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

[BruceM]

That's oversized, all right!  What I'll need is the rating on the field coil. 

[mbryner]

OK, here are the numbers from faceplate on my ST 7.5:

120/240 Volts
62.5/31.3 Amps
60 Hz, 1 phase
Excite V = 61 V
Excite current = 2.7 A


But, I think I scored an even better find than those e-bay toroid transformers.   A great big autotransformer from an engineer friend, gratis.   Check this bad boy out:





It's built for 3 phase, but couldn't I just disconnect the transformers from each other and gang the secondaries together in parallel?   Then current limited by the charge controller, no?

Faceplate says:
Input volts:    480
Output volts: 0-560
Amps:    10
kVA:       9.7
Phase:    3

Since I don't know enough about these, why does it list 10 amps but 9.7 kVA?   560 x 10 = 5600.    Of course 240 x 10 = 2400 watts.   Or am I limited to the amperage coming out of the secondary?   Can I multiply by 3 for parallel connection, or is the faceplate referring to all 3 transformers in the existing setup, thereby limiting to 10 Amps total?   Showing my ignorance of 3 phase equipment and variacs in general.   I mean, I do get the concept of common lead, non-electrically isolated like a normal transformer, etc.

[mobile_bob]

because it is 3 phase

5600 watts x 1.73= 9688 watts or 9.7kwatt


560volts per phase x 10 amps gets you the 5600watts

the power therefore in a three phase circuit is 1.73 times that amount

the forumla is

volts averge per phase x amps average per phase x pf (see note) x 1.73 = total power

*note*  pf= power factor, which is assumed to be at unity for the sake of rating, if the load is not unity the
kva will derate accordingly

bob g

[BruceM]

There are two issues with variacs/adjustable transformers/autotransformers.

The first is that there is no transfomer isolation, so you don't get a floating AC output which you can reference to something else, like your battery ground.  With 240V input, this makes using these near impossible.  You'd have to apply not just 240V but also your neutral, to a balanced set of windings, appropriate to 240VAC (near half of the winding).   Then you'd have to use the neutral as your battery ground, and the ouput at your desired voltage from the neutral.  You may have to modify the unit to achieve this- you can check the various taps resistance to see how the windings have been divided.

Second, you will only get 10 amps out of each transformer, since this is an autotransformer, and there is no secondary winding to do a power conversion.  So if rewired for parallel, you will be limited to 30 amps total, regardless of voltage, or 2400 watts at 80 volts. 

It would avoid a dropping resistor in the field coil, or a mechanical governor speed reducing lever, if the first issue can be addressed successfully.

[mbryner]

OK, guys.   Thanks.   I got all excited for nothing!     30 Amps is what we have already with the toroids.  Now I have a big variac getting in the way.   Anyone want it?

So I guess we can continue with dropping a resistor in the field coil...

[BruceM]

Two things about the variac- no excitation change, and you can use the toroid secondaries in parallel for a choke filter.  But whether you can get the windings sorted without modification is another issue.

My guess is that a 5 ohm resistor is about right, but I guess you can buy a 10 ohm adjustable and find out how far off I am.  You can buy some cheap 25watt values for testing...

I really like the approach of slowing down the engine (governor); that would solve the problem and give you a bit more fuel efficiency.  I'm not sure how to best do it mechanically...you need a simple lever type speed selector for the wife.

[BruceM]

http://cgi.ebay.com/Large-Toroidal-Transformer-39-0-39-v-new-heavy_W0QQitemZ170397140538QQcmdZViewItemQQptZLH_DefaultDomain_2?hash=item27ac76063a

This seems like it might be a match for your existing toroid transformers, assuming it has dual primaries so it can do 240V in;go to  4 total  and then only your Lister is the limit to how much power you pump into your batteries. 

[mobile_bob]

Marcus

if you end up not using the 3phase powerstat unit, and want to sell it let me know.

i could use it in testing of my modified alternators.

having that setup would be really handy in working out ratio's for transformers and for testing
different parameters.

thanks
bob g

[mbryner]

OK, Bob, but how would I get it to you.  It weighs a ton.  >50 lbs.  You coming down through Oregon on I-5 anytime?

Bruce, I e-mailed the e-bay seller to ask if that toroid has dual primaries.   A 4th transformer would simplify the whole situation.  The charge controller would limit the Listeroid charging current to what it is set at the 3400 W of the solar panels.

[mobile_bob]

sorry not coming to oregon anytime soon,

do you have a ups store? if so just take it to them and they will package and ship it.

let me know how much you need and i will get you the money however you want it,

the more i think about it, the more i think having it would provide a level of matching needed
to greatly simplify my testing.

bob g

[BruceM]

Perhaps you should be watching ebay for a while to see if we can come up with a 3400 watt transformer solution, one way or another.  Just remember we can do series or parallel on the various windings.  Then use your existing toroids for the filter choke.

[mbryner]

Something like this?

http://cgi.ebay.com/GE-Transformer-9T21B1008G02-480-Pri-120-240-Sec-7-5KVA_W0QQitemZ330407964497QQcmdZViewItemQQptZBI_Circuit_Breakers_Transformers?hash=item4cedd96f51

[BruceM]

Yes, in 3KW plus sizes, a conventional transformer is pretty efficient.  you need a 240V primary, with 80 to 100V secondary. 

[mbryner]

So, correct my math if I'm wrong before I buy it.

It has 480, 456, 432 VAC input (H4, H3, H2) and 240 or 120 VAC on secondary.  
432/120  = 3.6 ratio.  
So if I input at 240 VAC through H2 (432 volt input), it should output 66.7 VAC.  Perfect for rectification.
Would be limited to 7500 watts / 120 vac = 62.5 amps,
Or 66.7 x 62.5 = 4170 watts when charging via listeroid.  Just about right.

All correct?

P.S.  Bob, I'll look into sending the autotransformer by UPS.   Had thought of using it for stepdown transformer for high voltage hydro (eventually), but not having isolation sort of scares me.   Can't think of anything else to do with it...

[mbryner]

Forget everything on the last post about the 7.5kw transformer:  Shipping to Oregon from Ohio is estimated at $355 !!!!!