Parallel transformer input and series the outputs

[mbryner]

I would like some advice from the best and brightest (which means all of you):

As always, my overall goal is to convert 240 or 120 V AC from the ST head to approx 80 V filtered DC to a MX80 Outback solar charge controller.   I had a prototype system working, except the DC neg was connected to ground (A "no, no". Dangerous error I now realize).  You may remember how I overheated the previous bridge rectifier, which lead me to troubleshoot, which inadvertently caused me to lock the ST head, which caused damage to the big end shell bearings, which caused a engine rebuild.   (Please, don't do that everyone else!  It's a PITA.)

I have 3 toroid 3:1 transformers (i.e. input 120 V and output 40 V) rated at 1200 watts each.   I was thinking of using them as isolation transformers for my DC charging system.   That way I can bond the negative of the rectifiers' output to the ground of the ST gen head w/o creating a short and the blowing diodes.   If I connect the primaries of the transformers in parallel and the secondaries in series, I will have 120 V AC in and 120 V AC out, for max of 3600 watts.  There are 2 bridge rectifiers in parallel just to divide up the current for better heat dissipation.  Am I correct in this thinking?   Sorry for the poor diagram again.



P.S.  I tried to post this in the "electrical generation" section but there is no "new topic" option, just "new poll".  Maybe the admin or moderator of that section can fix it.

[mike90045]

Don't try to parallel diodes, it just doesn't work. (you can use resistors to help split the power between them, but lots of losses that way. use diodes of adequate amperage & wattage.     You CAN put diodes in series, but you have some extra losses from that.   Best to get the right diodes, and only have to deal with them once.    I like the Schottky diodes in To220 case format.  Easy to heatsink the power tab, and some are dual diodes, but I just ignore the spare one, for ease of use.   Pay attention to the center lead, which is usually connected to the metal heatsink tab.   

Here's a $2 one, 250V, 40A
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=MBR40250GOS-ND
http://www.onsemi.com/pub_link/Collateral/MBR40250-D.PDF

[mbryner]

Thank you both.

Yes, I see the error in the bridge rect diagram.   Hasty sketch.

I'm not using individual diodes -- I have 100 amp/1000 V bridges x 2.   They are exactly the same.   I figured that since they are rated for way more current than I'm putting through, it would be OK to put them in parallel.  Guess not....   Will use one as backup then.

I'll make sure the transformers are actually isolation transformers; they should be.   Good tip.  Thank you.

The way I came up w/ 80 V is to slow down the 6/1 to 450 rpm.  Gen head is then producing about 80 V AC, output from rectifier is about 120 V DC, but after going through big choke it's less.   120 V DC is still OK for MX80, though.

Every time I run 120 V through a rectifier, the neg output is always 40 V diff than system ground.  Seems dangerous to me, but I don't understand why it's happening.   The circuit is about as simple as can be:  grounded generator, 120 hot and neutral/ground output to rectifier, measure at output +/-.

?   

[Crofter]

I dont see that you are gaining by transforming down to 40 v then hook three in series to get back to 120 (aside from possibly isolating from ground. If you remove the grounding from the neutral at your generator, cant you simply rectify this output then the DC out will have no potential to ground (unless you choose to).

[mbryner]

The generator case is grounded, and the neutral goes into the subpanel where it is bonded to the whole land/house system neutral.   

Seems like if I isolate the generator/'roid ground and neutral from the whole electral system ground, that could be dangerous???   Don't know.  Over my head now.   Grounding is a complex subject.

[Crofter]

Yes, now I see why you are grounding the center tap, being that it is going into your main system. I had blinders on just for the battery charging. It would be a pain to have to put in more transfer gear to reconfigure just for charging. Things can get hairy when you start combining different systems. It gets real hairy for someone else to come in in the middle of the movie and try to figure it out. Make good instructions for the butler!