Any reason this wouldn't work? Generator backup power through a midnight classic

[Vector]

Im trying to maintain a radio/microwave repeater site that suffers from add-on-itis. it started out with a small 1000w polysilicon system (24v battery bank). which was then added to with another amorphous 2kw system running on a midnight classic 200. then a second midnight classic 200 was added when the polysilicon systems charger died. On top of this there are dual 1000W chargers (24v) being powered by 120v diesel generator part time in the winter when there isnt enough sun. this generator is on a controller that monitors battery voltages and fires the generator when the voltage drops below 50% capacity in the batteries.

ok, so total mismash of systems, oh and the operator is an non profit internet provider/society that is the only connection to the outside world for a remote community. so doing things on the cheap and being reliable make this a near impossible but very important job.

So here is my mad plan.. One of the AC chargers has died, the other is on its way out. Is there any reason that rather than going though the chargers i couldn't simply rectify the ac voltage into DC and pipe it into one of the classics as 170Vdc (120vrms gives 170v dc when rectified)  generator has a 15A ac breaker so should limit to 12a avg or 15a peak, just in case something really bad happens. there is no way the battery bank could absorb that much wattage in any case so the classic should limit below that amperage.

if i were to parallel this into the bus the polysilicon panels are attached to (very under utilized classic) im thinking at most i would need to add a blocking diode to the polysilicon combiner to prevent the rectifier from pushing voltage through the panels.

how crazy is this? the reason i ask is it would be a simple replacement for two bad ac chargers and allow me to actually see how much current is being pushed into the batteries, which the ac chargers don't let me do currently.

[mike90045]

You would have to configure the classic to some fixed MPPT mode I think, or it will try to find the MPPT point of an alternator, and smoke something.   PV panels as a Current Source, can be loaded and IV curve played with, an alternator, not so much.  Wind alternators have a pre-programed curve , so maybe someting like that ?

[TomW]

If I recall correctly, Classic wind curves are a voltage / current curve so it seems you could set that up to be fed by the fuel generator? Maybe? Say a flat curve of 170 volts / XX amps suitable for the source? You could sneak up on the loading by tweaking it gradually until you get a proper load on the engine in its sweet spot for efficiency, etc.  Maybe even easier if it uses a common vehicle type alternator that you can manually tweak the field strength.

I am pretty certain using an active MPPT such as solar or Legacy P&O on it will draw out the magic smoke as it tries to pull every milliamp it can from a very stiff source.

Maybe one of the engineers or Ryan will jump in on this. Personally. I would try it but I am relatively experienced in power electronics and understand what I need to be aware of to use The Classic in a non approved manner.

Wait, wait. The coffee just kicked in. Scratch the above, at least for now. Might be sensible to trade or modify  some gear and do this right from the get go?

I see folks get caught up trying to use what they have thinking it is economical when it really is just the opposite and they could swap some proper gear in to do this right. If you spend a hundred dollars to reuse that hundred dollar piece of existing equipment or you could just buy the $150 piece of gear to do it right and sell the existing piece of gear and be better off.

If that makes sense?

Lots of thoughts in my head on this, too bad I got stuff to do just now and have to cut this short.

Good Luck.

Maybe get back to you with a few ideas later if time allows.

Tom

[Powerplay]

So here is my mad plan.. One of the AC chargers has died, the other is on its way out. Is there any reason that rather than going though the chargers i couldn't simply rectify the ac voltage into DC and pipe it into one of the classics as 170Vdc (120vrms gives 170v dc when rectified)  generator has a 15A ac breaker so should limit to 12a avg or 15a peak, just in case something really bad happens. there is no way the battery bank could absorb that much wattage in any case so the classic should limit below that amperage.

Would it be easier to power your loads directly from the diesel(s) when there is no sun?  And switch back to battery when there is some sun.

Is all this equipment in a relatively climate controlled area? 

[harryn]

Full wave rectified 60 Hz AC is pretty noisy power and generators are notorious for making very spiky power on top of that. 

MPPT controllers deal with "shifts" in voltage and current from the panels, but usually they are not intended to deal with things that happen at 120+ Hz, which is what you are asking it to do.

I suspect that in order to make the rectified power clean enough to feed into the classic, (without risking damaging it), it would take at least some amount of capacitance and inductive filtering to be sure of not damaging it.  By the time you do that, it might be 1/2 way to a DC charger.

I know that you are trying to help them save money, but supposed that Classic dies for some random reason?  Even if your idea had nothing to do with it, people will wonder if your modification caused their $800 classic to die and at least in their heart, hold you accountable.

An IOTA 24VDC, 40 amp charger is around $400.  That is  50% of the cost of a Classic if it had to be replaced.

Another path, that might even turn out to be cheaper is to remove the AC generator head from the diesel generator and replace it with a 24 volt alternator.  24 volt alternators are common in heavy duty trucks, some school buses, and the marine industry.  Marine industry equipment tends to be expensive, but semi trucks are very common so there are a lot of parts around.

Some of the high output 24 volt alternators use an external voltage regulator, but I think some are built in.

100 amps from a 24 volt alternator is easy - as long as you achieve a minimum rpm.  Below that minimum, they more of less generator zero power, but the engine does not have to run at a constant frequency like an AC generator.

[ZoNiE]

Marine stuff is only expensive because it is painted white 

[tecnodave]

Vector,

Solar controllers are meant for "soft sources" of power that change in voltages when loaded down, a generator does not do that! In my opinion you would be risking an expensive controller to do a cheap job. A good battery charger can be had for less money than the replacement cost of a good MPPT controller.

David