single vs. 3 phase

[mivey]

So now move these taps from the 57.7% location to the 50% location. Do you need to change this phasor diagram yet?

Just a magnitude change for the L-L phasors as the angles would stay the same. My metering phasors would just need a magnitude and angle change for the new tap phasors.

This will continue to hold true unless I wanted to look at another phasor, say from b to c'. Then I would just add in either phasor Vbc' or phasor Vc'b, depending on what I wanted (or just use the difference of the Vc'n and Vbn phasors from my metering setup).

 

[mivey]

I am pretty sure that 'rotation' as being applied here as a differentiation between a single phase and polyphase power.
Single phase power, energizing a motor stator, cannot develop a rotating magnetic field. Polyphase power can.

Which agrees with your earlier statement that the physics and definition are two different things. The 120/208 has the "bowstring" neutral but the 120/240 does not. I was trying to illustrate that the V1 and V2 of the delta and wye are the same type line conductors and I could show that by finding a "bowstring" reference point for the 240 leg.

...what phasors are used to describe the _three_ outputs of a 240/120 high leg single phase system? Similarly, if you have a 'scott-T' secondary to derive a 208/120 wye output, what phasors should be used to describe the output?...If in the three phase systems which include single phase center tapped secondaries you use _different_ phasors to describe the output, why?

For 3 phase voltages, my thought is that if you have a 4-wire, your metering would reference the neutral and you have the "T" oriented phasors. If you have, 3-wire 3 phase, you would use L-L for metering and have the triangular oriented phasor diagram. As for the loads, it would depend on what you were doing. I still don't see one method as the absolute over the other in all cases.

 

[jim dungar]

Just a magnitude change for the L-L phasors as the angles would stay the same. My metering phasors would just need a magnitude and angle change for the new tap phasors.

This will continue to hold true unless I wanted to look at another phasor, say from b to c'. Then I would just add in either phasor Vbc' or phasor Vc'b, depending on what I wanted (or just use the difference of the Vc'n and Vbn phasors from my metering setup).

So if the 3-wire vectors, now with center taps, stay the same except for magnitude, why do you change them when you switch to 4-wire? Physically they haven't changed, the construction of the transformer doesn't change just by adding a 4th conductor.

 

[mivey]

So if the 3-wire vectors, now with center taps, stay the same except for magnitude, why do you change them when you switch to 4-wire? Physically they haven't changed, the construction of the transformer doesn't change just by adding a 4th conductor.

Because in the 4 wire, I usually am referencing to the neutral. These are the voltages and currents I will get from my metering equipment.

 

[mivey]

Jim, Let me add: If I have a grounded neutral system, I usually need it for something, and it just seems natural to use a grounded neutral reference.

As far as if the center tap was not being used, it would NOT seem natural to try to reference the center tap by splitting the phasors in that coil.

As for why the grounded neutral seems like a natural reference, maybe because as a child, I grew up in a single phase world. I never new of a three phase world until I was much older. When I was a child, my circuits were simple L-G and used a ground reference. When I moved to a center-tapped AC world, the grounded reference still felt natural. Since I saw the second leg as an inversion of the first, it would have felt "backwards" to not use the grounded center tap as a reference.

When I had a combination circuit that used the grounded center tap, I used the ground as a reference because it just seemed like a natural extension of the old L-G world I started with. I guess with the L-G-L circuit and having + and - values, the bottom section just seemed more like an "inversion" of the top instead of a "stack". A multi-tap coil did seem like a stack but I would have had a "+" stack and a "-" stack if I grounded the center of the multi-tap coil.

 

[jim dungar]

Because in the 4 wire, I usually am referencing to the neutral. These are the voltages and currents I will get from my metering equipment.

Is a correct summary: with no 4th wire all of the voltage phasors are connected "head to tail" at the center taps of each coil, once you add a 4th wire the voltage phasors suddenly become re-connected "tail to tail" but only in the coil that has the 4th wire?

 

[mivey]

No, I don't understand because I am of the opinion that rotation of whatever it is has nothing to do with a steady state analysis, and no one can explain it to me, so I must be right.

argumentum a silentio

 

[rattus]

Just hoping:

Just hoping:

argumentum a silentio

Hoping someone would become non-silentio on the subject!

 

[mivey]

Is a correct summary: with no 4th wire all of the voltage phasors are connected "head to tail" at the center taps of each coil, once you add a 4th wire the voltage phasors suddenly become re-connected "tail to tail" but only in the coil that has the 4th wire?

Not really. Where is the sudden change? Who has gone in and "added" a 4th wire to the system?

Are you saying that we are in the middle of working on a 3-wire circuit and all of a sudden someone comes in and adds a 4th wire? These are two separate circuits. If someone wants to add a 4th wire to a 3-wire circuit, they must have a reason. Maybe they want a neutral reference.

 

[jim dungar]

Not really. Where is the sudden change? Who has gone in and "added" a 4th wire to the system?

Are you saying that we are in the middle of working on a 3-wire circuit and all of a sudden someone comes in and adds a 4th wire? These are two separate circuits. If someone wants to add a 4th wire to a 3-wire circuit, they must have a reason. Maybe they want a neutral reference.

What has changed in the system to cause the phasors to suddenly switch from head-tail to tail-tail? If the 4th wire is removed do the phasors switch back? The 4th wire is strictly arbitrary it can be connected to any of the winding center taps so why would the directions of the voltages in the coils depend on the prescence or absence of this conductor?

I am trying to get the answer to: does the simple addition of a 4th wire cause an actual change in the direction of the voltages across the winding or only the appearance that they have changed?

 

[rattus]

Let me answer this one:

Let me answer this one:

What has changed in the system to cause the phasors to suddenly switch from head-tail to tail-tail? If the 4th wire is removed do the phasors switch back? The 4th wire is strictly arbitrary it can be connected to any of the winding center taps so why would the directions of the voltages in the coils depend on the presence or absence of this conductor?

I am trying to get the answer to: does the simple addition of a 4th wire cause an actual change in the direction of the voltages across the winding or only the appearance that they have changed?

No voltages change at all, and there is no appearance that they have changed. As I understand it, the 4th wire is a CT/G/neutral which may be used as a reference node, and mivey opts to connect the tails of two of the phasors to this reference. Of course, the phase angle of the flipped phasor must be shifted by 180 degrees. Any phasor arrow can be drawn in either of two directions whether it makes sense or not.

 

[mivey]

What has changed in the system to cause the phasors to suddenly switch from head-tail to tail-tail? If the 4th wire is removed do the phasors switch back? The 4th wire is strictly arbitrary it can be connected to any of the winding center taps so why would the directions of the voltages in the coils depend on the prescence or absence of this conductor?

I am trying to get the answer to: does the simple addition of a 4th wire cause an actual change in the direction of the voltages across the winding or only the appearance that they have changed?

What has changed is the reference point. Just adding a wire will not change the direction. With or without a 4th wire, the direction of voltage is relative to a reference frame. If I change my reference point, then phasor direction changes. It is a relativity issue.

Once I have picked a reference point for a circuit, I would say the reference point decision is over and I don't think I would move it back and forth. I'm saying that my choice of a reference point may be different for a 3-wire vs a 4-wire, depending on what I was doing. I don't think we are continuously adding and removing the 4th wire and/or changing the load type. The configuration at the time I start my analysis will determine what I want to do.

If we come back later and add a 4th wire to a 3-wire circuit because we are going to have some neutral load, I may very well change my mind about what I use for a reference point. I'm sure it is not un-heard of to change the metering type when the service type and load type change.

[edit: typo]

 

[coulter]

... If I change my reference point, then phasor direction changes. It is a relativity issue. ...

How could that be? The physical source didn't change. The universe didn't warp. The physical current direction didn't change. No question you can draw the arrows any way you want and take care of the math with rattus' missing minus signs.

Jim talks about the reversal on one of the phasors in a 4WDelta and then comments on why don't you also show the other two of the Delta phasors split and half flipped. That was already discussed two threads ago. Remember Yum, Yuk, and Puk.

And now we have another reason to not like yuk or puk. The current 4WD mivey developed from a phasor drawing suspuciously similar to a Scott Tee. So far, miveys response was (paraphrased translation), "I can make my reference any way I want. I can even redefine a Scott Tee." (Yes, you did that, see post 253).

Recommend reading jon's post on Scott Tee (post 260).

The only reason I have heard to draw the single phase vector flipped over is to make the scope picture understandable to one that chooses to not take in to account the scope lead polarity.

carl

 

[coulter]

... I'm sure it is not un-heard of to change the metering type when the service type and load type change. ...

This significance of this statement completely eludes me.

Are you saying that the metering will change if the service changes from say single phase 120/240 to 240/120 4WD? Cause that's true, but near trivial for this discussion.

Or are you saying the metering changes if the service changes from 240/120 4WD to 208/120 4WY? It likely would not - have to ask one of our utility guys. I think it uses the same 7Jaw meter, as long as the voltages are in range.

I must not understand what you meant here - none of my translations fits the discussion.

carl

 

[coulter]

... Any phasor arrow can be drawn in either of two directions whether it makes sense or not.

I'd agree with one small change.

"Any phasor arrow can be drawn in either of two directions. One will make no sense at all."

carl

 

[crossman]

Indeed the vectors are all relative. They depend on what we are using as reference.

For example, take a moving train. From a certain reference point, the train velocity vector could be 100 mph and pointing to the east. But from another reference point, the train velocity vector could be 3000 mph and pointing west. I could choose a suitable inertial reference to make the train's velocity vector pretty much anything I wanted (less than C).

Choosing the neutral point as reference is an obvious choice even if it appears to make the analysis harder.

One thing that puzzles me is that when we use RMS Vab and Vbc and Vca for the 3-wire delta vectors, we don't really have a set reference point to measure voltage to. yeah, I realize it all works out, and perhaps it is easier to analyze than the neutral reference vectors.

Anyway, good discussion.

 

[engy]

I'd agree with one small change.

"Any phasor arrow can be drawn in either of two directions. One will make no sense at all."

carl

Which one? :smile:

 

[crossman]

Which one? :smile:

Brings up another issue of the delta 3-wire vectors. You can draw it with arrows going clockwise. Or counterclockwise.

 

[coulter]

I think I see one reason for the difference in viewpoints.

... I grew up in a single phase world. I never new of a three phase world until I was much older. ...

Me Too. I helped with a couple of houses prior to 1964. Then went to service/ small new work in small petroueum bulk plants. 240/120 4WD was the most common. Half-dozen small pumps, 2 - 10 hp, couple kw of office load, yard lights on 240, 1ph. By 1970 we were changing the electrical out to 480Y with a small 120/240 xfm for the office load. Yard lighting went to 480 single phase.

... When I moved to a center-tapped AC world, the grounded reference still felt natural. Since I saw the second leg as an inversion of the first, it would have felt "backwards" to not use the grounded center tap as a reference. ...

I can see your reasoning here. However, for me, once we went away from 240/120D, I never looked back. And, I don't ever remember doing a fault current analysis on 240/120D that included the neutral connection - not ever.

... When I had a combination circuit that used the grounded center tap, I used the ground as a reference because it just seemed like a natural extension of the old L-G world I started with. I guess with the L-G-L circuit and having + and - values, the bottom section just seemed more like an "inversion" of the top instead of a "stack". ....

Okay, I see that. And I would say, "Why?" Fault current analysis doesn't have much to do with the neutral. 3ph load calcs don't have anything to do with the neutral. 1ph load calcs really don't care. About all that matters is making sure the 1ph loads are with in the load rating of the xfm - most are limited to 5%.

So, the thought never occured to me to flip half of one of the arrows around just because I added a point marked "N".

Still doesn't.

carl

 

[coulter]

Which one? :smile:

Good question. You could pick the one you like from yum, yuk, puk. Luckily for me, I'm okay with which ever you choose.:grin:

carl