Dear Studiot (Sir),

The field strength in iron will be the strongest as compared to air and vacuum.

Yes! If the permeability of vacuum and air is almost the same, you are justified in saying that air and vacuum are essentially the same.


Dear BillO,

Do not bow out of this discussion, as it might lead you to a better understanding of E.E. topics. Knowledge is infinite. No human being can possess 100% knowledge. Of course, you can always perfect it and that is my intention!
Please do not misunderstand me. I am not doubting your knowledge or veracity. It is just a suggestion.

Thanks & Regards,
Shahvir

 

To look ahead at the magnetising current in a transformer you might like to look at post 52 in this thread.

http://forum.allaboutcircuits.com/showthread.php?t=15741&page=2

Ignore the silly arguments between some of the members which make it such a long thread.

 

Dear Studiot,

I went thru the link you had provided and have come to understand about the non-linearity of magnetic conditions. However, i was unable to relate it to our present case. I would be grateful if you could elaborate further on this concept and make me understand the factors contributing to my doubts.

P.S. I would be grateful if you could go thru my thread on 'Two PM Alternators in Parallel' in the 'Physics Forum' and give some comments on the same as i have high regards for your knowledge.

Thanks & Regards,
Shahvir

 

Dear BillO,

Do not bow out of this discussion, as it might lead you to a better understanding of E.E. topics. Knowledge is infinite. No human being can possess 100% knowledge. Of course, you can always perfect it and that is my intention!
Please do not misunderstand me. I am not doubting your knowledge or veracity. It is just a suggestion.

Thanks & Regards,
Shahvir

Unfortunately I have to. For a number of reasons.

One is, I don't think we are communicating.

I don’t know whether you are talking about ‘ideal’ or ‘real’ situations, or something in between. So, I have no idea how you are modeling the transformer you are talking about.

You seem to be confusing Im and Imag, or at least your confusing me, or perhaps the texts you are working from are different than the ones I studied so many years ago.

Since you a talking about resistance I had assumed you are looking at some non-ideal model for the transformer. But even then you neglect the effects capacitance. Fair enough. Then I had thought you might be talking about the typical model where R is in series with L in the primary. But on the contrary, you talk about Ir as though you were modeling it as R in parallel with L, because you were talking about Ir (or Iw as stated in the beginning) as though it might be something different than Im (or even Imag in the case of an open secondary)…..

So now, let me set the ground for my last (there I go again) comment on this. I will use the standard ‘real’ model minus the capacitance. So, the primary is just a coil in series with a resistor. The secondary is open and the core is loss-less. We apply a nice sinusoidal voltage to the input Vi and we measure a small input current Ii.

So, in this case the primary is just a like an inductance, or coil (solenoid, whatever) with a series resistor. The current is then entirely due to the magnetization of the core.
So:

Ii=Imag, or the magnetizing current.

Now we note, according to Kirkoff’s current law that indeed:

Ii=Ir, or the current through the resistor.

Low and behold, Ii=Imag=Ir. In other words, there is just the one current to worry about. So, Ir being identical to Imag you would not need to consider it separately. Vr is a more interesting value to the study of transformers. Vr is in phase with Imag and not with Vi.

To address another one of your original concerns the phasor diagram for the primary of an open secondary transformer would be essentially the same as that of the standard LRC circuit.

I don’t know of a link on the web that will provide any real enlightenment. Not that one doesn’t exist, I just don’t know about it.

When I was a first year under-grad we studied this stuff as a part of our core physics program. We used what I consider to be a great introductory text on all this stuff. Fundamentals of Physics by Halliday and Resnick, 2nd edition, published by Wiley. I’d suggest you get a hold of that book. Later editions (5, 6, 7) suck. Anyway if you can get a hold of that one, chapters 32 – 36 should give you all the background you need to understand transformers. The chapters take you from Farday’s law through to alternating current discussions. The chapters are short, so it’s not too much to go through. Figure 36-5b on page 641 is the phasor diagram you seek.

Good luck.

 

Dear BillO, ​

I apologize! I had indeed confused you since i had messed up different concepts. “The standard ‘real’ model minus the capacitance” you presented was exactly the one i was trying to depict, you have got it right anyways. Actually, I did not want to mix the R-L series ckt. doubt with the Xmer query and hence I had considered it separately under CASE 2.
What I have interpreted from your explanation is that Vr would lag Vi since the major component of R-L series circuit is ‘inductive reactance’. Please correct me if I am wrong.​

Final re-verification of transformer theory;
As per your explanation, considering ‘non-ideal’ conditions (with series impedance), the magnetic flux in the ferrous core of a ‘loaded’ Xmer (VT or CT) will reduce due to the voltage drop across the impedance in series with the ‘non-ideal’ Xmer winding as a result of load currents flowing thru the Xmer windings (ref - typical Equivalent ckt. of a loaded ‘non-ideal’ Xmer). As a result , the iron losses would also reduce!​

So; can the magnetic flux in the ferrous core of a loaded ‘non-ideal’ Xmer be completely reduced to zero if the load currents in the 'non-ideal' Xmer windings are sufficiently large? If so, the iron losses would also be completely eliminated (reduced to ‘zero’)! Please comment.​

P.S. Thanks very much for your suggestions on the reference materials, extremely grateful.​

Thanks & Regards,
Shahvir​

 

Dear BillO, ​

I apologize! I had indeed confused you since i had messed up different concepts. “The standard ‘real’ model minus the capacitance” you presented was exactly the one i was trying to depict, you have got it right anyways. Actually, I did not want to mix the R-L series ckt. doubt with the Xmer query and hence I had considered it separately under CASE 2.​

What I have interpreted from your explanation is that Vr would lag Vi since the major component of R-L series circuit is ‘inductive reactance’. Please correct me if I am wrong.

Yes, in this particular case. Remeber, resistance will not affect phase. The current in this system (Imag) is 90 degrees lag out of phase with the input voltage. Since this current is passing trough the resitance Vr is in phase with it.​

Final re-verification of transformer theory;

As per your explanation, considering ‘non-ideal’ conditions (with series impedance), the magnetic flux in the ferrous core of a ‘loaded’ Xmer (VT or CT) will reduce due to the voltage drop across the impedance in series with the ‘non-ideal’ Xmer winding as a result of load currents flowing thru the Xmer windings (ref - typical Equivalent ckt. of a loaded ‘non-ideal’ Xmer). As a result , the iron losses would also reduce!​

So; can the magnetic flux in the ferrous core of a loaded ‘non-ideal’ Xmer be completely reduced to zero if the load currents in the 'non-ideal' Xmer windings are sufficiently large?​

​

No, not in the real world. ​

 

No, not in the real world.
[/quote]
Could you elaborate, why?
Also, if time permits i will send you a set of vector diagrams which was the cause of this particular query.

Thanks & Regards,
Shahvir

 

Because in the real world you have losses of various kinds, as well as resistance in the transformer, in the source and in the load, as well as non-linear problems like hysteresis. It is not a simple thing I can relate in a comment or two, or possibly ever as I’m not a transformer design engineer. The real life of transformers, to me, is horribly complex.

Why don’t you attach the diagram to your next post so that everyone can see it?

 

Why don’t you attach the diagram to your next post so that everyone can see it?

Dear BillO,

I have already started the process of constructing the diagrams & its associated explanation, but i would require some time. Also, i will surely attach it to my next post alongwith the explanation.

Thanks & Regards,
Shahvir

 

Dear All,

Attached is a set of vector diagrams for a non-ideal transformer at no-load or a R-L series ckt. in general, with a ferrous core.

Fig. (a) depicts a vector diagram commonly used to explainconditions prevailing in a non-ideal Xmer at no-load as per Textbook theory.
Where;

Vi = Applied input AC voltage

Imag = Magnetizing current

Iw + ir = Currents supplying Iron loss + small amount of Cu loss

Ii = Resultant input current through Primary wdg. of Xmer with open secondary [Ii = Imag + (Iw + Ir)]

As is evident, the magnetic flux (phi) is in-phase with Imag (magnetizing current).


Fig. (b) depicts a vector diagram which is my interpretation of the foregoing theory as per the subsequent discussions held in this particular thread. Iw (Iron loss current) has been neglected in this case for simplicity.

As per the explanations presented in this thread, Ii = Imag = Ir and hence, the magnetic flux (phi) in the ferrous core of the Xmer should, in my opinion, appear in-phase with the resultant input primary current Ii.
Please correct me if my interpretation is incorrect and elaborate on the same.

Thanks & Regards,
Shahvir

 

Dear BillO,

Please reply to my post. Attachment forwarded as you had suggested. Inputs by other experts are also welcome.

Thanks & Regards,
Shahvir

 

Shahvir,

Diagram (a) is not consistent with a series R-L circuit. It is more consistent with a parallel R-L circuit. Here Iw+Ir ≠ Imag which would not be the case for a series circuit.

Diagram (b) is confusing. I'm not sure how you would represent the flux in a ferrous core on a phasor diagram.

 

Shahvir,

Diagram (a) is not consistent with a series R-L circuit. It is more consistent with a parallel R-L circuit. Here Iw+Ir ≠ Imag which would not be the case for a series circuit.

Diagram (b) is confusing. I'm not sure how you would represent the flux in a ferrous core on a phasor diagram.

Dear BillO,

That's okay, then consider Fig. (a) to be that of a 'non-ideal' transformer at no-load and please comment on the same.

About Fig. (b), then what should be the case, please suggest. I would be very grateful.
Sorry for trouble!

Thanks & Regards,
Shahvir

 

Dear BillO,

That's okay, then consider Fig. (a) to be that of a 'non-ideal' transformer at no-load and please comment on the same.

About Fig. (b), then what should be the case, please suggest. I would be very grateful.
Sorry for trouble!

Guys please reply!

Kind Regards,
Shahvir

 

Someone please reply!

Kind Regards,
Shahvir

 

studiot and billo,

would it be safe to say that Ir never gets 'through' the coil due to the losses? I've always wondered if picturing little colliding electrons was a valid way to describe current, and that Ir goes in, but due to classical mechanics on an atomic scale, never actually gets out and thus can be considered as 'eaten'?

The analogy here would be the exact same as current flowing through a resistor. If that resistor is dissipating heat, the current dissipated as heat energy never returns to the source for redistribution.

If so I believe I can answer what Shahvir is after.

 

If so I believe I can answer what Shahvir is after.

Dear James,

Please go ahead and answer. Thanks for help.

P.S. I think BillO and Studiot have given up on me!

Kind Regards,
Shahvir

 

studiot and billo,

would it be safe to say that Ir never gets 'through' the coil due to the losses? I've always wondered if picturing little colliding electrons was a valid way to describe current, and that Ir goes in, but due to classical mechanics on an atomic scale, never actually gets out and thus can be considered as 'eaten'?

The analogy here would be the exact same as current flowing through a resistor. If that resistor is dissipating heat, the current dissipated as heat energy never returns to the source for redistribution.

If so I believe I can answer what Shahvir is after.

This has come full circle. The phasor diagram (a) where there is an Ir+Iw in phase with the input voltage is consistent with a parrallel resistance. Imag is just that portion of the current that wuld normally go to magetizing the core given a no-load secondary in an ideal transformer. It, by itself, does not give the full story about the actual flux in the core. Diagram (b) might be pretty close to what you'd see in a air-core, but that's a guess on my part. I would have no idea how this would play out with an iron or ferrous core. I do not think the flux in a real core can be depicted by a phasor.

If you can help, go for it.

 

This has come full circle. The phasor diagram (a) where there is an Ir+Iw in phase with the input voltage is consistent with a parrallel resistance. Imag is just that portion of the current that wuld normally go to magetizing the core given a no-load secondary in an ideal transformer. It, by itself, does not give the full story about the actual flux in the core. Diagram (b) might be pretty close to what you'd see in a air-core, but that's a guess on my part. I would have no idea how this would play out with an iron or ferrous core. I do not think the flux in a real core can be depicted by a phasor.

If you can help, go for it.

Hey Bill,

Lets focus on answering the Ir question first. My question above was attempted to state that Ir is dissipated as heat. Therefore one might state, as far as current flow is concerned, it never 'returns' to the source, as such can't be considered a 'flow' and thus cannot be categorized with the magnetizing currents.

Would you agree, or am I missing some primary school level concept regarding current flow

 

Well, once you bring non-ideal elements into it, it is not clear how to deal with them.

If R can be seen as a pure resistance of zero phyical length in parallel with L, then, in that model we can probably ignore the contribution of Ir to the magnetic flux. Indeed the power dissapated acrross the resistor will be lost to heat. But that is a pretty strange model indeed.

In any event, the current still flows through the 'R' circuit and returns to source. That has to happen, but what flows through the strange resistor will not flow through the coil.

You need to keep some pretty basic concepts in mind. First, any way you model this transformer it must follow the simple circuit laws expressed by Ohm and Kirchhoff. And second, any current flow through any element must create a field around that element in accordance with Farraday's law of induction.

A much more practical and realistic model is for R to be in series with L. Then you have only one current to worry about and you don't have to worry about zero length resistors.