open delta

[electricalperson]

can anybody explain to me what an open delta system is and what is it used for? or at least link me to a website that does explain?

 

[peter d]

Usually it was used by poco's to supply very limited 3 phase loads. It saves them a transformer.

I still see them used extensively in agricultural areas of the country where there is a single 3-phase pump or motor that needs a service. But here in New England poco's will not supply an open delta or even "regular" delta service anymore.

 

[chris kennedy]

Here is a start. About ? way down the page.

 

[electricalperson]

Usually it was used by poco's to supply very limited 3 phase loads. It saves them a transformer.

I still see them used extensively in agricultural areas of the country where there is a single 3-phase pump or motor that needs a service. But here in New England poco's will not supply an open delta or even "regular" delta service anymore.

so you can use 2 transformers single phase to make an open delta 3 phase?

 

[zbang]

No, you have to start with 3-phase to get 3-phase (unless you use a converter of some sort).

 

[chris kennedy]

Check my link above.

The open delta transformer connection can be made with only two transformers instead of three (figure 1-8)

 

[winnie]

You have to start with three phase power (which can be two phases plus the neutral of a three phase system), but you can use only two single phase transformers to create an open delta service.

-Jon

 

[electricalperson]

thanks guys

 

[Jraef]

Another good resource.

http://www.3phasepower.org/3phasetransformers.htm

 

[LarryFine]

A bit more info:

Let's say you have a 120/240v center-tapped, single-phase transformer, just like the one supplying your house. You have a grounded neutral and two hot wires, with 240v between them and 120v from either to the neutral.

Now, let's say you want to add one or two 3-phase motors to the existing service. The power copmpany only has to add one more 240v transformer, with one end tied to one end of the first transformer.

Now you have 240v between the first two hot wires as before, and also 240v between one of the first two and the new one. So far, it only looks like two 240v sources wired end-to-end, or in series.

However, there is also 240v between the two open ends of the two transformers, almost as if a third transformer was there, forming a triangle, the Delta. You could call it a virtual Delta, but they call it an open Delta.

Because of the placement of the center tap, the phase wire on the opposite side of the triangle has a higher voltage to neutral. This is equally true of both full Delta's and open Delta's, by the way.

The major difference between the two is that the open Delta has a lesser ability to keep the voltage between the hot wires at 240v across the open side of the triangle, although it can be used for 240v 1ph loads.

The advantage of the open Delta is that only two primary wires are required, because the system neutral gives the two transformers the timing difference required to make the secondaries behave as three phases.

Nobody would intentionally request an open Delta as a new service, but its value was obviously in the ability to convert an existing single-phase service into a three-phase service with minimal alteration and materials.

 

[mull982]

A bit more info:

Let's say you have a 120/240v center-tapped, single-phase transformer, just like the one supplying your house. You have a grounded neutral and two hot wires, with 240v between them and 120v from either to the neutral.

Now, let's say you want to add one or two 3-phase motors to the existing service. The power copmpany only has to add one more 240v transformer, with one end tied to one end of the first transformer.

Now you have 240v between the first two hot wires as before, and also 240v between one of the first two and the new one. So far, it only looks like two 240v sources wired end-to-end, or in series.

However, there is also 240v between the two open ends of the two transformers, almost as if a third transformer was there, forming a triangle, the Delta. You could call it a virtual Delta, but they call it an open Delta.

Because of the placement of the center tap, the phase wire on the opposite side of the triangle has a higher voltage to neutral. This is equally true of both full Delta's and open Delta's, by the way.

The major difference between the two is that the open Delta has a lesser ability to keep the voltage between the hot wires at 240v across the open side of the triangle, although it can be used for 240v 1ph loads.

The advantage of the open Delta is that only two primary wires are required, because the system neutral gives the two transformers the timing difference required to make the secondaries behave as three phases.

Nobody would intentionally request an open Delta as a new service, but its value was obviously in the ability to convert an existing single-phase service into a three-phase service with minimal alteration and materials.

Are you saying that you can have 3-phase with only two primary wires? I always thought that you needed at leaset three primary wires for an open delta arrangment, but only 2 transformers were needed

 

[jim dungar]

Are you saying that you can have 3-phase with only two primary wires? I always thought that you needed at least three primary wires for an open delta arrangement, but only 2 transformers were needed

You are correct.

You need all three phase conductors for an open delta arrangement. You need two phase conductors + the neutral conductor for an open-wye.

For service upgrades to 480V, where there is a need to feed the old 240/120V system I still specify open delta arrangements.

 

[winnie]

You need all three phase conductors for an open delta arrangement. You need two phase conductors + the neutral conductor for an open-wye.

I am pretty sure that with two phase conductors + neutral you can feed an open-wye primary, open-delta secondary transformer bank.

-Jon

 

[gar]

080917-0920 EST

To create an open delta output you need three wires on the input side. The source can be either Y or delta.

.

 

[jim dungar]

I am pretty sure that with two phase conductors + neutral you can feed an open-wye primary, open-delta secondary transformer bank.

-Jon

Just like any other transfromer the primary conductors determine the primary connection,the secondary connection is independent of the primary. An "open" transformer bank is basically just like a "closed" bank except for its capacity. An open delta ouput does not mean the voltage is 240/120 3PH 4W.

 

[charlie]

. . . An open delta output does not mean the voltage is 240/120 3PH 4W.

Let me count the ways:
120/240 delta, 3?, 4w
240 corner grounded delta, 3?, 3w
480 corner grounded delta, 3?, 3w
480 un-grounded delta, 3?, 3w
and those are just the common ones.

 

[LarryFine]

Are you saying that you can have 3-phase with only two primary wires?

I am indeed. Each transformer primary is supplied line-to-neutral, like two lines from a 208-120v Y system. In fact, around here, just about every modern transformer primary is wired line-to-neutral, also known as single-ended; there are very few new few line-to-line primaries.

There is only one HV bushing (and one fuse, etc.) per transformer; the other end of the primary winding is fed through a tiny bushing on the side of the can, and is connected to the grounded neutral. The primary can be one, two, or all three phases, as with any Wye system.

I always thought that you needed at leaset three primary wires for an open delta arrangment, but only 2 transformers were needed

I almost went into this in my previous post, but I wanted to keep it simple. The use of the neutral as one end of each primary is the reference that allows only two line conductors the ability to produce the timing needed for three phases.

Remember, there are only two secondaries, also. If you ignore the center tap for a moment, the primaries and secondaries are wired the same way. The only difference is that the point where the two primary windings meet is grounded, and the point where the two secondaries meet is used as a line conductor.

In fact, you could ground that common point of the secondaries, instead of the center tap, and you'd have an open, corner-grounded Delta. Grounding a center tap instead of a corner is the only reason there is a high leg. But we're here to talk about getting three phases from two.

If you took a regular, closed three-transformer Delta secondary system, and opened one corner, the voltage across the opened corner would theoretically be 0v. Likewise, if you removed one transformer from the Delta, the voltage across the two open points would still be 240v (or 480v).

So, in actuality, there are only two phases present with an open Delta. There are two hot wires relative to the common point, but to a 3-phase load, it doesn't matter which, if any, conductor is grounded. If there is the same voltage among the three conductors, you can power a 3-phase load.

 

[LarryFine]

An open delta ouput does not mean the voltage is 240/120 3PH 4W.

No, it's the bonding of the center tap of one secondary does that. In fact, Delta's are often made up of three identical center-tapped transformers. They simply only bond one of them, and ignore the other center taps.

With open Deltas, though, the 3-phase load is relatively small, so the second transformer is usually smaller than the main, center-tapped one, which again is identical to the 1-phase transformer feeding your house.

 

[jim dungar]

With open Deltas, though, the 3-phase load is relatively small, so the second transformer is usually smaller than the main, center-tapped one, which again is identical to the 1-phase transformer feeding your house.

If I have an open-delta with a 480V secondary, I would not consider the 3-phase load to be relatively small. I have seen many 240/120V 3Ph 4W services where the 120V load is relatively small (i.e. center pivot irrigation pumps).

A three phase transformer bank requires three primary conductors even if one of them happens to be a neutral conductor or dirt.

 

[LarryFine]

If I have an open-delta with a 480V secondary, I would not consider the 3-phase load to be relatively small.

But then, you won't have a center tapped secondary, either. Nor will you be supplying line-to-neutral loads.

I have seen many 240/120V 3Ph 4W services where the 120V load is relatively small (i.e. center pivot irrigation pumps).

And they were open Deltas?

A three phase transformer bank requires three primary conductors even if one of them happens to be a neutral conductor or dirt.

Dirt aside, I agree. Two lines and a neutral are three conductors.