Post by: romy on February 16, 2023, 07:30:59 PM
i do most of the electrical work myself and have some practical knowlegde of electronics (like cutting an igtb out the welder or replacing a cap in a vfd) but consider myself totally clueless from the perspective of a professional. i have 3x380vac mains on 40aac breakers. so here goes:
1) power ratings
if i have this: https://www.ebay.com/itm/174540923486? and see 40adc at 37vdc its 1'480w.
however this: https://www.ebay.com/itm/225251784975? is confusing. i found out (they are reluctand to give this out) it consumes 28aac (6'440w) and will run on a 32a breaker. the gage seems to be showing a max of 800a*. what are they trying to say? that 15'000w/800aac = 18.75vac is the voltage accross the work coil?
*i see this says 600a, i cant find the 800a-contraption right now. they all show 800a on youtube.
would this perhaps be real 15kw?
https://www.ebay.com/itm/324023764415?hash=item4b715245bf:g:4BUAAOSwyWZZXgqL
2) does anyone own this? how does it look to you? i assume it would run on two phases without the neutral?
https://www.aliexpress.com/item/1005002033519057.html?
3) i saw a thread on here where a member was building an induction heating circuit (must have a different title). it was 10+ pages but i cant find it. any idea what it might have been? (maybe im dreaming and it was somewhere else.)
im considering eventually getting a professional quality unit (support, spares etc.). 10kw is what i have in mind, as the prices seem to explode further up (>$20'000).
im reading all the other threads and have more technical questions also, but this is it for now.
Post by: klugesmith on February 16, 2023, 07:57:06 PM
Post by: romy on February 16, 2023, 08:05:46 PM
i also believe the gain can be an order of magnitude in the coil, if the system is in resonnance and the curve is "sharp", so that the 15kw is way underrated in case is supposed to express whats going on in the coil. but maybe these contraptions are so lousy that its correct?
Post by: klugesmith on February 17, 2023, 12:44:38 AM
Suppose unloaded work coil and IH capacitor are working at 600 A, 100 V.
That's 60 kVAR, which might need 1 kW of power from wall to keep it going.
It would be a 1 kW heater for the stream of IH cooling water.
If the coil has 5 turns, then a 1-turn secondary in ideal transformer might theoretically approach 3000 A or 20 volts.
If a real workpiece had induced eddy current of 1200 A at 5 volts, it would receive 6 kW of heat.
Wallplug input power to IH would go up by a little more than that. Coil current, voltage, and frequency would change by some percentage.
Post by: davekni on February 17, 2023, 08:37:27 PM
Quote
3) i saw a thread on here where a member was building an induction heating circuit (must have a different title). it was 10+ pages but i cant find it. any idea what it might have been? (maybe im dreaming and it was somewhere else.)Perhaps one or more of these threads:
https://highvoltageforum.net/index.php?topic=530.msg11972#msg11972
https://highvoltageforum.net/index.php?topic=1561.msg12104#msg12104
https://highvoltageforum.net/index.php?topic=1976.msg14755#msg14755
The induction units you show appear to be almost all dishonest about power. Claiming more power than is drawn from the line. The one possible exception is the "2500W" 48V unit. Under ideal conditions you might get close to 2500W. However, you will need a 55A 48Vdc supply in addition to the unit, and adjust work coil and load to draw no more than 55A else unit fries.
At least the claims aren't as absurd as flashlights that could double as rapid camp fire starters:
https://www.ebay.com/itm/204177345517
900000 lumens is at least 2500 watts, all from a single 18650 cell.
Post by: petespaco on February 18, 2023, 12:48:51 AM
Post by: romy on February 18, 2023, 06:50:55 AM
dave, unfortunately it seems even professional units are rated that way. my conclusion is that the dc units show real power, as you say, and the mains-units need degrading by 1.73 to be comparable. yes?
pete, are you referring to the 230vac board? does it seem to have a smoothing stage? it looks like a cooker to me (sure no expert here). what do you think? i wonder if i eventualy get any reply from them.
so nobody has bought/used it?
4) oscillating frequency of apparatus
i understand how energy "vibrates" between the capacitor tank and the the work coil. it gets exited externally to compensate for the energy transfer to the work and internal losses. this input has to be in phase. frequency is defined by this hardware and is inversely proportional to the roots of C and L including (!) the work. at perfect resonnace the current is limited by the resistance of the coil and connections only.
how come then that:
a) apparently F can be adjusted externally? i see tuning from above to reach resonnance
b) i see external "self-tuning" cirquits being added
c) apparently the resonnant apparatus can be hooked up to the toroid of a welding inverter and F adjusted by a variable resitor in its cirquit
d) in the case of the "1800w-aliexpress" contraption F remains (seemingly?) unchanged when work is inserted into coil? im seeing a drop from about 51khz to about 48khz. no idea if its real, as im manually handling the workpiece.
i have come accross a circuit, where there is an inductance parallel to the work coil for the purpose of stabilizing F, without gaining deeper insight of how that works.
im i just confused? what are your thoughts.
lets see if i can get a schematic in here. this is what im looking at. what would be the correct name for this, btw? royer/jensen/baxdall/mazilli seem to be used loosely (sinus/square) and all have a center tapped coil, if im not mistaken.
Post by: davekni on February 18, 2023, 07:11:20 PM
Quote
dave, unfortunately it seems even professional units are rated that way. my conclusion is that the dc units show real power, as you say, and the mains-units need degrading by 1.73 to be comparable. yes?Are you the professional units you reference fed 3-phase line power? If so, they are probably accurately rated. If AC line input voltage is listed as phase-to-phase, total power from all three wires is sqrt(3)*V*A = 1.73*V*A, presuming unity power factor and equal current from all three phases. None of the links in your initial post appeared to be three-phase line input, though I couldn't tell for sure. If one of them was 3-phase input, then perhaps it was accurately specified.
Quote
d) in the case of the "1800w-aliexpress" contraption F remains (seemingly?) unchanged when work is inserted into coil? im seeing a drop from about 51khz to about 48khz. no idea if its real, as im manually handling the workpiece.Seems reasonable. Presume your work piece was iron/steel to cause a frequency decrease. Frequency will increase a little with non-magnetic work pieces, and with steel once above its Curie temperature. How much frequency shifts depends on coil and work piece geometry, especially how tightly the coil fits around the work piece.
Quote
lets see if i can get a schematic in here. this is what im looking at. what would be the correct name for this, btw? royer/jensen/baxdall/mazilli seem to be used loosely (sinus/square) and all have a center tapped coil, if im not mistaken.I don't know the correct name either. I tend to call them ZVS oscillators for Zero Voltage Switching. I gather "royer" is a square wave version without resonant caps and relying on inductor saturation to trigger switching.
The schematic you show does not have a center-tapped coil. This is the most common version, presumably used in the 1800W unit you mention. If L1 is center-tapped, then L2 and L3 can be combined into a single slightly-smaller inductor.
Post by: Alberto on February 19, 2023, 11:16:04 AM
It came without coil. i made one but I'm not sure how many turns it should have. How can I calculate the number of turns? What effect does it have changing the number of turns?
Post by: petespaco on February 20, 2023, 06:50:56 AM
Quote
pete, are you referring to the 230vac board? does it seem to have a smoothing stage? it looks like a cooker to me (sure no expert here). what do you think? i wonder if i eventualy get any reply from them.Yes, the 220 board. Without a schematic or a better description, I, personally wouldn't touch it with a ten foot pole. That "Description" chart doesn't make any sense at all to me.
To: Alberto-
That first link offers several different models. If you go there and select the one you bought, several thumbnails for it show up to the left of the main picture. One of them shows the coil and I can count the turns and estimate the diameter, so I am sure that you can, too.
Post by: romy on February 20, 2023, 07:52:50 AM
so there is no real answer to (4)? that probably means im confused. am i mixing up different topologies?
ad 4b): oscillating F is inversely proportional to sqrt of L. L is proportional to relative magnetic permeability. if i insert a piece of steel into the coil i fill half of the area with material that can have a permeability up to 2 orders of magnitude higer than air/free space. (did i get it right?) so why do i see a negligible (if at all) drop in F on the "1800-contraption"? it cant be that much out of phase, right?
5) penetration deph and appropiate coil frequency
standard pd is defined as the distance where magnetic flux reaches 37% (a little less, where does such a number come from?*) of max. the recommendation is to use 1/3 of diameter max. this makes sense, after that the flux tends to cancel out.
so taking steel (e.g 1045) 30mm diameter and 60 mm long (360g) we would aim for 10mm pd. this would be achieved at way below 500hz at room temp. pd would rise to about 25mm at currie and 30mm at 1'200°c. to get it down to 10mm F has to increase to around 6khz. this is even more pronounced with thiner work. flat stock of 6mm needs 2mm pd. this is the case at 500hz at room temp and way above 100khz at 1'200°c.
the above are very rough empirical numbers, they greatly vary with the type of alloy, the exact setup and probably all other (interdependent) factors you might think of. there are formulas, but my understanding is they are not especially usefull. however, fact is we need a frequency controll of at least an order of magnitude for efficien operation.
how do we manage to do that?
i see small (5-45kw) commercial units operate on 35-80khz, 20-150khz. so they clearly are not efficient for thinner stock or small parts. on top of that, those i looked at will "tolerate" those F depending on the working coils used, but will not adjust F during operation in any way. is this the reason you need a 45kw (45kva?) unit to heat up 1"x1" stock i short time? im getting the impression, much less power is neccessary if the system is tunable/autotuning.
thoughts?
*oh, 1/e.
Post by: klugesmith on February 20, 2023, 07:06:31 PM
In EE trade, what you call pd (penetration depth) is more commonly called skin depth.
For induction heating, it's very common to use frequencies where skin depth is a tiny fraction of the workpiece thickness. Whether we are heating a frying pan or a 25-mm-thick bar for forging.
On the plus side, that makes the sheet resistance relatively high, so we get more power for a given amplitude of eddy current. High frequency also means more power density from magnetic hysteresis loss in ferrous material below Curie temperature.
On the minus side, sometimes, that thick bar could get incandescent at its surface while still cold at its core.
Post by: Alberto on February 20, 2023, 10:43:27 PM
To: Alberto-
That first link offers several different models. If you go there and select the one you bought, several thumbnails for it show up to the left of the main picture. One of them shows the coil and I can count the turns and estimate the diameter, so I am sure that you can, too.
Thank you!
Yes, I didn´t think about that
Post by: romy on February 22, 2023, 06:53:44 AM
ad (5)
so in a nut shell: a much higher frequency is needed to get steel to forging temp than offered by most commercial units (at least as i see it).
in industrial applications this apparently is compensated by large power densities and short heating times. this strategy is wastfull for the obvious reason and additionaly for two others, that mittigate the effect:
- p.d. increases with magnetic fiel density
- "magic wave" phenomenon: second (or even first) max. of power density under the surface (→max. in permeability/critical field intensity/currie)
question: why are there no devices that automatically increase working F from say 10 to 500 khz (that i understand is about the limit for current igbts)? is it technically impossible or is it because maybe they were designed before it was possible?
Post by: klugesmith on February 22, 2023, 05:00:55 PM
It might work if you abandon the use of resonant circuit in induction heater. To get a 50:1 change in resonant frequency, without changing the coil geometry, you would need to change C by a factor of 2500.
Post by: Anders Mikkelsen on February 23, 2023, 12:21:19 AM
Start with a coil, just a winding of copper tubing. This coil will have some inductance and some effective series resistance. Inductance is more or less constant with frequency, while the resistance increases roughly with the root of frequency. 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.
Let's say we wind a 1 uH coil of 5 mm brake tubing, this will be some five turns of 50 mm diameter with a 8 mm pitch. At 160 kHz, this coil will have a reactance of 1 ohm, and a resistance of 10 mohm, giving a Q factor of 100, or a power factor of 0.01. If we feed the coil with 100 V, it will draw 100 A, and dissipate 100 W, while circulating 10 kVAR.
Now we put a piece of steel pipe inside the coil, and the effective resistance has gone up. Some of the field is coupled into the workpiece, and the workpiece has higher resistivity and higher permeability, giving much higher eddy current losses than in our coil. Now the series resistance has gone up to 100 mohm, of which 90 mohm is from the workpiece. If we feed the coil with the same 100 V, the current will still be around 100 A, but now we have 100 W of losses in the coil and 900 W in the workpiece. The Q factor is now 10, power factor is 0.1, and coil efficiency is 90 %.
I like to use Q factor and coil efficiency, because the former will illustrate how much reactive power we need to transfer a certain amount of power, and the latter will tell how much power will end up in the coil and how much will end up in the workpiece. Both can be measured without involving high voltages, high currents or expensive equipment, as long as you have a coil and a capacitor to resonate it with.
If we change the frequency, the ratio of workpiece and coil resistance will stay the same, so it has no impact on how efficient the system is, but once we go low enough then the eddy currents in the workpiece go deep enough to start cancelling out. Below this critical frequency, coil efficiency goes down. This frequency depends on the size, conductivity and shape of the workpiece
[ Invalid Attachment ]
So why would we want to operate much above this critical frequency? Firstly efficiency will still continue increasing up to around the workpiece being 10 - 15 skin depths. Secondly a given coil will have a higher impedance at higher frequencies, meaning you need less current and more voltage to drive it. It's generally easier to make a driver that provides 500 amps at 500 volts, compared to one that provides 5000 amps at 50 volts. One could also wind the work coil with thinner wire to trade current for voltage, but there are limits to how far you get with this.
[ Invalid Attachment ]
So now we have a coil, where hopefully most of the losses come from our workpiece. The inductance is still much larger in magnitude than the resistance is, so we need to provide 10 - 100 times the reactive power to the coil, for a given amount of power delivered. The cheapest way to do this is often with a capacitor, and if the frequency is chosen so that f = 1/(2*pi*sqrt(LC)) then we are left with pure resistance. The higher the Q factor is, the narrower the frequency window is, and inductance changes with the workpiece, which is the reason for the active feedback used in a lot of circuits. There are two concerns here, one is that the phase (and therefore power factor) changes rapidly with frequency with high-Q tanks, which firstly impacts the power factor as seen by the inverter, but more critically also affects the conditions under which switching happens. Voltage fed transistor bridges are a lot more efficient when a controlled amount of inductive power factor is present in the load, due to zero voltage switching, and one of the main tasks of the control circuit is to maintain this condition.
ad 4b): oscillating F is inversely proportional to sqrt of L. L is proportional to relative magnetic permeability. if i insert a piece of steel into the coil i fill half of the area with material that can have a permeability up to 2 orders of magnitude higer than air/free space. (did i get it right?) so why do i see a negligible (if at all) drop in F on the "1800-contraption"? it cant be that much out of phase, right?
Permeability of steel is high, but for higher frequency, the field doesn't penetrate deep into the material (skin effect), so most of the core does not contribute to the path reluctance. At higher frequencies, a tiny fraction of the outer skin is contributing high permeability, but the inside of the core is contributing zero permeability, so putting an iron core in a coil can actually reduce inductance. Putting conductive non-magnetic materials in the core always reduces inductance.
Quote
question: why are there no devices that automatically increase working F from say 10 to 500 khz (that i understand is about the limit for current igbts)? is it technically impossible or is it because maybe they were designed before it was possible?
It's possible, but due to the fact that the coil will have a very different impedance at the two frequencies, then completely different levels of current and voltage are needed at the two operating points, making it hard to do a single inverter that can do it. There are heaters that run at two frequencies at the same time, but that's mainly to tailor the heating pattern for things like case hardening of gears.
Post by: romy on February 24, 2023, 08:57:56 AM
The high frequency volts and amps in work coil are almost 90 degrees out of phase, so their product is measured in VAR, not in watts. Typically the VAR is many times higher than the power input from wall, and the power output to workpiece. It's because the coil and internal capacitor are operating at their resonant frequency. But VAR is not hard to measure, and can fairly be used to compare the possible heating ability of different induction heaters.
"The high frequency volts and amps in work coil are almost 90 degrees out of phase":
i thought at resonance in the work coil the effect of inductance and capacitance cancel out and the coil behaves as a resistive load. i can see how some shift occurs because in case we are not at perfect resonnance (low quality factor?), but 90°? please help me there.
ad (4a/c): im realizing this is a misunderstanding on my part. they are not changing the inherent resonant frequency of the oscillator. they are adjusting the frequency of the exiting input to match the former. it applies to bridge-circuits like below. the "zvs" circuits are self tuning. yes?
Post by: klugesmith on February 24, 2023, 12:32:59 PM
The combination of coil and capacitor behaves as a resistive load at resonance. Current in coil always lags the tank voltage; current in capacitor always leads the voltage. At resonance the reactive currents in L and C cancel.
Anders's excellent description puts some numbers on an example coil. I-V phase is very close to 90° with no workpiece, and slightly less close to 90° with workpiece.
Post by: romy on February 24, 2023, 08:07:46 PM
6) 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.
Post by: klugesmith on February 26, 2023, 02:04:25 AM
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.
Post by: romy on February 26, 2023, 05:15:44 AM
" 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?
Post by: Anders Mikkelsen on February 26, 2023, 03:52:49 PM
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!
Quote
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.
Quote
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.
Post by: romy on February 26, 2023, 06:59:21 PM
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?
Post by: Mike on February 28, 2023, 02:56:12 AM
Quote
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(Xl / R) combined with the definition of Q, i.e. Q = Xl / R
Post by: romy on March 02, 2023, 07:56:01 PM
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.
Post by: klugesmith on March 02, 2023, 09:41:34 PM
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.
Post by: klugesmith on March 03, 2023, 03:34:38 AM
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.
Post by: romy on March 04, 2023, 05:22:37 AM
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):
Post by: petespaco on March 04, 2023, 07:30:23 PM
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.
Post by: romy on March 05, 2023, 07:35:19 PM
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.
Post by: romy on March 07, 2023, 07:28:44 AM
/>
another channel i find interesting:
https://www.youtube.com/@Fluxtrol
Post by: petespaco on March 07, 2023, 07:30:11 PM
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.
Post by: romy on March 10, 2023, 10:27:36 AM
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.
Post by: petespaco on March 10, 2023, 05:13:44 PM
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.
Post by: romy on March 14, 2023, 02:03:42 PM
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?
Post by: petespaco on March 14, 2023, 05:01:44 PM
Re: induction cooktop:
I have no idea. But why would you short the loop anyway?
Post by: romy on March 15, 2023, 03:51:10 PM
- thinking i would be able to measure current.
Post by: petespaco on March 15, 2023, 11:53:29 PM
Quote
- please see (11).Re: please see 11: Ahha- so the highest voltage that was ever applied to the system was 32 volts, right?
- thinking i would be able to measure current.
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?
Post by: romy on March 16, 2023, 08:12:37 AM
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.
Post by: romy on March 27, 2023, 08:14:00 AM
Post by: romy on April 04, 2023, 07:46:44 PM
Handbook of Induction Heating, Second Edition. 2017
CRC Press;Cook, Raymond L., CRC Pr I Llc, Loveless, Don, Rudnev, Valery
Valery Rudnev, Don Loveless, Raymond L. Cook
Practical Induction Heat Treating, Second Edition. 2015
ASM International
Richard E. Haimbaugh
Induction and Direct Resistance Heating: Theory and Numerical Modeling, 2015
Springer International Publishing
Sergio Lupi, Michele Forzan, Aleksandr Aliferov (auth.)
Elements of Induction Heating: Design Control and Applications (06522G), 1988
S. Zinn, S. L. Semiatin
Conduction and induction heating, 1990
P. Peregrinus Ltd. on behalf of the Institution of Electrical Engineers
Davies, John
Optimal Control of Induction Heating Processes (Mechanical Engineering, 201), 2007
CRC/Taylor & Francis
Edgar Rapoport, Yulia Pleshivtseva
Heat Transfer Modeling: An Inductive Approach, 2015
Springer International Publishing
George Sidebotham (auth.)
Optimal Control of Induction Heating Processes, 2006
CRC Press
Zdenek Dostál
Basics of Induction Heating (vols. 1&2), 1960
John F. Rider
Chester A. Tudbury