Effects of gaps in inductors and transformers

[MassiveProng]

On Thu, 22 Feb 2007 10:00:54 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

Even today people ...
http://www.davidberning.com/siegfried.htm

Is there no limit to audio insanity? Don't look like it.

John

He was off by one or even two places on the decimal for the value of
them though.

There's a sucker born every minute.

 

[MassiveProng]

On Thu, 22 Feb 2007 18:08:44 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> Gave us:

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Right. The only reason to add gaps is if there's DC present, or to
better define the inductance.

Even today people ...
http://www.davidberning.com/siegfried.htm

Is there no limit to audio insanity? Don't look like it.

John

For 6500 $ a piece, I was thinking, 'how many does he sell'?
I can hire a guy to assemble many of these a week.

Did you notice he feeds the heaters with 250 kHz AC to prevent hum?
hehe

Ever seen the switching frequency of the Class D amp chips
available?

Most are 200kHz to 300kHz.

This being used as the front end for a CAT scanner PS I made in
first proto was why we couldn't use them. That made the ripple spec
fail. That spec was 20mV @ 1500 VDC.

We ended up making an audio amp with a string of IGBTs, and it ran
at 17kHz flawlessly. 11mV ripple and 1Volt regulation at 250Watts at
1500Volts. Beat the competition by a factor of two in both ripple and
voltage regulation.

 

[MassiveProng]

On Thu, 22 Feb 2007 18:43:27 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> Gave us:

On a sunny day (Thu, 22 Feb 2007 18:30:29 GMT) it happened D from BC
myrealaddress@comic.com> wrote in
b5nrt254p5hn7ta0bvs3u59ohu3e52l2u4@4ax.com>:

On Thu, 22 Feb 2007 10:00:54 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 22 Feb 2007 17:31:11 GMT, Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote:

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped;

In class A audio amps the transformers are normally gapped.
this to prevent saturation because of the DC component.
Class A audio amps were _very_ popular in the tube ages, and later
in small transistor radios.
Even today people ...
http://www.davidberning.com/siegfried.htm

Right. The only reason to add gaps is if there's DC present, or to
better define the inductance.

Even today people ...
http://www.davidberning.com/siegfried.htm

Is there no limit to audio insanity? Don't look like it.

John

Saw a small tube amp for ipod at London Drugs
http://www.londondrugs.com/Cultures/en-US/Product+Detail/Electronics.htm?CatalogNavigationBreadCrumbs=Electronics;Electronics;Hom
e%20Audio;Home%20Theatre%20Systems;Vuum%20Audio%20Vacuum%20Tube%20Amplifier%20System%20with%20Speakers,%20iPod%20Docking%20Stati
on%20and%20Remote%20-%20VTI-B1&CS_Catalog=Electronics&CS_RootCategory=Electronics&CS_Category=Home%20Theatre%20Systems&CS_Produc
tID=2094647&ProductTab=1
or use "vacuum" as keyword..
$800.00CAD!!!
I actually saw somebody stare at this thing for 5 minutes!!
D from BC

*S-Video* output?????????????????????????????
On an audio amp?

Simply the connector style. Not all pins used thing.


Probably standard line level signals passed over that as a
convenient cable choice.

common sense.

 

[MassiveProng]

On Thu, 22 Feb 2007 22:23:03 GMT, Rich Grise <rich@example.net> Gave
us:

The rest of ferro design, of course, is Black Magic.

To you, a simple tester.

 

[John Larkin]

On Thu, 22 Feb 2007 20:29:03 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:

On Thu, 22 Feb 2007 10:00:54 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

Right. The only reason to add gaps is if there's DC present, or to
better define the inductance.


Nope. Saturation can occur in transformers that do not have any DC
component as well. It all comes down to Ampere Turns,

OK, say we have an ungapped iron-core transformer, with some fixed AC
voltage and frequency applied to a primary winding. Primary
ampere-turns are some value X. Now add an air gap. What happens to X?

What happens to the peak flux density?

and whether or
not the switcher has any overlap on its pulses. (most do) Gapping
opens that overlap up, and actually improves efficiency by reducing
the losses caused by said overlap.

Sounds like leakage inductance to me. Gapping may increase leakage
inductance.

John

 

[John Larkin]

On Thu, 22 Feb 2007 20:39:50 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:

On Thu, 22 Feb 2007 22:23:03 GMT, Rich Grise <rich@example.net> Gave
us:


The rest of ferro design, of course, is Black Magic.


To you, a simple tester.

Please explain to us what a ferroresonant transformer is for, and how
it works.

John

 

[MassiveProng]

On Thu, 22 Feb 2007 21:29:09 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

OK, say we have an ungapped iron-core transformer, with some fixed AC
voltage and frequency applied to a primary winding. Primary
ampere-turns are some value X. Now add an air gap. What happens to X?

Switchers typically do not provide sinusoidal AC. They typically
provide high slew rate DC pulses, which alternate in polarity.

 

[MassiveProng]

On Thu, 22 Feb 2007 21:29:09 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

Sounds like leakage inductance to me. Gapping may increase leakage
inductance.

Gapping ALWAYS increases leakage inductance. That is why the gap is
not very large, at all. I have always maintained this qualifier.

 

[MassiveProng]

On Thu, 22 Feb 2007 21:30:49 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

On Thu, 22 Feb 2007 20:39:50 -0800, MassiveProng
MassiveProng@thebarattheendoftheuniverse.org> wrote:

On Thu, 22 Feb 2007 22:23:03 GMT, Rich Grise <rich@example.net> Gave
us:


The rest of ferro design, of course, is Black Magic.


To you, a simple tester.

Please explain to us what a ferroresonant transformer is for, and how
it works.

The term for today is:

MAG AMP.

 

[Bruce Varley]

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:56mrt2hvm16tuv98m41n2vmt2sgsnthpf4@4ax.com...

On Thu, 22 Feb 2007 17:31:11 GMT, Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote:

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped;

In class A audio amps the transformers are normally gapped.
this to prevent saturation because of the DC component.
Class A audio amps were _very_ popular in the tube ages, and later
in small transistor radios.
Even today people ...
http://www.davidberning.com/siegfried.htm

Right. The only reason to add gaps is if there's DC present, or to
better define the inductance.

Even today people ...
http://www.davidberning.com/siegfried.htm

Is there no limit to audio insanity? Don't look like it.

John

Get the THD specs .....

 

On Feb 23, 12:58 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On 22 Feb 2007 02:57:13 -0800, bill.slo...@ieee.org wrote:

The maximum magnetic field you can build up in the magnetic path is
independent of the gap - it is limited by the saturation flux for the
core material.

Small, impractical nit: Won't the flux continue to increase with
current, at a declining effective permeability, approaching u=1 at
full saturation of the iron? Admittedly, this is a pretty low slope,
but I don't think it goes to zero.

Sure, but you don't usually want your transformer or inductor to go
into saturation. Currents go through the roof, the magnetic field
extends outside the core material and so forth. Of course I once built
a starter for a xenon lamp where the step-up transformer spent most of
its time during start-up - probably a few microseconds - in
saturation. I still eventually got my 20kV across the secondary,
enough to get the electrodes to spark over, initiating the discharge -
initially as a glow discharge, rapildy developing into an arc.

The number of ampere-turns of current through the
winding required to generate that flux depends on the magnetic path
length. The magnetic path length through the core itself is divided by
the relative permeability of the core (about 1000 times air for
ferrites, and 10,000 times air for iron) so even a small air-gap can
dramatically increase the magnetic path length.

I know a guy who has a secret process for treating metglas, up to a
permeability of about 1e6.

For many years Telcom metals have sold a bunch of funny alloys - mu-
metal amongst others - offering permeabilities of the order of 1e5.

You've got to anneal them after you've deformed them into the shape
you want to get these high permeabilities. National Standards labs use
this sort of material as cores in precision ratio transformers. Hardly
anybody else can afford it.

http://en.wikipedia.org/wiki/Annealing_%28metallurgy%29

--
Bill Sloman, Nijmegen

 

On Feb 23, 12:41 am, MassiveProng
<MassivePr...@thebarattheendoftheuniverse.org> wrote:

On 22 Feb 2007 02:57:13 -0800, bill.slo...@ieee.org Gave us:

A gap in a transformer core increases the leakage inductance, which is
usually undesirable.

The gaps are very small. For pot cores, it can be anywhere from
half a mil to around 5 or even 10 mils. Without it, problems do
occur.



Moreoever, a transformer isn't usually used as an energy storage
device, so increasing the energy storage capacity is rarely a design
priority.

In a switcher, it keeps the crossover from banging into each other.
A big source of switcher noise, and LOST efficiency.

This sounds like nonsense to me. For simple, centre-tapped windings a
high leakage inductance, corresponding to poor coupling between the
two sides of the centre-tapped winding, leads to a higher turn-off
dissipation, which can be a major source of lost efficiency, not to
mention over-heated switching transistors.

What you seem to be thinking off would seem to be shoot-through
losses, which are best dealt with by a break-before-make switch
driver.

--
Bill Sloman, Nijmegen

 

[Tony Williams]

In article <1172226326.252329.20030@k78g2000cwa.googlegroups.com>,
Bill Sloman <bill.sloman@ieee.org> wrote:

For many years Telcom metals have sold a bunch of funny alloys -
mu- metal amongst others - offering permeabilities of the order
of 1e5.

You've got to anneal them after you've deformed them into the
shape you want to get these high permeabilities. National
Standards labs use this sort of material as cores in precision
ratio transformers. Hardly anybody else can afford it.

http://en.wikipedia.org/wiki/Annealing_%28metallurgy%29

Ha! I have a stack of Telcon HCR (square loop) and Mumetal
toroids in stock, and even 1000:1 400Hz CT's wound on HCR.
You can have some to play with..... cost you though.

--
Tony Williams.

 

[Tony Williams]

In article <vt0tt2pg9i917bcl669h5athguln1bul7p@4ax.com>,
MassiveProng wrote:

On Thu, 22 Feb 2007 21:30:49 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:
Please explain to us what a ferroresonant transformer is for,
and how it works.

The term for today is:
MAG AMP.

Nothing to do with a magnetic amplifier, which is
the active controlling of AC or DC power via a DC
control winding.

A ferroresonant CVT is a passive device and is a
clever design of a core, with a non-saturating
primary limb, a magnetic shunt, and a saturating
secondary limb. The winding on that secondary limb
inherently produces a constant output voltage.
The output is square, (mainly third and fifth
harmonics), and the capacitor is there to resonate-
-out those harmonics. There are even more clever
designs that use compensating windings to reduce the
secondary harmonics.

The CVT is adjusted by "shimming the shunt". I've seen
it done on some special CVT's we were buying, and that
is definitely a black art.

<http://www.research.ibm.com/journal/rd/316/ibmrd3106H.pdf>

--
Tony Williams.

 

[MassiveProng]

On Fri, 23 Feb 2007 18:54:26 +0900, "Bruce Varley"
<bxvarley@weastnet.com.au> Gave us:

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message
news:56mrt2hvm16tuv98m41n2vmt2sgsnthpf4@4ax.com...
On Thu, 22 Feb 2007 17:31:11 GMT, Jan Panteltje
pNaonStpealmtje@yahoo.com> wrote:

John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped;

In class A audio amps the transformers are normally gapped.
this to prevent saturation because of the DC component.
Class A audio amps were _very_ popular in the tube ages, and later
in small transistor radios.
Even today people ...
http://www.davidberning.com/siegfried.htm

Right. The only reason to add gaps is if there's DC present, or to
better define the inductance.

Even today people ...
http://www.davidberning.com/siegfried.htm

Is there no limit to audio insanity? Don't look like it.

John

Get the THD specs .....

4.3% !!!

I take it back... move the decimal TWO places back!

They are worthless!

 

[Fred Bloggs]

Paul E. Schoen wrote:

In a thread in SEB there was a discussion on transformer failure modes that
also mentioned gaps in the magnetic path. I never fully understood the
function of gaps in the core, but I observed that they are generally
present in iron core inductors, but not in most transformers.

I found some information at
http://www.micrometals.com/appnotes/appnotedownloads/id4hf.pdf, where it is
explained that the gap size can maximize energy storage in an inductor by
balancing the point of magnetic saturation (and core heating) with winding
losses. It seems that a wider (or longer) core gap extends the point of
magnetic saturation by allowing more current to flow through the windings,
so the effect is to lower the inductance. A smaller gap will have higher
inductance, but will saturate the core much more quickly, resulting in less
energy storage.

As an inductor is used more for energy storage, a gap (whether actually cut
in the magnetic material or distributed as with powdered iron), allows more
energy storage by allowing more current flow, and energy is proportional to
the square of the current. For a transformer, as I understand it, the
energy is transferred from the primary to secondary by mutual inductance,
so the absense of a gap results in higher inductance and a higher volts per
turn.

More information can be found at
http://ece-www.colorado.edu/~ecen4517/course_material/Exp6/Inductor.pdf,
which describes filter inductor design.

I would like to get a better understanding of the characteristics of
transformers and inductors to know how best to design high current 50/60 Hz
transformers as well as switch mode boost converters using inductors.

The transformers I have made use toroidal primary cores with 120/240 VAC
windings, and secondaries consisting of several turns of bus bar or welding
cable to produce up to 10s of thousands of amps. They will usually produce
15 to 30 times their nominal output currents for short pulses.

The switch mode boost converter I have designed uses a 10 uH inductor at
100 kHz to boost 12 VDC to 25 or 45 VDC at about 800 mA. However, I
recently found that a small pot core inductor rated at 6.7 amps seemed to
work better than a larger toroidal inductor rated at 10.8 amps. I think
this might be because the smaller inductor starts to saturate sooner,
lowering its inductance but allowing more current to flow, resulting in
higher energy storage. The larger inductor is probably allowing much less
current and hence less energy, so it cannot produce the power for the
higher voltage load. I can probably drop the frequency to 75 kHz or 60 KHz
and maybe get the output I need.

Thanks for any thoughts and discussion.

Paul

There are many different reasons as to why magnetic circuits are gapped.
For the switching power supply application, the main reason is to
prevent core saturation under a DC bias and the gap is usually sized by
volume so that the 1/2*mu*B^2 due to the DC is stored in the
non-saturating gap. In an ac-transformer application, the load current
has little to do with core flux density theoretically. The flux density
is only determined by the primary voltage as this sets the dB/dt
magnetization in the core, and the net flux due to the load current in
primary and secondary is zero.

 

[Fred Bloggs]

Paul E. Schoen wrote:

bill.sloman@ieee.org> wrote in message
news:1172141833.429696.85370@v33g2000cwv.googlegroups.com...

On Feb 22, 8:28 am, "Paul E. Schoen" <pst...@smart.net> wrote:

In a thread in SEB there was a discussion on transformer failure modes
that
also mentioned gaps in the magnetic path. I never fully understood the
function of gaps in the core, but I observed that they are generally
present in iron core inductors, but not in most transformers.

I found some information
athttp://www.micrometals.com/appnotes/appnotedownloads/id4hf.pdf, where
it is
explained that the gap size can maximize energy storage in an inductor
by
balancing the point of magnetic saturation (and core heating) with
winding
losses. It seems that a wider (or longer) core gap extends the point of
magnetic saturation by allowing more current to flow through the
windings,
so the effect is to lower the inductance. A smaller gap will have higher
inductance, but will saturate the core much more quickly, resulting in
less
energy storage.

This is not quite the right way to describe what is going on.

The maximum magnetic field you can build up in the magnetic path is
independent of the gap - it is limited by the saturation flux for the
core material. The number of ampere-turns of current through the
winding required to generate that flux depends on the magnetic path
length. The magnetic path length through the core itself is divided by
the relative permeability of the core (about 1000 times air for
ferrites, and 10,000 times air for iron) so even a small air-gap can
dramatically increase the magnetic path length.

A ten-fold increase in magnetic path length allows a ten fold increase
in current through the winding before you ht stuaration, and reduces
the inductance of the assembly by a factor of ten, thus allowing a
factor of ten increase in the energy stored in the inductance (LI^2)
before saturation sets in.



OK. That is very helpful in understanding the principles involved. The gaps
I saw in some large C-core inductors I have are about 0.1", and the
laminated steel has a length of about 10". So if the magnetic path length
is increased to 1000, that is a 100 fold decrease in inductance, allowing
100 times the ampere turns, and thus 100 times the energy. This is a 100 mH
inductor rated about 10 amperes.


As an inductor is used more for energy storage, a gap (whether actually
cut
in the magnetic material or distributed as with powdered iron), allows
more
energy storage by allowing more current flow, and energy is proportional
to
the square of the current. For a transformer, as I understand it, the
energy is transferred from the primary to secondary by mutual
inductance,
so the absense of a gap results in higher inductance and a higher volts
per
turn.

A gap in a transformer core increases the leakage inductance, which is
usually undesirable.

Moreoever, a transformer isn't usually used as an energy storage
device, so increasing the energy storage capacity is rarely a design
priority.



Thank you for that information. I had posted on SEB that some small
transformers may be made with a gap to make them impedance protected in
case of an output overload or short. There are probably other ways to
achieve this effect with looser coupling. As I had posted there, it comes
at the price of low efficiency and poor regulation, but that's what is
desired for self-protection.

If you look at any hunky arc welder, even a small one is 200A rated
secondary, you will notice that the current adjust dial is mechanically
linked to a chunk of laminated iron that is moved in and out of a big
gap in the core, directly adjusting the reluctance of the magnetic
circuit and the magnitude of flux linked with the secondary.

More information can be found
athttp://ece-www.colorado.edu/~ecen4517/course_material/Exp6/Inductor.pdf,
which describes filter inductor design.

I would like to get a better understanding of the characteristics of
transformers and inductors to know how best to design high current 50/60
Hz
transformers as well as switch mode boost converters using inductors.

The transformers I have made use toroidal primary cores with 120/240 VAC
windings, and secondaries consisting of several turns of bus bar or
welding
cable to produce up to 10s of thousands of amps. They will usually
produce
15 to 30 times their nominal output currents for short pulses.

The switch mode boost converter I have designed uses a 10 uH inductor at
100 kHz to boost 12 VDC to 25 or 45 VDC at about 800 mA. However, I
recently found that a small pot core inductor rated at 6.7 amps seemed
to
work better than a larger toroidal inductor rated at 10.8 amps. I think
this might be because the smaller inductor starts to saturate sooner,
lowering its inductance but allowing more current to flow, resulting in
higher energy storage. The larger inductor is probably allowing much
less
current and hence less energy, so it cannot produce the power for the
higher voltage load. I can probably drop the frequency to 75 kHz or 60
KHz
and maybe get the output I need.

At 100kHz you probably need to worry more about inter-winding
capacitance. The detailed structure of the windings can get to be very
important at this sort of frequency. The pot core may well have a two
or four section former with the windings built up as two or four
successive sections, while the toriod is more likely to have its
windings built up as successive layers, one on top of another, which
gives a higher winding capacitance and a lower self-resonant
frequency.

At even higher frequencies, you have to restrict yourself to single-
layer windings to keep the interwinding capacitance within bounds, and
eventually you have to go over to transmission line transformers.



The toroid has only about 10 turns of approx #16 wire on a core about 0.75"
x 0.37". I don't know the internal construction of the pot core, but it is
only about 0.5" square and 0.3" high. It most likely has several layers of
windings.

Thanks,

Paul

 

[John Larkin]

On Thu, 22 Feb 2007 22:02:20 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:

On Thu, 22 Feb 2007 21:29:09 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:

OK, say we have an ungapped iron-core transformer, with some fixed AC
voltage and frequency applied to a primary winding. Primary
ampere-turns are some value X. Now add an air gap. What happens to X?


Switchers typically do not provide sinusoidal AC. They typically
provide high slew rate DC pulses, which alternate in polarity.

Did I say "sinusoidal"?

John

 

[John Larkin]

On Thu, 22 Feb 2007 22:03:25 -0800, MassiveProng
<MassiveProng@thebarattheendoftheuniverse.org> wrote:

On Thu, 22 Feb 2007 21:29:09 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> Gave us:


Sounds like leakage inductance to me. Gapping may increase leakage
inductance.


Gapping ALWAYS increases leakage inductance. That is why the gap is
not very large, at all. I have always maintained this qualifier.

I can imagine a geometry in which increasing a gap reduces leakage
inductance.

John

 

[Kevin]

On Feb 22, 9:31 am, Jan Panteltje <pNaonStpealm...@yahoo.com> wrote:

John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped;

In class A audio amps the transformers are normally gapped.
this to prevent saturation because of the DC component.
Class A audio amps were _very_ popular in the tube ages, and later
in small transistor radios.
Even today people ...
http://www.davidberning.com/siegfried.htm

I don't think that any mass production transistor radios used class A
audio output stages - from the late 50's onwards the push-pull
transformer coupled class B output stage was the most common, until
the transformerless designs cam in. The early transistors had very
limited power dissipation capability and the low idle consumption of
class B suited battery power better. Car radios often used class A
even in hybrid designs where the RF and early audio stages were
implemented with valves/tubes.

kevin