Experimental 48VDC Generator

[Chris Olson]

I've been trying to figure out what sort of head to use for a 48VDC genset, as I've wanted one for a long time. Then the lightbulb came on over my head today - use a little Honda conventional generator and rectify it. The AVR that Honda uses in their gensets will maintain 110VAC no-load output with the engine even at dead idle. I tried it with a MidNite Classic 150 and it did not work because it puts out 165VDC. It works beautiful with the 2:1 step-down transformers on our Bergey wind turbine, simply hooking the single phase output from the generator to the wye primary neutral of the transformers, and the hot to the primary on one transformer. It steps the AC voltage down to 55-60VAC and then rectifies it to 77-84VDC open circuit before being "clamped" by battery voltage.

I was surprised how well it works - actually beyond anything I imagined:

http://youtu.be/DEXEtjjiu20

[Rob Beckers]

Chris, that's surprising. I'm kinda scratching my head that this works as well as it does. That genny is putting out about as much as it can deliver. Considering the very simple setup that is amazing. Somehow it must be a good impedance match between batteries and alternator, with the help of that transformer. It's actually very efficient.

I'll keep this in mind in case someone wants to charge batteries (and doesn't have an inverter that can do charging).

-RoB-

[Chris Olson]

RoB, since I made the video of it I got it to work with a Classic 150 controller with no transformer and just using a full-wave bridge. This is the power curve I'm using for the little Honda generator:

https://lh4.googleusercontent.com/uMxAeT5ooogUhFzX849rfVoSUhgPhT1LpAUbf2XdvCo=w640-h440-no

The problem was getting the Classic to "kick in". With the generator idling I plug a 1500 watt electric heater into it, which causes the generator voltage to "sag" due to the exciter not being able to put out enough current to keep the output voltage up. This lets the Classic come out of HyperVOC and "lock in" on the genset. Once it has the genset "clamped" then it works fine even at full throttle. The only caveat is that if the batteries get to Absorb V a guy would need to control a AC SSR with the Aux output of the Classic and power a substantial load with the SSR (preferably a resistive heating load) to act as a "voltage clipper" to keep the DC voltage under 150VDC.

Pretty much the same as with a wind turbine, with the exception that this tiny generator can produce as much power as a fairly big wind turbine with no problem. I tried all the solar tracking modes, and I tried Hydro Mode with the genset. The Hydro Mode kind of worked but the AVR in the genset kept trying to compensate when the Classic attempted to track it, so the power output was very uneven. Using a wind curve shown above it works perfectly and puts out nice steady, even power - and you can adjust how much you want with the throttle setting.

[Chris Olson]

Longer term, it seems that single phase is not ideal for a DC generator. It causes my SSR's to buzz. In theory the battery bank should act as a capacitor of sorts. But there is definitely a problem with "dirty" DC power when the SSR's are buzzing.

[Rob Beckers]

I've not measured the frequency behavior of batteries, but bet that they are low-frequency 'capacitors' only. The higher frequencies will likely not get attenuated all that much. So, yes, all the noise produced by various devices will stay on the DC line. Adding some real capacitors in parallel (some electrolytics, some ceramics for the higher frequencies) should help.

-RoB-

[Chris Olson]

The only thing is, I wouldn't know what type or size of capacitors to use for that. I'm not very good when it comes to electronics. I'm sure the Classic has capacitors on its input that try to smooth that single-phase power too, and it's probably hard on those, I would imagine.

I think ideally a guy should use a three or four-phase generator. The problem is finding one that puts out enough volts (a small one). I have a Delco 22SI alternator, 140A, four-phase. But it is designed for 12V and I don't think the field windings in it would hold up if a person shoved more current into them to get the voltage up to 80-90VAC.

This is the problem with DC generators - at battery voltage the cables from the genset to the battery have to be huge. And it limits how far the genset can be from the battery. If you can find or build a unit that puts out 140VDC, then the wire run is not an issue and the wire size can be much smaller. The Classic 150 works beautifully to step that down to battery voltage.

I have stopped using the single phase generator for now because I believe the power is too "dirty" from it for my equipment. I'm going to look into either rewinding the field on that 22SI, or building a different polyphase unit that will put out over 100V.