Author Topic: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"  (Read 20910 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

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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 »

 


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