jumper
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Center-tap Transformer Voltages
- Thread starter mivey
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mivey
Senior Member
Been there, done that, never considered the polarity.[/B]
Either way, it still hurts.
The worst is learning that the neutral is a conductor. It ain't neutral at all: it takes sides, and it ain't yours.
mivey
Senior Member
I was thinking about hooking these up and getting some scope shots then thought better of it.
However, I was curious about the impact of feeding a DC signal, like a rectified 5-10 volt AC into the primary side of a 120:30 transformer I have laying around my workbench somewhere. I would not anticipate any damage, but then am I left to demagnetize it? I don't ever recall purposely feeding a DC signal into a transformer and have not given it much thought other than knowing that it is not what we want to do and can saturate the transformer (ignoring ferroresonant transformers for the moment).
jumper
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- 3 Hr 2 Min from Winged Horses
Been there, done that, never considered the polarity.
The worst is learning that the neutral is a conductor. It ain't neutral at all: it takes sides, and it ain't yours.
Dat true.
Hear, hear!
Hear, hear!
As gar said, more or less, that those with an engineering background tend to think in terms of phasors, loop and node equations, and common reference nodes. Those without that background tend to think in terms that work for them and that is OK as long the job gets done.
Years ago, we had this same argument and I was given a lecture on the "rotating magnetic field" in the transformer which is of no concern once the voltages are defined.
dan, you need to understand that it is perfectly valid to define the voltages at L1 and L2 as being 180 degrees apart. We arrive at the same result either way.
Hear, hear!
As gar said, more or less, that those with an engineering background tend to think in terms of phasors, loop and node equations, and common reference nodes. Those without that background tend to think in terms that work for them and that is OK as long the job gets done.
Years ago, we had this same argument and I was given a lecture on the "rotating magnetic field" in the transformer which is of no concern once the voltages are defined.
dan, you need to understand that it is perfectly valid to define the voltages at L1 and L2 as being 180 degrees apart. We arrive at the same result either way.
gar
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- Ann Arbor, Michigan
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- EE
111110-2341 EST
mivey:
When you put a single diode on the secondary of a transformer and connect a load resistor to the output, then you introduce a DC current in the secondary and this biases the core and causes an unbalanced hysteresis curve.
In a course on industrial electronics, a graduate level course, taught by Dr. Joseph A. Boyd, we were assigned the problem to hand sketch the transient change of flux (or flux density if you wish) as a function of time for the above 1/2 wave rectifier circuit with an assumed 0 residual flux density at the time of switch closure on a voltage zero crossing of a sine wave source voltage, and for several cycles after the switch closure. I got it approximately but not totally correct.
Joe Boyd was assistant director of the Electronic Defense Group at the U of M at that time, then became director. Later he became president of Radiation Corp., expanded the company, next merged it into Harris Corp. and became its chairman.
For dan and others I suggest that you find some university level books on AC and DC circuit analysis and study these. A good book on AC Analysis is "Analysis of A-C Circuits", by Melville B. Stout, 1952. But I have no idea of where you could obtain a copy. This covers steady-state and transient analysis. According to Google only 3 libraries in the world have a copy of this book. Whereas 750 libraries have a copy of his book on "Basic Electrical Measurements".
.
mivey:
When you put a single diode on the secondary of a transformer and connect a load resistor to the output, then you introduce a DC current in the secondary and this biases the core and causes an unbalanced hysteresis curve.
In a course on industrial electronics, a graduate level course, taught by Dr. Joseph A. Boyd, we were assigned the problem to hand sketch the transient change of flux (or flux density if you wish) as a function of time for the above 1/2 wave rectifier circuit with an assumed 0 residual flux density at the time of switch closure on a voltage zero crossing of a sine wave source voltage, and for several cycles after the switch closure. I got it approximately but not totally correct.
Joe Boyd was assistant director of the Electronic Defense Group at the U of M at that time, then became director. Later he became president of Radiation Corp., expanded the company, next merged it into Harris Corp. and became its chairman.
For dan and others I suggest that you find some university level books on AC and DC circuit analysis and study these. A good book on AC Analysis is "Analysis of A-C Circuits", by Melville B. Stout, 1952. But I have no idea of where you could obtain a copy. This covers steady-state and transient analysis. According to Google only 3 libraries in the world have a copy of this book. Whereas 750 libraries have a copy of his book on "Basic Electrical Measurements".
.
The CRUX of the FLUX:
The CRUX of the FLUX:
The problem arises I believe because some are taught the relationship between the core flux and the induced voltage in the secondary. Then they are taught that adding a CT creates two identical voltages which, as they are defined, are in phase and therefore ADD.
They are not taught however that both voltages can be defined relative to a common reference in which case one is subtracted from the other to get the potential DIFFERENCE between the two points. Or course to do this one must study trig and phasors which are not for the faint of heart.
The CRUX of the FLUX:
The problem arises I believe because some are taught the relationship between the core flux and the induced voltage in the secondary. Then they are taught that adding a CT creates two identical voltages which, as they are defined, are in phase and therefore ADD.
They are not taught however that both voltages can be defined relative to a common reference in which case one is subtracted from the other to get the potential DIFFERENCE between the two points. Or course to do this one must study trig and phasors which are not for the faint of heart.
jjkind
Member
- Location
- Las Vegas, NV
I am using the free, 2D version of MagNet FEA software to design a switched reluctance motor (software available here: http://www.infolytica.com/en/products/magnet/ ). If you are curious as to what would happen in the core of the transformer with an pulsed DC signal, you could easily simulate it here by importing a simple core and coil geometry and running a 2D simulation under varying currents. The software gives you numeric and graphical output so you can see how and where the core is saturating under different currents. I have used it to optimize the geometry of my SRM to avoid points of excessive saturation and ensure even flux throughout the rotor/stator during the firing of each phase. Unfortunately, you won't be able to investigate or visualize the very interesting transient behavior...the 'demagnetization' of the core via flux leakage and the effects of hysteresis...with this free version (I understand that you can with the 3D dynamic version of the software, but it is around $20k). If you now of something that I could use to visualize these dynamic processes...something that doesn't cost as much as a new car!...let me know. This is one of those areas where the science becomes a bit of an art.
gar
Senior Member
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- Ann Arbor, Michigan
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- EE
111111-2334 EST
rattus:
I personally had Stout as my teacher for our measurements course, and a couple other courses. I can draw a direct correlation between his measurements course and a circuit patent I received.
What was your connection with the Signal Corp?
.
rattus:
I personally had Stout as my teacher for our measurements course, and a couple other courses. I can draw a direct correlation between his measurements course and a circuit patent I received.
What was your connection with the Signal Corp?
.
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Rick Christopherson
Senior Member
Pardon my flippancy, but I can't help but make the observation. This discussion is kind of like going to the dance at an all-boys school. Has it not occurred to you guys that you are all standing alone on one side of the room?
:jawdrop:
You've been carrying on an argument for 30 postings, and none of you have noticed that the other side isn't even participating? :lol::lol::lol:
I'm sorry. Please carry on with what you were saying. :angel:
:jawdrop:
You've been carrying on an argument for 30 postings, and none of you have noticed that the other side isn't even participating? :lol::lol::lol:
I'm sorry. Please carry on with what you were saying. :angel:
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Did you have something useful to add?:? If so, I missed it.:roll:
Let's say each winding turn adds 1 volt per turn. The secondary winding has 120 turns and 120 volts out. Starting from the winding end, which is either L1 or L2 the winding turns add 1 volt out each in series to the winding center, system grounded point, which yields 120 volt.
As I contiue adding winding turns past the neutral point, what happens. The winding turns continue adding identically in phase, in series. Output voltage progresses 121 volt, 122, 123 volt ..
If the subsequent winding turns past the neutral point were "180 out of phase" each added turn, the output voltage would go 119 volt, 118, 117 .. In fact, this paradigm fails this test.
Some part of your audience wants to know "what happens" and you have the O O P repeating gobbledegook 240 is "the voltage are 180 out of phase" when he could have been saying, the "the wiindings are in series (and matched)". It makes my ears hurt.
As I contiue adding winding turns past the neutral point, what happens. The winding turns continue adding identically in phase, in series. Output voltage progresses 121 volt, 122, 123 volt ..
If the subsequent winding turns past the neutral point were "180 out of phase" each added turn, the output voltage would go 119 volt, 118, 117 .. In fact, this paradigm fails this test.
Some part of your audience wants to know "what happens" and you have the O O P repeating gobbledegook 240 is "the voltage are 180 out of phase" when he could have been saying, the "the wiindings are in series (and matched)". It makes my ears hurt.
I agree completely and when you get done the voltage between Line #1 and Line #2 would be zero
and you would still have 120 volts from neutral to line #1 and line #2.
No, not quite:
No, not quite:
The proper way to do this is to start at neutral when you are adding turns. Then for L1 say, you are adding 1V at 0 until you reach 120V.
But when adding turns between N and L2, the sense of the windings is reversed, so you are adding 1V @ 180 with each turn.
This is the whole idea of establishing a reference point.
Got it?
No, not quite:
The proper way to do this is to start at neutral when you are adding turns. Then for L1 say, you are adding 1V at 0 until you reach 120V.
But when adding turns between N and L2, the sense of the windings is reversed, so you are adding 1V @ 180 with each turn.
This is the whole idea of establishing a reference point.
Got it?
You are making my point for me. The description leads to confusion.
The winding does not start at the neutral. It starts at an end, L1 or L2. This is the physical reality of the device as fabricated by the manufacturer. The neutral is at the middle of the winding and the physical turn direction continues homogeneous. The fact of the winding does not reverse.
Reversal comes as an artifact of how the meter leads or loads are attached to the sources. The leads reverse after the output, between the source and the load. Winding turn direction does not reverse.
It is in the way you look at it:
It is in the way you look at it:
Doesn't matter where the winding starts. I choose to start at the neutral in my analysis. If one could stand at the neutral and look at the windings, one winding would appear to be clockwise; then turn around and look at the other winding; it would be counter clockwise. Think about it.
Consider a right hand screw. If you look at it from the head, the threads are clockwise; if you look at it from the other end, the threads are counter clockwise. Try it.
Or consider a piece of threaded rod. It is threaded right hand or left hand. Try it.
It is in the way you look at it:
Doesn't matter where the winding starts. I choose to start at the neutral in my analysis. If one could stand at the neutral and look at the windings, one winding would appear to be clockwise; then turn around and look at the other winding; it would be counter clockwise. Think about it.
Consider a right hand screw. If you look at it from the head, the threads are clockwise; if you look at it from the other end, the threads are counter clockwise. Try it.
Or consider a piece of threaded rod. It is threaded right hand or left hand. Try it.
Most power Transformers we use in the electrical trade have two seperate
secondary windings.
They are marked x1, x2, x3 and x4
You can put them in parallel for twice the current at 120 VAC or in series
for 240 VAC.
The normal series configuration is x1 to line #1 x2 and x3 tie together for the neutral
Line #4 is line #2.
If you really reverse one of the secondary windings in the series configuration it will be
180 degrees out of polarity from the other one. Doing this would cause the secondary
to null and produce zero volts between line #1 and line #2.
If you reverse it in the parallel configuration it will make a very loud pop and let the
smoke out of it.
secondary windings.
They are marked x1, x2, x3 and x4
You can put them in parallel for twice the current at 120 VAC or in series
for 240 VAC.
The normal series configuration is x1 to line #1 x2 and x3 tie together for the neutral
Line #4 is line #2.
If you really reverse one of the secondary windings in the series configuration it will be
180 degrees out of polarity from the other one. Doing this would cause the secondary
to null and produce zero volts between line #1 and line #2.
If you reverse it in the parallel configuration it will make a very loud pop and let the
smoke out of it.
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