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Understanding AC phase relationship...

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devronious2009

New Member
If I induce a voltage in a coil via a magnetic field, will the amperage follow the voltage with some delay? Or is the graph of the voltage and amperage occure at the same time just at different levels?

I know that inductive reactance limits the current to less than the coil resistance would allow. But is there any phase delay between the voltage and current?
 

crutschow

Well-Known Member
Most Helpful Member
If there is no external or stray inductance in the circuit then the current will be in perfect phase with the voltage. This is true for a transformer also.
 

Papabravo

Well-Known Member
A changing magnetic field will induce a voltage in a coil and the usual:
Code:
V = L*(di/dt)
will apply. This means that the voltage and current will not be in phase. I don't know what the previous answer was referring to but certainly not the question you posed.
 

crutschow

Well-Known Member
Most Helpful Member
A changing magnetic field will induce a voltage in a coil and the usual:
Code:
V = L*(di/dt)
will apply. This means that the voltage and current will not be in phase. I don't know what the previous answer was referring to but certainly not the question you posed.
The answer was indeed referring to the question asked.

V = L*(di/dt) refers to the self induced voltage by a changing current in an inductor, not a voltage induced by an external changing magnetic field in the coil. They are two quite different things.

The self-inductance of a coil does not affect the phase of the current and voltage induced in that coil by an external field. That's why a transformer may have Henrys of inductance but the input and output voltage and current are in-phase with no lag from that inductance. Similarly the output inductance of a generator does not affect the phase of the output voltage and current.
 

devronious2009

New Member
crutschow,

So if I get this right, considering only a single transformer in a circuit as the only component, which is pretty much what I have, the voltage and amperage are in phase with eachother, and the secondary is also in phase with the primary. So that all the phases of all the voltages and currents are all in phase with eachother?

Papabravo,

The inductive reactance should just limit the amperage due to the voltage in the coil, not throw off the phase? Isn't that right? Crutschow is that what you mean too?
 

Papabravo

Well-Known Member
The answer was indeed referring to the question asked.

V = L*(di/dt) refers to the self induced voltage by a changing current in an inductor, not a voltage induced by an external changing magnetic field in the coil. They are two quite different things.

The self-inductance of a coil does not affect the phase of the current and voltage induced in that coil by an external field. That's why a transformer may have Henrys of inductance but the input and output voltage and current are in-phase with no lag from that inductance. Similarly the output inductance of a generator does not affect the phase of the output voltage and current.
In any AC circuit the only way that current and voltage can be in phase is if the reactance is zero. Slice it any way you want but what you are describing cannot happen.
 

ke5frf

New Member
In any AC circuit the only way that current and voltage can be in phase is if the reactance is zero. Slice it any way you want but what you are describing cannot happen.

You are correct.

Impossible for voltage and current to be in phase when the transformer winding is in practically a short when current is at its peak. Voltage is potential, potential is zero across a short, therefore no voltage when current is peak, therefore E,I out of phase.
 

smanches

New Member
LTSpice shows the input winding powerfactor is bad, but the output is near 1.0.
 
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devronious2009

New Member
So how do I calculate the phase of the current, and lets pretend there is a load resistor and not a total short so long as it doesn't effect the phase, which it shouldn't right?

[edit] just so you know I need to plot the phase and need to know how to calculate the amperage phase behind the voltage then.
 
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ke5frf

New Member
So how do I calculate the phase of the current, and lets pretend there is a load resistor and not a total short so long as it doesn't effect the phase, which it shouldn't right?

[edit] just so you know I need to plot the phase and need to know how to calculate the amperage phase behind the voltage then.


No, a resistor WILL effect the phase, as will any capacitive reactance in the circuit.

This is all fundamental in AC electronics, it is how radios, oscillators, antennas, notch and bandpass filters all work.

The formulas are ALL OVER the internet. I'm not big on doing the math, though I HAVE in the past (for tests LOL). There are calculators that make it just as easy.

You just have to now the impedance of your coil to begin with.

Google RLC and phase and you'll fiind your formulas and some calculators too.
 

smanches

New Member
If you are inducing the current with a magnetic field, then they are in phase, just as crutschow stated. If you are inducing the current with a voltage through a coil, then they will be out of phase based on the inductance of the coil.

This is what LTSpice is showing, and I doubt they got such a fundamental principle wrong.
 

crutschow

Well-Known Member
Most Helpful Member
You are correct.

Impossible for voltage and current to be in phase when the transformer winding is in practically a short when current is at its peak. Voltage is potential, potential is zero across a short, therefore no voltage when current is peak, therefore E,I out of phase.
That is not correct.

Apparently you've never worked with transformers. The voltage and current out of a transfromer are indeed in phase. Measure it if you don't believe.

If there were no voltage when the current was at its peak (90 degrees out of phase), the power factor would be zero and the transformer would be delivering no power.

The transformer is not a short to it's own voltage. Where did you ever get that idea?
 

Papabravo

Well-Known Member
I have worked with transformers. I have measured them and simulated them and I'm here to tell you that the current and voltage on the secondary of a transformer are most definitely not in phase with each other at least as far as I understand the meaning of that term.

One of them may be in phase with the alternating magnetic field that generated the output on the coil. It is also the case that the output of a generator does not have the current and voltage in phase with each other. If it was possible to get a generator to do that, the whole power factor correction industry would be a solution looking for a problem.

You can continue to insist on any particular fantasy that you like, but you should be mindful of the damage you are doing to people trying to learn.
 

ke5frf

New Member
OK, I have no qualms with disclosing that I don't know everything. Send that to the 5 o'clock news if you wish :)

However, I have worked plenty with transformers, especially baluns and other impedance matching transformers.

And they are, in fact, reactive elements in an AC circuit, and must be accounted for.
They, in fact, have impedance characteristics and thus have an ohmic value

And they are, in fact, transformers.

Having thought the matter through a bit, I think you are strictly thinking of power transformers, and to be honest I am not sure I've ever given them as much thought as matching transformers, being my pet interest has always been radio, antennas, and transmission lines.

Adding to that, in my 15 years of maintaining electronic equipment at various capacities, I'm quite certain I've never had a power trasformer fail on me, though it isn't unheard of to have shorted windings it is fairly rare. So yeah, in that regard I haven't given it much thought.

Perhaps, if you will, without math, with more of a mental model, you could describe why there is no reactance accounted for in a power transformer's performance.

I am here to share, but believe me when I say I'm even more eager to learn.
 

Papabravo

Well-Known Member
OK, I have no qualms with disclosing that I don't know everything. Send that to the 5 o'clock news if you wish :)

However, I have worked plenty with transformers, especially baluns and other impedance matching transformers.

And they are, in fact, reactive elements in an AC circuit, and must be accounted for.
They, in fact, have impedance characteristics and thus have an ohmic value

And they are, in fact, transformers.

Having thought the matter through a bit, I think you are strictly thinking of power transformers, and to be honest I am not sure I've ever given them as much thought as matching transformers, being my pet interest has always been radio, antennas, and transmission lines.

Adding to that, in my 15 years of maintaining electronic equipment at various capacities, I'm quite certain I've never had a power trasformer fail on me, though it isn't unheard of to have shorted windings it is fairly rare. So yeah, in that regard I haven't given it much thought.

Perhaps, if you will, without math, with more of a mental model, you could describe why there is no reactance accounted for in a power transformer's performance.

I am here to share, but believe me when I say I'm even more eager to learn.
Think about the magnitude of the reactance at power line frequencies versus the DC resistance of the windings or the load resistance. 10 mH -> 4 Ohms Reactance at 60 Hz. When it comes to delivering power at DC the reactance no longer has much influence on the situation.
 
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ke5frf

New Member
That makes sense. Yes. So if all AC circuits operated at 50 or 60 HZ, then the "no phase change" argument would be valid.

But AC is not limited to line power, thankfully, or we would have a pretty boring hobby :)
 

devronious2009

New Member
OK to be honest since this seems complex. I have a secondary coil only to consider. it is in direct short. The magnetic field from the primary is generating the voltage and thus amperage in the secondary. I don't reall care or am concerned with the voltage or amperage phase of the primary. All I'm concerned about is the voltage and amperage phase of the secondary. So its really just voltage to amperage thru the secondary coil to consider. Does this help. And I need a formula cause I'm building software to chart the data. I can't use online calculators really since they don't plug into the program. I need formulas. And thanks for your guys help, I'm a little confused at this point.

How is the amperage affected? Isn't it limited by the inductive reactance only?
 

devronious2009

New Member
OK to be honest since this seems complex. I have a secondary coil only to consider. it is in direct short. The magnetic field from the primary is generating the voltage and thus amperage in the secondary. I don't reall care or am concerned with the voltage or amperage phase of the primary. All I'm concerned about is the voltage and amperage phase of the secondary. So its really just voltage to amperage thru the secondary coil to consider. Does this help. And I need a formula cause I'm building software to chart the data. I can't use online calculators really since they don't plug into the program. I need formulas. And thanks for your guys help, I'm a little confused at this point.

How is the amperage affected? Isn't it limited by the inductive reactance only?

so I guess the real question is: How do I calculate the inductive reactance at different phase angles. Or how to I calculate it at different voltage points?
 

ke5frf

New Member
Another point.

I'm not sure why it is so "shocking" to consider that a coil, choke, or transformer in a circuit, at some point in an AC cycle, is seen at the supply as (in essence) a short.

Maybe my ability to describe those conditions is lacking. I'm not an engineer or physics professor. I think in more "animated" or graphic terms.

I also do not follow, precisely, how voltage and current being out of phase equals no power.

Just because they aren't in phase, this doesn't mean they aren't still "there" and calculable, right? I mean if a voltage of y exists in x circuit, and a current of z exists in x circuit, then ohms law still applies (P=ExI), no matter what point in time they occur, correct? Wouldn't this be because in AC, we talk of AVERAGE current and AVERAGE voltage, not peak...right?

Maybe I'm missing something, but I am here to learn.

I think of a dead short, which in actuality is never a perfect short as all conductors have resistance...but this is true whether AC or DC....As minimal as the resistance is, when the current has spiked and all those electrons are bursting through the seams to get through, if I measured the voltage across the short with a meter, would it not be close to zero and out of phase with the current? Yet, nobody can argue that there is no power there. There might even be a small fire!

Voltage is potential, or electrical pressure...and when there is a short, it is no longer potential but rather an occurance whose potential has been released.

I think of a cylinder of compressed air with a pressure gauge and a valve. I open the valve a little, there is flow. There is still some potential because there is resistance in the valve. As I open it, that potential is exchanged for increased flow. The pressure drops a bit at the gauge. If my valve had a large enough oriface to create a large enough flow, I might see the needle on the gauge drop to near zero lbs of pressure, depending on what the supply pressure was to begin with.

Maybe I think in too simple terms for this hobby :p
 
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