MH's ideal coil and voltage question

[poynt99]

I didn't ask what happens when R=0, I asked what happens as R is made smaller and smaller.

The journey can be as enlightening as the destination.

Does everyone acknowledge that at t=0 VL>>>VR and that VL=Vin?

[partzman]

Absolutely not!

If you were to replace the inductor with another voltage source producing an equivalent CEMF of 4 volts, the only way you would be able to measure that there are two 4 volt sources connected in parallel would be to measure the current flowing thru the circuit.  In the case of using two voltage sources, we know the second source is 4 volts when the current flow is zero.  In the case of the 5H inductor, we know the CEMF is 4 volts when we see a rate of change of .8 amps per second happening to the measured current.

The CEMF of 4 volts is generated when, and as soon as, the rate of change of the current flow thru the inductor is .8 amps per second.

I don't mean to be a PITA and I know you must be frustrated along with some others with my insistence, but I don't think we are that far apart in our reasoning! For example, when I refer to a Cemf of zero at a dI = .8 amps/sec, my mental equivalent model is a series connected Emf and Cemf where yours is parallel connected. The end result is the same in the fact that the Cemf is basically neutral during normal operation.

I think the parallel model requirement of producing a positive Cemf to match the source Emf with a positive induced current is problematic as compared to the series model Cemf operating around zero especially if one agrees that Cemf controls dI/dt.

The evidence of the interaction of EMF and CEMF is via the measured .8 amps per second rate of change to the current flowing thru the circuit.  It is the induced CEMF that prevents the rate of change from being faster or slower than the .8amps per second rate, exactly as defined for a 5H inductor with 4 volts applied across it.

PW

At this point in time, I still have a problem with Cemf determining dI/dt in an inductor because I can't derive an equation to support this but I'm not the greatest mathematician. I can find support however for field interactions with physical dependencies for determining inductance.

pm 

[tinman]

Now once again, what happens as we make R smaller and smaller?

As you make R smaller,the exponential current curve will become a more linear/straight line.
As long as there is a winding resistance,there will still be a curve.
This curve shows that as from T=0 the CEMF induced current value is not the same as that of the EMF induced current,and current will start to flow.

We place a voltage across the coil,and the current from that EMF is set to a maximum value when there is winding resistance. The self induced CEMF(if it actually exists)creates a current that is in opposition to that which the EMF induced,but is not as great,as it to now must obey ohms law,and that current is also limited to a maximum value due to winding resistance,and so we have a loss plus a loss,and there for the self induced current value can never be the same as the EMF induced current value,and so current flows.

When you take away winding resistance,there is no loss in either the EMF induced current value,nor the CEMF induced current value--they are equal and opposite.


Brad

[tinman]

MileHigh

I specifically asked you in no uncertain terms to give an example with a coil, not with a motor.  This same thing has happened before, this is the second time we are doing this.  A spinning motor is a different kind of thing with a different model that is more complex.  The principle difference that comes to mind is that the spinning rotor is an active generator of CEMF and is more akin to a battery than a coil.

First off,i dont really care to much for demands from some one that dose nothing him self when asked. If you think i am here to bow to your commands,then you have a big shock coming your way.

Second-BackEMF and CEMF are produced by the very same mechanism-the rate of change of the magnetic field imposed on a conductor--in both cases,a coil.
An ideal DC PM motor would not draw any current--that is a fact,and i can show this on the bench with a real world motor--even the value of the BackEMF being identical to the EMF the moment the motor draws no current.

So you bait and switch instead of giving a simple, tangible example of CEMF needing to be less than the EMF in a coil for current to flow, because you can't.

You physically show me the CEMF value being equal to that of the EMF value,then i may consider your request.
It is the value of the CEMF induced current that must be less to allow current to flow,and the self induced current can only be less that the EMF induced current,if there is winding resistance.

Personally, I am not overly interested in this discussion.  There are two equations that relate the current and voltage for a coil.  No matter how you want to model it, the two equations hold true.  And they have their direct analogies in the real life physical world and nobody questions them.

Thats a lye,as there is more people on this thread that has questioned them,than there is people that actually believe they hold.
Like it or not,thats the truth.


Brad

[tinman]

I think the parallel model requirement of producing a positive Cemf to match the source Emf with a positive induced current is problematic as compared to the series model Cemf operating around zero especially if one agrees that Cemf controls dI/dt.

At this point in time, I still have a problem with Cemf determining dI/dt in an inductor because I can't derive an equation to support this but I'm not the greatest mathematician. I can find support however for field interactions with physical dependencies for determining inductance.

pm

Because there is not one.
Once again,we see nothing more than an assumption based on what is said to be true,but with no experimental data to back it up. Non ideal equations being used to derive an answer for an ideal coil.

The fact remains with all components-no current will flow if there is no potential difference.

At T=0,there is no potential difference between the EMF and CEMF value--apparently,but current still flows because the book says it will.

If we look at the diagram below,we have the CEMF and EMF marked accordingly.
In the second diagram,we have a cap that has the same voltage across it as the battery--no current will flow.
In order to get that current to flow,we must remove some of the stored energy in that cap,or we must have a leaky cap in order for a small set value of current to keep flowing.
The very same must apply for the coil. Once the CEMF value is the same as the EMF value,no current will flow until we either remove some of the stored energy,or the coil can dissipate some energy. In this case,neither happens,as the coil is ideal.

So my question was-and still is,where is this loss that creates an imbalance,so as current can continue to flow?


Brad