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I have a home (in Ukraine) which is wired for three phases, but only ever use the three together for the electric heating system (which, for obvious reasons at this point we are not using).

So, I bought a diesel generator with three phases. I connected it to the house via an isolator switch, but this just doesn't work well at all. The three outputs measure at 230 each, (how is it 690?) but when I turn anything on, even low-power devices, the output from the generator goes haywire, and the generator runs roughly, circuit breakers close, displays output of 280, 190, and 210 for example.

I have a funny feeling that three-phase generators are only good for powering high voltage tools, like a 400 V table saw, but NOT three-phase homes.

Sure would love to hear some opinions. A local generator repair guy says that his shop of full of three-phase diesel generators that have failed because they were used to supply 220 V to three different home circuits.

So, what's the right way to supply power to my home during our many rolling blackouts? Three single-phase generators? We're getting maybe 3-6 hours of electricity per day, not much sunlight, and it's getting really cold, and the generators provide much needed power to the pump and fans that run my wood-burning central heating.

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    This might be a stupid suggestion and not sure if it can work. Was wondering if you hook the generator up to the where the three phase wires come in, instead of the panel. Will need some kind of transfer switch, which probably cost big bucks/rubies/money.
    – crip659
    Dec 20, 2022 at 16:22
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    There are a couple of 3 phase configurations - wye (4 wire) and delta (3 wire). Does the generator output match the home? Note that the phases are 120 degrees apart, so the voltages are not just added together.
    – red_menace
    Dec 20, 2022 at 17:13
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    The symptoms you describe make be think that you've wired the 3 phases from the generator to your house, but no neutral. All of the single-phase loads in your house need that neutral to work properly.
    – brhans
    Dec 20, 2022 at 18:01
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    Can you add photos of the generator's data plate, and the isolator hookup? Try for a well-lit clear photo as much as possible.
    – Criggie
    Dec 21, 2022 at 2:07
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    Also a disconnected neutral is highly dangerous: As it can feed 400V into devices that are designed for 230V. Lot of property got destroyed that way..
    – Martin
    Dec 21, 2022 at 10:29

6 Answers 6

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Neutral MATTERS

I connected it to the house via an isolator switch, but this just doesn't work well at all. The three outputs measure at 230 each, (how is it 690?) but when I turn anything on, even low power devices, the output from the generator goes haywire, and the generator runs roughly, circuit breakers close, displays output of 280, 190, and 210 for example.

Because neutral is missing/not connecting/not wired. European homes are wired 230V phase to neutral and then with multiple phases. If neutral is not connected to the generator, then neutral will bounce all over the place.

Look. Thomas Edison fired Nikola Tesla because Edison did not understand how all this stuff works in 2-dimensional multiple phasing with AC power. I can barely explain it myself. However, Edison had a very good grasp on the simple 1-dimensional version of the same thing, and used it when wiring the USA for DC electric power, and that became our simpler split-phase system. So here it is.

enter image description here

I love this video because the thumbnail says it all.
White = neutral.
Black and Red = 2 of the phases. (well there are only 2 phases in this system).

The red wire is POSITIVE 6 volts, and the neutral is 0 volts, and the black is NEGATIVE 6 volts. All the lights are 6 volts (except the bottom 2).

Think about what is happening here. Because the polarity is opposite, very little power flows on neutral (normally) - only the difference/imbalance current. This works very well, and that is why Thomas Edison used it.

But now, what happens if you LOSE THE NEUTRAL? If you have 2 lights on both sides, it is balanced already - and so the neutral does next to nothing. However, when the load is imbalanced, as in the thumbnail, crazy stuff starts to happen. The neutral voltage gets pulled to one side. And the video is all about that if you want to watch it.

enter image description here

In North America, everyone has split-phase 120/240V like this. All our lights are 120V but motors can be connected 240V. A "Lost Neutral" to a home is a regular occurrence, and it drives us crazy. However we are obsessed with putting earth spikes on everything, so neutral simply re-routes through the dirt, which means voltage difference is only 10-30% typically. People can go months without knowing they have a problem. This is no help with a generator, though.

The same thing is happening to you, in 3-phase.

The only real difference is that with AC power, we are able to manipulate phasing in a way that gives us functionally two dimensions. And that gives us Tesla's invention of 3-phase AC power, which took the world by storm (once someone figured it out). 3-phase comes in 2 flavors: "Wye" (for Y) and "Delta" (for the Greek letter Δ).

enter image description here

I am showing standard European voltages here. Notice how "wye" has 4 connections, but "delta" has only 3.

A very large motor takes only the corner connections and ignores neutral. However, every load in a residence - including your range/hob - is wired from one phase to neutral. Why? So the range/hob can work in a single-phase home.

Now, how does it work with 3 phases sharing one neutral wire? That is Tesla's genius. Tesla figured out that with AC power phased correctly and this type of "wye" wiring, neutral current will never exceed current of any one live wire. And so the neutral wire can be the same size as the live wires.

When a European single-phase customer loses neutral, their power just fails entirely. But when a 3-phase customer loses neutral, voltages go "bonkers" just as you expect them to. See what happens when you have all single-phase loads and lost neutral?

enter image description here

Nothing holds neutral in the middle. It just bangs around like a pinball based on the momentary loads on each phase.

So, you need to sort out your neutral wire on that generator. If it is able to provide neutral, you need to wire it and use it. If it is a "delta" generator incapable of providing neutral, then you will need to either get rid of it, or use balancing transformers to synthesize a neutral.

There's one other way to synthesize neutral

and I'm only suggesting this because you are on a war footing (because it's risky And Definitely Not Code). And also because you need heat. This technique will give it to you!

Remember in the above video where the disconnected neutral did not matter if both phases had equal load... And, the imbalance wasn't that bad if phases had near equal load... (E.G. 4 and 5 lights).

That works because the heavy loads are acting like a "resistor ladder". If you have lots of balanced load on all three phases, it will also act like a resistor ladder and keep neutral reasonably near the middle.

Ukraine has mostly nuclear and hydro so Ukraine uses a lot of electric resistive heating - baseboards, that kind of thing. Most of those are wired 230V live-neutral. So I assume they are very abundant. There are also plug-in space heaters, but those are a bit dangerous.

If you put 3 matched heaters, each of equal wattage, each connected from a phase to neutral -- that will act as a "resistor ladder" and will pull neutral toward the middle. The heaters can be combined, so say two 1500W heaters could be used on one phase, and a 2000+1000W heater used on a second phase, and a 2500+500W heater on the 3rd phase.

The heaters need to be the largest loads.

Further, the thermostats must be disabled. You can't have heaters turning on and off - they need to be on whenever the generator is on.

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    I think it's fairly unlikely that OP's generator wouldn't be able to provide a neutral (as that would make it practically unusable for 99% of the loads you might want to connect to it in Europe. Unlike the US where 240V loads are somewhat common, most European houses don't even have a single 400V load.) Also worth pointing out that a lost neutral in a 3-phase house is a positively scary situation as it can expose all kinds of cheapo devices to >600V peaks, likely way above what their insulation is designed to handle (if designed at all).
    – TooTea
    Dec 21, 2022 at 7:47
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    “most European houses don't even have a single 400V load” What about electric stoves? Or heat pumps? Photovoltaic solar panels (not a load but a source) are sometimes also connected using a 3 phase 400V plug/wiring.
    – Michael
    Dec 21, 2022 at 9:29
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    @Michael : no, a 3-phase European stove has 230v elements connected to different phases.
    – grahamj42
    Dec 21, 2022 at 10:18
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    @ferrybig Sure, and other variants too. But we're talking Europe, where having 230V as your basic household supply removes any need for the variety that America has, since 230/400V can cover all commercial and light industrial requirements. Dec 21, 2022 at 20:07
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    @TooTea Fair chance OP's generator was Soviet era, built by a world-class superpower for institutional or military application, where the load was entirely motor/3-phase. There's a whole lot of that old stock floating around Ukraine, Belarus, Moldova, Russia etc..... Some of it glows in the dark! The Chornobyl equipment graveyards were aggressively pilfered by thieves. Dec 21, 2022 at 20:27
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Harper has identified the issue correctly. I suggest another way of getting the neutral in case your generator is wired as a delta. Many small motors/generators allow both star and delta wiring, something like this: 6-pin motor wiring diagram

Your neutral line will be at W2, U2, V2.

Those six terminals are to be found in the box mounted on the motor/generator housing. The voltage and frequency sensing wires, if they exist, must be connected as before. Example photo:

photo of motor connection

Hang in there!

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  • +1 for mentioning 'star' as an alternative to the mainly American 'wye' description. Dec 22, 2022 at 10:56
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Use a single phase generator and a transfer switch

I would consider using a single phase generator with a 3 phase transfer switch, like this:

3 phase transfer switch image

source

Wire up your three phase supply to input ports 1/3/5 on the normal side, and the supply neutral to N. Join each output port 2 together and that's your first load phase, the same for the other phases for ports 4 and 6 and the bottom neutral terminals. Like this (yellow/green/red = phases, blue = neutral):

3 phase transfer switch wiring

source

On the backup side, join all three inputs (ports 1/3/5, yellow/green/red) together and join them to the live of your single phase generator. The generator neutral goes to the backup neutral terminal.

In normal mode you'll run them as 3 phase loads normally (120 degrees out of phase), and in backup mode all the loads are run in-phase. The transfer switch has a motor that moves the switches from position N to position R, which should leave enough dead time for desynchronising not to be a problem.

The motor is powered from the green terminals at the bottom/top, so you can wire it to switch either when the normal power supply goes away or when the backup supply comes on stream.

I think this should work but I haven't tested it.

Edit: This won't work if you have 3 phase loads which are expecting a 120 degree phase relationship, eg motors, but should be fine if your phases are independent (eg separate heating elements). One thing I might watch is whether there might be components that need 120 degrees - eg a fan heater: the heating elements are fine with the change in phase, but maybe the fan isn't.

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    Hello, my friend. My setup looks exactly like the photo you sent. I connect L1,2,3, Z into the transfer. When the power comes back on (we got a Christmas gift this morning of 1.5 hours of electricity!) it transfers automatically to the city power. The difference is I haven't tried connecting the the single phase 220volt from the generator to the three phase inputs to the house. The 'Z' from the Gen is also connected to the transfer switch. It seems to me that whenever one of the phases is pulling more amps that the others, the AVR boosts the voltage on all three phases. But not equal. Dec 25, 2022 at 10:34
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    I switched off the utility room along with a few other things, (at the moment, I've only lights, internet, refrigerator ,etc, and the three phases are pretty close, 233, 222, 221. If I understand correctly, the output voltage of the generator's three phases should be within 1-2% of the average, or things start to get damaged, burned etc. For another example, if I see 235, 200, and 200, I think the second two phases are consuming more power and the gennie speeds up the motor to manage that, but the first phase also produces higher voltage. Dec 25, 2022 at 10:40
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When you add other small loads, while "The heaters need to be the largest loads", try to keep the system as balanced as possible, by distributing the connection of the "small loads" among the three phases, i.e. try to add the same amount of small loads wattage to each of the three pairs of (Phase <-to-> "new synthetic neutral") points.

So when ANY of these small loads is turned-on it would be too small to un-balance the system, or when these small loads are ALL turned-on the system will remain balanced even though the total power drain from the many connected small loads may be relatively large.

The idea is to keep the "synthesized neutral" at the same potential with respect to each of the 3 phases. When all three phases are supplying similar amounts of power, if you use an AC voltmeter, you should be able to measure very similar voltages between each phase coming out of the generator and the "synthesized neutral" connection point represented by the tied-together heaters contact points.

A final warning, voltage-sensitive low wattage equipment connected to any of the phases may be damaged if the arrangement becomes even momentarily un-balanced (for example, disconnecting one of the three main heater loads).

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First, I want to thank everyone who has responded to my plea for advice. The generator (a generic chinese diesel) does indeed have a separate neutral. I used a 4-pole automatic transfer switch, with each of the three phases to its corresponding pole, along with the N. I have three stabilizer boxes which adjust the voltage outputs to the home. Example, if I am getting 240 volts in (from the street or the gennie), it reduces it somehow to 220. They can even adjust for drops to as low as 160 volts (some kind of magic if you ask me). I suggested the single-phase to three phase backup to an electrician who says that it will cause issues, and I am no one to argue with him. I connected the generator again, and switched off everything on the panel. According to the stabilizers, the three inputs were all dead on 230 volts. I began to switch things on, one by one, until the three input voltages began to change. Currently 200, 220, 240. I can't run the pumps to the central heating without the voltage levels going crazy. In any case, I wonder if maybe the AVR is perhaps working less than optimally? After all, is IS a cheap (for the USA, anyway $2000) unit. In any case, my seltup looks exactly like the diagram posted above

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  • I don't have much insight into the details but I suppose that 3 independent UPS devices (I think you are meaning those) on a tree-phase circuit make trouble as the phases aren't synchronous anymore. The entire 3 phase system relies on the fact that the phases are always 120° apart. Should one phase have a slightly different frequency you are inviting trouble..
    – Martin
    Feb 3, 2023 at 12:18
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I would suggest that it's your voltage stabilizers that are causing the issue. I also live have a house in the Ukraine with a 3 phase supply and recently added a generator for the power outages we are suffering. I had a lot of ups devices attached and these sent the generator crazy and the voltage fluctuating rapidly. Once I'd bypassed these the generator works quite well if a little underpowered at only 3.3kw total. But it works, we run computers, lights, central heating, internet etc no problems. I intend to but a more powerful 9kw unit soon if the outages continue.

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