Is it much mor awkward working on 3 phase

[Paul_C]

Do they therefore distributes with the neutral conductor having a lower CSA?

Not normally, but it still results in a saving. Consider a case where you have only normal domestic 240V services connected to a local transformer and need to allow for a maximum load of 200A. With a simple 2-wire distribution system, then obviously you would need two conductors, each capable of carrying 200A.

With a 3-wire single-phase 240/480V system, the loads can be split so that neither pole of the supply will be called upon to deliver more than 100A. That means that the neutral will never be called upon to carry more than 100A either, since the worse-case scenario is full load on one pole and no load on the other. So the same amount of power can be distributed using three conductors each capable of carrying 100A.

Although most connected loads are probably 2-wire 240V, there are still some farms and similar consumers in places such as this which have a 3-wire supply and run heavier single-phase motors on 480V.

 

[JohnW2]

Do they therefore distributes with the neutral conductor having a lower CSA?

Not normally, but it still results in a saving. .... With a 3-wire single-phase 240/480V system, the loads can be split so that neither pole of the supply will be called upon to deliver more than 100A. That means that the neutral will never be called upon to carry more than 100A either, since the worse-case scenario is full load on one pole and no load on the other. So the same amount of power can be distributed using three conductors each capable of carrying 100A.

That's certainly true. However, given that we appeared to be talking about rural locations, where consumers might well be widely spaced, I was thinking that much of the cable runs would probably be 2-wire (again only needing to carry 100A, even without the 'split phase' setup).

Although most connected loads are probably 2-wire 240V, there are still some farms and similar consumers in places such as this which have a 3-wire supply and run heavier single-phase motors on 480V.

That's obvioulsy different, and a good argument for the system. I must say that I've never heard of a 480V single-phase motor!

Kind Regards, John.

 

[sparkticus]

So the transformer is single phase, has an 11kV primary and a 480V centre tapped secondary. I say 480V because I don't think any have been installed in the past 20 years.

As a matter of interest, why did they do that rather than just have two separate 240V secondaries? The only difference would be that the 480V secondary to which you refer would be broken at the centre point, but that would then remove the issue of 480V bewteen the two 'split phases'.

Kind Regards, John.

I'm having one of those "not so bright" days today. Though my wife might argue that I am permanently on "dim"

From the secondary winding: Are 3 wires delivered to the end user? If so are those both ends of the secondary (480V PD between them) plus the centre tap (240V PD between it and each end)?

If the above is true, then: Is the centre tap connected to earth either at he transformer or at the end user or somewhere in between?

For the sake of the other questions I will call the centre tap: neutral but apologise if I am wrong.

For Rural Areas Requiring 480V
I am assuming that in rural areas which have 480V equipment (single phase motor for example) then only the two ends (480 V PD between them) is used and not the neutral (centre tap)? In this case the question of any neutral current is a mute point because it is not used. is that right?

Or are there two windings in these motors with one end and neutral on one winding and the other end and neutral on the other winding? (240V windings) In this case the neutral will carry current under all circumstances, will it not? (the sum of the RMS current in the two windings, neglecting phase difference for a moment)

For Rural Areas Requiring 240V
For normal domestic use in rural areas I assume that one end and the neutral are used to provide 240V single phase?

 

[JohnW2]

As you will understand, I'm learning myself here, too, but if I understand correctly what is being said to me ....

From the secondary winding: Are 3 wires delivered to the end user? If so are those both ends of the secondary (480V PD between them) plus the centre tap (240V PD between it and each end)?

Only if the consumer requires both 480V and 240V supplies, I believe.

If the above is true, then: Is the centre tap connected to earth either at he transformer or at the end user or somewhere in between?

I would assume so - I can't see that they could leave it floating.

For the sake of the other questions I will call the centre tap: neutral but apologise if I am wrong.
For Rural Areas Requiring 480V
I am assuming that in rural areas which have 480V equipment (single phase motor for example) then only the two ends (480 V PD between them) is used and not the neutral (centre tap)? In this case the question of any neutral current is a mute point because it is not used. is that right?

... except that, in practice, they would almost certainly need the 240V supply as well, so they would have to have all three wires.

From Or are there two windings in these motors with one end and neutral on one winding and the other end and neutral on the other winding? (240V windings) In this case the neutral will carry current under all circumstances, will it not? (the sum of the RMS current in the two windings, neglecting phase difference for a moment)

As above, I was assuming the single-winding scenario, but I may be wrong.

For Rural Areas Requiring 240V
For normal domestic use in rural areas I assume that one end and the neutral are used to provide 240V single phase?

That would be my understanding.

Hopefully Paul or someone else will put us right!

Kind Regards, John.

 

[Paul_C]

That's certainly true. However, given that we appeared to be talking about rural locations, where consumers might well be widely spaced, I was thinking that much of the cable runs would probably be 2-wire (again only needing to carry 100A, even without the 'split phase' setup).

Well, if the consumers are very widely spaced then a single-phase transformer with a simple 240V secondary is likely to be used for each isolated house. Here's a typical example of where the 240/480V 3-wire system is used in this rural area:

http://maps.google.co.uk/maps?.....

If you turn on the satellite view, you can see a line of houses spread along Chapel Road, just south of the junction with Honing Rd. & Drabbs Ln.

This link should take you to the street view where the transformer is located:

http://maps.google.co.uk/maps?.....

You can more or less see how far the 3-wire network extends along this little lane on the street views.

 

[sparkticus]

Or are there two windings in these motors with one end and neutral on one winding and the other end and neutral on the other winding? (240V windings) In this case the neutral will carry current under all circumstances, will it not? (the sum of the RMS current in the two windings, neglecting phase difference for a moment)

Oh wait, I have made an obvious mistake here. If the two windings were as above then the neutral current would not be the sum of the two windings since the two windings would essentially be in series with one another (joined at the neutral connection) the two ends are out of phase (W/R to one another) so the neutral would would not carry the sum of winding currents. I don't know what the phase angle difference is but this would be a similar concept to a balanced three phase load but not as efficient.

 

[westie101]

I say 480V because I don't think any have been installed in the past 20 years.

We still install them at 100kVA ratings on single phase HV, any of lower ratings are selectable as either single or split single.

It was possible to get suitable motors at one time but I doubt it is the case now.

As a matter of interest, why did they do that rather than just have two separate 240V secondaries? The only difference would be that the 480V secondary to which you refer would be broken at the centre point, but that would then remove the issue of 480V bewteen the two 'split phases'.

See above the smaller types are twin windings that can be configured as a centre tap or both in parallel.

In the 240 - 0 - 240 case the centre tap is indeed the neutral and is earthed in the "normal" way.
I do not know of any cases where a customer has a 2 wire supply formed of the two 480V wires, in such cases if they need that voltage they will have a 3 wire supply. (apart from anything else the meters need a neutral to work!)

 

[Paul_C]

From the secondary winding: Are 3 wires delivered to the end user? If so are those both ends of the secondary (480V PD between them) plus the centre tap (240V PD between it and each end)?

If the above is true, then: Is the centre tap connected to earth either at he transformer or at the end user or somewhere in between?

Yes, the center-tap of the transformer winding is connected to earth and is then distributed as the neutral. It may also have additional earth connections along its route, just as for PME with a 3-phase 4-wire system.

As John has suggested, where 480V service is provided all three wires are brought to the premises, so that 240V is available as well for "normal" loads. Residential supplies at 240V are each taken from one of the outer poles and the neutral, the load being distributed as nearly evenly as possible between the two outers (just as with a 3-phase LV network houses requiring only 240V single-phase supplies are distributed between the red, yellow, and blue phases reasonably evenly).

This is effectively the a.c. version of the old 3-wire d.c. systems which were installed in some urban areas in much earlier times.

For Rural Areas Requiring 480V
I am assuming that in rural areas which have 480V equipment (single phase motor for example) then only the two ends (480 V PD between them) is used and not the neutral (centre tap)? In this case the question of any neutral current is a mute point because it is not used. is that right?

Yes, any load such a motor which has a simple 2-wire connection to 480V and no neutral connection cannot give rise to a neutral current.

Or are there two windings in these motors with one end and neutral on one winding and the other end and neutral on the other winding? (240V windings) In this case the neutral will carry current under all circumstances, will it not? (the sum of the RMS current in the two windings, neglecting phase difference for a moment)

Motors can complicate matters slightly due to the inductance involved, but remember that relative to the neutral, the sinusoidal waveforms on the two live 240V lines are 180 degrees out of phase with each other (which is how you get 480V between them).

Assuming no complications from phase differences (pretend we're dealing with purely resistive loads to keep it simple), the neutral will carry not the sum of the individual currents but the difference.

Example: Connect a 2400W 240V load between one live pole and neutral, and obviously the current in that live supply line will be 10A. Connect an 1800W 240V load between the other outer and the neutral, and the current supplied from that live outer will be 7.5A. But because the two currents are 180 degrees out of phase, the neutral current will be the difference of 2.5A.

Edit: Was busy typing while the above posts were being made.

 

[westie101]

Oh and if you get the wiring wrong, 480V fed to a few houses ensures that everyone has a bad day!

 

[sparkticus]

Example: Connect a 2400W 240V load between one live pole and neutral, and obviously the current in that live supply line will be 10A. Connect an 1800W 240V load between the other outer and the neutral, and the current supplied from that live outer will be 7.5A. But because the two currents are 180 degrees out of phase, the neutral current will be the difference of 2.5A.

Thanks Paul_C, yes I realised my mistake just as I pushed submit! The neutral current will indeed amount to only the difference of the phase imbalance.

 

[sparkticus]

Oh and if you get the wiring wrong, 480V fed to a few houses ensures that everyone has a bad day!

Are the conductors colour coded or is there a label indicating split phase at the intake position? I have not done a lot of domestic and even less out in the sticks so this is quite intriguing. I could easily imagine someone putting 480V across their nice new Sony 50inch 1080P

 

[JohnW2]

Well, if the consumers are very widely spaced then a single-phase transformer with a simple 240V secondary is likely to be used for each isolated house. Here's a typical example of where the 240/480V 3-wire system is used in this rural area: ....

Thanks; that's interesting.

Kind Regards, John.

 

[Paul_C]

The neutral current will indeed amount to only the difference of the phase imbalance.

That's the whole purpose of using the 3-wire system. If you wired two 240V secondaries such that the two lives were in phase with each other, relative to earth/neutral, there would be no benefit, since the neutral would have to be large enough to carry the sum of the currents (and in fact it would not be a neutral in the true sense of the word).

It's worth pointing out that this is exactly the same single-phase 3-wire arrangement which is used to provide power to homes across North America, just at different voltages, and with the "full" 3-wire supply being the norm even for a regular house.

The standard residential supply in the U.S. & Canada is delivered over 3-wires at 120/240V, and distribution boards have two live busbars. Lighting & general-purpose outlets for all small appliances are 120V, connected between one outer and the neutral, with the load distributed as evenly as possible between the two sides of the board. Large fixed appliances (stove, clothes dryer, larger air-conditioning units & heaters etc.) are fed with 240V by way of a double-pole breaker.

 

[westie101]

Are the conductors colour coded or is there a label indicating split phase at the intake position

Not to any standard, in fact if a cable is used it will be a 3 phase type with brown, black grey.
It's not unknown to connect two of the cores to the same phase.
I've never seen notices so the only way to tell is by testing and not assuming

 

[Paul_C]

So long as you identify the neutral correctly, it doesn't matter which way round the two outers are connected anyway. It's not like three-phase connections where rotation is important and swapping two over will make a motor run backwards. Swapping the two lives to a motor or other device will make absolutely no difference as they are 180 degrees out of phase with each other.