Searching \ for '[EE] Three questions on inductors' in subject line. ()
Help us get a faster server
FAQ page: www.piclist.com/techref/index.htm?key=three+questions
Search entire site for: 'Three questions on inductors'.

Exact match. Not showing close matches.
'[EE] Three questions on inductors'
2010\05\16@050401 by

Hi,
trying to improve my knowledge of inductors.

The energy stored in an inductor is proportional to the current that is flowing
through it in that certain moment. If the current increases, then (part of it or
all?) gets stored as additional energy in the magnetic field, if the current
decreases, part of the energy gets released to oppose this.

I'd three questions for Your brains:

1) The "(part of it or all?)" already written above part.

2) Suppose I increase the current in an inductor till 1A, then keep it constant
for 1 second. What happens during this second? Do I simply waste power?

3) If I simply wasted power (continuing from the previous question), then I
suppose I will waste some power even during the "current is increasing" part.
Then, when current is increasing (that is, dI/dt is not zero), how much energy
I am storing and how much I wasting (as heat, etc..)? What is the formula?

I fail to make an analogy with capacitors, as once charged they get (theoretically)
infinite impedance. Inductors instead get (theoretical) zero impedance, and this
may imply the "waste" problem stated above. As I said, I fail to make an analogy
with capacitors on this particular issue.

Thank You!

Greets,
Mario

On 16 May 2010 21:03, Electron <electron2k4infinito.it> wrote:
>
> Hi,
> trying to improve my knowledge of inductors.
>
> The energy stored in an inductor is proportional to the current that is flowing
> through it in that certain moment. If the current increases, then (part of it or
> all?) gets stored as additional energy in the magnetic field, if the current
> decreases, part of the energy gets released to oppose this.
>
> I'd three questions for Your brains:
>
> 1) The "(part of it or all?)" already written above part.

The energy stored is equal to 1/2 x L x I^2.  i.e. proportional to the
square of the current. If the current increases, then the energy
stored increases also. Unless the core is "saturated" in which case no

>
> 2) Suppose I increase the current in an inductor till 1A, then keep it constant
>   for 1 second. What happens during this second? Do I simply waste power?
>

With an ideal inductor there is no DC resistance. Energy is only
stored when the current is changing and this is the only event that
will cause a voltage drop. When the current is stable no energy is
being absorbed or (ideally) dissipated. So no power is being wasted.

> 3) If I simply wasted power (continuing from the previous question), then I
>   suppose I will waste some power even during the "current is increasing" part.
>   Then, when current is increasing (that is, dI/dt is not zero), how much energy
>   I am storing and how much I wasting (as heat, etc..)? What is the formula?
>

Power is not being wasted., energy is being stored (or released) only
while the current is changing. The formula above E=1/2 x L x I^2 is
the one used. While the current is changing, there will be a voltage
drop across the inductor equal to V = L x di/dt

> I fail to make an analogy with capacitors, as once charged they get (theoretically)
> infinite impedance. Inductors instead get (theoretical) zero impedance, and this
> may imply the "waste" problem stated above. As I said, I fail to make an analogy
> with capacitors on this particular issue.
>

With the analogy to capacitors, voltage converts to current. So while
a capacitor stores energy with a voltage across it, and inductor
stores energy when current is passing though it. It's a little harder
to visualize, partly because we are more used to dealing with voltage
sources, and partly because capacitors tend to be more "ideal" than
inductors.

Hope this assists

RP
Electron wrote:
> Hi,
> trying to improve my knowledge of inductors.
>
> The energy stored in an inductor is proportional to the current that is flowing
> through it in that certain moment. If the current increases, then (part of it or
> all?) gets stored as additional energy in the magnetic field, if the current
> decreases, part of the energy gets released to oppose this.
>
> I'd three questions for Your brains:
>
> 1) The "(part of it or all?)" already written above part.
>
> 2) Suppose I increase the current in an inductor till 1A, then keep it constant
>    for 1 second. What happens during this second? Do I simply waste power?
>
If the voltage across the inductor is not changing, then "all" of the
energy possible is stored. Assuming ideal inductor.

If it's a perfect inductor it has zero "real" resistance. Imaginary axis
"resistance" is invisible at DC.

Thus no power is wasted. IN THE INDUCTOR. Other parts of the circuit
will  / may dissipate power. The actual power wasted in a non ideal
inductor for steady state current is P = I^2 x R, where R is the DC
resistance (Real axis Resistance).

> 3) If I simply wasted power (continuing from the previous question), then I
>    suppose I will waste some power even during the "current is increasing" part.
>    Then, when current is increasing (that is, dI/dt is not zero), how much energy
>    I am storing and how much I wasting (as heat, etc..)? What is the formula?
>
Wasted power when current changing is these main parts:
P = I^2 x R, where R is the DC resistance (Real axis Resistance). Mainly
winding DC resistance. At high frequency the current in a conductor
flows closer to surface. Skin effect. This makes "real" resistance
higher even at 1MHz.  By  400MHz a flat strip will have much lower
"real" component (lossy resistance) than a circular wire. High power RF
coils may be wound with 4mm hollow copper, possibly silver plated as
almost no current flows in the  core.  Polished copper with lacquer to
avoid oxidising is lower loss than tin or tin/lead plated or gold.

Radiated RF (large air core coil is higher, on ferrite toroid is lower).
Formula is complex and depends on frequency, construction, size
Losses in the Magnetic material if not air core.
Magnetic/Induction losses: Eddy current with in core and windings, worst
for solid conductor, less laminated, lower insulated dust iron or
ferrite and least on air core. Coupling to magnetic materials or other
nearby coils.
Core Saturation
Magnetic hysterises

Losses are usually lowest with sine wave and highest with  Square wave
due to frequency dependent eddy current loss, skin effects on
conductors, RF emission and such.

Interwinding capacitances may not increase losses directly but affect
self resonant  frequency and actual inductance.

> I fail to make an analogy with capacitors, as once charged they get (theoretically)
> infinite impedance. Inductors instead get (theoretical) zero impedance, and this
> may imply the "waste" problem stated above. As I said, I fail to make an analogy
> with capacitors on this particular issue.
>
>
harder to make a "good" inductor that's millihenries. A perfect
capacitor with DC (fixed/steady) voltage has no current. A perfect
inductor with DC(fixed/steady) current has no voltage  across  it.

For C   P =  V x 0  =  0
For  L   P =  0 x I  =  0
no power dissipated.

>Interwinding capacitances may not increase losses directly but affect
>self resonant  frequency and actual inductance.

>From what I've seen (which is not a great deal :-)) self resonant
frequencies can be a lot different for the same rated inductance (between
different inductors). I'm in the process of looking for an SMD power choke,
around 100uH, and the rated maximum SRFs seem to vary between 5 and 12 MHz,
a bit lower than I expected. I suppose unless you take this kind of thing
into account, you could up with performance quite a bit different from that
expected in an ideal situation. Not sure how this would translate to the
stator scenario though (I assume this post is related). Maybe a rough sketch
in spice might be helpful for all this, with temperature coeffiecient,
saturation, winding capacitances/resistance etc.

Thank You all for Your replies, very complete, very clear and very explicative!

Have a nice sunday,
Mario

At 12.23 2010.05.16, you wrote:
>>Interwinding capacitances may not increase losses directly but affect
>>self resonant  frequency and actual inductance.
>
>>From what I've seen (which is not a great deal :-)) self resonant
>frequencies can be a lot different for the same rated inductance (between
>different inductors). I'm in the process of looking for an SMD power choke,
>around 100uH, and the rated maximum SRFs seem to vary between 5 and 12 MHz,
>a bit lower than I expected. I suppose unless you take this kind of thing
>into account, you could up with performance quite a bit different from that
>expected in an ideal situation. Not sure how this would translate to the
>stator scenario though (I assume this post is related).

Not again! Please if the thread is different do not assume things not explicitly
stated.. else only confusion may arise. ;)

Cheers,
Mario

>Maybe a rough sketch
>in spice might be helpful for all this, with temperature coeffiecient,
>saturation, winding capacitances/resistance etc.
>
>
>-
>>Not sure how this would translate to the
>>stator scenario though (I assume this post is related).

> Not again! Please if the thread is different do not assume things not
> explicitly
> stated.. else only confusion may arise. ;)

Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing
to do with either :-) I was just guessing at why you might be asking..

Oli Glaser wrote:
>  >>Not sure how this would translate to the
>
> Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing
> to do with either :-) I was just guessing at why you might be asking..
>
>
>
Because people here might know the answer and he wants to know?

or  Wikipedia. They are  easy  when you sort of half know the answer or
have simply forgotten the details but you understand it. You need at
least the correct terminology.
At 13.30 2010.05.16, you wrote:
> >>Not sure how this would translate to the
>>>stator scenario though (I assume this post is related).
>
>> Not again! Please if the thread is different do not assume things not
>> explicitly
>> stated.. else only confusion may arise. ;)
>
>Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing
>to do with either :-) I was just guessing at why you might be asking..

Inductors have such a wide range of applications that it was just a general
question. I have other projects going on as well, not just the CDI one. ;)

Also, as inductors knowledge is generally in "shadow" in many hobbyst's
minds, I think that making such a question, the eventual answers may be of
general interest.

In this specific case, the answers were simply great. More than to the point.

To be honest I hate to go to the supermarket to get some milk and get a lot
of questions like "what do you have to do with it?", "are you sure you really
need milk?". I mean, if you ask a very specific question, then you probably
need a very specific answer. Of course, even "what do you need to achieve?"
is the think to ask, sometimes, but always - not. Sorry for the OT.

Greets,
Mario

At 13.45 2010.05.16, you wrote:
>Oli Glaser wrote:
>>  >>Not sure how this would translate to the
>>
>> Sorry, I meant Bike/CDI related really, but i appreciate it may be nothing
>> to do with either :-) I was just guessing at why you might be asking..
>>
>>
>>
>Because people here might know the answer and he wants to know?
>
>or  Wikipedia. They are  easy  when you sort of half know the answer or
>have simply forgotten the details but you understand it. You need at
>least the correct terminology.

I use Wikipedia and - as a more general rule - Google a lot, I actually think
I consumed it during the years.

However, sometimes you have a doubt and even if you read tens of pages that
explain everything about inductors, they repeat the same things but none of
them clear the little, sometimes strange, doubt you have in your mind.

Then and only then I resort to ask things here, and I do it with an ease
directly proportional to my belief that bringing up the matter and causing
the production of answers may be of general interest for many other readers,
who maybe didn't even need to know, but that will be enriched by the answers
anyway - without even needing to ask. :D

Electron wrote:
> trying to improve my knowledge of inductors.

Then Google and find a tutorial first.  Surely there must be many basic
introduction to inductors out there.  Do your homework first, then ask here

> The energy stored in an inductor is proportional to the current that
> is flowing through it in that certain moment.

No.  Go look it up.

> If the current
> increases, then (part of it or all?)

All unless you hit saturation.

> gets stored as additional energy
> in the magnetic field,

> I'd three questions for Your brains:

Phew.  Good thing they are not for my foot or left elbow.  Those aren't very

> 2) Suppose I increase the current in an inductor till 1A, then keep
>    it constant for 1 second. What happens during this second? Do I
> simply waste power?

In a ideal inductor, no.  However, most (all you can hold in your hand)
inductors have a non-zero resistance.  During the 1 second of constant
current this resistance will dissipate power.

> 3) If I simply wasted power (continuing from the previous question),
>    then I suppose I will waste some power even during the "current is
>    increasing" part. Then, when current is increasing (that is, dI/dt
>    is not zero), how much energy I am storing and how much I wasting
> (as heat, etc..)? What is the formula?

Model the real inductor as a ideal inductor in series with a resistor and it
will become obvious.

> I fail to make an analogy with capacitors, as once charged they get
> (theoretically) infinite impedance.

Huh?  Go read a tutorial on capacitors too.

> (theoretical) zero impedance, and this may imply the "waste" problem
> stated above. As I said, I fail to make an analogy with capacitors on
> this particular issue.

This is too confused to try to correct here.  Go read some introductory
material on inductors and capacitors and then come back after having done

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
At 15.23 2010.05.16, you wrote:
>Electron wrote:
>> trying to improve my knowledge of inductors.
>
>Then Google and find a tutorial first.  Surely there must be many basic
>introduction to inductors out there.  Do your homework first, then ask here

This is what I did*. You like to be uselessly mean, as always.

>> The energy stored in an inductor is proportional to the current that
>> is flowing through it in that certain moment.
>
>No.  Go look it up.

No amount of good technical advice is worth having someone like you in a list.
Proof is that I got other answers which cleared up my doubts and even went
beyond, and in doing do have also taught it to many other people who don't
even bother to ask questions here, maybe just because they "fear" you may

* just to name one:

You remain an uselessly mean person anyway. I wonder if you treat people the
same way also when you have them face to face, or if you do this only through
the keyboard.

But I don't really care.

Welcome to my killfile Olin, it's the right tool for the job.

Electron wrote:
> To be honest I hate to go to the supermarket to get some milk and get
> a lot of questions like "what do you have to do with it?", "are you
> sure you really need milk?".

You are paying them and not asking for advice, and they just want to sell
you milk, not spend time on advice about how to use it.

> I mean, if you ask a very specific
> question, then you probably need a very specific answer.

Maybe, but often enough we have people here asking the wrong question in the
first place.

> Of course,
> even "what do you need to achieve?" is the think to ask, sometimes,
> but always - not. Sorry for the OT.

First, people are here for their own entertainment and other reasons.
that serves their ends so they do help.  Second, anytime you don't like the
free advice you receive here, you can have a full refund.  Third, often
enough we get people here asking about specific details to the wrong
approach.  The right answer is to find the true problem one or two levels
up, then discuss how to solve that.  That's how it works, get over it.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

Electron wrote:
>> Then Google and find a tutorial first.  Surely there must be many
>> basic introduction to inductors out there.  Do your homework first,
>
> This is what I did*.

You certainly didn't give any clue to that.  To me it sounded like you
hadn't looked up anything.  You've been around here long enough to know that
showing you've done your homework is as important as doing it.

> You like to be uselessly mean, as always.

I like people to do their homework, then help those that have tried to help
themselves.

>>> The energy stored in an inductor is proportional to the current that
>>> is flowing through it in that certain moment.
>>
>> No.  Go look it up.
>
> No amount of good technical advice is worth having someone like you
> in a list.

Oh get over yourself.  Just telling you that your statement is incorrect
should have been useful information.  I could have told you the answer, but
I felt you would learn more by looking it up.  This should not be difficult
since you now know what specifically to look up.

********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.

I'm sure you did look it all up on google so I won't suggest that :-)

Here's a useful book though (at least I found it quite useful) on basic
theory, general electronics, useful circuits etc:

http://www.amazon.co.uk/Practical-Electronics-Inventors-Paul-Scherz/dp/0070580782

It may also help to get a couple of books on calculus, trigonometry etc, for
the math behind all the theory. Personally, I try and avoid
"generalisations" if I can, and dig down to see what's really going on with
stuff (time permitting)

And I like LTspice for simulation:

http://www.linear.com/designtools/software/ltspice.jsp

>>> The energy stored in an inductor is proportional to the current that
>>> is flowing through it in that certain moment.

>>No.  Go look it up.

> No amount of good technical advice is worth having someone like you in a list.
> Proof is that I got other answers which cleared up my doubts ...

Be very wary of being so dismissive of any known competent person's
technical input.

Olin's style may be lacking, and he sometimes gets the wrong idea of
what someone is saying in his enthusiasm to 'correct', BUT the
technical basis for his utterances is almost invariably sound and it
is highly advisable to sort the wheat from the chaff and understand
what the technical point was that he was making. It's very usually a
valid one.

You said:
>>> The energy stored in an inductor is proportional to the current that
>>> is flowing through it in that certain moment.

He said:
>>No.

He was, of course, correct.
Although, as you tell us,  you have looked extensively at Wikipedia
etc before asking questions here, you have managed to form a
fundamentally incorrect impression. Olin sought to correct this. To
ignore his input, no matter how badly it is put, is to allow yourself
to become blind to technical truth and to resist correction.

He said:
>>Go look it up.

+"The energy stored in an inductor is"

Produced a large number of good quality hits.
The first 5 or 6 give the actual formula or are obviously directly
relevant. Many others will be too.

The top result says

Electronics Forum -> Energy Stored In The Inductor.
The energy stored in an inductor is given by the formula = (L*I*I)/2.
http://www.dutchforce.com/~eforum/index.php?showtopic=31528... - Cached

ie energy stored is proportional to current squared.

An extremely important thing to know about the internet is that you
get good and bad information. Pages which LOOK escellent may be
rubbish. Checking a few sites against each other is always wise.

Unfortunately, the sight that you cite
has some good material BUT also some misleading material and also some rubbish.
Unfortunately, I'd go so far as to say that despite its helpfulness
and the effort that they have gone to that that is effectively a bad
page for beginners.

They NOWHERE explicitly mention the relationship between inductor
current energy and current, although they easily could and certainly
should have given the detail they go into. They make one correct
related  which may mislead in what it says
ie " The more rapidly current is decreased, the more voltage will be
produced by the inductor, in its release of stored energy to try to
keep current constant." This may have mislead you in coming to your
conclusion.
Unfortunately you were wrong and Olin  was correct. You need to be
able to both expect that and to  pick up on his "clues" even if you
think that his mother wears army boots.

So -

> ... they "fear" you may give them one of your "silly" ... answers.

Learn to take the wheat and throwaway the chaff.
Expect the technical content to very very largely be correct.
(just like technical content from me :-) - very very largely correct,
sometimes rubbish for whatever reason just to keep you alert :-) ).

> * just to name one:

Good attempt, as above. . It shows you tried. Compare several sources
and find ones which expressly address the point being studied. ie in
this case energy stored is not actually addressed even though they
imply a relationship.

> You remain ...
> I wonder if you ...

use the appropriate channels.
Nobody is above proper behavior requirements, but there are
appropriate ways to complain set out in the rules.
Personal slanging matches on line often end up with the person
complaining getting more hurt than the person being complained about.
Really.

> Welcome to my killfile Olin, it's the right tool for the job.

That would probably be a mistake on your part if you want to learn.

ALL: If you MUST continue anything on the personal aspects of this and
other matters DO IT IN [OT]. Even there it may, at this rate, end up
causing you to be subject to admin action if it relates to mothers
wearing army boots or anything approaching ad hominem attack.

Hat off ...

Russell

2010\05\16@105332 by

ALL: If you MUST continue anything on the personal aspects of this and
other matters DO IT IN [OT]. Even there it may, at this rate, end up
causing you to be subject to admin action if it relates to mothers
wearing army boots or anything approaching ad hominem attack.

At the rate we are going at least two people are at risk of being
subject to admin action if they so much as blink an eye.
_____________

FIAT:       BICKERING, ADVISING AND GENERAL SLANGING IF DONE AT ALL
SHOULD BE DONE IN OT.

Even there it may not be acceptable IF  it violates list rules.

Either take it outside OR change the tag to [OT] and keep the subject
the same if you must bicker.

_________

Hat off ...

Russell
> Because people here might know the answer and he wants to know?

I meant the reason behind the question, what project it might be related to
etc. I get the "obvious" reason for the question :-)  I wasn't suggesting
for a minute it was a silly or unecessary question, I was just guessing at
the "reason" he might be asking it, sorry if it may have looked that way..

> or  Wikipedia. They are  easy  when you sort of half know the answer or
> have simply forgotten the details but you understand it. You need at
> least the correct terminology.

Indeed, that's exactly what these kind of things are great for, as you get
an intelligent, focused answer rather than a list of possibly
related/unrelated stuff to sort through.

More... (looser matching)
- Last day of these posts
- In 2010 , 2011 only
- Today
- New search...