Semi-OT: Crazy half-baked phase conversion question

Oh wizards of polyphase power, please accept this virtual offering of three different but oddly related single-malt whiskys and prevent Fried
Fred.
I (will shortly) have an engine-driven 3-phase 220V generator of roughly 20kW capacity. [On Topic: this is a half-ton of metal and it will be used to power metalworking Stuff!] For one of its several duties I would like to use it to power the household during outages. It is 3 phase, the household is single phase at the same voltage.
Now for the half-baked part: If I were to take a suitably sized laminated iron donut, wrap a goodly number of turns of stout wire around it and connect the two ends of that wire to each other, install three taps to the winding spaced at 120 degrees (measures in turns) and label them 'Input', install two more taps at mutual 180 degrees and label them 'Output' ...
would I have a device to pull single phase from three phase (delta connected autotransformer) or merely a short-lived heater?
Also, I would like to think of the neutral for the 3-ph as the neutral for the single-phase but I can't prove to myself why it would be. A 'Y' connection would make that more explicit but then I can't figure out the 180 degree taps and phase load balancing.
Neither my references or Google helped and I'm about ready to build a baby version to check my thinking; asking Those Who Know seemed more effective. Thanks.
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Fred R
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Fred R wrote:

No, you don't have it right. If you wrap turns around iron and then connect the ends, no potential can exist anywhere along that wire or enormous currents would flow.
If you consider your 3 phases L1, L2 and L3, then L1-L2=L2-L3=L3-L1 = 220VAC, or L1-GND=L2-GND=L3-GND = 208VAC. My intuition is you'd use 3 buck-boost transformers to boost each single-phase leg from 208-220 and then try to balance the load as best you can when you hook it to the house wiring circuits.
This can be real dangerous and to do it right might cost you $1200-1500 but you might consider if you had a fire when it was connected they might refuse to pay because you had some non-code electrical device connected.
GWE
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Grant Erwin wrote:

OK, I was thinking originally of three primaries of a delta-connected set of isolation (1:1) transformers.(Three different cores.) Then a variation on the Scott-T to get only 1 phase instead of 2. Did a grand over-simplification. Will ponder further.

Got that part right!

The balancing part really has me baffled; 'solving' the phase difference keeps pushing me toward a rotating converter which might as well be a separate single-phase generator.

Ouch! Good advice. Even if I work it out, getting the equivalent of UL approval on a strange one-off concoction could be .. challenging. The separate generator might be the best overall solution.
Thanks, Grant.
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You have three phases - that is nice. You only need one for the house. If your primary was this one it would induce a field . Then the secondary is a wire.
Take a wire - fold it in half - and then with it - feed the folded end in and around and around. The folded end is the common or neutral. Each end turns out to be 180 degrees out of phase.
This is an electrical phase thing - not a mechanical phase thing. IIWY - I'd buy a 240v to 120 0 120 transformer.
Good luck, Martin Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder
Fred R wrote:

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Others are more knowledgeable, but let me assure you that neutral on the three phase is not going to be 110VAC. Someone will tell us what the voltage is, but it will be higher than 110VAC
The simplest solution is to buy an autotransformer or transformer to split the 220VAC to two 110VAC legs.
I don't know of any easy way to use more than one 220VAC leg if what you really need is 110VAC.
I will be interested in what the real experts have to say.
Richard
Fred R wrote:

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On Fri, 16 Dec 2005 22:35:11 GMT, Richard Ferguson

Richard, I'm no electricity guy but I think the only thing that makes the neutral wire coming off the pole "neutral" is the fact that it is tied to ground. So that there is no potential to ground. If I'm wrong someone here will surely correct me. I'll be looking for that correction. ERS
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Eric R Snow wrote:

You are right, neutral is tied to ground, generally at the entrance box. The complication is that the voltage between hot and neutral in a three phase system is not half the voltage between hot and hot, as it would be in typical house wiring.
If you have a portable generator, I think that you would normally tie the generator ground to the building ground. If you were to use a transformer to "split" 220V into two 110V legs, you would tie the transformer center tap, neutral, to the building ground. Thinking further about this, I can see that an autotransformer would not be a good solution, since it would tend to make the generator "hot" relative to ground, a hazard. A regular transformer would allow the transformer to be grounded normally, since it would isolate the generator from the building wiring.
Doing fancy stuff with electricity is generally dangerous, keep it simple and don't cut corners.
Richard
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On Fri, 16 Dec 2005 21:56:58 GMT, Fred R <"spam

You're out of luck - there's no passive way of transforming 3 phase to single phase. The nearest you can come to this (and it's more trouble than it's worth) is to use a Scott transformer pair to reduce it to a 2 x 10KW 2 phase output instead of 3 x 6.7kw.
One thing that's worth remembering is that the ouput of this sort of generator thermally limited. If you leave two phases unloaded the iron loss is unchanged but is there is a major reduction in total copper loss. The heat generation is down so the single utilised winding can safely deliver quite a bit more than its normal rating. I would be happy with 10Kw from a single winding and if you took the trouble to directly check the winding temperature you might well be able to push it nearer 15Kw.
Jim
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snipped-for-privacy@yahoo.com wrote:

Thanks Jim. I think the best solution - particularly considering insurance coverage - is a separate single-phase generator head. Much simpler than balancing loads, etc. and then trying to prove that it is 'safe'.
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Do you need Resilient Weapons Mounts for it, as pictured here:
http://cgi.govliquidation.com/auction/view?ids9744
Each was $100 originally, never used. Meant to be used for weapons, but could be used for gensets too.

I think that the latter is the answer, as the windings cancel out. They are out of phase and when you add them, they add up to 0.

It would not be.

This is not an easy problem and tere is no easy answer. A motor generator could be one solution (look for a single phase genend, which is hard to find at this power), splitting your house electrical panel may be another.
I am may be missing something important and also I am not an expert on this.
i
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Ignoramus20852 wrote:

Ooooh yes! I'm sure it would make the power juicier. Actually they would be better engine mounts than what I have now. There really are some astonishing buys mixed in with the dreck on GovLiq - it is where I got the 25HP motor-generator for $1.40/HP.

Thanks, that is where all answers are pointing. I still want to figure out where I derailed my train of thought on the three-phase transformers, just on generous principles.
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adding to previous:

Got it: three flux loops 120 apart are not equivalent to one flux loop.
Thanks, everyone.
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exactly, they cancel out.
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These mounts are mine. I am the winner of that auction. I will soon pick them up (tuesday). So if you want, say four, I can sell them to you for not too much.
They are rated for 400 lbs each. For military, a 400 lbs rating means that it can definitely handle 400 lbsload easily, there are no BS civilian style over-ratings.
So if your genset is really half ton, four mounts should handle it nicely. They are cadmium plated.
They cost $104 each.

Motor generator? Is it like from a super old torpedo welder?

Generosity is a great human quality!
You can rectify your 3 phase (easy) and then use a used industrial inverter to make inverted single phase AC. It is worth thinking about. You need to consider wave quality issues with that though, but there are better inverters out there. You can see if you can buy a used monster UPS (nobody wants them) and use its inverter part. You have no easy route, but this may be a comparatively easy one.
i
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wrote:

I'd try it this way: If the generator can't be jumpered for 120/240V 1-Ph, configure the generator for 120/208V Y 3-Ph 4-wire, and hook two hot legs and the Neutral to the house loads. Make sure to have breakers or fuses sized to protect the interconnect cabling and/or the generator windings, whichever is lower.
That will give you the full 120V on both main busses for the house lighting and convenience loads, and the few 240V loads in the average house should run happily at 208V, with a slightly higher current draw on motors and a slightly lower output on resistive loads like a water heater.
This is how they wire large Condo complexes - they bring in 120/208V and the first stack of three or four meters are fed A-B phases, the second stack B-C, the third C-A, the fourth A-B, the fifth B-C...
You can reserve the third unused 120V leg for cord connected loads to do some load balancing. Things like the freezer in the garage, and powering the electric chainsaw you're using to cut up the tree that fell on the main power line to your house in the storm...
--<< Bruce >>--
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Bruce L. Bergman wrote:

Thanks, Bruce. Is the leg-to-leg load balancing primarily a thermal issue or does the torque loading during each revolution need to be smooth? I'd think the rotor inertia would be a powerful smoothing agent.
If other words, if the entire unit is operating significantly below full capacity is leg load balancing critical?
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wrote:

The load balancing is not critical as far as I know. The inertia /should/ cover it. Thermal issues should only mean don't overload any one winding past it's ratings.
But if you were going to do this a lot and were pushing full load on two windings (which is 2/3 of the total rated load of the generator!) with nothing on the third, I'd call Onan/Cummins (or whoever built the unit in question) and make sure they don't have any objections that I can't foresee.
The definitive answer is to "Read The (Friendly) Manual", but you already knew that... ;-)
--<< Bruce >>--
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wrote:

Google Philidephia Experiment.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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Fred R wrote:

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Fred R wrote:

Fred, On your generator each of the 3 output legs to ground will measure about 106-109vac. Across any 2 phases it will read close to 220vac. But don't worry. Power companies that provide 240 only adhere to like a 14 percent tolerance which is 226/106vac. Therefore you can wire 2 of it's 3 hot wires into a breaker panel via a dryer pigtail or whatever and backfeed your house. Providing you ALWAYS disconnect your commercial main input feed first. And be sure to also ground your generator through your breaker panel hookup. Your refridgerator and water pump... will work just fine. I will cite a real world example. UCA Airport is fed with 3 phase 208 comercial from Niagara Mowhawk. Instead of providing my FAA suff with proper single phase 240/120vac. They feed me with 2 legs of 3 phase 208. These are used for everything single phase in the Air Traffic Control Tower. My emergency generator is designed for single phase 240. I must dial down the voltage regulator to match the 220/109vac commercial. So when the commercial fails the Emergency power matches the commercial and it is a smooth transition when it switches over. Everything in the tower runs fine on 2 legs of the 3phase 208vac.

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