Alternator Field Dump:Hobby Project Hits A Sour Note (Long)

Why not just fix the connection problem? Are you using the automotive connector and a suitable cable? I can't imagine this vibrates more than a car engine. If you do want to crowbar this thing use one of those 275a SCRs they use in big switcher supplies. Surplus places come up with them now and then. Hook your overvoltage circuity to the gate.

Reply to
Greg
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Well, here's the deal. I have an off-the-grid neighbor that has a 12v solar/hydro/generator system for his electric. We built a little board from a magazine circuit that regulates the output of a 12v ambulance alternator by varying the field in proportion to the battery charge....to accomplish battery charging for the dreary weeks. The alternator is driven by a 5 hp Honda engine with an appropriate step-up pulley arrangement. Nominal current with low batteries is

30-50A.

It's a little PWM circuit implemented with an LM723 and a 555 timer to make a quick-and-dirty PWM output. The output of the 555 switches a

30A rated series-pass transistor to supply about 2A max to the alternator field. Surprisingly, the circuit works quite well....most of the time.

With the engine comes vibration, and with Tennessee weather, 70F one day, 7F the next things get loose. Like the connections to the alternator field, which defines our failure mode. When the field connections get loose, the field will dump ~300V onto the dc 12V bus. Despite a hefty flyback diode (6A4) across the field and a fat 24V MOV at the output of the board, the board is destroyed when this happens.

The 6A4 will fail shorted, the Output transistor (like a 2N3055, but PNP) will be open E-B, the MOV is a cinder and even the 50v rated .1 bypass caps around the board will short. Of course, the 723 and 555 are toast.

We have tried fuses (make a handy indicator to tell the board has blown), a couple ohms to the field circuit as a dump (no joy there), bigger MOV's, and assorted blue language. Just for s**ts-'n-grins, we put an NE-2 across the field and wire-wiggled. Now I know how to make black Neon lamps.

I HAVE changed the 6A4 diode to a stud-mounted 200A fast-recovery type commonly used in induction heaters, but I have to wait for some parts from Digikey to try again. Short of soldering the wires everywhere external (not practical), I dunno.

Spike and surge experts come out come out where ever you are....thanks.

Reply to
Steve Cothran

It would seem simpler and cheaper

Even simpler than that, my first thought was to use a 1-wire alternator, with built-in regulator. Most offgrid folks don't like these, though, because of some difference in the charge of their deep-cycle batteries vs. car batteries. I have found that charging batteries properly (for home power use) is somewhat of a black art. Bulk charging is simple but "topping off" the batteries without boiling them is tricky, and this circuit (not my design) does a great job of that.

The user likes also this scheme because when the genny is running, the board will sense any applied load and source power to it while it charges the batteries. The threshold window on the board from "on" to "off" is ~ 0.2v. He can also adjust the charge rate (current) and the cutoff voltage with a couple of pots. A car regulator or one-wire alternator might do the same, but offgrid guys stay away from them for reasons I don't know.

I know we need to address the root of the problem, ie the field connections at the alternator and at the controller, but still I wanted the board to be robust enough to handle this kind of oops. Things shake and rust, especially outside where a Honda engine belongs.

Reply to
Steve Cothran

We're working on that. Corrosion causes more problems than vibration, but vibes are still there. This is a little 1-cylinder engine on a metal plate that doesn't have all the frame mass a car has, so it indeed vibrates more than a car engine.

The problem isn't every day. More like a couple of years between blowouts...after the wires/terminals shake and corrode.

Reply to
Steve Cothran

Set up a maintenance schedule and adjust as required.

Al

Reply to
Al

anti oxidant grease for the corrosion and locktite/locknuts for the vibe?

Reply to
no_one

No technical advice from me. I'm just lurking on that stuff...

But, vibration a problem? Why not just secure the thing to a heavy base, like concrete? Maybe add a flywheel?

And what's with the corrosion? A grounding mismatch issue? Tennessee salt spray?

Reply to
Wayne R.

Wayne R. wrote in news: snipped-for-privacy@4ax.com:

If he's in E. Tenn, the problem is all the lakes, lotta moisture in the air most of the time.

Reply to
Anthony

I am on the water in Florida, we have our share of weather and critters. You just need to put the same amount of engineering into the protection of your machine as you did in it's design. Finding a junkyard NEMA 3R cabinet will go a long way towards saving your electronics and pouring a few bsags of concrete under your gen set will take out most of that vibration. Build a little dog house over it will keep out a lot of the weather. Just be sure the engine can breathe. Maybe a garage sale dog igloo?

Reply to
Greg

Tennessee has very unique weather. Today, f'rinstance, it was near 60F at noon. By 7 pm it was 20 and snowing. Condensation plays havoc with a lot of stuff here. Ring terminals and spade lugs rust. Terminal strips without barriers between the screws used in this service will eventually short. Remember, we're talking stuff that sits in the equivalent of a woodshed all its life.

BTW, did I mention the assorted vermin that like to chew on wires? Or the ladybugs that like to nest between the armature and coil of your favorite contactor?

Reply to
Steve Cothran

Steve Cothran wrote in news: snipped-for-privacy@4ax.com:

Hey....I live here...believe me, I know.

Reply to
Anthony

why not just use wingnut type screw on connectors or some shit and peen the last couple of threads? or double nut them, or junk the stupid ambulance alternator and go with an old generator from a vw and an external regulator , thinking of it , you could use an older dodge alternator with the associated external regulator, you could even put an inverter on that and run a smart charger from the inverter. Or , go back on the grid like everyone else........... MLM

Reply to
JohnQ.Public

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