We have a situation with two 50kva pole mounted transformers, each recieves a phase power lead and a ground from the local power grid, so assume the primary is 7.2KV. On the 480v secondary of each of the transformers, there are 3 terminals: One goes to the service entrance switch, the center one ties to ground (and connected to the other transformers center lug), and the third terminal ties to the third terminal of the second transformer and then goes to the service entrance switch. That makes 3 leads and a grounded guy wire going from the pole to the service entrance switch. The drawing shows a 3 phase 480v service entrance. Is that possible with this configuration of transformers and if so, how?
Can you get 3 phase power out of two single phase pole mounted transformers?
- Thread starter eruotanen
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ptonsparky
Tom
- Occupation
- EC - retired
Try looking for an "Open Delta" configuration. One I am familiar with also has a grounded phase.
You can, but I can't think of a reason that a building would need 480/240 open delta, unless there is a small 480 load and some dry pots inside the building. You sure it isn't 240/120 open delta?
This wouldn't be a grounded phase bank.
"there are 3 terminals: the center one ties to ground (and connected to the other transformers center lug),"
This can't be right. The X2 is disconnected on one pot to tie the two coils in series. X1 on pot "a" goes to load. X3 on pot "a" ties to X1 on pot "b" and then goes to load, X 3 on pot "b" goes to load. The one with the grounded X2 is the lighting pot. The other with the ground lifted from the X2 is the power pot.
This wouldn't be a grounded phase bank.
"there are 3 terminals: the center one ties to ground (and connected to the other transformers center lug),"
This can't be right. The X2 is disconnected on one pot to tie the two coils in series. X1 on pot "a" goes to load. X3 on pot "a" ties to X1 on pot "b" and then goes to load, X 3 on pot "b" goes to load. The one with the grounded X2 is the lighting pot. The other with the ground lifted from the X2 is the power pot.
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Let me clarify something about the installation
Let me clarify something about the installation
Let me clarify-this is an older existing installation for a remote well pump (so it is a "small load" which was installed 40+ years ago. The old paperwork indicates the service from the utility is 480v 3 phase, and the pump design dwgs show a 3 phase starter and pump. The utility owns the two pole mounted transformers and we don't know more than what we can see which indicates 50KVA with the 2 primary leads on top (a phase and a ground) and the 3 secondary lugs described in the original thread. Hope this helps.
Let me clarify something about the installation
Let me clarify-this is an older existing installation for a remote well pump (so it is a "small load" which was installed 40+ years ago. The old paperwork indicates the service from the utility is 480v 3 phase, and the pump design dwgs show a 3 phase starter and pump. The utility owns the two pole mounted transformers and we don't know more than what we can see which indicates 50KVA with the 2 primary leads on top (a phase and a ground) and the 3 secondary lugs described in the original thread. Hope this helps.
richxtlc
Senior Member
- Location
- Tampa Florida
You can only get three-phase using two transformers if the primaries of the transformers are connected to a three-phase source. i.e., transformer 1 to A-B, Transformer 2 to B-C phases. then you can connect them open-delta and get three-phases on the secondary.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
yes two transformers can and are commonly used to get three phase. The primary has two hots and neutral, the secondary is a deltla, most common is a mid point grounded delta, 120/240. POCOS liked this as it gave single and three phase from one bank, for a house and shop. if the load picked up, they would add a third transformer.
Our POCO will no longer supply as new anything but 120/240, 120/208 and 277/480.
the open delta systems had problems with voltage imbalance.
And for 480 corner or open delta, the voltage to ground is 480 not 277, causing a much higher voltage stress on conductors.
Certainly more dangerous for workers.
And you can't use slash rated breakers.
Our POCO will no longer supply as new anything but 120/240, 120/208 and 277/480.
the open delta systems had problems with voltage imbalance.
And for 480 corner or open delta, the voltage to ground is 480 not 277, causing a much higher voltage stress on conductors.
Certainly more dangerous for workers.
And you can't use slash rated breakers.
I beg to differ on this point. You can get three phase secondary with transformers connected to only two phases of the primary. It is the whole design of an open delta bank.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
In which case, you have the neutral connection and are configuring the transformers as open-wye to open-delta.
Regardless, a three-phase primary connection requires 3 conductors.
OK, I fully understand that. I never stated otherwise. I didn't agree that the bank had to have three phase conductors (A,B,andC)
Hence, the statement "connected to only two phases of the primary."
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