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Old 11-11-2012, 03:51 AM   #21
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In addition to some of the things already mentioned, using two single pole breakers for 240V circuit is a dangerous situation (and quite likely a Code violation) unless the handles are tied for common trip (essentially making it a 2-pole breaker). If only one leg tripped out using separate breakers the circuit could/would still be hot through the other leg.

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Old 11-11-2012, 12:59 PM   #22
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FWIW, 240 isn't two phase power. It's just single phase power, typically with a center-tapped secondary on the transformer. Using the full winding of the transformer gives you 240V, using the center tap to either end gives you the two 120V legs.
Technically it is. It is just that the two phases are 180 ° apart as opposed to some other angle such as the (approximately) 90° phase shift between the phases in a 2 phase motor that uses a running capacitor. It it often referred to as bi-phase, as opposed to three phase or polyphase and sometimes as split phase to recognize that it can be obtained from a center tapped winding as explained in #8 with the >E 'diagram'. If you ask an electrician what the black, red and white wires connected to a 240 V 4-wire outlet are he will answer 'phase, phase and neutral'.
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Old 11-11-2012, 01:09 PM   #23
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In addition to some of the things already mentioned...
This was mentioned explicitly in #16 and alluded to in #8. But as the potential hazard is great enough it is probably worth mentioning again. Another aspect of it is that it would be impossible to use GFCI breakers in the box though one could use a GFCI outlet (in a single outlet arrangement - not the originally proposed dual outlet scheme).
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Old 11-11-2012, 01:15 PM   #24
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Technically it is.
No, it is not two phase power. It is a single, split phase. Just because lots of people call the legs "phases" doesn't make the correct. Almost everyone refers to the neutral wire in a 120V circuit too, but technically it's the grounded wire. There is no neutral wire in a 120V circuit yet that's the parlance.

L1 and L2 being 180 out of phase is merely a matter of the reference point. Using the full winding of the transformer to obtain 240V is decidedly single phase with no relevance to 180 in or out of phase.

The system is 240V, center tapped, which facilitates also providing two 120V legs; not two 120V legs that can also provide 240V.
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Old 11-11-2012, 03:40 PM   #25
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No, it is not two phase power.
Yes, actually it is. Any system with 3 or more wires in which two or more (called phases) are at potentials with respect to one designated as the 'neutral' which peak at different times is a polyphase system. If there were a total of 4 wires and their voltages with respect to the neutral peaked 1/3 of a cycle apart I think you'd agree that we have a 3 phase system. Now suppose there are 2 wires and the voltage on one phase wire peaks 1/4 cycle later than the voltage on the other. This is a biphase system with 90 ° shift. Power used to be generated and distributed this way. Each phase was developed by a coil on the stator of a generator and these were at right angles. If I took a notional generator in which the coils could be moved to any position and shifted the one at right angles to 120 ° we'd have biphase with a 120 ° phase relationship. Thus is exactly what is done in 3-phase generation except that an third coil at 240 ° is also added and that is connected to the 3rd phase wire (we're ignoring number of poles considerations here). If I moved the 2nd coil to 179 ° we'd have a biphase system with a 179 ° relationship and if we moved it to 180 ° then we'd have a biphase system with 180 ° phase shift.

As my generator is a bit impractical and I'd have to build it I'd probably just buy a three phase generator, connect one end of one stator coil to neutral, connect the other end to a red wire, then take the coils from the other two phases and connect them in series with one end of the series chain connected to neutral and the other to a black wire. This would give me biphase with a 180° phase relationship. I actually worked briefly at facility where someone installed a trailer wired for 120 biphase 60 Hz and the facility had 408 three phase 50 Hz. They got the 120V biphase they needed by wiring the generator on an MG set exactly as I described.

Simpler than generators or MG sets is to center tap the coil of a secondary of a transformer. You still get a neutral and 2 phases and its still biphase with a 180° relationship. The fact that you can get it by center tapping a single winding doesn't somehow remove the biphase properties.

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Originally Posted by whoaru99 View Post
It is a single, split phase.
You can get it by 'splitting' a phase but you can also get it by combining 3 phases or by combining 2 as with my hypothetical generator or by wiring two inverters to a common (neutral). In your house you may know that it came from splitting a phase but at my overseas site it didn't and if you are in an industrial setting or large apartment building it probably comes from 2 out of 3 phases of a Y connected 3 phase distribution with the neutral being the neutral of the 3 phase system (in which case you would have two 120 V circuits and/or one 208 V circuit).

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Just because lots of people call the legs "phases" doesn't make the correct.
What makes it correct is wide acceptance by engineers who work with this stuff. And it's probably in the IEEE dictionary because I know I didn't invent it.

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Almost everyone refers to the neutral wire in a 120V circuit too, but technically it's the grounded wire. There is no neutral wire in a 120V circuit yet that's the parlance.
The NEC calls the white wire the 'grounded conductor' and the bare or green wire the 'grounding conductor'. Little wonder that most of us prefer to refer to the neutral as the neutral.

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L1 and L2 being 180 out of phase is merely a matter of the reference point. Using the full winding of the transformer to obtain 240V is decidedly single phase with no relevance to 180 in or out of phase.
Yes that's so and once that reference is available you have created a polyphase system as I explained above at some length above.

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The system is 240V, center tapped, which facilitates also providing two 120V legs;
As I noted above I have worked in several places (including here in the US) where it is not 240V center tapped. To assume that it always is (though that may almost universally be the case for home brewers in the US - I have been to one country where I noticed that the homes are wired 3-phase) is not only incorrect but obscures the fact that a split phase system is indeed a polyphase (biphase) system even in this case.


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Originally Posted by whoaru99 View Post
...not two 120V legs that can also provide 240V.
But that's exactly what they are that's exactly how they are used in house wiring! There are two windings on the transformer on the pole that are connected in series to provide 240 and are used separately to provide 120 circuits relative to the point where those windings interconnect. The two windings may be made by spooling on half the turns, bringing out a loop and then winding the other half of the turns but they are in fact two series connected windings with their common point available.

Perhaps it's all just semantics. If I point out a dog to you and you say no, it's not a dog, it's a Leonberger you are right to some extent but the fact of it being a Leonberger does not mean it is not a dog.
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Old 11-11-2012, 03:49 PM   #26
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No. It is not two phase and those two legs are not 180 deg out of phase.

Perhaps the transformer drawing I marked up (showing just a half cycle to keep it less cluttered) will clarify it. It's clear the two 120V legs are in phase. They have to be because it can't go both ways simultaneously as would be the case in the lower illustration. If that even worked, the neutral would see the sum of the currents not the difference which is further evidence to support the single, split phase illustrated at the top, not the 180 degrees out of phase myth.

It's all about the transformer windings in series, nothing about phase in this case.

If you want to run that 2-phase thing higher up the flag pole I suggest trying it on a dedicated electrical forum such as forums.mikeholt.com.

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Old 11-11-2012, 04:29 PM   #27
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I'm not your EE prof. but what your drawing is missing is the little dots that go at one end or the other of the coils. If you wind two separate windings on a core in the same direction the start of the windings would be unmarked on the drawing and the end of the windings would be marked with a dot (or the dots could go at the start ends - it's arbitrary as long as it's consistent). A biphase transformer, as found on a power pole, would have the dots at the tops of the two windings i.e. on your top drawing there would be dots at the top of the upper winding and at the top of the lower winding (adjacent to but below where you have the neutral wire). If you separate the place where the winding are connected and put an oscilloscope (triggered from the primary - carefully, it's 13.8 kV) with the common on the start wire and the probe on the finish (dotted) wire you will observe a sine wave in phase with the primary (assuming the dot is at the top of the drawing for the primary winding too.) If you move the scope leads to the bottom winding you will see the same thing (again with the probe at the dotted wire) you would see the same thing. The two windings, considered separately, each produces 120V. At the peak of a cycle they are like 120V batteries. If you join the windings (the dot on one to the unmarked on the other) they are like two batteries connected in series. The voltage from the bottom most wire to the topmost wire is 240 Volts. If, in this case, you connect the common wire of the oscilloscope to the common connection and move the probe to the top wire you will see + 120V (at the peak) i.e. a waveform in phase with the primary. But when you move the probe to the bottom wire you will see -120V at the peak (a waveform that is 180° out of phase with the top coil and the primary. Thus: 3 wires, 2 phases. It's a biphase system.

Your bottom drawing is intended to represent the case where the dot is at the top for the top winding and at the bottom in the bottom winding. It is like connecting two batteries in series with the two negative terminals in common. The potential relative to the connection (reference) of the one battery is +120 and the potential referenced tothe common point of the other battery is +120. The potential difference between the two (non common) terminals is 120 - 120 = 0. The same thing happens if you connect transformer winding in a 'bucking' connection.

As we've just seen, transformer winding in series is all about phase. I mentioned in my last post that you can also get 120/240 biphase from a 3 phase system by connecting 3 transformer windings in series but they must be phased correctly. I brought that up to get you to think in broader terms IOW to understand that a center tapped transformer isn't the only way to get 120/240. This should help you to see that the center tapped transformer is indeed a biphase hookup.

If this is a myth I've surely known hundreds of engineers, not only in power but audio and rf design where exactly the same principles apply, who have been taken in by it and a lot of text books will need to be burned.

[EDIT] Where I say 'peak' here I am neglecting to mention that the actual peak voltages are sqrt(2)*120. As we're confused enough here without bringing in that aspect I though this would best be omitted but in retrospect I think it should at least be mentioned. Thus this edit.

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Old 11-11-2012, 05:42 PM   #28
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Originally Posted by stlbeer
AC current has a frequency. That's the alternating part of AC. If I explain this correctly, 120v AC uses a single phase. It runs at 60Hz. It requires a neutral to complete the connection. With 240v, there are 2 120v lines, but they run in 2 phases which are 180 degrees from each other. There is no neutral. Since they are out of phase from each other, the opposite leg of each is the effective neutral. And because there are 2 legs there of 120v there is 240v.

Wiring 2 120v lines from the same circuit will likely overload the breaker for that circuit, be in the same phase and not produce the power you expect.
Wiring 2 120v from 2 separate circuits has a probability of not working because they might be fed from the same phase and thus not capable of producing 240v.

If you are considering using 240v for your ebrewery, you should really lay the foundation for it correctly and put in 240v for it. That way you know it's correct. Otherwise you'll likely be on here asking us to troubleshoot why your element isn't getting hot.
That is about the best explanation I have ever read.
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Old 11-11-2012, 06:09 PM   #29
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The neutral is the reference point with respect to which phase voltages are measured (where you connect the black wire from the scope). As in a 240V outlet you only have 2 wires either one or the other has to be the neutral. But if I only give you 2 wires in a 120V circuit it is exactly the same thing. You have to connect the black wire somewhere. In the 240V case you can choose either the red or black wire to be the neutral because neither is the system neutral. That's the white wire which you don't find in older clothes dryer installations, for example (but the earth wire is connected to the neutral at the panel). In a 120V circuit you don't have a choice. The neutral is the white wire though in terms of making a measurement you could just as well connect the black wire of the scope to the black wire in the outlet and the probe to the white wire.

So the explanation is a fairly good one except for the 'uses a single phase' part. Each 120V circuit is connected to one or the other of the two antipodal (or approximately antipodal) phases available in a biphase system. Each 240 V circuit is connected across those two phases in series.
[EDIT] But connecting across one of the available phases is using that phase so I guess what I'm 'objecting' to is failure to mention that the other phase is there and that a different 120V circuit may be connected to it.

As phase is so confusing think of two AA batteries connected in series. You can get several voltages from that connection simultaneously
1) + 1.5 V between common point and + terminal on one battery
2) - 1.5 V between common point and - terminal on the other.
3) + 3 V between - terminal one battery and + terminal on the other.

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Old 11-11-2012, 07:48 PM   #30
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Originally Posted by ajdelange View Post
I'm not your EE prof. but what your drawing is missing is the little dots that go at one end or the other of the coils. If you wind two separate windings on a core in the same direction the start of the windings would be unmarked on the drawing and the end of the windings would be marked with a dot (or the dots could go at the start ends - it's arbitrary as long as it's consistent). A biphase transformer, as found on a power pole, would have the dots at the tops of the two windings i.e. on your top drawing there would be dots at the top of the upper winding and at the top of the lower winding (adjacent to but below where you have the neutral wire). If you separate the place where the winding are connected and put an oscilloscope (triggered from the primary - carefully, it's 13.8 kV) with the common on the start wire and the probe on the finish (dotted) wire you will observe a sine wave in phase with the primary (assuming the dot is at the top of the drawing for the primary winding too.) If you move the scope leads to the bottom winding you will see the same thing (again with the probe at the dotted wire) you would see the same thing. The two windings, considered separately, each produces 120V. At the peak of a cycle they are like 120V batteries. If you join the windings (the dot on one to the unmarked on the other) they are like two batteries connected in series. The voltage from the bottom most wire to the topmost wire is 240 Volts. If, in this case, you connect the common wire of the oscilloscope to the common connection and move the probe to the top wire you will see + 120V (at the peak) i.e. a waveform in phase with the primary. But when you move the probe to the bottom wire you will see -120V at the peak (a waveform that is 180° out of phase with the top coil and the primary. Thus: 3 wires, 2 phases. It's a biphase system.

Your bottom drawing is intended to represent the case where the dot is at the top for the top winding and at the bottom in the bottom winding. It is like connecting two batteries in series with the two negative terminals in common. The potential relative to the connection (reference) of the one battery is +120 and the potential referenced tothe common point of the other battery is +120. The potential difference between the two (non common) terminals is 120 - 120 = 0. The same thing happens if you connect transformer winding in a 'bucking' connection.

As we've just seen, transformer winding in series is all about phase. I mentioned in my last post that you can also get 120/240 biphase from a 3 phase system by connecting 3 transformer windings in series but they must be phased correctly. I brought that up to get you to think in broader terms IOW to understand that a center tapped transformer isn't the only way to get 120/240. This should help you to see that the center tapped transformer is indeed a biphase hookup.

If this is a myth I've surely known hundreds of engineers, not only in power but audio and rf design where exactly the same principles apply, who have been taken in by it and a lot of text books will need to be burned.

[EDIT] Where I say 'peak' here I am neglecting to mention that the actual peak voltages are sqrt(2)*120. As we're confused enough here without bringing in that aspect I though this would best be omitted but in retrospect I think it should at least be mentioned. Thus this edit.
It is not two phase power and the legs are not 180 out of phase.

The explaination with the oscilloscope is a common one, albeit flawed, because both leads have to be moved, not just one. Using the center tap as reference is the reason that oscilloscope example is flawed. This is akin to putting the black lead of a multimeter on the positive battery post and the red lead on the negative post, the proclaiming the battery polarity has miraculously reversed because the multimeter shows negative voltage.
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