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Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:40pm 02 May 2019
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Think about it Boppa.
With two phases and two diodes, each supply phase conducts only half of the time and has a complete rest the other half of the time. But the neutral conducts on EVERY half cycle.
Its even worse with three phases and three diodes. Each phase only supplies a third of the total dc current, but all of it has to return through the neutral.Cheers, Tony.
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 10:03pm 02 May 2019
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I was talking about a single phase system, (afaik there isn't a 3 phase variac made) ;-)
You can get 3 phase installations specifically designed for that situation (called 'solid neutral' or 'bar neutral' 3 phase- heard it called both over the years) where the N is actually capable of carrying the summed actives loads and is common in installations where unbalanced active loads may be likely to occur
And I still maintain that the active and neutral will be balanced on that phase at the load (assuming its a star connection), the sum total to the N might be higher overall, but it will always be the sum of all the active currents in a star 3 phase- L1 is 1A, L2 is 4a and L3 is 10a, the N will be 15a, but measured at the rectifier neutral, it will be balanced both sides, even if the combined current is higher downstream ie N1 will always be at 1a if L1 is at 1a; N2 at 4a if L2 is at 4a and N3 at 10a if L3 is 10a...Edited by Boppa 2019-05-04
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 11:00pm 02 May 2019
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There certainly are three phase variacs made, I have a 3 x 15amp one right here. Although strictly speaking its three single phase units ganged together onto a single shaft.
As you say, in "normal" three phase systems, the neutral carries only any unbalance current, and can never be higher than any individual phase current.
The problem with running three half wave rectifiers, is that the neutral current is dc and there can be no balancing out effect. There is with full wave rectification though, which is usually the only permitted method of direct off mains rectification.
For instance, variable frequency motor drives powered from three phases use a six diode bridge rectifier straight off the mains to generate balanced +ve and -ve dc voltages. No problem there at all.
Its only half wave rectification that causes the dc back down the neutral problem.
My own three phase off grid rectifier uses a transformer, so I am fully legal. Cheers, Tony.
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 11:32pm 02 May 2019
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Hmm never seen a 3 phase one before- cool (I still got one I bought second hand back in the eighties-3000w and cost me $5 at a garage sale- they didn't even know what it was, so stuck a $5ono on it... I didn't have the heart to even haggle with them...)
The 1/2 wave rectification is why I said to use a bridge rectifier, cheap and fixes all the issues
Why I suggested the fully isolated variac was that you not only stop any possibility of DC going back to the grid (which most power companies HATE, understandably so as it can cause major issues with electrolysis etc) is that by feeding the bridge rectifier with it, you can 'adjust' the voltage being fed to the system from the grid side, allowing a much better control over the system running voltage and load sharing points...
azhaque Senior Member Joined: 21/02/2017 Location: PakistanPosts: 117
Posted: 01:36am 03 May 2019
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Natasha Salaams,
Normally available off the shelf variacs are wound in the auto transformer configuration which would not solve the neutral current issue.
The good part is that there is a servo mechanism inside. That can be controlled bu a uC. An ESP32 based unit would mean feedback over radio waves to move the servo mechanism back and forth to balance voltages.
So if you are taking this route, please be sure about it before $$$.
azhaqueEdited by azhaque 2019-05-04
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 01:48am 03 May 2019
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Which is why I have emphasised several times that it must be a fully isolated model, although by correcting the mistake (ie the N to pv - link), that issue should disappear and using a bridge rectifier correctly should also fix it ie full wave rectification rather the current 1/2 wave rectification caused by that link (which basically shorts one diode out altogether, and renders two more totally ineffective...
Effectively using a bridge rectifier directly over the mains supply to provide high voltage DC is what every switchmode power supply worldwide does... so it's not exactly an unknown technique
azhaque Senior Member Joined: 21/02/2017 Location: PakistanPosts: 117
Posted: 02:01am 03 May 2019
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I agree Boppa.
But then a switchmode supply does not cause islanding, an the potential to kill someone working on the pole mounted transformer(pmt).
azhaque
Boppa Guru Joined: 08/11/2016 Location: AustraliaPosts: 814
Posted: 02:14am 03 May 2019
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hence the isolation transformer/isolated variac AND bridge rectifier combo, belts and braces... (personally I would probably also suggest a volt sense system on the mains side that when the mains goes down, positively disconnects the grid side of the transformer/ isolated variac from the grid- maybe even using it to disconnect another isolation relay on the DC side as well (2 contactors, one AC, one DC)- belts and three braces...
In theory you should never be able to get raw DC back onto the grid system, but should a bridge diode fail s/c and then the grid go down (not entirely outside the bounds of possibility should say a nearby lightning strike overvolt the diodes in the bridge and drop the grid supply offline...
Either resetable DC breakers or fuses should also be on EVERY voltage source leg on the DC side- be it the grid dc supply, each of the panel legs, the generator should one be connected later- plus imho, the AC side should also have its own AC breaker/fuse AS WELL as the household 'grid' breakers existing
Edited by Boppa 2019-05-04
azhaque Senior Member Joined: 21/02/2017 Location: PakistanPosts: 117
Posted: 12:33pm 03 May 2019
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Here is another idea:
$5
The triac should be about US$5 for 25 amps. Provides isolation. Control thru radio link. Scalability- very high. Just change the triac transfo and bridge.
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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$5