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Author Topic: Royer induction heater  (Read 1725 times)

Offline afk

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Re: Royer induction heater
« Reply #40 on: July 27, 2017, 01:55:19 PM »
IIRC Mads or somebody said that 10000µF or so should be able to support 20 amps... Since I'm aiming for 60 amps I went all out for the filter. Having more doesn't hurt for me, since I can't afford having strong spike at start that cause the voltage to drop too much. I got the 1000µF/450V for $1.53 each... I can't find any high voltage cap that has higher capacitance.

Big ripples shouldn't be much of a problem I think, as long as the voltage is well above 12V (iirc the minimum working voltage, I once tested a ZVS heater at 12V before). The issue is that the starting spike can cause a big voltage drop that can latch both MOSFETs and short the drain to the ground, risking a big blow (I once got this which dropped my 20V to 8V and fried up my circuits). The transient current can damage your cap bank when you start the circuit, which is also a factor. If you are using tri-phase transformer it shouldn't be much of a problem since the initial ripple of the rectified input is fairly small compare to mine, but you should calculate a bit.

I planned to use 470 Ohm for the gate resistor but I couldn't find any at 5W at the store, so I decided to get 630. Apparently it works at 20V and obviously it still works at 60V, which is my application. The gate resistor is to limit the current charging the gate, so having it small can overload the gate. It depends on your MOSFETs IMO, but I played it safe and chose 630.

I'm disassembling the circuit to tweak my coil. I'll try to take gate signal when I finish.
« Last Edit: July 27, 2017, 01:59:52 PM by afk »

Offline kamelryttarn

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Re: Royer induction heater
« Reply #41 on: July 27, 2017, 02:32:54 PM »
I thought that the zener was fast enough to make sure the gate would not overload. My maximum voltage is 37VDC and my zener's will drop about 14V so 100 Ohms should make the rise of the gate voltage much faster I think.

Transient current will definitely not be an issue for me as the caps are rated at 5kA peak each and distrubute the load over 5 of them :D

Looking forward to seeing some gate waveforms and compare to mine as soon as I get my PSU together

Offline Mads Barnkob

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Re: Royer induction heater
« Reply #42 on: July 27, 2017, 11:53:26 PM »
Really great results when it finally got all the power it could get at a higher voltage, the losses are really massive, 33% is high, maybe if you tried lower gate resistors to turn on faster, but only if your gate voltage allows it, or you could also add a separate gate voltage supply so that its independent of the supply for LC circuit. How is the MMC holding up temperature wise?

You are right that tap water will perform some electrolysis, but if this is just hobby work and not something that has to run 24/7, do not worry about it. Distilled water for batteries/ironing clothes can be bought in almost any super market in Denmark, don't you have the same options?

Your current transformer looks like its 1:20 ratio, so if you have 100A in the LC circuit, that would give you 5A * 0.5 Ohm = 2.5Vpeak sine wave on you scope, the current transformer will be floating, as you just measure the voltage across the resistor, it does not have any polarity important to ground. Just remember that a current transformer must never be used open loop, its so easy to forget a CT in a test setup, it will destroy itself while attempting to reach infinity Volt.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline afk

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Re: Royer induction heater
« Reply #43 on: August 04, 2017, 03:31:05 PM »
So I just tweaked around with the coil (making it smaller diameter-wise and adding one more turn). I also increased the number of caps, from 11 caps to 15 caps to achieve 4.96µF. I first ran the machine on 22V. As usuall, yellow and blue are the two branches of the work coil, while the red is their difference - the tank voltage.


With f = 38.91kHz I got L = 3.37µF which is quite high. Probably I should get one or two turn less later. The input voltage stayed on 21.36V so having 139V in the tank circuit was well between the estimation.

The idle consumption rose to 2.667A from 2A before, which is a bit undesirable. However the working consumption also raised up:
  • Heating up the door hinge: current rose from 7.73A to 12.93A
  • Heating up 8 steel screws: current rose from 8.4A to 13.73A

So probably the wasted energy has reduced (but not so much). I should liberate one turn or two to see how it fares later.

Anyway this is the graph I got when trying to heat up 8 screws at 22V; I measured the temperature while having a stopwatch. Thing is, the thermometer I have can only go to 250°C, but frankly at 22V it is insufficient to bring the steel screws to red hot. However it should be good for tempering at 300°C or so.


Anyway, it is time to try out the circuit in 60V+. When idle, I got pretty much the same thing except for the tank voltage. The machine also heated up immensely. I need to buy some distilled water next week. The next two are the gate voltage (yellow) and its corresponding drain (blue).



I got 62V input for 8.8A idle current (rose from 8A) which is as expected. Tank voltage was 392V. All good. Drain voltage should be at 12V.

And finally the performance of the induction heater when heating up one steel bolt:

Up close and personal:


The machine was running at 60.34V/12.267A which is very, very inefficient but I'll do another experiment with more pieces to heat up. However it only takes around 35s for the bolt to start getting red (which should be around 550-600°C), and judging from the brightness and color of the bolt it should reach 800-900°C.
« Last Edit: August 04, 2017, 04:22:48 PM by afk »

Offline petespaco

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Re: Royer induction heater
« Reply #44 on: August 04, 2017, 05:58:30 PM »
Hello, afk.
  You said that "The machine also heated up immensely."  Tell us more about this.

-Mosfet or IGBT heatsink temperatures
-Tank capacitor temperatures, and, did the temperature vary much between capacitors
-Temperature of the cooling water.  Talk about your method of moving the cooling water through the system and what you are doing to cool the water
-temperature of anything else that got hot.
-Did any of these temperatures stabilize, or did they keep climbing?

Tell us about current flow changes as the metal heated toward the curie point.

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

Offline Mads Barnkob

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Re: Royer induction heater
« Reply #45 on: August 06, 2017, 08:17:41 AM »
Thank you for sharing all the oscilloscope measurements, it all looks very clean, stiff and stable. I might just be wrong or my own Royer was much worse, but I seem to recall that the gate waveform was not as nice as yours.

Have you tried heating a large diameter water pipe? Just to try something with a larger diameter to get a better coupling to the work coil. I think that bolt is simply too small to be effectively heated in such a big coil.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline afk

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Re: Royer induction heater
« Reply #46 on: August 06, 2017, 10:35:06 AM »
Hello, afk.
  You said that "The machine also heated up immensely."  Tell us more about this.

-Mosfet or IGBT heatsink temperatures
-Tank capacitor temperatures, and, did the temperature vary much between capacitors
-Temperature of the cooling water.  Talk about your method of moving the cooling water through the system and what you are doing to cool the water
-temperature of anything else that got hot.
-Did any of these temperatures stabilize, or did they keep climbing?

Tell us about current flow changes as the metal heated toward the curie point.

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

My mechanical design is temporary so there are a lot of drawbacks when it comes to surveillance of the machine... Measuring temperature is quite hard. I'll get down to the details once I overhaul the design again.

On the other hand, both MOSFETs and tank caps were below 90°C in the video... On extended usage that will be quite a problem, but so far there isn't any degradation yet. Work coil, however, could reach 200°C so there is a dire need for water cooling... I'm gonna buy some distilled water next week along with some graphite crucibles. I plan to freeze a part of water so that it can keep the cooling water at good temperature.

Regarding crucibles I'm thinking about DIYing some steel crucible instead. Steel can reinforce the heating before Curie temperature but the problem is to find a steel can with correct shape. Else, graphite crucible should be good enough.

Thank you for sharing all the oscilloscope measurements, it all looks very clean, stiff and stable. I might just be wrong or my own Royer was much worse, but I seem to recall that the gate waveform was not as nice as yours.

Have you tried heating a large diameter water pipe? Just to try something with a larger diameter to get a better coupling to the work coil. I think that bolt is simply too small to be effectively heated in such a big coil.

The graph might not reflect correctly what is going on, actually. I used average sampling to get the measurements. The signals weren't this stable and were fluctuating quite a bit. The efficiency increases if I put a bigger workpiece, that is pretty much no-brainer already. I just wanted to test the machine with that bolt. I'll do with a bigger object but I have to make the machine neater first.

Offline petespaco

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Re: Royer induction heater
« Reply #47 on: August 08, 2017, 04:41:52 PM »
I suggest that you be very careful when you insert a graphite crucible or a large iron tube for the first time.  The current will increase dramatically and you might instantly blow your output Mosfets.  I think I'd prefer the graphite crucible over a steel crucible. I have tried a couple of graphite crucibles and they do couple pretty well, and, of course, aren't subject to the Curie point issue.  Get some fan cooling on the electronic components before you proceed.  So far you haven't shorted any turns on your work coil, so I suggest you insulate it electrically before you put larger work pieces in it.

Offline afk

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Re: Royer induction heater
« Reply #48 on: August 18, 2017, 03:56:05 PM »
I suggest that you be very careful when you insert a graphite crucible or a large iron tube for the first time.  The current will increase dramatically and you might instantly blow your output Mosfets.  I think I'd prefer the graphite crucible over a steel crucible. I have tried a couple of graphite crucibles and they do couple pretty well, and, of course, aren't subject to the Curie point issue.  Get some fan cooling on the electronic components before you proceed.  So far you haven't shorted any turns on your work coil, so I suggest you insulate it electrically before you put larger work pieces in it.

Actually I don't think Curie point is actually an issue... Being magnetic below Curie point, steel heats up a lot faster so this can save time for heating (and somehow reducing the heat loss). But well, if graphite crucible works well, I don't mind about that.

I want to ask about the cost of the crucible. Here I found a retail company selling one that is used to melt 1kg gold for $13.2. Is that price good, giving that it is imported product?

Another topic: I'm thinking of making a secondary circuit using a temperature controller to turn on/off the gate voltage via an electronic switch like a relay. I want to consult whether this is feasible and not blowing up everything. Transient current when turning on with workpiece in the coil can somehow be damaging to the MOSFETs, and switching on/off the gate continuously might risk latching both sides' gate at the same time (though the temperature controller usually has a hysteresis to avoid turning on/off continuously).

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Re: Royer induction heater
« Reply #48 on: August 18, 2017, 03:56:05 PM »

 


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