it's not arbitrary
gnd/neut is 0 V or the reference
if you touch g/n and a driven ground/earth rod, nadda
if you touch L1 or L2 to the rod, zap!!!!!!!!
May I ask a question about the single vs two phase stuff
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it's not arbitrary
gnd/neut is 0 V or the reference
if you touch g/n and a driven ground/earth rod, nadda
if you touch L1 or L2 to the rod, zap!!!!!!!!
wwhitney
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I agree it is not an arbitrary choice, as many objects in the vicinity of electrical installations are earthed, and so when one circuit conductor is earthed, it may be convenient to choose that conductor as the 0V reference. But that is still a choice.
Here's a thought experiment: say I have a bunch of equipment that operates at 40 VAC. I choose to use a 120V : 120V isolation transformer where the secondary is tapped twice, producing three equal segments and 4 circuit conductors. Call the circuit conductors A-D in order. The voltage A-B, B-C, or C-D is 40V; the voltage A-C or B-D is 80V; and the voltage A-D is 120V.
Now NEC 250.20(B) requires me to ground one of the circuit conductors, but 250.26 doesn't specify which conductor to ground. I can choose to ground conductor A, take it as 0V, and then the conductor voltages are (0V, 40V, 80V, 120V). I think we can agree that in this case legs B, C, and D are all in phase.
Or I can chose to ground conductor C and take it as 0V. Then the conductor voltages are (-80V, -40V, 0V, 40V). Are legs B and D now out of phase simply because I've moved both the earthing point and the 0V reference? No, they are still in phase, their voltage ratio has simply changed from 3/2 to -1.
Cheers, Wayne
gar
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180723-1543 EDT
Consider a full wave center tapped rectifier circuit with a resistive load.
Label the two coils of the secondary L1-L2 and L3-L4. Each coil has the same output voltage. Place a phasing dot on L2 and L4, and connect L2 to L3. Now you have a conventional center tapped secondary with L2L3 being the center tap. The voltage across L1-L4 is twice that of either L1-L2 or L3-L4.
Connect one end of the load resistor to L2L3, the center tap.
Connect the cathodes of the two diodes together, and to the second end of the load resistor.
Connect a scope to measure the load resistor voltqge.
Connect the anode of diode D1 to L1. The load voltage is now a pulse of a positive 1/2 sine wave.
Connect the anode of D2 to L4. The load voltage is now two positive 1/2 cycle pulses. The phase difference is 180 degrees.
Move D2 anode to L1 and now we have just one 1/2 cycle output pulse.
The time position of where the second pulse is is where it should be, but the second pulse is 180 degrees out of phase with respect to the first pulse.
.
Consider a full wave center tapped rectifier circuit with a resistive load.
Label the two coils of the secondary L1-L2 and L3-L4. Each coil has the same output voltage. Place a phasing dot on L2 and L4, and connect L2 to L3. Now you have a conventional center tapped secondary with L2L3 being the center tap. The voltage across L1-L4 is twice that of either L1-L2 or L3-L4.
Connect one end of the load resistor to L2L3, the center tap.
Connect the cathodes of the two diodes together, and to the second end of the load resistor.
Connect a scope to measure the load resistor voltqge.
Connect the anode of diode D1 to L1. The load voltage is now a pulse of a positive 1/2 sine wave.
Connect the anode of D2 to L4. The load voltage is now two positive 1/2 cycle pulses. The phase difference is 180 degrees.
Move D2 anode to L1 and now we have just one 1/2 cycle output pulse.
The time position of where the second pulse is is where it should be, but the second pulse is 180 degrees out of phase with respect to the first pulse.
.
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K8MHZ
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Not enough to trip a breaker, but enough to make night crawlers surface so they can be snatched up and used as fishing bait. People really used to do that around here.
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- Electrical Engineer
Gary Glaenzer
Member
Actually it is neither.
It is a POLARITY relationship.
Opposite polarities.
Phase has absolutely nothing to do with a 120-N-120 set-up.
the limiting Z will be you
you 1000 Ohm
earth 25
120/1025 ~ 117 mA << 20 A no trip (unless gfci)
should kill you in ~
from Dalziel
mA = body wt/sqrt time
117 = 175 / sqrt t
t = (175/117)^2 = 2.34 sec
unless pulled free and cpr is started
mivey
Senior Member
Because using pre-determined, fixed-polarity components to model a system without a fixed polarity doesn't work well.
120/0 deg = 120 + 0j
120/180 = -120 + 0j
a 180 deg phase delta IS, or results in, opposite polarity of 0 deg
and of a different ph ang, hence 2 phases
mivey
Senior Member
Perhaps it is just too simple. It really is such a basic thing to go back to how a voltage is defined.What!!! You trying to bring some basic common sense to this here cat fight!
It will never work...unfortunately.....sigh....
The field increases across the whole winding, not propagating from one end to the other. One man's voltage rise is another man's voltage drop. It really is that simple.
mivey
Senior Member
Aieeeeee!We control the horizontal. We control the vertical.
wwhitney
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For any real world waveform (not a perfect sine wave), that will not be true. The relationship is multiplication by -1, not a time shift by 1/2 cycle.and 0 deg is one phase and 180 is another
it really is that simple
Cheers, Wayne
winnie
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That is the crux of the argument.
In a single phase transformer there is no time shift. All of the terminals are being energized at the same time by the same magnetic flux.
To some of us, '180 degree phase difference' means there must be a time shift.
To others of us, the fact that inversion of a sine wave is indistinguishable from a 180 degree time shift is enough to call inversion a 180 degree phase angle.
I figure that as long as you are self consistent and get your math right, it doesn't matter which camp you fall into.
-Jon
wwhitney
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So if a load introduces noise on the 60Hz voltage signal, that noise will be delayed by one half-cycle on one leg versus the other?
Cheers, Wayne
the FACT is if you scope each line-g/n they are not in phase, ie, not of a single phase, hence TWO phases
hard to determine
but the fundamental phase delta will remain the same subject to any impact of a reactive load
mivey
Senior Member
Not really the crux when we are talking about a service supply. We are talking about a set of supply voltages.
However, if the "noise" occurs on a common shaft (someone dropped a cell phone in the generator gearbox), the 180 degree displaced windings on the common shaft will produce the noise in both phases at the same time.
The center-tapped transformer can be a source of two 180 degree displaced voltages like a 180 degree physically-displaced set of generator windings.
Some have argued in the past that two phases can't exist with a 180 degree difference and that is simply not so. The center-tap transformer is a means to supply those voltages without having the actual gen-set right there.
We use this exact principle of phase differences to produce a 3-phase set of voltages using an open wye-wye transformer bank. The missing 3rd phase is produced by using half of the two windings with voltages that are 180 degrees different from the other halves. We have:
a = 0.5<0d
c = 0.5<120d
b = 0.5<180d + 0.5<300d = 0.5<240d
and we get a 3-phase set of voltages with a 120 degree displacement even though one phase is made up of two "reverse polarity" voltages. The phase displacement is real and works just fine as a 3-phase supply.
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