Convert 420VDC StepUp to 210VDC

[FabianLPHD]

Hey there,
i'm currently working on a project to use my APC Online UPS with 220VDC Battery Pack as a 12VDC->230VAC Inverter.
As StepUp i would like to use my broken Power Inverter, its StepUp is working fine, but the problem is it's for 230VAC, so ~380VDC.
If i screw down the voltage to 220VDC, i only have a efficency of 50%, that's to bad! So i looked up the circuit to mod it to 220VDC with high efficency.
I saw that 2 pairs of transformers seems to be in series, so i thought i can break up the pcb and solder them in parallel, to half the DC Output, without lowering the efficency.
Done all these things and wow, 220VDC! I was happy but as soon i wanted to draw current from this setup, the voltage breaks down and the transformers getting warm. Something isn't working as wanted.
Maybe you can give me an idea.

I draw a schematic, very poorly.

[moffy]

It looks like you have two push/pull drives driving two transformers. How are the two push/pull drives synchronised? I can imagine all sorts of possible cross conduction issues. If you ran the outputs of each transformer through its own bride rectifier then paralleled the outputs of both bridges +/- might solve some issues but it really depends upon the controller circuitry.
P.S. I hope the way that you have drawn it that the windings are paralleled the correct way? You need to use "dots" to denote polarity of windings.

[FabianLPHD]

Thanks for you answer.
Well, today i tested the circuit under load and my messurements are:
Without Load output is around 210V(trimmer) and under load (up to 10amps) it sits at around 199V.
While pushing 800 Watts it drops to 185V. I think this is normal, because  it's near the 380V before my mods.
All 6 Transformer Cores gets warm around 40°C with 300Watts and no air flow, seems normal?
Without any load only idle the circuit draw 0,4A at 12V. I think it works fine?

Is it possible that these transformer can't keep up the voltage, because in 115VAC operation you only need around 160-180VDC?
That would make sense to me, why it's dropping to 185V while doing over 60Amps.

Don't know anything of polarity of transformer. Just broke up the connection like in the picture.

[moffy]

Your first and second posts kind of contradict one another:

1st: " the voltage breaks down and the transformers getting warm. Something isn't working as wanted."
2nd: " I think it works fine?"

What is your problem if any?

[FabianLPHD]

Okay, okay. I understand the confusion.

First i thought it wasn't working at all, because i wanted the voltage to be 230VDC, but it breaks down to 185VDC under high load.
The temperature of the transformer is always around 40°C, i thought that the transformer overheats.

Now i know after some testing that my circuit is working, more or less.

The 230VDC i wanted is because the UPS uses 16x12V Batterys and full charged at 14V it would be around 224VDC.
The breaking down voltage of 185VDC is problematic in one way, because the UPS says that the batterys are empty and is close to shutdown.
My first idea was to hold up the charge of the batterys in the UPS, but with this low end voltage that's not possible and now i'm direct feeding the ups through this DC-DC StepUP.

Now, my questions are, are the transformer the limiting factors, that the voltage is dropping under load?
Is it normal that they (6x) heat up to 40°C under slightly operation 10amps input?
Under high load (60amps) the core gets to around 47-50°C. Seems for me ok.


Maybe you know where i can buy a 230VDC Stepup with high current (3kw)? I can't find those types of stepup.

[moffy]

So if I understand your setup is:

3kW system
1. Source: 12V batteries
2. 12V DC to 210V DC, modified broken power inverter
3. UPS with 16, 12V batteries in series, hopefully fed and charged by 2.
4. Output of UPS 230V AC @ 50Hz.

Problems:
a. Output of 2 is drooping to much under load, 60A @ 12V gives 185V DC output out of 2.
b. Is Transformer temp of 47-50C OK @ the load of (a)?

Advice/Questions
1. Transformer temps sound fine, I would expect much hotter out of the transformers for (2) @ 60A.
2. Are you trying to build some sort of hybrid UPS or just a simple 12V to 230V AC inverter?
3. What is the power rating of the 12VDC to 210VDC modified inverter?
4. Would you not be better off if you just need a 12VDC to 230VAC inverter buying one? If so I would configure the batteries as 24VDC and buy a 3kW, 24VDC to 230VAC inverter.
5. Buying a 3kW, 12VDC to 210VDC PSU is quite possibly going to be more expensive than buying a full 230VAC inverter, because it sounds unusual and likely to be uncommon and expensive as a result.

[FabianLPHD]

So if I understand your setup is:

3kW system
1. Source: 12V batteries
2. 12V DC to 210V DC, modified broken power inverter
3. UPS with 16, 12V batteries in series, hopefully fed and charged by 2.
4. Output of UPS 230V AC @ 50Hz.

Problems:
a. Output of 2 is drooping to much under load, 60A @ 12V gives 185V DC output out of 2.
b. Is Transformer temp of 47-50C OK @ the load of (a)?

Advice/Questions
1. Transformer temps sound fine, I would expect much hotter out of the transformers for (2) @ 60A.
2. Are you trying to build some sort of hybrid UPS or just a simple 12V to 230V AC inverter?
3. What is the power rating of the 12VDC to 210VDC modified inverter?
4. Would you not be better off if you just need a 12VDC to 230VAC inverter buying one? If so I would configure the batteries as 24VDC and buy a 3kW, 24VDC to 230VAC inverter.
5. Buying a 3kW, 12VDC to 210VDC PSU is quite possibly going to be more expensive than buying a full 230VAC inverter, because it sounds unusual and likely to be uncommon and expensive as a result.

Sooo the System you describe is right.

Your Advice/Questions:
1.6x Transformer are in the DC-DC StepUp
2. Well the Backstory in short, bought a used Edecoa Sine Wave Inverter with 3,5kW/7kW Power... After a week, ir2110s broken, after 2 weeks igbt blown, after 3 weeks whole control circuit blown. Had a APC UPS SURTD3000RMXLI that is not in use, and it works with 16x 12V Batterys/234VDC. The idea was to fed in 234VDC from Inverter to have a good Sine Wave Inverter.
3. it was a 230V 3,5kW/7kWpeak Inverter.
4. 24VDC is a problem, because i use this whole system in my van
5. I think that too.


I have to test it more, how much the Voltage is dropping under high load (>800Watts). Maybe it's working close to cut-off voltage.

[moffy]

Thank you for your clarification, it helps. If you are running this from your van and your battery/alternator I think you are going to have to limit the maximum power for a couple of reasons:
1. Maximum current draw on the alternator, 50A is a heavy accessory load. I don't know how you are going to manage switching/control so that you don't overload your alternator or flatten your battery and leave you stranded.
2. Voltage drop due to high currents, your cables can drop significant voltage at these currents, joins etc. In fact your voltage droop could be due just to this, a sum of external and internal voltage drops on the 12V side of the step up converter. That is why I was asking for its power rating.

I cannot vouch for the product, just using it to illustrate pricing, but a 12V to 230VAC sine wave inverter, 3kW with UPS function for $380 AUD: https://au.renogy.com/renogy-3000w-12v-to-230v-pure-sine-wave-inverter-with-ups-function/?gclid=EAIaIQobChMIncmh5rm69wIVAq-WCh10dgl2EAQYAiABEgKFEvD_BwE
I really wouldn't trust the specs without verifying, but economically what you are doing just doesn't seem value for money/effort and end result.
 

[FabianLPHD]

Thanks for your effort.
I'm aware of the currents, but i know what i'm doing (in this section).
At the moment i only have a 14V/90A alternator, but i'm upgrading to 14V/150A so there is no problem.
The 3kW are only, to size the system correct, but i will never use this much power. Only up to 1kW.
All wiring is thick enough.

Well, now to the DC-DC StepUp. When the voltage is 14V, the output is stable at 185V while doing 2000Watts.

So this thread is solved, for me.
Sorry for "wasting" time.