MH's ideal coil and voltage question

[picowatt]

Pursuant to simulating an inductance, I have some questions and comments.

If you are going to try to simulate the PW model, you must simulate Cemf=L*di/dt

In our ideal inductor of 5H, it is stated that Emf = Cemf when di = .8A/s. Can we then say di = Emf-Cemf?

No...

But again, do you at least understand this negative feedback loop?  If the problem is one of expression, that can be dealt with.  But it does not materially change the model or its action, just the way of looking at it.

1.  When CEMF<EMF, di increases.

2.  As di=.8A/s, CEMF=EMF.

3.  However, if CEMF=EMF, di<.8A/s

4.  If di<.8A/s, CEMF<EMF

5.  Loop to 1 

What is it you are trying to do?  "Understand" the feedback mechanism or just "describe" it mathematically in a form you can agree with?

It would help if I knew what you disagree with or do not understand.

If it is your desire to express the CEMF as a negative voltage, that can be done, but it was hoped that as I described it, the concept of the induced CEMF being a negative feedback mechanism would be readily understood.

It is interesting to read Wiki's definition of Counter-electromotive force and I quote in part  "For example, the voltage drop across an inductor is due to the induced magnetic field inside the coil.[1][2] The voltage's polarity is at every moment the reverse of the input voltage.[1][3]"  Is this not contradictory?
pm

Contradictory to what?  In the model, are not the positive terminals of both the EMF and CEMF facing each other?  That is, the polarity of the CEMF is such that it "is at every moment the reverse of the input voltage".

If you want to see the CEMF as a negative voltage, measure around the loop KVL style...

I have work related chores to attend to and have little time right now.  But please do let me know whether it is the entire concept of CEMF being a negative feedback mechanism that is troubling you or just the desire to mathematically express the CEMF as a negative voltage.

PW   

[partzman]

If you are going to try to simulate the PW model, you must simulate Cemf=L*di/dt

No...

But again, do you at least understand this negative feedback loop?  If the problem is one of expression, that can be dealt with.  But it does not materially change the model or its action, just the way of looking at it.

1.  When CEMF<EMF, di increases.

2.  As di=.8A/s, CEMF=EMF.

3.  However, if CEMF=EMF, di<.8A/s

4.  If di<.8A/s, CEMF<EMF

5.  Loop to 1 

What is it you are trying to do?  "Understand" the feedback mechanism or just "describe" it mathematically in a form you can agree with?

It would help if I knew what you disagree with or do not understand.

Well, I must admit that at this time, I do not believe I can simulate the PW model. I do understand the feedback loop but what you are saying in essence is that the function of inductance is dependent on the applied voltage and Cemf feedback. If so I disagree. I think the feedback is an effect that is the result of a deeper cause and that is flux interaction between windings with said flux generated by the current flowing in the wire due to the applied voltage source. Everything else follows this IMO.

Surely in 3) above you meant to say "if CEMF>EMF, di<.8A/s".

If it is your desire to express the CEMF as a negative voltage, that can be done, but it was hoped that as I described it, the concept of the induced CEMF being a negative feedback mechanism would be readily understood.

Contradictory to what?  In the model, are not the positive terminals of both the EMF and CEMF facing each other?  That is, the polarity of the CEMF is such that it "is at every moment the reverse of the input voltage".

If you want to see the CEMF as a negative voltage, measure around the loop KVL style...

I don't wish the Cemf to be negative or positive. I am/was trying to generate a single expression (if possible) to simplify the simulation.

I still find the Wiki wording confusing, sorry!

I have work related chores to attend to and have little time right now.  But please do let me know whether it is the entire concept of CEMF being a negative feedback mechanism that is troubling you or just the desire to mathematically express the CEMF as a negative voltage.

PW   

I understand what you are stating and your description of the feedback mechanism. I do have a problem with it being the controlling factor of inductance as I stated earlier. So, I don't think I can really contribute anything useful to the matter from this point forward and my comments may be confusing without any substance to back them up.

Best to all.

pm 

[picowatt]

Well, I must admit that at this time, I do not believe I can simulate the PW model. I do understand the feedback loop but what you are saying in essence is that the function of inductance is dependent on the applied voltage and Cemf feedback. If so I disagree. I think the feedback is an effect that is the result of a deeper cause and that is flux interaction between windings with said flux generated by the current flowing in the wire due to the applied voltage source. Everything else follows this IMO.

Surely in 3) above you meant to say "if CEMF>EMF, di<.8A/s".

Actually no.

If CEMF<EMF, di increases.  Consider CEMF=0, at which time the full 4 volts is applied across the conductor and current rises rapidly.  Even if CEMF=3.99volts, current would rise very rapidly (both the EMF and CEMF sources are considered ideal in the discussion)

If CEMF=EMF, current does not rise so di<.8A/s. (technically, di=0 would be true if CEMF=EMF.  However, this condition also causes the CEMF<EMF, so I would not state di=0 when CEMF=EMF in an isolated statement.  It is an unstable term in the feedback loop and would be confusing to state outside of that loop)

If the CEMF>EMF, the CEMF would tend to reverse the flow of current and the rate of change, di, would go negative as the current flow would be decreasing.

Due to the feedback mechanism, di is "trapped" at precisely .8A/s and cannot be greater or lesser than that amount.  Any attempt by di to be greater or lesser than .8A/s will affect the CEMF such that di is forced to return to .8A/s.

We know, by the very definition of the Henry (1A/s thru 1H produces an EMF of 1volt that opposes the current that induced it) that the 5H inductor will generate a an EMF of 4 volts when .8A/s is flowing thru it.

That induced EMF will oppose the direction of the current flow that induced it, hence it is referred to as a CEMF.  In all the models proposed, including yours, the CEMF polarity has been consistent, with the + terminals of the two representative sources facing each other.

I don't wish the Cemf to be negative or positive. I am/was trying to generate a single expression (if possible) to simplify the simulation.

I still find the Wiki wording confusing, sorry!

I understand what you are stating and your description of the feedback mechanism. I do have a problem with it being the controlling factor of inductance as I stated earlier. So, I don't think I can really contribute anything useful to the matter from this point forward and my comments may be confusing without any substance to back them up.

Best to all.

pm

I had hoped that if you read my proposals for a simple simulation using either an arbitrary current source or another 5H inductor for a .8A/s current reference, that perhaps the action of the CEMF might become more clear.

Even if you do not intend to attempt a simulation, perhaps if you scratched out the proposed simulation circuit on paper it would help...

PW

[tinman]

Well, I must admit that at this time, I do not believe I can simulate the PW model. I do understand the feedback loop but what you are saying in essence is that the function of inductance is dependent on the applied voltage and Cemf feedback. If so I disagree. I think the feedback is an effect that is the result of a deeper cause and that is flux interaction between windings with said flux generated by the current flowing in the wire due to the applied voltage source. Everything else follows this IMO.

Surely in 3) above you meant to say "if CEMF>EMF, di<.8A/s".

I don't wish the Cemf to be negative or positive. I am/was trying to generate a single expression (if possible) to simplify the simulation.

I still find the Wiki wording confusing, sorry!

I understand what you are stating and your description of the feedback mechanism. I do have a problem with it being the controlling factor of inductance as I stated earlier. So, I don't think I can really contribute anything useful to the matter from this point forward and my comments may be confusing without any substance to back them up.

Best to all.

pm

I believe that what wiki is saying,is that the polarity of the CEMF voltage will cause a current to flow in the opposite direction to that of the source,and so there for,the polarity is opposite to that of the EMF,even though the polarity is the same 

The confusion for me is,PW keeps assuming that the EMF will produce a current of 800mA/second,an so some how the CEMF induced current is less than this 800mA.

In a real world inductor,we have first a current flow induced from the EMF to charge the capacitance of the coil,and so at this point (T=0),the CEMF is not equal to the EMF,as a magnetic field is yet to be produced that creates the CEMF. Once capacitance is charged,the current drops back down to a 0 value,and then current starts to flow that produces the magnetic field that induces the CEMF.
So with a real coil,we have a small time delay between applied EMF ,and produced CEMF. This is enough time to create an offset between the EMF induced current,and the CEMF induced current.

With an ideal coil,we get no such offset,as there is no parasitic capacitance,and so to me,there is an instant reaction to any current that tries to flow as a result of the applied EMF,by the CEMF.
So i am still stuck as to how PW still insists that the EMF will win out in this battle,and produce a current flow greater than that of the CEMF induced current.

If at T=0,the EMF induced current flow value is 800mA/second,and at that very same instant the CEMFs induced current flow is also 800mA/second,then you have no current flow.
The very instant current tries to flow in one direction,it is counteracted against by a current trying to flow in the other direction,so how was it decided that the EMF induced current would win in this battle?,as it would seem to me that every action is met with an equal and opposite reaction.

Ideal or not,the current rise time could not be faster than the speed of light,and so the EMF induced current would have to start of with a value at !lets say for example! 1mA,and so at that very time,the CEMF induced current would also have to be 1mA. We know this current variation in time exist,as it has been denoted as rising at 800mA/second,and so that is less than 1mA per microsecond.
So why are we looking at the result at the 1 second mark?,why not at the instant of connection?.


Brad

[synchro1]

@Tinman,

Here's a quote from you:

"I believe that what wiki is saying,is that the polarity of the CEMF voltage will cause a current to flow in the opposite direction to that of the source,and so there for,the polarity is opposite to that of the EMF,even though the polarity is the same  "


This is the point I tried to make to you on Luc's moderated thread that got me 86'd off this website!