240v debate....

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Besoeker

Senior Member
Location
UK
Both legs are not 180 degrees apart but are on the very same cycle together with twice the magnitude between the two than between one and the center tap of the transformer.
To get twice the magnitude they have to be 180 degrees apart:

120V-0-120V.jpg


Blue is L1-N
Magenta is L2-N
Red is L1-L2
 

Rick Christopherson

Senior Member
To get twice the magnitude they have to be 180 degrees apart:
Have to be? No. They only "have to be" because you choose to reverse the polarity of your definition. That's a choice. Not an absolute. It's a choice that you have made, but it is not a choice that is universally made by all.
 

Besoeker

Senior Member
Location
UK
Have to be? No. They only "have to be" because you choose to reverse the polarity of your definition. That's a choice. Not an absolute. It's a choice that you have made, but it is not a choice that is universally made by all.

For a single-phase transformer.e with a centre tap, they are and that's the point I was addressing.
You wouldn't get 120-0-120V with 240V end to end otherwise.
From post #21:
center tap of the transformer.
 

Rick Christopherson

Senior Member
For a single-phase transformer.e with a centre tap, they are and that's the point I was addressing.
You wouldn't get 120-0-120V with 240V end to end otherwise.
From post #21:
center tap of the transformer.
Whoa there little Besoeker Buckaroo! (Sorry, couldn't help myself :D) Are you, as a fellow electrical engineer, honestly suggesting that the electrons are suddenly reversing themselves when they get to that mythical midpoint in the transformer? Do they go zipping along on their merry way down the wire, and then they suddenly see this big red stop sign in front of them at the center tap, and say, "oops, we can't go any farther this way, we gotta turn around and go back the other way"?

I hope you're not, because that is how your words are coming across in your posting.

The 180? phase shift exists only because you have hidden the negative sign caused by reversing the polarity of the power source's reference. If during your circuit analysis you fail to acknowledge (or discover) this hidden negative sign, then one half of the transformer's secondary coil will be a power source and the other will be a power sink (load). One half will have current opposing voltage, and the other half will have current conforming to voltage. These are electrical Laws that we are not free to break.

You are free to choose the personal preference of labeling your voltages A-N and B-N, but in doing so, you cannot ignore the resulting negative sign from not choosing the A-N-B true convention of the actual voltages. If you insist on using the 180? designation, then you must include the negative sign with either your A-N or B-N voltage definition. Those two negatives must cancel out somewhere in your circuit analysis.
 

mivey

Senior Member
Are you, as a fellow electrical engineer, honestly suggesting that the electrons are suddenly reversing themselves when they get to that mythical midpoint in the transformer? Do they go zipping along on their merry way down the wire, and then they suddenly see this big red stop sign in front of them at the center tap, and say, "oops, we can't go any farther this way, we gotta turn around and go back the other way"?

I hope you're not, because that is how your words are coming across in your posting.

The 180? phase shift exists only because you have hidden the negative sign caused by reversing the polarity of the power source's reference...
There is no electron reversal as the results are the same as you can get with two separate fluxes. The waveforms are working in concert.

Ponder this: Take two identical single-phase transformers with single bushings (i.e., the tank serves as the other bushing, a grounded bushing if you will). Let's say the bushings are H0-H1, X0-X1.

At the primary of one transformer connect bushing H0 to ground and H1 to a sinusoidal voltage source of magnitude M and frequency F that is at a positive peak at time zero (call it voltage VH). On the second transformer, connect H0' to ground and H1' to a sinusoidal voltage source of magnitude M and frequency F that is at a negative peak at time zero (call it voltage VH'). In other words, VH' is a voltage source that is 180 degrees displaced from VH (easily created by using a delay box or by using the physical displacement of a second winding on the same generator shaft, etc).

Tie the tanks (the XOs) of the secondaries together and consider the secondary waveforms between the terminals.

The single-phase center-tap transformer produces the same waveforms but only uses one primary voltage. We could use two primary voltages and two transformers but that would not be economical so we don't.

I hope you can see that the choice of reference point is completely arbitrary. In deciding which terminal you pick as a reference, you may use the fact that there is only one flux in the center-tap configuration to say we must use X1 or X4, but that is a preference, not a universal choice (neither is using the tank grounds in the two-tank scenario).

Even in the single-flux case, you can take the secondary waveforms of the center-tap, run them through two isolation transformers, and get two separate waveforms and fluxes just like you would have in the single-bushing case (FWIW, you could also do this with the secondaries of the single-bushing set-up even though we just need to separate the transformers to "separate our fluxes", so to speak).

Also consider that in the single-flux case, the presence of the center-tap makes it possible to have two different currents in the two halves of the windings (i.e. with unbalanced loading). Without the center-tap, we can only have the single waveform (the X1-X4 voltage). With the center-tap, we have the capability to supply multiple waveforms.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Before we get too buried in discussions of electrical theory, let's go back to the original question.
But... I get this a lot from people, 240v is more dangerous. So I'm looking for a better way to explain to people that its just as safe as a 120v circuit but with better benefits.
The statement that 240 volts is more dangerous than 120 volts is equivalent to saying that 120 volts is safer than 240 volts. And both statements are nonsense. It is possible for a person to get electrocuted by a 24 volt truck battery. The amount of current that a 120V source can push through a human body is many times higher than is needed to kill the person, and many, many more times higher than the amount of current that would contract a person's muscles so tightly that the person would not be able to let go of the energized wires. You don't get any "more dead" by touching even higher voltages.
Electricity can be your servant; it will never be your friend. Famous saying by some famous person.
 

iwire

Moderator
Staff member
Location
Massachusetts
The statement that 240 volts is more dangerous than 120 volts is equivalent to saying that 120 volts is safer than 240 volts. And both statements are nonsense.

I do not think those statement are nonsense at all.

It is possible for a person to get electrocuted by a 24 volt truck battery.

It is possible but much less likely, to me that tells me in most cases 24 volts is safer than 120, which is safer than 240.


'Safer' is a relative term, not a black and white one. :)
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Bob, I call the two statements nonsense, in the sense that energized electrical equipment is not safe, and voltage levels do not change that statement. It is the same sense in which I would declare that a pistol is never unloaded, that a stop sign is not capable of causing a driver to stop his car, and that it is unwise to stand under a load being lifted by a crane. We are surrounded by dangerous circumstances. We improve our chances of coming home at the end of the day by recognizing the dangers, and by taking specific precautions to mitigate the dangers. You are far more aware of that than I could ever be, since your chosen profession puts you closer to such dangers than mine does. But the basic notion of ?this voltage level is safer than that voltage level? simply runs contrary to my notion of the meaning of safety.
 

jwelectric

Senior Member
Location
North Carolina
To get twice the magnitude they have to be 180 degrees apart:

120V-0-120V.jpg


Blue is L1-N
Magenta is L2-N
Red is L1-L2

Interstate 95 has two lanes running from North to South and two lanes from South to North.

Using one side as A-N and the other side as B-N and N being the grassy part between the four lanes of traffic one can easily see that all four lanes of traffic are traveling in the same direction.

Looking east we can see the traffic that is coming from the south and going north. Looking to the west we can see that the traffic is coming from the north and going south.

As we stand there looking all the traffic is coming from the right and leaving to the left no matter which way we are looking. The same is true on a center tap transformer. The sine wave from either the A or B line to N is exactly the same as the sine wave of A to B. There is no 180 degree phase shift. We only see it on the screen of the scope due to both being referenced to the center tap.
 

Rick Christopherson

Senior Member
I hope you can see that the choice of reference point is completely arbitrary. In deciding which terminal you pick as a reference, you may use the fact that there is only one flux in the center-tap configuration to say we must use X1 or X4, but that is a preference, not a universal choice (neither is using the tank grounds in the two-tank scenario).
There is a difference between having a chosen representation for a system, and defining the system. All too often, people forget that it is a chosen representation, and they try to define the system based on the representation. This is where you run into problems.

Saying that the two waveforms are the equivalent of being 180? out of phase is not the same as saying the two wave forms are 180? out of phase. This chosen representation works only because we have a symmetrical waveform. The symmetrical waveform allows you to represent one of them as though it had an 8 ms time delay with a magnitude reversal. However, if the waveform was not symmetrical, you cannot represent them with this time delay. There is no time delay. It is just a chosen representation.

Let's say your mechanical generator has some sort of flaw that introduces an artifact into the waveform every 360 degrees. This artifact is going to appear in both of your output waveforms at the same instant in time, yet your chosen representation says the two waveforms are time shifted by 180?. This is a situation where the chosen representation fails.

There is nothing wrong with using whatever chosen representation someone wants. It's their choice. The problem arises when someone forgets that it is simply a chosen representation, and tries to define the system as though the representation was real.
 

Besoeker

Senior Member
Location
UK
Whoa there little Besoeker Buckaroo! (Sorry, couldn't help myself :D) Are you, as a fellow electrical engineer, honestly suggesting that the electrons are suddenly reversing themselves when they get to that mythical midpoint in the transformer? Do they go zipping along on their merry way down the wire, and then they suddenly see this big red stop sign in front of them at the center tap, and say, "oops, we can't go any farther this way, we gotta turn around and go back the other way"?

I hope you're not, because that is how your words are coming across in your posting.

The 180? phase shift exists only because you have hidden the negative sign caused by reversing the polarity of the power source's reference. If during your circuit analysis you fail to acknowledge (or discover) this hidden negative sign, then one half of the transformer's secondary coil will be a power source and the other will be a power sink (load). One half will have current opposing voltage, and the other half will have current conforming to voltage. These are electrical Laws that we are not free to break.

You are free to choose the personal preference of labeling your voltages A-N and B-N, but in doing so, you cannot ignore the resulting negative sign from not choosing the A-N-B true convention of the actual voltages. If you insist on using the 180? designation, then you must include the negative sign with either your A-N or B-N voltage definition. Those two negatives must cancel out somewhere in your circuit analysis.

I'll try again.
Take the instant in time 90 deg into the cycle. At that point, L1 is positive with respect to the centre tap, the neutral, and L2 is negative.
Here, we don't usually use a centre-tapped configuration except for some specific applications like hexaphase rectifiers. These are three phase units with the three secondary windings centre tapped and the centre taps connected together to and are the the negative leg. The other ends go to SCRs and are controlled to to provide the voltage on the positive leg.
If you take any one of the three windings, they are in anti-phase. Just like a single phase centre tapped transformer. That's just how it works.

If you'd like, I'll try to find a bit of time to capture real-time waveforms on a storage 'scope. Would that convince you?
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Besoeker,

I don't know what was so controversial about what you said. In a single phase center tap setup as described, the voltage as referenced from A-to-neutral and from B-to-neutral always have a 180? phase difference. If Van = 120<0 then Vbn = 120<180. If Van = 120<90 then Vbn = 120<-90. If Van = 120<-35 then Vbn = 120<145.
 

Besoeker

Senior Member
Location
UK
The sine wave from either the A or B line to N is exactly the same as the sine wave of A to B. There is no 180 degree phase shift. We only see it on the screen of the scope due to both being referenced to the center tap.
If they were all the same and in phase, how would you get 240V end to end on the transformer?
 

kwired

Electron manager
Location
NE Nebraska
I am not a big expert on phase angles and reading sine waves and that type of thing but this debate all comes from this post:

With both legs being 180 degrees apart, is it even possible to be hit with a 240v shock?

If it is not possible to be hit with the 240 volt shock as it suggests then how would a load ever see 240 volts?

I don't know that we need to technical of an answer to realize that the suggestion in that post is not possible based on the fact that we do connect loads to this source and operate them at 240 volts.
 

jwelectric

Senior Member
Location
North Carolina
If they were all the same and in phase, how would you get 240V end to end on the transformer?

By not using the center tap we get one 240 volt sine wave.

It is just like a 2 D cell flashlight. The total voltage is three volts but we could use the same two batteries to light the 3 volt bulb at the same time lighting two 1.5 volt bulbs using a center tap between the two batteries.
 

Besoeker

Senior Member
Location
UK
By not using the center tap we get one 240 volt sine wave.

It is just like a 2 D cell flashlight. The total voltage is three volts but we could use the same two batteries to light the 3 volt bulb at the same time lighting two 1.5 volt bulbs using a center tap between the two batteries.
OK.
Let's go with that as an analogy.
Here's what you said in post #21:
Both legs are not 180 degrees apart but are on the very same cycle together with twice the magnitude between the two than between one and the center tap of the transformer.
For your two 1.5V batteries to have a "centre tap" and produce 3V end to end, they would have to be connected in series with the positive of one connected to the negative of the other. Just like in your flashlight. Measure from one battery to the centre point and you get +1.5V and from the other to the centre point and you get -1.5V.

The analogy works quite well for a centre-tapped transformer. Look back at the diagram I posted. At 90 degrees in, one voltage is +Vpk, the other -Vpk.
Does that help your understanding any?

BTW, "centre" is the correct spelling for British English which is why I spell it that way.
 
Last edited:

Rick Christopherson

Senior Member
Besoeker,

I don't know what was so controversial about what you said. In a single phase center tap setup as described, the voltage as referenced from A-to-neutral and from B-to-neutral always have a 180? phase difference. If Van = 120<0 then Vbn = 120<180. If Van = 120<90 then Vbn = 120<-90. If Van = 120<-35 then Vbn = 120<145.
A 180? phase difference is an 8ms time delay at 60Hz (or 10ms @50 Hz). If this assertion that a 180? phase shift truly existed, then it would mandate that if a waveform artifact (like a spike) were to occur in the primary winding at To then the transformed spike would allegedly occur in the secondary A-phase at To, but in the B-phase at To + 8ms.

In reality, if there is a spike or other anomaly in the primary, it will be transformed at both secondary terminals at the same instant in time. There will not be an 8ms delay (180? phase shift) with one phase versus the other.

Besoeker and many others have chosen to invert one of the signals as a convenience for visualizing the system, and that is perfectly fine. The error comes about when you define the system as though it has this inversion and time-shift. The inversion and time-shift are not real, but are only mathematical tools used to make their analysis fit their needs.

Besoeker, my apologies for assuming that this was something patently obvious to anyone with an engineering degree. However, if you spend a little time giving this some critical thought, I am sure that you will see the distinction between how a system is perceived versus how it is defined, and it cannot be defined with this artificial time delay.
 

jwelectric

Senior Member
Location
North Carolina
The analogy works quite well for a centre-tapped transformer. Look back at the diagram I posted. At 90 degrees in, one voltage is +Vpk, the other -Vpk.
Does that help your understanding any?
The reason why you are seeing this as one 170 peak + and 170 peak ? is due to the manner in which you are connecting the two channels of your meter to the system not because there are two phases that are 180 degrees out of each other.

Just what type of phase system is it you are working with? Is it a two phase system or a single phase system? If you say a single phase then there is only one phase and nothing that is 180 degrees out of anything.


By the way I think I got it down pretty good.
 
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