208 V Single Phase?

[g9s0x]

I'm new to the boards so let me first introduce myself. I work as a telecommunications engineer, particularly with central office-type exchange equipment at the network level. As such I'm inherently involved with inside plant power from a functional standpoint. Most of my power work is in the -48VDC realm, as well as DC microelectronics. I am not quite as familiar with AC, and have a few questions that I thought would best be addressed by the members of this forum.

Say I have equipment that needs 220 VAC single phase @ 50 Hz, and the inside plant AC is wired as 208Y/120. From a functional and theoretical point of view, is there any way to generate single-phase 208 V or 240 V from a three phase configuration, not necessarily wye? The equipment has some tolerance built in to accept anywhere from 190 to 250 V single phase. My initial assumption is that L-L voltages will still be three phase and won't work. The only single-phase is 120 V L-N, but this is not sufficient. I have seen the split-phase delta configuration that will generate 240 V but this is a line to line (L-L) measurement so it is still three phase correct? The equipment expects single phase so the 240 V it's expecting is measured from line to neutral (single phase) opposed to line to line (three phase).

Am I getting that correctly? In other words, does L-L measurement versus L-N measurement bear any indication on the number of phases involved? 120 V L-N is single phase, but 240 V L-L is still single phase?

My assumption is that the only solution would be to run 240 V single phase from two 120 V single phase legs - ignoring the discrepancy in line frequency for this discussion. This 240 VAC single phase is still measured line to line, correct? The phase angles would be 180 degrees off between the two 120 V legs that make up the 240 V. This would seem to the novice learning AC theory that there are two phases here, but it is classified as single phase.

I do need to read a bit more on three phase power, so if you could recommend a good resource I would appreciate it. I hope this isn't too basic for this particular forum, and if so I apologize.

 

[winnie]

A discussion of the meaning of the phase angle difference in a 'single phase' system will quickly derail this thread; so I suggest that you search past forum threads on that point.

A voltage measurement must always be made between two points.

When you describe AC phase angle, you are talking about the _timing_ of the AC cycle in a given circuit. Phase angle only has meaning if you have a reference point, for example two AC circuits with different timing.

A device connected to _two_ and only _two_ circuit conductors has only a single circuit path. While the AC current flowing in that circuit may be at just about any 'phase angle' relative to that flowing in some other circuit, the device only sees that single AC circuit, and thus only 'sees' a single phase of power.

It does not matter if those two conductors are a hot and a neutral or the two 'hot' legs of a single phase transformer, or two 'hot' legs selected from a three phase supply. If you connect a device between two supply terminals, it is a single phase load.

A related consideration is ground reference. Many European single phase supplies are 230V L-N. If the device is designed with the assumption that one of the terminals will be connected to a grounded conductor, it might not work correctly on a 240V (or 208V) L-L supply, where _both_ supply terminals are ungrounded.

-Jon

 

[charlie b]

From a functional and theoretical point of view, is there any way to generate single-phase 208 V or 240 V from a three phase configuration, not necessarily wye?

You already have that ability with the 208/120 system.

My initial assumption is that L-L voltages will still be three phase and won't work.

Incorrect assumption.

The only single-phase is 120 V L-N, but this is not sufficient.

Again, incorrect. There is also a single phase 208V L-L, and that is sufficient.

In other words, does L-L measurement versus L-N measurement bear any indication on the number of phases involved?

Yes. It is derived from two of the phases, but it results in only one phase.

My assumption is that the only solution would be to run 240 V single phase from two 120 V single phase legs. . . . - ignoring the discrepancy in line frequency for this discussion.

That is what you want to do, but it will not give you 240 volts. It will give you 208 volts. As to the frequency, I would expect that your equipment's tolerances include a range of frequencies that would take a 50 Hz input and give you the output that you need.

The phase angles would be 180 degrees off between the two 120 V legs that make up the 240 V.

No. The phase angle between one of the 120V legs and the neutral point will be 120 degrees different from the phase angle between the other 120V leg and the neutral point. Try this experiment: Pick a point on a piece of paper, and draw a line 120 millimeters (about 4.5 inches, to give you an idea of scale) to the right. Go back to the starting point and draw a second line the same length, but at an angle of 120 degrees from the first line. Those will represent Phase A voltage of 120V at a reference angle of 0 degrees, and the Phase B voltage of 120 V at a phase angle of 120 degrees. Now measure the distance between the end points of the two lines. If you drew the picture accurately, you should get an answer of about 208 mm (about 8 inches). That represents the voltage of 208V between Phase A and Phase B.

This would seem to the novice learning AC theory that there are two phases here, but it is classified as single phase.

Not to repeat myself, but it is derived from two of the phases, but it results in only one phase.

 

[deee]

3 phase and single phase

3 phase and single phase

in a 208 three phase wye 4 wire line to line voltages will read 208 volts and line to neutral will read 120 volts. The only time you will be considered having three phase load is if you are using all three wires and I don't mean the NEUTRAL. if you are using two of the three phases, then you are considered single phase. You wont use the neutral in a three phase or a 240 or a 208 single load phase "typically".

 

[deee]

line to line, if you are only using two wires is basically two phase

 

[GoldDigger]

line to line, if you are only using two wires is basically two phase

What the load sees is one phase. Period.
Now if the OP derives a high leg delta, then the two lines on either side of the center tapped neutral will be 180 degrees out of phase from each other with respect to neutral and ground. That 180 degree shift is generally considered to be single phase. Occasionally called split phase. Rarely and disputedly called two phase, which actually properly refers to a 90 degree phase shift.

 

[g9s0x]

Thank you for the responses gents. That was very helpful and I do appreciate it.

charlie b - when I was referring to using 2 x 120V legs to get 240V, I was referring to the single-phase setup as seen at traditional wall outlets, and not the 208Y/120. Sorry for not making that clear. Knowing that, are my previous comments/assumptions correct? That is, both 120V single-phase legs are 180 degrees out of phase and from L-L I'd have 240V single phase. This is how 240V circuits are created in residential applications, correct?

It is derived from two of the phases, but it results in only one phase.

That was the key bit of information I was missing in trying to understand this.

 

[g9s0x]

Within the United States, is it possible to have an electrician wire 240V (or some value relatively close to that) as measured from line to neutral, to mimic a European supply voltage? Disregard line frequency differences, i.e. 50 Hz vs. 60 Hz.

 

[charlie b]

You can use two 120 volt legs from a panel that is rated 120/240 volts single phase (typical for residential panels), to obtain a 240 volt, single phase source. But you told us the building your equipment is in has a 120/208V three phase distribution system. That will not do what you are trying to describe. If you use two 120 volt legs from that type of system, you will get 208 volts, single phase, as I said before. So you need to verify what type of panel will be supplying your equipment. Is it single phase or three phase?

 

[charlie b]

Within the United States, is it possible to have an electrician wire 240V (or some value relatively close to that) as measured from line to neutral, to mimic a European supply voltage?

No. But why do you think that your power source has to come via a line to neutral configuration? If your building has a 120/240 single phase distribution system, then use a line-to-line configuration and you will get 240 volts, single phase. If your building has a 208/120 three phase distribution system, then use a line-to-line configuration and you will get 208 volts, single phase.

 

[charlie b]

line to line, if you are only using two wires is basically two phase

That is not true; it is basically single phase. There exists a two-phase distribution system. The two legs are separated from each other by 90 degrees of phase angle. This is rarely used; I have never actually encountered it myself.

 

[g9s0x]

No. But why do you think that your power source has to come via a line to neutral configuration?

I will need to determine what the equipment is expecting for sure, but say the inputs are expecting 220 VAC L-N where one of the conductors, in this case neutral, would be grounded, correct? I'd be supplying it with 208 VAC L-L where both conductors are not grounded. So a line connection would be made where a neutral connection was expected. To throw out some more terminology I've come across in my readings, I'd be supplying the equipment with line voltage, but it is expecting phase voltage. This seems very relevant. Thanks again for all the responses as I'm learning throughout all of this.

 

[GoldDigger]

If the equipment expects one input conductor, of the two which differ in potential by 240V, to be grounded, then your solution is to use a transformer. The primary of that single phase transformer can either be connected line to neutral or line to line, as you choose

 

[g9s0x]

If the equipment expects one input conductor, of the two which differ in potential by 240V, to be grounded, then your solution is to use a transformer. The primary of that single phase transformer can either be connected line to neutral or line to line, as you choose

I would need a 1:1 transformer to keep the discussion simple - although I could adjust the N-value in the secondary to step-up the voltage slightly - and connect the 208 VAC line voltage (L-L) to the primary side. This would generate a phase voltage of 208 VAC L-N at the secondary? Can you point me in the direction of some good material on transformers that covers this?

 

[GoldDigger]

I would need a 1:1 transformer to keep the discussion simple - although I could adjust the N-value in the secondary to step-up the voltage slightly - and connect the 208 VAC line voltage (L-L) to the primary side. This would generate a phase voltage of 208 VAC L-N at the secondary? Can you point me in the direction of some good material on transformers that covers this?

It would generate a voltage difference of 208 volts between the two secondary terminals. And if you then supplied a reference voltage by connecting one terminal of the secondary to ground/neutral you would have you a voltage of 208 volts between line and ground.
I do not have any specific recommended references for you. You could start as far back as basic physics to see how moving charges create a magnetic field and changing magnetic fields induce voltages over a loop of wire.
Or you could start by taking as given that the voltage across one coil around a magnetic core and another coil on that same core will be proportional to the turn ratio of the two coils. And that no current will flow from one isolated coil to the other.

 

[winnie]

I was the one who mentioned the ground reference issue. I apologize for not expanding further: _If_ the equipment in question cares about having one of the supply conductors at low voltage, then using line-line 240V could cause a problem. An example in the US would be a simple table lamp with a screw shell, where the screw shell is supposed to be energized with the _grounded_ conductor.

However most equipment will not care about which conductor is grounded, or even if you have a grounded conductor. In this case 240V L-N or 240V L-L will be equally acceptable.

You will need to carefully review the specification of the equipment in question.

-Jon

 

[jumper]

I was the one who mentioned the ground reference issue. I apologize for not expanding further: _If_ the equipment in question cares about having one of the supply conductors at low voltage, then using line-line 240V could cause a problem. An example in the US would be a simple table lamp with a screw shell, where the screw shell is supposed to be energized with the _grounded_ conductor.

However most equipment will not care about which conductor is grounded, or even if you have a grounded conductor. In this case 240V L-N or 240V L-L will be equally acceptable.

You will need to carefully review the specification of the equipment in question.

-Jon

I am pretty sure you meant ungrounded conductor. The term "energized" would usually refer to the "hot" conductor, not the grounded conductor attached to the screw shell to complete the circuit path.

 

[deee]

What the load sees is one phase. Period.
Now if the OP derives a high leg delta, then the two lines on either side of the center tapped neutral will be 180 degrees out of phase from each other with respect to neutral and ground. That 180 degree shift is generally considered to be single phase. Occasionally called split phase. Rarely and disputedly called two phase, which actually properly refers to a 90 degree phase shift.

GoldDigger, I know this, that is why I used the word basically. Just simplifying the statement

 

[winnie]

I am pretty sure you meant ungrounded conductor. The term "energized" would usually refer to the "hot" conductor, not the grounded conductor attached to the screw shell to complete the circuit path.

I shouldn't have been writing quite so late at night

I did specifically mean the _grounded_ conductor, but I was using the term 'energized' to mean 'connected to the supply circuit'.

Of course you are correct, the whole point of having the screw shell connected to the grounded conductor is to provide a closed circuit without 'energizing' it relative to ground.

-Jon

 

[kwired]

I will need to determine what the equipment is expecting for sure, but say the inputs are expecting 220 VAC L-N where one of the conductors, in this case neutral, would be grounded, correct? I'd be supplying it with 208 VAC L-L where both conductors are not grounded. So a line connection would be made where a neutral connection was expected. To throw out some more terminology I've come across in my readings, I'd be supplying the equipment with line voltage, but it is expecting phase voltage. This seems very relevant. Thanks again for all the responses as I'm learning throughout all of this.

If your equipment is expecting to see an input of 220V with one of those conductors at ground potential, you about need a separately derived source - which could be the secondary of a transformer that only has two output leads or is arranged to not utilize any center tap, you can ground either output lead and have such a system. This is a voltage that would be common in European installations so chances are if you need that particular input voltage, your equipment is likely European made. They have 415/240 (400/230) wye systems for most instances. With dwellings or light commercial they just run one phase and grounded conductor to single phase services and if you are on one of those services you have 240 volt two wires - with one of them being grounded.

I've been working lately on a machine that is that way and it European made. This one has 120/240 source at the site, but near this machine is a 240 x 240 isolation transformer - with secondary having two conductors - one of them is grounded. Same thing the machine would see if installed in majority of European countries except frequency is different here.

You may or may not need special transformer if the 208 vs 240 is an issue or possibly a buck boost transformer to get desired output voltage from commonly stocked transformers. A single phase transformer with 208 volt primary may not be something that is all that easy to find in stock from any supplier and would likely be special order.