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Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 01:02am 04 Feb 2021
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Don't know what else to call it but here is what I want to do and would appreciate the knowledgeable people telling me if it is feasible or laughable?
I have a 3 phase Load. I want to take power from one phase ( that will be connected to the phase I want to supplement) and feed/ boost it from another phase.
The Idea, flawed as it may be, be is to use a 100A, 1000V 3 phase rectifier on one or 2 Legs of the 3 phase AC supply ( NOT the one I want to feed to) Put the resulting power into the input of a solar inverter ( SB 1200, max input voltage 400V) and then connect the output to the phase I want to feed.
Specs of the inverter are:
PV generator connection data SB 1100 SB 1200 SB 1700 Max. input voltage UDC Max 400 V a) Input voltage, MPP range UPV 139 V ... 320 V 100 V ... 320 V 139 V ... 320 V PV start voltage, adjustable UPV Start 180 V 120 V 180 V Max. input current IPV Max 10 A 12.6 A 12.6 A Max. input power PDC 1210 W 1320 W 1850 W Voltage ripple UPP < 10 % of the input voltage.
I have several of these rectifiers so was thinking to use these:
So the Idea/ question would be to take the AC, turn it into DC with the rectifier which I am thinking will eliminate the phase sequencing of that leg, feed it into the inverter which will Sync it with the leg I want to feed which will be powered and then output it's power to that leg reducing the draw from the mains it is connected to.
Questions:
What will the rectified output voltage be from the AC and can I take the input from 2 Legs of the AC ( preferable) or will I have to take it from one and Neutral to keep under the 400V input of the inverter? I know the rectified voltage goes up ( I think by 1/3rd??) but not sure what the resulting output will be?
Will the Voltage from the rectifier be clean enough ( 10% Ripple specified) or will I have to add caps and if so, any suggestions how much.
I spose the big question is would this work at all and or what am I missing that would cause Fire works?
Thanks.
InPhase Senior Member Joined: 15/12/2020 Location: United StatesPosts: 178
Posted: 01:19am 04 Feb 2021
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I can't comment on whether it is legal by any stretch of the regulations, I suspect not. But could it work? Yes. What is the rectified DC voltage? Well that depends on if you get power from two phases or between a phase and neutral. If you have a 400/230 system, then 400 VAC becomes 566 VDC and 230 VAC becomes 325 VDC. After capacitor smoothing, that is. Edited 2021-02-04 11:21 by InPhase
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 02:02am 04 Feb 2021
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Thanks.
Finding some conflicting info on what the rectified Voltage will be. Does Frequency make a difference? Maybe that is the reason for the different info I'm finding?
With solar feed, voltage can get a little high on my mains but I figure voltage drop with the load will be my friend here and the Inverter won't kick in till the load is already running so should be fine.
With the caps, they go across the DC output I take it and what sort of capacitance would you recommend for the inverter being 1200W?
Barring any other conflicting input, I might try and Find a little 3 phase motor I have up the back and set up a test with some barely adequate Circuit breakers just to verify my setup if nothing else.
I have never seen anyone talk about this before which seems a bit surprising but then again, it's not single to 3 phase so I spose the application is very limited.
nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 05:52am 04 Feb 2021
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Hi davo99, 2 weeks ago I did something similar, I had to repair a gti for someone. The input I needed was 150-450VDC, but I don't have easy access to my solar panels that are connected in series.
So I first used an isolation transformer, this step is very important with inverters that don't have a transformer inside but only have inductors in them. And maybe also for gti's with transformer, I don't know, it will depend.
Then I took a Variac and a 35A 1000V rectifier in to a 1000uF capacitor, as input for the gti, so I had a variable DC output. I only used 1 230VAC connection so I was "backfeeding" in the same phase that I was loading, and that worked perfectly. I used a 40 w bulb to precharge everything. And that was on my " ozzinverter grid".
Some inverters test all kinds of things like earth connection and L an N, so this could complicate some things.
So injecting from one phase to another won't be a problem I think. For capacity start with 1000uF per 1A draw.
Your actually maximum voltage becomes AC x 1,41 = DC with a bridge rectifier, this is important for choosing your capacitors voltage rating. When you load it it will drop. Edited 2021-02-04 15:54 by nickskethisniks
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 09:19am 04 Feb 2021
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Thanks Nick.
Pretty sure this GTI is a low frequency transformer type. At 23 Kg for a 1200W inverter, either it's a transformer or built out of WWII surplus amour Plating!
Topology Low frequency transformer
The isolation transformer could greatly complicate the issue, especially the installation. I was hoping the inverter itself would provide the phase rotation isolation in changing the DC to AC through the galvanic isolation.
The capacitance sounds a lot but if that's what is needed that's what I'll have to get.
Voltage times 1.41 Gives me 352V off 250 V which I think is way more than this will see especially under load and gives 50V margin on the inverters limit anyway so that's all good.
nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 02:50pm 04 Feb 2021
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You can probably get away with less capacity, because there is already in the gti itself. But normally it expects a clean solar DC input, now you will inject a DC with 100hz ripple. So I don't know how it will behave on the long-term.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 03:32am 05 Feb 2021
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Yes, I was wondering about that. The specs say Under 10% so might be OK if I throw enough caps at it.
Do you think it will be OK without the isolation transformer?
nickskethisniks Guru Joined: 17/10/2017 Location: BelgiumPosts: 458
Posted: 05:18pm 06 Feb 2021
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If there is already a toroid inside, it's probably ok.
mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209
Posted: 07:37pm 06 Feb 2021
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It's feasible, I rectify my hydro generator (single phase - 28Hz to 62Hz) and feed into a SB1200.
But: as my hydro generators output is ground referenced on one side (as your AC input neutral is) you will get an 'earthfault' warning as the SB expects the d.c. input to be floating - not an issue in itself, but I find that if my hydro generator is pushing >160v into the SB it won't start inverting - just sits there saying 'grid-mon' or 'waiting' (and 'warning earthfault').
i don't use additional caps but I've only gone up to 400w. I don't use an inrush limiter as the hydro output can't produce that many amps, but if your using rectified mains...
Oh, and yes, there is a big isolation tx in the SB1200. Edited 2021-02-07 05:45 by mab1
mab1 Senior Member Joined: 10/02/2015 Location: United KingdomPosts: 209
Posted: 08:23pm 06 Feb 2021
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This is not answering your questions on this thread Dave but I can't help thinking that a rotary phase convertor would be a simpler way of doing what you want - little or no electronics:-
You have 2 phases to draw power from, so if you had a 3phase motor wired 400v 'Y' (not delta) you could connect two of it's lives to the two grid phases and neutral to the star point, and it should start and run and generate phase 3 - independent of the grid's phase 3 - on it's remaining live connection.
It would be better still with some motor run caps to the new phase 3 - but here my knowledge of phase convertors runs out and I would be guessing it I tried to advise what caps where.
It could be done with a 400v Delta wired motor, but you would definitely need the caps right. It may be easier to google a rotary phase diagram for that though ( i can't google much at the mo' - my broadband is currently running at 0.1Mbps)
Then all you'd have to do is make sure the control circuitry of your AC was connected to the 'real' phases as the generated phase would have poor voltage regulation.
Davo99 Guru Joined: 03/06/2019 Location: AustraliaPosts: 1578
Posted: 10:11am 07 Feb 2021
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That could be a problem as the rectified 240 Is going to be way higher. If I remember right as I haven't played with it for a while and my mind is not improving any, if I go between 2 live phases my voltage will be 440v and rectified that will be out of spec for a lot of inverters.
Oh, and yes, there is a big isolation tx in the SB1200.
Least thats a positive.
I have played around a lot with 3 Phase motors in the past and making rotary converters. I forget more than I knew now, as with a lot of things. I'm not sure it would work in this case though with out a bit extra gear.
If I have 2 phases connected which will also be on the Compressor and the 3 rd one going to the motor which would also be on the compressor, I'm just going to be running the motor as all 3 Phases would be hooked up.
If I take the 3rd phase off the compressor, I'm not sure the control circuitry of the AC will Kick it in because I'm pretty sure that is monitored. Might just be supply though to the board but I got an idea that mate said at least some test for connections present and correct phase rotation.
I think I'd need a delay on the compressor with the 3rd leg coming off the motor because if it tries to Kick in when there are only 2 phases live, that probably won't work and may overload it. If the motor and the compressor all try to start at the same time, that could be a big inrush current and cause voltage drop issues as well. The motor I believe would have to start up and then the compressor would have to fire once the 3rd phase coming from the motor was present. Might be helpful to put a big pulley on the motor or a flywheel to help with the reactive power handing as well. Compressors can hit hard when they start up.
I was just wondering if there might be a pre start on the condenser fans I could tap with a realty but pretty sure there is not on this one.
I think a motor is going to need a decent and I perceive none to cheap delay relay to work. The amp draw running per phase is about 9A, the LRA which doesn't really tell one a lot is about 60 but the delay contactor would have to be pretty Sturdy. I could ask my Mate that does Commercial and Industrial AC and heat pumps if he has any laying round though. There is usually a treasure trove of Goodies in his yard waiting for the scrappy to bring the bins. Got some great taperlock pulleys as well as things like relays and other bits and pieces. I got a 12 Kw 3 phase motor off him a while back, was changed out because it had a slightly Noisy bearing. He had the bearings but then the customer changed their mind and upgraded the system. We forked the thing up on his workbench and changed the bearings and into my ute.
Was going to to an IMAG with that but another project that I'm going to have to 157 years old to get to as long as I don't take any more on in the mean time.
I played years ago with a 3 phase motor Driven by a little Diesel, rectified the power and fed it into an inverter and back fed with that years ago. Got a Vid on YT and all. The only thing wrong with it which wasn't a great problem was the tracker in the inverter was hunting around and caused a bit of load variation on the engine. Wasn't a problem but it wasn't smooth either. I did work out a fix for that although I can't remember what it was now. I know it wasn't caps as they pushed the motor voltage up, I tried bypassing the governor on the engine and that helped because there was a see sawing effect between the tracker on the inverter and the Governor but it wasn't perfect. Can't remember what I did now. It was only playing around at the time so I probably forgot soon as I solved the problem.
I guess from here all I can do is set up The inverter and try it. If it doesn't work I'll just run the inverter off some panels. Won't be much good at night but will at least take the day load off and then I'll go back to looking at a commercial single or twin phase to 3 phase converter.
Although I wonder if I could get away with a single just on that Leg?
Thanks all for the feedback and Help. Greatly appreciated.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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