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48V 5000W Oil cooled inverter build
#11
Yeah, he needed some floating plastic fish in the tank !

Still trying to work out the effectiveness of the seal / construction of electrolytic capacitors as the heatsink end of the inverter board may end up in the tank as well so that it eliminates the need for a fan there and helps with the cooling of the board. Capillary creep of the oil up the board to the variable resistors may make this a no go though...


Confirmation back that the FET's are HY3810 so all good, order going in shortly....


Next up, quick scan for Baby Oil and there is a deal on at Tesco (2 x 500ml for £3) so £3 per litre, which is cheaper than engine oil (but not as cheap as used engine oil from my very first experiment) ! So will be placing an order once I have a bit more of the volumes worked out which brings me to the actual toroidal trnasformer selection... The wife will probably give me a slap when the oil turns up for thinking that she is on massage duty for the next 5 years. lol. just waiting for the "WTF is all this ?"

Basic thought at the start of all this was why pay for a 10kW inverter when your only going to be using 10kW for a few minutes at a time or that some of that 10kW can be offloaded to a separate 2kW grie tied battery inverters switched on only when needed (to avoid no-load losses and extend the lifetime). This may sound a bit wrong at the moment, however a little diagram may help clear up what I'm trying to do....


Remember this is a "and now for something completely different" monty python build....

Overall if I have two of the 2kW units (capable of 1900W) that can add 3.8kW to the 5kW of "base" I should then run my load upto about 12kW for a few minutes, if not longer, at a time while the inverter alarm bleeps away and the oil slowly warms up. The main limmiting factor is then only cooling if I keep the peak switching load below 15kW.

The two solar and one wind untis are the ones I have available at the moment and have so far tested a setup like the one above with just one solar inverter and the wind turbine grid tied inverter for a few weeks without any issue. Chargers are a work in progress after decoding the serial formatting for the Eltek units in a separate post.

This is all being posted now, but I have been doing research for several months so far and just getting to the actual nut's and bolts physical build stage.....
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#12
Now that 20 bottle of baby oil are on the way (10 litres)... lol.... I need a transformer...

 
This appears to be the "standard" off the shelf product from one suppliers, however they appear to be very willing to make transformers to request at a reasonable price.

Modifications needed (or lack of build steps for the manufacturer) are :
1. No core filling. This is nice in a normal install as the core then provides a nice bolt hole to fix the unit to the chasis, however my unit will be "free floating" in a sense... and will be bound to a fixing plate/mesh with cord.
2. No outer wrap. The final wrapping mormally provides a protective layer to the outer windings from scratches that remove insulation and provides an additional layer of insulation. In my install the oil will provide the extra insulation and as long as it is not damaged in transit or by me dropping it on the floor it should be all good.

Fixing the transofmer to something suitable will then be done with some paracord with thin layers/strips of padding to protect the windings from excessive pressure. This will leave the outer windings fully open to the oil and the inner windings with less distance for heat to transfer into the oil.
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#13
You could see if they'd be willing to wrap it in masking tape instead of the plastic, and then only on the outside of the 'donut'. Would make it easier to remove once you got it and was ready to drop it in the oil and would keep it protected from your Linus (LTT) moments Wink

How much are the toriods?

(02-24-2019, 03:13 PM)completelycharged Wrote: Next up, quick scan for Baby Oil and there is a deal on at Tesco (2 x 500ml for £3) so £3 per litre, which is cheaper than engine oil (but not as cheap as used engine oil from my very first experiment) ! So will be placing an order once I have a bit more of the volumes worked out which brings me to the actual toroidal trnasformer selection... The wife will probably give me a slap when the oil turns up for thinking that she is on massage duty for the next 5 years. lol. just waiting for the "WTF is all this ?"

LOL I'd love to see the video reaction of this moment Tongue
Then again, she might get a twinkle in her eye thinking you guys are going to start acting like teenagers again Wink
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#14
LOL, that made me laugh !

Critical aspect for the transformer is turns ratio....

With the previous 139 page post the early conclusion was 8 to 1 to get a primary to secondary ratio (and later on slightly different). This is also where the FET selection Amp ratings come back in, as the calculations as to the current handling are not really based on just 48V if considering the average current delivered at the effective PWM sinewave voltage....


23kHz PWM switching into the transformer creates the sine wave and a feedback loop from the output also alters the timing so as to cope with the not so nice loads and with a reverse power flow of energy being pushed back in at the 230V side of the transformer back through the FET's to the battery.

The chart shows a simplified "slow" PWM just to give the idea (nice chart lifted from electric bike forum post by non other than.... spinningmagnets). The chart also shows where the FET's in a H-Bridge arrangement switch the phase aroud on the input to the transformer.

The "headroom" between the DC voltage and the peak of the sinewave is where your low battery voltage needs to be considered and all the other voltage drops along the way.
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#15
The sale page on aliexpress indicates a 220V to (24-28V) transformer selection... another inverter board quotes 26V and another quotes 24-32V, however a fixed toroidal transformer is not variable. So key criteria is then what voltage range do I want to handle at minimum charge ?

For my battery back my minimum charge will be 45V / 22 series cells = 2.05V per cell as they are LTO chemistry and below 2.05V you don't get much when you fall off a cliff and upper charge is 56V, which gives me something like this....


The black line represents the combination of volt drop caused by the current on each FET, the inline chokes and drop to the busbar on my system. This says that a 30V secondary is the highest voltage I can use, but this again does not always leave much room for high crest factor loads and a bit of de-rating still to cope with so 28V is looking like the choice of secondary.


My design consideration will also take into acount a load derating when the battery pack is below 48V as I will want to avoid continuously pulling 8kW when the battery is not above 48V. This still allows for a very comfortable draw of 5000W down to around 45.5V or about 10% full without distortion.

Efficiency means loss, so if I want to see 8000W on the output terminals I will have to put more than 8000W in so I re-run the sheet with 9000W as an inpout and now get a rough indication as to what the losses will be as a starting point. The actual losses should be more than this because the switching currents will be higher and the core losses of the transformer are also not taken into acount in the chart.


Still, as a starter the low load efficiency should be ok and with 8kW out 1kW of heat will be generated in various places with 800W at the heatsinks of the inverter board when the battery is at 49V before de-rating, so the inverter board may yet end up in the oil with the transformer after replacing the post with fixed resistors once the levels are set.

There is still yet the option of two smaller toroids as per many manufacturers (like Victron) using two smaller units rather than a single larger unit.

For reference a standard 2000VA Toroidal in the UK with 230V Primary and 30V secondary is listed as £132 each from Airlink Transformers. They each weigh in at 12Kg.... the 5000VA units are around 23Kg and a big difference in fixing load in a case. One transformer gives less no-load losses, which is the plan..
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#16
Looking further into the ratings and heat dissipation of the FET's in the various data sheets the critical chart can be the drain current de-rating due to "package limitations" or put another way watching the legs melt off.

The HY3810P is "rated" for 180A (5mOhm Rds) but the "package limitations" mean only 75A in the real world, there is another near identical board made with HY4008W FET's that are "rated" for 200A (2.9mOhm Rds) and again the "package limitation" is 90A, but the main downside is the voltage rating with the HY4008W which is 20V lower at 80V compared to 100V on the HY3810. Running the 80V FET's at 56V on a full charge does not leave much room for back EMF spikes on shutdown, so would possibly look to add surge suppression between the board and transformer.

This is the HY4008W FET based inverter board of choice so far
The Rds of 2.9mOhm vs 5mOhm also drops the losses by 42% to 470W on the FET's at 9000W input (30A per FET) and improves overall efficiency by 3%.

At 3000W I'm then hoping for over 95% discharge efficiency to grid (LTO full cycle efficiency at 5A is around 98.5%) on 3.5A per cell.

 
Modifications to make it "oil" proof.... so it can then join the transformer.
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#17
I'm curious about the losses in the FETs you're calculating?
The transformer has a single low voltage winding not a centre tapped winding right?
The board has an H bridge design with 6x FETs on each leg of the H right?
So at conduction time, you'd have 6 FETs in parallel to ground on the "low side H bridge leg" & on the other "high side H bridge leg" you'd have another 6 in parallel.
So a total of 12 FETs would see the average current for each half cycle.
All values approx.
Working back from a nominal 8000W output on the AC side @230VAC, I get:
230VAC side current = 34.78A RMS @230VAC.
Assuming turns ratio of 8:1, a peak DC side voltage of 40.7 & RMS current of 278.3 A
So 278.3 divided across 6 FETs per leg = 46.4A/FET
At 5mOhms Rds, that's a power dissipation P = I(squared)*Rds = 46.4 x 46.4 x 0.005 = 10.75W per FET
At any one time, 12 FETs carry this current & so dissipation is 12 x 10.75 = 129W
This is for conduction losses & doesn't include switching transition losses.
Efficiency losses also not included for simplicity.
129W is a lot less?
Did I get something significantly wrong?
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#18
You are indeed correct, the lack of sleep + using watts as voltags in a column = way more than it should be... will re-work the sheet after a large cup of coffee. lol.


Found out the transformers in the Victron are in parallel but one is apparently switched in as the load increases, also part of the toroid is left exposed to increase leakage to provide some additional inductance that then eliminates the need for an second in line inductor. learnt something new on that one...
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#19
Delivery no 1 complete.... but only 8 litres as I guess they ran out of stock for delivery. Think I may need some more given the case size and a circuation allowance for cooling when EV charging for 3-4hrs at 6kW.


I was going to carry out some more detailed flamability tests, although I guess this is about as far as I need to know... lol.
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#20
Summing up so far, this is the effective circuit, components and wiring


The 50uH inductor core will then need 3-6 turns or with two smaller cores, one per secondary leg, which may be more practical.
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