Author Topic: Royer induction heater  (Read 2255 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.
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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
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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.
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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).

Offline petespaco

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Re: Royer induction heater
« Reply #49 on: August 22, 2017, 04:37:21 PM »
Crucibles:
  You might visit Banggood.com and search there for graphite crucibles to get an idea about prices.  You need to make sure that the crucible fits your coil, of course, so you need dimensions, not just the weight of material that you will be melting.
Watch this video for some information about my usage of crucibles:
/>You will see there, that I had to reduce power supply voltage to keep the current DOWN for a crucible that just barely fit into that work coil.

Regarding a temperature controller:
See this video where "radio mechanic" builds and tests a solder pot with a temperature sensor and a power supply relay:
/>
Pete Stanaitis
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Offline afk

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Re: Royer induction heater
« Reply #50 on: August 23, 2017, 03:45:23 PM »
Interesting video. I just don't know if the guy separated the gate input and the coil input? I plan to use the temperature controller for only the gate signal while keeping the coil input on (the other end of the two chokes on voltage). Since the gate signal is low, the MOSFETs are OFF, thus no current will conduct. This way I can get a general purpose relay at low voltage, low current to turn on/off the gate voltage. A general purpose relay is a lot cheaper than getting a contactor for higher current rating. I just don't know if I can leave the coil on voltage while cutting off gate signal. Theoretically there shouldn't be a problem but I can't really be sure of everything in practice.

It is also necessary to wire a RC snubber circuit in parallel with the contacts of a relay/contactor to suppress arc. The first relay in the video got welded because of that, and also due to the transient current surge I think.

Also, thanks for the banggood. It looks like the prices are a lot cheaper than what I could find here, only at around 50% without counting shipping fee.
« Last Edit: August 23, 2017, 04:05:44 PM by afk »

Offline kamelryttarn

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Re: Royer induction heater
« Reply #51 on: August 23, 2017, 04:11:05 PM »
That setup is basically my end goal :)

Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?

Offline afk

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Re: Royer induction heater
« Reply #52 on: August 23, 2017, 05:09:29 PM »
That setup is basically my end goal :)

Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?

I think that it should be possible. The second video in petespaco's post was using a temperature controller to control the gate via a general purpose relay and a contactor. It is just I don't know if I can separate the supply for the gate and the supply for the LC circuit, with LC circuit being on voltage all time as I switch on/off the gate. Theoretically it shouldn't pose any problem if the latching resistors just work out well. There is also the problem with the gate being switched on/off too frequent, and the surge at every ON.
« Last Edit: August 23, 2017, 05:11:28 PM by afk »

Offline Mads Barnkob

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Re: Royer induction heater
« Reply #53 on: August 23, 2017, 07:24:28 PM »
That setup is basically my end goal :)

Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?

I think that it should be possible. The second video in petespaco's post was using a temperature controller to control the gate via a general purpose relay and a contactor. It is just I don't know if I can separate the supply for the gate and the supply for the LC circuit, with LC circuit being on voltage all time as I switch on/off the gate. Theoretically it shouldn't pose any problem if the latching resistors just work out well. There is also the problem with the gate being switched on/off too frequent, and the surge at every ON.

I have used separate supply for gates and power electronics, with shared ground, that can be done.

However I would not turn off gate drive, unless by turning off you mean shutting it down, but still powered up to keep it at zero Volt, only then would I dare having the power electronics supply on all the time, if you turn the driver supply off, you do not know the state of the switches.
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Offline ldm314

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Re: Royer induction heater
« Reply #54 on: August 24, 2017, 12:18:09 AM »
I am using an ignition IGBT to turn a Royer on/off fairly quickly. So far it has been reliable and works at 42V only getting slightly warm. The IGBT is ISL9V5036P3 and is controlled by 5V.

Offline afk

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Re: Royer induction heater
« Reply #55 on: August 24, 2017, 04:04:46 AM »
That setup is basically my end goal :)

Wouldn't it also be possible to control the gates via some kind of logic gate circuit and mosfet driver circuit?

I think that it should be possible. The second video in petespaco's post was using a temperature controller to control the gate via a general purpose relay and a contactor. It is just I don't know if I can separate the supply for the gate and the supply for the LC circuit, with LC circuit being on voltage all time as I switch on/off the gate. Theoretically it shouldn't pose any problem if the latching resistors just work out well. There is also the problem with the gate being switched on/off too frequent, and the surge at every ON.

I have used separate supply for gates and power electronics, with shared ground, that can be done.

However I would not turn off gate drive, unless by turning off you mean shutting it down, but still powered up to keep it at zero Volt, only then would I dare having the power electronics supply on all the time, if you turn the driver supply off, you do not know the state of the switches.



Since it is only the gate drive is being turned off (B1 and B2 above being cut off from the circuit) while the rest (ground, power electronics) is still on, I believe that the 10kOhm pull-down resistors in the circuit will bleed the gate voltage down to ground quite quickly. The supply on LC circuit can stay on voltage (like 60V) while the gate signal being cut off via a general purpose relay.

Frankly, it is possible to get a contactor to turn off the power electronics as well, but a contactor is a lot more expensive than a general purpose relay. I also need a RC snubber that is more robust (hence more expensive) for a contactor, since the current surge and arc are much stronger. Instead of controlling a lot of current, just turning on/off the gate with little current would be more preferable. The only question is whether the cutoff is stable and there is no latching (or latching happens but mostly negligible before the pull-down resistors kick in). Also the question of current surge with a workpiece inside the coil when the gate signal is ON again.

It is also interesting to note that a temperature controller has a delay time of a few seconds before it can engage the general purpose relay. This delay time can serve well to stabilize my power supply once I activate the power supply.
« Last Edit: August 24, 2017, 04:07:54 AM by afk »

Offline petespaco

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Re: Royer induction heater
« Reply #56 on: August 24, 2017, 02:46:08 PM »
Regarding Radio mechanic's solder pot:
  As far as I can tell, he is NOT controlling the Mosfet gates with his relay.  He is turning the DC power to the induction heater on and off.

Pete Stanaitis
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Offline afk

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Re: Royer induction heater
« Reply #57 on: August 25, 2017, 05:40:19 AM »
Regarding Radio mechanic's solder pot:
  As far as I can tell, he is NOT controlling the Mosfet gates with his relay.  He is turning the DC power to the induction heater on and off.

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

Regarding that I can see it. If the relay had been used to control the gate signal the contacts would not have been welded together after only one use. Only a strong current drawn by the main power electronics can create an arc powerful enough. Also even with arc suppression a general purpose relay cannot endure that much amps (depends on type but at most only around 10A or so).
« Last Edit: August 25, 2017, 11:29:59 AM by afk »

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Re: Royer induction heater
« Reply #57 on: August 25, 2017, 05:40:19 AM »

 


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