Center tapped delta 240V 3-phase transformer connections

[Open Neutral]

240/120 is the correct way to describe this voltage. My personal preference is to add the descriptors 3PH and 4W (unless I am joining a conversation), but they are not part of the IEEE/ANSI methodology.

In context I was not sure if you meant 240/120 single or 3 phase service.....

 

[iceworm]

In context I was not sure if you meant 240/120 single or 3 phase service.....

Open -
Adding to Jim's response:
"120/240" is defined as single phase
"240/120" is defined as centertapped 240 delta (open or closed - doesn't matter)

ice

 

[Open Neutral]

Fair enough....

 

[iceworm]

... is defined as ....

Fair enough....

Practically speaking - I screw the order up plenty trusting to context to make it clear. As you noted, if it isn't apparent through context, I should be adding the extra.

ice

 

[Smart $]

Practically speaking - I screw the order up plenty trusting to context to make it clear. As you noted, if it isn't apparent through context, I should be adding the extra.

ice

Just remember for 3?, the higher line-to-line voltage is first. For 1?, the lower voltage is first (or backwards compared to 3?)... and this also includes 120/208 "hybrid" 1? systems.

 

[JoeStillman]

I would just use a 480V-delta to 240/120-4-wire-delta. It's readily available from most manufacturers and you don't have to fool around hooking up two transformers. The "open delta" is a red herring. I don't imagine there is much economic advantage over closed delta when the primary and secondary are low-voltage.

 

[dicklaxt]

I got a question,,,,,,are there any liabilities in creating an open delta in the field and putting it into service without a listing?

I'm sure it's safe and has been done and will be done again but what about the what if's ???????

dick

 

[kwired]

I would just use a 480V-delta to 240/120-4-wire-delta. It's readily available from most manufacturers and you don't have to fool around hooking up two transformers. The "open delta" is a red herring. I don't imagine there is much economic advantage over closed delta when the primary and secondary are low-voltage.

If secondary load is balanced then why wouldn't you want the primary balanced? With open delta you are only using two primary phases and the neutral. I guess you could use all three primary phases - but if open delta you still have more load on one phase than the other two.

I got a question,,,,,,are there any liabilities in creating an open delta in the field and putting it into service without a listing?

I'm sure it's safe and has been done and will be done again but what about the what if's ???????

dick

Doesn't POCO do the same thing when they build an open delta bank?

 

[jim dungar]

If secondary load is balanced then why wouldn't you want the primary balanced? With open delta you are only using two primary phases and the neutral. I guess you could use all three primary phases - but if open delta you still have more load on one phase than the other two.

It is totally acceptable to build an open delta bank using the three phase voltages instead of two plus a neutral, especially when the primary is 480V.

 

[jim dungar]

I got a question,,,,,,are there any liabilities in creating an open delta in the field and putting it into service without a listing?

It is absolutely permissible to connect single phase transformers in order to create multi-phase banks. I cannot think of a major manufacturer that does not publish 'field' connectable wiring diagrams.

 

[kwired]

It is totally acceptable to build an open delta bank using the three phase voltages instead of two plus a neutral, especially when the primary is 480V.

I was responding to the question of economic advantage for open delta when the primary is also a low voltage system (like 480 volts to 240/120) I did mention it could be all three lines and no neutral - but it still will not have balanced current on the primary side - assuming balance on the secondary, one phase is common to the other two and should carry 1.73 times what the others carry. A closed delta will be balanced.

 

[dicklaxt]

It is absolutely permissible to connect single phase transformers in order to create multi-phase banks. I cannot think of a major manufacturer that does not publish 'field' connectable wiring diagrams.

True enough but my point was,,,,, when this is done and a mishap occurs without this as a tested/listed stand alone unit,,,,,,,what happens to the little guy? The POCO is big enough to fight the battles..........just a what if type thing,stranger lawsuits have been filed.

dick

 

[Smart $]

... I did mention it could be all three lines and no neutral - but it still will not have balanced current on the primary side - assuming balance on the secondary, one phase is common to the other two and should carry 1.73 times what the others carry. A closed delta will be balanced.

When you have a open-delta primary and secondary, the primary line conductors will not be unbalanced.

For the sake of discussion and to make it easier to understand, let's say you have 1:1 open-delta to open delta transformer. Secondary load is balanced at 100A. In a closed delta, each secondary winding would see 100A ? 1.732. In an open delta, each secondary winding sees 100A. However, the "extra" current on one winding (100A – 100A?1.732) is also the same extra current on the other winding. Since that current enters and leaves the common node via the two windings, it is not passed on to the line conductor (KCL). Being a 1:1 transformer, the same effect occurs on the primary windings and line conductors.

 

[kwired]

True enough but my point was,,,,, when this is done and a mishap occurs without this as a tested/listed stand alone unit,,,,,,,what happens to the little guy? The POCO is big enough to fight the battles..........just a what if type thing,stranger lawsuits have been filed.

dick

Stranger lawsuits have been filed - true. That is one reason we purchase liability insurance. Even if you do everything the way you should bad things do happen at times. Getting insurance to pay a claim sometimes can be just as bad or end up being a lawsuit in itself though. Their job is to determine your risk, set a premium based on that risk, then find any technical reason to not pay claims they can come up with.

 

[iceworm]

Question to the Wizards of Open Delta:
Why would one ever install open delta as a new installation?

Makes no sense to me what so ever.

ice

 

[mivey]

Question to the Wizards of Open Delta:
Why would one ever install open delta as a new installation?

Makes no sense to me what so ever.

ice

Makes sense for:

Combination loads with large amounts of single-phase 120/240 and small amounts of 3-phase 240.

2-phase (v-phase) utility feeders.

Relatively small 3-phase loads.

 

[iceworm]

Makes sense for:

Combination loads with large amounts of single-phase 120/240 and small amounts of 3-phase 240.

2-phase (v-phase) utility feeders.

Relatively small 3-phase loads.

So does 208/120.

However, yes, the v-phase is a bit more difficult. So, I'll give you this: A new open delta makes damned little sense.

So let's look at the OPs issues. The original open delta is bieng replaced with a 480Y. The OP is working on dealing with the existing loads that were serviced from the open delta. Unless there is some freaky load, there is no reason to consider 240/120 open or closed.

The only place I see have seen new open delta installs is where the utility has old transformers and intentionally installs a cobbed up snot ball for an unsuspecting customer.

Note to Hv: I'm not pointing at existing 120/240 services that are changed to 240/120 open delta at the customer's request for minimal cost, three phase, small, service.

ice

ice

 

[hillbilly1]

So does 208/120.

However, yes, the v-phase is a bit more difficult. So, I'll give you this: A new open delta makes damned little sense.

So let's look at the OPs issues. The original open delta is bieng replaced with a 480Y. The OP is working on dealing with the existing loads that were serviced from the open delta. Unless there is some freaky load, there is no reason to consider 240/120 open or closed.

The only place I see have seen new open delta installs is where the utility has old transformers and intentionally installs a cobbed up snot ball for an unsuspecting customer.

Note to Hv: I'm not pointing at existing 120/240 services that are changed to 240/120 open delta at the customer's request for minimal cost, three phase, small, service.

ice

ice

Cost,cost,cost! With the wye you have to use three transformers, with the open delta, only two transformers are needed. Most POCO's stock only certain size transformers, because they are required to have spares on the lot for emergencys. The cost between hanging three 10 kva transformers versus hanging a 15 kva and a 10 kva is considerable when the payback due to low usage is a lot longer. Especially if only two phases are run on the poles from the main lines. If you have to add another line for a couple of miles, it gets even more expensive.

 

[Hv&Lv]

So does 208/120.

However, yes, the v-phase is a bit more difficult. So, I'll give you this: A new open delta makes damned little sense.



The only place I see have seen new open delta installs is where the utility has old transformers and intentionally installs a cobbed up snot ball for an unsuspecting customer.

:lol::lol: Cobbed up snotball...:lol:

We install brand new open delta's with new transformers, and actually use a bracket made for three transformers. http://www.alumaform.com/ClusterMts.pdf

Occasionally you will have a business that is mostly single phase that has one or two motors that needs 240 three phase. Sometimes this motor will not be used everyday, but the bank has to be there, just in case. Sort of like demand charges, the capacity has to be available just in case you want it.

We have one built way out in the country on a single phase line at a two way feed pole. One phase comes in from the south, one phase comes in from the north. The nearest V or 3 phase is 6 miles away. It would cost "a bunch" to reconductor the line just for that small of a load. In today's economy, least cost planning is or should be at the top of the engineering dept's priorities. Linework gets expensive fast. $50-200k per mile depending on construction chosen.


There is also the problem of no load losses to consider. There is one extra pot up there consuming energy that isn't being paid for. That pot humming along for 8759 hrs. a year is money down the drain for a poco. (we can't keep the power on for the whole year...)

Even with the bad power factor of an open delta, it may still be worth it. It is a balancing act of whether the no load losses or power factor losses are greater as to whether to leave the delta open or to close it.

 

[mivey]

So does 208/120.

Not always

A new open delta makes ... little sense.

For some situations it does not make sense to install an open delta.

The only place I see have seen new open delta installs is where the utility has old transformers and intentionally installs a cobbed up snot ball for an unsuspecting customer.

I have seen both good and bad installs. Even if you have never seen a good install, a little objectivity on the subject would allow you to run the numbers and see an open delta works just fine for some installs.