48V charging via ST generator and solar charge controller (no ST mod)

[mobile_bob]

what specifically are you looking for in a transformer?

what voltage input, and what voltage output? and of course what kva capacity

maybe we can find something a bit more local

bob g

[mbryner]

Well, it has not changed since the beginning of this extended thread.   To recap:  I want to take 240 VAC from a 6/1 Listeroid w/ an oversized ST (7.5 kw), transform down to 60-80 VAC, rectify, filter and feed into Outback MX80 MPPT charge controller for 48 V battery bank charging when there is inadequate solar insolation.   I have 3400 watts of solar array, which is what the max current the charge controller is set to (should be at least).

I don't want to run the ST head at 120 VAC because I want to have it available at 240 V to bypass the batteries and inverters to run the house and well pump if anything goes wrong w/ electronics in the inverters, etc.

A good portion of this thread discusses using toroid transformers from e-bay.   Now there's the problem of nothing to limit the current:  the toroids that I bought would be max out at 2400 watts.

Don't want to have to flip a lot of switches.   Trying to keep it simple for the wife.

Looks like Bruce's idea of putting a resister in the excitation circuit may be best.

Thanks, Marcus

[mobile_bob]

sorry to put you through memory lane, its just that this thread has got pretty long and sometimes the specifics
change from one end to the other.

so a 6/1 driving an st7.5, the limit of the 6/1 is about, what?  3.5kwatt?

would a 3kva transformer be big enough, or do you need 5kva (the next step up in size generally)?

what i am thinking is maybe we can find a 480/240 to 240/120 transformer in a 3kva or if need be a 5kva
and feed the primary 480v winding with 240 from your genhead, you would then have 60vac off the 120secondaries

if that sounds reasonable, i will keep my eyes open for such up here in the seattle tacoma area for ya.

bob g

[mbryner]

3 kva and 5 kva are too small, unless my calcs are wrong. 

3 kva at 120 Vac is 25 amps rating, correct?
25 amps x 60 volts is 1500 watts, worse than where we are now.

5 kva at 120 Vac is 42 amps.
42 amps x 60 volts = 2500 watts, about the same as using the toroids I have already.

Both above calcs are using 4:1 windings ratio, i.e. the 480:120 v configuration to go 240:60.

That's how I came up with a 7.5 kva transformer.   If you find one, it would be appreciated.

[BruceM]

Marcus's MX80 is NOT a boosting charge controller, so it would be better to have 80-100 volts, not 60. You won't be able to finish a charge with 60 volts, unless adding capacitance to the filtering, which screws your PF, and then the Listeroid won't be able to match the PV current.

A set of smaller transformers would also get you to 3400 watts, so watching ebay for a while might pay off.  The current set of toroids can be the choke filter, secondaries in parallel.

Nothing wrong with the relay or switch for dropping the ST voltage, either.

I don't know about operating a 440V primary on 240V, I haven't ever tried that. It seems unlikely to work at even half the rated wattage.

[mobile_bob]

Bruce:

i am going to ask because i don't know for sure

what happens to the 60vac when you rectify it with a full wave rectifier?

does it not end up being 60 x 1.414 = 84.84vdc?

or does that fall off nearer to 60 vdc in a single phase system under load?

or after filtration?

bob g

[yellowhead]

Since I've been running through similar calculations for my system I thought I'd jump in.

Assuming the MX80 delivers the full rated 80A into a 48V nominal battery bank being bulk charged at 57.6V thats a maximum input power of ~4600W isn't it? So it might work with a 5kva transformer although I'd probably be inclined to bump up to 7.5kva as well; I hate running things at their limit (although you could argue the same for the MX80 in this case 

But if I understand all this correctly, the Lister will be blowing smoke long before the MX80 gets to pulling 4600W off the charger. So I guess we need a way to get the MX80 to back off before this happens. Assuming that dropping the current limit for the MX80 to say 60A isn't acceptable (and it would be less than ideal if you forgot to reset it and then limited the charging from the solar array) I'm thinking we need a way to start reducing the input voltage to the charge controller when the current reaches some defined limit. That way the MPPT tracking smarts can hopefully be persuaded to back off the charging current in an attempt to restore 'maximum power' to the batteries.

For the ST/rectifier/filter approach your are planning reducing the field current on the ST might be a way to persuade the head to start dropping the output voltage. For the switch mode power supply approach I am hoping to use it looks like I can drive a control input to achieve much the same effect ...

Or maybe I'm just confused 

[rl71459]

"I don't know about operating a 440V primary on 240V,"

I have a bit of experiance on this subject. The Plant's I work at (There is 2 on the same Road) have different Voltage services (#1 is 240 vac 3 Phase) and (#2 is 480 vac 3 Phase) at both plants we have
many machines of different voltages. We frequently swap transformers from shop to shop and often
use secondaries as primaries and vice versa. as well as use primary voltages lower and higher than
tagged.

We have never had a problem and even have been advised by the one of the transformer companies
that this is a common practice...

Rob

[BruceM]

Thanks very much Rob, on the transformer info.  I've done secondaries as primaries before- but not primary at half rated voltage.  That will make finding a 3400 watt output unit much easier for Marcus.

Bob- With full wave rectification and a large choke as filter, AC volts out are equal to DC volts out. Some capacitance before the choke can be used to raise the voltage, but since Marcus is trying to get the max output of the Lister (3400 watts) then I wouldn't, as this lowers your PF.  In a pinch it will do the trick for a modest voltage increase; you have to use motor run caps, though, as nothing else can take the massive ripple current.

I use the method of switching in (dynamically) capacitors for regulating the output my 120VDC battery bank charger, but I'm only putting out 1824 watts (12A at 152V), so can afford the PF loss.
(I have multiple transformer taps that are controlled dynamically, also.) 

Marcus-  I forgot to mention, you were wrong on the transformer requirement and output calculation of the MX80.  The PWM buck regulation process gives you an inductive conversion of power, so that like a transformer, you will get close to watts in equals watts out. So a 5KW unit would be plenty. 

Yellowhead-  The MX80 has a programmable charge current limiting feature, which you can set to whatever you want.  Marcus would like to get his AC charger to match his PV system output of 3400 watts, so that the MX80 will handle it with no changing it at the keypad; his concern is for simplicity of operation for his wife.  This also has the advantage of reducing charging time and getting the best fuel efficiency out of the Listeroid. 

[yellowhead]

Yellowhead-  The MX80 has a programmable charge current limiting feature, which you can set to whatever you want.  Marcus would like to get his AC charger to match his PV system output of 3400 watts, so that the MX80 will handle it with no changing it at the keypad; his concern is for simplicity of operation for his wife.  This also has the advantage of reducing charging time and getting the best fuel efficiency out of the Listeroid. 

Apologies. I thought the solar array was bigger than the potential capacity of the charger. I just checked back to the start of the thread and I see that this isn't the case ...

FWIW, I've PM'ed the guy who was using that 48V power supply I mentioned a while back. He claims to be able to use it just fine for bulk charging a 48V battery bank so he must have been able to push the output to 57.6V (spec. sheet notwithstanding). Obviously not enough for equalizing though. I've opted to go with a 48V battery bank after all so I'll give this a go at some point in the next couple of months. If I get no luck (or smoke) I'll be chasing down the same path as Marcus ...

[BruceM]

Yellowhead- For somewhat cold wet lead acid batteries (45F?), the cyclic charge voltage is about 15.2 volts. This gets to 60.8V for a 48V nominal battery.  So for winter charging, you'd want to start with at least 65 volts.  Unless you have one of the new fancy boosting charge controllers by Midnight Solar.

[BruceM]

Here's just he 5KW transformer that's needed, 240V in 80V out.
http://cgi.ebay.com/Step-Up-Down-Transformer-5KVA-5-Kva-Output-80v-160v-GUR_W0QQitemZ360237030941QQcmdZViewItemQQptZBI_Circuit_Breakers_Transformers?hash=item53dfccde1d

[mobile_bob]

that looks to me to be about the best solution yet, and where the heck would a guy find another like it?

and it is in california so shipping to Marcus should not be too bad?

i don't think i have ever seen one like that.

bob g

[BruceM]

It's in Los Angeles.  I know it's not a super bargain, but it is exactly what is needed. 

[mobile_bob]

after talking to Mark today,  about this project he reminded me that we have some SD5 generators

the sd5 is a dual use generator, looks like an st5 but produces 240 vac and up to 160amps of welding current

i don't know for sure what the open circuit voltage is but i bet it might be sufficient to charge a 48volt bank
and provide a bunch of amperage, certainly 60-80 or even 100amps would not be a problem.

i have never worked with one of these yet, we have 3 such heads and would part with one for a very good price
should anyone want to experiment with one of these for battery charging.

fwiw

bob g