Author Topic: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"  (Read 25273 times)

Offline davekni

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #260 on: November 25, 2019, 02:37:09 AM »
I could picture frequency rising if the copper shape changes significantly during melting, spreading out to a shape that blocks more of the magnetic field within the work coil.  If the frequency goes up, then the impedance goes down, not up.  The capacitors stay the same, so higher frequency comes from lower inductance, which is lower impedance.

Copper resistance roughly doubles as it melts.  That's the most likely explanation for power dropping.
https://www.sciencedirect.com/science/article/pii/S0022369717304341

At the start, before the copper heats significantly, the electrical conductivity of the copper being heated and the work coil are roughly the same.  That makes direct induction heating quite inefficient.  I don't know much about the graphite crucibles, what their electrical conductivity may be.  Conductivity may be too low for much heating from the induction field.  If the graphite is conductive enough to generate much heat, that will reduce as the copper melts and its shape fits the crucible inner wall.
David Knierim

Offline T3sl4co1l

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #261 on: November 25, 2019, 05:28:44 AM »
Did I say up, I meant down... ::)

In any case, it's a matching thing, always is.  If the load resistance is rising, you simply need fewer turns or more volts to match it, that's all.

Tim

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #262 on: December 02, 2019, 04:10:14 AM »
Some interesting recent comments, but still don't have a good handle on best frequency for melting copper in a graphite crucible.
   It certainly isn't all that simple, is it?

I have found a few "Induction Heating Calculators" around on the internet.  I think they probably do a fairly good job of estimating a few variables when it comes to iron and lower alloys of steel, but they seem to miss the target when getting into non-ferrous metals.
  For instance, here's one of the "calculators":
https://www.plustherm.com/power-calculation.html

It took me some time to figure out how it works.  (Note that you put in the whole number for the "Inductor Efficiency", not the percent as a decimal.  That is- if your material is  Aluminum, enter "40" not "0.40").

In the "Results" area, I am very dissapointed that it usually outputs an extremely low frequency in the "Minimum required frequncy" box.  Why doesn't it know what frequency it actually used for that particular calculation?
  And--- I am pretty certain that the results from their "minimum" frequecny and the ideal frequency would produce differing readings.

At first, I left the "Working Frequency" blank, so the calculator could choose, but even when I did enter a number, it didn't seem to help much.

After fooling around with the thing for a while, I realized that we aren't actually melting copper anyway with these 1800 watt to 2500 watt ZVS induction heaters, we are heating a graphite crucible!  Well, there's no graphite in the Material choices box, so now what?  I made a few guesses in the Inductor Efficiency box, estimating the graphite to be somewhere between "60" and "85", but no conclusive results.

It seems that some of the parameters in the  "Details" area not closely related to the input parameters at all.

What do you guys think?

Pete Stanaitis
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Offline T3sl4co1l

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #263 on: December 02, 2019, 10:51:33 AM »
Minimum frequency is just informational; note it's driven by the minimum size of material being heated, which is quite a bit smaller than the crucible if the crucible itself isn't doing the heating.

There isn't really an ideal frequency because any frequency works.  Nonmagnetic metals aren't more or less lossy with respect to frequency, they just have a resistance.  (Steel is notably more lossy below a few MHz, because that's around the magnetic relaxation rate for mild steel.)

This calculator does not produce any coil data, so it doesn't do anything with resistivity.  It's assuming you know how to design a coil and match it to a power supply.  Not very helpful I'm afraid.

It also doesn't ask for insulation thickness and quality, it's only using material for heat capacity; and in the solid phase at that (i.e., not including latent heat of fusion!).

Tim

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #264 on: December 24, 2019, 11:01:26 PM »
Cobalt in your future?
Many readers of this thread have asked about methods to melt copper faster.
  We typically use ceramic insulated graphite crucibles, which work pretty well, but
1. don't push the 2500 ZVS induction heaters to their power limit, even when on 48 volts
2. their graphite is comsumed during the process, which limits one crucible to about 10 "heats" before it gets so thin that it is dangerous to use.

So I was thinking that a steel crucible might melt copper a lot faster than do the graphite crucibles due to their magnetic/hysterisis characteristics.  Problem is that they lose their rapid heating abilities well below the melting point of copper.

I just realized that Cobalt has a curie point of 2067 degrees F, well above the curie point of iron  (1418 F, or thereabouts).  I wonder if there's a cobalt crucible in my future, but I won't lose any sleep thinking about it, since that metal costs about USD$75.00 per pound.   And, there probably aren't a lot of Cobalt crucibles on sale at Ebay.

Offline klugesmith

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #265 on: December 25, 2019, 01:39:02 AM »
I had a summer job in 1973, at a place which made lithium niobate for optoelectronic applications, & related single-crystal materials.
They grew boules from scratch, using graphite resistance-heated elements.  Imagine 0 to 10 volts at up to 1000 amps, 60 Hz three-phase, from conventional power transformers very close to the station.

Anyway, some of the  crystal materials were initially prepared from high-purity powders,
melted by induction heating in crucibles of pure platinum.
Crucibles got dented and battered from removing residual solid between batches.
Eventually they were weighed very carefully & traded in for new ones from the precious metal supplier.

Looks like Pt crucibles are still a thing, 46 years later.
http://www.platinumcrucible.com/
« Last Edit: December 25, 2019, 01:45:17 AM by klugesmith »

Offline rikkitikkitavi

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #266 on: December 25, 2019, 06:41:29 PM »
PT crucibles is da thing when used for dry ashing, ie heating to white hot to render all C N O H into gas, leaving only non volatiles left. These can be dissolved in fex nitric acid and analysed fex with ICP /AAS. Safer than using perchloric acid and nitric acid etc directly on the cellulose filters used for collecting water samples...
Fex heavy metal content in the blow down water on a nuclear reactor. Especially content of Fe-56,Co-60,Ni-58 and other activation products as a measurement of corrosion on the actual reactor itself.

Amazing that they let a summer worker do that kind of work, but they wear us down milli Sivert for milli Sivert :)

Sorry for Off Topic...
A man can not have to many variacs

Offline John123

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #267 on: December 25, 2019, 08:08:52 PM »
Anyone tried running a flyback LOPT on one of these? Bet it runs in the audible range with all those tank capacitors.

Offline klugesmith

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #268 on: December 25, 2019, 08:39:46 PM »
Pete's thread makes me question whether to buy an induction heater driver, or finish making one from scratch.  The latter project, stagnant for a few years, is just a collection of capacitors, documented ferrite toroid cores, and some copper for tank electricity and coolant conduction. Oh, and some Cerrobend (low melting-point metal) in case it helps when making tight coils of copper tubing.

Back to the tangent about Pt crucibles. RTT's account led to failed search for "fex" in that context.
Found FEX as a line of turbo expanders used in chemical plants.  Their blades might not run as hot as those in jet engines, but have to contend with process fluids that would be very corrosive even in an open crucible at room temperature.

Ref. this industrial service newsletter, with a drawing in Christmas colors. :-)
Axial flow with scroll cases on both ends! Maybe to share parts with other machines.
https://rotatingmachinery.com/pdf_files/newsletter-april-june-2014.pdf
« Last Edit: December 25, 2019, 08:53:04 PM by klugesmith »

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #269 on: January 02, 2020, 01:38:14 AM »
A little more  information for people who want to make specialized work coils:
I just updated my work coil spreadsheet.
It is here:
http://www.spaco.org/Blacksmithing/ZVSInductionHeater/Work%20Coil%20Data.xlsx

I added an area down toward the bottom that I call "guessitmates.  This is where I enter the data that I do have, usually  physical coil dimensions and measured (not necessarily actual) inductance.    As I have said many times before, if the coil's inductance is higher than about 1.0 microhenry, then the ZVS driver will run at a low enough frequency to avoid overheating of a properly cooled system.


Pete Stanaitis
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Offline Mads Barnkob

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #270 on: January 11, 2020, 12:28:35 PM »
Thank you Pete, for continuesly sharing your calculations and findings to optimize and get the most out of these units :)

I bought some other versions and sizes of the "Chinese induction heaters" and just finished looking at the 150W unit.

http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #271 on: January 12, 2020, 03:18:17 AM »
Thank you for your kind words, Mads.
   Nice demo of the little 150 watt unit.
Several members of our blacksmith club enjoy making things from horseshoe nails. This unit is just right for that.
If you power it from a 12 volt car battery (or appropriate rechargeable battery pack), you have a very portable heat source.

Salt spoons and finger rings are but two examples.
Works well for making tiny swords from double headed (duplex) nails, too.

Pete Stanaitis
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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #271 on: January 12, 2020, 03:18:17 AM »

 


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