Buc Boost 208 to 120/240 with a Neutral

[ronaldrc]

Rattus

That would be like saying if I hooked one 120 volt transformer to Line #1 and the xo and another 120 volt transformer two Line #2 and to the XO it would burn the system out.

The only other difference here is one is one core and the other is two cores and that shouldn't make any differents.

 

[winnie]

The sort of transformer that we are discussing has 4 coils placed on a single core. Two of these coils are rated 120V, and two of these coils are rated 16V.

As shown in the diagram in post 29, with the transformer connected as in the original post, the 'center tap' of the transformer will be at 60V relative to the grounded neutral of the wye source.

In theory, if you have a 120/240V single phase load, it will operate just fine between the center tap of the buck-boost transformer as connected. However the center tap will not be a grounded conductor, and will in fact be at 60V relative to ground. For some devices this makes no difference at all, because there are no exposed terminals and all supply conductors are switched. But anything that depends upon the neutral being a grounded conductor could be a hazard. Using this 'derived neutral' is probably a code violation.

If you attempt to connect the center tap of the transformer to the grounded neutral of the wye source, you will see lots of 'circulating current' in the transformer set. Without the center tap connected to the grounded neutral, you only have a pair of connected terminals, and thus a single phase connected to the buck-boost transformer. But with the center tap connected, you have two phases with a real phase angle difference. These two phases drive two separate 120V coils, but the two coils are on the same magnetic core. The net result is that the magnetic flux induced in the core will not be in phase with the voltage applied to the coils, and current will flow to compensate. Another way to look at this is that current will flow to reconcile the fact that the buck-boost transformer is trying to hold the 'center tap' at 60V relative to the grounded neutral that it is connected to.

In the original drawing, assuming a 208/120V wye source, the voltage between X1 and X4 is about 236V, and the voltage between X1 (or X4) and ground is about 132V. If instead you connect both of the X coils on one 'side' of the transformer, and just use one of the supply legs directly as a supply to your 240V load, then that leg will also be at 120V to ground. The _other_ leg, however will be at 145V to ground. If you know which leg serves both 120V and 240V loads, then you could use a buck-boost transformer to supply a 120/240V load from a 208/120V wye system. Of course if you get this wrong than the 120V components will be significantly overloaded.

-Jon

 

[ronaldrc]

Line #1 to Line #2 does not give you two phases

Line #1 to line #2 is A phase
Line #2 to line #3 is B phase
Line #3 to line #1 is C phase

 

[Volta]

Line #1 to Line #2 does not give you two phases

Line #1 to line #2 is A phase
Line #2 to line #3 is B phase
Line #3 to line #1 is C phase

If it is a wye system aren't the phases (utility transformer secondaries):

A to neutral,
B to neutral, and
C to neutral?:-?

 

[ronaldrc]

Not from what I was taught and have learned.

I think what might be confusing some if they tried this, is there will be a arc when the neutral is hooked because you can't balance a three phase system with a two wire circuit without the third leg loaded equally.

 

[winnie]

Line #1 to line #2 is A phase
Line #2 to line #3 is B phase
Line #3 to line #1 is C phase

I quite agree that this is the normal usage, for example in the windings of an electric motor.

However you can measure and plot voltage waveforms for any pair of terminals in the system. If you measure and plot the following:

Line #1 to Line#2
Line #2 to Line#3
Line #3 to Line#1
Line #1 to Neutral
Line #2 to Neutral
Line #3 to Neutral

You will find that each has a _different_ phase angle.

If you take a transformer coil and connect it to a supply, the thing that prevents excessive current flow in the coil is the emf induced in the coils by the changing magnetic flux in the core. If you have two phases on two different coils on the same core, then the flux cannot match the applied voltage and you will see large current flows.

-Jon

 

[ronaldrc]

Larry,Smart,Rattus and Winnie and all concerned.

I have to say this is a very complex subject more complex than a lot of us can put into words.

I understand what you are saying about the generation of a 2nd current being induced in the core but what we need to be concerned with is the resulant wave form which results from these two phase being mixed.

Does that make any sense?

 

[LarryFine]

One core or three cores shouldn't have anything to do with this.

But, it does. I read through the other responses, but let me explain in my own words. Join me as we explore the depths of the abyss: (Be afraid. Be very afraid!)

The only effect the three phase wye system has is the interaction of the three phase mixing and beating it down from 240 volts to 208 volts.

But keep in mind that, when you grab the two opposing "phases" (re: the neutral) of the 120/240v 1ph, and "bend" them from 180 deg to 120 deg apart, which reduces the L-L voltage from 240 to 208, you are altering the timing of the alternating magentic field(s).

I added the 's' because the 1ph transformer has but a single core which 'locks' the fields in sync. (Without that phenomenon, the autotransformer wouldn't work.) Remember why 3-ph transformers must have a separate core for each phase. The fields don't peak at the same time; they have the same 120 deg spacing as the voltages.

You can't simply apply two different phases to a single core. Something has to give. If you forcibly 'bend' the 180-deg 120-0-120 vector into a 120-deg 120-0-120 vector, the two 'neutral' points must separate from 0v to 60v. If you could 'force' the 60v point back to 0v, the vectors must 'un-bend' back to 180 deg.

Make sense? (So much for under-simplifying. )

 

[LarryFine]

I understand what you are saying about the generation of a 2nd current being induced in the core but what we need to be concerned with is the resulant wave form which results from these two phase being mixed.

Does that make any sense?

Sure, but the concern as I'm seeing it is that the mixing of the waveforms will cause extreme overcurrents, not the other way around. One phase will attempt to force the other one to adopt its timing.

Remember, when you energize any winding, or winding segment, with its design voltage, every other winding and winding segment will be energized at its design voltage (compensations and other variables aside) and in synch.

 

[ronaldrc]

Like I said Larry mind bending ain't it? Goodnight

 

[winnie]

It is possible to wind coils from different phases on to the same core, and the flux through the core will be some composite created by the current flowing in the different coils. But when you do this, something other than the back-emf created by the flux in the core is needed to limit the current flow.

You can find examples of having coils driven by multiple phases placed on a single core if you look at 'zig-zag' transformers, in certain types of motor winding, and I believe in certain types of phase shifting transformers.

-Jon

 

[rattus]

Rattus

That would be like saying if I hooked one 120 volt transformer to Line #1 and the xo and another 120 volt transformer two Line #2 and to the XO it would burn the system out.

The only other difference here is one is one core and the other is two cores and that shouldn't make any differents.

You are OK with two transformers because the phase angles of the magnetizing currents and hence the magnetic fluxes in the two cores are independent of each other, and these angles differ by 120 degrees.

You cannot do this with a single core because you have built an autotransformer with your B-B transformer. In the ideal case, you have, in effect, a dead short between L1 and L2.

 

[ronaldrc]

Okay you all have been very courteous and considerate

I have been down this road before as I said Larry. Now I remember, I learned this years ago all of you are right.

And I believe I am going completely senile. I keep forgetting that each winding of the wye are 120 volt one per phase and in series.Mainly that each winding is a phase.

I completely understand but if I live long enough like a year or so I will forget about these arguments.

 

[glene77is]

Ron,

This has been an interesting discussion.

Makes this forum worth reading!

 

[glene77is]

Ron,
I was looking at NEC 210.9, Handbook exhibit 210.19.
Your diagram and this one from the NEC Handbook have many similarities.
Yours is to be sourced by a single phase, and the Handbook from a 208 Open Delta.
Tell me what you think.
Glen

 

[glene77is]

Ron,
Am I reading this right?
Your Autotransformer has a neutral ("Secondary Neutral")
which is insulated from the 'Primary Neutral'.
This 'Secondary Neutral' is 'floated' to the secondary load.

 

[Coopman24]

208v/120v

208v/120v

I have a question and this seems to be the place to ask it. Sorry if it's elementary. I understand that from a 208/120 3 phase four wire system the following are possible.

120 single phase
208 single phase
208 three phase

My question is why can you not get 240v (single phase) from connecting two hots like in a 120 single phase system. Same thing right if your connecting to the same phase twice?

 

[Coopman24]

Nevermind

Nevermind

I answered my own question above. I figured that because a 2-pole breaker was on one side of the panel it was on the same lead.

Now I see the panel alters between the two supplies and since they are 180 degrees apart they have the same potential differance at any given time. So you can add them. Got it!

 

[kbsparky]

Using a 2-pole breaker in a 208Y/120 3 phase panel will not result in a 240 Volt circuit.

Since the lines are 120 degrees out of phase (not 180), you will only realize a potential difference of 208 Volts in such a scenario.