Induction Cooking Table : IGBT keeping to short !

[Franc Zabkar]

On Fri, 30 Nov 2012 09:31:06 +0100, cLx <clx.kat@jaimail.com.invalid>
put finger to keyboard and composed:

On 29/11/2012 11:23, Franc Zabkar wrote:
On Wed, 28 Nov 2012 19:22:28 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

You got 0.68uF and you are afraid to use 1uFs ? Just use them as long as the current capabilities are up to snuff. The value does not mean shit as long as it doesn't go too low.

Wouldn't increasing the capacitance from 0.68uF to 1uF result in a 50%
increase in cooking energy?

Also, wouldn't each capacitor take longer to charge, and if the
capacitor wasn't fully charged when the IGBT switched off, wouldn't
this result in an interruption of the current in the coils, with a
potentially damaging back-EMF? Or am I completely misunderstanding how
this appliance works?

It's a serie LC resonnance driven by a half bridge switcher, if I'm correct.

Yes, I see that now. I should have researched the topic instead of
relying on the OP's rough circuit diagram. The absence of the flyback
diodes from the drawing left me wondering how the coil current would
decay after the IGBTs switched off, so I assumed that the current had
to be zero when this happened.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Fri, 30 Nov 2012 00:18:28 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

"The second IGBT's body diode allows the coil current to decay
gracefully when the first IGBT switches off."

No. Don't you understand that ground is a human thing, not an electronic thing ? There is no up and down.

The only possible difference is if one side of the coil has more capacitance to ground, which would make an inbalance. This COULD happen, but if it did, the bottom Xsistr failing would be such a common failure mode it would be on Fox fucking news. Switching the leads to the inductor could prove it, because then the top Xsistr would fail first.

We are talking about a ground fault condition here, without that, no anomality in the load could be imbalanced after it is running. Failing on startup is a different story, and this ain't it.

Get a grip, or a firm base of theory. Somehting. I'm surprised people can tie their shoes,,,, oh wait, they can't.

So much for that.

I don't understand what triggered your "ground" rant. Of course the
circuit is balanced, and of course there is no connection to terra
firma. Did I suggest anything else?

Are you perhaps misunderstanding what I meant by "second" and "first"?
All I'm saying is that after the first IGBT turns off, the second
IGBT's flyback diode allows the coil current to decay. Then the second
IGBT turns on. After the second IGBT turns off, then the first IGBT's
diode allows the coil current to decay, and so on.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Thu, 29 Nov 2012 09:31:06 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

"Wouldn't increasing the capacitance from 0.68uF to 1uF result in a 50%
increase in cooking energy? "

No, that is a common misconception in the business. Do you remember the formula for capacitive reactance ? Apply it and just guess the frequency is over an octave above the sonic range.

The fact is that those caps are not being used as reactive components like in a tuned system, they are being used as coupling caps.

XC = 1/wC = 1 /(2 x pi x 20000 x 1.36 E-6) ~ 6 ohms

My research would suggest that a typical inductance value for the coil
would be of the order of 50uH.

XL = wL = 2 x pi x 20000 x 50 E-6 ~ 6 ohms

So XL = XC, ie resonance.

In that circuit they are effectively in parallel. Ground and the power supply rail are effectively at the same AC potential, so it's not 0.68, it's aready 1.36 uF. That is almost a piece of wire at 20 Khz. Almost, but we are dealing with a quite higher frequency here.

If those 0.68uF capacitors were coupling capacitors, then it wouldn't
matter how large they were. In fact the larger, the better. So let's
assume for the sake of analysis that they were infinitely large. This
means that the voltage at their junction would be constant (Vsupply /
2), irrespective of the induction coil current.

So when the upper IGBT is on, the voltage across the coil would be
Vsupply - Vsupply/2 = Vsupply/2. Similarly, when the bottom IGBT is
on, the coil voltage would be -Vsupply/2. This results in a linearly
increasing coil current, first in one direction and then in the
reverse direction, ie a symmetrical triangular current.

Vsupply/2 = L . dI/dt

I would think that a triangular current would be undesirable.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Thu, 29 Nov 2012 10:21:43 +0100, cLx <clx.kat@jaimail.com.invalid>
put finger to keyboard and composed:

Now i'm stuck at finding the polypropylene caps. They are expensives,
and what i found can handle only 17 amps. Seem a bit underrated for such
that case.

Doing measurements is not easy as, without load or transistors, the
drive shuts itselfs (not detecting anything), just trying shorts pulses,
and i would not wasting my replacements parts.

I would measure the voltage at the junction of the two 0.68uF
capacitors, with the coil disconnected, and confirm that it sits at
half the DC supply voltage. This might identify any imbalance in the
capacitor characteristics, ie leakage, reduced capacitance.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

"I don't understand what triggered your "ground" rant..."

I didn't mean it like it sounded I guess.

Anyway I understand what you say about the Xc, but I am assuming that the frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture. The Xc of the caps would be much lower and the Xl of the coil much higher. If the whole shebang was 12 ohms total, one burner would pull almost 27 amps providing the supply is 320 DC. The numbers fall into line at around 75 Khz or so.

At that point, there is nothing tuned about it. I didn't mean to offend, I just know that so many have a few misconceptions about these things, actually not even these things pty se, SMPSes that use a similar configuration. If you don't have those misconceptions great.

I try to think from the engineer's standpoint. Why would I waste energy on Xc ? There is a reason to do it for example in the horizontal sweep ciscuit of a TV but usually it's all or nothing. Block the DC and pass the AC.

J

 

[Franc Zabkar]

On Fri, 30 Nov 2012 16:20:16 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

Anyway I understand what you say about the Xc, but I am assuming that the frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.

That Fairchild design note I alluded to in an earlier post discusses
several topologies (including the OP's), all of them based on
resonance principles. The resonant frequency was chosen as 24kHz while
the operating frequency "of the system is set at 28kHz, which is
higher than the resonance frequency, in order to avoid noise generated
within the audio frequency band".

Wikipedia also suggests that 24kHz is a common design target:
http://en.wikipedia.org/wiki/Induction_cooking

The above article tabulates the skin depth of various materials at
24kHz.

However, Wikipedia also states that "Panasonic Corporation in 2009
developed a consumer induction cooker that uses a higher-frequency
magnetic field, and a different oscillator circuit design, to allow
use with non-ferrous metals."

As for some real examples, here are two service manuals for induction
cookers:
http://www.garland-group.com/docs/uploaded/gar/products/g_ec_sm_inducrtcs_4521635.pdf
http://www.garland-group.com/docs/uploaded/gar/document_catalog/Document%20Catalog%20Files/Service%20manual%20Fajita%20Heater.pdf

Both manuals refer to a diagnostic mode whereby the microprocessor is
accessible via an RS232 interface. There are several sample outputs
which indicate that the operating frequency is 20080 Hz.


- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Sat, 01 Dec 2012 16:27:24 +1100, Franc Zabkar
<fzabkar@iinternode.on.net> put finger to keyboard and composed:

On Fri, 30 Nov 2012 16:20:16 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

Anyway I understand what you say about the Xc, but I am assuming that the frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.

I found this interesting application note:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00115561.pdf

"Induction cooking functions based on the principle of the series L-C
resonant circuit, where the inductance L is the cooking element
itself. By changing the switching frequency of the high voltage
half-bridge driver, the alternating current flowing through the
cooking element changes its value. The intensity of the magnetic field
and therefore the heating energy can be controlled this way."

It describes the OP's circuit as a resonant circuit consisting of an
L-C resonant tank. It states that "the IGBTs are driven by high
frequency complementary square waves with 50% duty cycle", and that
the frequency of "the PWM signal applied to the driver input pin ...
varies in a range between 19 kHz and 60 kHz". The two capacitors are
680nF 600V types, and the IGBTs are STGY40NC60VD.

The design has 9 power levels, with a frequency of 60kHz for lowest
power, and 25kHz for highest power. At 60kHz the plate current appears
to be roughly triangular, while at 25kHz it appears sinusoidal.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Sat, 01 Dec 2012 17:12:28 +1100, Franc Zabkar
<fzabkar@iinternode.on.net> put finger to keyboard and composed:

It describes the OP's circuit as a resonant circuit consisting of an
L-C resonant tank. It states that "the IGBTs are driven by high
frequency complementary square waves with 50% duty cycle", and that
the frequency of "the PWM signal applied to the driver input pin ...
varies in a range between 19 kHz and 60 kHz". The two capacitors are
680nF 600V types, and the IGBTs are STGY40NC60VD.

The datasheet for the STGY40NC60VD states that the IGBT is good for
"high frequency operation up to 50kHz". Hmmm ...

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003462.pdf

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

Very interesting Frank. You do good research. However there is no real contradiction. The inductance must be higher than we assumed because if the Xl is only 6 ohms, it would pull a hell of alot more current. It had to be one or the other.

Even more interesting is how they're making it work with non-ferrous vessels. Technically it should work with anything that conducts electricity, coils in a transformer are not ferrous and you can induce current in them right ? It actually does work on non-ferrous I guess, just extremely inefficiently. They aimed to increase the efficiency. It says reduced efficiency and I saw a chart, but it wasn't boiled down to just HOW inefficient a given unit will be on specific cooking materials - as if the layman would usually even know. It would still be trial and error.

Now if they can make it work on glass cookware, they get my vote for the Nobel prize.


Very interesting, thanks for bringing that in.

 

[josephkk]

On Sat, 1 Dec 2012 13:42:16 -0800 (PST), jurb6006@gmail.com wrote:

Very interesting Frank. You do good research. However there is no real contradiction. The inductance must be higher than we assumed because if the Xl is only 6 ohms, it would pull a hell of alot more current. It had to be one or the other.

Even more interesting is how they're making it work with non-ferrous vessels. Technically it should work with anything that conducts electricity, coils in a transformer are not ferrous and you can induce current in them right ? It actually does work on non-ferrous I guess, just extremely inefficiently. They aimed to increase the efficiency. It says reduced efficiency and I saw a chart, but it wasn't boiled down to just HOW inefficient a given unit will be on specific cooking materials - as if the layman would usually even know. It would still be trial and error.

Now if they can make it work on glass cookware, they get my vote for the Nobel prize.


Very interesting, thanks for bringing that in.

Making it work with glass would impress must anybody. However i wonder if
they can be made to work on slightly salty water (saline{?}).

Just a thought.

?-)

 

Actually if they can really penetrate the non-ferrous they can heat the food directly. I gues an upside down microwave or something.

Hell, it might be all the rage range now because your steak heats the pan, not the other way around.

By now you know most of that "Visions" type cookware got recalled right ? The stuff exploded on a few folks and they went back to metal pots and pans.

Funny what it takes to induce common sense sometimes eh ? My Mother had them but got rid of them over that shit. Really, they would probably be alright for boiling and casseroles etc, but I think trying to fry crispy chicken in a glass pan might not be the smartest idea.

But then lead could be put in the glass, or barium. Maybe it would glow or something.

Colored glass I guess in a way could be considered "doped". A study of the various types' reaction to bombardment like this would be intereasting.

 

[cLx]

On 01/12/2012 06:27, Franc Zabkar wrote:

On Fri, 30 Nov 2012 16:20:16 -0800 (PST), jurb6006@gmail.com put
finger to keyboard and composed:

Anyway I understand what you say about the Xc, but I am assuming that the frequency is much higher than 20 Khz. I'd bet it's somewhere around 75 Khz which takes the tuned circuit out of the picture.

That Fairchild design note I alluded to in an earlier post discusses
several topologies (including the OP's), all of them based on
resonance principles. The resonant frequency was chosen as 24kHz while
the operating frequency "of the system is set at 28kHz, which is
higher than the resonance frequency, in order to avoid noise generated
within the audio frequency band".

When it was brand new, i've measured the frequency at ~35KHz, modulated by
100Hz (double rectification of the mains' frequency).

See:
http://clx.freeshell.org/view.html?f=Tech/induction/img_0809.jpg
http://clx.freeshell.org/view.html?f=Tech/induction/img_0812.jpg
http://clx.freeshell.org/view.html?f=Tech/induction/img_0813.jpg

Wikipedia also suggests that 24kHz is a common design target:
http://en.wikipedia.org/wiki/Induction_cooking

The above article tabulates the skin depth of various materials at
24kHz.

However, Wikipedia also states that "Panasonic Corporation in 2009
developed a consumer induction cooker that uses a higher-frequency
magnetic field, and a different oscillator circuit design, to allow
use with non-ferrous metals."

As for some real examples, here are two service manuals for induction
cookers:
http://www.garland-group.com/docs/uploaded/gar/products/g_ec_sm_inducrtcs_4521635.pdf
http://www.garland-group.com/docs/uploaded/gar/document_catalog/Document%20Catalog%20Files/Service%20manual%20Fajita%20Heater.pdf

Both manuals refer to a diagnostic mode whereby the microprocessor is
accessible via an RS232 interface. There are several sample outputs
which indicate that the operating frequency is 20080 Hz.


- Franc Zabkar

 

[William Sommerwerck]

By now you know most of that "Visions" type cookware got recalled,
right? The stuff exploded on a few folks and they went back to metal
pots and pans.

I've been using Corning glassware and Corningware for years, and have never
had a piece break while cooking. Dropping them is a different matter.

 

[Michael A. Terrell]

William Sommerwerck wrote:

By now you know most of that "Visions" type cookware got recalled,
right? The stuff exploded on a few folks and they went back to metal
pots and pans.

I've been using Corning glassware and Corningware for years, and have never
had a piece break while cooking. Dropping them is a different matter.

The older stuff was real 'Pyrex'. Now they use 'Soda Glass' and call
it 'Pyrex'. Soda glass will explode if heated unevenly.

 

[Michael A. Terrell]

jurb6006@gmail.com wrote:

"When it was brand new, i've measured the frequency at ~35KHz, modulated by
100Hz (double rectification of the mains' frequency). "

So they don't bother filtering, like a microwave. Hmmm.

"I've been using Corning glassware and Corningware for years, and have never
had a piece break while cooking. Dropping them is a different matter. "

Visions were different. Some sort of superglass. The commercial showed them melting a conventional metal pan in their glass pan. I would imagine it was not stainless, IIRC the melting point is quite high compared to aluminum or steel, or cast iron.

But these things didn't just break, they exploded. They are no longer on the market, at least with the old formula.

It is pretty cool to be able to see what you're cooking like that. There was always Pyrex as well. One of the things visions touted was an extremely high thermal mass. In most cases if you wanted to cook say spaghetti, once it came to a boil again with the pasta in the water, you supposedly could shut the heat off and it would cook fully.

I've done that with steel cookware on a stove, and in a glass
casserole in the microwave. The must be covered to help hold in the
heat, and let them sit about eight minutes, depending on the thickness
of the pasta. In fact, I cooked some medium sized shell pasta in my
microwave a few days ago.

 

"When it was brand new, i've measured the frequency at ~35KHz, modulated by
100Hz (double rectification of the mains' frequency). "

So they don't bother filtering, like a microwave. Hmmm.

"I've been using Corning glassware and Corningware for years, and have never
had a piece break while cooking. Dropping them is a different matter. "

Visions were different. Some sort of superglass. The commercial showed them melting a conventional metal pan in their glass pan. I would imagine it was not stainless, IIRC the melting point is quite high compared to aluminum or steel, or cast iron.

But these things didn't just break, they exploded. They are no longer on the market, at least with the old formula.

It is pretty cool to be able to see what you're cooking like that. There was always Pyrex as well. One of the things visions touted was an extremely high thermal mass. In most cases if you wanted to cook say spaghetti, once it came to a boil again with the pasta in the water, you supposedly could shut the heat off and it would cook fully.

 

[William Sommerwerck]

I've been using Corning glassware and Corningware for years, and have
never had a piece break while cooking. Dropping them is another matter.

The older stuff was real "Pyrex" [borosilicate glass]. Now they use "soda
glass" and call it "Pyrex". Soda glass will explode if heated unevenly.

I complained about this a few years ago, and was told that the newer glass
was as good. I doubt it.

Several years ago I dropped a Visions baking dish and it shattered like you
wouldn't believe. (I'm still finding the pieces.) Corning paid the postage
to return it, but they never told me what they found (if anything). They
made no offer to replace it.

See the Wikipedia article on Pyrex, section on Composition.

 

[Phil Allison]

<jurb6006@gmail.com>

"When it was brand new, i've measured the frequency at ~35KHz,
modulated by 100Hz (double rectification of the mains' frequency. "

So they don't bother filtering, like a microwave. Hmmm.


** Microwave ovens have similar line frequency modulation and for the same
reason.

The high voltage PSU is not filtered.

Electronic transformers for 12v halogen lighting are the same too.


.... Phil

 

[Cydrome Leader]

William Sommerwerck <grizzledgeezer@comcast.net> wrote:

By now you know most of that "Visions" type cookware got recalled,
right? The stuff exploded on a few folks and they went back to metal
pots and pans.

I've been using Corning glassware and Corningware for years, and have never
had a piece break while cooking. Dropping them is a different matter.

beware that anything made in the past some years isn't what it used to be.
it's just some sort of cheapo fake stuff and not the real pyrex as used in
labware.

You can even tell from the color of the stuff that it's nothing more than
melted bottles and windows.

 

[Cydrome Leader]

William Sommerwerck <grizzledgeezer@comcast.net> wrote:

I've been using Corning glassware and Corningware for years, and have
never had a piece break while cooking. Dropping them is another matter.

The older stuff was real "Pyrex" [borosilicate glass]. Now they use "soda
glass" and call it "Pyrex". Soda glass will explode if heated unevenly.

I complained about this a few years ago, and was told that the newer glass
was as good. I doubt it.

Several years ago I dropped a Visions baking dish and it shattered like you
wouldn't believe. (I'm still finding the pieces.) Corning paid the postage

I wattched a Visions lid from the large pasta pot get dropped recently
onto a wooden floor.

I would not doubt there was more energy stored in that lid than just it
falling into the ground. It din't break, it became hundreds of pieces, but
sharp ones unlike a windshield.

We noticed that those 1980s white corningware plates with the yellow
printing on the edges were unbreakable when new, but became extremely
brittle over the years. They seemed to heat in the microwave oven, so
maybe that has something to do with it.

The extra plates and bowls from the set that never really saw use can
survive a kitchen floor drop test. The rest just shatter.

to return it, but they never told me what they found (if anything). They
made no offer to replace it.

Did you send it to the real corning or the fake place called "world
cookware" or something like that that now just licenses the corning name?

See the Wikipedia article on Pyrex, section on Composition.