Aargh! Inductor Coupling Question...

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Aargh! Inductor Coupling Question...

Some may remember my post about coupling between adjacent inductors
some time back.

In that case the inductor cores were face-to-face and I found a
coupling equation that matched lab tests, and the customer's product
worked.

Now the customer has changed to surface mount inductors facing
upwards, so the coupling problem is side-by-side, and the product quit
working :-(

Can anyone point me to a coupling equation for side-by-side inductors?

Thanks!
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Aargh! Inductor Coupling Question...
On Fri, 09 Dec 2016 10:30:56 -0700, Jim Thompson

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What kind of inductors?


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Aargh! Inductor Coupling Question...
On Fri, 09 Dec 2016 10:30:56 -0700, Jim Thompson wrote:

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Without knowing the inductor construction I don't think this can be  
done.  Maybe you can come close if they're unshielded bobbin cores, but  
otherwise I think you need to measure the coupling.

Come to think of it, once you do all the fun math, you'll need to measure  
the coupling ANYWAY, to make sure your assumptions were all correct.  So  
why not just cut to the chase?  Get a few boards and a handful of  
inductors, hand-solder them on one board as close as they'll go, centered  
on their pads on the second board, and as far away as they'll go on the  
third.  Then measure the coupling to get min, typical, and max.

And for a second come to think of it -- I could be wrong, but if there's  
cores involved I suspect that you would not be able to get a nice tidy  
symbolic solution anyway -- it's probably a job for a 3D FEM program,  
which would just boil down to experimentation with virtual boards rather  
than the real thing.  Moreover, if there are ground planes involved then  
predicting coupling is going to get even hairier, particularly if you  
care about any frequency dependencies.

--  
www.wescottdesign.com

Re: Aargh! Inductor Coupling Question...
On 09/12/2016 17:44, Tim Wescott wrote:
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I think its worse than that. Unlike Jim's initial end-to-end mounting of  
inductors, the field outside a solenoid coil, and hence coupling, is  
going to be low. It would then depend very much on the proximity of  
magnetic and diamagnetic materials just as much as the accuracy in the  
inductor's mounting.

I also have no idea of the constructions of these inductors. If off the  
self there's a fighting chance they will be designed to minimise  
external fields.

--  
Mike Perkins
Video Solutions Ltd
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Re: Aargh! Inductor Coupling Question...
On Friday, December 9, 2016 at 10:47:07 AM UTC-8, Mike Perkins wrote:
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Yes, that's a good approach

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Very true.
So, can you convince the customer to spring for two C-cores mounted side-by-side?

Seriously, the internal construction of a surface-mount inductor is going to have  
significant impact (even the mounting pad design counts)    There's little likelihood
that a FEM can be constructed without doing experiments on the actual parts.
By the way, you DO realize that vertical inductors are likely to have unknown
polarity?  Swap the pin 1 and pin 2, and the polarity of the coupling changes.

Re: Aargh! Inductor Coupling Question...
On Friday, December 9, 2016 at 3:30:40 PM UTC-5, whit3rd wrote:
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Oh that last is a good point.  
Picture the fields of two dipoles,
head to tail the fields are in phase,
side by side the fields are 180 out of phase.  

JT I don't suppose they can glue a piece of ferrite across the coils?  

George H.  

Re: Aargh! Inductor Coupling Question...
On Fri, 9 Dec 2016 12:37:19 -0800 (PST), George Herold

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I thought of that but was vetoed by the customer who is in Hong Kong.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Aargh! Inductor Coupling Question...
wrote:

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Modulation is amplitude, not phase.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Aargh! Inductor Coupling Question...
On 12/9/2016 3:01 PM, Jim Thompson wrote:
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You've never heard of phase modulation?

Re: Aargh! Inductor Coupling Question...
On Saturday, December 10, 2016 at 4:31:10 AM UTC+11, Jim Thompson wrote:
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The coupling equation for side-to-side inductors is exactly the same as for face-to-face inductors, though the mutual coupling coefficient is a whole lot less.

The flux of interest forms a toroid around each coil, and only that bit of the toroid going through the second inductor creates coupling - the off-axis flux has a lot more places to go that by-pass the second toroid than the on- axis flux ever did.

The equations are still the transformer equation

V1 = L1. dI1/dt + M. dI2/dt

V2 =  M. dI/dt  + L2. dI2/dt  

where M is some fraction of the square root of L1.L2 - a smaller fraction for side by side coils than coaxial coils.

Nobody ever seems to teach the transformer equation - even Win Hill couldn't squeeze it into AOE-3 - but I've been pushing it here for years.

Bill Sloman, Sydney


Re: Aargh! Inductor Coupling Question...
On 12/9/2016 3:10 PM, snipped-for-privacy@ieee.org wrote:
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Bill, I must have missed your pushing. Will you please present the  
transformer equation again?

Thanks

Re: Aargh! Inductor Coupling Question...
On Saturday, December 10, 2016 at 8:46:35 AM UTC+11, John S wrote:
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Why? The two lines of algebra say it all, which is most of the charm.

The voltages have to be alternating, since you can't increase current indef
initely without blowing up the coils (or generating a magnetic field big en
ough to destroy super-conductivity, which comes to the same thing).

L is just common-or-garden inductance in volts per rate of change of curren
t, and M is mutual inductance - the volts in one coil induced by changing t
he current in the other coil. The fact that it is the same for any two pair
ed coils ought to be obvious, but takes thinking about.

You seem to have to fiddle with inductors for a bit before you can properly
 take it on board - I must have seen the equation long before it made sense
 to me, and I had to spell it out to the guy who was my boss at the time. H
e had trained as a physicist, and was one of the better electronic engineer
s I know, at least in terms of understanding what a circuit did, though I n
ever had much trouble changing his circuits to get better performance, whic
h did miff him from time to time.

--  
Bill Sloman, sydney

Re: Aargh! Inductor Coupling Question...
On 10/12/2016 01:50, snipped-for-privacy@ieee.org wrote:
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While I might agree with you, the real question here is how would you  
calculate the mutual inductance? What is your magic fraction?

--  
Mike Perkins
Video Solutions Ltd
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Re: Aargh! Inductor Coupling Question...
On Saturday, December 10, 2016 at 12:52:56 PM UTC+11, Mike Perkins wrote:
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Since Jim hasn't posted any actual dimension - if fact it sounds a bit as i
f his customer hasn't told him about them - there's not a lot to calculate  
with.

My understanding is that once you have the dimensional information you do s
ome sort of complicated integration to get the right answer. There are elec
tromagnetic field-plotting software packages that will do it for you, and I
've got a copy (reprint) of Frederick W. Grover's "Inductance Calculations"
  
ISBN-13:978-0-486-47440-3 which tubulates a lot of hand-cranked calculation
s - it was first published in 1946.

Chapter 18 is on the mutal inductance of circular elements with parallel ax
es, which is probably what Jim needs.

Chapter 19 goes on to circular elements whose axes are inclined to one anot
her, which may be what he will end up needing, if his customer was vague en
ough to get Jim to work things out for coaxial inductors when the board was
 laid out with inductors side-by side ...

The book does go on a bit, and my experience is that winding and measuring  
is just as quick as plowing through the tables, and the results are easier  
to sell to management.

--  
Bill Sloman, Sydney

Re: Aargh! Inductor Coupling Question...
On 12/9/2016 7:50 PM, snipped-for-privacy@ieee.org wrote:
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Oh. I see that it comes from a freshman physics textbook. I though you  
had something more informative for us.

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Well, that is sort of correct in a perfect universe.

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Yes, you should do that.

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 From 1980 to about 1995 I designed magnetics for inverters ranging from  
40Hp to 500Hp to drive 3-phase motors. The magnetics included DC filter  
bus chokes, inductors, and tapped transformers for those inverters.  
Also, I designed current transformers for use for current sampling of  
those motors. At one point, my career was on the line. I had to find a  
solution for a tightly coupled center tapped inductor but not so tight  
that di/dt would destroy the thyristors. I specified the inductor lead  
length such that it would add enough series inductance to protect the  
devices from di/dt. It was the 500Hp inverter which IIRC, had to  
commutate about 2000A. It worked!

I'm not trying to impress you or anyone else with my 30 year-old  
magnetics experience. I'm just saying that you could offer more  
information rather than a freshman physics textbook example. We could  
benefit from your detailed knowledge. In other words, how would you  
handle a design for a 10w, 120V to 12V output transformer using EI lams?  
Please show the M in your so-called equation.

Thanks for your indulgence.





Re: Aargh! Inductor Coupling Question...
On Friday, December 9, 2016 at 10:16:14 PM UTC-5, John S wrote:
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Grin... I'm a physicist so admittedly biased,  
but for me, "freshman physics"* is my basis for
understanding the world.  
Of course there is lots more.    

George H.  
*Fenyman lectures and Berkeley series count.


I though you  
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Re: Aargh! Inductor Coupling Question...
On Saturday, December 10, 2016 at 2:16:14 PM UTC+11, John S wrote:
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The Art of Electronics is a text-book evolved to teach physicists enough el
ectronics to let them understand the instruments they use.

It doesn't include the transformer equation. If a freshman physics textbook
 does, it won't be embedded in enough electronics to make the idea remotely
 useful. Final year secondary students can understand the relativity equati
ons. The fact that a magnetic field is a consequence of Lorentz contraction
 won't strike them until they get a little further into the subject.

<snip>
  
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That's fine. You won't have done so.

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Sure, but I couldn't post it here and expect to see it read.

This is about creating threads that end up in useful places, rather than po
sting a giant slab of stuff that might be useful

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Jim couldn't - he kill-filed me years ago.

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Why would I indulgence you with anything quite so irrelevant? That kind of  
transformer sells off the shelf from broad-line distributors.

The voltage makes it clear that you are talking about driving it with 50/60
Hz mains. 400Hz parts for aircraft use wouldn't be all that different.

The coupling factor in that kind of design is between 0.99 and almost 1, an
d usually doesn't matter. M is the geometric mean of L1 and L2 less a usual
ly inconsequential sliver.

--  
Bill Sloman, Sydney

Re: Aargh! Inductor Coupling Question...
On Fri, 09 Dec 2016 10:30:56 -0700, Jim Thompson wrote:

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Are they isolated from one another?  If they're both referenced to the  
same ground you can couple them easily enough by running their cold ends  
to a small inductor, making a 'T'.  Presto -- instant coupling.

--  
www.wescottdesign.com

Re: Aargh! Inductor Coupling Question...
On Fri, 09 Dec 2016 15:30:08 -0600, Tim Wescott

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There's a whole X-Y array daisy-chained, so that won't work.

I'm toying with, instead of sending 1:0, send 1:0.5, leaving carrier
always present, allowing coherent AGC and coherent detection.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Aargh! Inductor Coupling Question...
On Friday, December 9, 2016 at 4:52:37 PM UTC-5, Jim Thompson wrote:
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consider PSK or FSK  

use a limiter in the receiver, no AGC needed.

m




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