3 phase 120/208

[mivey]

A is the high leg and AN is 90 degrees from CN and BN. CN and BN are 180 degrees apart exactly like a center-tapped single phase systen.

AB and CA and BC have a 60 degree separation like any normal three phase system, either wye or delta.

The 120 degree separation is not for line to line phases but is for the line to neutral phases when the neutral is at the middle of the wye configuration. You lose that when the neutral move to the center tap position in a delta.

No. You lose the 120 degree LN relationship in the high-leg delta. The LL phases do maintain the 60 degree separation.


A is the high leg in the posted pic.

90, 180, 90 for LN phases (actually two different phase systems consisting of two 120 volt phases in one system and one 208 volt phase in the other system)

60, 60, 60 for LL phases (three 240 volt phases in one system).

The lines representing the line-line coils meet at a 60 degree angle. That does not mean that the corresponding phasors are 60 degrees apart!
-60 + 180 = 120.

That is true.

GoldDigger corrected my posts. The phasors have their normal 120 degree separation in either the wye or delta system. I got hung up on the angles at the common points and erroneously assigned that to the phasors.

Thanks for correcting that.

 

[electrofelon]

You are looking at the secondary connections. That document is a little unclear because they show two primary connections, not the third, then they discuss the neutral in context with the secondary. SCL is one of the few places that will derive a 240/120 3 phase 4 wire service from an open delta bank. I had one at my shop in Seattle, it confused people a lot, even me, because it’s somewhat rare. But there STILL must be 3 primary phases. The open delta just saves them the cost of one transformer.

Yes I agree the primary connections are not super clear, but SCL is a MGN so the primary neutral and secondary neutral land in the cans. I didn't realize it till mivey pointed it out, but this is an open wye not an open delta.

As to 240/120 3 phase 4 wire services in general, utilities that still offer it will often limit the 120V loading to 5% of the total load, because it is inherently unbalanced. The intent of that service is for customers with MOSTLY 3 phase loads who need a SMALL amount of 120V lights and receptacles, NOT the other way around. If most of the loads are single phase with a few 3 phase, they will want you to accept 208Y120 and balance the loads. MOST 230V motors 5 HP and under are designed to allow for 208V input, but if not, use buck-boost transformers for them.

That was likely the original intent, but I see tons of high leg open deltas in seattle and most of them don't have any three phase loads. In fact, many I come across don't utilize the third phase at all! Changing times I guess.

 

[mivey]

I know little about the POCO side of things for the most part. As I said earlier, I really have only paid attention to the secondary since that is what I deal with.

I would totally be the one scratching my head also. “ Use a noodle on a primary side, huh?”.

The whole wye primary was a bit confusing to me, I have heard of wye-wye, but this wye-delta caught me by surprise. I can understand now for the open delta set up.

For small trannies I have hooked up, it has mostly been delta-wye.

I like the delta-wye myself for a lot of things. Most distribution pads come wye-wye now-a-days and you have to specify a delta-wye when you want one. I do like a wye-delta-wye in a substation when I want to avoid a phase shift.

Lots of interesting configurations available. J&P is a good resource for many.

 

[jumper]

That was likely the original intent, but I see tons of high leg open deltas in seattle and most of them don't have any three phase loads. In fact, many I come across don't utilize the third phase at all! Changing times I guess.

We see that on the East coast also, I call them legacy or leftover systems.

Installed way back when 3 phase was needed for most AC, but not anymore.

AC is for air conditioning in the above.

 

[mivey]

Yes I agree the primary connections are not super clear, but SCL is a MGN so the primary neutral and secondary neutral land in the cans. I didn't realize it till mivey pointed it out, but this is an open wye not an open delta.



That was likely the original intent, but I see tons of high leg open deltas in seattle and most of them don't have any three phase loads. In fact, many I come across don't utilize the third phase at all! Changing times I guess.

There was a push at one time to install 3-phase A/C in homes so you used to see a lot of high-legs in residential areas. That is going away. We also see them a lot where you have a mix of small commercial along with large commercial and industrial.

Could be the high leg is not needed anymore as people move to 208/120 three-phase.

Really depends on if the customer's single-phase needs are 120/240 or if they are happy with 120/208.

If they are not using the high-leg anymore it could be a sign of down-sizing where smaller industry moved into older buildings.

 

[kwired]

Joined the party late and multiquoted some items as reading through, sorry if I repeat something already mentioned:

180105-1627 EST

mannyb:

Apparently all present machines are single phase 240 V.

Any that do not have high initial loads, and are not generally run at full load can probably run at 208 with no problem. This would include drill presses, sanders, small tool grinders, and hand tools.

Anything with a high inertia load, for example a radial arm saw with a large diameter saw blade might be a problem on start up.

An air compressor may have a high initial load. The air compressor motor probably should be replaced with a 3 phase motor, at least if it is in the 5 HP or higher rating.

If the 208 source is very stiff, little voltage drop under load, and you use low impedance wiring from source to the load, then the high initial loads may not be a problem.

At no steady-state condition do you want to run a load current higher than rating.

A future CNC machine should have input taps that allow 208 operation.

.

Radial arm saw and other similar high inertia loads often don't start "loaded" That helps get them up to speed faster even if voltage applied is right on rating. Air compressors also usually have an "unloader valve" so the next start isn't under pressure.

One problem with compressors often noted on this forum is that the HP rating stated on the units seem to be misleading in many cases.

I think mostly happens with 5 HP and less single phase. HP on motor nameplate often just says "SPL" it is the signs on the tool that state an exaggerated HP, which is not false but is only what it can develop for a short time.

Not sure I understand.

An open High leg Delta would still need three primary conductors but only two trannies, no?

So I understand that one only needs two hots and a neuter for an open delta, but I dont see the logic in a POCO running two phases and neuter. Why not just run three phases? I guess if they want to make it a MGN is that it?

Have several single phase distribution lines in rural areas here, occasionally you will find a pole with two pots on it - it is supplying an open delta service. If you look carefully the top line is ungrounded but does not tie to the top line coming from other direction - they are two phases of same system but if only single phase services are along the two routes to get to an isolated three phase service why add all the cross arms additional hardware and a two other conductors, they add up in cost over several miles.

We also have open delta quite often on limited load applications. Many irrigation services that don't have a large pump motor only need 30 amp max supply. third transformer not worth investment to POCO to such a limited load.

That's the pic ^ I was looking for. Are the phases still 120* apart with open delta? A to N and N to C are 180* total, correct? Does that make A to B and B to C 90* ea vs 120*, or is the neutral on the split phase on the small pot (A to N to C) splitting one phase in half and the phase angles between A, B, and C all 120* like every other 3 phase application?

Phase angle between only two points is always 180 degrees. 277 volt circuit on a wye secondary is 180 deg between any phase and the neutral. Throw in a third point and you can get an angle other then 180.

 

[MAC702]

The lines representing the line-line coils meet at a 60 degree angle. That does not mean that the corresponding phasors are 60 degrees apart!
-60 + 180 = 120.

That was a good way to explain that. Thank you.