AC Efficiency: Fact or Fiction?

[anorlunda]

Switch over to the split phase system and you add only one wire to double the power available.

I agree, but he misspoke. He said the same number of wires as single-phase, but as you point out we must add a third wire. A three-phase system delivers 3x power with 3 wires. The split system delivers 2x power with 3 wires.

 

[russ_watters]

I've run across clothes dryers with 240 volt heating element and 120 volt motor.

Thanks, that's what I was guessing -- the chart on the wiki page lists clothes driers as having 4-prong plugs (2 hot, neutral, ground). I figured it was so they could provide both 120 and 240V.

 

[russ_watters]

I agree, but he misspoke. He said the same number of wires as single-phase, but as you point out we must add a third wire. A three-phase system delivers 3x power with 3 wires. The split system delivers 2x power with 3 wires.

No, averagesupernova was not correct about what I meant (in fairness, the first writing of the post was not very clear) and no, you don't have to ("must") add a 3rd wire to get 240V. Indeed: you only "must" have a 3rd wire if you want to supply two voltages instead of just the 240V.

 

[jim hardy]

Thanks, that's what I was guessing -- the chart on the wiki page lists clothes driers as having 4-prong plugs (2 hot, neutral, ground).

that's a relatively new code requirement for buildings, so they can accommodate such dryers.
It's handy of you're using the dryer outlet with a big extension cord for a welder or aircompressor - you can get both voltages where you're working,.
One should have a GFCI in such an extension cord
One should never use the green wire to return normal operating current.

 

[Averagesupernova]

Ok this has gotten pretty nutty. About as basic stuff as I can think of and people who I have had a fair amount of confidence in are in some kind of disagreement/misunderstanding or whatever. :(

 

[sophiecentaur]

Note that in the US, residential 240V service is "split phase", with two hot wires at 180 out of phase with each other. This is an even more efficient use of wires as you've doubled the power you can deliver with the same number of wires (vs normal single phase).

This is pretty obvious to me but one has to compare apples with apples. You have doubled the voltage so the available power is double, for the same current. The centre tap doesn't have to play any part in that arrangement and I think it's a red herring. The US system really does seem to bring in another layer of difficulty for people to get their heads round, with a range of strange conclusions about the consequences of such a system.
But I think there could be a similar 'misdirection' in claiming an advantage for three phase and single phase - because the effective volts are different for the two systems. The voltage limit is less obvious than the Current situation because the relevant I is RMS but the relevant Voltage is Peak. I realize that insulation can be a lot cheaper than copper to install.
I will now read those helpful links and try to get myself sorted.

@Old Jim: Thanks for your description. It is getting me there! I will search for a suitable version of the argument with some diagrams and Maths.

 

[sophiecentaur]

Ok this has gotten pretty nutty.

It could appear so but I think it is such a multi-faceted business, with some facets carrying more weight than others, that it's not surprising such a wide open thread title has led us all over the county.

 

[Averagesupernova]

This is pretty obvious to me but one has to compare apples with apples. You have doubled the voltage so the available power is double, for the same current. The centre tap doesn't have to play any part in that arrangement and I think it's a red herring. The US system really does seem to bring in another layer of difficulty for people to get their heads round, with a range of strange conclusions about the consequences of such a system.
But I think there could be a similar 'misdirection' in claiming an advantage for three phase and single phase - because the effective volts are different for the two systems. The voltage limit is less obvious than the Current situation because the relevant I is RMS but the relevant Voltage is Peak. I realize that insulation can be a lot cheaper than copper to install.
I will now read those helpful links and try to get myself sorted.

Huh? Voltage is spec'd as peak in 3 phase but not single or split-phase? Since when?
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Without the center tap we are no longer maintaining the original voltage as we do in split-phase. It is quite relevant. Also, in a 3 phase wye system there is a center tap and I think it is quite relevant as well.

 

[sophiecentaur]

Huh? Voltage is spec'd as peak in 3 phase but not single or split-phase? Since when?

It is the peak voltage that determines whether or not the insulation will fail. That's what I meant. Insulation for an AC 120V system need not be as good as for an AC 240V (or the split phase that the US use) system. That is a hidden cost for the 'advantage' of doubling the Power capacity in this case.
The three phase 'advantage' is harder to calculate and I am scouring the info that's been given on this thread. Bare assertions won't help my understanding of this.

 

[russ_watters]

Ok this has gotten pretty nutty. About as basic stuff as I can think of and people who I have had a fair amount of confidence in are in some kind of disagreement/misunderstanding or whatever. :(

If you haven't re-read my post #55 since I edited it, please do. It should clarify this. Perhaps people are used to doing split phase 240V with 3 wires (plus ground) and aren't aware it can be done with 2, just like 208V single phase is done with 2 legs of a 3 phase circuit, with no neutral. Otherwise, I think we're probably just talking past each other.

The basic issue we are discussing is whether a 240V split phase system must have a neutral. It can have a neutral if you also want it to provide 120V power, but it doesn't need to have a neutral. Just like if you are using 2 phases of a 208V three-phase circuit to get 208V single phase.

 

[russ_watters]

This is pretty obvious to me but one has to compare apples with apples. You have doubled the voltage so the available power is double, for the same current.

Kind of. You've doubled the voltage because you have two "hot" wires out of phase with each other -- you haven't changed anything about the phases themselves (they are each still 120V vs ground).

Insulation for an AC 120V system need not be as good as for an AC 240V (or the split phase that the US use) system.

Not true. Per the above, both 120V single phase and 240V split phase are 120V vs ground and therefore need exactly the same insulation.

But I think there could be a similar 'misdirection' in claiming an advantage for three phase and single phase - because the effective volts are different for the two systems. The voltage limit is less obvious than the Current situation because the relevant I is RMS but the relevant Voltage is Peak...
[separate post]
...It is the peak voltage that determines whether or not the insulation will fail. That's what I meant.

You got yourself chasing a red herring and you're still on it. In both cases, we're considering the RMS voltage and we're considering it to be the same. There's nothing "hidden" here: for the same voltage (peak or RMS), you need less "wire" with 3-phase to carry the same power. The √3 factor used when calculating three-phase power has nothing to do with RMS voltage. It's there because of the phasing of the waveforms (120 degrees out of phase with each other).

 

[jim hardy]

I will search for a suitable version of the argument with some diagrams and Maths.

lots of them about

http://ece.k-state.edu/~starret/581/3phase.html

 

[Averagesupernova]

The whole 240 volts thing without the neutral is just adding more confusion to the thread. If I am not mistaken, we are talking about adding the most power for the least cost in additional wire while maintaining the same voltage into the devices that are the load. You cannot do this unless we keep the neutral which means we go to three current carrying wires. While it is true that 240 volt devices such as a residential water heater do not need a neutral and only require 2 current carrying conductors the rules of the game have changed since we raised the voltage in that particular load.

 

[nsaspook]

The whole 240 volts thing without the neutral is just adding more confusion to the thread.

Reminds me of this thread. https://www.physicsforums.com/threads/total-amperage-in-a-service-panel.705961/

 

[sophiecentaur]

I now understand those formulae about three phase currents and voltages. But I still don't see why people are making out that there's a magic 'profit' to be made by using three phase, because the amount of current is reduced. Using three phases requires higher voltage specification, afaics. So the power capacity of a single phase system could also be increased by increasing its running voltage. The system voltages are chosen, in the same way that the currents are matched to the wire gauges used. I remember my old Dad pointing out that you had to be much more careful in places where there were more than one phases supplied. In the UK we had / have just a 240V supply to most dwellings and offices. It is not easy to find yourself with two appliances in the same house, operating on different phases. The safety practices are based on that fact. In premises with three phase supplies, the whole system is specified differently, with expensive, different connectors and switch panels - because of the higher voltages.
You can always get more capacity by increasing the volts but this costs real money.
The choices that have been made are, presumably, optimal but copper is not the only factor involved.

 

[Averagesupernova]

I now understand those formulae about three phase currents and voltages. But I still don't see why people are making out that there's a magic 'profit' to be made by using three phase, because the amount of current is reduced. Using three phases requires higher voltage specification, afaics. So the power capacity of a single phase system could also be increased by increasing its running voltage. The system voltages are chosen, in the same way that the currents are matched to the wire gauges used. I remember my old Dad pointing out that you had to be much more careful in places where there were more than one phases supplied. In the UK we had / have just a 240V supply to most dwellings and offices. It is not easy to find yourself with two appliances in the same house, operating on different phases. The safety practices are based on that fact. In premises with three phase supplies, the whole system is specified differently, with expensive, different connectors and switch panels - because of the higher voltages.
You can always get more capacity by increasing the volts but this costs real money.
The choices that have been made are, presumably, optimal but copper is not the only factor involved.

The highest voltage between 240 volt split-phase or single phase conductors is no less than it is for the conductors in 240 volt 3-phase delta for instance. The wire used is the same concerning voltage rating. Been there, done that, passed inspection. A single phase 240 volt motor rated at 10 horsepower we will say for the sake of argument uses about 10000 watts. When estimating power consumption it is not unusual to estimate 1000 watts per horsepower. So in this hypothetical case the motor draws about 42 amps. #6 copper is the minimum size to be run for this load. A 3 phase motor will still be estimated at 1000 watts per horsepower but we compute the current draw differently for 3 phase so the current draw will be less. It may sound like it is not a big deal but any time we are able to reduce wire size we take advantage of it. It is not just a concern of the wire safely carrying the current but reducing the loss in a long length of wire.
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Now concerning non-motor loads utilizing a 3 phase source. The split phase system in the US with balanced currents eliminates current in the neutral wire. This is a savings in that there is no loss in the neutral conductor. In the US it is permitted to size the neutral one size smaller than the hot conductors in residential service. This is a savings. It is no different in a 3-phase system. If all 3 phases are loaded similarly then the neutral current is very low if not zero. Do this with 3 separate sets of feeders for 3 different loads which means a total of 6 current carrying conductors and you have just doubled the power loss in the conductors. 3 phase makes sense no matter how you look at it. The wire insulation is the same, the conduits are often the same since the wire size can shrink, motors are more inexpensive, etc. You will get more power through a 3 phase load center panel than you will a single or split-phase panel rated at the same voltage and current so the extra material required will not cost as much as it first appears when you figure cost per watt.

 

[jim hardy]

Using three phases requires higher voltage specification, afaics...
...but copper is not the only factor involved.

One can have 240 or 120 3 phase let's take a look

three 240 volt 15 amp single phase circuits could move 240 X 15 X 3 = 10,800 VA over six conductors = 1800 VA per conductor

a single three phase circuit could move 240 X 15 X √3 = 6235 VA over three conductors = 2078 VA per conductor,

half as many conductors carrying 6235/10800 = 58% the power seems modest gain indeed

where three phase really shines is in rotating machinery
motors need no starting capacitor and there's no pulsating torque

a 6235 VA 240 volt motor would need three wires sized for 15 amps
or two wires sized for 26 amps and a starting switch
and a start winding that's idle 99.9% of the timeold jim

 

[sophiecentaur]

The highest voltage between 240 volt split-phase or single phase conductors is no less than it is for the conductors in 240 volt 3-phase delta for instance.

That comment is not really relevant to my case. So many of the arguments on this subject seem to hang on whichever system the proponent happens to have in their home. The UK system has to have a Star arrangement because everyone down a street gets the neutral and just one of the (240V) phases. You couldn't do it with a delta system. The Volts between any two of the phases is 415V, which can be consider significantly more 'dangerous, compared with the standard 240V supply. This bears no comparison with a split phase 240V system, where there is never more than 240V between conductors (even for connections between neighbouring premises). But the US domestic split phase system is a complete red herring in the context of the '3Phase current' discussion afaics. It is such a totally different arrangement.
When considering Power capacity of a 3 phase star system, the voltage between the phases is, imo, the relevant voltage to be considering when deciding how the power is being carried and not the 'phase Voltage. Any transmission system design has to consider that PD, surely (insulation and separation of conductors). 70% extra is a very relevant amount of Volts. Perhaps my point is really that the '240V' figure is not the relevant one but that, comparing system with system, one should really be talking 415V. (This is the Voltage value that's printed in the warning sign on the front of 3 Phase panels in the UK). If you take that inter-phase voltage, then the advantage of using smaller cables is explained simply in terms of the V that's used in the VA.
The calculations and diagrams that show how the 'return' current is taken care of by the other two phases are fine but they do show that current flows due to an 'extra' net voltage that's additional to the volts of the phase in question. I think my problem is that I am trying to explain it to myself more or less from a fresh perspective and I don't actually see a conflict with my view and 'the rest of you', who got taught it conventionally.
Interestingly, I came across this page which has explained a lot to me about the WYE and DELTA systems and also sneaks in a comment about the different number of windings for the two systems. IN the caption ro Fig 2 it says "(Note: In a Wye transformer, line-to-line voltage encompasses two phases that are electrically 120 degrees apart.)" and this seems to support what I have been droning on about- i.e. the relevant voltage is, in fact higher than is acknowledged in this thread.
I believe that "you don't get owt for nowt" (ancient Yorkshire saying) and I have been desperate to explain away this case where it is claimed that you can. It seems that it may just be the way that it's being looked at. (I am not arguing that the systems are not engineered properly, btw, they are obviously getting it right, in practice)

 

[Averagesupernova]

But the US domestic split phase system is a complete red herring in the context of the '3Phase current' discussion afaics. It is such a totally different arrangement.

I disagree. And as someone who lives in the US and is very familiar with the split-phase system I feel that my opinion that there are enough similarities between the systems to warrant its inclusion at this point in this thread should carry some weight.
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You say:

But I still don't see why people are making out that there's a magic 'profit' to be made by using three phase, because the amount of current is reduced. Using three phases requires higher voltage specification, afaics.

Then I say:

The highest voltage between 240 volt split-phase or single phase conductors is no less than it is for the conductors in 240 volt 3-phase delta for instance.

Then you say:

That comment is not really relevant to my case.

Really? How much more contradictory can you get? It doesn't matter what the arrangement is. The voltage is what it is.

I believe that "you don't get owt for nowt" (ancient Yorkshire saying) and I have been desperate to explain away this case where it is claimed that you can.

So far, I would say you have not succeeded. Per my post #76:

It is no different in a 3-phase system. If all 3 phases are loaded similarly then the neutral current is very low if not zero. Do this with 3 separate sets of feeders for 3 different loads which means a total of 6 current carrying conductors and you have just doubled the power loss in the conductors.

Does this fit not getting owt for nowt?
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Edit: For the record I am not in complete agreement about what they call legs and what they call phases in your link sophie.

 

[jim hardy]

nice link there sophiePublic Service of Colorado(central US) will no longer install figure 1, though it used to be common with phase B known colloquially as "Wild Leg"

fig 2 is what I'm accustomed to for small commercial installations
208 volt 3 phase HVAC and elevator motors, and 120 volt lighting/, all from one service drop

 

[russ_watters]

The whole 240 volts thing without the neutral is just adding more confusion to the thread.

Unfortunately yes. I thought it would help - it did not. I didn't expect so much resistance to what is a fairly basic (if quirky) concept in electricity, particularly from people who I know understand the issue. It certainly is bizarre.

If I am not mistaken, we are talking about adding the most power for the least cost in additional wire while maintaining the same voltage into the devices that are the load.

Constraints were never really specified and that's a lot of the problem here. Yes, we were discussing why three-phase circuits can carry more power with less "wire" than single phase circuits. When I brought-in split-phase, it added a new wrinkle that confused things: 240V single phase and 240V split (single) phase are not the same and as a result, I changed the constraint.* Perhaps people picked-up on it (or tripped over that), but several of the responses I got were wrong and continue to be wrong:

You cannot do this unless we keep the neutral which means we go to three current carrying wires.

That's wrong. If you want to compare 240V split phase and 240V single phase, both need two wires. 240V single phase uses a hot and a neutral and 240V split phase uses two hots. The third wire is only needed for a dual-voltage circuit. No one, that I've seen here, has specified they want to discuss dual-voltage circuits, so I need us to be absolutely clear on this, so I'll say it again: both 240V single phase and 240V split (single) phase can be done with two wires. I even provided an example receptacle, with description that explicitly states this.

While it is true that 240 volt devices such as a residential water heater do not need a neutral and only require 2 current carrying conductors the rules of the game have changed since we raised the voltage in that particular load.

Raised it from what? You seem to be arguing opposite sides of the same point at the same time. The fact that 240V split phase can be done with two wires is true regardless of anything else we are discussing.

*And the reason why I changed it is that I'm more interested in real-world situations than hypotheticals. In the real world you may have complete control over what you can do (in new construction) or you may have limited control based on a pre-existing system. In a real world system that already exists, 240V split phase and 240V single phase are not equivalent and equally available choices. In a real-world situation, you have one or the other, but pretty much never both. Heck, you almost never have 240V single phase as an option at all. The nearest typical is 480/277V: the 480V can be single (split) or three phase and the 277V is single phase to ground (or neutral). For low voltage in commercial or large residential systems, you have 208V single (split)/three phase with 120V single phase. In small residential, you have the split phase I brought-up. In both of the low voltage situations, the higher voltage is a phase-to-phase split and is available from the same panel/transformer as the 120V single phase. That's why I compared them as otherwise equals. If that change confused things, I apologize.

But the switching of voltage constraints wasn't the initial objection, the number of wires was. Ironically:

Reminds me of this thread. https://www.physicsforums.com/threads/total-amperage-in-a-service-panel.705961/

Yeah. That's the thread/diagram anorlunda cited in post #53 and in post #5 of that thread, Nugatory pointed out the same issue I did:

The diagram also has a small mistake in the 240v receptacle - that's supposed to be a green grounding wire, not a white one, out of the ground plug. The white wire would only be present if this were a receptacle for a combined 120/240 appliance, and then would be four prongs and four wires: hot 1, hot 2, white grounded and green grounding.

I'll be more charitable and call it a truncation or oversimplification: If they are going to show the ground wire on the 120V circuit, they should show the ground (green) wire on the 240V circuit. So either one of those wires is the wrong color or they glossed-over the fact that that isn't just a 240V circuit, it is a dual-voltage 120/240V circuit.

 

[Averagesupernova]

I'll get back to you russ. I can now see why you think I am being contradictory. There are some things I need to clarify but don't have the time at the moment. Already took too much time. LOL

 

[russ_watters]

I now understand those formulae about three phase currents and voltages. But I still don't see why people are making out that there's a magic 'profit' to be made by using three phase, because the amount of current is reduced.

Generally, the current drives the wire size, not the voltage. So you save on copper in the wires: less current means you can run smaller wires.

Using three phases requires higher voltage specification, afaics.

Actually, no. If anything it would require less because while the phase to phase voltages can be made equal (in the calculation example I gave), the phase to ground is lower.

I run into situations at work occasionally where I have to work with the electrical department to pick the specs of a device such as an electric heater or motor. The cost may not be much different for different voltages/phasings. I might have 120V and 208V single phase and 208V three phase available from the same transformer (very common). The one requiring the least amount of wire is 208V three phase.

 

[russ_watters]

I'll get back to you russ. I can now see why you think I am being contradictory. There are some things I need to clarify but don't have the time at the moment. Already took too much time. LOL

Understood -- I'm sure we're just talking past each other and can see why my changing constraints in the middle of the discussion confused things.

I recognize that you know what you are talking about. And frankly, this all came from trying to help Sophiecentaur, and since we haven't yet, that should be the main focus!

 

[Salvador]

i wonder why all the fuss , after all , all this phase this phase that talk is simply a matter of how one wires the secondary of the last step down transformer in the line to residental buildings and/or factories.
I think I totally get the US split phase system , the secondary of this system is basically wired like almoust all audio power amplifier transformer secondaries , where you have one secondary winding with a given voltage value but you simply have an extra pinout made at the exact middle of this winding, so if the winding itself is wound to be 240 volts then connecting at each side and the middle you get half of that while when connecting across the wholw winding you get the whole voltage across it.
and it needs only two wires I don't see why would it need an extra third wire , after all its only a single phase winding , after all a single winding can only be single phase since it hs only one wire aka two wires coming out at each end.simply connect the load across those wires and voula, you get the full winding voltage.
I do realize that its not just a single secondary winding even in the US split phase, its a three phase coming in but for simplicity I analyzed only one ot the three secondary (main 240 V) windings from which each has a center tap and each supplies two split phase 120v outputs.

I personally don't see why this split phase arrangement would be more efficient ? after all you have the same single phase winding with basically the same " european" 240 V across it , the only difference is that instead of simply using that whole winding with its whole 240 V you split it in half and then use one half and the other half , but in theory I think this means that there can be situations were one half od the winding is more loaded than the other and doesn't this cause unequal loading in the winding whic then causes unequal heating in the coil and currets and whatnot?
But anyway I think using the whole 240v across the winding is more efficient than using eac half of the winding , because due to lower voltage from that each half you have more loss in the circuits attached, also you need thicker wire for 120v to get the same power than for 240.As for the three phase system argument , having higher voltage hence need for better insulation , well pardon me if I say something wrong here as I'm no 50 years experienced expert but I tend to agree with sophie on this as I have heard and read many times that three phase has higher overall voltage.Yes each pahse is still only 220 or 230 or whatever the value (differs from country to country) but as I read on the internet (please correct me if this is wrong) the vectors from the phases add up and the total sum is larger than the individual 240 volts from a single phase.

also while on this , I guess every single secondary winding single phase system has both wires hot with no neutral, the only times we get the so neutral is when as in the US system they have the center tap into the single secondary coil , or in other places in the world like in all of eastern Europe and Russia and probably many other places in europe , you get three phase power as the lowest power coming from the last step down transformer along the line , but in the buildings these three phases are split evenly , and then they use for example phase 1 and neutral, phase 2 and neutral and phase 3 and neutral, so the neutral is the same in all building but the phases are not.
for many industrial applications the three phase system doesn't have that neutral.

 

[jim hardy]

i think the higher voltage question becomes moot
i don't rightly recall seeing any wire rated less than 600 volts
and in the plant we tested insulation at twice rated plus a thousand voltsjust looked at a piece of NM-B Romex bought at local lumberyard for household wiring
it's stamped12AWG 600 V and wrapper says 600 volt

 

[sophiecentaur]

I disagree. And as someone who lives in the US and is very familiar with the split-phase system I feel that my opinion that there are enough similarities between the systems to warrant its inclusion at this point in this thread should carry some weight.

Split phase system (two extremes):
1. Totally balanced = 240V into a 1kW load (= 4.166A)
2. Totally unbalanced = 120V into 1kW load ( = 8.33A)
Doubling the volts uses half the current. I really can't see that's very relevant. I'm sure you know and love the split phase system - as do most US consumers but that doesn't make it a good model for proving the relationships in the 3 Phase system. I must say whilst we are on the split phase topic, I have read more confused statements about it on PF than I have about the much simpler single phase with neutral system, used in Europe. Perhaps the statistics are biased there, though (not a proper controlled test)

Are you familiar with the UK 3 phase distribution system? There is 415V between the phases of the '240V' system. "My case" was the WYE system and you were quoting the delta system. That's why I said your comment was not relevant.

It doesn't matter what the arrangement is. The voltage is what it is.

Which voltage, though? 'Phase voltage' or 'inter-phase voltage'? This is my point and they are not always the same.

 

[sophiecentaur]

Do this with 3 separate sets of feeders for 3 different loads which means a total of 6 current carrying conductors and you have just doubled the power loss in the conductors. 3 phase makes sense no matter how you look at it. The wire insulation is the same

That is the nearest thing to a good argument - apart from the last bit. There is 415V voltage difference between pairs of 240V phases in the same piece of equipment. Whether or not that is as relevant as I think it is, may be arguable but it has to mean that the insulation needs to be thicker.

 

[sophiecentaur]

nice link there sophie

I nearly spilled my Christmas drinks when I saw the "Wild Leg" system. You'd need to be very careful to make sure no one else was trying to connect another Wild Leg to the system. Talk about the Wild West! There seem to be more different standards over there than I've had hot dinners. All we have is 240V single phase or 415V 3 phase and everything we have will use one or the other - except for railway traction (afaik).

 

[sophiecentaur]

this all came from trying to help Sophiecentaur, and since we haven't yet, that should be the main focus!

Thanks for your indulgence. I am getting there but, on the way, I have turned up new issues which seem to be unresolved in the minds of many (non expert) people. I think I will just have to turn it over in my head a bit more and let you guys get on with your lives. I guess that, in my worry about getting something for nothing, I had forgotten that efficiency can often be improved. You can often get something from being smart.