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

[fonz]

This is quite a frustrating problem for me so hopefully somebody can describe it in such a way that it settles in my mind.

It is the concept of voltage and current being out of phase. Inductors are frequently described as responding to changes in current but does this really make sense?

The big problem for me is how the inductor is reacting to changing current on an alternating voltage supply. It is the voltage that is changing so how is it not responding to the changing voltage?

Trying to visualise what is happening when a voltage is applied across an inductor. Initially when the voltage is applied what will the current be? My own intuition would suggest that since initially there is no magnetic field to oppose the change there will be a large current inrush to build up the magnetic field correct? But how is this current 90 degrees out of phase with voltage?

It is difficult for me to explain what it is I cannot understand but hopefully by providing an explanation for these two questions I will begin to understand.

Thanks
Dan

 

[FOIWATER]

I think I can explain it to you as I thought about it when it did not make sense to me.

You will have to read each thing I write to make sense of it, and if something doesn't make sense you will have to learn more about each thing.

Once you connect AC voltage across an inductor, it draws a specific amount of current, based on resistance. The current through the inductor creates a magnetic field around the coil. As you said, the voltage is what is constantly changing. But because the voltage is constantly changing, the current is constantly changing (in magnitude) and so the magnetic field is constantly changing around the inductor as well.

Think about it, as the voltage changes the current changes, (it is increasing and decreasing) so to the magnetic field is growing (expanding away from the inductor, and then collapsing onto the coil as the current (which creates the magnetic field) is decreasing in value.

Now as the field collapses, it CROSSES THE COIL. now the conditions for induction exist. we have a voltage induced back into the coil each and every time the current value is on the negative half cycle of the ac waveform. The voltage that is induced, OPPOSES the source voltage (see lenz' law) so, you now have a LOWER value of voltage applied.

This is the nature of inductive reactance, inductors have both resistance (due to the resistance of the wire composing the coil) and reactance. You can see, the reactive part is simply due to the voltage that the inductor creates when the current changes. It is created in such a direction that it opposes the source voltage, and as so it seems to drop voltage. But it does not it simply stores energy in the form of a magnetic field and every so often (60 times per second) returns the energy to the circuit.

Now think about what you commonly hear people say, inductors opposes current changes. How can this be so, is your question. Do you now see? this can be so because as current passes through a coil, and if the current is sinusoidal, it creates a magnetic field that is also sinusoidal. Which means the field expands and collapses on the coil depending on frequency (which is why inductive reactance depends on frequency X = 2pifL) And each time it collapses on it, it induces an opposing voltage into the coil that appears to impede the source voltage.

The current is out of phases ninty degrees (and so too, is the field) because it physically takes time to build the magnetic field around the conductor, and it stores the energy there. This does not happen at the same rate that electrons travel through a circuit.

Hope this helps,

PS: This accounts for why inductors appear as shorts in DC. Imagine, if the current is not changing, the field is now static (not changing as well) If it doesn't change, then the field can not cut the inductors and induce a voltage into it that opposes source voltage. IE, at 0 frequency inductive reactance is zero as well. but as soon as you open the circuit, or stop providing voltage across the coil, the field WILL collapse.. the voltage that is induced will cause a current to back feed in the circuit. This is the purpose of connected a diode reverse biased across coils in automotive circuits containing computers (many applications). It is called a free-wheeling diode, or flyback diode and allows the current produced by the coil to return to the other end of the coil rather than feeding through the circuit and damaging other equipment.

 

[Averagesupernova]

Trying to visualise what is happening when a voltage is applied across an inductor. Initially when the voltage is applied what will the current be? My own intuition would suggest that since initially there is no magnetic field to oppose the change there will be a large current inrush to build up the magnetic field correct? But how is this current 90 degrees out of phase with voltage?


Thanks
Dan

You have it backwards. When voltage is first applied to an inductor the current will be nearly zero. Then it steadily ramps up to max. In order for there to be a 90 degree phase shift it has to be this way.

 

[FOIWATER]

is the reason the current is initially zero because this is the time the field is expanding? obviously not, because the magnetic flux is in phase with the current, and is out of phase with the voltage lagging by ninty degrees.

fonz, the physical reasoning behind lagging / leading currents in inductors/capacitors respectively (especially including resistors) has been a sort of hot topic on a few threads here, the math becomes especially important when describing WHY they are out of phase. My previous post just describes the action of an inductor in an ac circuit, which isn't really what you asked but may give you some understanding that you didn't have prior to reading it not sure.

 

[DragonPetter]

Here is one way to think of it mathematically. I don't know if this is of any use or already obvious to you. It can at least show you that this magic number of 90 is a result of differentiation

given the inductance relation:
$V(t) = L\frac{di(t)}{dt}$

let $V(t) = sin(t)$

then rearrange:

V(t)dt = Ldi(t)

integrate:

$\int{sin(t)}dt = L\int{di(t)}$

$cos(t) = Li(t)$

$i(t) = \frac{cos(t)}{L}$

Now, knowing that sine and cosine are 90 degrees out of phase, we know $cos(t) = sin(t-90°)$

so we can say
$i(t) = \frac{sin(t-90°)}{L}$ which shows a phase shift of -90 degrees from the voltage function, so the current is lagging behind the voltage.

 

[psparky]

A similar thread about caps and inductors was discussed earlier today.

Give it a good read...lots of good stuff in here as well:

https://www.physicsforums.com/showthread.php?t=174615&page=2.

 

[psparky]

I think I can explain it to you as I thought about it when it did not make sense to me.

You will have to read each thing I write to make sense of it, and if something doesn't make sense you will have to learn more about each thing.

Once you connect AC voltage across an inductor, it draws a specific amount of current, based on resistance. The current through the inductor creates a magnetic field around the coil. As you said, the voltage is what is constantly changing. But because the voltage is constantly changing, the current is constantly changing (in magnitude) and so the magnetic field is constantly changing around the inductor as well.

Think about it, as the voltage changes the current changes, (it is increasing and decreasing) so to the magnetic field is growing (expanding away from the inductor, and then collapsing onto the coil as the current (which creates the magnetic field) is decreasing in value.

Now as the field collapses, it CROSSES THE COIL. now the conditions for induction exist. we have a voltage induced back into the coil each and every time the current value is on the negative half cycle of the ac waveform. The voltage that is induced, OPPOSES the source voltage (see lenz' law) so, you now have a LOWER value of voltage applied.

This is the nature of inductive reactance, inductors have both resistance (due to the resistance of the wire composing the coil) and reactance. You can see, the reactive part is simply due to the voltage that the inductor creates when the current changes. It is created in such a direction that it opposes the source voltage, and as so it seems to drop voltage. But it does not it simply stores energy in the form of a magnetic field and every so often (60 times per second) returns the energy to the circuit.

Now think about what you commonly hear people say, inductors opposes current changes. How can this be so, is your question. Do you now see? this can be so because as current passes through a coil, and if the current is sinusoidal, it creates a magnetic field that is also sinusoidal. Which means the field expands and collapses on the coil depending on frequency (which is why inductive reactance depends on frequency X = 2pifL) And each time it collapses on it, it induces an opposing voltage into the coil that appears to impede the source voltage.

The current is out of phases ninty degrees (and so too, is the field) because it physically takes time to build the magnetic field around the conductor, and it stores the energy there. This does not happen at the same rate that electrons travel through a circuit.

Hope this helps,

PS: This accounts for why inductors appear as shorts in DC. Imagine, if the current is not changing, the field is now static (not changing as well) If it doesn't change, then the field can not cut the inductors and induce a voltage into it that opposes source voltage. IE, at 0 frequency inductive reactance is zero as well. but as soon as you open the circuit, or stop providing voltage across the coil, the field WILL collapse.. the voltage that is induced will cause a current to back feed in the circuit. This is the purpose of connected a diode reverse biased across coils in automotive circuits containing computers (many applications). It is called a free-wheeling diode, or flyback diode and allows the current produced by the coil to return to the other end of the coil rather than feeding through the circuit and damaging other equipment.

Really cool explanation without using math.

Bravo.

 

[Antiphon]

As an exercise for the advanced student I pose a little brain teaser.

1) All real wires exhibit some inductance even if very tiny.
2) The differentiation carried out above with sines and cosines is obviously correct and the phase is always 90 degrees regardless of the tiny nature of the inductance.
3) therefore you can NEVER have voltage and current in phase!

 

[Fuxue Jin]

A piece of wire consist of three component, DCR, inductance L and capacitance C.

With DC current, frequency is zero, V and I follows V = IR, always in phase.

With AC current, when frequency is low, L dominate, when frequency is very high, C dominate.

Phase shift between V and I depends both on L and C for that specific frequency.

As an exercise for the advanced student I pose a little brain teaser.

1) All real wires exhibit some inductance even if very tiny.
2) The differentiation carried out above with sines and cosines is obviously correct and the phase is always 90 degrees regardless of the tiny nature of the inductance.
3) therefore you can NEVER have voltage and current in phase!

 

[Antiphon]

A piece of wire consist of three component, DCR, inductance L and capacitance C.

With DC current, frequency is zero, V and I follows V = IR, always in phase.

With AC current, when frequency is low, L dominate, when frequency is very high, C dominate.

Phase shift between V and I depends both on L and C for that specific frequency.

Ok- so far so good. Then by all means, let us ensure that the inductance dominates any capacitance by setting the frequency F = 1 femto-Hertz. Clearly then at this frequency we will have AC voltage that takes 31.6 million years to complete a cycle with the voltage and current around 7 million years apart!

 

[fonz]

Ok thank you for contributing so far, I've had a think and I'm starting to get to grips with this however I seem to have come across a bit of a paradox

If an unmagnetised inductor is connected to a voltage source I would say that the initial current is zero because the magnetic field of the inductor induces an emf to counter the source voltage. This would mean the voltage drop initially is equal to the source voltage. As the source voltage decreases the inductor generates an emf to sustain it's magnetic field and current increases?
How can an emf be induced initially to counter the source if there is no magnetic field?

 

[FOIWATER]

Here is one way to think of it mathematically. I don't know if this is of any use or already obvious to you. It can at least show you that this magic number of 90 is a result of differentiation

given the inductance relation:
$V(t) = L\frac{di(t)}{dt}$

let $V(t) = sin(t)$

then rearrange:

V(t)dt = Ldi(t)

integrate:

$\int{sin(t)}dt = L\int{di(t)}$

$cos(t) = Li(t)$

$i(t) = \frac{cos(t)}{L}$

Now, knowing that sine and cosine are 90 degrees out of phase, we know $cos(t) = sin(t-90°)$

so we can say
$i(t) = \frac{sin(t-90°)}{L}$ which shows a phase shift of -90 degrees from the voltage function, so the current is lagging behind the voltage.

Do you know this for a capacitor? would it be possible to PM it to me?

 

[Antiphon]

Ok thank you for contributing so far, I've had a think and I'm starting to get to grips with this however I seem to have come across a bit of a paradox

If an unmagnetised inductor is connected to a voltage source I would say that the initial current is zero because the magnetic field of the inductor induces an emf to counter the source voltage. This would mean the voltage drop initially is equal to the source voltage. As the source voltage decreases the inductor generates an emf to sustain it's magnetic field and current increases?
How can an emf be induced initially to counter the source if there is no magnetic field?

Your thinking is confused in that the voltage applied to the inductor never changes.

If you connect a 1 Volt source to a 1 Henry inductor the voltage will be 1 Volt forever and the current I as a function of time t will be I=t.

The magnetic field will grow linearly in time as well.

 

[fonz]

The trouble I am having is that the argument for why when full voltage is applied at t=0 the current is zero is that the magnetic field in the inductor induces an emf to counter the applied source voltage.

Initially where does the magnetic field come from to generate this back emf?

By Faraday's law, induced emf is a result of current flow but no current is flowing?

My only explanation of this is that when the voltage is applied, the inductor offers virtually no resistance so maximum current tries to flow. The inductor reacts to this by absorbing this current into it's magnetic field. This changing magnetic field produces the counter emf to prevent current from flowing.

Can somebody please confirm this explanation?

 

[Antiphon]

There is no "back" emf.

When the voltage is applied to the inductor, that is the EMF.

For a 1 volt source connected to any inductor, the EMF is 1 Volt.

 

[psparky]

The trouble I am having is that the argument for why when full voltage is applied at t=0 the current is zero is that the magnetic field in the inductor induces an emf to counter the applied source voltage.

Initially where does the magnetic field come from to generate this back emf?

By Faraday's law, induced emf is a result of current flow but no current is flowing?

My only explanation of this is that when the voltage is applied, the inductor offers virtually no resistance so maximum current tries to flow. The inductor reacts to this by absorbing this current into it's magnetic field. This changing magnetic field produces the counter emf to prevent current from flowing.

Can somebody please confirm this explanation?

The entire explanation lies right here.

L*di/d(t)=i(t)

It covers everything you said above...

 

[psparky]

Do you know this for a capacitor? would it be possible to PM it to me?

Anyone should be able to derive this. Follow his same steps but start with:

C*dv/d(t)=i(t)

I find derivation to be most satisfying.

 

[fonz]

The entire explanation lies right here.

L*di/d(t)=i(t)

It covers everything you said above...

Thanks!

 

[psparky]

An inductor's reactance of JωL fills in the rest nicely as you now realize after reading all these wonderful threads.

 

[Fuxue Jin]

Don't we need to separate the initial state and steady state for the original question?

For steady state, the voltage and current simply follow the rule V = L di/dt. If the voltage is sine wave AC, then it will be 90 phase shift for a PURE ideal inductor.

For initial state, no matter how quick the voltage ramps up from zero to whatever the level is, it STILL takes time, even though it is very short. AND the current will be starting from ZERO.

 

[Antiphon]

Don't we need to separate the initial state and steady state for the original question?

For steady state, the voltage and current simply follow the rule V = L di/dt. If the voltage is sine wave AC, then it will be 90 phase shift for a PURE ideal inductor.

For initial state, no matter how quick the voltage ramps up from zero to whatever the level is, it STILL takes time, even though it is very short. AND the current will be starting from ZERO.

No, this is not correct. You do not need to make a distinction between steady state and initial conditions here.

For an ideal inductor, voltage changes can happen in zero time without anomalies.

Here is how the inductor works, step by step.

The voltage across the inductor is determined by either the voltage source it is connected to, or by the minus of the time rate of change of magnetic flux or by a combination of the two in a more complex circuit. Electric current has not entered the picture yet.

When an inductor is connected to a voltage source the voltage source *sets* the time derivative of magnetic flux. This is the EMF. The magnetic flux begins to grow linearly and will continue to do so forever for an inductor with no resistance in the wires.

The magnetic flux B is related to the magnetic field H through a material relation. For air core inductors it is mu, the permeability of free space. For ferrites mu is a few hundred times more that that of air.

When the flux begins to grow linearly then through B=mu.H, the magnetic field H also begins to grow. By Ampere's law the magnetic field is a measure of the current.

The physics then goes like this: applied voltage causes flux to increase linearly. Material medium translates flux into magnetic field which is proportional to current. Therefor the current begins to rise linearly.

After a time, the relation B=mu.H will cease to be linear because the material becomes "saturated". Mu will become smaller and smaller as the flux increases. At that point, as the flux continues its simple linear increase in time, the current through the inductor will start to increase more and more to maintain the relation B=mu.H with decreasing mu.

This is called a saturated inductor. Its still an inductor but resembles an air inductor now more than a material one.

Magnetic saturation is a function of the flux, not the magnetic field. It is possible to saturate an inductor that has very little current flowing in it by applying a large enough electric field (voltage) to its windings.

 

[Fuxue Jin]

I am a magnetic component designer, design transformers and inductors all day everyday, so there is no problem to understand what you discussed.

However, I think, to my understanding, voltage doesn't generate magnetic flux but current. You have current first, then you have H (NI, ampere-turn), then depends on the material (the core), you have B. Not you have B first, then H.

No, this is not correct. You do not need to make a distinction between steady state and initial conditions here.

For an ideal inductor, voltage changes can happen in zero time without anomalies.

Here is how the inductor works, step by step.

The voltage across the inductor is determined by either the voltage source it is connected to, or by the minus of the time rate of change of magnetic flux or by a combination of the two in a more complex circuit. Electric current has not entered the picture yet.

When an inductor is connected to a voltage source the voltage source *sets* the time derivative of magnetic flux. This is the EMF. The magnetic flux begins to grow linearly and will continue to do so forever for an inductor with no resistance in the wires.

The magnetic flux B is related to the magnetic field H through a material relation. For air core inductors it is mu, the permeability of free space. For ferrites mu is a few hundred times more that that of air.

When the flux begins to grow linearly then through B=mu.H, the magnetic field H also begins to grow. By Ampere's law the magnetic field is a measure of the current.

The physics then goes like this: applied voltage causes flux to increase linearly. Material medium translates flux into magnetic field which is proportional to current. Therefor the current begins to rise linearly.

After a time, the relation B=mu.H will cease to be linear because the material becomes "saturated". Mu will become smaller and smaller as the flux increases. At that point, as the flux continues its simple linear increase in time, the current through the inductor will start to increase more and more to maintain the relation B=mu.H with decreasing mu.

This is called a saturated inductor. Its still an inductor but resembles an air inductor now more than a material one.

Magnetic saturation is a function of the flux, not the magnetic field. It is possible to saturate an inductor that has very little current flowing in it by applying a large enough electric field (voltage) to its windings.

 

[FOIWATER]

There is INDEED a back EMF. It is induced during the negative half cycle of the AC wave form, and accounts entirely for inductive reactance.

I agree that there is no back EMF when there is no established magnetic field. But one cannot argue against the fact that current creates a changing magnetic field, and it is field collapse that induces a voltage which provides the opposition we refer to as inductive reactance. In fact, the amount of voltage for a specified amount of current change is what constitutes a henry. one volt second per ampere. usually millivolts are induced on a change of one ampere. which is why henries are most commonly found in mH.

 

[FOIWATER]

The entire explanation lies right here.

L*di/d(t)=i(t)

It covers everything you said above...

Should be an equation for voltage?

V = L di/dt

 

[Fuxue Jin]

This is how a filter (choke) works, basically.

There is INDEED a back EMF. It is induced during the negative half cycle of the AC wave form, and accounts entirely for inductive reactance.

I agree that there is no back EMF when there is no established magnetic field. But one cannot argue against the fact that current creates a changing magnetic field, and it is field collapse that induces a voltage which provides the opposition we refer to as inductive reactance. In fact, the amount of voltage for a specified amount of current change is what constitutes a henry. one volt second per ampere. usually millivolts are induced on a change of one ampere. which is why henries are most commonly found in mH.

 

[Antiphon]

There is INDEED a back EMF. It is induced during the negative half cycle of the AC wave form, and accounts entirely for inductive reactance.

I agree that there is no back EMF when there is no established magnetic field. But one cannot argue against the fact that current creates a changing magnetic field, and it is field collapse that induces a voltage which provides the opposition we refer to as inductive reactance. In fact, the amount of voltage for a specified amount of current change is what constitutes a henry. one volt second per ampere. usually millivolts are induced on a change of one ampere. which is why henries are most commonly found in mH.

An inductor connected to a voltage source never has a back EMF ever. It simply has an EMF.

You are thinking possibly of two separate cases; an electric machine where the mechanical rotation of a coil in a magnetic field creates an EMF (back emf) due to its rotation, or the case of a series RL circuit where the voltage on the inductor can differ from the voltage source because of the resistor drop. In the first case (motor) the term back-emf makes sense because the voltage on the armature has an emf associated with its natural inductance and an emf (the back emf) associated with the rotation of the armature.

In the second case this is confusing because there is no "back emf" in the sense of a second source of flux variation as in the motor. The voltage developed across an inductor (emf- not back emf) is always such as promote the continuity of the current- including for the example you gave of an inductor on an AC circuit.

 

[Antiphon]

I am a magnetic component designer, design transformers and inductors all day everyday, so there is no problem to understand what you discussed.

However, I think, to my understanding, voltage doesn't generate magnetic flux but current. You have current first, then you have H (NI, ampere-turn), then depends on the material (the core), you have B. Not you have B first, then H.

Which one comes first depends on the circuit. If you have a short-circuited inductor with 1 Amp flowing through it and you break the circuit, the Ampere-turns were already there but there was no EMF. The large EMF (not a back-emf!) develops via Faraday's law.

On the other hand the example I've been using is an inductor with no current that is connected to a voltage source.

Voltage is related to the (negative) time rate of change of flux (Int B.da), and current is related to the Ampere-turns available due the magnetic field H.

Here's a very readable reference geared for magnetics designers.

http://my.ece.ucsb.edu/bobsclass/194/References/Magnetics/slup123.pdf

Note particularly the right hand portion of the paragraphs on page 1-4.

 

[DragonPetter]

The entire explanation lies right here.

L*di/d(t)=i(t)

It covers everything you said above...

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

 

[DragonPetter]

Do you know this for a capacitor? would it be possible to PM it to me?

Its the same procedure, except now its i(t) = Cdv(t)/dt for a capacitor

let v(t) = sin(t), then differentiate, instead of integrate from the first example,to get i(t)

 

[FOIWATER]

An inductor connected to a voltage source never has a back EMF ever. It simply has an EMF.

You are thinking possibly of two separate cases; an electric machine where the mechanical rotation of a coil in a magnetic field creates an EMF (back emf) due to its rotation, or the case of a series RL circuit where the voltage on the inductor can differ from the voltage source because of the resistor drop. In the first case (motor) the term back-emf makes sense because the voltage on the armature has an emf associated with its natural inductance and an emf (the back emf) associated with the rotation of the armature.

In the second case this is confusing because there is no "back emf" in the sense of a second source of flux variation as in the motor. The voltage developed across an inductor (emf- not back emf) is always such as promote the continuity of the current- including for the example you gave of an inductor on an AC circuit.

I strongly disagree, but encourage anyone to have input here as well.

Connect an inductor across a voltage source with an ammeter in series, once it draws its max DC current, shut it down, and you will see the ammeter go negative.

This is because the field is collapsing onto the inductor, inducing a counter EMF (back EMF) into it and is supplying current to the source. It is the purpose of having diodes in parallel with coils (inductors) They provide safety to sensitive electronics once power is removed from inductors, the back EMF causes current to flow into the rest of the circuit. \

I know that back EMF is generated in a generator/motor, but you have to realize that this back EMF caused by motion is the opposite side of the same inductive coin.

Faraday's law states you need relative motion between a magnetic field and a conductor to induce a voltage in the conductor.. you are saying back EMF is produced by motion of the conductor in a machine, not a coil... but I am saying that it can be either. As long as the motion is relative there will be a voltage induced, whether it is in an electric motor and the motion is caused by a rotating conductor in a magnetic field, or whether a back EMF is generated in an inductor because the magnetic field is time varying, it does not matter.

I'm sure I can get some positive feedback from others on this?

Surely back EMF is exhibited in inductors. It accounts for nearly all coil behavior.

PS: Chokes are just applications of inductors, they are not any specific kind of inductors, so the fact they act like filters lends itself to the fact that voltages are induced in coils, and due to lenz' law they oppose the source voltage.

I mean if you research lenz' law even, you will see what I am talking about. The reason why polarity of voltages induced by collapsing fields are opposite to currents that produce the fields is exactly because of that, the field is COLLAPSING, how else could an inductor oppose current, if not by producing a voltage? it consists of something much more than resistance. IE, inductive reactance.

 

[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.