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Forum Index : Solar : Simplified Solar Hot Water

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Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3370
Posted: 12:27am 04 Aug 2020
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  BenandAmber said  

I also am trying to get a external 48 volt voltage regulator designed

It need to work just like a regular car truck van alternator work just higher



That is not going to work.

Standard alternator needs to turn about 2,000 rpm before it can even reach 12v and the alternator light goes out. May have to go to 3,000 alternator rpm for full rated current.

So alternator may need to turn at 12,000 rpm to charge a 48 volt battery.
Max rated speed of alternator usually 20,000 rpm.

Its not going to work in a vehicle because even if you could get it to turn that fast, it would only work over a very limited engine speed range, that would make the vehicle undrivable on the road.

A normal alternator typically runs about three times engine speed.
Normal car engine speed might be 800 rpm idling, 6,000 rpm flat out.
Alternator x3 that, or 2,400 rpm to 18,000 rpm. works pretty well like that.
Cheers,  Tony.
 
Jacob89
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Joined: 10/09/2017
Location: Australia
Posts: 17
Posted: 04:09am 04 Aug 2020
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Success! Sort of. I set the lower value at 100k as you said Tony, and it actually started switching!

All the problems I've experienced seem to stem from these cheap and nasty aliexpress transient voltage suppressors. I think they are polarised despite not having the ring marking them. I can orient them by which way up the writing on the side is, and I found by putting them the opposite way to what I have been previously they let go immediately.

So with one soldered in the "right" way, and the low voltage trip resistor at 100k it finally started working.

A few things:

The circuit does not work without the TVS in place! I cannot explain it but without it, all kinds of weird stuff happens.
It some really weird really high frequency switching. 400khz - 1mhz depending on which meter I use. The mosfets get hot and I was getting hf burns from the heatsink. The input(array voltage) stays high, and the element sees much lower voltage, which varies according to the polarity I connect the multimeter in?
I connected an incandescent bulb to get some visual representation of whats going on. It actually glows, but doesn't seem generate any appreciable heat and nor does the element.

When the circuit is working properly, it cycles quite slowly. I can see the bulb flashing at around 2-3 times a second. Using the 100k lower and 220k upper means it doesn't turn off until the voltage drops to around 100v and turns on again around 220v. Today has been intermittently cloudy which is perfect for testing this. When the sun comes out the array is big enough that the voltage doesn't drop low enough and it won't switch off until another big cloud comes. In full sun it will stay over 200v but with partial cloud it will dip right down to 110v and still not turn off.

So I added a 47k resistor in series with the 100k one. This worked briefly but with the much narrower on/off window, the switching frequency was much higher and the TVS doesn't seem to be able to cope with the faster switching and let the smoke out again.

I think this explains why the first two units kept blowing the voltage suppressors, with the higher and closer trip points the switching would have been much faster and they couldn't take it.

Nonetheless I'm pretty pleased to see it run, and I'm going to order some better transient voltage suppressors and hopefully I can better optimize the switching points.
 
Jacob89
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Joined: 10/09/2017
Location: Australia
Posts: 17
Posted: 04:24am 04 Aug 2020
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  Warpspeed said  
That is not going to work.

Standard alternator needs to turn about 2,000 rpm before it can even reach 12v and the alternator light goes out. May have to go to 3,000 alternator rpm for full rated current.



They will output much higher voltage though if you feed higher voltage to the field. I was able to get over 60v out of a 12v alternator by bypassing the regulator and feeding 12v straight to the field. Spinning about 5000rpm.

Granted, that was open circuit no load voltage though. I don't know what you could expect with a load on it. And I don't know if the field is saturated at 12 volts, or if you could put more in it, or if its already way past saturation at that point.

I seem to remember an old thread by Oztules where he'd made an external regulator for an old 12v alternator so it could be used to charge a 24v bank.

A 24v alternator might be a better candidate for modifying to charge a 48v bank though.

Whether or not you could actually charge a 48v bank from a modified 12v alternator, I'm not sure, but I am sure it would be quite inefficient, and noisy, however you're going to get it spinning at 6000+rpm.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3370
Posted: 04:27am 04 Aug 2020
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Great stuff there Jacob

The transient suppressor does exactly that, reduces any very high voltage spikes which may be getting back and confusing the control system.

Definitely sounds like you have the dc type polarised suppressors, if they burn up when turned around.

With possibly fake Chinese parts you can never be quite sure what you are actually getting. The part number on it is not always a guarantee of whats inside.

If the suppressor runs hot, which it may well do if the switching frequency is very high, you could parallel up some suppressors, or add more electrolytics to slow the whole beast down. Those suppressors are very effective, but can only handle a watt or two.  As you may be switching kilowatts, it does not take much of a voltage spike to heat them up.

If you spread the trip points further apart, that will slow it down as well. Its really a case of experimenting.  Anyhow you are making real progress and that's the main thing.

As you have found, plenty of sun, the load will stay on continuously, and that is a good thing. If you add on extra load it will start to switch again.

Once the two voltage trip points are set appropriately, it should self adjust and "do its own thing" under all solar conditions.
Cheers,  Tony.
 
Davo99
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Joined: 03/06/2019
Location: Australia
Posts: 537
Posted: 06:34am 05 Aug 2020
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I'll have another play with mine and pay particular attention to the resistors and set up a bank of panels to match as close as I can.

I asked the local Auto elecs about using a 12 V alternator for higher voltages a while back. They do rewinds and both the father and son in the business are older than god so should know something.

I suggested spinning the alts a lot faster and they said the input voltage and Current would need to be a LOT higher as well. They said this would cause overheating. They said you could lower the amps to keep the heat down however time you got it to a workable level the amps would be very low.

I have seen a white paper where an external Reg was used to get a decent 24V output from a 12V leece Nevile  alternator. The external regs are used a lot in marine applications and are stepped charge I think the term is, which is a lot more efficient than the built in regs. A big bonus with the external regs is getting something like 80% output at engine idle speeds. There is one controller I saw which has a temp probe and will do 100% output for a time at least as the alts tend to heat up due to lack of airflow so the controller backs off the output when the alt gets too hot.  Still be a big jump on charging a depleted battery ASAP at low engine speeds.

I also read a number of times car alts themselves are in fact very efficient as they are of the same Claw pole type construction type as power station alts. The thing with car alts is the internal regulation controlling them is basically crap.  One of the reasons given is the extreme conditions a car alt works in with under bonnet temps and widely varying RPM.  A stationary application is almost always a lot easier on the alt because they work in free air not air that can easily be 100C before it is asked to cool the alt. With an external Reg a lot more control can be applied and the efficiency can take a huge leap as well as the variance in application.

I have played a LOT with the Mitsubishi Enclosed ( twin) fan alts and they are virtually unbreakable. I have run them for hours upon hours on my lister engine at absolute FULL power and had them on a battery as well as Direct connected to a load with no batt which is supposed to be a Big No No.  Hasn't seemed to worry them a Bit.  Besides Running them dead shorted, I have put them through about every torture test I could think of to try and blow the things but haven't succeeded.

The Twin enclosed fan alts I think are a Huge step up from the external fan type. On the twins, one fan cools the windings and one fan cools the electronics.
You can feel the Temp coming out of them like a fan heater but they seem to run hot but stable even flat out which the ones I have tested  all output slightly more , 5-7A, on an 80 A alt all day.

I have only run them stock with the internal regs and they are pretty inefficient with the mechanical input power. I'd guess they are pulling about 3 Kw ea for an 80A or 1000W load. I have pulled them apart and it seems tricky to isolate the regulator and the Diodes with the way they are built but no doubt a smarter person could manage it OK.

They do seem to be tough as nails though and I would think that if one were going to over clock one, The twin fan ones would definitely be the first choice as the Cooling seems very good indeed.
 
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