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48V 5000W Oil cooled inverter build
After buying various inverter options I'm now opting for the monty python option of "and now for something completely different"

My plan now is to go non conventional, mixing commercial distribution transformer oil cooling with off the shelf parts and hopefully not create a glorified deep fat chip fryer.

Basic plan, 5kVA toroidal transformer, inverter board and about a gallon or so of whatever oil is knocking around at the time (less water).

Once tuned, hopefully with no load power use 40W or less and with the oil cooling I should be able to run the output at around 6-7kW for around 10 minutes or so providing the FET's are all ok. The following posts will be the progress so far....

First up and main issue the actual inverter board to use and the first selection criteria is it must be low frequency H-Bridge.

Not H-Bridge - might be good for 5000W at 48V but I can't charge my battery through it. Forgot to add the reason for H-Bridge is so that I can push power back from wind and solar grid-tie inverters back through the inverter to charge the battery..

H-Bridge and lovely Chinese writing to follow (must learn Chinese some day). BUT this unit only has 4 FET's per active leg, so the 6000W rating (special Chinese factory rating) of this unit at 48V would mean each FET is dealing with 31A (averaeg not peak switching amps) each.

Now this is starting to look better, 6 FET's per leg (12 x 2) and 5000W "rated" at 48V using RIFB4710 FET's, which are rated at 75A and have a gate resistance of 11mOhm. Not ideal, but a good start. Still, that 5000W at 48V spread across 6 FETs then drops to 17A per FET. Surge current per FET of 70% would then be 15.1kW. Hmmmm...

The layout of the PCB, however, may not be able to spread out the current effectively between the FET's due to the traces though.... without modification or additional "helper" wires...

Another board indicates that it can handle 7500W at 48V with 5 FET's and a surge capacity of 21000VA.... hmmm.. thats 21000/48 = 438A over 5 FET's = 88A per FET. Even the 7500W rating is 31A per FET.


The FET's indicated are either HY4008 or HY3810 and from the non specific indication of a board sold as 24V, 48V and 60V the HY4008 at 80V rating is not going to go into a 60V board.

The HY4008 is rated at 200A and would be curious to see the legs of the TO-247 cope with 200A for a long period of time.... the HY4008 does, however have a Rds of only 2.9mOhm so way lower losses.

The H3810, which I'm guessing/hoping would be in the 48V unit is rated 100V and 180A with Rds of only 5mOhm, brilliant..... but....

Using the same PCB layout as the board with 6FET's per leg AND the board in the previous post actually shows the under side PCB trace of a different board altogether with 5FET's per leg !!!!!!!

The powerstar W7 3000W units use 6 FET's per leg and I know that these units can take some abuse, my older unit can start my 3hp circular saw and cope with it stalling with my poor method of cutting pallets up, which jams the blade on the odd occasion..


The 6000W version, which incoporates two transformers in the case rather than a single larger transformer, still uses 6 FET's and is a proven inverter for abuse but not so great on no load power consumption due to the E type transformers lossy design.

So, this makes me think that the 6 FET board is the way to go (can't find any larger boards yet....) and it should be able to cope with 5000W continuous without any problem.
chuckp, hbpowerwall, MGPTWEGB like this post
As for larger boards....


This is rated at 10kW at 48V with the 24 FET's but they are not H-Bridge switched and will not allow power to flow back through the unit, which is one of may main criteria....

So, this appears to be my board of choice..... "rated" for 10000W at 48V (I'm designing for 5000W) and it has 6 FET's per leg so hopefully it will put up with my abuse.

The PCB trace also appears to be the same board this time round as well, showing the 6FET's per leg on one side and the 12 on the other. The traces look to have some better soldering additions as well.

Unfortunately the actual FET used is not listed so awaiting confirmation back as to the FET's to work out if this really is the 100% right board to order.

Next up is the transformer and this is the second critical item, ok there are really only two main big parts the board and the transformer....

From my time in the electricity industry many moons ago and playing around with a coil tyring to measure the magnetic field strength around buzzing transformers when I was even younger with no real idea what I was doing (not much changed) pushing the coil into the cooling loops on the metal case.

The basic options that I'm after are :
1. Better coolling which will provide the ability to run a 5000VA unit at 7000VA or more for longer periods of time.
2. Eliminaton of some buzzing noises due to wire vibrations.
3. Additional electrical insulation of the windings (not really needed or relevant just a nice to add in my mind)

Using an E type build trnsformer can work, however they tend to have higher magnetic losses and the grade of steel used in cheaper transformers adds to the losses, so forces a much more expensive transformer.

Toroidal transformers have lower losses but are not so easy to source for a specific voltage level, needed for the output of the inverter board.

They are normally are supplied wrapped and sometimes have the center hole filled with compound to help fix the toroid into a case, however I will be looking to unwrap the cover to allow the oil to circulate better, athough may have to compromise with some string binding in places if there is none underneath at the ends of the coils.

Which should then leave me with something like this....
I've often wondered about these boards. Then build the transformer or buy it locally. This would safe boat loads of the cost of high output inverter. But I have been leery as I don't know enough to go that route.
Glad someone is Big Grin

I will be watching the progression closely.
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This is one of the differencs in scale I'm hoping to get as a reduction in less wasted energy.

Lower no load power consumption

And higher efficiency when the cooker, TV, microwave, PC, Iron, washing machine, dehumidifier, fridge, lights, playstation are all overloading the inverter, bleeping away in the background getting ready to double up as a deep fat fryer..

The aim is to have an inverter that is more efficient overall and completely non compliant with any grid code around the world, while being built in line with the standards that can be applied.... or not. lol.

Final goal is to take the house off grid 9 months of the year or more and only charge for 1 hour per day in winter (at 6kW) at most.

There is another thread on a forum where they take out the toroids from solar grid tie inverters, unwind the secondary and replace it with about 16 turns of very think wire like 2 or 3AWG...

Enough posting for now, have to actually get something done now, lol
Korishan and hyperborean1 like this post
So with the transformer type, E-core type are more forgiving to loads with some DC offset vs a torroidal one.
You can easily have loads like a typical hairdryer that half-wave rectify the AC when on half heat, etc. This means the AC wave gets current drawn on one half-cycle not equally on each + & - wave peak. Many other loads can have some DC offset too.
This will make the transformer saturate (becomes nearly a short circuit)

It's the losses that let the big power grid transformers soak up a few hair dryers without going into saturation.
Torroidal cores with less losses don't cope so well with the DC and get into saturation.
I built a few types many years back & saw all this in action.

To properly drive loads like this you really need a design with two transformers & FET systems one drives each half cycle separately.
My Victron inverter has two transformers.

Also when looking at the above FET drive boards, hake sure they're proper sine-wave drive not "modified square wave" ones
Running off solar, DIY & electronics fan :-)
Oil cooling is not necessary for such a low power inverter - much debate on the use of ready made controller boards started elsewhere a little over 10 years ago, eventually resulting in the production of a viable open source inverter.

Most current thread -
The control board that is used (EGS002) has transformer output feedback loop to alter the switching through the full cycle so that any offset does not occur and is true full sine, with carrier frequency around 23kHz.

The larger the transformer the larger the energy stored in the magnetic flux and this is what allows the larger grid transformers to cope better with high harmonic inducing loads switching part way through the waveform. This is also why for a larger inverter you can't just power the transformer part way through the waveform as the inrush (to charge the magnetic flux) would blow the FET's. The control board in this instance has a 3 second slow start approach. Zero crossing would also work in theory....

Are you sure the transformers in the victron are not in parallel or setup to handle split phase output ? Its more typical to install two smaller toroids/transformers so that they fit into a smaller casing with lower fixing load.....

For those interested in reading the original 139 pages from 2012 of the forum thread I found really interesting and what originated an open source inverter desing (using the same control boards in the pre-built board Iintend to use) you can read it here:

It's long and well worth a read to understand the behaviour of the inverter transformer and how to avoid saturation and decrease the no load power consumption.

Bit more research, learning chinese (or not) via google translate via the camera on a phone...

The 4 variable resistors on the board are for :

1. Output voltage adjustment - this has an impact on saturation if your output is being driven too high as the input voltage waveform will be pushing the primary into saturaction. Voltage sag on the secondary may also cause the primary into saturation if the primary to secondary winding ratio is not selected well for your battery voltage range. The inverter needs to be able to cope with your minimum and maximum expected battery voltage as an input and it is the minimum voltage that can create the real constraint on a true sine output at load.

2. Under voltage - battery protection shutdown level, basic and easy to adjust (in theory!)

3. Over voltage - not needed if your charge controller is dealing with things.... in my case ths pot will be set to a above max battery voltage level because the inverter can't switch off if the battery is being charged through the inverter..... automatic dump loads will be a last resort to prevent over voltage rather than shutting down the inverter as this would also switch the house off !

4. Overload setting - an interesting one to tune and set as I'm not entirely sure as to what the exact details are relative to any "level". Immediate shutdown, alarm level, 3x alarm level for x seconds ???? One for more research.... or more likely experimentation once built.
Very interesting threads there :-)
Re my Victron unit I just squinted through the side, but it sure looks like there's two whole transformers in there.
Interested to see how the single transformer versions do handle a hairdryer - must be a trick to stopping the core saturating!
Running off solar, DIY & electronics fan :-)
The cooling of a transformer as a result of the No-Load and Load losses is typically done with fans in your normal off the shelf inverter purchase or if your unlucky no fans and a shutdown sensor and if your really unlicky a shutdown when the magic smoke or fireworks appear.

Frome some peoples quantative experiments on the effectiveness of different cooling materials to add to my childhood experience of fortunately not burning the house down with a transformer, in a plastic tub, filled with used engine oil being short circuited for long periods of time.

This gives some indication towards how much "overloading" above the air cooled rating of a transformer can be achieved for short durations of time without damage.

The main thing to to remember at this point is that the load losses and resulting heat are all related to the current and as long as the voltage stays low enough so that the core is not saturated. This means that your loading will increase the wire losses as a square of the current but have very very little impact on the actual core and you can wind the current up as high as you can "cope" with.

There is a good overview of oil cooling in a presentation "Properties behind effective Transformer Oil Cooling" given at a Transformer Life Management Conference in 2013....

The chart shows how much of a difference some oils can make, which are a lot thinner and therefore can flow and transfer the heat away more effectively.

Faced with the choice of cooling medium the ideal option is actually de-ionised water from a cooling effifiency perspective but due to the items being nice materials that would create some interesting electrolytic reactions the water would end up a highly toxic and conductive medium after a short period of time, besides making a big mess of everything. Which means for me it has to be some form of mineral oil, readily available and ideally clear.... which leads me to this...

Korishan likes this post
That's what I was thinking, mineral oil. And there's several videos where ppl have made transformers, used a vacuum chamber to pull all the air out while submerged in the oil. Works pretty good.

Hahah, that oil cooled computer, 4 minutes of watching the system run in the fish tank Tongue
Didn't think about baby oil. That's actually cheaper than just straight mineral oil, for some odd reason. Altho, buying the MO in 5 gallon or larger quantities might drop the price drastically.
completelycharged likes this post
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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