Understanding AC phase relationship...

[smanches]

I would agree with that as well, but in this case you're inducing the current from the voltage in the line. And it would be inline with how an inductor works when being induced by voltage. It's the back EMF that is causing them to go out of phase.

OP asked if he generates the current from a magnetic field; that is a generator. In this case, everything I can find (which I admit is almost nothing so far) shows them both to be in phase. In this case, there is no back EMF, so there should be no phase difference.

The real question is whether generators produce voltage and current waveforms that are in phase with each other. I would have to think yes, otherwise PF correction would be dependent on the specific generator as well as the load, not just the load.

 

[devronious2009]

OK I modeled the software via the PDF and used the arctangent of the Inductive Reactance. I get the following:

**broken link removed**

Does that seem right? The amperage is 90 degrees behind the voltage. And the frequency only very slightly changes that by like a hair. The lower the frequency the further off from 90 it is, but again only slightly.

Also I'm aware that the amplitudes are not correct, but I'm really only concerned with the angle.

 

[smanches]

Yes, that would look correct if you are sending a signal through an inductor. But didn't you mention that you are supplying the magnetism itself to induce the current/voltage?

 

[crutschow]

4-6 1. A current flows in the antenna with an amplitude that varies with the generator voltage. 2. A sinusoidal distribution of charge exists on the antenna. Every 1/2 cycle, the charges reverse polarity. 3. The sinusoidal variation in charge magnitude lags the sinusoidal variation in current by 1/4 cycle.

A 1/4 cycle is 90 degrees out of phase.

I believe the confusion is the phrase "variation in charge magnitude lags the variation in current". It does not say the variation in current lags the voltage. The variation in current is in phase with the voltage, but out of phase with the variation in charge. Make sense?

Also again regarding a generator, transformer or any induced voltage in a coil: Smanches is correct. The voltage is in phase with the magnetic change. Any phase shift at the output is due to the reactive charactistics of the load. It is not due to any inherent inductance of the transformer/coil/generator (ignoring stray or parasitic inductance).

 

[ke5frf]

I believe the confusion is the phrase "variation in charge magnitude lags the variation in current". It does not say the variation in current lags the voltage. The variation in current is in phase with the voltage, but out of phase with the variation in charge. Make sense?

Also again regarding a generator, transformer or any induced voltage in a coil: Smanches is correct. The voltage is in phase with the magnetic change. Any phase shift at the output is due to the reactive charactistics of the load. It is not due to any inherent inductance of the transformer/coil/generator (ignoring stray or parasitic inductance).

No, I don't think there is confusion.

Further reading from the same article:

Figure 4-6.—Standing waves of voltage and current on an antenna

"Look at the current and voltage (charge) distribution on the antenna in figure 4-6. A maximum movement of electrons is in the center of the antenna at all times; therefore, the center of the antenna is at a low impedance. This condition is called a STANDING WAVE of current. The points of high current and high voltage are known as current and voltage LOOPS. The points of minimum current and minimum voltage are known as current and voltage NODES."

And if you refer the diagrams, the point of high current "loop" is at the place where voltage is zero, or the voltage "node", and where current is zero, there is a voltage loop...and the voltage loop corresponds to areas of high charge magnitude.

I realize that voltage and charge are two different things, but that doesn't mean they aren't related, or rather aren't UNRELATED.

 

[devronious2009]

Smanches,

Yes I did mention that. I was under the impression that the voltage induced by a magnetic field was delayed 90 degrees and if the amperage is delayed 90 degrees from the voltage then there is a distribution of 180 degrees between the three roughly. Isn't that correct?

The voltage reflects the change in magnetic field, from full positive to full negative of a magnetic field the the voltage is one polarity and then from full negative to full positive the voltage is the other polarity. its out of phase 90 as well.

 

[crutschow]

Well, I'm certainly no expert on antennas so I have no real argument about what happens inside them, and the current and voltage may very well be out of phase. But fundamental AC circuit theory says the voltage and current driving the antenna at the coax interface must be in phase for power to be delivered to the antenna.

Edit: After some thought I realized that an antenna apears as a lossy resonant circuit with the loss being mostly the radiation into space. That's why the voltage and current are out of phase in the antenna. But at resonance, which is the design frequency of the antenna, it appears to be purely resistive to the drive circuit (see Antenna resonance and bandwidth :: Radio-Electronics.Com).

 

[smanches]

Yes I did mention that. I was under the impression that the voltage induced by a magnetic field was delayed 90 degrees and if the amperage is delayed 90 degrees from the voltage then there is a distribution of 180 degrees between the three roughly. Isn't that correct?

The voltage reflects the change in magnetic field, from full positive to full negative of a magnetic field the the voltage is one polarity and then from full negative to full positive the voltage is the other polarity. its out of phase 90 as well.

Voltage and current are NOT out of phase when induced by a magnetic field. This is what I've been researching all day now.

The problem is there is almost no direct information out there about this. I think it must just be taken for granted for those that work with it every day. There is a TON of information about load power factor, but nothing for generator output power facter, which is what you are looking for. I'm sure everyone probably just learns it in school, and being so simple really, it's never touched on again.

 

[mbarazeen]

Hi,

i couldnt fully read all your discussions, but just my knowledge in this title is..
if we analyse by taking a mathematical model for a transformer.... it has a series inductance and a resistance wit the load seen from either of side.

so when there is an open circuit(no load) it acts like an open circuit for the souce thus doesnt consume current(infact small current is there which is highly in phase with voltage) when you provide a small load the current and voltage will have small phase difference that is negligable. when there is a short circuit the inductance plays the role and it gives large phase differences. its like an inductor that time.

i hope it may clear the doubt or confusion some one had on this tittle.

razeen

 

[crutschow]

so when there is an open circuit(no load) it acts like an open circuit for the souce thus doesnt consume current(infact small current is there which is highly in phase with voltage) when you provide a small load the current and voltage will have small phase difference that is negligable. when there is a short circuit the inductance plays the role and it gives large phase differences. its like an inductor that time.

The primary inductance (ignoring leakage inductance) has no significant effect on the short circuit behavior. The output short circuit current is limited solely by the transformer winding and line resistance. The transformer primary inductance is only seen by the primary and it determines the magnetizing current for no load conditions.

Transformers basics are fairly simple. An AC voltage on the primary generates a magnetizing flux in the transformer core that induces a corresponding in-phase voltage in the secondary. With no output load the only current is the primary magnetizing current needed to generate the flux. Any current draw from the secondary will slightly reduce this flux and the primary current increases to maintain the flux level, thus the flux stays constant, independent of the current, and determined only by the voltage. Both input and output voltage are always in phase with the flux. The transformer input current is also in phase with the output current. The phase between the voltage and current is determined by the reactive characteristics of the load. Thus an ideal (or near ideal) transformer is transparent to the source and load, except for whatever transforming of voltage and current it does.

 

[mbarazeen]

crutschow

i agree with what you mentioned, on my post i mentioned the leakage inductance only. any how its clear for a resistive load, the out put current from an ideal transformer is in phase with the voltage.

 

[devronious2009]

crutschow,

my primary inductive reactance is around 12,000 ohms, my secondary inductive reactance is around .5 ohms.

The magnetic field of the primary generates a voltage that is out of phase 90 degrees from the magnetic field, which is natural to how magnetic fields induce voltage. The current of the secondary is nearly in perfect phase with its voltage. This since I'm only running the circuit at around 1khz and with such a small inductive reactance it makes little effect on the current.

Now the amperage of the primary has to be out of phase with the voltage by about 90 degrees per the formula for the phase relationship which says:

theta=atan(inductive reactance)

So the original angle plus theta for the primary is quite large and very near to 90 degrees.

since the voltage and amperage are out of phase in the primary naturally from the inductive reactance, then how can the secondary match that? Inductive reactance is physics and a law that must be observed, is it not? How would this be circumvented or rendered null? Is there something I don't understand about how the inductive reactance is cancelled? Perhaps that can be explained so I can understand? Perhaps that is the key that I'm missing?

I saw something about mutual inductance and if that played a role then indeed you would be correct. But since these are separate circuits and indeed a transformer where one is driven solely by the magnetic field of the other, which creates the amperage and voltage of the secondary I determined for the time being that the inductance of the primary wasn't influencing the secondary other than to create its voltage and amperage.

in other words, since the inductance of the primary was creating the amperage for the secondaru it couldn't influence with a resistive effect, since its in fact pushing the amperage and not resisting it. This idea would then put the two out of phase since then both inductance would be independantly observed rather than a mutual observance. So I ruled out mutual inductance. But perhaps I'm wrong?

So we have two separate influences, the magnetic field of the primary driving the current forward and the inductive reactance of the secondary, resisting the current.

I see how the voltages are matched because of the magnetic field coupling. But the amperage would be limited by the inductive reactance law. So wouldn't the voltage be the only thing in phase?

So considering what I said earlier about how different the inductive reactances are between the primary and seconday, wouldn't the voltages match, but the amperage be off? And I guess what I'm really asking again is why the inductive reactance, which is the cause of this, does not play a role?

 

[crutschow]

The transformer inductance is only seen by the primary and determines the no-load magnetizing current. It is not seen by the load current or the primary current due to the load. Only leakage inductance is seen by those currents. The 90 degree phase shift due the magnetizing inductance is seen only by the primary magnetizing current, it is not seen by secondary load current or the primary current due to the load current.

The important thing to remember is that inductive impedance to AC signals is generated by a change in flux in the magnetics. As I previously stated, there is no change in magnetic flux due to the load current since it is matched by an equal and opposite flux from the source current. This maintains a constant flux in the magnetics based solely upon the voltage. The point is that the current directions in the primary and secondary always generate opposing flux, thus cancelling any inductance due to these currents.

To see this, think of primary and secondary windings on a transformer and they are both wound in the same direction. Then the primary positive portion of the sine wave will generate a positive current going into the winding. The primary voltage also generates a positive voltage at the secondary. If you attach a load to the secondary the positive voltage will cause a current to exit the winding, opposite to the primary current direction. Thus the magnetic flux by the two currents cancel each other and no magnetic inductive effect is seen. The transformer inductance is thus transparent to the load current and the source current due to the load.

Make sense?

 

[devronious2009]

The important thing to remember is that inductive impedance to AC signals is generated by a change in flux in the magnetics

I read that inductive reactance is generated by the current itself. The current creates a magnetic field which opposes its flow. I've read this in numerous places. its actually the change in amperage that causes the reactance, is what they say. I went to hyperphysics and also read another site and went to the book store and read out of a radio circuit book. All say its created from the change in current. Which indirectly creates a magnetic field, and perhaps the magnetic field resists the flow change, but current is what the text books seem to say.

Go here: HyperPhysics

and go to electricity and then inductors and it brings you to the physics laws.

 

[Sceadwian]

Inductive reactance doesn't exist without the magnetic field, so it can't logically be just the current that causes it. The reactance comes from the current interacting with the magnetic field when the current changes, so the cause is the interaction between the energizing or collapsing magnetic field and the charge flowing through the wire, not one or the other. You're misinterpreting what you're reading.

 

[mbarazeen]

as crutschow said during no load on the transformer only current would be magnetizing current that is very small since the AC primary inductance is very high(in ideal transformer its infinity thus takes zero current to establish flux). it is for sure 90 degree out of phase of voltage. Mean time secondary would be excited to have a voltage in phase with primary.

When secondary is connected to a resistive load then primary current increases to keep the flux constant in the core. now the secondary current would be in phase with its voltage.

A transformer just transforms energy from primary to secondary, when we say inductors they establish a magnetic circuit that is decided by its reluctance. since transformers are having closed cores they are approximated to have low reluctance thus to give high impedance and draw less current only to have its magnetic field not changed by the losses inside and leakages.

If we neglect the magnetizing current of the transformer, then for any kind of load, the phase of primary current is in phase with secondary current. For resistive load it has zero angles with primary and secondary voltages.

Furthermore to trigger the idea in mind you can compare capacitors with inductors. An inductor connected in parallel can be compared with a capacitor connected in series. Large capacitors make nearly shot circuit (low impedance) and very large inductors make nearly open circuits. A transformer is like a large inductor it stores energy in its field (core). When we draw this energy by secondary load then it draws more current on its primary to keep it constant.

 

[Sceadwian]

They day I found out that transformers weren't actually electrically connected to each other and that the energy was transmitted via a magnetic field was one of those defining moments that got me interested in electronics, it had never occurred to me before that such a thing was possible.

 

[smanches]

I read that inductive reactance is generated by the current itself. The current creates a magnetic field which opposes its flow. I've read this in numerous places. its actually the change in amperage that causes the reactance, is what they say. I went to hyperphysics and also read another site and went to the book store and read out of a radio circuit book. All say its created from the change in current. Which indirectly creates a magnetic field, and perhaps the magnetic field resists the flow change, but current is what the text books seem to say.

Go here: HyperPhysics

and go to electricity and then inductors and it brings you to the physics laws.

This is correct, except that in a transformer, the magnetic field is already created by the primary winding. The above only applies if the current through the inductor has to create the magnetic field as well. It's the creation/change in the magnetic field that causes the lag in current. This is why the primary winding is not in phase (must create the magnetic field) where the secondary is in phase (does not have to create the magnetic field itself.)

 

[crutschow]

Inductance is simply the effect of energy stored in a magnetic field. As the current in a inductor increases due to an increase in voltage, the energy being extracted from the current to create the magnetic field (and stored in the magnetic field) generates an impediment (inductive reactance) to the flow of current, which causes the current increase to lag the voltage increase. When the inductor voltage is reduced, the collapsing magnetic field now transfers this energy back to the current to keep if flowing, causing the current decrease to lag the voltage decrease.