Induction heating / cooking - and where to get low ESR high

Ian Stirling
Guest

Fri Mar 12, 2010 1:10 am

Tim Williams <tmoranwms_at_charter.net> wrote:

Quote:

"Ian Stirling" <usenet_at_mauve.plus.com> wrote in message
news:avadnQgF6p7tsQTWnZ2dnUVZ7tmdnZ2d_at_brightview.co.uk...
Some rough calculation said that skin depth for non-magnetic stainless
at 50KHz was ~2.5mm.

As expected, most of my thick non-magnetic stainless steel pans diddn't
work at all.
snip
Speaking of aluminum, give it a try -- pick it up and wave it around, you
should be able to feel a repulsive "bubble" over the coil due to eddy
currents!

Aluminium pans don't work - none of them.
However.

Regular kitchen foil is indeed levitated.
Moderately on medium, and quite strongly on high power.
It also glows red, and fails quite rapidly.

A disk of foil in the bottom of a plastic bucket could be a handy
water heater.

I do not have any fine enough copper foil to try - I would guess that
foil about the thickness of PCB foil would levitate just fine (while
also heating lots).

Dirk Bruere at NeoPax
Guest

Fri Mar 12, 2010 1:36 am

On 12/03/2010 00:00, Michael A. Terrell wrote:

Quote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 20:42, Michael A. Terrell wrote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 19:19, Michael A. Terrell wrote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 17:46, Ian Stirling wrote:
Tim Williams<tmoranwms_at_charter.net> wrote:
"Ian Stirling"<usenet_at_mauve.plus.com> wrote in message
news:nrOdnawA0P55BhLWnZ2dnUVZ8nednZ2d_at_brightview.co.uk...
http://www.st.com/stonline/products/literature/an/12443.pdf

The above app-note uses 2*.68uF caps in parallel, for the
bridge.

Hmm, those are nice and tasty...

However - all the film caps I can seem to source easily
- digikey, mouser, rs, farnell, ... seem likely to promptly
explode - at least based on the datasheets, they sharply limit
voltage at high frequencies.

Nah, they won't explode -- hell, I've put 2A through some dinky MKPs (10mm
lead spacing) I found for cheap at Allied. No failures, although they did
get warm!

Interesting.

These Epcos parts (15mm lead spacing) are *rated* 2.3A, so I put a few
together for my induction heater:
http://webpages.charter.net/dawill/tmoranwms/Elec_CapBank7.jpg
200 x 0.1uF = 20uF, 250VAC, 460A (max. VAR at ~15kHz). I've had them up at
snip
That bank was only $70 in parts, but maybe you'd like something a little
less labor intensive.

naah - not really a problem - at least for playing around with.
But the ones in the chinese cookers are _MUCH_ smaller yet.

The Chinese don't need no steenkin' ratings. Smile

Also, they've been known
Or they get ratings that are actually real - not produced by someone
looking at a cap and thinking 'let's spec it to 100mA'.

to be exactly as cheap as I am... I've seen more than a few Chinese
induction supplies that use a big mess of MKPs soldered on boards for the
tank cap!

Been playing with my new induction device today, violating its specs.
:)

Some rough calculation said that skin depth for non-magnetic stainless
at 50KHz was ~2.5mm.

As expected, most of my thick non-magnetic stainless steel pans diddn't
work at all.
I was not completely surprised to find that thin non-magnetic cookware -
bowls, pans, ... worked fine.

Which is handy.

Pondering modding it to add auto-simmer. (high power until noises indicate
boiling, then turn it down)

On a more DIY note - do you have any sources for the ferrite bars used to reduce the field under the pan?

So, if you stick your head close do your metal fillings explode?

A modern version of the old 'Hot Foot'? ;-)

And wedding rings, metal watch straps etc

How resonant would a wedding ring or watch band be at 60 KHz? The
wavelength is 5,000 meters. A quater wave antenna would be 1,250
meters.

I've heated the tips of small screwdrivers to red heat at 100kHz

They are a solid, not a small loop.

If anything, a small loop would heat faster

--
Dirk

http://www.transcendence.me.uk/ - Transcendence UK
http://www.blogtalkradio.com/onetribe - Occult Talk Show

Michael A. Terrell
Guest

Sat Mar 13, 2010 12:29 am

Dirk Bruere at NeoPax wrote:

Quote:

On 12/03/2010 00:00, Michael A. Terrell wrote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 20:42, Michael A. Terrell wrote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 19:19, Michael A. Terrell wrote:

Dirk Bruere at NeoPax wrote:

On 11/03/2010 17:46, Ian Stirling wrote:
Tim Williams<tmoranwms_at_charter.net> wrote:
"Ian Stirling"<usenet_at_mauve.plus.com> wrote in message
news:nrOdnawA0P55BhLWnZ2dnUVZ8nednZ2d_at_brightview.co.uk...
http://www.st.com/stonline/products/literature/an/12443.pdf

The above app-note uses 2*.68uF caps in parallel, for the
bridge.

Hmm, those are nice and tasty...

However - all the film caps I can seem to source easily
- digikey, mouser, rs, farnell, ... seem likely to promptly
explode - at least based on the datasheets, they sharply limit
voltage at high frequencies.

Nah, they won't explode -- hell, I've put 2A through some dinky MKPs (10mm
lead spacing) I found for cheap at Allied. No failures, although they did
get warm!

Interesting.

These Epcos parts (15mm lead spacing) are *rated* 2.3A, so I put a few
together for my induction heater:
http://webpages.charter.net/dawill/tmoranwms/Elec_CapBank7.jpg
200 x 0.1uF = 20uF, 250VAC, 460A (max. VAR at ~15kHz). I've had them up at
snip
That bank was only $70 in parts, but maybe you'd like something a little
less labor intensive.

naah - not really a problem - at least for playing around with.
But the ones in the chinese cookers are _MUCH_ smaller yet.

The Chinese don't need no steenkin' ratings. Smile

Also, they've been known
Or they get ratings that are actually real - not produced by someone
looking at a cap and thinking 'let's spec it to 100mA'.

to be exactly as cheap as I am... I've seen more than a few Chinese
induction supplies that use a big mess of MKPs soldered on boards for the
tank cap!

Been playing with my new induction device today, violating its specs.
:)

Some rough calculation said that skin depth for non-magnetic stainless
at 50KHz was ~2.5mm.

As expected, most of my thick non-magnetic stainless steel pans diddn't
work at all.
I was not completely surprised to find that thin non-magnetic cookware -
bowls, pans, ... worked fine.

Which is handy.

Pondering modding it to add auto-simmer. (high power until noises indicate
boiling, then turn it down)

On a more DIY note - do you have any sources for the ferrite bars used to reduce the field under the pan?

So, if you stick your head close do your metal fillings explode?

A modern version of the old 'Hot Foot'? ;-)

And wedding rings, metal watch straps etc

How resonant would a wedding ring or watch band be at 60 KHz? The
wavelength is 5,000 meters. A quater wave antenna would be 1,250
meters.

I've heated the tips of small screwdrivers to red heat at 100kHz

They are a solid, not a small loop.

If anything, a small loop would heat faster

If it is inside the loop of an induction heater.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

Ian Stirling
Guest

Sat Mar 13, 2010 6:52 pm

Michael A. Terrell <mike.terrell_at_earthlink.net> wrote:

Quote:

Dirk Bruere at NeoPax wrote:
snip

How resonant would a wedding ring or watch band be at 60 KHz? The
wavelength is 5,000 meters. A quater wave antenna would be 1,250
meters.

It's not resonance. Anyway.
Some tests with 'worst case' realistic tests.

A pan at the low edge of the diameter limit - with a stainless steel
washer of diameter 25mm OD, 15mm ID, placed right next to the (merrily
boiling) pan, the washer did not heat noticably.

There is very little induced current in a loop of a small diameter.

Michael A. Terrell
Guest

Sun Mar 14, 2010 12:44 am

Ian Stirling wrote:

Quote:

Michael A. Terrell <mike.terrell_at_earthlink.net> wrote:

Dirk Bruere at NeoPax wrote:
snip

How resonant would a wedding ring or watch band be at 60 KHz? The
wavelength is 5,000 meters. A quater wave antenna would be 1,250
meters.

It's not resonance. Anyway.
Some tests with 'worst case' realistic tests.

A pan at the low edge of the diameter limit - with a stainless steel
washer of diameter 25mm OD, 15mm ID, placed right next to the (merrily
boiling) pan, the washer did not heat noticably.

There is very little induced current in a loop of a small diameter.

If the load was near resonance it wouldn't have to be right against
the source.

--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

Tony
Guest

Sun Mar 14, 2010 2:10 am

On Thu, 11 Mar 2010 12:01:20 -0600, "Tim Williams" <tmoranwms_at_charter.net> wrote:

Quote:

"Ian Stirling" <usenet_at_mauve.plus.com> wrote in message
news:avadnQgF6p7tsQTWnZ2dnUVZ7tmdnZ2d_at_brightview.co.uk...
On a more DIY note - do you have any sources for the ferrite bars used to
reduce the field under the pan?

Nope. You can find ferrite tiles and EMI slabs at various places, for $$$.

I've wondered if some of those EMI shielding products are useful (like the
flexible mats that claim mu_r ~ 10), but it would probably saturate too
easily, plus it would take a lot with the low permeability. And it's even
more expensive than sawing your own tiles, oh well.

Sorry for chiming in late here. I have no knowledge of this subject, so maybe this is
already done; but I would try concentric anti-series coil sections.. With the right choice
of turns ratio the far-field flux should drop off quite quickly, so an ordinary steel case
would be enough to contain it.

Cheers, Tony

Ian Stirling
Guest

Sun Mar 14, 2010 5:30 pm

Tony <tony_at_nowhere.com> wrote:

Quote:

On Thu, 11 Mar 2010 12:01:20 -0600, "Tim Williams" <tmoranwms_at_charter.net> wrote:

"Ian Stirling" <usenet_at_mauve.plus.com> wrote in message
news:avadnQgF6p7tsQTWnZ2dnUVZ7tmdnZ2d_at_brightview.co.uk...
On a more DIY note - do you have any sources for the ferrite bars used to
reduce the field under the pan?
snip
Sorry for chiming in late here. I have no knowledge of this subject, so maybe this is
already done; but I would try concentric anti-series coil sections.. With the right choice
of turns ratio the far-field flux should drop off quite quickly, so an ordinary steel case
would be enough to contain it.

Perhaps.

It's not so much containing it that's worrying me - it's losses in the case.

Somewhat pulling this out of thin air - just thoughts.

Winding differently may be interesting for a number of reasons, but
has some issues.
You really want the pan to be in the very near field - this does several
nice things.

The pan is a single turn load, with a resistance of something like
k*thickness/r^2 .

k is the resistivity of the material, and the thickness depends on
whichever is smaller - the pan thickness, or a skin depth.

The skin depth varies with frequency - but you really want to be at the
highest frequency you can.
This is as many losses in the circuit - the ESR of the resonant caps and
the plate inductor, are dependant on the circulating current in the
resonant circuit. (well, its square).

Your ideal cooker would have a very good coupling between the pan and
the coil, enabling it to operate at high frequency, minimising skin
depth (and coincidentally enabling use of copper pans).

The ferrite helps with this by vastly improving the coupling to the pan
in the face of the required ~15mm space that's taken up by the actual
top of the cooker (glass or whatever), and some space or insulator over
that to stop the coil overheating when the stovetop is at 250C.

If you made the coils multi-regional (or indeed much smaller) and
maintained that 15mm gap, then you've effectively lifted the pan off
the surface a bit more, worsening the coupling, and meaning that
it doesn't heat nearly as well, or efficiently.

Hmm. I need to think some more about this.
It is improving my knowledge of transformers Smile