Quick question - Reg number prohibiting SP RCBOs on TT

[ban-all-sheds]

Exactly.

Different directions.

Of course everything is relative to the midpoint.

 

[bernardgreen]

If the equipment has only two wires ( ignoring the safety earth ) then no matter where those two wires come from the equipment can only be single phase.

Indeed, but that doesn't stop the supply from being 3-phase.

Where there are three wires it is possibly the supply is 3 phase with 120 degree phase angles and no neutral

Or it could be 2 phase and neutral with 180 degree phase angle.

That is the SUPPLY format. which is not always the same as the equipments format.

Single phase equipment ( 2 wires ) can be connected to any two wires of a 3 phase (120 degree) or any two wires of a two phase + neutral ( 180 degree ) or any two wires of a three phase + neutral ( 120 degree ) and the equipment sees and uses only ONE phase. The last one being the domestic set up in the UK The voltage between which ever two wires are used has to be suitable for the equipment.

 

[JohnW2]

If you are happy with the concept that at any point in time the potential difference between the centre point and one end will be a positive value and between the centre point and the other end it will be a negative value, then you must be happy that the currents have different directions.
If you could provide me with a supply with a voltage low enough, and a high speed video camera, I could connect two loudspeakers as the loads, and when filmed and played back at a slower speed you would see the diaphragms moving out of phase.

That was my original thinking, but I think that I have now been persuaded otherwise.

Consider first two equal resistive loads in series across the whole of the transformer secondary, with no connection to the centre tap of the transformer. It is apparent that exactly the same current is flowing through both of the loads, so the currents through them are clearly 'in phase'. If those loads were loudspeakers, your camera would show the diaphragms moving in phase.

As I now see it, and again assuming resistive loads, the common point where the two loads are connected will have a potential (relative to anything you choose) equal in magnitude and phase to that at the centre tap of the transformer. Hence, if you join that common point to the centre tap, no current will flow through the connection and hence absolutely nothing will change - the currents through the two loads will still be 'in phase'.

That's how I now see it, anyway - have I convinced myself of something which is wrong?

If the two loads are unequal, then things obviously get more complicated, since current then would flow through a connection between their common point and the CT of the transformer - but I need to think about that situation a bit more.

Kind Regards, John.

 

[bernardgreen]

Hence, if you join that common point to the centre tap, no current will flow through the connection and hence absolutely nothing will change - the currents through the two loads will still be 'in phase'.

That's how I now see it, anyway - have I convinced myself of something which is wrong?

That is perfectly correct.

 

[ban-all-sheds]

Or it could be 2 phase and neutral with 180 degree phase angle.

Which is what we're talking about.

 

[ban-all-sheds]

If those loads were loudspeakers, your camera would show the diaphragms moving in phase.

You think so?

The - terminal of each one connected to the centre point, and the + terminals connected to the end points, and the diaphragms would be moving in phase?

I don't think so.

 

[Tech99]

No - The currents will be in phase (assuming we're not complicating things with non-resistive loads).

No.

But they will. Call the current as measured at one end of the supply winding I1 and the current as measured at the other end I2. Take two loads, R1 & R2, connect them in series across the full supply (i.e. no neutral connection). I1 must be the same as I2 and they must be in phase. They can't not be.

Assume that R1=R2. Now connect a neutral from the centre tap of the supply transformer to the point between R1 & R2 - No current will flow in that neutral because the two halves of the load are balanced. Has the magnitude of either I1 or I2 changed? Has the phase relationship between I1 & I2 changed in any way? The answer to both questions is no. With the balanced load, adding the neutral connection hasn't changed anything because there's no current flowing in it.

If R1 & R2 are different values, then the magnitude of I1 & I2 will change when you add the neutral connection. But the phase relationship between them won't change, it's just that one will be less than the other because part of the total current is now flowing through the neutral.

But even if it was the IET going it alone, do you really think that all those engineers with all that collective knowledge would not have had a good reason? Do you really think they would make a mistake?

I'd like to know what the supposed reason is. And yes, why shouldn't they make mistakes? If you're saying that this sudden decision is the correct one, I could ask whether you think that thousands of electrical engineers over the last hundred and something-odd years have all been making mistakes, and whether the thousands today who will continue to refer to this arrangement as single phase are wrong.

Sorry - I worded my question badly. From your perspective, do you think that the phase angle has to be other than an integer multiple of 90° for it to be a polyphase system?

Not at all. I mentioned the 2 phase system with 90° phase difference before.

In everyday life if you take one thing and split it into two then you have two things. Not with everything, granted, but if I have a piece of wood and I split it into two I then have two pieces of wood. And if I split it in the middle I have two pieces of wood of equal size, each one half the size of the original

Remember that the 3-wire single phase arrangement is called split phase. But that doesn't create a two phase system.

Rather than your wood analogy I'd suggest something like a motor. You could have a simple induction motor (say a shaded pole type) with a single winding, which is clearly a single phase motor. If you split the winding into two halves for manufacturing convenience and wire those two halves in series, you haven't created a two phase motor, have you?

If you are happy with the concept that at any point in time the potential difference between the centre point and one end will be a positive value and between the centre point and the other end it will be a negative value, then you must be happy that the currents have different directions.

No. You are only seeing anti-phase voltages because you're using a mid point as your reference. It doesn't change the direction of the current in any way, or introduce a current which is out of phase with any other current.

If you could provide me with a supply with a voltage low enough, and a high speed video camera, I could connect two loudspeakers as the loads, and when filmed and played back at a slower speed you would see the diaphragms moving out of phase.

It depends which way round you connect the speakers. Connect +ve of each speaker to the respective outer of the supply and join -ve of each together to the neutral and yes, as one cone moves towards you the other will move away and vice versa. But connect -ve of one speaker to +ve of the other to use as the neutral, then the cones will move forward and backward in phase. The current though each speaker would be in phase in both cases, it's just a matter of which way the resultant magnetic field at any given instant causes the speaker to be driven.

You'd get exactly the same result with the two speakers in series and a simple two wire connection to a suitable AC source without the neutral.

 

[JohnW2]

That's how I now see it, anyway - have I convinced myself of something which is wrong?

That is perfectly correct.

Which is perfectly correct - what I actually wrote, or my suggestion that I may have convinced myself of something which is wrong?

Kind Regards, John.

 

[JohnW2]

If those loads were loudspeakers, your camera would show the diaphragms moving in phase.

You think so? The - terminal of each one connected to the centre point, and the + terminals connected to the end points, and the diaphragms would be moving in phase? I don't think so.

I walked straight intoi that one, didn't I?

Yes, of course, if one has a 'directional' load such as a louspeaker, one can get them to perform 'out-of-phase' if one reverses the connections of one of them, even when (as in what I was talking about) they are in series across a supply so that exactly the same current is passing through both.

However, what you've just described (getting two louspeaker diaphrams to move out of phase) can be equally achieved by connecting them back-to-back across a 1-phase 2-wire supply - and I presume that even you would not then call it a 2-phase supply!!

Kind Regards, John.

 

[Tech99]

Consider first two equal resistive loads in series across the whole of the transformer secondary, with no connection to the centre tap of the transformer. It is apparent that exactly the same current is flowing through both of the loads, so the currents through them are clearly 'in phase'.

Posted the above before getting to your message, but that's the point which it sounds as though you've got now.

If those loads were loudspeakers, your camera would show the diaphragms moving in phase.

As above, it depends entirely upon which way round the speakers are connected.

Supply ----- (+) Spkr-1 (-) -------- (+) Spkr-2 (-) ----------- Supply

will result in the speakers moving in phase.

Supply ----- (+) Spkr-1 (-) -------- (-) Spkr-2 (+) ----------- Supply

will result in the speaker moving in anti-phase.

But in both cases the current through both speakers will be the same, and the current through spkr 1 will be in phase with the current through spkr 2. With a simple series connection, how could it be anything else?

And you'd get those same results by connecting the two speakers in those different ways without involving any sort of mid point connection.

As I now see it, and again assuming resistive loads, the common point where the two loads are connected will have a potential (relative to anything you choose) equal in magnitude and phase to that at the centre tap of the transformer. Hence, if you join that common point to the centre tap, no current will flow through the connection and hence absolutely nothing will change - the currents through the two loads will still be 'in phase'.

Again, I hadn't got to this before posting the above. But that's exactly right.

To put it back into power terms, connect two identical lamps in series across the ends of the supply transformer. The current in each lamp must be the same, and the currents through each lamp must be in phase, agreed?

Put that neutral connection in now between the centre tap of the xfmr and the mid point of the lamps and, with the load balanced, absolutely nothing changes because there will be no neutral current. The currents through each lamp remain the same, and they remain in phase, just as they were without the third wire in place.

Change one of the lamps for a larger one, and the instantaneous direction of the current at any moment in time doesn't change, nor does the phase relationship between the currents in the two lamps. All that changes is the magnitude of the currents, i.e. I1 = I2+N or I2=I1+N, depending upon which lamp is drawing the larger current.

That's how I now see it, anyway - have I convinced myself of something which is wrong?

No, you've convinced yourself that the over-a-century old reference to a 3-wire single phase system is correct.

 

[ban-all-sheds]

However, what you've just described (getting two louspeaker diaphrams to move out of phase) can be equally achieved by connecting them back-to-back across a 1-phase 2-wire supply - and I presume that even you would not then call it a 2-phase supply!!

No I wouldn't, but as we established earlier on, the nature of the load does not affect the supply, it only has a bearing on how you connect it to the supply.

 

[ban-all-sheds]

Supply ----- (+) Spkr-1 (-) -------- (-) Spkr-2 (+) ----------- Supply

will result in the speaker moving in anti-phase.

But in both cases the current through both speakers will be the same, and the current through spkr 1 will be in phase with the current through spkr 2. With a simple series connection, how could it be anything else?

If the currents are going in the same direction then why are the diaphragms moving in opposite directions?

You still keep failing to look at the two voltages, and two currents, with respect to the common point, i.e. the mid point.

And at any point in time the currents flowing in each load with respect to the mid point are out of phase.

To put it back into power terms, connect two identical lamps in series across the ends of the supply transformer. The current in each lamp must be the same, and the currents through each lamp must be in phase, agreed?

Yes.

Put that neutral connection in now between the centre tap of the xfmr and the mid point of the lamps and, with the load balanced, absolutely nothing changes because there will be no neutral current. The currents through each lamp remain the same, and they remain in phase, just as they were without the third wire in place.

No, because you are observing the currents flowing in the lamps with respect to the mid point, not with respect to the same end of the output winding of the transformer.

Change one of the lamps for a larger one, and the instantaneous direction of the current at any moment in time doesn't change, nor does the phase relationship between the currents in the two lamps. All that changes is the magnitude of the currents, i.e. I1 = I2+N or I2=I1+N, depending upon which lamp is drawing the larger current.

Yes, but when measuring the currents with respect to the mid point then they will be out of phase.

No, you've convinced yourself that the over-a-century old reference to a 3-wire single phase system is correct.

Do you and JohnW2 claim to be greater electrical engineering experts than the IET?

 

[ban-all-sheds]

I'd like to know what the supposed reason is. And yes, why shouldn't they make mistakes? If you're saying that this sudden decision is the correct one, I could ask whether you think that thousands of electrical engineers over the last hundred and something-odd years have all been making mistakes,

No, because it's a phrase used to describe a particular arrangement. The arrangement has not changed, but the name we give it has.

and whether the thousands today who will continue to refer to this arrangement as single phase are wrong.

By definition they will be, just as if they started calling it Norman.

It's a name change, and the point I have been trying to make is that the new name is not without merit. Maybe for completeness the name should include the phase angle.

Remember that the 3-wire single phase arrangement is called split phase. But that doesn't create a two phase system.

Yes it does. It creates what is now officially defined as a 2-phase 3-wire system.

Rather than your wood analogy I'd suggest something like a motor. You could have a simple induction motor (say a shaded pole type) with a single winding, which is clearly a single phase motor. If you split the winding into two halves for manufacturing convenience and wire those two halves in series, you haven't created a two phase motor, have you?

No, but as we have already established, the characteristics of the load do not affect those of the supply, only whether, and how, the load can be connected to that supply.

You can make a 2-phase motor which works on a 4-wire 2-phase system with a 90° angle.

No. You are only seeing anti-phase voltages because you're using a mid point as your reference.

That's the whole point.

It doesn't change the direction of the current in any way, or introduce a current which is out of phase with any other current.

It does change the direction of one of them with respect to the mid point.

The mid point is all that is different from a single-phase 2-wire system, so it makes perfect sense to recognise it's existence.

But connect -ve of one speaker to +ve of the other to use as the neutral, then the cones will move forward and backward in phase. The current though each speaker would be in phase in both cases, it's just a matter of which way the resultant magnetic field at any given instant causes the speaker to be driven.

But then you are not referencing the current with respect to the mid-point.

 

[JohnW2]

No I wouldn't, but as we established earlier on, the nature of the load does not affect the supply, it only has a bearing on how you connect it to the supply.

Exactly - but you were trying to use your 'out-of-phase' loudspeaker diaphragms to support the postion that what some people call a 1-phase (or split phase) supply is actually a 2-phase one. That argument wouldn't work if you reversed the connections to one of the loads (speakers) - and, by your own admission above, the nature of a load (which I presume includes which way around it is connected) doesn't affect the nature of the supply.

Kind Regards, John.

 

[JohnW2]

That's how I now see it, anyway - have I convinced myself of something which is wrong?

No, you've convinced yourself that the over-a-century old reference to a 3-wire single phase system is correct.

Well, as we seem agreed, I think that I've now got my mind straight about the currents. What is, and is not, ideal terminology (regardless of history) is perhaps a different matter. In particular, if one used such a three wire supply by distributing the CT (as N) and one of the L's to various circuits in an installation, with unbalanced loads, I think it might perhaps be more logical (if one were inventing the terminoogy from scratch) to call that 2-phase.

However, what I do agree, and always have (although maybe I haven't said it) that to change terminology such as this after it's been established for decades is, at best, meddlesome.

Kind Regards, John.