Using both 50Hz and 60Hz output leads on ST head

[mbryner]

I alluded to this on a different thread, but thought it might like a thread of its own.

Background:
Most MPPT charge controllers have difficulty w/ high voltage VOC's.   Outback MX60 and MX80 can handle 140 volts.   Some of us are using AC generators charge batteries after rectification.   Everyone knows I take 240 VAC from a ST, run it through an isolation and stepdown transformer, a bridge rectifier, and filter choke, then charge using the resulting DC through Outback charge controller.   To placate the MX80, I decrease the voltage by decreasing the excitation current in the ST head w/ a variable resistor.  

Idea:
What if you use the U3 & U4 leads that are designed for 50hz 230 VAC @ 1500 rpm, and run at 1800 rpm instead.   That would yield 276 VAC at 60Hz.   After going through a 4:1 transformer, you would have 69 V AC (RMS of course).   After rectification, it would be 98 V DC (peak) -- just perfect for the charge controller.   Here's a diagram.

But, now there's the problem of the AC radiator fan, WMO pump, heating elements, etc. often running often running at the same time when I want to charge the batteries.   Normally we bond the U5 & U6 leads together here in the USA to give 230 VAC @ 60 Hz @ 1800 rpm.   Is there any way to use both outputs at the same time?   On my diagram it looks like a big short circuit if I were to try.   Thanks.

[mbryner]

Oooh, after posting my question, I figured it out.   As long as I don't bond U3 and U4 together, I think I can still use U1 & U3, and U4 & U2 as separate outputs of regular 120 VAC.   But what would neutral be then?

edit:  and I still wouldn't have 240 to the heating elements in the WMO tank.    Guess I can't have it both ways....

[Tom Reed]

I'd concur with your assessment.

[Ronmar]

But what would neutral be then?

And that is where you will run into problems...  Strictly speaking, if running 120 only, you do not need a neutral.  If you think about it, does 240 have a neutral?  no, 2 hots and a ground.  Neutral is a function of the way we wire US domestic power.  Neutral is the center tap of the pole mounted step down transformer that feeds a typical US home and gives us both 120 and 240 off of one transformer.  It gets tied to ground as a safety measure, and all 120V appliances are built to deal with this convention safely.  Again, if the windings are not shorted to ground anywhere, either end of a 120V winding could be tied to ground/neutral as the reference just as the transformer center tap is.  Both 120V windings could be tied, but then the legs bocome phase sensitive, IE: you need to connect the proper ends together otherwise they will be out of phase and fight each other/short.  Hence the instructions for setting up the generator for 120/240 using certain contacts, and a different setup to put the 120V windings in parallel for 120V only operation.  Not a horrible problem, but if you want to tie it to a conventional 120/240 distribution panel, you may have issues if you are not carefull about how you connect/switch things. 

[Thob]

You should be able to make it work if you reverse the windings.  Tie U1 and U2 together to form the center tap, or neutral.  U5 to U6 will give you the 230V, and U3 to U4 will give the 276V.  More or less.  You'll have to stay withing the total load capability of the generator.

[mbryner]

You should be able to make it work if you reverse the windings.

Awesome idea!  So simple.  Thanks!

If you think about it, does 240 have a neutral?  no, 2 hots and a ground.  Neutral is a function of the way we wire US domestic power.

Thank you, Ron.   That became obvious to me as soon as you asked over on Simtech's thread why I was using an isolation transformer:  because I had tied the center tap to ground via the 120/240 selector box.   Of course, if I want to keep using 120/240 output to a pump and heating element, and simultaneous battery charging via a rectifier, then the center tap will have to stay neutral (and, by extension, ground).

[Ronmar]

You are correct.  It can do direct from generator rectified DC/battery charging, or it can provide 120/240VAC, but not at the same time with that "center tap" from the generator windings tied to ground...  You know, there might be a way, let me think about this a bit.  How much DC do you want feeding the inverter/charger?   

[mbryner]

Ron, you have a lot more experience w/ this than I do, but I don't understand why charging w/ 276 V from U3 & U4 leads, and 240 AC from U5 & U6 w/ center tap ground won't work concurrently.   It would still need an isolation transformer on the U3 and U4 leads, of course, otherwise there would be a voltage across ground and short circuit.   I'll wait for your opinion before I experiment w/ it, though.

Edit:

How much DC do you want feeding the inverter/charger?

2000-3000 watts

[mbryner]

[Ronmar]

Ron, you have a lot more experience w/ this than I do, but I don't understand why charging w/ 276 V from U3 & U4 leads, and 240 AC from U5 & U6 w/ center tap ground won't work concurrently.   It would still need an isolation transformer on the U3 and U4 leads, of course, otherwise there would be a voltage across ground and short circuit.   I'll wait for your opinion before I experiment w/ it, though.

Edit:

How much DC do you want feeding the inverter/charger?

2000-3000 watts

I am sorry, what I was looking for was what DC voltage range you want to feed to the controller.  You said that the outback can handle 140VDC, how low can it go?

[mbryner]

Above 60 to be able to equalize.

[mbryner]

What a disappointment.   I spent part of Xmas afternoon dismantling the ST and could not find those U3/U4 leads *anywhere*!   My gen head came from Powersolutions and specifically says 240/120 V 60 Hz on the faceplate, but I thought all ST heads had all the leads somewhere inside.  ()   There are 2 small leads that were taped and isolated but when checking w/ a multimeter they have only 6 VAC across them and 12 V AC between each and the case ground.   (O V DC.)  Who knows what they're for?  Don't remember them on the schematic.   

The concept should work if you have all the leads -- just like multitapping a transformer.   You just have to stay within the rated limits of the gen.   I think it would be a great way to charge DC and run AC loads concurrently.   Anyway, thanks for humoring me.   I ordered a Midnight Solar Classic 200 last night.   I'll plug in regular rectified 120 V AC to it when it arrives and update you, probably on a different thread.

Marcus

[Ronmar]

Not all the ST heads have them.  I don't think mine does either.  I seem to recall PS specced that in their order, basically useless in most US domestic applications.  That extra pair of wires were for the cheesy little red light that indicates the generator was generating on the original ST doghouse.  I think PS replaced the original doghouse with a NEMA electrical enclosure so those leads were just taped back and secured... I bought one of their heads along with the 6/1 before they started replacing the doghouse.

Was thinking about your setup a little more and don't see an easier way.  You could add another gen head, dedicated and hardwired for DC generation, which would allow you to ditch the transformer.  3 phase might even allow you to ditch the filtration:)