here are some good graphics of the subject might help
http://www.mhprofessional.com/downloads/products/0071467890/0071467890_ch15.pdf
http://www.mhprofessional.com/downloads/products/0071467890/0071467890_ch15.pdf
Transformer Delta to Y
- Thread starter brother
- Start date
- Status
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I'm 100% sure that, whatever emerges from that open winding, it won't be the 'missing' phase somehow 'synthesized' from the other two phases.
Even if it exhibits such properties, I don't believe it will be capable of any significant loading.
weressl
Esteemed Member
The supply NEUTRAL IS CONNECTED TO GROUND at the supply transformer. Since there is no L/N loads connected, onle a three phase transformer it is unnecessary to bring the neutral. The source's connection to ground assures that the power supply is NOT floating in relationship to the ground.
Elementary my dear Watson.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Laszlo,
I believe that you are missing a key point.
In the situation that you describe, with a wye primary, with the common terminal of the wye 'floating', what is the voltage between that primary common terminal and the supply neutral?
If, instead of a wye connected transformer primary, you had a set of wye connected resistive loads, than the answer is quite clear: the 'floating' common terminal voltage would depend upon the particular load balance, and with perfectly balanced loads, the 'floating' common terminal voltage would be 0 relative to the supply neutral.
If you have a wye to wye transformer, with the primary common terminal 'floating', what mechanism holds the primary common terminal at neutral voltage? Does this mechanism work even if there is unbalanced loading on the secondary side? If the primary common terminal is not at neutral voltage, won't the secondary voltage be unbalanced relative to the secondary neutral?
-Jon
weressl
Esteemed Member
Following the same argument, you should ground the 'common' at the end of any unbalanced lighting circuits that employs single phase ballasts(transformers).
Either you bring the supply transformer neutral along and NOT ground it at the transformer, or not and still not ground the common point. Since you do NOT have a neutral you should NOT use a Y/Y trasnformer.
The supply transformer grounding will force the U/I ratio to follow the supply. Grounding both the secondary and primary of a transformer will introduce dangerously high voltages on the secodnary side in case of failure.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I believe that the above points to a language misunderstanding. I have not been suggesting that the H0 terminal needs to be _grounded_ in a Y/Y transformer. I have been saying that the HO terminal needs to be connected to the source _neutral_ conductor.
Note that going back to the first page, the original post states that the source is a delta with no neutral, meaning that the whole discussion of connecting the supply neutral to H0 is moot with respect to the original post. (whoops)
I don't understand the above. Assume a grounded wye source feeding a Y/Y transformer with the secondary X0 properly grounded and the primary H0 not connected to anything.
1) What does the source grounding have to do with anything?
2) Given unbalanced loading on the secondary, won't the H0 voltage shift in order to maintain balanced current through all three legs on the primary side? Phase A current on the primary side must balance phase B and C, after all.
3) Is there some mechanism by which the phase B and C current on the primary side can cause phase A current on the secondary side, so that you can have balanced current flow on the primary side with unbalanced current flow on the secondary side?
Thanks
Jon
weressl
Esteemed Member
1.) The source neutral grounding will establish the sink point and the voltage reference.
2.) The unbalanced secondary loading will casue the CURRENT change in each phase of the primary.
3.) See 2.) It is maintaning the load-corresponding flux in each phase that will cause the needed power transformation in each phase and it is primarily dependent on the delta current, not the voltage.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Please forgive the 'hand waving' in this comment, but after thinking about it overnight, and without a rigorous analysis, I've become convinced that a connection to the primary H0 is _not_ needed in a 3 leg common core wye-wye transformer.
In my mind, the key is the common core. If you have three separate single phase transformers in a wye-wye bank, then I still believe that a neutral conductor must be brought to the primary common terminal in order to maintain voltage balance.
I know that I have to be wrong somewhere, either here or in my previous posts on the subject.
Okay, the hand waving. My rough intuition of transformer function as follows:
1) Alternating voltage applied to the primary coil causes alternating magnetizing current to flow. The magnetizing current produces an alternating magnetic flux in the core. The alternating magnetic flux induces voltage in all coils around the core. The voltage _induced_ in the primary coil must be equal to the voltage applied to the primary coil (less a tiny bit of resistance drop).
2) If anything acts to reduce the flux in the core, then more current must flow in the primary to raise the flux back to the point where the induced voltage equals the applied voltage.
3) Current flow in the secondary tends to reduce flux in the core, thus 'demanding' more current flow on the primary side to maintain flux.
How this applies to a three leg common core wye-wye transformer.
Okay, on a three leg common core transformer, the flux through each leg must be the sum of the flux through the other two legs (disregarding leakage flux through the air).
The current through the phase A primary coil _must_ be equal to the sum of the current through the phase B and C primary coils.
A load on a _single_ secondary coil will reduce the flux threading through _all_ of the core legs and thus all primary coils.
Thus loading on a _single_ secondary coil will cause current flow on _all_ the primary phases that keeps the primary H0 terminal roughly neutral. (Remember: in the case of a _common_ three phase core, not separate cores.)
-Jon
In my mind, the key is the common core. If you have three separate single phase transformers in a wye-wye bank, then I still believe that a neutral conductor must be brought to the primary common terminal in order to maintain voltage balance.
I know that I have to be wrong somewhere, either here or in my previous posts on the subject.
Okay, the hand waving. My rough intuition of transformer function as follows:
1) Alternating voltage applied to the primary coil causes alternating magnetizing current to flow. The magnetizing current produces an alternating magnetic flux in the core. The alternating magnetic flux induces voltage in all coils around the core. The voltage _induced_ in the primary coil must be equal to the voltage applied to the primary coil (less a tiny bit of resistance drop).
2) If anything acts to reduce the flux in the core, then more current must flow in the primary to raise the flux back to the point where the induced voltage equals the applied voltage.
3) Current flow in the secondary tends to reduce flux in the core, thus 'demanding' more current flow on the primary side to maintain flux.
How this applies to a three leg common core wye-wye transformer.
Okay, on a three leg common core transformer, the flux through each leg must be the sum of the flux through the other two legs (disregarding leakage flux through the air).
The current through the phase A primary coil _must_ be equal to the sum of the current through the phase B and C primary coils.
A load on a _single_ secondary coil will reduce the flux threading through _all_ of the core legs and thus all primary coils.
Thus loading on a _single_ secondary coil will cause current flow on _all_ the primary phases that keeps the primary H0 terminal roughly neutral. (Remember: in the case of a _common_ three phase core, not separate cores.)
-Jon
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
My post above was written not having seen your response above.
Please check my understanding of the above statement. I read this as saying that load change on a _single_ secondary phase will cause the current to change on all _three_ primary phases.
I can now see how this works on a common core transformer, but I cannot see how it would work with a bank of single phase transformers with separate cores.
-Jon
Please check my understanding of the above statement. I read this as saying that load change on a _single_ secondary phase will cause the current to change on all _three_ primary phases.
I can now see how this works on a common core transformer, but I cannot see how it would work with a bank of single phase transformers with separate cores.
-Jon
- Status