| Author |
Message |
Rich Grise
Guest
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 Posted:
Fri Feb 25, 2005 2:28 am Post subject:
Re: 14-3 shared neutral |
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On Wed, 23 Feb 2005 18:43:26 -0800, John Larkin wrote:
| Quote: | On Wed, 23 Feb 2005 21:11:22 GMT, Rich Grise <richgrise@example.net
wrote:
On Wed, 23 Feb 2005 09:40:23 -0800, John Larkin wrote:
On Wed, 23 Feb 2005 13:38:01 -0330, "Terry" <tsanford@nf.sympatico.ca
wrote:
Someone wrote:
" Right, so long as the two branches are on different phases".
Err? Different 'Legs' might be a better term?
Well, I'm an electrical engineer, and we don't refer to "legs". By
"different phases" I meant that the voltages are, well, not in phase.
But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.
Cheers!
Rich
As I said, to see if they're in phase, merely connect them together
and see what happens. Report back and I'll interpret the experiment
for you.
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By this logic, the positive pole of a battery is 180 degrees out of
phase with the negative pole.
Thanks,
Rich
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John Larkin
Guest
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| Posted:
Fri Feb 25, 2005 5:24 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 20:28:24 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Wed, 23 Feb 2005 18:43:26 -0800, John Larkin wrote:
On Wed, 23 Feb 2005 21:11:22 GMT, Rich Grise <richgrise@example.net
wrote:
On Wed, 23 Feb 2005 09:40:23 -0800, John Larkin wrote:
On Wed, 23 Feb 2005 13:38:01 -0330, "Terry" <tsanford@nf.sympatico.ca
wrote:
Someone wrote:
" Right, so long as the two branches are on different phases".
Err? Different 'Legs' might be a better term?
Well, I'm an electrical engineer, and we don't refer to "legs". By
"different phases" I meant that the voltages are, well, not in phase.
But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.
Cheers!
Rich
As I said, to see if they're in phase, merely connect them together
and see what happens. Report back and I'll interpret the experiment
for you.
By this logic, the positive pole of a battery is 180 degrees out of
phase with the negative pole.
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No, phase is meaningless for DC.
John |
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John Fields
Guest
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| Posted:
Fri Feb 25, 2005 5:48 am Post subject:
Re: 14-3 shared neutral |
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On Wed, 23 Feb 2005 21:11:22 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Wed, 23 Feb 2005 09:40:23 -0800, John Larkin wrote:
On Wed, 23 Feb 2005 13:38:01 -0330, "Terry" <tsanford@nf.sympatico.ca
wrote:
Someone wrote:
" Right, so long as the two branches are on different phases".
Err? Different 'Legs' might be a better term?
Well, I'm an electrical engineer, and we don't refer to "legs". By
"different phases" I meant that the voltages are, well, not in phase.
But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.
|
---
No, they're not.
If you use one end of the transformer secondary as a reference, the
center tap will be in phase with the other end of the winding, (that
is, one positive-going peak will occur at the same time as the other
positive-going peak) but if you use the center tap as the reference
the ends of the winding will be 180° out of phase with each other.
That is, the positive-going peak of one will occur when the
negative-going peak of the other occurs.
--
John Fields
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John Fields
Guest
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| Posted:
Fri Feb 25, 2005 5:52 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 20:28:24 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | By this logic, the positive pole of a battery is 180 degrees out of
phase with the negative pole.
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---
More like the case in point, the ends of two batteries connected in
series-aiding will be out of phase with each other with respect to the
common connection between them.
--
John Fields |
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Peter Bennett
Guest
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| Posted:
Fri Feb 25, 2005 6:08 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 20:27:26 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Thu, 24 Feb 2005 09:00:16 -0500, Beeper top-posted (see below for
context)
Terry, you just said what I was hoping to hear. It is OK if the two seperate
circuits are mechanically connected(if you will) If one trips, the other one
trips also. This was not true in my case and that's why I questioned it. How
about everyone else who knows residential code. Mechanically connected? yes
or no?
They should be mechanically connected (ganged) if you will be using the
two hots for a 220 (240) load down the line. If you don't have anything
like that, and are absolutely certain that you never will, then
independent breakers should be fine. The most the neutral will ever have
to carry will be the current to the one side; otherwise they subtract,
as Terry has pointed out.
|
If two circuits are run in a single 14/3 cable, the breakers feeding
that cable _must_ be mechanically interlocked, so that they will
switch on or off together, whether or not you are using them as a 220
V supply. The reason for this is so that if you turn off one circuit
to work on it, you won't get burned by the other, which will terminate
in the same box.
--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq |
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Rich Grise
Guest
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| Posted:
Fri Feb 25, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 17:48:28 -0600, John Fields wrote:
| Quote: | On Wed, 23 Feb 2005 21:11:22 GMT, Rich Grise <richgrise@example.net
But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.
No, they're not.
If you use one end of the transformer secondary as a reference, the
center tap will be in phase with the other end of the winding, (that
is, one positive-going peak will occur at the same time as the other
positive-going peak) but if you use the center tap as the reference
the ends of the winding will be 180° out of phase with each other.
That is, the positive-going peak of one will occur when the
negative-going peak of the other occurs.
|
Let me put it this way.
A 180 degree phase-shifted sine wave is _visually indistinguishable_
from an instantaneously-inverted sine wave, that is, they _look_
exactly the same, but they are not the same, because they got
created by a different process.
To put it to an extreme example, consider a pulse train:
_ _ _ _ _ _
___| |_____| |_____| |_____| |_____| |_____| |__
Phase-shift it 180 degrees, and you get this:
_ _ _ _ _ _ _
|_____| |_____| |_____| |_____| |_____| |_____| |__
But if you invert it, you get this:
____ _____ _____ _____ _____ _____ ___
|_| |_| |_| |_| |_| |_|
You see, when you're not using a sine wave, it's very easy to tell the
difference between a phase shift and a polarity inversion.
Using a sine wave, however, they _look_ exactly the same, which is
misleading, and leads to tech school instructors (and probably college
profs) teaching their students that they _are_ the same, while in truth,
they're not.
Thanks,
Rich |
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Rich Grise
Guest
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| Posted:
Fri Feb 25, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 16:08:35 -0800, Peter Bennett wrote:
| Quote: | On Thu, 24 Feb 2005 20:27:26 GMT, Rich Grise <richgrise@example.net
wrote:
On Thu, 24 Feb 2005 09:00:16 -0500, Beeper top-posted (see below for
context)
Terry, you just said what I was hoping to hear. It is OK if the two seperate
circuits are mechanically connected(if you will) If one trips, the other one
trips also. This was not true in my case and that's why I questioned it. How
about everyone else who knows residential code. Mechanically connected? yes
or no?
They should be mechanically connected (ganged) if you will be using the
two hots for a 220 (240) load down the line. If you don't have anything
like that, and are absolutely certain that you never will, then
independent breakers should be fine. The most the neutral will ever have
to carry will be the current to the one side; otherwise they subtract,
as Terry has pointed out.
If two circuits are run in a single 14/3 cable, the breakers feeding
that cable _must_ be mechanically interlocked, so that they will
switch on or off together, whether or not you are using them as a 220
V supply. The reason for this is so that if you turn off one circuit
to work on it, you won't get burned by the other, which will terminate
in the same box.
|
I stand corrected. I keep forgetting that regulations are written in
hopes of minimizing the Darwin effect.
Thanks,
Rich |
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John Larkin
Guest
|
| Posted:
Fri Feb 25, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Thu, 24 Feb 2005 17:48:28 -0600, John Fields wrote:
On Wed, 23 Feb 2005 21:11:22 GMT, Rich Grise <richgrise@example.net
But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.
No, they're not.
If you use one end of the transformer secondary as a reference, the
center tap will be in phase with the other end of the winding, (that
is, one positive-going peak will occur at the same time as the other
positive-going peak) but if you use the center tap as the reference
the ends of the winding will be 180° out of phase with each other.
That is, the positive-going peak of one will occur when the
negative-going peak of the other occurs.
Let me put it this way.
A 180 degree phase-shifted sine wave is _visually indistinguishable_
from an instantaneously-inverted sine wave, that is, they _look_
exactly the same, but they are not the same, because they got
created by a different process.
To put it to an extreme example, consider a pulse train:
_ _ _ _ _ _
___| |_____| |_____| |_____| |_____| |_____| |__
Phase-shift it 180 degrees, and you get this:
_ _ _ _ _ _ _
|_____| |_____| |_____| |_____| |_____| |_____| |__
But if you invert it, you get this:
____ _____ _____ _____ _____ _____ ___
|_| |_| |_| |_| |_| |_|
You see, when you're not using a sine wave, it's very easy to tell the
difference between a phase shift and a polarity inversion.
Using a sine wave, however, they _look_ exactly the same, which is
misleading, and leads to tech school instructors (and probably college
profs) teaching their students that they _are_ the same, while in truth,
they're not.
|
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
Thanks for the education.
John |
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Rich Grise
Guest
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| Posted:
Fri Feb 25, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 19:53:39 -0800, John Larkin wrote:
| Quote: | On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
|
It's more a matter of definition than telling them apart. Like I said,
once they're generated, they look exactly the same.
You look at where they came from. For example, an ordinary inverting
amplifier's output is only phase shifted by whatever reactance the
opamp shows - the output is actually just inverted. Vo = -A * Vi. (plus
some j omega stuff, which they try to compensate out, AIUI.)
In a phase-shift oscillator, the feedback is literally phase-shifted
180 degrees, giving the effect of an inversion at that particular
frequency, ergo positive feedback. The inverted signal and the 180
degree phase-shifted signal add together.
Of course, to throw a monkey wrench into the thing, if you have three-
phase delta, and just put a transformer between e.g. phases A and B, but
center-tap the secondary, _then_ you could call them phases, but not with
respect to the center-tap, only with respect to phase C. (or the
center-point, if it's a 'Y'). And they're 120 degrees from it and each
other, if you look at it that way. But at two ends of the transformer,
with just the center-tap in the middle, there is no third phase to get the
graph up off the X axis!
Yeah, that's it! ;-)
Hope This Helps!
Rich |
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John Larkin
Guest
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| Posted:
Fri Feb 25, 2005 6:10 am Post subject:
Re: 14-3 shared neutral |
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On Fri, 25 Feb 2005 05:19:19 GMT, Rich Grise <richgrise@example.net>
wrote:
| Quote: | On Thu, 24 Feb 2005 19:53:39 -0800, John Larkin wrote:
On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
It's more a matter of definition than telling them apart. Like I said,
once they're generated, they look exactly the same.
You look at where they came from.
|
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
Got it. Thanks.
John |
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Michael A. Terrell
Guest
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| Posted:
Fri Feb 25, 2005 6:32 pm Post subject:
Re: 14-3 shared neutral |
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John Larkin wrote:
| Quote: |
On Fri, 25 Feb 2005 05:19:19 GMT, Rich Grise <richgrise@example.net
wrote:
On Thu, 24 Feb 2005 19:53:39 -0800, John Larkin wrote:
On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
It's more a matter of definition than telling them apart. Like I said,
once they're generated, they look exactly the same.
You look at where they came from.
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
Got it. Thanks.
John
|
John, I think this may be a better explanation:
If you use a scope to look at the unfiltered output of a full wave 60
Hz rectifier you see 120 bumps in a one second sweep. If there are not
two different phases you would see 60 with wider spaces between them.
This is like the argument of a diode in series with a resistor: Is the
power or the voltage reduced to half. Most people seem to think the
diode will reduce the voltage 50% and refuse to look at the math that
the effect voltage will be reduced from, Say, 120 VAC to an effective 84
VAC rather than 60 VAC.
--
Beware of those who post from srvinet.com!
Michael A. Terrell
Central Florida |
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Robert Monsen
Guest
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| Posted:
Sat Feb 26, 2005 5:32 am Post subject:
Re: 14-3 shared neutral |
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Michael A. Terrell wrote:
| Quote: | John Larkin wrote:
On Fri, 25 Feb 2005 05:19:19 GMT, Rich Grise <richgrise@example.net
wrote:
On Thu, 24 Feb 2005 19:53:39 -0800, John Larkin wrote:
On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
It's more a matter of definition than telling them apart. Like I said,
once they're generated, they look exactly the same.
You look at where they came from.
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
Got it. Thanks.
John
John, I think this may be a better explanation:
If you use a scope to look at the unfiltered output of a full wave 60
Hz rectifier you see 120 bumps in a one second sweep. If there are not
two different phases you would see 60 with wider spaces between them.
This is like the argument of a diode in series with a resistor: Is the
power or the voltage reduced to half. Most people seem to think the
diode will reduce the voltage 50% and refuse to look at the math that
the effect voltage will be reduced from, Say, 120 VAC to an effective 84
VAC rather than 60 VAC.
|
Actually, in Rich's defense, he was talking about the voltages at the
ends of the secondary of a center tap transformer w.r.t. the center tap,
and indicated that these voltages were inverted in relation to the
center tap, not 'out of phase'. He seems to believe that phase shifts
can only be generated by timing delays. Thus, the argument is really one
of definitions, as in "what is a phase shift?".
For pure sine waves, there is simply no way to a 180 degree time shifted
sine wave from an inverted sine wave, which is what JL's remarks were
pointed at. If it quacks like a duck...
However, if you put a square wave with a duty cycle of 25% into the
primary of the transformer, Rich's point will become apparent. You won't
be able to make the waves match, even with a suitable time shift. You
will need to invert one, and then they will match without any timing delay.
One question I have is can you generate three phase output with the
secondaries of a transformer?
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God. |
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Fred Abse
Guest
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| Posted:
Sat Feb 26, 2005 7:02 pm Post subject:
Re: 14-3 shared neutral |
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On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
| Quote: | Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
|
That's only true for relatively large values of 3
;-)
--
Then there's duct tape ...
(Garrison Keillor) |
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Fred Abse
Guest
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| Posted:
Sat Feb 26, 2005 7:02 pm Post subject:
Re: 14-3 shared neutral |
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On Fri, 25 Feb 2005 05:19:19 +0000, Rich Grise wrote:
| Quote: | On Thu, 24 Feb 2005 19:53:39 -0800, John Larkin wrote:
On Fri, 25 Feb 2005 02:21:57 GMT, Rich Grise <richgrise@example.net
OK. Given a reference sine wave A, please tell us how to tell the
difference between signals B and C, where
B = "A" inverted, and
C = "A" shifted 180 degrees.
It's more a matter of definition than telling them apart. Like I said,
once they're generated, they look exactly the same.
|
If you substitute "phase difference" for "phase shift", I think it might
help.
Signals at the same frequency are either in phase, or they're
not. Causality is not ascertainable without additional information.
--
Then there's duct tape ...
(Garrison Keillor) |
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Kitchen Man
Guest
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| Posted:
Sun Feb 27, 2005 12:02 am Post subject:
Re: 14-3 shared neutral |
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|
On Sat, 26 Feb 2005 13:02:19 +0000 in sci.electronics.basics, Fred Abse
<excretatauris@cerebrumconfus.it> wrote msg
<pan.2005.02.26.12.09.39.752208@cerebrumconfus.it>:
| Quote: | On Thu, 24 Feb 2005 21:48:13 -0800, John Larkin wrote:
Ah, I see now. Suppose two different computers calculate the number
"3". One adds 2+1, and the other takes the square root of 9. These 3's
are different, so there are actually two distinct kinds of "3".
That's only true for relatively large values of 3
|
But to get true phase shift, you have to take the limit as 3 approaches
zero.
I believe that Terry's original comment was meant to draw a distinction
between what's available on the two big black wires coming in to the
house, and the three 120 degree phases on the big transmission lines.
Some people are under the mistaken impression that two of those phases
are fed into the residence, rather than one phase split and inverted.
And yes, once you split and invert, you've got a 180 degree phase shift,
as well as a 50% attenuation, on *ahem* each leg. Or should I say bus?
--
http://www.xmission.com/~tiger885/motorbike/NART/nart.html |
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