MH's ideal coil and voltage question

[poynt99]

I guess i must have,as i have taken the 1 ohm resistor as the circuit model that represents the winding resistance,and as such,assumed we were now dealing with a non ideal inductor in one of your two examples.

Yes, that is the non-ideal inductor without any capacitance, just like we've been discussing the last couple of weeks.

As we have been talking about the CEMF difference between ideal and non ideal being the same at T=0,i thought i would point out that it is not the same,for the very reason you just listed-the ideal coil will have no parasitic capacitance,and the non ideal one will.

As up until now we have not discussed parasitic capacitance (or at least I don't recall it), the point being made was that the two are equivalent at t=0. Now that your real-world inductor measurement is indicating some artifact, this may not be the case with your inductor. If you tell me what you used, I may be able to try it myself.

[tinman]

Brad,

What you are describing might be core saturation. With parasitic capacitance you will have a short sharp spike of current right at connection, then the current rise will resume as per normal from a low value.

What type of inductor are you using? Voltage? DC resistance? Resistor value? Inductance?

Yes,that is what i see-a sharp rise in current ,where the current traces the voltages virtical rise across the inductor.-both current and voltage trace track virtical on scope,unless i narrow the time devisions right down,at which point the voltage trace is not vertical,but starts to slope--so even the voltage takes some time to rise across the inductor,but we are now looking at Pico seconds in time with a 480 mH inductor.

I am using the primary side of a large MOT,as that is the largest in L value i have-short of pulling my welder apart.
The circuit is just the circuit model for an inductor, where the R is being used as the CVR.
Value of the CVR dosnt seem to make any difference,other than a reduction in voltage across it of course,as we decrease the R value of the CVR.

I must say,it is getting hard to know what we are discussing at anyone point in time here now.
I must have missed the bit on looking at a real coil that has no parasitic  capacitance,which of course means it is not a real coil.

So ATM, i am looking at a real coil-with it's  parasitic capacitance,so as to see what we have ,that we would not have in an ideal coil.
To me,this makes sense,and from there we can work out if the ideal coil would act the same as a real coil at T=0. So far,it would seem not,because at T=0 with an ideal coil,there would be no insthant current to charge the parasitic capacitance,such as we have with a real coil. I would also suspect that the parasitic capacitance  value would increase with the increase of windings-such as that guitar  picup you were talking about some time back.

I will try and get those scope shots up tonight.
P.S-the coil/inductor has a resistance value of 1.7 ohms,and is being pulsed with a square wave from my FG,with 10% duty cycle. I also have a diode on the positive input,and the CVR is on the negative side of the inductor,due to my common ground issue between scope and FG.

Brad

[tinman]

OK,the scope shots below.

In the first picture,we have the 470mH inductor,and circuit attached.
In the top scope shot,we see the current trace as expected.
In the bottom scope shot of that same picture,we have decreased the time base P/D,and we now see at T=0,the current trace track the voltage trace,and then settle back down to a 0 value. After a short time,it then starts to show the current trace seen in top pic.
At T=0,the winding capacitance seems to charge first.

In the second pic,i have swapped over to a 540mH inductor,but with a much higher winding resistance value--> lots more turns of smaller gauge wire. Looking at the two scope shot's,it would seem that although the inductance value is greater,so is the winding capacitance,and so at T=0,once again we see the current trace track the voltage trace,and for a short time,the current actually leads the voltage. This would mean a clear indication of capacitance being charged.
And once again,we see the current value return to 0,once the capacitance is charged.

My point is this.
At T=0,with regard to a real world inductor,the CEMF is not equal to the EMF,due to capacitance charging. This means that the ideal coil will react differently to that of a real world coil that has parasitic capacitance.
At higher frequencies,this parasitic capacitance would greatly alter the CEMF to EMF ratio

The frequency used in these tests was 10 Hz,with a 10% duty cycle.


Brad

[poynt99]

So what do you suppose your traces would look like if we could somehow first remove the capacitance, then the resistance?

[tinman]

So what do you suppose your traces would look like if we could somehow first remove the capacitance, then the resistance?

I do not know,nor do i have the means to find out.

My point is this--
We would assume that we would not see this with an ideal coil,and we know we do have this situation with a non ideal coil,and so the two will not act in the same manor.
As far as i can see from bench tests,the CEMF dose not equal the EMF at T=0 ,with real world inductors--am i incorrect in saying that?,and if so,what is the initial current spike at T=0,if it is not the charging of coil capacitance ?.


Brad