closed delta diagram
- Thread starter fwomack
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
Re: closed delta diagram
The answer is a qualified yes.
You conceivably could get two 3 2:1 xfmrs, and wire them up delta-delta.
To get a neutral you could make one of them have a 3 wire output and ground the center of the secondary of that xfmr.
I am not a huge fan of doing this kind of thing but for a very small amount of single phase power it is acceptable.
it sort of depends on just what you are trying to accomplish.
The answer is a qualified yes.
You conceivably could get two 3 2:1 xfmrs, and wire them up delta-delta.
To get a neutral you could make one of them have a 3 wire output and ground the center of the secondary of that xfmr.
I am not a huge fan of doing this kind of thing but for a very small amount of single phase power it is acceptable.
it sort of depends on just what you are trying to accomplish.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: closed delta diagram
The transformer that supplies your neutral should be oversized compared to the other transformers.
When using three similar transformers in a closed delta it is common practice to limit the single phase (120V) load to 5% of the total KVA. Open delta configurations are not usually this limited.
The transformer that supplies your neutral should be oversized compared to the other transformers.
When using three similar transformers in a closed delta it is common practice to limit the single phase (120V) load to 5% of the total KVA. Open delta configurations are not usually this limited.
charlie
Senior Member
- Location
- Indianapolis
Re: closed delta diagram
Jim, if you are using 3 separate transformers, the 5% doesn't apply.
Jim, if you are using 3 separate transformers, the 5% doesn't apply.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: closed delta diagram
Charlie,
You are correct. This is one of my bad (?) habits. I like to use the 5% limit when all three transformers are the same size. This forces the designer to make sure they have correctly allocated capacity based on the mix of 1 and 3 phase loads.
Charlie,
You are correct. This is one of my bad (?) habits. I like to use the 5% limit when all three transformers are the same size. This forces the designer to make sure they have correctly allocated capacity based on the mix of 1 and 3 phase loads.
beanland
Senior Member
- Location
- Vancouver, WA
Re: closed delta diagram
A 240/120 4-wire delta service is common in the utility industry. The problem is one of the power leg transformers can fail, turning the bank into open delta, and continue to supply power but overload the surviving (for awhile) two transformers. However, 240/120 grounded delta is much preferrable over corner grounded delta.
A 240/120 4-wire delta service is common in the utility industry. The problem is one of the power leg transformers can fail, turning the bank into open delta, and continue to supply power but overload the surviving (for awhile) two transformers. However, 240/120 grounded delta is much preferrable over corner grounded delta.
charlie
Senior Member
- Location
- Indianapolis
Re: closed delta diagram
Beanland, that will not happen without notice if you connect the transformers into a wye/delta and float the center point of the wye. We connect ours that way and get called immediately if we lose a transformer.
Beanland, that will not happen without notice if you connect the transformers into a wye/delta and float the center point of the wye. We connect ours that way and get called immediately if we lose a transformer.
Re: closed delta diagram
How about a diagram?
Charlie, I am missing something. Doesn't this connection play havoc with the secondary voltages?Originally posted by charlie:
Beanland, that will not happen without notice if you connect the transformers into a wye/delta and float the center point of the wye. We connect ours that way and get called immediately if we lose a transformer.
How about a diagram?
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: closed delta diagram
I agree with Charlie, that an ungrounded wye/delta is much preferable to a grounded wye/delta. The latter will overload as beanland indicated, but worse, it will attempt to balance any imbalance in the primary system, which is often more serious than the chance of losing a transformer and overloading the open bank.
If you backfeed a 480 delta to 120/208 wye stepdown transformer you will witness the same problem if you don't lift the 208 ground. It will tend to balance the 208 (primary) side and often trip breakers with no load on the 480 (secondary) side.
Charlie's ungrounded connection will significantly change the secondary voltage when a transformer goes out, and that is probably why he gets calls.
Jim T
I agree with Charlie, that an ungrounded wye/delta is much preferable to a grounded wye/delta. The latter will overload as beanland indicated, but worse, it will attempt to balance any imbalance in the primary system, which is often more serious than the chance of losing a transformer and overloading the open bank.
If you backfeed a 480 delta to 120/208 wye stepdown transformer you will witness the same problem if you don't lift the 208 ground. It will tend to balance the 208 (primary) side and often trip breakers with no load on the 480 (secondary) side.
Charlie's ungrounded connection will significantly change the secondary voltage when a transformer goes out, and that is probably why he gets calls.
Jim T
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