for high voltage standoff. \$\endgroup\$ – Whit3rd Sep 12 '16 at 21:40

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