Author Topic: Building a 6kw pure sine wave inverter using power jack boards part1 transformer  (Read 24931 times)

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Offline oztules

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It's time to put some practice down in a how to style rather than a Q and A thread.
To make a nice 6kw unit that will truly do the 6kw rather than the not so real figures bandied about by PJ, we need first and foremost a better transformer.

This is easily done in Australia by procuring  "inspire" grid tie inverters off ebay for 50-80 dollars, and extracting the huge torroids from them. In this article we will assume you have procured two of them, and we will make a much better transformer than both of them put together, by making them a single core and then rewinding them.

This is a simple procedure, but one that requires patience and methodical action.... it is a true pain in the butt to do, but is simple, and a perfect foundation for your inverter.
The transformer you will create would cost around the $1800 region, if it was purchased in this country... this one I am winding will cost $120, and i will have two stainless steel boxes with ip55 rating as well left over, plus a range of very nice filters to quieten down the hash from the inverter if you want to.

Here are the two transformers after pulling them from the box, and knocking the center out of them... I use a smallmash hammer and a big drift to simply knock it straight through the center hole... you can drill it out if you wish like I did the first time... but it is both dangerous to the wiring and to you unless you have a good way of holding it very securely.... best just to knock it through... takes seconds, and no mess.
3948-0

Once that is achieved, it is time to look at what we have got. A huge torroid the like I have never seen before. The figures on the side say 130v:240v 50hz.

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The 240v winding is on the outside, and the 130v winding is on the inside.... there are several ways we can tackle this.
We can simply tape the two together, join the 240v windings in parallel, ignore the 130v windings, and wind a secondary around the two transformers, and were done.

This will work, but you will struggle to get decent wire through the center for your new primary... so not worthwhile.

Next option.... we can strip the two 240v windings off the outsides, and be left with the 130v windings on each. This is not so silly, as the 130v winding is 2 in hand, so if we stack them together, tape them up, we can series these windings and get a 2 in hand 260v winding, and a much bigger hole in the center. This is an ok arrangement, and will work well... and very easy to do.

This gives us a 260v secondary.... this is a good thing, as the reason we are doing this anyway, is to get the magnetizing currents down... otherwise we need only buy the Powerstar W7, and we get the same kind of honest power, but 5.5kwh/day losses because of the E I transformers they use... very lossy. It's that simple, if they used big decent torroids, we would not need to change anything.

So if we can "overwind" a bit, we decrease the strength of the transformer field needed to run the thing, so we can get the idle currents down even further... ie for a given field, it is amp turns... so more turns, less amps..... and the magnetic field in the steel can be decreased as well, and this will mean the magnetising currents will be less.

It is likely that the original configuration will be close to saturation, and a look at the graphs for this will show a non-linear relationship with current and field strength as we get progressively closer to saturation.

So if we can back off from that point even a little bit, we will get a more than linear decrease in magnetising current.. so all good.

One draw back with this approach is that you have to live with the transformer noise. This may not be an issue if you locate it well away from your living environment, but can be a nuisance if you work next to it all day.

The second draw back, is that you can get rid of a pound or two of copper, if you rewind it as a single transformer from the start........ ie those wires that are pressed together between the cores where they meet, can be gotten rid of entirely. Each transformer can then loose 1/4 of it's resistance from copper loss, and that pushes your power handling up.

Saturation: When you have designed the transformer, and it runs fine, you cannot saturate if from overload if you have a sine wave source... contrary to popular opinion.

If you look at the equations, you need a frequency change or a voltage change to saturate the core... input current makes no difference.. nix... nill nada.
So providing your inverter does not clip, you should not saturate the core, no matter how hard you press it. All this extra current put into the system will be working against the wire fields as MMF and back MMF, the magnetising current remains the same... so I think the load losses are in the copper... so if we can get copper R down, we are well on the way to more power for longer. ( eddy currents and hysteresis will also play a part, but not much in a torroid comparatively.)

ie any transformer ( because we know the core does not saturate from over loads) can take large currents...so the output current is only limited by the heating and regulation ie can be massively overloaded for short periods. The voltage will sag in proportion to the copper losses, but provided it is released before it burns up, it will survive... but it won't saturate.... so.... a 100w tranny will stand say 10kw..... but only for maybe  milliseconds before the copper fuses.. this is extreme, but you can see what we are driving at. Basically the difference between a 1kw and a 10kw transformer is how fast we can keep temp from rising beyond safe. ( alright, the voltage regulation will be total crap too )

So a 2kw transformer may handle 10kw fine if it is...... say .....oil cooled, and so becomes rated at 10kw. The grid tie ones are inside a sealed box, so are "under rated" so to speak because they have little chance to get rid of the heat, but must run all day at full power or for a fair bit of it, so although they are rated at 1.5kw inside the box at 100% duty cycle, once outside, and with most of the windings gone, they are very much increased in power handling ability.

Edit: I edited the above, but it is still yuk, the idea is there, the expression is not.. will edit it again soon.

Thats why the PJ can do the 8kw for a short period of time, sadly, the transformer gets hot quickly... but the cores will do it fine... the copper won't. PowerJack has taken this to extremes, and sometimes uses a single transformer with a fan instead of two transformers... obviously hoping for a very low duty cycle.

So after all that, the next possibility is to unwind it totally... and start from scratch.. with three cores used,  we could loose a complete transformer worth of copper, with 2 cores we get rid of 2 sides of wire..... so our copper losses get less, and if we want we have the option of more copper as well, as the hole size remains the same, but the turns is decreased by half or 2/3rds ( depending on core count)

I have done all three styles. A complete rewind is best for noise and performance.. so the suffering is worthwhile.

So, moving right along....

Here we are with some of the tape coming off. We use a small former of some type to roll the plastic on. This is to be reused, and is an easy way to unwind, and rewind... and is boring as hell....
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And here we are with the plastic off the outer layer:

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Here we can see the unwinding stick. It is about 800mm of 6mm x 38mm steel, with notches in the end to take the wire. It must be fully insulated with a few layers of tape so that the steel cannot touch the enamel as you weave the thing back and forward through the core.

There is a right and a wrong way to use this, and if you look at the wire as the stick is passing from  right to left, you can see that the wire being unwound is on top of the stick, this will stop horrible kinking, and make it very easy...... you'll be surprised how unfit you are until you try this for a few hours.
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Notice how kinked the wire is on the stick... once we get a layer off, we need to straighten it out.... first step is to hook it onto something solid....and give it s stretch..... thusly

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Delicate is not my strong point..... unroll your wire off your stick, and stretch it a bit to get the main kinks out of the way.. then use this next thing... simple but effective.

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It can take two passes on the inner windings, as their corner kinks are far more pronounced than the outer wires corner bends.......
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Then wind it up on a spare former  like this.... and try to keep it very neat, so the wires do not cross, this will help to not introduce more ripples in the wire.

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Then it is a matter of just plugging away until you find the core itself.

..............oztules




Flinders Island...... Australia

Offline oztules

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So we have now found our core and it looks like this:
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By the time you get it all off you will have over 7 lbs of wire there 2mmsq...
Now we strip off the last layer of plastic, and we get left with this after we pull off the glass layers from one end. There were a few manufacturers fo these for inspire, and they may be slightly different in how they terminate etc, but the cores will be very very similar

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So we get two of these cores with glass on one end only of each, so we can join the cores together with glass and epoxy like this.

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and then we simply rewind on two or three layers of tape that we have on our bobbins from the unwind... and we have this..... ready to start again.

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Well thats as far as I got today, will try to get some more done tomorrow.. it is the rewind time.

I think I will wind this with 114 turns on the secondary and around 14 turns on the primary.

......... oztules



Flinders Island...... Australia

Offline frackers

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Perfect timing

I've just last week ordered a set of 48v 15kw boards and (weather permitting) Wednesday lunchtime will be 'skip diving' round the scrap yards to find suitable transformers.

Must tap up a couple of old mates to get access to a decent crimp tool (and maybe even the cable and crimps) to handle the cable for the primary. I'm thinking 50^2mm as the (high quality) stuff I've used on my current system is good for over 200 amps.

Some detailed dimensions of the Inspire toroids would help in identifying suitable substitutes :)

Cheers
Robin Down Under (or are you Up Over!)

Offline oztules

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50mmsq will be fine for this application.. it is air cooled, and will rarely be driven for long periods where this amount of copper will be pushed.
dimensions:
outer diameter = 185mm
inner diameter = 85mm
hight of roll = 65mm
width of stack = 50mm

weight is 10.1kg with no wire, near 15kg complete.


The cross section is of interest only really.. although a big core hole helps immeasurably. The cross section can be calculated simply, and if you start stacking them, say smaller torroids 5 high may get the same results.... but the hole size may make winding them iffy.

The original power jack torroids would do 25% better if they were rewound and glued two transformers together too.

Apparently torroids are twice as easy to magnetize as EI transformers, so good old fashioned welding transformers may have iron good enough for maybe twice the magnetizing current. My testing bears this out too. A big  EI transformer from the Hydro here runs at about twice the idle current as the torroids... it would be high quality.

..............oztules
Flinders Island...... Australia

Offline oztules

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Got some time to do a bit today.

I know this is a bit like watching paint dry, as the pics all look the same... but if your doing it for the first time, you probably want to see every stage... so here we go.
First we wind the secondary one in hand. There will be three of these... this is what the first one looks like. It is one in hand... even though it looks two. I followed their pattern of placing two wires side by aide rather than making each wire equidistant. This allows us to use the inter winding tape to advantage... ie the next layer has a depression to lay into.. two wires again.

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It took two reels of wire, so there is a join in there. It is helpful to put some tape/ texta/ whatever on the 45 degree positions, and calculate how many turns you should have by the time you get to each point. I wanted 114 turns so roughly 114/8=14.25 turns to each station. This is important if you want a tidy and easy to work with transformer. It means the second layer goes in easily between the first layers wires.

This  next one is after the epoxy has been painted on all over. This sticks the whole mass together, but you don't want too much that is takes up any space... just glue everything together, don't let it run and gather anywhere.... space in valuable

You can see the tape under the epoxy I used as reference points for the 14 turns

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Now we put a layer or two of tape to make sure there will be no chance of wires touching the first winding. They would be almost the same potential, but we don't want these to have any chance of failure.. it looks like this, and we can see the outputs there too.

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and here

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And here is a pic of the second layer going on. Showing the former that we roll inside the center to pull the wire through smoothly. The wire is orientated so that it unrolls against the walls.. We can see how the winding lays between the first wires now.... very useful reference.

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here is the second layer finished

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Before you join the layers together in parallel, it is important you check that the wire turns are the same, and the phasing is correct.

So join one set of ends  together as you would to parallel the windings, and put neutral on that, leave the other two wires open.

Now attach an active on one of the floating wires.........with a globe or similar in series as a fuse and soft start. Without the globe, you will blow any size fuse your house has..... it will blow any fuse if you get anywhere near zero crossing... once running it uses almost no power.

Turn it on. Now measure the input voltage across that live winding... then on the other winding that is loose at one end. Both voltages should be the same .. exactly the same.
Now measure between the active of the winding and the floating end of  the other winding.. they should be the same potential, so the measurement should be zero volts.

If all good, then turn off and connect the windings in parallel for keeps ( twist and solder) ...... otherwise add or subtract turns until it is right.  Minimum is 114 turns... don't subtract below that, add instead.
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For the benefit of Andy, I assume that the aerosharp has 1.6mm wire , not 2mm, so I wound this with 1.6 just to see.
It will require at least one more winding for power carrying. The aerosharp 3kw one I have has a bigger hole than the inspire, and that helps lots, hopefully yours does too.

It is much easier using the 1.6 rather than the 2mm stuff, but now there is an extra layer to do..... always something.....

So, more epoxy, then more wire tomorrow.... then the primary.

One interesting thing of using the epoxy, is that it helps the cooling, stops the noise, fills in any micro cracks in the second hand wire.... and also gets slimy when still runny. This allows you to fairly easily move the wires around on the plastic substrate very easily... for cosmetic purposes. So while the wire is tight and does not want to move in that picture, tomorrow when the epoxy goes on, it will be easy to tidy it up some... and hopefully make it symmetrical. This looks better... but importantly makes making the third layer the same as the second, as the wiring will simply fit in the gaps again, making it easy to keep the turns the same...

Remember, a turn is counted as a wire going through the hole. Don't care how it ends up, but has to be through the hole... thats where the magic happens. So this takes away the confusion of half turns at the start or finish.... if it goes through the hole, it counts as 1 turn. If you only count the wires in the core, you will be fine.

This will have to carry 8 amps for 6kw on each secondary wire. Should be no problem for 1.6 wire fairly well cooled. Thats at 100% duty cycle.

In this application, thats fairly unlikely, as the batteries will start to grizzle if we keep that up hour after hour, so full power will likely only be transient... maybe 30 mins max for 6kw-9kw cont... and even that is hard to see. This one will spend it's life at 500 watts average for 12 kwh/day, but should handle 10kw fairly comfortably for short bursts. For off grid, it seems 400 - 900 watts average  is the normal usage. So providing your inverter can do very large bursts for say 15 mins, then it is unlikely you will ever feel like your off grid. So if the hot water is running, the microwave running, the toaster is going and the kettle goes on.... the inverter just runs as normal.... but we can see that these loads will not run forever. So providing our transient ability is huge ( 15-20kw), and our short term high power is in the 30 min range, it is highly unlikely that we will ever resort to overload shut down from temp, and the boss will never complain of not being able to vacuum while the washing is on and the water heating  and boil the jug etc etc etc.

If you expect more, then another layer will give you better duty cycles. ...... ie 4 layers will mean that each secondary need only carry 6 amps for a 6kw load. 6 amps in a 1.6mm wire is pretty conservative, and only takes another hour to accomplish.... it's roll your own territory.

Your limiting factor will become how much copper you can get in the hole for the primary. This one will probably only get 50mmsq wire, but 6kw will be only 120 amps, and air cooled means that 50mmsq will be easily enough..

If anyone else has anything to add, please speak up, all this is from scratch, so I have had to invent every step. There are probably better ways of doing this stuff, and I'm all ears.


................oztules


Flinders Island...... Australia

Offline andymack

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Still unwinding the secondary.  I measured the wire to be 1.8mm dia. including enamel.  Its look like their bobin couldn't hold enough wire as there is a join.  Keep up the great photos - much appreciated.
Andrew

Offline oztules

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Well.... different again. Interested in the hole size. So far the aerosharp I had (3kw) had larger hole than the inspire, same outer diameter, but higher profile to make up for the less width.

"still unwinding the secondary"..... are we having fun yet???

..........oztules
Flinders Island...... Australia

Offline andymack

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AeroSharp 2k5

The transformer was 15kg, I've taken off pretty much 3kg of secondary, if the primary is another 3kg that's leave the core at about 9kg. So yes, bit smaller.  I  suspect the inside dimension is a bit smaller than your core as well.  We will know soon.

So i'm looking at 246volts /2 = 123 turns ?

I've haven't got any left over thread seal rolls (threw them out) nor empty solder rolls.  I figured I might make some spools from 20mm conduit and 4mm plywood sides (use hole saw to cut them) - bolt through the middle

Offline oztules

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Hopefully your hole size is bigger than mine, and so a homebrew former will fit. What did you put the tape onto??... you must have miles of it running around the place without a former for that.

Yes, it looks like you will have 123 turns on the secondary, and 15 turns on the primary.

Doing a transformer rewind with these is quite a  physical enterprise isn't it.


...............oztules
Flinders Island...... Australia

Offline andymack

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It really is  :)
Admittedly I'm a slow worker, its taken 1 1/2 days of work just to get to where the secondary winding is off (and yes, other odd jobs).  Yeah lots of mylar tape on the floor  :) Would definitely be easier using a spool.   I think the extra joins actually made it more work.  I used a 3 plywood 800 x 35mm stick as unwinder former, duct tape on the V cuts each end.  If some else it working one of these AeroSharps I'd recommend have 2 sticks available - I had to unwind one wire to continue unwinding another  ::)

Offline oztules

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Ok This is where we have wrapped the first two windings up, and now ready for the last and final hv winding.

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And here is the final winding before epoxy

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and with epoxy

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Whew glad thats over for this one.
Now I we only have to wind the 14 turns of heavy wire and we are DONE :) :) :) :) :)

That may be tomorrow.


.............oztules

Flinders Island...... Australia

Offline andymack

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AeroSharp 2k5

More unwinding  :)
Shield layer I presume.  Do the inspires have this ?

3999-0

Offline oztules

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yep .. sheild.

No, the inspire has no shield thank goodness..... but my aerosharp 1.5's did.

now the primary.... hehehe...


Sadist Oztules
Flinders Island...... Australia

Offline andymack

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Okay, unwinding the 2nd layer of mylay tape, this time i'm using the bobbin when I should be.
I didn't have anything suitable so I made these.  Seems to work, 20mm dia conduit x 20mm ish with 50mm round cardboard hot glued on.


4003-0

Offline oztules

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I found single width is best for this.. It will make a big difference for you now, without it, there is plastic all over the place..... and it makes rewinding easy .

............oztules
Flinders Island...... Australia