I have a situation where I need to power a intermittent single phase large load with a three phase transformer so I will not be using one leg. Its short term hook up for two to three years at best so I do not want to buy a transformer ($$$$$) when I have a 3 phase dry transformer collecting dust of sufficent size. I have not come up on this type of config before so I have my conserns. Such as
*Would it be kosher to wire and tap only two of the three coils?
*if not, what issues would I have if I were to wire the high side 3 phase and only use two legs where the amperage in theory be upto 100 amps, 90 more realistically and only intermittenly?
I've seen this done over the years, as having 3 phase wired as normal, secondaries going to a disconnect/panel, and single phase load being connected.
Can't say I heard of any problems.
IMHO, as long as the xfr is of sufficient KVA, you should be OK. Efficiency? May be an issue.
Scott....jump in here please!
Does the transformer have each end of the secondaries available for separate connection? If so, you could hook them up in a "dog leg" connection to get single phase out of a three phase source.
It is the same method that is used on three phase generators when they need to supply single phase loads.
I timed out while editting. Use the following "Scott" drawing.
12 lead generator drawings Basically it is a open delta where Phases A & B are connected in opposing series and Phase C is connected in aiding series to the combined A & B. The output "L1"is connected to the free end of A, the "neutral" is the connection between B & C, and "L2" is the free end of C. The connection between A & B is not used.
This limits the unit's maximum power to 2/3 of full rating but is does load the 3 phase input equally.
Larry C
I am not sure if it's a 12 wire transformer or not. I currently do not have access to it.
Larry, I think the the 12 lead generator link is hinky. It takes me to some web site. I do have schems of generators and I didn't thInk of looking at it from your perspective.
Thanx for everyone's input
Maybe I am missing something but why not terminate in a 3 pole disconnect and feed a single phase panel from there or just use a 3p panel with one 2 pole breaker plugged into it.
The transformer won't care.
That is what I'm planning to do. By maxing two of three legs while having nothing on the third leg, I dont know such an unbalance would cause any issues for the transformer and or the circuits or not. That would be a rather expensive smoke test if overlooked something
The link is hinky because address got changed from electrical-contractor .... to electrical.contractor ....
The link is supposed to go to the technical drawings on this site.
Larry
Ah ha! That did it. Thanx Larry.
Btw, I'll get the rest of my barge pics posted some day. Got super busy. Finally getting on top of things.
I am thinking that using two phases of the three phase transformer will only get you 208 vs 240 Volts.
Unless the transformer or the load has adjustable taps, I assume that there will be some voltage mismatch problems.
Just something to be aware of.
Although the "dog leg" configuration is more complicated, it does a better job of matching voltages.
I don't think we ever heard whether the transformer was delta or wye on the secondary.
Transformers are rated for 100% unbalance on each winding (basically 1/3 the 3-phase kVA). You will lose some efficiency, but is that a big deal?
Is this a wye connected transformer?
Although the "dog leg" configuration is more complicated, it does a better job of matching voltages.
In general you should stay away from using generator connections on 'single core' 3-phase transformers,
(among other things, the magnetic flux distribution in a single core transformer is different). Is usually perfectly fine to use generator connections with multiple single-phase transformers.
JBD
I was not aware of the magnetic flux distribution issues for a single core three phase transformers when connected in a "dog leg" configuration. Is there a magnetic saturation concern? I am just curious.
Thanks.
Larry C
It will depend on the construction of the transformer core. But, you will notice that no major transformer manufacturer includes this as a 'standard recommended' connection. Of course, the fact that you do not have access to all of the winding ends, may also factor into it.
Back when I was a fresh-out engineer, I remember discussing this connection with Jerry Frank, from the old Sorgel Co. He 'politely' reminded me there was a difference between theory and practical application.
The transformer has a delta primary and a wye secondary. It also have voltage taps as well. I have not seen the inside of it. It's in storage at another site. I'll try to get some pics from the inside. I got a pic of the data plate but I can't upload it to the Internet from my phone. I'll get on my computer later and post it
Spark...
Have a couple Qs per your Scenario...
Q #1: Is there a Three Phase System available to drive the Primary, or only Single Phase?
Q #2: Will the Transformer be "Reverse-Connected", with the Delta Winding being the Secondary; or will the Transformer be connected Delta Primary / Wye Secondary?
Q #3: Will there be only (1) Single Phase L-L Connected Load, or is it possible to make Two (or possibly Three) individual Single Phase L-L Load Connections?
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Per "Q #1":
If the Currently Active System is 3 Phase 3 Wire, I suggest to connect the Primary to all Three Lines, even if the Secondary side will connect via Two of the Three Windings.
If only Single Phase is available, suggest either using another Transformer, or attempt to reconfigure the Transformer to appear as Open Delta - provided the Windings are on individual Cores, as opposed to being wound to a Single common Core. The latter of these is most common.
(I need to do some thinking on this part - the Single Core would be normal for a Split Coil Transformer, but the unused Windings have me concerned).
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Per "Q #2":
If the Transformer will be Connected as Wye Primary, Delta Secondary, I was thinking to feed the Primary with all Three Lines, then Corner Ground the Secondary.
Grounding the Secondary is something to consider, in order to keep the Voltage to Ground stable (reduce the issues of Insulation Breakdown).
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Per "Q #3":
If there are / will / can be Two or more L-L Load Connections, then these may be split up between the Three output Lines.
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In my opinion, the most simple way to drive a Single L-L load via the available Transformer would be to connect the Transformer to a Three Phase 3 Wire Circuit, and use Two of the Three Secondary Output Lines to feed the L-L Load.
bond the Wye Connected Secondary Windings' Star Point ("X0") to a Grounding Electrode System, and derive an Equipment Grounding (Bonding) Conductor from the same Star Point.
Driving the Primary via an L-L Connection (as opposed to L-L-L Connection), would create the same Voltage issues as would be found with a Blown fuse in One Phase Line.
Additionally, attempting to reconfigure the Transformer may be Counter-Productive.
Feeding the Primary via a Three Phase L-L-L Branch Circuit will drive the Delta Windings as normal.
Let me contemplate this scenario in more detail, along with verify some points via text manuals.
-- Scott (EE)