open delta
- Thread starter kimrichi
- Start date
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
An open delta is a three phase system.
SG-1
Senior Member
- Location
- Ware Shoals, South Carolina
I know it does not look like it could be 3-Phase with only two single phase transformers, but it is.
See this website:
http://www.3phasepower.org/3phasetransformers.htm
See this website:
http://www.3phasepower.org/3phasetransformers.htm
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Cheap, economical, and efficient 3 phase service. Less power, 57% I think.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Old wives tale, kind of.
If you have a bank of three transformers and then remove one of them, your are correct the result will be 57.7% of the designed amount you started with.
However, a bank designed using only 2 transformers can deliver 100% of its design.
BAHTAH
Senior Member
- Location
- United States
Open-Delta Transformer Connections
Open-Delta Transformer Connections
As a transformer bank the use of open-delta is relatively inefficient where most of the loads are three-phase since the bank has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of the closed delta bank of three units. When the open-delta bank is used because the three-phase loads are small, you find transformers of different sizes being used. A larger transformer for the single-phase load and a smaller transformer for the lesser, three-phase loads.
Open-Delta Transformer Connections
As a transformer bank the use of open-delta is relatively inefficient where most of the loads are three-phase since the bank has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of the closed delta bank of three units. When the open-delta bank is used because the three-phase loads are small, you find transformers of different sizes being used. A larger transformer for the single-phase load and a smaller transformer for the lesser, three-phase loads.
kwired
Electron manager
- Location
- NE Nebraska
This is true with three transformers of the same size and then you remove one. There are cases where there is larger transformer for the two 120 volt to ground phases because there is more load on those phases - then 57% changes even more, or there are even three transformer banks that still have a larger unit for 120 volt phases but two smaller units for the others - again because the 120 volt load is expected to be larger than the loads connected to the 208 volt phase.
Could you explain how the 57.7% changes based on the size of XF's? If I need more than a 25 kVa for the power pot, it needs to be closed IMO.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
Are you talking about the delta/wye 30? phase shift?
No, the 30? current shift in an open delta. The 86% efficiency reason...
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
In simple terms the 57.7 percent of the open delta is a bit short of the 2/3 (66.6%) rating that you might logically think it would be is.
The current that would normally be created by the the third closed leg, is now being created and carried by the two remaining transformers.
And that an increase in current increases the heating by the square of the current. I think:huh: Don't go designing anything with that answer lol
kwired
Electron manager
- Location
- NE Nebraska
Going back to Jim's post where if you have three units and remove one of them that the result is 57.7% of what you started with.
This assumes that all three were the same size to start with. If they were not and you remove one of them the result probably is not 57.7 less total kVA than it originally was to start with, and will depend on what size the units are in relation to each other.
I have seen many large pots with a small one added to pick up a pretty limited three phase load where necessary - usually feeding multiple single phase services and that one customer happens to need three phase but does not have a very high load for the third phase.
But the 57.7% comes from 1.73 divided by 3 (or (2*√3/2EI)/(3EI)). I haven't seen XF size come into play on any calculations.
This site has more information on this question.
http://www.vias.org/matsch_capmag/matsch_caps_magnetics_chap6_12_05.html
http://www.vias.org/matsch_capmag/matsch_caps_magnetics_chap6_12_05.html
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
The exact XF size is immaterial, as long as they are equally sized, removing one will yield 57.7% of the starting 3-phase kVA.
Joethemechanic
Senior Member
- Location
- Hazleton Pa
- Occupation
- Electro-Mechanical Technician. Industrial machinery
The transformer is wound around iron, it takes current to magnetize and then current to demagnetize and reverse magnetize.
If you have ever worked on a scrap yard magnet. The controller upon de-energizing the magnet briefly closes reverse polarity to collapse the magnetic field. If the reverse polarity function of the controller fails, the scrap will slowly drop off the magnet as the magnetic field dies a slow natural death.
Due to the amount of iron in the transformer the magnetic field must be forced to change faster than it would if just naturally left to collapse and reverse. I believe you will find that for a given transformer, if you slow things down (lower frequency) that 86.6 number will improve.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Yes I understand what you meant. Thanks.- Status