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Forum Index : Electronics : A compact Inverter
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Murphy's friend Guru  Joined: 04/10/2019 Location: AustraliaPosts: 648 |
So I thought I build an inverter for my caravan, 24V system. It had to be portable, only used for that long trip with lots of free camping. This is what I came up with, the 24 can drink box is for size comparison.  It was built completely from ideas seen on posts in this forum. My thanks go to wiseguy who kindly posted a schematic for the driver & final power stages. Also to poida for publishing his nano inverter progam, I made my own PCB (100x100mm) for this control board. Lastly to warpspeed for his very clever opto isolated idea. The inverter uses a toroid salvaged from a 1.5Kw Latronics inverter, I left the original secondary in situ and re wound the 96V primary so it can now be connected to a nominal 24v or 48v battery. The fully opto isolated output uses a DC-DC up converter to supply 50V to the little 15v isolated power modules as they won't start reliably on 24V. The output stage has a totem pole final drive. The inverter uses 16 HY4008's, and 60,000uF of capacitors. The idea was to connect it in sync to my generator as the genny (1800VA cont, 2200 peak) won't run the A/c in my caravan. Here are a few pictures what it looks like:       A few precautions have to be observed when messing about with synchronising a generator to my inverter. The correct phase connection of the little AC sensing transformer is, as poida pointed out, VERY important. The reason being is the Nano program also locks at 180 degrees out of phase. The frequency sync lock works very well BTW, almost instant. My generator is the inverter type so it always puts out 50Hz regardless of the motor RPM. It also has an ECO switch that senses the load and adjusts the throttle accordingly. As these type of inverters can work backwards, an uncontrolled battery charge would result once paired to the genny. To prevent this I installed a 120A Schottky diode in series with the positive battery connection so power can only flow from the battery to the inverter. The diode (dual 4 terminal block type)is on the small heat sink visible in the top left corner on one picture. So, did it work? Well, the inverter by itself works very well  but the fun started when I threw the switch that closes two relay contacts which link the inverter & generator AC.It works for a very short time only then my inverter trips to shut down the nano. Its a voltage thing, the inverter AC voltage has to be set just so and there has to be a small inverter load (I used a 100W flood light) for the short sync session to happen. But there are power surges as the genny senses the load and gives a burst of revs. That is when the inverter shuts down. Trying to re start it gives the inverter frequency the 'wobbles' and I hit the off switch fast. The genny never misses a beat BTW  .I must say wiseguy, you came up with a *very* robust design, one that refuses to die despite my numerous attempts to kill it with those synchronising experiments. I was worried I had killed it when the first 'wobbles' happened but doing a hard re start (disconnecting the battery) and re connecting it always seems to bring it back to normal working frequency & voltage. No busted mosfets, just two blown floodlights?? thanks wiseguy  .The outcome of all that is that I have to give up that synchronising idea for plan B unless somebody comes up with a better voltage regulation that takes into account what the generator is doing. (plan B is my itechworld RB2 genny has a provision to connect two of them together with a special cable to double the output. I'm investigating if that also works with an inverter & the genny). |
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| brucedownunder2 Guru Joined: 14/09/2005 Location: AustraliaPosts: 1548 |
A lovely build, congratulations Murphy,s,(whoever he is)lol., You have made a very Robust unit, having no caravan experience, but my brothers have gone around Oz a few times ,and discovered that your robust construction is the only way to stop disasters on the rough dirt roads, . Nice that you posted the pics, gives us all good ideas, thanks. Bruce Bushboy |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
A lot of thought and experience has obviously gone into planning this, and it definitely shows. Great attention to detail and magnificent workmanship. Thank you for sharing ! Cheers, Tony. |
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| johnmc Senior Member  Joined: 21/01/2011 Location: AustraliaPosts: 282 |
Thanks for sharing your inverter project. Most impressive. Cheers john johnmc |
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Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1024 |
I can see a lot of time and effort gone into arranging components to fit in neatly and work nicely. Nice to see another successful inverter build. Cheers Aaron Off The Grid |
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wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1156 |
Well done - looks like we are going to have to re-write Murphys law ! It really looks the part - what was in the box initially ? Thanks for the up vote and I am glad it is working well for you too. Syncing with an external generator seems to be fraught with issues. I have pointed out before that when you have 2 asynchronous signals it is easy to lock one to the other. The issue is that when you now connect the 2 outputs together physically, the inverter synch circuit only sees the sum of the two signals even though the generator may be now drifting away, our inverter just drives harder to fix the output voltage which is also trying to be be controlled by the generator inverter and they both fight for output control while the drift away just continues to worsen. The inverter and generator started to run asynchronously again as soon as the outputs were physically locked together and the synch signal that our inverter initially locked to is now a combination (sum) of two asynchronous signals and the sync signal we are feeding to our inverter is not really the generator voltage true zero crossings any more. A possible method of working is to put a current transformer between the generator and the load. Our inverter output has to be fed into the load side of the generator current transformer. Use the sinewave created by the generator current transformer to generate the zero crossing to sync the inverter to (1 pulse per 360 degrees). This sinewave is also mixed with a current transformer signal from our inverter output to create a feedback signal to the inverter (output sense) input. Now the fun part, use the current transformer on the inverter output such that the output of our inverter matches the generator inverter output and our inverter is either driven harder or throttled back to always match the generators, always keeping the current transformer signals equal. In this manner the generator is essentially the master and our inverter is the slave if a 500W load is attached to the setup the generator will settle supplying 250W and the inverter will match it with 250W. To my mind I cant see it ever working any differently, unless we could access the generators inverter drive signal internally, to provide a real zero crossing synch pulse from the generator inverter, that is unaffected through tying the outputs together. If we did this, then our inverter should always be synching properly to match the generators output and simplifying the power sharing issue. Edited 2020-10-03 00:22 by wiseguy If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1156 |
BTW, I believe the latest van Ibis? air conditioners have a compressor soft start built in which will now allow them to start and run from the 2kW Honda inverter generators. I bought a 3kW Hyundai remote start unit for our van - which is a real pig to start at times when it has not been used for a while, but once running is excellent. We have the latest Ibis anyway and I should have just got the Honda - my desire for overkill worked against me this time. Do you think the aircond could run from the 24V inverter only? Then use the generator output to input 1.8kW (75A) into the 24V battery ? Piece of cake  If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
My thoughts exactly. Cheers, Tony. |
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renewableMark Guru Joined: 09/12/2017 Location: AustraliaPosts: 1678 |
Well that certainly looks a hell of a lot neater than anything I have made. Looks very much like a Klaus build. Very nice work, lots of time has gone into that. When I run A/C in my van it just gets hooked direct to a small Kipor generator. Even "she'll be right" Marky fella hasn't tried the sync yet, frightens the crap out of me! Blowing up a good gen and an inverter isn't on my to do list. Edited 2020-10-03 20:05 by renewableMark Cheers Caveman Mark Off grid eastern Melb |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Thanks wiseguy & warpspeed. I've read wiseguy's current transformer suggestion, several times , but I think this is a bit beyond me.The van has an Ibis a/c unit but its as old as the van (7 years), I bought that van second hand last year. I have not yet tested it if it has soft start - doubt it as the generator shuts down when the a/c comes on. I did run the a/c just of the inverter to see what happens, it seems to start & run set on the heating cycle(it was too cold for cooling) but my clamp meter showed more than 60 Amps going out of my battery so that test was quickly terminated. I see a little problem with 1.8Kw battery charging, good plan B though. My van's 240V battery charger only does 20 amps. So I need something more powerful but good enough not to wreck my very expensive 120Ah/24V lithium batteries. So, if you guys know of a suitable charger I'm all ears, meanwhile I'll do a bit of ebay surfing  to see what's available.Oh, before I forget wiseguy's question, the inverters enclosure was built from scratch to fit exactly what's inside, got pretty crammed full as shown in the pictures. I have since removed the sync relay parts & mounting brackets so the inverter PCB's are more accessible now. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You might be able to do something clever with diodes, so that the inverter can draw DC current from either the 1.8Kw petrol powered dc rectifier, or the battery. The battery could then help mightily with any startup surge of the air conditioner, without any risk of overcharging the battery. If the rectifier voltage is set suitably higher than maximum battery voltage, that would be the primary power source when the generator is running. It could also charge the battery via a suitable additional current limited charging circuit. If its all thought through, I think a suitable overall control strategy may be possible. Only real down side would be the losses through the diodes. But in the great scheme of things, if you are running both generator and air conditioner together its not a huge disadvantage. Under more normal conditions the diode in series with the inverter could be switched out. Cheers, Tony. |
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| Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578 |
Very Nice and professional Looking Build. You have put a lot into a small compact package as well. I know this is a ham fisted solution but I'm no where near the level of you guys. What I have done in the past is simply couple up an alternator ( car) to a battery and run an inverter off that. As long as the alternator has the output the inverter is drawing, the battery is basically " Ballast" and you can run things as long as you like and the battery(S) will still be fully charged. You don't even need a very big battery, I have done it off a couple of 12AH SLA's with the inverter pulling a bit under 2 KW with a suitable couple of alternators and was no trouble. If you could get an 80 A 24V alternator ( 160a Leece Neville's are common) you could just use that coupled to a 6.5 Hp stationary engine and by pass all the problems with the inverter genny. You can increase the efficiency of the Alt with an external controller either building one your self or with one of the step Chargers that are common in marine applications and aren't that exy at all. Leeces are easy to hook up with these external regs and give a lot more control. It's not the alternators themselves that are inefficient, it's the very rudimentary charge Controllers they have built in now days. You could then gear up the alt to run the engine quieter and slower. An engine that size would have capacity for about 3Kw of electrical generation so plenty of headroom. Would be another setup but at least you could dedicate this to the ac and have power from the generator to run whatever else you wanted in the van. Either that or Build another inverter and run it all off the 24V genny. |
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| Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 648 |
Thank you all for your suggestions. I decided that trying to sync this inverter to my genny is too difficult so the inverter got re connected to 48V nominal and serves now to power my house lights off the battery bank. Perfect for that job too.. I did some more tests with that inverter type generator, there is a large (>20VAC)) momentary voltage drop when a big load gets switched on, I think that is what upset the synced inverter. Otherwise the generator voltage remains between 243V (idle) and 241v full load. I also did some measurements on the caravan A/c unit, it takes just around 1000W to run, so well within the generators 1800W capacity. But the momentary load as it switches on must be larger than the rated 2200W of the genny since it disconnects the output - but keeps on running. It has to be stopped to turn the output on again. So I was thinking, what about using an UPS? It only needs to cover the brownout for a second or two and I could possibly use my big super capacitor (250F @ 30VDC) instead of a battery in it. But I know little about UPS's and there are absolutely lots on ebay. Could a smaller (<1000VA) model be used? the bigger ones are a bit too expensive for me. Or it it possible to build such a beast from scratch, based on the super cap storage idea? |
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| noneyabussiness Guru Joined: 31/07/2017 Location: AustraliaPosts: 512 |
UPS will work as long as it is rated for it, the apc 1500w jobs should be enough, or there abouts... why i said them, is the ones with LCD on front, you can set their voltages when they trip to a point of course, but im sure any UPS rated enough should work. I use 2 on critical loads in my house, so if i have to change over inverters ( i have 2 backup inverters) for maintenance or something there is no drop out. I built the batteries myself out of scrap laptop batteries, as the ones in them were dead. Hope this helps... I think it works !! |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Only way to know for sure is to connect up your air conditioner to the grid through a current transformer, and measure the inrush peak current under various start up conditions. A digital oscilloscope (set to a very low sweep speed) or a multimeter with peak hold should be able to record a peak value. Prepare yourself for a real surprise... Cheers, Tony. |
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poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1418 |
I did this a few months back see this post wronger than a phone book full of wrong phone numbers |
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