transformer core material

55 views
Skip to first unread message

Tom Del Rosso

unread,
Aug 24, 2021, 4:49:56 AM8/24/21
to
AIUI you use iron cores for low frequency and ferrite for high frequency
because ferrite doesn't get magnetized, so why couldn't aluminum do the
same?


--
Defund the Thought Police


Phil Allison

unread,
Aug 24, 2021, 5:32:48 AM8/24/21
to
Tom Del Rosso wrote:
==================
>
> AIUI you use iron cores for low frequency and ferrite for high frequency
> because ferrite doesn't get magnetized,

** LOL - wrong.

Most loudspeakers use FERRITE magnets !


> so why couldn't aluminum do the same?

** Or a piece of wood ?

FYI

the whole point of a core is that it DOES get magnetised.
And as easily as possible and with the least energy losses when cycled in each direction.
Iron / Silicon alloys are is the only materials that do that well at low frequencies.

Ferrite has very low losses at at ALL frequencies but cannot compare with iron at low ones for sheer brute force.


.... Phil






Jeroen Belleman

unread,
Aug 24, 2021, 9:01:17 AM8/24/21
to
Tom Del Rosso wrote:
> AIUI you use iron cores for low frequency and ferrite for high frequency
> because ferrite doesn't get magnetized, so why couldn't aluminum do the
> same?

You *want* a transformer core to be easily magnetized! You don't
want it to *stay* magnetized when the current goes to zero.

Iron is good in low-frequency transformers because it has a high
saturation field and high permeability, so you can get away with
relatively few turns for the windings. Its disadvantage is that it
is conductive, so there will be eddy current losses, which get
rapidly worse with higher frequency. Those losses can be reduced
by making the core out of thin insulated laminations, but this
gets impractical quite fast.

Ferrite has a lower permeability and lower saturation field, but
it's an insulator, so it doesn't sustain eddy currents.

That's the simple view. Magnetic materials are complicated and
lots of effort has been spent on finding the best materials for
specific applications. There are hundreds of different kinds of
magnetic materials, maybe thousands.

Jeroen Belleman

Phil Allison

unread,
Aug 25, 2021, 3:40:58 AM8/25/21
to
Jeroen Belleman wrote:
===================
>
> Iron is good in low-frequency transformers because it has a high
> saturation field and high permeability, so you can get away with
> relatively few turns for the windings. Its disadvantage is that it
> is conductive, so there will be eddy current losses, which get
> rapidly worse with higher frequency. Those losses can be reduced
> by making the core out of thin insulated laminations, but this
> gets impractical quite fast.
>

** Typical iron core transformers intended for 50/60 Hz can be used to 20kHz and beyond with no such issue.
As the operating frequency rises, core magnetisation falls cancelling any rise in losses.



....... Phil





Jeroen Belleman

unread,
Aug 25, 2021, 5:35:39 AM8/25/21
to
Why isn't laminated iron good for RF transformer cores then?

Jeroen Belleman

Phil Allison

unread,
Aug 25, 2021, 6:04:15 AM8/25/21
to
Jeroen Belleman wrote:
> Phil Allison wrote:
-----------------------------------------
> >> Iron is good in low-frequency transformers because it has a high
> >> saturation field and high permeability, so you can get away with
> >> relatively few turns for the windings. Its disadvantage is that it
> >> is conductive, so there will be eddy current losses, which get
> >> rapidly worse with higher frequency. Those losses can be reduced
> >> by making the core out of thin insulated laminations, but this
> >> gets impractical quite fast.
> >>
> >
> > ** Typical iron core transformers intended for 50/60 Hz can be used to 20kHz and beyond with no such issue.
> > As the operating frequency rises, core magnetisation falls cancelling any rise in losses.
>
> >
> Why isn't laminated iron good for RF transformer cores then?

** You have misunderstood my post.




...... Phil




Jeroen Belleman

unread,
Aug 25, 2021, 8:36:47 AM8/25/21
to
It's true I assumed constant Bmax. I should have said so.
Constant voltage, as you assumed, is indeed more natural.

I measured the frequency response of a few iron core
transformers: A 75VA rectangular-core worked well up
to 40kHz, while a similar sized toroid went up to only
about 10kHz (-3dB), measured between the two independent
15V windings of each. This was with a tiny excitation
voltage, which may mess up the results.

Jeroen Belleman

Phil Allison

unread,
Aug 25, 2021, 5:21:19 PM8/25/21
to
Jeroen Belleman wrote:
==================
>>>
> >>> ** Typical iron core transformers intended for 50/60 Hz can be used to 20kHz and beyond with no such issue.
> >>> As the operating frequency rises, core magnetisation falls cancelling any rise in losses.
> >> Why isn't laminated iron good for RF transformer cores then?
> >
> > ** You have misunderstood my post.
> >
>
> It's true I assumed constant Bmax. I should have said so.
> Constant voltage, as you assumed, is indeed more natural.
>
** Well, I deal lot with audio transformers - from mic input to hundreds of watts.


> I measured the frequency response of a few iron core
> transformers: A 75VA rectangular-core worked well up
> to 40kHz,

** Yep. Audio output types go to about 60kHz or more.


>while a similar sized toroid went up to only
> about 10kHz (-3dB), measured between the two independent
> 15V windings of each.

** That is odd, toroidals are usually the best with -3dB responses to 100kHz.
Just the fact the secondary is wound all over the primary does the trick.

What cannot be done is having a tiny laminated iron core running at 100kHz and high power.

As well as ferrite there are "powdered iron" cores and toroids that will.
Another material is "amorphous steel" which as very low losses.


...... Phil



Jeroen Belleman

unread,
Aug 25, 2021, 6:05:23 PM8/25/21
to

Jeroen Belleman

unread,
Aug 25, 2021, 6:24:54 PM8/25/21
to
On 2021-08-25 23:21, Phil Allison wrote:
> Jeroen Belleman wrote:
> ==================
>>>>
>>>>> ** Typical iron core transformers intended for 50/60 Hz can be used to 20kHz and beyond with no such issue.
>>>>> As the operating frequency rises, core magnetisation falls cancelling any rise in losses.
>>>> Why isn't laminated iron good for RF transformer cores then?
>>>
>>> ** You have misunderstood my post.
>>>
>>
>> It's true I assumed constant Bmax. I should have said so.
>> Constant voltage, as you assumed, is indeed more natural.
>>
> ** Well, I deal lot with audio transformers - from mic input to hundreds of watts.
>
>
>> I measured the frequency response of a few iron core
>> transformers: A 75VA rectangular-core worked well up
>> to 40kHz,
>
> ** Yep. Audio output types go to about 60kHz or more.
>
>
>> while a similar sized toroid went up to only
>> about 10kHz (-3dB), measured between the two independent
>> 15V windings of each.
>
> ** That is odd, toroidals are usually the best with -3dB responses to 100kHz.
> Just the fact the secondary is wound all over the primary does the trick.

It surprised me too. I also measured an inter-winding capacitance of
2nF, which strikes me as high. The drop-off was a resonance dip.

>
> What cannot be done is having a tiny laminated iron core running at 100kHz and high power.
>
> As well as ferrite there are "powdered iron" cores and toroids that will.
> Another material is "amorphous steel" which as very low losses.

I use transformers for RF. I've used ferrite, of course, but also metglas
and similar materials. The cores are mostly there for the low end of the
frequency range. Beyond a few MHz, you really want to keep the flux out
of the core, which is done by using transmission lines for the windings.
Some of my transformers are good to 9GHz, but those bear little resemblance
to a traditional wound transformer.

Jeroen Belleman

Phil Allison

unread,
Aug 25, 2021, 8:39:56 PM8/25/21
to
Jeroen Belleman wrote:
====================
>
> > ** That is odd, toroidals are usually the best with -3dB responses to 100kHz.
> > Just the fact the secondary is wound all over the primary does the trick.
>
> It surprised me too. I also measured an inter-winding capacitance of
> 2nF, which strikes me as high. The drop-off was a resonance dip.

** You did have a suitable resistive load on the secondary ??



...... Phil


Tom Del Rosso

unread,
Aug 25, 2021, 10:26:57 PM8/25/21
to
Jeroen Belleman wrote:
> Tom Del Rosso wrote:
>> AIUI you use iron cores for low frequency and ferrite for high
>> frequency because ferrite doesn't get magnetized, so why couldn't
>> aluminum do the same?
>
> You *want* a transformer core to be easily magnetized! You don't
> want it to *stay* magnetized when the current goes to zero.

Of course that's what I meant. It has to conduct a magnetic field but it
must not fight the induced field when it reverses.

I asked about the behavior of ferrite vs aluminum.


Jeroen Belleman

unread,
Aug 26, 2021, 2:18:57 AM8/26/21
to
Just the 50 Ohm ports of my network analyzer.

Jeroen Belleman


Jeroen Belleman

unread,
Aug 26, 2021, 2:26:52 AM8/26/21
to
The short answer is that aluminium is worse than nothing as a
transformer core. It *will* fight changing fields.

Jeroen Belleman

Phil Allison

unread,
Aug 26, 2021, 2:36:59 AM8/26/21
to
Jeroen Belleman wrote:
===================
>
> > ** You did have a suitable resistive load on the secondary ??
> >
> Just the 50 Ohm ports of my network analyzer.
>

** So you paralled the windings or had them in series ?

15V or 30 V ?

Suitable = close to full VA *if* the primary was operated at rated V.

Unloaded trannys always ring like a bell.


...... Phil

Jeroen Belleman

unread,
Aug 26, 2021, 2:56:15 AM8/26/21
to
An RF network analyzer is a voltage source with a 50 ohm internal
impedance and a receiver with another 50 Ohm internal impedance.
I connected the source to one of the 15V windings of my transformer
and the receiver to the other. The source voltage is well below 1V
rms. Pretty far from the normal operating conditions of the transformer,
is true.

Jeroen Belleman

Phil Allison

unread,
Aug 26, 2021, 3:10:28 AM8/26/21
to
Jeroen Belleman wrote:
===================
>
>
> An RF network analyzer is a voltage source with a 50 ohm internal
> impedance and a receiver with another 50 Ohm internal impedance.
> I connected the source to one of the 15V windings of my transformer
> and the receiver to the other. The source voltage is well below 1V
> rms. Pretty far from the normal operating conditions of the transformer,
> is true.

** Drive voltage makes no difference at mid and high frequencies.
Only a low ones when approaching core saturation.
Toridals use GOSS wound cores with tiny Imags.

Fun fact:

GOSS = grain oriented silicon steel.
First invented by an engineer who's surname was Goss.

https://en.wikipedia.org/wiki/Norman_P._Goss


..... Phil

Tom Del Rosso

unread,
Aug 27, 2021, 2:07:08 AM8/27/21
to
That implies that it will "stay magnetized" as you put it, so the answer
is too short but thanks for trying.

Jeroen Belleman

unread,
Aug 27, 2021, 4:53:03 AM8/27/21
to
Aluminium is a good conductor. There will be eddy currents induced
in it that will oppose any /change/ of magnetic field. Lenz law and
all that.But once external fields are removed and enough time has
passed for eddy currents to decay, there will be no field left over.

Jeroen Belleman

Phil Allison

unread,
Aug 27, 2021, 5:42:39 AM8/27/21
to
Tom Del Rosso Total Fuckhead wrote:
============================
>
> > The short answer is that aluminium is worse than nothing as a
> > transformer core. It *will* fight changing fields.
>
> That implies that it will "stay magnetized" as you put it, so the answer
> is too short but thanks for trying.
>

** FUCK OFF you vile, arrogant, POS, wog asshole

Never dream of coming back.




..... Phil

Helmut Wabnig

unread,
Aug 27, 2021, 6:29:16 AM8/27/21
to
On Tue, 24 Aug 2021 04:44:02 -0400, "Tom Del Rosso"
<fizzbin...@that-google-mail-domain.com> wrote:

>AIUI you use iron cores for low frequency and ferrite for high frequency
>because ferrite doesn't get magnetized, so why couldn't aluminum do the
>same?
An aluminium core would act as a short circuit.
Will heat up and eventually melt.

w.

Tom Del Rosso

unread,
Aug 27, 2021, 8:38:01 AM8/27/21
to
I know what you mean, but since the current only has the length of the
core to travel it's hard to grasp how that produces more than a very
short pulse.

Tom Del Rosso

unread,
Aug 27, 2021, 8:38:49 AM8/27/21
to
More so than an iron core?



Tom Del Rosso

unread,
Aug 27, 2021, 8:40:18 AM8/27/21
to
HI PHIL!

You know, in that study you're involved in, I think you're getting the
placebo.



Tom Del Rosso

unread,
Aug 27, 2021, 8:47:44 AM8/27/21
to
Is that so even if we assume that it's laminated?


Jasen Betts

unread,
Aug 27, 2021, 12:00:52 PM8/27/21
to
what else being equal?

--
Jasen.

Jeroen Belleman

unread,
Aug 27, 2021, 3:33:18 PM8/27/21
to
I'm getting a bit tired of this. Learn about magnetic fields in
conductors. You're in for some surprises, I'm sure.

Jeroen Belleman

Phil Allison

unread,
Aug 27, 2021, 7:02:49 PM8/27/21
to
Tom Del Rosso = TROLLING FUCKWIT ASSHOLE
====================================
>
> > ** FUCK OFF you vile, arrogant, POS, wog asshole
> >
> > Never dream of coming back.
> >
> >
> HI PHIL!
>
> You know, in that study you're involved in, I think you're getting the
> placebo.

** Hi Tom,

know that brain tumor you have ?
Are you enjoying your daily seizures ?


..... Phil

Tom Del Rosso

unread,
Aug 28, 2021, 7:59:03 AM8/28/21
to
Jeroen Belleman wrote:
>
> I'm getting a bit tired of this. Learn about magnetic fields in
> conductors. You're in for some surprises, I'm sure.

That's fine. You don't have to answer at all. I don't know why people
enter a 'basics' group though, if not for basic questions.


whit3rd

unread,
Sep 11, 2021, 2:04:47 PM9/11/21
to
On Friday, August 27, 2021 at 5:38:01 AM UTC-7, Tom Del Rosso wrote:
> Jeroen Belleman wrote:
> > On 2021-08-27 08:07, Tom Del Rosso wrote:
> >> Jeroen Belleman wrote:
> >>> On 2021-08-26 04:25, Tom Del Rosso wrote:
> >>>> Jeroen Belleman wrote:
> >>>>> Tom Del Rosso wrote:
> >>>>>> AIUI you use iron cores for low frequency and ferrite for high
> >>>>>> frequency because ferrite doesn't get magnetized, so why couldn't
> >>>>>> aluminum do the same?

> >>> The short answer is that aluminium is worse than nothing as a
> >>> transformer core. It *will* fight changing fields.


> > Aluminium is a good conductor. There will be eddy currents induced
> > in it that will oppose any /change/ of magnetic field. Lenz law and
> > all that.But once external fields are removed and enough time has
> > passed for eddy currents to decay, there will be no field left over.

> I know what you mean, but since the current only has the length of the
> core to travel it's hard to grasp how that produces more than a very
> short pulse.

The problem that a core solves, is flux coupling in multiple windings. The
magnetizability of a core means that it contains and directs almost all the magnetic flux.
A conductor will exclude flux, which is counterproductive; even the conductivity
of iron is detrimental (so lamination, or iron powder, or nonconducting ferrite is
employed).

In induction motors, where the flux is intended NOT to change in the rotor (so the
alternation of current rotates the rotor instead of changing its magnetization) there
are aluminum parts to enhance the available torque.

When/if you don't allow the rotor to move, those rotors burn up. Almost all induction motors have
thermal protection components that open if/when the motor is stalled.
Reply all
Reply to author
Forward
0 new messages