MH's ideal coil and voltage question

[partzman]

Ideal inductors do exist in our society today. I am certainly no expert in the subject but examine the superconducting electromagnets used in MRI. Once below their critical superconducting temperature, certain materials exhibit zero resistance and yet maintain inductance. The inductive fields exist outside the confines of the wire but they do exist. A 5 henry superconducting coil is still 5 henries even with zero resistance and they will "store" their current with zero voltage drop for extremely long periods of time. There are qualifications for these ideal conductors to work as they do but they are in use everyday.

Resistance of a coil does not determine it's inductance, it simply hinders pure inductance.

I have attached another sim using a coil resistance of 1e-110. This parameter may have passed a preset limit in LtSpice however.

partzman

[tinman]

The Tau business is fairly simple to explain.

When you transition from a finite Tau to an infinite Tau the current waveform goes from an inverse exponential curve to a straight line.  Note that it is a straight line with a constant slope of V/L.



So Tau being infinity does not mean stopping current flow, it means linearly increasing current flow.

Since we are discussing limits, the only possible way for the current to flat-line at zero "forever" would be for the inductance to be infinity.  Then you have a "more real" Tau = infinity because this time L/R becomes infinity/R.

So when Tau = infinity/R that gives you the horizontal current trace stuck at zero with a slope of zero (V/infinity), whereas when Tau = L/0, you get a current trace that is a straight line with with a slope of V/L.

MH

Your explanation above only explains the trace or wave form seen on the scope. It dose not explain away the actual resultant math value. The math is precise,and defines the actual time taken for the current to rise,and mathematically that time is infinite--you cannot redefine math at your will.

So Tau at infinity just means the current trace is a perfectly straight line.  Since it is a straight line the concept of "reaching 63% of the maximum value" does not apply anymore because that concept does not exist when the current waveform is a perfectly straight line.  i.e.; "There is no time constant."

Tau at infinity dose not just mean the current trace is just a flat line. Tau at infinity is a mathematical calculated value using the the equation that is always used to calculate the Tau time constant--> Tau=L/R.
There is a time constant,and that time constant is infinity. A flat line on a scope dose not just dismiss this mathematically calculated time constant. In fact,that flat line seen on the scope ,represents the infinite flat line that would be seen as an infinite time value. If the scope had an infinitely long screen,then you would see that trace remain flat at a zero value throughout the entire time a voltage is placed across the ideal inductor.


Brad

[Magluvin]

So I guess the question is how much time does it take for an ideal inductor to reach a particular current over time if resistance was not an obstacle. This would mean that the ideal inductor is functional as an inductor.....

Well the ideal inductors current rise when the ideal input voltage is applied will be a straight line increase and not a curve because the absence of voltage division because of no resistance. So the current could rise indefinitely over time, directly related to time and the resistance value does not need to be in the equation L/R.   Correct?


That is if the ideal inductors bemf ends up not being equal to the input and the ideal inductor actually works.

Mags

[Magneticitist]

the superconductors are a good example but they are like comparing 0 resistance with .0000001 or any of the other ridiculously low resistances mentioned to sim with.
and how do we know if R was 0 that line wouldn't be level flat or plumb straight up.

when we super cool the conductors we are probably just reallocating that resistive variable elsewhere. maybe much less measurable resistance but maybe a much less hindered ability to create
a field. idk I'm no expert either lol. at any rate it makes me think of how the curie point is sort of like
the opposite effect and maybe we can take magnetic inductor with very little resistance and heat it to a point where it has very high resistance and has a harder time creating a field if that doesn't sound stupid.

[Magneticitist]

So I guess the question is how much time does it take for an ideal inductor to reach a particular current over time if resistance was not an obstacle. This would mean that the ideal inductor is functional as an inductor.....

Well the ideal inductors current rise when the ideal input voltage is applied will be a straight line increase and not a curve because the absence of voltage division because of no resistance. So the current could rise indefinitely over time, directly related to time and the resistance value does not need to be in the equation L/R.   Correct?


That is if the ideal inductors bemf ends up not being equal to the input and the ideal inductor actually works.

Mags

yea my beef is if the ideal coil aka absolute 0 R coil cannot dissipate or radiate anything whatsoever, then to me that's like another way of saying it has immeasurable current. infinite, 0, immeasurable either way and might as well be non existent. you would never have the field or current to begin with and the lack of a time constant would just basically make all the equations equal 0 since progress over time is out of the window and no longer even discernible from 0 time progression at all. it's like we are talking about trying to apply current to something that can perfectly and absolutely resist current change aren't we?