I want to build or buy a power supply which will in effect mimic the output of solarvoltaic panels. I want to feed the output into an OutBack MX60 charge controller. The maximum input is 141 VDC and 60 amps. I can use any voltage above about 63 VDC (up to 141) to charge my 48 volt battery bank by programming the MX60. This charge controller will automatically adjust the absorb, float, and equalize charge rate voltages and times. I plan to power it with a 120 VAC 60 Hz generator.
What I had in mind is a step-down transformer, a couple of rectifying diodes, and a smoothing capacitor. The problem that I expect to encounter is ripple. The MX60 is only designed for solar panels so it may behave incorrectly (or worse) if there is any ripple or pulsing present. Any suggestions? I don't want to do anything that might damage my rather expensive charge controller. I'd be happy to buy a power supply if it's not too expensive but so far all of my searches have not turned up anything. Are there any web sites that have high current power supply plans? I couldn't find any.
You might save yourself a lot of trouble by locating a couple of 48 V ACDC supplies that can be put in series to provide 96V. Or, you might find a 72V supply. Search for something like a Vicor MegaPac. There's not much competition for the 48V units, so you'll probably find some kind of a deal. If your nominal output voltage is 72V for a transformer/rectifier running on single-phase 60 Hz power, at 60 Amps output, you'd need about 60,000 microFarads capacitance to keep ripple down to about 8 volts peak to peak (11%). This is not out of the question, but you'll end up spending more than the cost of some surplus switchmode power units. Paul Mathews
That's a lot of solar panels you're wanting to mimic. At 10% efficiency, I suppose it's about 25 to 30 square meters of surface. Do you really need that much current? If you really want your source to look like solar panels, you should have it look like a moderately high impedance. Into a short circuit, solar panels look about like constant current sources. Also, if a shadow passes over the panels (even the right small portion of them) the output drops drastically. Controllers have to be able to handle that sort of thing.
A mains-frequency transformer that size won't be a small piece of iron either. You'd probably be a lot better off with switching supplies, and you can filter the output to get the ripple to a low value with a lot smaller components than it will take to do it with a mains- frequency supply. Now if you have three phase mains available, the ripple can be much more tolerable, but it's still a lot of capacitance (or a little less capacitance and some inductance; a little inductance goes a long ways in reducing ripple especially if you want to get to a very low ripple percentage.
There are surplus houses that deal in power supplies...though 4kW supplies aren't all that common anyway. Also, I'm not too surprised you're not finding plans for them (switching type, at least) on the web. That's a lot of power, potentially kinda dangerous to be playing with, and because of the currents involved, the physical construction details and proper selection of components is critical.
Knowing a fair bit about why you are trying to do it this way, several things come to mind. The obvious-but-it-hurts-the-wallet method to get an off the shelf charger is to drop bucks on another inverter (which contains a nice high current switched-mode supply, filter capacitors, charge control smarts so the MX60 can stay put on the solar panels and not get involved), and set it up such that you are not using the inverter section (where all that irritating pass-through-the-generator stuff comes up when feeding the house power). Could be another Outback (which would give you a spare to move around in case of any issues), could be a less spendy brand.
Another off-the-shelf option is to visit the local golf cart dealer, as
48V golf carts need 48V (really 60+V, same as your "48V" bank) chargers, but I don't know that they pay much attention to efficiency when designing those.
The best method for fuel to DC is fuel to DC, not fuel to 60 Hz AC to DC. In reality, what's commonly available (other than esoteric and overpriced "48V" DC gensets for the cell tower market) is fuel driven welders, most of which are doing some form of 3 phase on the way to DC, though there may be "true" direct DC versions (sounds like a lot of brush wear issues, though). One with a decent OCV (in the 80 V range) should work for you, though the main output power is typically intended to be in the 24V range; but if you get one without too many new-fangled smarts, it should be possible (perhaps with some taming of the MX60's MPPT settings) to operate on the higher-voltage part of the curve. Might take some fiddling with (or at least monitoring) the control arrangement, I'd think.
Another approach is to grab a high-amperage car/truck alternator that uses an off-board regulator, drive it with a fuel engine, and crank up the voltage. These are also typically 3-phase AC, with a 6-diode bridge rectifier. So long as the diodes are not overly low voltage, and you keep the current within/well below the current rating, they are not overly picky about what voltage they are putting out. The insulation should be good for it, and the heat generated is a function of current.
Perhaps a heavy-duty truck alternator that's already intended for 24V operation would be a good starting point, but I recall people talking about ramping 12V units up to 70 volts for various purposes with either no trouble, or frying low-spec diodes and replacing them with higher-voltage units when they die. In the case of feeding your MX60, I'd replace them at the start. 100-200 amp units are rather common, and should not cost much at the junkyard.
Thanks for the pointers. Having a name (Victor MegaPac) to search for should help a lot. I've tried searching with every combination of words I could think of and I usually end up finding stuff around 1-2 amps.
I have something like a 24,000 uF capacitor in my junk box but I didn't think it would be enough. 60,000 eh? Time to start digging through ALL of my boxes of stuff in the garage. I have an old NCR cash register somewhere that has a lot of good stuff in it if I can only find it...
If I was going to be using that many solar panels, then yes, it would be quite a few. That just happens to be the maximum amount that the charge controller will accept. I could go down to, say, 40 amps and still ge reasonably content. I'm just trying to cut down on generator run-time and hopefully gasoline consumption.
I just have single-phase AC. Someone suggested a surplus line transformer (has a catchy name but I can't think of it right now) but there don't seem to be any such things available near where I live. The closest town is basically a huge trailer park and I can't find any junk yards or places that sell surplus electrical stuff. No scrap metal places either. Before I buy a huge transformer I want to be reasonable sure I can deal with the frequency/ripple problem effectively. But then I could always find a use for a big 2:1 transformer...
Ah, the upiquitous Ecnerwal ;-) Someone uses your "cats" signature on the OB forum so I assumed it was you.
I'm using Inverter #2 only to charge the batteries right now. Yes, it was an expensive battery charger and not all that great (as far as charging currnet goes) for the money. I just completed my gravity-fed water system with float switch and I want my other inverter back so I can have my water on automatic. With only one 3600 watt inverter (OB VFX 3648) no doubt some bad stuff will happen if someone is using the vacuum cleaner or washing machine when the well pump comes on. I don't have any solar panels yet (other dreams of mine have come true) so in the meantime I'm looking for a better way the charge the batteries. Of all the things I've thought of so far using the MX60 seems to make the most sense as far as ease of use and properly charging the batteries. It has all the smart stuff built into it. OutBack says just get a bigger (12,000 watts or more) generator. Well, like you said, running the house from a generator stinks unless I spend $4000 on a 5000 watt inverter generator and I'd still have the "where the hell is all the power going?" issue. Right now I'm charging my batteries with a $200
2000 watt generator via Inverter #2. That seems to be about as small as I can get away with. I usually have to run it about 3-4 hours a day (evening) to get the batteries up to 100%. Sometimes a little longer.
The good news is that since I switched to propane refrigeration and have my well pump on it's own generator I could probably get by nicely with only six
175 watt panels. Hopefully I could even squeeze in the well pump as I only run it about 15 minutes a day.
I found something like what you described but it didn't seem to have a float or equalize setting. It's in my referece file if all else fails.
Welders eh? I was looking at welders a while back and I don't even remember why now. They seem to be fairly inexpensive compared to a lot of things, at least the stuff Harbor Freight sells. I'll take a look.
I have a 12 volt 63 amp alternator that I was using to charge a 12 volt battery bank. A couple of people on the homepower NG told me I could use it for 48 volt charging if I replace the diode packs with heftier diodes and heat sinks and somehow modify the voltage regulator. Well, I'm almost smart enough to do it, but not quite. I tried putting a VR in series with the field control and managed to get about 30 volts. My understanding is that I have to remove the voltage regulator and bypass the circuit to get the full output. Then I have to control the voltage one way or another. Controlling the field would seem to be the way to go, but I'm not sure. If I could just find the instructions somewhere I could do it. I've looked at some permanent magnet alternators and that seems like it would work just fine. The voltage could be controlled by engine speed and I could probably adjust it at no load for equalizing too. The problem is that I've only found one source and a couple of people told me "don't buy anything from them." I have the booklet that I downloaded from the wind generator guy (can't think of his name) and building a PMA and driving it directly from a small engine is yet another option. It's not yet clear to me just what determines how much current you will get--the voltage part is apparent.
In any case I'm not going to connect anything to my MX60 unless I'm pretty darn sure it won't hurt it. Some kind of alternator seems like a good option and could charge the batteries directly, with some futzing.
Sounds like the safest way for you to do this, given the lack of resources, is find a big 2:1 transformer and rectify and filter it. Unfortunately, a "pole pig" is very unlikely to be 2:1, though you could use the secondary as a center-tapped winding that you put 120V across and get 60V from. Does the output have to be ground- referenced? If so, things get ugly again because you don't have a grounded center-tap you can work from -- unless you can float the 120V input. You'll need to float either the input or output, to make a center-tapped full wave rectifier. A half-wave rectifier would be just too awful to try to filter.
For the filter, I'd highly recommend using a choke in your filter, if you want to get to seriously low ripple. A quick simulation using an
0.1 farad filter cap with 1 milliohm effective series resistance and a
40 amp load shows about 1/4 volt ripple, which is probably plenty low enough for you. But if you can get a 20 millihenry 60 amp choke, you can use just a couple 4700uF caps (less than 1/10 as much total capacitance as in the first case) and have about 1/10 volt ripple. Not that the choke would be either cheap or easy to find, but it can help a lot in the filtering.
It's VICOR, not Victor. A couple of 2 kW 48V supplies in series should do it, and most work fine in series. (Not so with parallel operation.) By the way, if you do use transformer/rectifier/capacitor, I doubt that the ripple needs to be much below 10% at 120 Hz. Some posters have suggested ways to get down to under 1% ripple. Paul Mathews
Oops. And I was being careful NOT to write Victor. Anyway, I found their web site and they have a model that sounds like it's in the right power range. It's just kinda scary because they don't have any prices posted. Thanks for the info!
Wow. Millihenrys. Sounds big. Makes Faraday sound like a midget. I asked about this on another NG a while back and a couple of people told me not to use full-wave rectification but to just use two diodes. What you are saying makes more sense to me. I've built several small (1-2 amp) power supplies but never anything this big. My problem seems to be that I have no formal education in electronics and it has always just been a hobby for me. Because if this I don't know what it is that I don't know.
By "choke" do you mean an AC capacitor connected across the outputs before it is rectified? And then the 4700uF caps after it's rectified? I think it's time to call my father-in-law. He just might have something like that.
From what everyone has said it sounds like the ripple can be dealt with satisfactorily. It also sounds like I'm going to have to break down someday and get a better oscilloscope. The one I have is a Heathkit from the 60s and the tubes are so weak I can't even get a good sine wave image any more.
This may sound too circular... but probably the most economical and versatile way of getting what you want will be a bunch of car/golf cart/etc. batteries connected in series, with some series resistance (paralleled car auto headlamps to get your desired current) to switch in/out as series drop elements if you want to mimic internal impedance of a solar panel.
Ironic, huh? Using batteries to power a battery charger!
If there is no need for isolation you could also do this from AC power with no transformer, and SCR's for voltage control. The output waveform will look like that from a giant light dimmer and I don't know if this can be adequately filtered to do what you want to test.
No, the choke is used as follows: diodes to a capacitor. The choke is an inductor (coil of wire, generally on an iron core for stuff like this), and it goes from that first capacitor to a second capacitor. The other side of both capacitors are connected together, and to the transformer center tap. If the diodes are connected anode to the transformer, and cathodes to the capacitor terminal, then that terminal should be the POSITIVE one (or you get exploding capacitors!).
You can do the filter with one big capacitance (maybe made up of several capacitors connected in parallel), or with a couple much smaller capacitors with a choke (inductor) in between them. What's most economical probably depends on what you can pick up surplus at a reasonable price. Maybe I'm biased toward using the choke because I have some kicking around. ;-)
Be careful with this stuff...it's not terribly high voltage, but the currents can melt things pretty quickly. I've heard of people doing serious damage to fingers when they get a ring across something that delivers enough current to make the ring really hot. You really don't want to be trying to hold liquid gold in your hand.
Well, I generally estimate how much it would cost to build something and then figure out how to do it with the stuff I already have. If you have enough boxes of junk....
This is one reason I don't wear any jewelry. Power saws are other reasons.
At least I'm not the only one. I've done something similar by using an alternator, inverter, and 12 volt battery to run a battery charger to charge
12 volt batteries. It also made for a cheap inverter generator (sorta).
That's about what I was thinking when I first got the idea. With a maximum input of 141 volts it seems like it the 120V AC from a generator could be rectified and filtered then it might work.
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