What is the reason behind inductive reactance in an AC circuit?

[FOIWATER]

that is the wrong equation, its Ldi(t)/dt = v(t), also I've never seen a differential written as d(t), just dt.

I agree, see post 24 as well

 

[FOIWATER]

@ antiphon,

It is possible you just do not lend yourself to the terminology I am using? Maybe I am incorrect to call it a counter emf,

But I think we both agree that field collapse in a coil induces a voltage into it, and that voltage is of opposite polarity of the voltage which caused the current that created the initial field.think about this... in the same way that an induction motor generates a voltage (counter emf), an inductor too MUST somehow generate a voltage, why can I not call this generation a counter EMF, as in a motor... i shall!

Best way I can explain it, and I'm sure it's correct to some degree, I have just always called that voltage, a counter emf.

 

[FOIWATER]

Let me make a post which in my opinion proves there is counter emf exhibited in inductors.

This is something I have done at work.

It is the neutral plane test for a DC motor.

There are four sets of brushes on my DC shunt wound motor, two positive and two negative.

Things you will need are 18 volt cordless drill battery, ammeter, dc shunt wound motor.

Connect the ammeter across a positive and negative brush set (IE, in series with the armature coil)

Remember, the neutral plane is the axis for the brushes where NO voltage is induced in that armature coil under the brush as the shunt field rotates (this is important as the brush short circuits armature coils, we would want no voltage there)

OK, so connect your 18 volt battery across F1 and F2. It draws current through it, and a magnetic field is developed around the coil...the magnetic field is now EXPANDING... and as it EXPANDS, if we are not in the neutral plane, the ammeter will KICK positive (also called a DC KICK test for neutral plane) The reason it kicked, is because the field expanding induced a positive voltage into the armature coil, which caused a positive current to flow for the duration of the flux increase. Now the magnetic field is static and not changing, and since there is no motion now, the meter returns to zero after kicking.

Now, remove the leads from the field. The current stops flowing, and the field COLLAPSES. As it collapses... it again crosses the armature coils... but now notice the meter kicks the opposite direction. This is because the voltage induced, is of the opposite direction, since the field now collapsed!

If one can see how this works, one can now make the logical jump to comparing the connecting of the dc supply, and the field EXPANDING around the field coil (which IS an inductor) to the positive half cycle of an AC waveform, the increasing current causes a field to grow outward from the conductor. One can now also imagine the removing of the 18 volt supply as the negative half cycle of an AC waveform, as the field COLLAPSES, so it will too in an AC negative half cycle. Each time it collapses... it will induce a voltage onto ITSELF...a self-inductance... a BACK EMF...this is the whole reason for the name inductor is it not... a voltage is induced into it...as current changes... which is the common conception for the term inductors oppose changes in current!

(some one like sophie, dragon, or Jim please read this and let me know if you agree, this is my thought process through the experiment we did, and I would certainly hope it is a correct way of thinking of what we did)

 

[psparky]

I agree, see post 24 as well

Whoopsy...yes, I obviously wrote the formula wrong!

L*di(t)/dt=v(t)

Sorry...rough weekend!

 

[Antiphon]

@ antiphon,

It is possible you just do not lend yourself to the terminology I am using? Maybe I am incorrect to call it a counter emf,

But I think we both agree that field collapse in a coil induces a voltage into it, and that voltage is of opposite polarity of the voltage which caused the current that created the initial field.


think about this... in the same way that an induction motor generates a voltage (counter emf), an inductor too MUST somehow generate a voltage, why can I not call this generation a counter EMF, as in a motor... i shall!

Best way I can explain it, and I'm sure it's correct to some degree, I have just always called that voltage, a counter emf.

I think this is the case that we disagree about the terminology.

The EMF developed by an inductor has exactly one relationship to the flux, a negative time derivative. Know how the flux changes and you know the EMF.

I reserve the phrase "back EMF" for motors but I acknowledge that others may use it to describe the EMF itself.

Let me conclude by saying that for an inductor the EMF and back EMF are one and the same. If there's still disagreement after this then it's not just a question of terminology.

 

[FOIWATER]

I disagree, I say there are two EMFs, which obviously can be added to yield one total EMF

One applied, and one self induced (when applied with AC, or any time there is relative motion, for that matter). The self induced EMF must be less than the applied obviously, since the current resultant of the former EMF is the reason the second EMF is induced. I say lenz' law determines the opposite polarity (since the fields are CONVERGING in a different matter), and the opposite polarity to what we can attribute inductive reactance. The ratio of the amount of voltage induced for a specific current change, multiplied by the angular frequency is where we get inductive reactance.

If you do not adhere to this, to what can you attribute inductive reactance?