gww - I guess I don't know enough about what the capacitors do in that situation. I'm not much on electronic circuits. The way I looked at it is that an AC 120V generator is really no different than a wind turbine generator - or any other DC generator. All generators produce AC current, including DC ones and the alternator in your car. The DC ones just have a rectifier - either a mechanical rectifier in the form of brushes and commutator, or a solid state rectifier in the form of a diode bridge. The only difference with the genset is that it is wound field instead of permanent magnet and you have to find a way to get the voltage down to more normal levels for a battery bank so it doesn't "clamp' it too far and overload the generator stator winding.
The little generator I used with a 2:1 stepdown transformer normally puts out 20 amps. The 2:1 stepdown transformer reduces the voltage by half and doubles the amps so I get 60VAC and 40A from it. That just happened to work perfectly on a 48V battery bank. The rectifier, I suspect, makes the sine wave look more square instead of a normal sine wave, so the DC ripple seems to be pretty minimal. If I reduce the throttle setting the exciter has less power available, it reduces the field flux in the gen head, and the voltage can "sag" more in the winding which reduces the power output of the generator.
I'm not exactly sure how the capacitors in your example could reduce the voltage enough to prevent the generator winding from overloading. And, in fact, I think I can get this work with a Classic 150 controller, without using the transformer, by plugging in some sort of resistive load to the generator with it idling to get the voltage to "sag" so the Classic will come online. Then use Hydro Mode in the Classic with the Vmp set at 140V max, 120V min. Once the Classic comes online in Hydro Mode it SHOULD keep it clamped below 140VDC and I should get better performance from it.
I think the key is a way to reduce the voltage and have a way to track the generator's power curve, and a Classic 150 should work perfectly for that. The only problem I had yesterday was the fact that I had 165VDC with the engine idling so the Classic wouldn't "kick in" as it was stuck in HyperVOC. Once I get it to "kick in" I'm pretty sure it will work and then the generator can run at its normal voltage with very little "clamp" and let the Classic do it's thing on the DC->DC conversion from 140V to 48-62VDC. If the Classic unloads the generator due to the battery getting to Absorb V I can have its AUX turn on a AC SSR and add a resistive load to its AC output to keep the voltage down so it the Classic stays online and the DC voltage doesn't exceed 150V.
That's the "vision" that I woke up with this morning, and is my next stage of the experiment for today. If it works, I've got success with a MPPT engine-driven DC generator, and at that point you could even get it to work on a 24V system. It is no different than what I've done with wind turbines with a Classic 150.
I think the other key is to use a very small generator. A 5,000 watt generator will put out ridiculous amps, approaching over 100A with the battery at 48V or lower. It's going to take some big stuff to handle 100A of DC current. Where with the little 2300 watt generator I'm using I only have to deal with 40-42A, which is plenty for what I need it for. Even a tiny engine-driven generator can rival a huge wind turbine, or thousands of dollars worth of solar panels, with ease for power output. If I can keep the amperage down to 20A thru the full-wave bridge, then the bridge won't get hot, it'll be more efficient, and the Classic 150 is the only way to do that.