induction heating

[romy]

so in the end Q is not Q? Q for quality factor and Q for reactive power?

" I-V phase is very close to 90° with no workpiece, and slightly less close to 90° with workpiece" (#17):

i still dont get how to look at it. I/V 90° out of phase means a powerfactor of zero (cos 90° = 0), no real power transferred. no? a resistive load has a power factor of 1, I/V in phase. so if the (empty) work coil is a resistive load, where do the 90° come from?

https://highvoltageforum.net/index.php?topic=2299.msg16805#msg16805: super thread, somehow i missed it so far. any updates on that?

[Anders Mikkelsen]

One minor correction is called for.
>> The ratio of reactance (2*pi*f*L) to resistance is called the Q factor, and this is also the ratio of stored energy to lost energy per cycle.
Nope, it's the ratio of stored energy to lost energy per radian time.  I was reminded of that when brushing up on Q a couple months ago. My example L and f were identical to those chosen by Anders!  https://highvoltageforum.net/index.php?topic=2299.msg16805#msg16805

Consider the example Anders gave a few posts back.  L = 1 microhenry, f = 160 kHz (radian frequency = 1 million/s), reactance Z magnitude is 1 ohm, resistance = 10 mohm.
So Q = 100.  The energy loss per cycle is not 1%, it's 2 pi %.

Let's check that.  Example current is 100 amps, RMS, so the real power loss is I^2R = 100 watts.
Peak energy in coil is 0.01 joule.  If we had a 1 uF capacitor for resonance at the designated frequency, peak energy in capacitor (with 100 V RMS) would also be 0.01 joule, and at all intermediate phases the L + C energy is 0.01 J.
Our 100 watt loss is 0.0001 joule per microsecond, or 0.000628 joule per cycle.

Thanks for the correction, with a very clear example even!

so in the end Q is not Q? Q for quality factor and Q for reactive power?

" I-V phase is very close to 90° with no workpiece, and slightly less close to 90° with workpiece" (#17):

i still dont get how to look at it. I/V 90° out of phase means a powerfactor of zero (cos 90° = 0), no real power transferred. no? a resistive load has a power factor of 1, I/V in phase. so if the (empty) work coil is a resistive load, where do the 90° come from?

The coil itself has a power factor close to zero yeah, more or less the inverse of the Q factor, and the phase angle is atan(Q), so for an unloaded coil with a Q of 100 the phase angle is 89.4 degrees. For reference, a coil like the one I've been describing would generally have a Q in the 100 - 150 range unloaded.

Once you connect a capacitor to the inductor, the power factor of the system will change, essentially the capacitor is power-factor-correcting your inductor, leaving a simple resistance at the one frequency where their reactances cancel out. Our example coil looks like 10 mohm of resistance in series with 1 ohm of inductive reactance at 159 kHz. Connecting a 1 uF capacitor in series with this, it will cancel out the reactance of the coil and leave a pure resistor of 10 mohm. Now the whole system has a power factor of 1, 1/100 the impedance of before, and for a given amount of power transfer, it will have to process 100 times as much apparent power.

Q is confusingly used to refer to both reactive power and quality factor. I've only used it to mean quality factor, and I'll stick to this to minimize confusion.

for now im not clear about the following: if over time we see constant current and sharply dropping induced power after curie, under what conditions/ceteris paribus is it the case? what variables of the system have been kept constant? if you have a constant voltage supply, does the supplied current simply drop along with the dropping voltage in the work coil? maybe im totally off the track.

This is related to the drive circuit. If we feed our series tank with a voltage source, then the current draw and power dissipation will be highest when the tank is unloaded. Putting a workpiece into the coil introduces more resistance and lowers the power transfer, which is the opposite of what we want. Why don't we make the tank a parallel resonant one then, to get it to behave more nicely? The problem is that we use switching circuits to drive the inverter, square waves and not sine waves. A square wave contains harmonics at odd multiples of the drive frequency. For the harmonics, our inductor and capacitor don't cancel out and a lot of current is drawn, ruining the efficiency of the circuit. The same problem applies for feeding a series resonant tank from a current source.

[romy]

ad (6)/#18: i was referring to the following relations. would we have to differentate between in series and parallel?





i thought most resonnant cirquits were parallel, as in the other case the all apparent power has to pass through the switches. interestingly the other day i had a visit by the manager of an austrian supplier of induction heaters and it turns out their smaller (up to 5kw) machines are series cirquits. the big ones are parallel.

would you kindly point me to some explanation on the "atangens(Q)" thing? its the first time i see this.

i wonder where currie temp is here. somewhere after the cross-over?

[Mike]

would you kindly point me to some explanation on the "atangens(Q)" thing? its the first time i see this.

This is just trig based on the reactive impedance of the inductor being at 90° to the resistance. theta = atan(X_l / R) combined with the definition of Q, i.e. Q = X_l / R

[romy]

a few short questions.

7) what is happening in case of a remote work coil where the power cables are wraped closely together? i assume the fields cancel out, but is there a power loss or an influence on resonating frequency? after all, it is recommended to keep the legs of the work coil as far appart as possible. a contradiction?

8 ) what is happening in the case where one of the coil legs passes through a toroid? is all the power transfered through this 1/4 or less of turns? is that a totally different topology than the "zvs" cirquit?

9) how does a thin walled cylinder behave in the work coil? are the the penetration deph considerations unchanged? i see that alowable p.d. is inceased in calculations i came accross (by maybe 30-50%) but i dont understand why. i was surprized seeing the acid handle heating up so fast in klugesmiths video*. p.d. must be about 10x wall thickness and it should be almost transparent to the energy input.

* #10: https://highvoltageforum.net/index.php?topic=2299.msg16832#msg16832

10) does anybody have resistivity, permeability (and other) data for crome and nickel? iv seen some for nickel but none for chrome so far. they are the main alloing elements in steel after all.

[klugesmith]

>>  8 ) what is happening in the case where one of the coil legs passes through a toroid? is all the power transfered through this 1/4 or less of turns? is that a totally different topology than the "zvs" cirquit?

Many amateur IH projects show a coupling transformer, with ferrite toroid core(s) through which the work-coil tube passes once.
In this case the tank circuit is series-driven.  The single-turn transformer secondary is in series with L and C, and carries the whole tank current at a small fraction of tank voltage.   Multiple-turn primary winding scales the current and voltage to levels more suitable for active driver.  Scaled impedance (looking into primary side of transformer) dips to a _minimum_ at resonance.   Look at the instructable by imsmooth.

[klugesmith]

>> 10) does anybody have resistivity, permeability (and other) data for crome and nickel? iv seen some for nickel but none for chrome so far. they are the main alloing elements in steel after all.

Probably not hard to find on Internet.  But knowing the resistivity & permeability of chromium would not help unless you want to induction-heat pure chromium, or a chrome-plated workpiece.   For resistivity of alloys, a weighted average of the elemental components won't even get you close.  Likewise for permeabilty - it doesn't take much to spoil ferromagnetism, but some well-explored formulations can enhance it in different ways.

Engineering Tool Box is one site that frequently pops up when I search for resistivity & permeability data.
Read the comments on this page about austenitic and other stainless steels.  (In my kitchen drawer are 2 sets of stainless-steel spoons, and only recently did I notice that one set is magnetic.)  https://www.engineeringtoolbox.com/permeability-d_1923.html

[edit] Just noticed that table in my link includes "Neodymium magnet".   Opportunity to point out that "Neodymium" or NIB magnet materials, just like our grandfathers' Alnico magnets, are more than 50% iron by atom & by weight.

[romy]

i looked at a lot of tables and cr is nowwhere to be found. you are of course right, in that electrical/magnetical properties of a material do not depend on the alloying proportions and not even on the composition of phases/precipitates (ferrite, perlite, austenite, accicular/plate martensite, upper/lower bainite, fe-carbide, complex carbides, widmannstätten structures). austenite is non-magnetic, thats about it. factors playing a role are such as grain size, grain shape, grain orientation, sub grain structuring, type/density of dislocations, stacking fault energy, type of recrystalization, type/shape/distribution of carbide etc. it seems the boundaries influence the properties more than the grains themselves. therefore the same alloy will be different depending on its thermo-mechanical history/heat treat (→workhardened/cold rolled, normalized, soft annealed, intercritical annealed, hardened, tempered  etc.). even the "same" material from anothr supplier can have substantially differen properties bacuse of miniscule variation.

what i have been musing about recently: fe-carbide supposedly looses magnetism at 210°. might materials with a large carbide fraction (e.g. cast irons) loose some magnetism at that temp. after all and should their fraction be considered by weight or volume? or is it again irrelevant and the exact cementite/ledeburite structure has to be looked at?

bt, the main difference in ss is if the are ferritic or martensitic.

some perhaps usefull data on materials not easy to find in condensed form (of course the dependency on field strenght is not accounted for):

[petespaco]

Going all the way back to your first post, where you say you want/need an induction heater in your shop:
Maybe you should go to this group with your original questions:
https://www.facebook.com/inductionheat/
They seem to be pretty good at their art.  I have known at least one of the regulars there for 30 years or so and he is very knowledgeable.

[romy]

thank you, unfortunately i dont have a facebook account, so i dont know if i see what you had in mind. oh, its a discussion group?

11) why did the 1800w heater burn?

it ran for a while on 2 car batteries. total run time about an hour. it would heat up steel, but only to about 600-650°, where permeability starts falling off sharply. then i tried 3 batteries and got it up to a max of 60adc/37vdc for a few seconds. i then wanted to investigate what might be gettimg hot, so i took off the fans and let it idle (5-6adc), checking the components every coulpe of minutes. nothing got even a little warm. then i let it idle some more checking on it from time to time and in the end, after what might have been 1-2 hours i came back and it had burned out.

all four fets were blown/cratered, one toroid burned. the other was fine as were the caps. the buck chip (probably lm2596hvs) has a deep hole in it, nothing else is blown as far as i can see, but who knows. i dont thing it gave up the ghost due to overheating, something else has happened. surely the contraption would be able to dissipate 200w, part of it throught the coil (right?).

what do you think could have happened? why did one toroid survive? also, why is the burned one undamaged on the underside? can we suspect the regulator as the culprit?

hopefully i can get some picts in here. o.k., no idea why one is large.

[romy]

12) how does this work?

/>
another channel i find interesting:

https://www.youtube.com/@Fluxtrol

[petespaco]

Re: "what do you think could have happened?"
My first thought is that the batteries became discharged. But pushing 60 amps might have stressed things before you ran your "idle" test.
Several people, me included have had pcb traces burned during excessive current runs or in situation where the gate circuits aren't getting full voltage.

 I have my 2500 unit set to trip off at about 47 amps, just to be safe.
What were you using as a workcoil?

Re: "can we suspect the regulator as the culprit?"
I don't know what the "regulator" is.

[romy]

i figured, if the deeply cratered 2596 chip failed first, it would have taken the whole cirquit with it.

https://hmsemi.com/downfile/LM2596HV.PDF

obviously i would like to know what the weak point of the contraption is.

still wondering why one toroid is unharmed. anybody?

oh, no way the batteries were discharged. they are 85 ah, have 6.5 a chargers on them and i didnt use the system that day, only for this failedd idle test.

edit: on second thought, mabe the question should be different. what makes this regulator blow up in such a way? it has 3-fold protection, right? iv seen cratered fets before, but never a regulator.

[petespaco]

Let's back up a bit.
What does that lm2596hv chip do in this circuit?  In my experience,  it (they) are only used in a circuit  that supplies 12 volts to  cooling fans and/or a water pump.

[romy]

yes, what does it have to do with the oscilator? i figured it was for fans and pump, neither of which were connected at the time of failure. is it on your board as well?

13) i have a pot with water on an 4000w induction cooker and a wire loop under it. i see a few volts on it. why does the cooker shut off if the loop is shorted?

[petespaco]

Re: lm2596hv chip :  The only thing that comes to mind is that the circuit experienced a severe over voltage at some point. And why would you run the machine without the fans and without cooling water?

Re: induction cooktop:
I have no idea.  But why would you short the loop anyway?

[romy]

- please see (11).

- thinking i would be able to measure current.

[petespaco]

- please see (11).

- thinking i would be able to measure current.

Re: please see 11:  Ahha- so the highest voltage that was ever applied to the system was 32 volts, right?
Well, it MIGHT be that the work coil heated up so much that it tranfered too much heat to the pcb and its components.  Was the work coil discolored when you returned to check the system?
Otherwise, sorry,  but I am stumped. I wonder if you could apply with the seller to return it under warranty.

Re:  thinking---:   Was that loop part of the original circuit or was it something you added for testing?

[romy]

i was checking on the sysem frequently for 2 hours. nothing got even warm incl. coil. then in my absence "something" must have happened. coil still looks like new. i dont bother returning stuff like that, on the contrary, i will eventually try and fix it. its been somewhere in a box for a month now. i will start playing with the 5kw contraption now.

cooker: just a piece of insulated wire under pan. i believe i saw 3vac (on a multimeter, whatever that really is). but 6 loops got me 35vac, so its somehow consistent. the short probably tripped the inductance detection, i would like to understend why.