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Inductors

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frankw

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I am wondering if anyone could help clarify my understanding of inductors. I would not be surprised if I have missed some important details due to some less then stellar studying in the past.

So what I know:

When an inductor circuit is closed, source voltage immediately gets transferred to the inductor as the electromagnet is "charged". Once this is over, the voltage quickly drops and current now runs through the circuit.

^^ I believe that, that is basically correct, but where I have some trouble is understanding the discharge function.

When an inductor discharges it can obtain voltages much higher then the source. I know this has to do with an inductor's opposition to current change, but I don't understand why and my book is doing a really poor job of making it clear.

The fact that the twelve volt battery in a car produces 25kV is amazing, but I just can't grasp the whys and hows..

Thanks Alot for any aid!

Frank.
 
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When you 'charge' an inductor you're actually storing the energy in a magnetic field in the space around the coil, this magnetic field is dependent upon the current flowing through the inductor. If the voltage that is driving that current is suddenly withdrawn the current will try to stop flowing, but the magnetic field requires this current to be flowing to exist, so it collapses by turning into a voltage in an attempt to keep itself going. In fact, super conducting magetics can be turn on and off (through some intersting mechanical linkages) Current is established in a coiled super conductor, then a separate piece of super conductor shorts the coil causing the current to flow in an endless loop around the inductor creating a magnetic field that is stable even without an applied voltage because it has no resistance.

The energy in the magnetic field itself is turned back into electromotive force (voltage) in the coil (generally) the energy HAS to go somewhere, so the peak voltages created can be incredibly high if no path is readily available, the voltage will eventually become high enough and unbalanced enough between two points to discharge. Keep in mind NO energy is gained, a low voltage high current is transformed into a high voltage current of a much shorter duration. So basically it's like condensing the time period the energy is released in. Total energy is the same (always less actually because of resistance) but no magic has actually occured.
the codependancy of electric and magnetic fields is quiet fascinating.

I remember the pure aw in my mind the first time I found out a transformer wasn't actually physically connected but was connected via magnetic flux only, and the two circuits were effectivly electrically isolated, but not magnetically.

Sorry for the babble, hope that provided some kind of helpful view on the subject.
 
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As the applied voltage increases the inductor current, energy is stored in the inductance from the magnetic field (E = 1/2 LI²). This current will keep increasing as long as the voltage is applied until the inductor resistance limits the current. If you then switch this inductor current to a high impedance the inductor voltage will rise as required to drive the current through the impedance. If you try to open the inductor circuit with the voltage will rise until an arc is generated to keep the current flowing. The energy for this is provided by the stored inductor energy.

An ignition coil in a Kettering ignition has a primary which acts as an inductor and transformer type secondary with many more turns (typically about 100:1). Then when the points open the primary circuit, the inductively induced voltage (200-300V) generates 20k-30KV at the secondary for the sparkplug.
 
What every-one has missed in this discussion is the fact that the voltage produced by a collapsing magnetic flux is in the opposite direction to the voltage producing the flux.
This is why a “damper diode” is fitted with the cathode facing the positive of the supply.

Rather than use complex terminology to explain the operation of an inductor, I prefer to state that an inductor produces an expanding magnetic flux when a voltage is applied and if you want to go down to microsecond conditions, the initial voltage will find it difficult to pump a current through the inductor due to the initial magnetic flux cutting the turns of the inductor and producing a voltage in the opposite direction. This means the resulting initial “charging voltage” is very small and the initial current is low.
As micro-time passes, the back voltage (called back emf) reduces (as the magnetic core becomes saturated) so that the effective forward voltage is higher and thus the current increases.
When the supply voltage (called the exciting voltage) is switched off, the magnetic flux collapses and produces a voltage in the opposite direction. Since the voltage produced is proportional to the speed of the collapsing magnetic flux, this reverse voltage can be very high.
If no load is connected to the inductor, the voltage will be very high and the ratio of the exciting voltage to the collapsing voltage is called the QUALITY FACTOR – commonly assigned the value “Q.” This can be as high as 1,000 or more.
If a load is connected to the inductor, the voltage across the load will be determined according to ohms law. The actual voltage produced (and the time it is produced-for), will be determined by the amount of energy the inductor produces via the magnetic circuit. This voltage could be higher than the exciting voltage or lower – determined by the resistance of the load.
 
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When an inductor discharges it can obtain voltages much higher then the source. I know this has to do with an inductor's opposition to current change, but I don't understand why and my book is doing a really poor job of making it clear.

The fact that the twelve volt battery in a car produces 25kV is amazing, but I just can't grasp the whys and hows..

Thanks Alot for any aid!

Frank.

You just discovered one of the great truths of our time: modern books on electromagnetic theory just plain suck, and were written by clueless hacks. You're better off ordering "antique" books on the subject from Lindsay Publications from the turn of the last century, when writers actually knew what they were writing about.

"The fact that the twelve volt battery in a car produces 25kV is amazing, but I just can't grasp the whys and hows".

This would be explained by Faraday's Law:

Vn(t)= d(phi)/dt

Where:

Vn(t): Instantaneous volts per turns ratio

phi= Magnetic flux in Webers

As for how that high voltage is produced, it requires a strong magnetic field and a fast time rate of change, and a secondary to step up the voltage even further. This usually means a primary with as low a self inductance as possible (speed) and resistance (moamps). As for induction coil design, this is something of a "black art" as there are no design equations that can be applied due to the loose coupling between the primary and secondary, and that externals -- such as winding insulation thickness -- can have a decided effect on actual operation.

In former times, most auto ignition coils were wound on open cores, with loose coupling. This loose coupling was a desireable feature, since the coil could produce a high voltage across a spark gap, with the only load being the gap's capacitance. Once the arc was struck, the current would pull the voltage down almost immediately. This kept the resulting arc small so that it wouldn't erode the electrodes too quickly. You often need this feature in other situations as well: starting laser tubes, running Giessler tubes, etc. A high current from a well regulated high voltage supply will poof these.

These days, the ignition coil is more like a conventional transformer, and they rely on resistance wire to limit current.
 
Thanks alot for the help, my understanding has greatly increased.

So basically the voltage reaction is like the magnetic field trying to protect itself and maintain current flow. Time is apparently a huge factor in this as is the strength of impedance. The faster you open the higher the voltage.. but the current is minuscule...

And from what Colin was saying, it sounds like the magnetic flux lines "cut" the coil in such a way that it interrupts the current as to cause it to reverse, caused by the inductor's protective voltage.

Would you be able to tell me what di/dt is? I see it occasionally in my textbook but I can't ever remember learning what it was. I looked through the indexes and couldn't find anything, so I guess maybe it is in one of the chapters my class skipped... :(

Thanks again!
 
di/dt is the rate of change of current with time.

d used instead of δ, the Greek letter "delta" and it means the change in some value. So di is the change in the current and dt is the change in time.

The primary of a car coil is an inductor. In old cars, the maximum current was limited by the resistance of the primary. Nowadays, the electronics that feed it limit the current.

Either way, when the 12 V is applied to the primary, the current builds up slowly.

L*di/dt = V

Here V is 12 V, L is just the inductance of the coil, so di/dt is small, which means that the current changes slowly. (It takes about 2 - 4 ms for the current to hit maximum in a car coil. That seems fast if you are looking at it yourself but for what we are talking about that is slow)

Then the current is stopped very quickly. The current goes from something to nothing very quickly. dt is very small, so di/dt is very large (and negative) so V is very big, often 200 - 300 V

The secondary of the coil, which up to this point has done nothing, has maybe 100 times as many turns. When the coil has 12 V on it, there will be maybe 1200 V on the spark plug. Sounds a lot, but in fact it's not enough to cause a spark. However, when the current stops and there is 200 - 300 V on the primary, the secondary produces about 20,000 to 30,000 V and you get a spark.

The primary of the coil is being used for two things. Firstly as an energy store, which gives out its energy as a 200 - 300 V pulse, and secondly the primary of a pulse transformer. Some ignitions systems (magneto and capacitor discharge systems) separate out these functions and the coils with the high-tension connection is only used as a pulse transformer)

I realise that I haven't said much that adds to what the others have said, but it can help to have it explained differently.
 
The faster you open the higher the voltage.. but the current is minuscule...
In most cases the current is turned off imediately so this fact is not taken into account when determining the high voltage produced by an inductor.
The thing that produces the high voltage is the "magnetic circuit" and its ability to turn the collapsing magnetic field into lines of flux that cut the turns of the coil. The current is not miniscule. It is in proportion to the current that was flowing before the voltage was switched off. If the voltage is increased 100 times, the current will be reduced to one-hundredth (less the losses).


And from what Colin was saying, it sounds like the magnetic flux lines "cut" the coil in such a way that it interrupts the current as to cause it to reverse, caused by the inductor's protective voltage.
The magnetic lines (flux) cut all the turns of the primary when the flux is expanding and this produces a voltage in each turn, in the opposite direction, that can be as much as 99% of the energising voltage. Thus the resulting voltage seen by the inductor can be very small and thus a very small current flows.
 
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And from what Colin was saying, it sounds like the magnetic flux lines "cut" the coil in such a way that it interrupts the current as to cause it to reverse, caused by the inductor's protective voltage.
You're misunderstanding something, because the current doesn't change direction. The reverse voltage is what's required to keep the current moving in the forward direction in the coil once the supply is removed. If you apply a diode to in reverse across the coil you'll see at switch off time that the current is still flowing around the coil in the same direction and will continue to flow dissipating as heat as the magnetic coils energy is turned back into an electric field. The reverse voltage is the magnetic field building energy up, it would cause current to flow in the opposite direction in the circuit it's attached to, NOT itself. Basically once that energy is removed from the circuit, that energy has to go somewhere so it'll find it's most direct route to gruond generating as much voltage as required to get there.
 
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How on earth did you come up with this:
The reverse voltage is what's required to keep the current moving in the forward direction in the coil once the supply is removed.
. There are so many mistakes in this reasong that it should be removed from the posting. Your reasoning is totally INACCURATE.


Where on earth does this come from:
Basically once that energy is removed from the circuit, that energy has to go somewhere so it'll find it's most direct route to gruond generating as much voltage as required to get there.
The voltage generated is a characteristic of the inductor - the amount and quality of the magnetic circuit, the number of turns, and the amount of energy in the inductor, in the form of magnetic flux. "As much voltage will be generated to get there" is totally inaccurate. The voltage may be very high or it may be quite low. It's like saying: "the money that will be despoted into my account will be sufficient for anything I want to buy this week!!!! How do the depositors know how much money I need this week???
 
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How on earth did you come up with this: The reverse voltage is what's required to keep the current moving in the forward direction in the coil once the supply is removed.
He came up with that because that is what happens. The initial reverse voltage will indeed go to whatever is required to keep the inductor current flowing when the charging voltage is removed.
 
The initial reverse voltage will indeed go to whatever is required to keep the inductor current flowing when the charging voltage is removed.
How can you have voltage flowing in one direction and current flowing in the reverse direction, AT THE SAME TIME?????
 
How can you have voltage flowing in one direction and current flowing in the reverse direction, AT THE SAME TIME?????

Voltage isn't normally considered to flow. A charged capacitor has a voltage on it when no current is flowing.

When there is voltage on an inductor, and current in the same direction, energy flows from the rest of the circuit to the inductor.

When the voltage is in the opposite direction to the current, energy flows from the inductor to the rest of the circuit, and the current in the inductor reduces.

The current in an inductor cannot change instantaneously. When the supply to an inductor is turned off, the current continues to flow and the voltage my reverse virtually instantaneously.
 
Would you be able to tell me what di/dt is? I see it occasionally in my textbook but I can't ever remember learning what it was. I looked through the indexes and couldn't find anything, so I guess maybe it is in one of the chapters my class skipped... :(

Start with Faraday's Law:

1) Vn(t)= d(phi)/dt

Next, convert to actual output voltage:

2) v(t)= N X d(phi)/dt

Next, apply the Chain Rule to break up that derivative into a product of derivatives:

3) v(t)= N X d(phi)/di X di/dt

Next, stipulate that d(phi)/di is a constant, meaning that the relationship between magnetic flux and current is a linear one. This does, in fact, hold true for an air coil. Absorb the two constants into another constant, and give it the symbol: L:

4) v(t)= L X di/dt

Now, you have gone from the "language" of field theory to that of circuit analysis. Once you know what L is, you no longer need be concerned about the geometry of the coil: what kind of core it has, how many turns, and so forth. In this case:

5) L= (N X phi) / I, or one Weber * turn per amp defines an inductance of one henry.

It should be noted that, strictly speaking, #4 applies to air coils only. If ferromagnetic materials are present, the relationship between magnetic flux and current is no longer linear. However, the concept is just so useful that that little detail is ignored. Thus, you might see a spec for a ripple choke like: 5H / 90mA. The 90mA doesn't refer to the max current capability of the wire, but rather the current that must flow through the coil to insure that it has the design nominal inductance of 5 henries. Try measuring that inductance with an RLC bridge (most often these use a sense voltage of 1.0Vp) and you might get a reading of many hundreds of henries. Try to figure the inductance by plugging that inductor into a wall socket while measuring the current, and you might figure an inductance of a few hundred millihenries. As per #5, the inductance changes greatly with core permeability and saturation.
 
So colin, do I get an appology? We've been on good terms for a while now =->

I think one important thing that needs to be understood about electronics in the first place is that it is entirely based on manipulation of one of the four major forces in nature.

Gravity being one, Electricity/magnetism combined being the second (you can't have one without the other) and the strong and weak atomic forces.

The strong and weak forces can generally be ignored except in some very special situations as they only interact in the subatomic world.

Gravity well that's the most publicized one as that's the one that holds all the matter together in the universe and one of the least understood forces.

The electromagnetic one however has serious implications for existence. All molecular bonds are based on the sharing of electron orbits between atoms. The entire reason two objects can't pass through each other is because the electrical forces repel each other when they get very close. I mean electrons are THAT important. All of chemistry and molecular science require it. Even though it's that pervasive a part of our world people don't generally think of it that way, most people would think that gravity is a more important force in the world, and they'd be wrong cause it's so much weaker than the rest of them (which is a completely different conversation)

I mean if you get right down to it, EVERY single last physical object you interact with every day owes it's entire existence to the electron. Every form of radiation is also based on alternating electric and magnetic fields. It's impossibly important. You can't simplify it too much..

If you try to dumb down electronics or real world physics too much it becomes impossible to learn, because the WHY of what's happening is actually the most important part of it, if you try to sweep that under the rug you can't actually learn anything useful.
 
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Yes. You were correct. It was just the poor description that led me down the wrong path.
I took it that the current "just kept flowing" when the power was switched off, as that was the way it was written. That confused me.
I have always concentrated on the voltage with any of my descriptions as one of the most important features of an inductor is to produce a high voltage and many times the very high voltage produced is used to advantage.
In addition, the fact that this voltage is produced in the reverse direction is employed too.
To make it clearer, I would have written that when the supply is removed, the current ceases to flow in the inductor and this causes the magnetic flux to collapse and produce a voltage in the opposite direction. According to the resistance of the circuit, a current will also flow that is determined by the resistance.
This makes it clear that the current is now produced by the collapsing magnetic flux, whereas everyone was stating that the current "continues to flow."
 
Wow, Sceadwian.

You have inspired me to go back and truly try to understand inductance and gain a stronger grasp of basic electron theory.

Thanks for all the responses I really appreciate the energy people have put in to help my understanding.

Would anyone have any recommendations of where to read of the basics of electron theory? My text is good at explaining stuff like sinusoids, but I find anything at the electron level ungraspable. I think for something like this I need something really written in an "Average Joe" tongue.

I know my shaky grasp of these fundamentals is the difficulty in comprehending. "You cant see" these things.

If anyone knows a good book (or online source) for fundamental electron concepts I would greatly appreciate it as I plan on now including this into my studies.

Cheers!
 
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colin, the description was technically correct, what more do you want?
I led you no where, you went there yourself by assuming something in what I said that I did not intend without first checking with me before attempting to lambast my statement.

I am not attempting to prove anything here, I am not trying to be correct, I am doing nothing but sharing the information I have gleamed in my time learning and sharing it as best as I know how.

You say you're trying to make it clearer however you are muddying the waters further. When the supply is removed the current continues to flow provided with the energy from the magnetic field collapse, the current never stops flowing even when it's originating voltage is removed, this is the basic key property of an inductor that is important, not voltage, a voltage is only produced if that current can't continue to flow by the new circuit conditions, because the energy has to go somewhere, it causes a backup in the EMF field which causes the voltage observed, the voltage itself is not however the actual reason for an inductors function.
 
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