2-phase source transformer to 3-phase load

Hello there,
if I have Dyn (Delta-Y with neutral) distribution transformer and one
of supplying phase is cut off (due to fuse link tripping out for
instance), will the transformer be able to supply three phase load
continuously? what about kva rating and voltage balance? is this the
same case with open delta connection?
what about Yzn or Yyn transformer?
thanks a lot in advance.
zip
Reply to
zip
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Hello there, if I have Dyn (Delta-Y with neutral) distribution transformer and one of supplying phase is cut off (due to fuse link tripping out for instance), will the transformer be able to supply three phase load continuously? what about kva rating and voltage balance? is this the same case with open delta connection? what about Yzn or Yyn transformer?
thanks a lot in advance. zip
Reply to
zip
in general, either the safety's (phase loss detector) shuts the load down or the load blows up or burns up.
without 3 phase on the input a transformer cannot supply 3 phase to the output.
of course single phase loads generally work OK when across the remaining two "legs".
Reply to
TimPerry
Trying to use a 3 phase distribution transformer when it is single phased is a recipe for disaster. I would not use it at all if the primary looses a phase. IMO your just screaming for an problem later on.
Reply to
SQLit
I had that happen to me on a job last summer. Blew a fuse on one of the 480 volt feeds to a three phase transformer. I didn't write the voltages down, but I had one leg at 120 volts, and the other two at something weird like 57 volts. I'm not going to be bothered with working out the math on it. It's not good.
--Dale
Reply to
Dale Farmer
When you blow one leg of 3 phase, you have single phase. There is no such thing as two phase. If you have single phase loads, like lighting, etc. you will still have power on those legs, but any three phase loads such as motors will not be too happy. The motors will do what is called "single phasing" and draw excessive currents, stall out, and you should end up blowing distribution side fuses. If you are continually blowing a specific fuse, you should lock out the system until you can correct the cause of the problem, not ask, "Will it work without the fuse."
If you are observing three phase power still on the secondary of the transformer, be aware that if you are running a lightly loaded three phase motor when one leg blows, it may actually generate the third phase which could be backfeeding some other devices on your power system, including the primary side of that transformer.
Reply to
ropenstein
You are correct about the single phase when one leg of the three phase opens.
However, there IS a two-phase system, that was used before three phase was available. The phases are at 90 degrees with respect to each other. It can be either three wire, with a common connection, or four wire with two separate windings. It gave the motor-starting advantage of three phase, but disappeared after the other three-phase advantages became apparent.
This is not the same thing as a center tapped winding, such as the commonly used 240/120, which is single phase. There have been many threads debating this subject, and there may be another one now!
Ben Miller
Reply to
Ben Miller
Yes, and you can get a true two-phase from three-phase by using a Scott-T transformer. I haven't heard of such a set-up since my P.E. license exam, over fifty years ago. :-)
Reply to
VWWall
Right on Ben
In lower Iowa and upper Missouri there used to be distribution called "V" phase. Some of it was above 1k volts some was not. Mostly the REA stuff installed in the 1930-40's It was as you describe above. Like you said most of this is ancient history now days. Them Missourians and Iowagians sure would never replace anything that was working.
Reply to
SQLit
! ! i i i /\ i i A / \ B i i / \ i i / \ i + --------+ C
You have voltage applied only at A-C and B-C (where the + signs are), therefore the coil C accepts "proper" supply, but coils A and B have half of the voltage, with current at some phase difference. Respectively, at the seconsary side coil "C" has got proper output, but coils A and B just deliver half of the voltage - as mentioned by Dale Pharmer. And, as mentioned by SQlit - it is a disaster to run three-phase load (especially a motor) at this conditions.
In the case Y/Y - two of the phases will have normal voltage, proper phase; third - zero. Again the motors may burn.
Reply to
bobc
There is also Y-Delta, where if one phase goes out, the transformer configuration will try to generate the missing phase and backfeed it up the primary and power everything downstream of the fault, which can cause blown fuses or the transformers to burn up. It's not used much for that reason, and when it is used, often the common point of the primary is _not_ connected to the system neutral to prevent that problem.
Reply to
Michael Moroney
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Reply to
sathio26

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