Power Distribution At Residential Level Understanding

[Ranes]

I was debating some of the theory with a coworker and realized I lack some understanding on a piece of how the power grid functions:

At a simple/brief level, power is generated with 3 phases, then stepped up and transmitted at high voltages, and then stepped down as it distributes and, with the circumstance my question lies, is split into single phase runs for sets of houses. I understand that these single phase runs are stepped down once more to become the 120/240V before being wired into houses, and I understand how 120/240V systems power devices (whether it be LN or LL). What I don't understand is how do we take a single phase with 60Hz frequency and break that into the two 120V phases that are 180° out of phase.

Just to be clear, I understand how we can step down the higher single phase run (before transformer) from higher voltage primary to 240V and run a neutral to divide into 120V "pieces" on the secondary side, but what I can't find is some clarification as to why these are out of phase with each other. Is this something that just inherently happens when we tap a transformer with the neutral to get two 120V phases?

Thanks for the help.

 

[gar]

180511-1346 EDT

Ranes:

Yes it inherently happens, if both halves of the secondary are wound on the same core, the secondary coil is one winding all wound in the same direction, and the center tap is in the center of the secondary winding.

If the secondary is made of two separate coils, then they must be phased correctly to get the result you describe. There are only two phasing possibilities.

.

 

[hbiss]

No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?

-Hal

 

[Ranes]

No offense, but you are an electrical engineer and you have to ask about the phase relationships of a center tapped winding of a transformer?

-Hal

I spent most of my degree on Controls, actually. I wound up in a semi-power job, so sometimes I have to ask questions to learn. Choosing to not ask questions when you don't understand something entirely is a bad life philosophy, in my opinion . Thanks for your input though.

 

[Ranes]

180511-1346 EDT

Ranes:

Yes it inherently happens, if both halves of the secondary are wound on the same core, the secondary coil is one winding all wound in the same direction, and the center tap is in the center of the secondary winding.

If the secondary is made of two separate coils, then they must be phased correctly to get the result you describe. There are only two phasing possibilities.

.

That makes sense, thanks for the info!

 

[gar]

180511-1429 EDT

Ranes:

If you look in the Handbook of Chemistry and Physics, or or Handbook of Tables and Formulas, there are lists of many trig equations.

An important concept is that the sum of two sine waves of identical frequency and that are phase related create a sine wave of the same identical frequency and with some possibly different phase relative to one of the original waves.

There is one degenerate case where the resulting output amplitude is zero. This is the case where the secondary consists of two separate coils of identical output voltage but are connected in parallel. Here, if you connect one end of each winding together, and then measure the voltage difference between the other two ends that difference voltage will be zero or near zero for the paralleling case. If the wrong phase for paralleling, then the voltage will be double that of one winding.

Get some small transformers, a scope, and AC voltmeter and play.

.

 

[hbiss]

I spent most of my degree on Controls, actually. I wound up in a semi-power job, so sometimes I have to ask questions to learn. Choosing to not ask questions when you don't understand something entirely is a bad life philosophy, in my opinion . Thanks for your input though.

Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

Guess it's just another symptom of our pathetic educational system.

But I digress...

-Hal

 

[JFletcher]

In a nutshell, when you Center tap (i.e., the neutral ) transformer, you provide a 120/240V secondary.

If you put the leads on a voltmeter on phase A and C, which is typically the leg that is Center tapped in a high leg Delta with B being 208 volts, you will read 240 volts. from A to neutral or C to neutral, you will get 120 volts.

if you think about where are you split that sine wave, say at the zero Crossing Point, one wave form will be going up while the other wave forum is going down. they are 180 degrees out of phase with each other in regards to the neutral, thus giving 240 volts from A to C.

 

[RumRunner]

Really, I'm not blaming you. I'm just trying to understand today's educational system that hands out what they like to call an EE degree without the recipient knowing fundamental concepts. Maybe it's because I'm an old grey haired guy who has worked with many EEs over the years. None would ever say "I don't understand analog because I spent most of my degree studying digital and computers".

Guess it's just another symptom of our pathetic educational system.

But I digress...

-Hal

You have not offered an answer to OP's question. . . instead, you spouted a sermon from your pulpit. . . which I think wasn't necessary.

 

[Ingenieur]

think of it as a 3 winding xfmr
1 prim
2 sec, opposite signs (or hand, dot convention, etc) and a common center point
this makes each sec v 180 deg out of phase

sometimes it is better to grab a text and teach yourself
I thought all abet bsee programs required basic power and machine courses?

I don't believe hbiss meant insult, but like me was surprised that a bsee program would not cover it or at least give you the tools to develop the model or at least look it up?
a xfmr is the most fundemental 'machine' and is the basis for understanding the application of fields, then on to gen/motors

 

[electrofelon]

180511-1429 EDT
An important concept is that the sum of two sine waves of identical frequency and that are phase related create a sine wave of the same identical frequency and with some possibly different phase relative to one of the original waves.
.

Yes I think that is the key, and how you choose to represent it graphically. Metering line to line could be thought of as just a "regular" single sine function. In this case, the x-axis is one line while the sine wave is the other line. But it could also be graphed as two sine functions, one reflected over the x- axis to make the second one. As gar said, that is mathematically the same as the first case and could be redrawn as such. In this case the x-axis is the center tap. Expressing it with the two functions illustrates the center tap to lines readings better.

 

[electrofelon]

Just to make sure my explanation was clear, I graphed the two methods in Maple. Yes, I took the 120 and 240 to be peak not RMS, and I didnt scale the x-axis to standard frequency.

 

[Ingenieur]

the 2 voltages
170cos(2 Pi 60 t)
170cos(2 Pi 60 t + Pi), 180 deg out of phase = -170cos(2 Pi 60 t)

when summed around the loop starting at common
v = 170cos(2 Pi 60 t) - (-170cos(2 Pi 60 t)) = 340cos(2 Pi 60 t)

phasors
120/0 = 120 + 0j
120/180= -(-120 + 0j)
sum = 240 + 0j = 240/0

 

[jaggedben]

...

Just to be clear, I understand how we can step down the higher single phase run (before transformer) from higher voltage primary to 240V and run a neutral to divide into 120V "pieces" on the secondary side, but what I can't find is some clarification as to why these are out of phase with each other. Is this something that just inherently happens when we tap a transformer with the neutral to get two 120V phases?

Thanks for the help.

I recall there was once a thread on this forum wherein a debate ensued as to whether the two 120V circuits were one phase or two phases and in phase with each other or not. It was basically semantics and the moderators decided that the conversation had gotten so stupid, or disrespectful, that they deleted the entire long thread. (Don't try to find it, you won't.)

Practically speaking, if you were to scope both 120V legs, you could say your measurements are '180 degrees out of phase' if you keep the same probe on the neutral for both measurements, or you could say they are 'completely in phase with each other' if you were to swap the probe on the neutral. So calling them 'out of phase' is really just semantics, or at most a matter of choosing the correct mathematical expression for wherever you decide to point your probes. But sin(x) = -sin(x+180). Finally, remember that if you measure hot to hot, you are just measuring one waveform, and utilities call it a single-phase service. 'Two phase' power is actually something else, which is old fashioned and rare but still exists I guess in Philly and maybe a couple other places.

 

[Gary Glaenzer]

180511-1346 EDT


Yes it inherently happens, if both halves of the secondary are wound on the same core, the secondary coil is one winding all wound in the same direction, and the center tap is in the center of the secondary winding.



.

Not to split hairs but.......

The two halves of the secondary are NOT 'out of phase' (although it is impossible to tell this from looking at the wave-forms in this instance).

They are OF OPPOSITE POLARITY, referenced to the center-tap.

To demonstrate:

Consider a complete cycle which consists of a positive going square wave for the first half-cycle, followed by a sine-wave second half of the cycle fed to the promary.

If the two secondary voltages at 'L1' and 'L2' (the two 'output' terminals, referenced to the secondary center tap) were 180 degrees out of phase, each cycle at one of the 'L' terminals would have a square-wave first half, followed by a sine second half; the other 'L' terminal would have a sine first half followed by square-wave second half. Shifted a half-cycle in TIME, in other words.

 

[roger]

I recall there was once a thread on this forum wherein a debate ensued as to whether the two 120V circuits were one phase or two phases and in phase with each other or not. It was basically semantics and the moderators decided that the conversation had gotten so stupid, or disrespectful, that they deleted the entire long thread. (Don't try to find it, you won't.)

Click here at your own risk

Roger

 

[Ingenieur]

L1-n nd L2-n are 180 deg out of ph relative to each other
this results in opposite polarity

scope, com and 2 measuring probes
com>n
probe 1>L1
probe 2>L2

you will see 2 traces, mirrored

reverse the winding on one coil and they will be in phase, 0 deg
the traces will overlay

if both were 120/0 then using kvl
L1 = 120 + 0j....n to line is pos reference
L2 = +(-(120 + 0j))....line to n is neg reference
L1 + (-L2) = 0, which is not the case

if L2 is 120/180 = -120 + 0j
then L1 + (- L2) = 240 + 0j


this does NOT make it 2-phase power

 

[Gary Glaenzer]

L1-n nd L2-n are 180 deg out of ph relative to each other
this results in opposite polarity

Sorry, no, that is backwards.

They are of opposite polarity, which in this case is indistinguishable from a 180 degree phase shift.

 

[Ingenieur]

Sorry, no, that is backwards.

They are of opposite polarity, which in this case is indistinguishable from a 180 degree phase shift.

incorrect
I have proven it mathematically
the reason they are indistinguishable is because they are the same thing


http://www.samlexamerica.com/suppor...ingleSplitPhaseandMultiWireBranchCircuits.pdf
** NOTE: The phase of Hot Leg 2 (Phase B) is in the
opposite direction - i.e., 180° apart from the phase of Hot Leg L1 (Phase A)


https://en.m.wikipedia.org/wiki/Split-phase_electric_power
[FONT=&quot]Current is transmitted in two [/FONT]sine waves[FONT=&quot], each of which is on its own "hot" wire, and each of which varies between +120V and -120V with respect to ground. Because the waves are 180° out of phase with each other, this means that when one is at a full +120V, the other is at full -120V, for a difference of 240V; both reach 0V at the same time. When added together, this results in a circuit which varies between +240V and -240V with the same frequency, but "split" across two hot wires rather than one hot and one neutral.[/FONT]

 

[Ingenieur]

one of you guys with a scope
how about trying this
neut to common
L1 and L2 to seperate measuring probes

what is the time delta between pos peaks (or ascending 0 crossings)?

close to 8.333 mSec?

1/60 sec = 360 deg so 1/120 sec = 180 deg