Author Topic: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"  (Read 20883 times)

Offline rikkitikkitavi

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #240 on: October 07, 2019, 08:24:21 AM »


I think the sole reason for unstable induction heaters is due to people using too small power supplies, most likely iron core transformers that are way too small, it gets overloaded, pulls the voltage down or saturates the core and boom goes the MOSFETs.

 
I am not following, how do you saturate a transformer by overloading it , ie taking to much current?
I totaly agree that if to much power is drawn, the rectified voltage on the caps drops and this leads to a very unstable operation of a Royerlike circuit, not unlikely that the high/low Mosfet latchup and crossconduct.

What is a reasonable power supply then?

@kamelryttaren- for a induction heater I would definiately look into the large gate capacitance on that. Those are probably best for a 12/24V switching converter.
But for a power supply on/off modulation to the converter it should work even if 75Vds is quite close to 50ish volts.
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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 #241 on: October 07, 2019, 08:46:10 AM »


I think the sole reason for unstable induction heaters is due to people using too small power supplies, most likely iron core transformers that are way too small, it gets overloaded, pulls the voltage down or saturates the core and boom goes the MOSFETs.

 
I am not following, how do you saturate a transformer by overloading it , ie taking to much current?
I totaly agree that if to much power is drawn, the rectified voltage on the caps drops and this leads to a very unstable operation of a Royerlike circuit, not unlikely that the high/low Mosfet latchup and crossconduct.

What is a reasonable power supply then?

You are right, overloading the transformer secondary side has nothing to do with core saturation, I wrote some gibberish there.

Since the Royer is super power hungry and f.ex. a specified 2kW unit can also draw exactly that, with changing size work coil and work piece, the current can suddenly be much higher than you expected. I would say that just to be sure, 25-50% current head room is preferable.

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.
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Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #242 on: October 07, 2019, 04:38:53 PM »
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
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Offline rikkitikkitavi

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #243 on: October 07, 2019, 05:09:36 PM »
Thanks, I got it. Sounds very plausible. And always very informative, thanks for the link.

So around 3kW rock stable server supply @51VDC should suffice for a 2kW Royer, I presume? And if not, I double that, haha :)
Assuming I keep it sufficiently away from the coil? No problems with induced voltages? AFIK they are quite shielded given you earth the chassis.

MOTs are due to cheapness almost always close to saturation (small core cross area and few primary turns) running with high magnetising currents.
I would only use them longer periods with a variac or similar.
They run hot in idle and hotter during load. I doubt many of them can take a full 230VAC max voltage according to spec without going into total saturation, therefore fuses...

Our kitchenette at the office smells of hot xformer insulation at around 1 pm every lunch...

They also have quite a high leakage, providing some current limit capability and therefore not so good regulation, ie they are not "stiff"
A man can not have to many variacs

Offline T3sl4co1l

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #244 on: October 07, 2019, 08:46:35 PM »
Yeah, you solve the regulation by knocking out the shunts.  (Which also can saturate, which happens under a heavy load.  Apparently they only need a soft (current-limiting) output over a certain range of load condition -- which is fine for a magnetron that drops some known voltage when forward biased, and if anything, the fault current under a short circuit would help by blowing the fuse faster.)

You can also solve the saturation by adding extra turns in the space where the shunts were; 10-20% more turns should do.

Tim

Offline kamelryttarn

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #245 on: October 08, 2019, 08:37:53 AM »
@kamelryttaren- for a induction heater I would definiately look into the large gate capacitance on that. Those are probably best for a 12/24V switching converter.
But for a power supply on/off modulation to the converter it should work even if 75Vds is quite close to 50ish volts.

I don't fully understand what you mean. I haven't paid too much attention to the gate capacitance yet but my power supply is close to 50V so I believe I am safe in regards to the Drain to Source maximum voltage. The gate signal will most likely be handled by a proper mosfet gate driver like MCP1407. The IXFN520N075T2 would be used to turn the induction heater on and off. I have other mosfets in the induction heater circuit. From my previous measurements I feel like time it takes for the trench fet IXYS mosfet module to fully conduct is probably shorter than the time it takes for the induction heater to start oscillating so I think that losses in the mosfet module will be reasonable.

When it comes to the power supply I am confident mine can supply 150A without sagging.

[sidenote] What is the "Kelvin Source ( Gate Return ) Terminal" mentioned in the picture?
« Last Edit: October 08, 2019, 12:49:22 PM by kamelryttarn »

Offline rikkitikkitavi

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #246 on: October 08, 2019, 09:06:54 PM »
I thought that the large FET would be not so good to use in the Royer oscillator as a switching element, but as a on/off switch instead. Just as you intend.
I totally agree that a mosfet with a rise time in 10s of nanos will fully conduct long before an oscillator at a couple of tens kHz ore slightly more is upp and running at full amplitude. Just as long as you smack the gate with a good drive signal.

I am interested in your induction heater, tell us more.
Låter intressant!.

Beefy supplies are always useful.
Ä
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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 #247 on: October 08, 2019, 09:15:28 PM »
It's probably better to hit it slowly (gate rising over 100s us?) in case there's a lot of inrush current.

Better still to put in a current measurement circuit and control the gate accordingly, but that's maybe not so big a priority with well behaved power supplies.

Tim

Offline rikkitikkitavi

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #248 on: October 08, 2019, 10:48:46 PM »
Nah, fast turnon. Let the power supply bleed :)

I think you want to raise voltage so fast before oscillator starts operating and it might show any kinds of unstable behaviour.
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Offline kamelryttarn

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #249 on: October 09, 2019, 09:56:00 AM »
Nah, fast turnon. Let the power supply bleed :)

I think you want to raise voltage so fast before oscillator starts operating and it might show any kinds of unstable behaviour.

My power supply almost scares me a little. The three phase 6kVA transformer is massive and the five parallel Kemet C4DEFPQ6380A8TK film-capacitors can supply an insane amount of peak current and I suspect that an unintentional short circuit would result in all sorts of smoke, flash and fire. My power supply is definitely NOT the weakst point of my setup.

I haven't done any work on my induction heater for about a year :( I bought some water cooling blocks for the mosfets but I haven't got around to putting it all together yet.

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #250 on: October 10, 2019, 08:58:36 PM »
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
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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 #251 on: October 11, 2019, 02:07:09 AM »
I mean you can go ahead and ignore the hesitance of a professional engineer but sure, damn the torpedoes right?

Tim

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #252 on: October 11, 2019, 04:45:55 AM »
Unless you are concerned about radiated EMI, I don't see any down-side to fast turn-on.  I'm still not clear if this FET is being used to switch the power input to a Royer oscillator, or is being used (a pair) to implement the Royer oscillator itself.  Either way, fast turn-on should be fine.  As a power switch, it's feeding the power-input inductors, so current rise will be ramped by the inductance and supply voltage, so not an instantaneous current load to the supply.  If within the oscillator, it's ZVS, so fast should be good there too.
David Knierim

Offline kamelryttarn

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #253 on: October 11, 2019, 08:51:11 AM »
It's probably better to hit it slowly (gate rising over 100s us?) in case there's a lot of inrush current.

Better still to put in a current measurement circuit and control the gate accordingly, but that's maybe not so big a priority with well behaved power supplies.

Tim

You are probably right but I don't have the "know how" to adjust switching time and to measure current at the speed probably necessary.

To better explain the intended use for the IXYS module I will use it between my power supply and the induction circuit. Mosfets inside the IH are NOT IXYS modules!

My power supply (as showed in another forum thread) is a 6kVA three phase transformer connected to 3+3 45L10/45LR10 rectifier diodes rated at 150A. After that I have a large choke from the ESAB welder the transformer came from and after that 5 pcs C4DEFPQ6380A8TK capacitors. The plan is to connect the IXYS module as close as possible to the ground rail of the capacitor bank of the power supply.

The water blocks are for the mosfets in the induction heater since these get quite hot and I will most likely water cool the gate resistors and the tank capacitors.

The main reason I need a robust switch to turn it on and a power supply that can supply a stupid amount of current instantly is that I must start it with the object intended to be heated already inside the work coil.

Offline T3sl4co1l

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #254 on: October 11, 2019, 11:33:19 PM »
Unless you are concerned about radiated EMI, I don't see any down-side to fast turn-on.  I'm still not clear if this FET is being used to switch the power input to a Royer oscillator, or is being used (a pair) to implement the Royer oscillator itself.  Either way, fast turn-on should be fine.  As a power switch, it's feeding the power-input inductors, so current rise will be ramped by the inductance and supply voltage, so not an instantaneous current load to the supply.  If within the oscillator, it's ZVS, so fast should be good there too.

If there are no capacitors then turn-on doesn't matter much, and a turn-on time of microseconds is fine -- this will help kick the oscillator into motion without generating much EMI.

Although it will take some time for the oscillator to start up, and I wonder if it should be ramped after all.  Cooking the oscillator at low amplitude and high voltage is going to put a ton of heat into the transistors...

Switching speed is adjusted by series gate resistance.  Rising and falling edges can be controlled by putting a diode in series with the one, and a resistor in parallel with the diode.  One direction sees the lone resistor, the other sees both in series.  Or use a diode each for independent control.

If the load is inductive (if there's no capacitors on there, it just goes right into the bias resistors and inductors), then rapid turn-off is likely to damage the switch itself.  At minimum a catch diode is suggested.

The biggest problem is fast turn-off.  Removing gate drive power shuts off the oscillator, and the inductors discharge into the transistor drains, which subsequently explode.  A TVS can be connected from GND to each drain, say a 1.5KE120 or SMCJ120A, which should absorb most of that energy before the transistor does.

Better is to have gate supply from a separate circuit so it stays always-on while the drain power is switched.

Tim

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #255 on: October 12, 2019, 06:16:12 AM »
Thank you for the clarification about FET use.  I do see that clearly in your earlier posts now, but was confused at some point.

BTW, don't see anyone answering your "Kelvin connection" question, so:  One source lead is for the high-current connection (source-to-drain current).  The other is for gate-drive return.  That way the inductive and/or resistive voltage drop of the high-current connection isn't added to the gate drive signal.  The two source connections are identical in this part (not typical for other packages).  However, you should use only one for the high-current path, and the other only for gate-drive return.  (Connect gate drive to gate and that second source terminal.)

As long as there's a catch diode from the drain to the incoming supply V+, and a good capacitor from V+ to V-, I'd go ahead with fast switching.  (Inductance of the diode/cap/FET loop needs to be LOW.)  If you need to slow down the switching, the simple way is to add a resistor in series with the gate.  Another small diode and resistor are often added in parallel with the main gate resistor to make turn-off faster than turn on.  If you need a more uniform voltage ramp on the output, adding additional miller capacitance (gate-drain capacitance) can help.  I've personally had trouble with RF oscillation when adding external miller capacitance, I think due to gate and source lead inductances, but it can work.  Any additional miller capacitance is a second step after inserting gate resistance.
David Knierim

Offline DICKEYBIRD

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #256 on: October 20, 2019, 10:34:22 PM »
Okay, I know this took forever from when I first mentioned it and till I put it all together last night :) The 1800W IH is now boxed up with all power supplies, water cooling, foot switch, current meter and insulated work coil.
Hi Mads, I enjoyed your video with the details about your putting the IH & support bits into an enclosure.  I noticed that you put a separate switch for each power supply & was curious why you did that.  I have the same modded PSU's (3) as you and have the mains power for all 3 wired through one switch.  I haven't finished the enclosure yet or actually powered up the the IH but I have powered-on the PSU's in series & then applied a test load of approx 200W for 5 minutes or so.  They seemed to be working OK.  Should I worry about not switching them on separately?

Thanks very much for all the good info you guys take the time to post here. :D
Milton from Tennessee ya'll

Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #257 on: October 21, 2019, 01:24:00 AM »
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
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Offline petespaco

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #258 on: November 24, 2019, 04:46:34 PM »
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.


Offline T3sl4co1l

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #259 on: November 24, 2019, 11:55:18 PM »
Guessing the frequency rises at the same time.  In other words, more magnetic field is being reflected than absorbed, when it melts.

The higher frequency means the work coil is a higher impedance, so draws less power, roughly speaking.  This could be compensated by switching in more capacitors.  Or running with more capacitors, but beware of controlling supply voltage to avoid burning up the transistors from too much current.

Tim

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Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
« Reply #259 on: November 24, 2019, 11:55:18 PM »

 


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December 03, 2019, 07:29:48 AM
post Re: HV resistor in oil
[Voltage Multipliers]
MRMILSTAR
December 03, 2019, 05:07:28 AM
post Re: HV resistor in oil
[Voltage Multipliers]
davekni
December 03, 2019, 04:34:46 AM
post Re: HV resistor in oil
[Voltage Multipliers]
MRMILSTAR
December 02, 2019, 09:06:07 PM
post Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
[Electronic circuits]
T3sl4co1l
December 02, 2019, 10:51:33 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
December 02, 2019, 05:49:05 AM
post Re: Help for people buying the "12-48 Volt 1800/2500 Watt ZVS induction Heater"
[Electronic circuits]
petespaco
December 02, 2019, 04:10:14 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
December 02, 2019, 04:09:37 AM
post Re: HV resistor in oil
[Voltage Multipliers]
davekni
December 02, 2019, 03:42:16 AM
post Re: HV resistor in oil
[Voltage Multipliers]
MRMILSTAR
December 01, 2019, 10:47:18 PM

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