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Messages - petespaco

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1
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
---------------

2
Effect of frequency on melting copper???

Recently, a guy asked me how to change his ZVS induction heater's frequency so he could keep the current up as the copper melted in a graphite crucible.  He has observed that the current drops at the moment the copper melts.  I think many of us have seen that effect.  My guess is that the molten copper shorts out the  carbide crucible or somehow reduces eddy currents in the graphite.  He must think that changing the frequency will get the current back up to where it was before the copper melted.  I don't think that's a viable option with these ZVS heaters.

Anyway:
A confession:
   I lack a practical understanding of the exact degree to which frequency affects the heating of copper and brass.
  After reading everything I can find on the internet, it seems generally accepted that higher frequencies are better for non-ferrous metals.
  That's nice, but it also appears to me that the people who are heating copper parts commercially almost always use induction heaters that have a lot more power than do the 1000 watt to about 2500 watt heaters that are the subject of this thread.
  Then there's the (apparently) very popular "15KW" induction heater of which this is one example:
(search this on Ebay)-
"220V 15KW 30-100 KHz High Frequency Induction Heater Furnace"
(Be warned, however, that this heater really only puts out about 7.5KW AND, you need a pretty good water cooler to use it!).

There are plenty of videos where the operator melts copper or at least heats some parts up for soldering or brazing.
  But we never, as far as I know, get to see what frequency the thing is running at when this takes place.


All that said, if appears to me that you need more than about 5KW to melt a kilogram of copper or more in a graphite crucible anywhere in the range of about 25 kiloHertz to about 100 kiloHertz.  Once you have a lot of power available, then you can start messing around with frequency, if your circuitry allows it.

Conclusion:
I don't have a good answer for that guy.  We can change the frequency of these ZVS heaters by changing the L (size and shape of the work coil) and C (adding or subtracting capacitors) in the circuit, but that's not what he thinks he  wants.  I told him that insulation and MORE insulation is his friend.

Your comments, please.


3
A positive thought for you guys who  are using multiple 12 volt power supplies:
 People are often asking me how to control the power that the  work coil transfers to the work of the ZVS heaters that I use....
If you are using a 48 volt power supply, as I do, then the two main ways to accomplish that is to either:
1. move the work closer to or farther away from the work coil until the current is at the level needed.
or-
2. Change work coil design to match the needs.
But, in many cases, the operator needs to adjust the power one direction or another as the work heats up.
An example of this would be when heating a piece of iron or steel-   Initially, if the work piece must be inserted into the work coil before the power is turned on, the current sucked up by the workpiece may exceed safe limits for the circuit, but once the part reaches the curie point, you need MORE POWER.
In this case, if you were to arrange your power supply switching so that you could start out with, let's say 24 volts and then switch to 36 volts or to 48 volts as needed, you'd have at least 3 power levels available.

Just a thought.   I am not saying that the switches would be simple, but it certainly isn't impossible.
And------- note that I did NOT suggest starting out with 12 volts.
I hope you all know why by now.

Pete Stanaitis
---------------

4
Just a Mosfet thought, relating to the recent posts about IGBT/Mosfet gate drive:
  I can't see ANY reason not to turn on these devices ASAP.  By that I mean that these devices, as far as I know, are SWITCHES and are NOT meant to be run in linear mode.  A main reason that they can handle such high currents is their low RDS(on).  So, if they are NOT fully on, very high amounts of power can easily be produced.
  Don't "baby" the gate.  Get it up to "full on' ASAP.  As previously mentioned, if the power supply can't hack it, get a better one. 

Pete Stanaitis
---------------

5
Quote
However, you need more current head room the lower voltage you are operating at. A 12VDC supplied homemade Royer with IRFP250N would get unstable below 10VDC, so it is far more prone to failure than a 36VDC suppplied Royer circuit from China.

Yes, I'd say that is exactly correct.

About 2/3 of the way down this page you can see 4 scope traces of gates turning on with differing input voltages from 12 to 48.
https://spaco.org/Blacksmithing/ZVSInductionHeater/1000WattZVSInductionHeaterNotes.htm

As far as the "core saturation" thing goes:
  I am no expert on transformers, but I have rewound a couple of microwave oven transformers and I have taken some data on their "regulation" or whatever you want to call it, under varying current  draws.
/>  In this video, I show two charts that demonstrate  transformer output voltage reduction as current increases.

Pete Stanaitis
--------------

6
Quote
When it comes to switching on and off

What will you be switching on and off?
What supply voltage?
what frequency?
what current?

Pete Stanaitis
---------------

7
Quote
For a PI control, I would not use a mechanical relay, but a solid state, a mechanical DC relay at this current would get worn out too fast.

What kind of insulation material did you use in that video?
------------------------------------------------------------------------------
I agree with using a solid state relay.  I have been looking at this one:
https://www.amazon.com/100A-Solid-State-Relay-SSR/dp/B00KO46YCU
but it is only rated for 32 volts.
I think they get pretty expensive when you get to 50 volts or so.

Insulation:
I use 8 pound density  one inch thick Kaowool that is rated for about 2300 degrees F service.
It is the material the we use in propane gas forges.  In that test I was simply tearing up small bits of it and stuffing them in beside the work.
If you search "kaowool" on ebay, you will find many sizes and thicknesses.
I see today that it is available in thicknesses down to about 1/4 of an inch.  I wish I had realized that when I bought another kind of "ceramic" insulation a few months ago. 
It was this stuff:
https://www.ebay.com/itm/INSWOOL-2300-PAPER-Flexible-Refractory-Paper-1-8-x-50-partial-rolls-also/131815404577?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
  I bought it because it is thin.  It insulates quite well, but this material has some sort of chemical binder that burns away leaving the ceramic material behind.  With the binder gone, the insulation has very little strength left and it falls apart if mishandled.

Pete Stanaitis
---------------


8
Hello Mads.
  You did a very nice job of packaging your 1800 Watt ZVS induction heater.
I like your idea of the big DC relay.  That should make it easy to add PID temperature control some day.

Several other induction heater experimenters have complained about ZVS heaters, saying that they do NOT start up reliably.  I have never had this problem.  Why do you think they say this?

I, too have been trying to get steel hotter than the curie point.  The only time it has worked for me is when I insulate between the work and the inside of the work coil, and cover the work, too:

You can see the high temperature part of this video at about 20 minutes in.

Pete Stanaitis
---------------

9
The making of induction heater work coils the way I do it and why-
  That's the focus of my newest ZVS 1000 to 2500 watt  12 to 48 volt induction heater video.
It is here:
/>
I'd be glad to entertain any comments, questions or criticisms.
  As they say somewhere:  "There's more than one way to skin a cat".
It's just the anyone who doesn't do it my way is wrong. <G>

Pete Stanaitis
---------------

10
Just to belabor the point about scoping the coil voltage:
I now remember that I was using my little DSO-112 battery powered scope early on when I had been taking readings.

Pete Stanaitis
---------------

11
Quote
Has there been any testing of the limits in terms of coil frequency with 48V 1,000W or 2,500W units?

Yes.
I have not found a lower frequency  limit yet, but lower seems to be better for the Mosfets.  Things still work well down to about 26 kHz.
It's the upper limit that I have spent most of my tine evaluating.

See my data and my opinions here:
https://spaco.org/Blacksmithing/ZVSInductionHeater/WorkCoilsForZVSInductionHeater.htm
--And click on the "Work coil, Observed" spreadsheet.

Pete Stanaitis
---------------


12
Quote
One question I still have; is it safe to touch the coil when running? I avoid it of course; but I've always been curious. You don't have to try that yourself of course...

I have touched the coil many times, but I would not recommend it.  After all, were see a few hundred volts across it, right? But, I am not dead.
I do touch the coil with "one hand in my pocket" from time to time just to verify that the water is cooling the work coil properly.  Occasionally, I have touched both coil leads at the same time WITH THE SAME HAND and I do feel an uncomfortable "tingle".
  I still think it is a DUMB idea, particularly since I already do have a temperature meter measuring water temperature on my radiators.
  I would NEVER attempt that if I had a board that was powered by 220 volt mains, even though the output ckt is "isolated".

Thanks for the scope "heads up", Mads.
  I should have realized that. 

Pete Stanaitis
---------------

13
Back on topic for the 12- 48 volt induction heaters, for a bit----
Here are two recent questions from one of my "pancake coil" youtube videos that I answered.  Hopefully I got the answers correct:

1st Recent question:
Hi, Is Coil Voltage also 48V or more than that?
If Yes, is there difference between 2500W, 1800W and 1000W?

Answer:
Yes, the voltage across the work coil is a lot higher than that of the power supply.
I just measured the voltage across a 12 turn 2 inch ID work coil on my 1000 watt heater with a multimeter and it is 220 volts rms.  I also looked at it with my old Tectronix 422 scope and then remembered why I don't pay much attention to the voltage across the work coil.  When I connected the 10X scope probe and turned on the power, I got a 300 volt peak to peak signal (at about 65kHz) that was approximately sinusoidal.  But, after about 30 seconds, the scope probe became too hot to touch, the insulation on the ground lead melted, and I almost burned it up.  The scope probe circuit's inductance (whatever it was) tried to act as a parallel work coil, but with only tiny conductors!!!
  I don't think these readings would change very much for the 1800 watt or 2500 watt heaters, but I have not measured them.  I really don't need to do that for my applications.
  For what it's worth, the experts say that the voltage across the Mosfets will equal
  the power supply input voltage X pi.

2nd Question:(In reply to my earlier answer)
I want to measure frequency with my multimeter it's up to 250V. I guess can't do with it.

Answer:
If your multimeter reads rms AC voltage, it should be okay.
 If you live in the USA, check the voltage at a  power outlet in your home.  If it measures approximately 120 volts, then your meter is reading rms.  If this is true (which is  quite normal for a multimeter) then you will be able to read the voltage across the work coil.
As always, use caution when measuring voltages this high.

(By the way, I find the posts about 220 volt AC input circuits interesting, but I that other readers realize the real dangers of working with power line voltages and currents.)

Pete Stanaitis
---------------

14

Those standoffs on the PCB have worked okay for me for quite some time, but I do use 1/4" OD copper tubing coils to connect to them and  I water cool the tubing.

Pete Stanaitis
---------------

15
For Bert911---
  Good that you have your induction heater running now.
What applications for this induction heater do you have in mind?
Can you tell us the frequency of operation and inductance of that pancake work coil?

Pete Stanaitis
---------------

16
Okay---  Here is the result of attaching the 10 inch OD "Pancake" work coil to the 2500 Watt ZVS Induction Heater and testing it:
/>
Pete Stanaitis
---------------

17
Quote
Probably just that the impedance is so high, from the high inductance.  Same overall dimensions but fewer turns, should be back with normal power levels I would think.
Regarding "should be back with normal power levels":
   For this 1000 watt heater, 22 amps at 48 volts IS the "normal" max. power level.

Regarding cooking with it:
   I think I'd rather buy an Ikea HOB (Hotplate Or Burner, as the English say) for USD$ 88.00.  It is clear the we CAN heat the bottom of a ferrous pan pretty well.

I will probably run this coil on the 2500 watt unit one of these days to see if and how the results differ.

Pete Stanaitis
---------------

18
Testing a large "Pancake" work coil.
   I just uploaded the test of my latest work coil.  It's a 10 inch diameter spiral or "pancake" coil.
It is here:
/>  There are lessons to be learned in this video, but I didn't find this coil to be as useful an everyday tool as I thought it might be.
I hope some of you  have some questions or ideas about this test.
At this point I think I have found the approximate range of frequencies that this 1000 watt heater can work successfully at.

Pete Stanaitis
---------------

19
Work Coil Lead Extension:
Short video on Swaging Copper Tubing is here:
/>
  Sometimes, when making a work coil, I don't make the leads long enough and have to add several inches to one or both ends.  Sometimes it's just laziness. I often don't premeasure the stock.  I just start with the  remainder of the tubing roll that I got from the hardware store and then cut the new work coil off that roll when I'm done.
  So sometimes I need to make minor adjustments to the length of coil leads to get them going in the right direction.  One way to deal with lengthening leads is to simply buy brass unions, with either flare or compression fittings.  Of course they cost several dollars each.  One could also use copper sweat tube unions, but they are hard to find for small tubing.  I even tried finding some 5/16" tubing to use for a union, with no success, even at a big box store!
---And, I have to go to the store to get them when I need them.
  So, in looking for alternatives, I found a few videos of guys swaging the end of the tube with any one of various tools.  The swage simply opens up the end of one tube so the other tube slides inside by about  1/4" and then you soft solder the joint.
  Well, I liked that idea but didn't want to spend anywhere from USD$ 20 to well over USD$ 100 to buy a swage or a swaging set.  So I made my own, just for the 1/4" tubing size that I usually use.

Pete Stanaitis
---------------

20
Thank you for your kind words, Mads.

Now I want to talk about a "Cheap" Frequency counter for the ZVS Induction heaters:

I am certain that most people who get into this  "World of  Electrical Oscillations" do have frequency measuring equipment.

I have an old Tektronix 422 Oscilloscope that I use to roughly measure frequency by counting  the number of cycles I see on the screen at a certain sweep rate.  I also have an inexpensive tiny battery powered DSO-112 touch screen  oscilloscope that has the ability to give me a digital readout of the incoming frequency (if I push the right spot on the screen at the right time and have my magnifiers on).

  That's not much in the way of frequency measuring equipment, but it's enough for my BENCH needs at present.

  However, my projects are often scattered about my shop and I can't always get my bench scopes to the unit under test.  And, the touch screen of the little scope isn't all that handy for big fingers when I am in a hurry.
  I currently have two different ZVS induction heater systems.  My testing has made me realize that it is important, for several reasons, to know the tank frequency at any given time, so I decided to dedicate a frequency counter to each machine so I'd instantly and always know the tank frequency.
  To that end, I went through an all-too-lengthy process to choose/build/connect "cheap" frequency counters to my heaters.  (I even built two extra sets while I was at it).
  Here is a webpage that I just put up which takes you through the whole process:
https://spaco.org/Blacksmithing/ZVSInductionHeater/MakingAFrequencyCounterForNoisyAndSineWaveInputs.htm

Pete Stanaitis
--------------

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