electricalperson
Senior Member
- Location
- massachusetts
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?
so you can use 2 transformers single phase to make an open delta 3 phase?peter d said: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.
The open delta transformer connection can be made with only two transformers instead of three (figure 1-8)
LarryFine said: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.
mull said: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
jim dungar said:You need all three phase conductors for an open delta arrangement. You need two phase conductors + the neutral conductor for an open-wye.
winnie said:I am pretty sure that with two phase conductors + neutral you can feed an open-wye primary, open-delta secondary transformer bank.
-Jon
Let me count the ways:jim dungar said:. . . An open delta output does not mean the voltage is 240/120 3PH 4W.
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.mull982 said:Are you saying that you can have 3-phase with only two primary wires?
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.I always thought that you needed at leaset three primary wires for an open delta arrangment, but only 2 transformers were needed
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.jim dungar said:An open delta ouput does not mean the voltage is 240/120 3PH 4W.
LarryFine said: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.
But then, you won't have a center tapped secondary, either. Nor will you be supplying line-to-neutral loads.jim dungar said:If I have an open-delta with a 480V secondary, I would not consider the 3-phase load to be relatively small.
And they were open Deltas?I have seen many 240/120V 3Ph 4W services where the 120V load is relatively small (i.e. center pivot irrigation pumps).
Dirt aside, I agree. Two lines and a neutral are three conductors.A three phase transformer bank requires three primary conductors even if one of them happens to be a neutral conductor or dirt.