Effects of gaps in inductors and transformers

[Paul E. Schoen]

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

 

[MassiveProng]

but I observed that they are generally
present in iron core inductors, but not in most transformers.

You observed incorrectly.

 

[Genome]

You observed incorrectly.

Ahem, since he is talking about low frequency power and has used the words
'generally' and 'most' his observation is not entirely incorrect.

In fact, since it is his observation and you do not know what he has
observed then he is perfectly correct.

DNA

 

[[email protected]]

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.

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.

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.

 

[Robert Latest]

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.

With the flyback converter being the counterexample for both. As more and
more electronics are powered by switchers instead of linear supplies, and as
most of those are flybacks, the day may come when transformers are indeed
"usually" used for energy storage.

robert

 

[Fred Bartoli]

Robert Latest a écrit :

With the flyback converter being the counterexample for both. As more and
more electronics are powered by switchers instead of linear supplies, and as
most of those are flybacks, the day may come when transformers are indeed
"usually" used for energy storage.

Sloppy word usage.
Flybacks don't use transformers. They use coupled inductors.

 

[Robert Latest]

Sloppy word usage.
Flybacks don't use transformers. They use coupled inductors.

I guess you're right.

robert

 

[John Larkin]

You observed incorrectly.

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped; ferrite power transformers are usually not; iron
core and ferrite inductors are usually gapped, either with a distinct
gap or just an open magnetic structure; it prevents dc saturation and
helps better define their inductance.

Ferroresonant power transformers were gapped, but they're pretty rare
these days.

John

 

[Paul E. Schoen]

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.

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.

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

 

[Jan Panteltje]

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

 

[John Larkin]

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

 

[Jan Panteltje]

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


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

 

[D from BC]

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


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...tre Systems&CS_ProductID=2094647&ProductTab=1
or use "vacuum" as keyword..
$800.00CAD!!!
I actually saw somebody stare at this thing for 5 minutes!!
D from BC

 

[Jan Panteltje]

Saw a small tube amp for ipod at London Drugs
http://www.londondrugs.com/Cultures...gationBreadCrumbs=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?

 

[Rich Grise]

No, he's generally correct. Power and audio iron-lam transformers are
almost never gapped; ferrite power transformers are usually not; iron
core and ferrite inductors are usually gapped, either with a distinct
gap or just an open magnetic structure; it prevents dc saturation and
helps better define their inductance.

Ferroresonant power transformers were gapped, but they're pretty rare
these days.

I've worked for a battery charger manufacturer who wound his own ferros,
and none of them was gapped. They do, however, have "magnetic shunts"
that let some of the flux bypass the secondary; apparently this has to
do with having a regulated output.

I was testing one one time, with a BMF variac and AC volt and ammeters.
I cranked up the variac, and the primary current went way high - it
worried me, so I mentioned it to the client (who designs the things, and
he beamed: "It's regulating!"

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

Cheers!
Rich

 

[MassiveProng]

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.

Thank you.

 

[MassiveProng]

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.

 

[John Larkin]

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.

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.


John

 

[MassiveProng]

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

Prevention of saturation is the main reason, even if there is no DC
component involved.

One can also affect how a switcher pulse is handled.

 

[MassiveProng]

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, 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.