High Voltage Forum

General electronics => Electronic circuits => Topic started by: afk on April 09, 2017, 04:57:35 AM

Title: Royer induction heater
Post by: afk on April 09, 2017, 04:57:35 AM
I HAVE REWORKED THE CIRCUIT PLEASE CLICK ON THIS LINK TO HEAD TO THE NEWEST CIRCUIT: https://highvoltageforum.net/index.php?topic=46.msg540#msg540

Hi guys,

First time coming here, though I have been in Kaizerpowerelectronics blog before.

Since it will be pointless to make another thread just for the same induction heater, I'm borrowing kamelryttarn's thread to post about my machine. If you don't mind, that is (I'll do another thread if you do mind).

First of all I have to thanks Mads for his advice on using multiple pairs of MOSFETS to deal with high-power circuit. I have made a prototype, and it worked out well:
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F1oWhjgo.jpg&hash=9c867294fba84742d11730b75b696c6f)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FKbw9kCK.jpg&hash=75afb127ee58e73615b827bedd60078a)

However, the consumption rate is bad. Even at no load, it consumed approx. 16A for 15.7V input (dropped from 18V). When I put a screwdriver in, the input current rose to  17.47A. Putting in a piece of stainless steel (3x6 cm²) it became 24A. Basically, the wasted energy into heat is just too big.

I assumed that the soldered joints and copper traces burn out a lot of energy, so I decided to redo the circuit (simpler, and wider traces for high-load). Also, getting a bigger work coil.

But for this time, it does not oscillate. I fried 3/4 MOSFETs when it failed to oscillate, and had to redo the circuit (again). Still no oscillation. Just in case, I only make the newest circuit to run under voltage for around a few seconds to observe the problem.

It seems that the input voltage has dropped very low, around 7.5V (from 18V) at 20A consumption. Something must happen badly with the power supply, so I'm posting it all here as well.

The transformer is one I wound by myself. It is very big (enough to be put inside a computer case) and personally I think it can support at least 2kW:
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F9vu1fgk.jpg&hash=0cab9328cb9cd450c815532ebbce32de)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FDpAoWkY.jpg&hash=964bdfb9a43a413ae6d0b2f29e0f9889)
Little final touch:
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F2f3RclY.jpg&hash=9d7f9d45b0b4d81e591f2dce3e644377)

Assembled with rectifier diodes. You can see below the two square components (the bridge rectifiers) that can deal 1kV-50A each. I used two so that it can work well to 100A (I'm using the transformer at 40A as my aim, btw). To protect the transformer I also added a 20A anti-leakage circuit breaker for the primary coil.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FyKmUjTB.jpg&hash=8192bc14af8c3951c0f42298b68d9809)

Basically I'm confident that my transformer can support a lot of current even for 18V secondary. I'm, however, doubting the filter I used.

I'm using capacitor-input filter (pi-filter with 1 capacitor bank + 1 coil + 1 capacitor bank) to increase the filtering efficiency. You can see the big-ass coil of 48mH underneath the capacitor bank (actually two banks of around 7000µF each I combined on one circuit board).
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FbNKDVhg.jpg&hash=987bb8fecd79778e606df9149ef61351)

I assume that with bigger coil the circuit will need stronger power spike to start up, and with that it overloaded the filter. Do you think that it is the case with mine?

EDIT: To OP: Where did you buy (or find) the choke? I'm trying to find a big one like that but it is hard to get one. If you bought it, how much did it cost? And btw, you are using the two chokes in the induction heater, or they are for the power supply?
Title: Re: Royer induction heater
Post by: Dylan Burger on April 09, 2017, 07:55:15 AM
Hi guys,

First time coming here, though I have been in Kaizerpowerelectronics blog before.

Since it will be pointless to make another thread just for the same induction heater, I'm borrowing kamelryttarn's thread to post about my machine. If you don't mind, that is (I'll do another thread if you do mind).

First of all I have to thanks Mads for his advice on using multiple pairs of MOSFETS to deal with high-power circuit. I have made a prototype, and it worked out well:


However, the consumption rate is bad. Even at no load, it consumed approx. 16A for 15.7V input (dropped from 18V). When I put a screwdriver in, the input current rose to  17.47A. Putting in a piece of stainless steel (3x6 cm²) it became 24A. Basically, the wasted energy into heat is just too big.

I assumed that the soldered joints and copper traces burn out a lot of energy, so I decided to redo the circuit (simpler, and wider traces for high-load). Also, getting a bigger work coil.

But for this time, it does not oscillate. I fried 3/4 MOSFETs when it failed to oscillate, and had to redo the circuit (again). Still no oscillation. Just in case, I only make the newest circuit to run under voltage for around a few seconds to observe the problem.

It seems that the input voltage has dropped very low, around 7.5V (from 18V) at 20A consumption. Something must happen badly with the power supply, so I'm posting it all here as well.

The transformer is one I wound by myself. It is very big (enough to be put inside a computer case) and personally I think it can support at least 2kW:

Little final touch:


Assembled with rectifier diodes. You can see below the two square components (the bridge rectifiers) that can deal 1kV-50A each. I used two so that it can work well to 100A (I'm using the transformer at 40A as my aim, btw). To protect the transformer I also added a 20A anti-leakage circuit breaker for the primary coil.


Basically I'm confident that my transformer can support a lot of current even for 18V secondary. I'm, however, doubting the filter I used.

I'm using capacitor-input filter (pi-filter with 1 capacitor bank + 1 coil + 1 capacitor bank) to increase the filtering efficiency. You can see the big-ass coil of 48mH underneath the capacitor bank (actually two banks of around 7000µF each I combined on one circuit board).


I assume that with bigger coil the circuit will need stronger power spike to start up, and with that it overloaded the filter. Do you think that it is the case with mine?


If it is drawing an insane amount of current at no load - you have too little turns on your work coil and/or your tank cap is too small/big.

Mads should be around soon to tell you about the thread thing.

If you'd like you can take a look at the troubleshooting guide at the bottom of my thread on the mazzili driver. Best of luck! and let me know how things go :)
Title: Re: Royer induction heater
Post by: kamelryttarn on April 09, 2017, 09:54:07 AM
afk, that is one HUGE toroid transformer. I'm amazed that you wound it yourself! Nice work.
Title: Re: Royer induction heater
Post by: afk on April 09, 2017, 10:57:29 AM
afk, that is one HUGE toroid transformer. I'm amazed that you wound it yourself! Nice work.

I got the core for around $44 (biggest spending for my project), and also need quite a lot of wires to wind it. It is a shame that I don't have much experience in transformer myself, so I did it mostly by consulting on Internet or people around me.

Winding it is a pain as well. The primary is still okay-ish (2.3mm-diameter wire), but for the secondary, it is 2.5x7mm² square wire, which is 1) thick af and 2) insulated by fiberglass, which can cause you a hell of itch. I added another secondary output at 14V for fan if my machine needs one for cooling.


If it is drawing an insane amount of current at no load - you have too little turns on your work coil and/or your tank cap is too small/big.

Mads should be around soon to tell you about the thread thing.

If you'd like you can take a look at the troubleshooting guide at the bottom of my thread on the mazzili driver. Best of luck! and let me know how things go :)

Hmm... I'm trying to tackle with my coil a bit... Still, I'm having a 6-turn, 10-cm-diameter work coil but it is currently not running. The old coil that worked had 5 turns with 7 cm of diameter, but the consumption rate was bad.

It is hard for me to find better tank cap. The best one I can find here is MKV cap (which is, honestly, heating up fast and that worries me). I don't have much experience in finding good cap myself... There is a retailer here that sold power cap (used for actual induction heater) but apparently it was not selling well, so currently they did not stock up the cap...
Title: Re: Royer induction heater
Post by: Dylan Burger on April 10, 2017, 02:11:25 PM

Hmm... I'm trying to tackle with my coil a bit... Still, I'm having a 6-turn, 10-cm-diameter work coil but it is currently not running. The old coil that worked had 5 turns with 7 cm of diameter, but the consumption rate was bad.

It is hard for me to find better tank cap. The best one I can find here is MKV cap (which is, honestly, heating up fast and that worries me). I don't have much experience in finding good cap myself... There is a retailer here that sold power cap (used for actual induction heater) but apparently it was not selling well, so currently they did not stock up the cap...

First off- if anything fails in this driver - it would be a good idea to check all other components too. Your transformer is probably fine. Just make sure your Zenner diodes arent shorted and that your ultrafast diodes are ok. Next step would be mosfet replacement. Then you can test tank cap - charge it up to 30v and see if it starts to discharge quickly. If it can hold its charge (and its capacitance always reads the same) then its good.

Alright , It may be that your frequency is inappropriate for your switches or if your driver isn't working at all anymore your zenner/fast diodes have blown. If that is the case your FETs will also be damaged. a multimeter on diode test can quickly ensure that everything is working and not shorted out.  If it only works with one coil, then i would check that it is not pushing the frequency up too far.

I see you have ~80Khz resonance - Thats not bad. Sometimes a bad cap can start to blow up mosfets. What size is your capacitor and how much capacitance does it have ?
Title: Re: Royer induction heater
Post by: afk on April 10, 2017, 04:27:06 PM
First off- if anything fails in this driver - it would be a good idea to check all other components too. Your transformer is probably fine. Just make sure your Zenner diodes arent shorted and that your ultrafast diodes are ok. Next step would be mosfet replacement. Then you can test tank cap - charge it up to 30v and see if it starts to discharge quickly. If it can hold its charge (and its capacitance always reads the same) then its good.

Alright , It may be that your frequency is inappropriate for your switches or if your driver isn't working at all anymore your zenner/fast diodes have blown. If that is the case your FETs will also be damaged. a multimeter on diode test can quickly ensure that everything is working and not shorted out.  If it only works with one coil, then i would check that it is not pushing the frequency up too far.

I see you have ~80Khz resonance - Thats not bad. Sometimes a bad cap can start to blow up mosfets. What size is your capacitor and how much capacitance does it have ?

As for the first try on the new coil, I burnt through 3 FETs and a few diodes. Most of the rest were still safe, but to be sure, I replaced all diodes and the remaining MOSFET as well.

With the second try, it is still not oscillating, but everything still looks okay (I did a quick measurement of resistance between important nodes and compared the values before and after the test). Within 5 seconds running, I checked that the source voltage dropped to approx 7.5V from 18V (before it was 15.7V), drawing approx 20A of current. I assumed that the problem lies in CLC filter, notably the capacitor banks could not hold the voltage high enough during the startup, so it fails to oscillate. I used 17 electrolytic capacitors of 1kV-1mF (though they aren't at good quality: most of them are 750µF and 900µF), creating two banks of around 7mF for the CLC filter. I'm planning to buy even more capacitors as well.

Tank cap is 16x0.22µF MKV cap and I measured 4.33µF (I assumed one cap got disconnected from the bank, I'll resolder the joints later). Since with this tank cap I was able to make the circuit oscillates and the capacitance didn't drop so much, I think it is still working. Though, I'll need better cap if I'm aiming for 2kW heating output.
Title: Re: Royer induction heater
Post by: Dylan Burger on April 10, 2017, 04:55:16 PM

I don't have any scientific knowledge on these drivers, only knowledge from experience. I can say that if the tank cap is heating, and your getting a high frequency from such a large tank capacitor, then your driver is basically focusing on pumping power into the cap. I theorize that it has to do with impedance. You want an equal load across caps and coil, so that you get a good resonant rise and so that your caps (or coil) doesn't suck up all the power.  That's just my theory anyway, so anyone who has proper knowledge is more than welcome to completely discredit what I have said.

If you are pulling 83 KHz from a 4.3uF tank cap, then you have a very low inductance primary coil. It makes sense that current draw is that high. If you would like to lower the current draw you can add a turn or two.


If 16x0.22 uF capacitors are heating, then there is a lot of tank current flowing (which makes sense since you are drawing 20A from your supply).  I would try adding a turn or two and see how that goes. You want it to not draw much when nothing is in your work coil (maybe 5A to 7A). Also keep an eye on your tank voltage- higher tank voltage means a stronger electric field in your coil. Which also means easier heating of objects.  For 2Kw induction heating you'll need low resistance caps (low ESR rating I think?) , or high RMS current rated caps. But I think if you can balance your tank current out between the work coil and  the caps you'll be fine.


Also, if zenner diodes keep blowing up, make em 18v 1W zenners. I've never had a zenner fail since :)
Title: Re: Royer induction heater
Post by: kamelryttarn on April 10, 2017, 04:57:34 PM
Are you sure your psu choke isn't part of the problem? 48mH is a LOT! My ginormous choke has an inductance value of 1.8mH. Maybe the DC-resistance in your choke is too high causing a voltage drop when you start to load it?

A choke that has 26 times the inductance of the one I got but is one tenth in size has got to have a lower current rating, thinner wires and therefore much higher voltage drop at high current. (I think... I'm no expert)
Title: Re: Royer induction heater
Post by: Dylan Burger on April 10, 2017, 05:02:15 PM
Are you sure your psu choke isn't part of the problem? 48mH is a LOT! My ginormous choke has an inductance value of 1.8mH. Maybe the DC-resistance in your choke is too high causing a voltage drop when you start to load it?

A choke that has 26 times the inductance of the one I got but is one tenth in size has got to have a lower current rating, thinner wires and therefore much higher voltage drop at high current. (I think... I'm no expert)

This is a very valid point. I did not consider that the resistance might  be inhibiting the supply power.
Title: Re: Royer induction heater
Post by: kamelryttarn on April 10, 2017, 06:41:06 PM
Even a DC resistance as low as 0.5 Ohm (500mOhm) constitutes a voltage drop of 10V at 20A. It doesn't take much when you are working with currents like these.
Title: Re: Royer induction heater
Post by: afk on April 11, 2017, 01:35:44 PM
Hmm? The choke has to be around 45mH or else I'll have to buy even more capacitors for the CLC.

The cutoff frequency of my CLC filter is approx 9Hz with approx 45mH choke and approx 7mF cap banks. The grid is at 50Hz and I used full-wave rectification.

I can actually liberate a few turns, but then I have to invest more on capacitors. Reducing inductance by a half, I will have to compensate by doubling the amount of capacitors. It is pretty hard to even find low-quality capacitors with high capacitance (as you can see, I bought 1mF caps, but their actual capacitance is only at 75-90%).

I wound it with 2.5x7mm² square wire (the choke is beneath the capacitor banks in the last image), so it is plenty even for 50A usage. Wires are insulated by fiberglass which is fairly resistant to heat, and also, since it uses a lot of E-T sheets the heat dissipation should be sufficient for a dozen minutes working.

I prefer to reduce the number of turns, though. Less turns will reduce inductance loss at high current (or so I read from various forums and documents). But then I don't want to spend so much money on capacitors. Even the wires I used for the choke are the leftover of my transformer. It is such a dilemma, haha.
Title: Re: Royer induction heater
Post by: kamelryttarn on April 11, 2017, 07:48:42 PM
You seem to have given the psu filtering a lot more thought and attention than I have so far, but ripple frequency was one of the reasons I opted for a three phase setup for my power supply. You can get away with a lot smaller capacitor bank that way. I will build a LC filter with my choke and a suitable capacitor bank.
Title: Re: Royer induction heater
Post by: afk on April 13, 2017, 12:43:06 PM
I have a small question regarding the toroidal cores for the two chokes in the driver.

I just read a few articles on the net and I found out that the mix in the core can affect the performance of the chokes. And you can determine the freq band that the core can work based on its paint color. I'd like to know more about this, in case if it is true.

I'm having two green cores, but they are quite small so I'm thinking of purchasing something bigger, so that I can use bigger wire.
Title: Re: Royer induction heater
Post by: kamelryttarn on April 13, 2017, 12:59:08 PM
AFAIK there is no actual color standard that every manufacturer adheres to. Micrometal for example has a color standard so as long as you buy an original Micrometals core you can determine the core material based on the color but you can not necessarily use the same information to decide the composition of a core from another manufacturer. I bought two Micrometals T300-60D-cores. I would have preferred T400-60D but they were harder to find and quite expensive. The T300 was a good compromise that I think will be sufficient for my experiments. At these power levels the T300-60D is pretty much the smallest core you can use according to their software.

http://www.micrometals.com/downloads/MicroRelease_March2010.exe
Title: Re: Royer induction heater
Post by: Mads Barnkob on April 14, 2017, 10:46:10 AM
You do have a oscilloscope, so you should wind yourself a cascaded current transformer (two ferrite ring cores with each 33 turns, 1 looped into the other and that gives 1:33:33 ~ 1:1000 turns ratio, so 1000 A through the first gives 1A out, then you can adjust shunt resistor for the voltage desired) to measure the primary circuit current, it could be that you actually have 200 Ampere flowing there and that explains your large current draw from the power supply.

It sounds like kamelryttarn has a good point in the large inductance of your filter, I never used a L part of the filter, just 4x 10000 uF in parallel. This circuit is paramount to fail at too low supply voltage and that is the true killer and also why this circuit sucks, you never know what happens at startup and you risk blowing it all up :)

About ferrite markings, there is no standard at all and there are numerous different names for the "same" materials as different manufacturers have copyrighted their own branding names. Micrometals does a two colour identification but mostly I have uncoloured cores that just has a small writing with the material type like "3C85" or "3F3".

I have a lot of ferrite cores for sale, check my thread that I'll make tomorrow :)


Title: Re: Royer induction heater
Post by: afk on April 22, 2017, 03:48:44 PM
Since I'm still looking for filter cap and better toroid for the two chokes, I'm posting the schematics of the driver circuit:

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FoGUQtMU.png&hash=6dbbfd2bd0506b7be78287fbbb9efd27)

I'm using double MOSFETs schematics so that it can support higher current consumption. I have tested its functionality before with 12V and IRF250N before and it did work, but just for confirmation whether it needs some tweaks.
Title: Re: Royer induction heater
Post by: petespaco on April 23, 2017, 07:42:38 PM
Are you actually using 3 different power supplies?

Do you plan on running the device on only 24 volts?

What maximum amperage/wattage are you expecting?

What Fres are you designing for, and why?

Pete Stanaitis
--------------
Title: Re: Royer induction heater
Post by: Mads Barnkob on April 23, 2017, 08:23:43 PM
The schematic looks good, this is the same way that I made it once, it was however not that successful, I kept burning one particular MOSFET in the setup and I can only think of  bad balance in the current sharing as the culprit. It was using two power supplies, 30VDC for the power circuit and 15VDC for driving the MOSFETs.

I think he just has 3 power supplies in the spice simulation, for the simulations/overviews sake.

Title: Re: Royer induction heater
Post by: afk on April 24, 2017, 01:17:01 PM
Are you actually using 3 different power supplies?

Do you plan on running the device on only 24 volts?

What maximum amperage/wattage are you expecting?

What Fres are you designing for, and why?

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

As Mads said this is just a simulation schematic. To view it easier, I modified the schematic a bit.

I'm expecting to use this at 50-60V for a consumption of at least 2kW. Since I'm still fiddling around with the work coil, I can't determine its inductance yet (my RLC meter isn't precise enough for this matter), but I'm aiming the freq to fall between 70-200kHz. It is used to melt copper and silver for metallurgy. Since I'm working with powdered metal, that frequency should be enough.
Title: Re: Royer induction heater
Post by: petespaco on April 25, 2017, 05:34:59 PM
"but I'm aiming the freq to fall between 70-200kHz. It is used to melt copper and silver for metallurgy. Since I'm working with powdered metal, that frequency should be enough.":
    This will be interesting to me, since the non ferrous metals that I have attempted to heat have done VERY POORLY so far with my Chinese 1000 watt device at anywhere from 43KHz up to about 113 Khz.  I can only get about 1 amp of NET current into a piece of 1/2" OD copper tube.  Same for a 1/2" X 5/8" solid tin bar.
So, at 48 volts input, that's only about 50 watts.
   I do see that people are melting small quantities of solder and aluminum, but they are using graphite crucibles.  In that case, I think it's the crucible that gets hot, imparting its thermal energy to the work.  There is one guy, "The Radio Mechanic", who seems to be successfully heating a small  pot of solder using a ceramic crucible.  He even has a cheap PID controller and thermocouple setup to control the induction heater's output in an on/off mode.
  From what little I have read, non ferrous metals need much higher frequencies and a LOT of power.
Title: Re: Royer induction heater
Post by: afk on April 26, 2017, 04:07:36 PM
"but I'm aiming the freq to fall between 70-200kHz. It is used to melt copper and silver for metallurgy. Since I'm working with powdered metal, that frequency should be enough.":
    This will be interesting to me, since the non ferrous metals that I have attempted to heat have done VERY POORLY so far with my Chinese 1000 watt device at anywhere from 43KHz up to about 113 Khz.  I can only get about 1 amp of NET current into a piece of 1/2" OD copper tube.  Same for a 1/2" X 5/8" solid tin bar.
So, at 48 volts input, that's only about 50 watts.
   I do see that people are melting small quantities of solder and aluminum, but they are using graphite crucibles.  In that case, I think it's the crucible that gets hot, imparting its thermal energy to the work.  There is one guy, "The Radio Mechanic", who seems to be successfully heating a small  pot of solder using a ceramic crucible.  He even has a cheap PID controller and thermocouple setup to control the induction heater's output in an on/off mode.
  From what little I have read, non ferrous metals need much higher frequencies and a LOT of power.

I understand that so I'm planning to use a metallic crucible (or any heat-resistant crucible with added metallic parts) to transfer heat instead.

Both copper and silver have melting point way below metal so this should be doable.

Still, even for metal when it passes its curie temperature the heating efficiency drops. For metal it is around 700-800°C or so, IIRC.

High frequency increases heating power, but the skin effect prevents eddy current from penetrating deeper for heating purpose. In fact, high frequency is only for heating up small objects and powdered metal.

There are also a few things to consider as well. For example, oscillating current can be calculated as I=V * Sqrt(C/L), and freq = 1 / 2pi * Sqrt(LC). To increase frequency, it is easier to drop C than L, but then, you will be reducing the oscillating current, in turn, reducing the heating effect.
Title: Re: Royer induction heater
Post by: petespaco on April 27, 2017, 04:49:07 AM
"oscillating current can be calculated as I=V * Sqrt(C/L)"
Thank you for this formula.  I have  seen  this phenomenon occur when I imbalance C and L from the "stock" values, to change Fres.  Now I should be better able to predict and optimize current flow.
  As you probably know, I have tested a couple of different graphite crucibles and they do heat up pretty well, as long as they are well insulated.  My main problem with them has been to match the ID of the work coil with the OD of the crucible to get enough current flow but not too much.   
Title: Re: Royer induction heater
Post by: Uspring on April 27, 2017, 11:46:45 AM
Quote
High frequency increases heating power, but the skin effect prevents eddy current from penetrating deeper for heating purpose.
That's true for a fixed coil inductance and current. A higher frequency will imply a higher voltage on the coil in this case, though. Often the constraints are different, e.g. the power supply will have a certain voltage and current capability or the transistors. For a given voltage and current rating, doubling the frequency e.g. implies halving the tanks inductance and capacitance. That will keep tank voltage and current unchanged.

Under this constraint, i.e. keeping f*L constant, there is a sweet spot for the frequency for the max power transfer. It's basically an impedance matching issue between the samples inductance and resistance. The frequency is lower for more conductive materials and for larger objects.
Title: Re: Royer induction heater
Post by: afk on April 27, 2017, 12:32:14 PM
Quote
High frequency increases heating power, but the skin effect prevents eddy current from penetrating deeper for heating purpose.
That's true for a fixed coil inductance and current. A higher frequency will imply a higher voltage on the coil in this case, though. Often the constraints are different, e.g. the power supply will have a certain voltage and current capability or the transistors. For a given voltage and current rating, doubling the frequency e.g. implies halving the tanks inductance and capacitance. That will keep tank voltage and current unchanged.

Under this constraint, i.e. keeping f*L constant, there is a sweet spot for the frequency for the max power transfer. It's basically an impedance matching issue between the samples inductance and resistance. The frequency is lower for more conductive materials and for larger objects.

Actually the oscillation/tank voltage is almost fixed. According to Mads, the peak-to-peak is pi times the source voltage, so if you use 60V, it should be around 188V peak (it is a criterion for choosing the MOSFET, since IRFP250 can deal with 200V, a safe value for our usage). Frequency will shift depending on the work coil and tank cap. The only thing that will influence the tank voltage is the voltage drop via parasitic impedances through out the circuit as the consumption current increases. Voltage drop that occurs with transformer is also another thing. However, in ideal PSU and ideal circuit, tank voltage shouldn't change. At least, I have confirmed that through a few experiments (I succeeded in making a weaker version of the driver, btw).

I believe most of us already know this, our circuit is in fact a LC oscillator with a charger that supplies the oscillator energy. In ideal situation, the LC circuit will keep oscillating without losing energy. In such case, the charger will not supply any energy at all, so the current consumption is zero. However, our situation is not ideal. There is always some loss throughout the circuit, like parasitic resistances that heats up when the oscillating current passes through them. The eddy current is also one of the losses that occurs, though this loss is intended. Energy loss of the LC oscillator will be resupplied by the charger, and there we can see the current consumption. To calculate this consumption we need to calculate the heating power of eddy current, but this is pretty hard and there is no general formula to calculate it (as I searched on the Internet). We only know that eddy current depends on a few things:

Mostly we work around on frequency and the tank current, but it is pretty much "hit-or-miss" at least for my case as I don't work in magnetic and related subjects, so I'm pretty much a noob in eddy current.

Although it is easy to say "halving the tank inductance and capacitance", it isn't easy in practice. Work coil is the most constrained, as it has to have a certain size for the intended purpose. When you make the work coil with copper pipe for water cooling as well, it is even harder to fine-tune the coil.

I haven't thought about keeping f*L constant, as it isn't easy to modify L, but your input gives me some thought. I'll try to calculate it out first, though.
Title: Re: Royer induction heater
Post by: Uspring on April 27, 2017, 05:59:13 PM
Quote
I believe most of us already know this, our circuit is in fact a LC oscillator with a charger that supplies the oscillator energy. In ideal situation, the LC circuit will keep oscillating without losing energy. In such case, the charger will not supply any energy at all, so the current consumption is zero.

I'm not familiar with this particular circuit. I believe, that the transistor will still carry current if there is no dissipation. But this current does not seem directly related to the tank current but only to the max expected power supply load. So in principle one could have an arbitrarily high current in the tank without overloading the transistors. So how would you optimize? By maximizing the ratio between power in the sample and power loss in the working coil itself? That seems to favor large frequencies. The effect flattens off at some frequency.

Edit: I thought a little about this circuit. It seems, that transistor current in the unloaded case just depends on the choke inductance (and voltage and frequency). So it is also not directly related to the tank current. There is one thing I don't understand: The max voltage on the transistors seems to be pi*Supplyvoltage in the steady state case, i.e. a constant load. But in a transient state, e.g. supply voltage is applied suddenly, there seems to be a large overshoot of up to twice the steady state value.
Title: Re: Royer induction heater
Post by: afk on April 28, 2017, 04:34:06 PM
Quote
I believe most of us already know this, our circuit is in fact a LC oscillator with a charger that supplies the oscillator energy. In ideal situation, the LC circuit will keep oscillating without losing energy. In such case, the charger will not supply any energy at all, so the current consumption is zero.

I'm not familiar with this particular circuit. I believe, that the transistor will still carry current if there is no dissipation. But this current does not seem directly related to the tank current but only to the max expected power supply load. So in principle one could have an arbitrarily high current in the tank without overloading the transistors. So how would you optimize? By maximizing the ratio between power in the sample and power loss in the working coil itself? That seems to favor large frequencies. The effect flattens off at some frequency.

Edit: I thought a little about this circuit. It seems, that transistor current in the unloaded case just depends on the choke inductance (and voltage and frequency). So it is also not directly related to the tank current. There is one thing I don't understand: The max voltage on the transistors seems to be pi*Supplyvoltage in the steady state case, i.e. a constant load. But in a transient state, e.g. supply voltage is applied suddenly, there seems to be a large overshoot of up to twice the steady state value.

I'm not familiar with ZVS driver either, so I just generalize it to simpler circuits that I understand the most. I see that there is an oscillation with a LC circuit, so I immediately characterize it as a LC oscillator. The FETs' role is like a charger, supplying this oscillator energy that got lost from heat (both from parasitic resistances and from eddy current). In ideal case, since no energy is lost, and the LC circuit is charged at maximum, it draws no energy from the charger. The mechanics behind is simple as that.

With the same generalization I can also calculate the current flow in the circuit, by calculating the possible energy can be stored as magnetic field and electric field, both in ideal case:

Welec = CV²/2 and Wmag = LI²/2

Energy of magnetic field in the coil and energy of electric field in the tank capacitors will periodically exchange into each other (hence the oscillation), so both Wmag=Welec. Expressing this further, I = V * Sqrt(C/L).

This calculation does not take account of the parasitic resistances, however. So the actual current flow will be less than that theoretical value. To calculate it you have to solve the differential equation of second order for RLC circuit, with R as parasitic resistance being an unknown variable.

This current, obviously, only flows within the LC circuit, and will not affect the FETs. Something like 200V and 200A+ flow would destroy any FETs before it can even oscillate: The functional ZVS driver shows that this current is independent from the FETs.

The best way to optimize is to cut off the parasitic resistance as much as possible (to reduce unneeded heat loss and also current drop), and choose the frequency at the right value depending on the material and the shape of your workpiece. The second is from the book - I'm not that knowledgeable in the field so excuse me for that. According to Wikipedia:

Frequency (kHz)Workpiece type
5–30Thick materials (e.g. steel at 815 °C with diameter 50mm or greater).
100–400Small workpieces or shallow penetration (e.g. steel at 815 °C with diameter of 5-10mm or steel at 25 °C with a diameter around 0.1mm)
480Microscopic pieces

There are also a few sources about induction heating as well, but I can't seem to find them again. Sorry about that, though.

As for the transient state, you have to charge up the LC circuit. There will be a great power spike seen at the PSU, which is typical, but in the actual LC tank, the voltage should be rising up gradually.
Title: Re: Royer induction heater
Post by: Mads Barnkob on April 28, 2017, 09:33:02 PM
One of the highest losses are properly proximity effect, to the adjacent turns in the work coil, together with the resistive losses from a low skin depth these two are the primary reasons for water cooling, maybe to some degree also the heat radiated from the work piece :)

I did describe the proximity effect / Medhurst factor in my chapter on Tesla coil secondary design and this can easily be transferred to the work coil: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/secondary-coil/

I thought I had more papers on induction heating, but there should be some good data to find one materials and frequencies in these old documents: http://kaizerpowerelectronics.dk/tools/file-archive/?drawer=application_notes*induction_heating (http://kaizerpowerelectronics.dk/tools/file-archive/?drawer=application_notes*induction_heating)
Title: Re: Royer induction heater
Post by: Uspring on April 29, 2017, 07:26:11 PM
Quote
The best way to optimize is to cut off the parasitic resistance as much as possible (to reduce unneeded heat loss and also current drop), and choose the frequency at the right value depending on the material and the shape of your workpiece.

Those might be conflicting goals. Lowest resistance from skin effect in the work coil implies a low frequency. That might not be best frequency for the work piece. A single figure of merit would be nice.

Quote
As for the transient state, you have to charge up the LC circuit. There will be a great power spike seen at the PSU, which is typical, but in the actual LC tank, the voltage should be rising up gradually.

I believe the tank voltage overshoots in some situations. Let's first consider the steady state case, i.e. constant amplitudes of voltages and currents:

Say, the current in one of the chokes (it doesn't matter which one, both have the same form) is the same for every tank cycle. Then the average value of of the voltage across the choke must be zero. A non zero average would imply a steadily rising current after each cycle. In equations:

integral Vchoke * dt = 0 (integral over a cycle)

If the cycle has length T, that amounts to two components:

Vsupply * T/2   +   (Vsupply-Vtank*2/pi) * T/2

For one half of the cycle, the FET is conducting, for the other half not. Putting the above sum to 0 we get

Vtank = pi * Vsupply

Now consider the case, that the tank has no energy, is not loaded and then the power supply is turned on. Since Vtank is near 0 during the first cycle, the V*dt integral won't be 0. After some cycles Vtank will have grown, so that the integral will become 0. During all this time the average choke current (averaged over a cycle) will have risen. It will stop rising at integral = 0, but it won't be 0 at this point. There will still be power fed to the tank, so its voltage will increase. The integral will become negative and average current will decrease but it will only stop rising, when the average current has fallen to 0. All this works out to an oscillating tank voltage envelope. Does this make sense?

@Mads: Interesting pdfs, but so looong.



Title: Re: Royer induction heater
Post by: Mads Barnkob on May 03, 2017, 10:17:22 AM
Uspring, from the measurements I did on my Royer ( http://kaizerpowerelectronics.dk/general-electronics/royer-induction-heater/ ) the switch voltage and tank voltage was the same, is this because the Vtank calculations for the LC circuit reflects its voltage envelope onto the power source?
Title: Re: Royer induction heater
Post by: Uspring on May 03, 2017, 12:14:28 PM
Hi Mads,

The closed FET switch grounds one side of the tank, so the other FET sees the whole tank voltage, so tank and FET voltage are the same. Under "Considerations" in the link you say, that the switches see pi times the supply voltage. That makes perfect sense to me for steady state operation. I'm assuming here the idealised situation, that the DC choke resistance is 0.

The one thing, that worries me is the transient situation of abrupt power up. With the idealised assumption of no losses, it seems, that tank voltage can rise up to 2*pi of the supply voltage. That would kill a FET specified only to pi times the supply voltage. My results are from theoretical deliberations only. I've tried a LTSpice simulation on a simplified circuit to confirm this, but my circuits don't want to oscillate. Do you have a working simulation?
Title: Re: Royer induction heater
Post by: afk on May 30, 2017, 04:42:02 PM
So I have been quite busy recently to continue on the project... But here I am, with a little progress!

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fr6QdJl5.jpg%3F1&hash=8960530accee1ec3b0498604458322cb)

I'm making a better tank cap bank using the X2 caps (as I have bought them, it is a waste not to use). It is hard to solder them directly on 0.5-mm-thick copper sheet (the heat dissipates too fast even with 100W soldering gun to have a focused heating point for soldering), so I get them on a PCB first, then bolt the two copper sheets on each side of the bank. I hope that the electric current will flow well through the bolts - I'm thinking of doing the same to the filter cap bank. The copper on the PCB is too thin so I'm thinking of reinforcing the flow with additional - and thicker - copper sheet, which also helps dissipating heat (as it did with my soldering attempt).

This tank cap bank consists of 11x 0.33µF, resulting in around 3.66µF measured.

My old tank caps are here, using 16x 0.22µF WIMA caps (3.52µF measured):
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FJ6ImVYa.jpg&hash=e92210f13414adee98ff6755d2c252b8)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FZ3GSQby.jpg&hash=8816c04c17e7b4f97024cedcd60104b7)

I'm redoing the circuit for now. I don't know if I should allow the current to flow through the heatsink of the FETs, but through my experiments before it does improve the current flow a lot. I'm thinking of coating the heatsink with a bit of paint after bolting the FETs on the heatsink so that it will prevent accidental shorting. There will be a lot of energy here so I want to avoid sparkling a bit.
Title: Re: Royer induction heater
Post by: petespaco on May 30, 2017, 08:15:48 PM
re: "I'm thinking of coating the heatsink with a bit of paint":
  Isn't it the job of the heatsink to take heat away from the Mosfets?  If so, then the LAST thing I'd do is to cover the heatsink with some coating.  You will probably want to fan-cool the Mosfets anyway, so just place the fan over the heatsinks so you can't get to them to short them out.
Title: Re: Royer induction heater
Post by: afk on May 31, 2017, 12:14:48 PM
re: "I'm thinking of coating the heatsink with a bit of paint":
  Isn't it the job of the heatsink to take heat away from the Mosfets?  If so, then the LAST thing I'd do is to cover the heatsink with some coating.  You will probably want to fan-cool the Mosfets anyway, so just place the fan over the heatsinks so you can't get to them to short them out.

I'll be covering the whole metallic parts with a thin layer of paint after I have connected everything intact. I'm thinking of using both watercooling and fancooling for it.

It is scary to leave big heatsink in air with voltage.
Title: Re: Royer induction heater
Post by: Mads Barnkob on May 31, 2017, 12:18:32 PM
You have some good surface area on the MMC to take away some heat from the capacitors and also low losses due to minimized skin effect. Good that current sharing is also taken care of by insuring a almost even current path from connection points to all capacitors.

It is better for the heat transfer from MOSFET to heat sink that it is mounted directly onto the heat sink without insulating pads, so I just mounted the heat sinks with acryllic or other plastic material to keep them floating with each MOSFET. I would also advise against painting the heat sinks, unless its with a special heat conducting paint, I have seen that used in some motor drives, but I doubt it has any insulating properties.
Title: Re: Royer induction heater
Post by: afk on May 31, 2017, 03:13:33 PM
You have some good surface area on the MMC to take away some heat from the capacitors and also low losses due to minimized skin effect. Good that current sharing is also taken care of by insuring a almost even current path from connection points to all capacitors.

It is better for the heat transfer from MOSFET to heat sink that it is mounted directly onto the heat sink without insulating pads, so I just mounted the heat sinks with acryllic or other plastic material to keep them floating with each MOSFET. I would also advise against painting the heat sinks, unless its with a special heat conducting paint, I have seen that used in some motor drives, but I doubt it has any insulating properties.

Is that so? Hmm... I'm torn, really. I also prefer mounting the MOSFET directly onto the heatsink without insulating pads, firstly to ensure good heat dissipation, and secondly, to use the large surface area of the thermal pad on the MOSFET to conduct electric directly to the LC circuit, bypassing the aluminium heatsink which has large cross-section for conducting electric. The issue is that the heatsink will be exposed to the air which is quite dangerous if there is an accidental shorting - and as a zealot of Murphy's Laws I prefer to avoid such risks as much as possible.
Title: Re: Royer induction heater
Post by: afk on July 21, 2017, 03:44:06 PM
So I'm back again!

After one month and a half that is way too busy to spend more time on this project, I finally finish this. The induction heater is below, heating up a piece of door hinge. The image next to it to remind you of my circuit, using double MOSFETs like the last time.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fs9yNFj3.jpg&hash=e02fc1f4bec0ed2103bb8b9ed53fede1)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FoGUQtMU.png&hash=6dbbfd2bd0506b7be78287fbbb9efd27)

This time I am using the following capacitor bank as tank. 11x0.33µF for 3.63µF measured capacitance. Surprisingly, the MKP-X2 caps work fairly well without heating much (I'll talk about the consumption later on) despite my abuse for a few dozens minutes. I soldered the caps on a PCB and complement them with thick copper sheets (0.5mm thick).

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fr6QdJl5.jpg%3F1&hash=8960530accee1ec3b0498604458322cb)

I replaced the old two chokes with bigger chokes and obviously, thicker wire (3 mm in diameter). I forgot about their inductance but they are all above 200µH (the previous two are 1mH, so I was worried if these new chokes couldn't do. Well, they work).

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FAhCNSDP.jpg&hash=256608fbc67b76d1cd9690f1a1b047f9)

Big improvement this time, I attached and soldered all components directly onto each other without using a PCB. I also used 12V/5W Zener (the black component soldered between the gate and source) instead of the old 2W to ensure their robustness. FR107 is still working fine as fast diode - there is no problem with it being a bit slow. Gate resistors are also 630Ohm/5W (the big white component I leave hanging in the air). The big thing with glassfiber tape and electrical adhesion tape is the ground line - I use 10x2.5mm² square wire to minimalize parasitic resistance and to increase heatsink.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F5VtaaZ0.jpg&hash=f14a9af6f73780e5490daac1951babc1)

Since the last time the filter cap bank was insufficient, I more than doubled the amount of capacitors. I also removed the 50mH filter choke as advised by Mads - mainly I think the choke will not be working well at high current and to reduce wasted heat. This capacitor bank has 35 capacitors of 400V/1mF. Since the capacitors are of low quality, I'm not having 35mF but it should be at least 30mF.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FcgrVg0m.jpg&hash=026245ce0d64d07b1d6ec78bd1187cd9)

And voilà! I have a good result on my oscilloscope. Yellow and Blue are the two ends of the tank cap/work coil, and the Red/MATH is the difference of potential a.k.a tank voltage. Btw I can't seem to measure the tank current despite having used two coils in cascade for a stepdown current transformer.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F82eteh1.jpg&hash=a0c69a1eca41854a4d705775edc86cf2)

We have f = 45.77kHz and tank cap is 3.63µF, so it is easy to calculate the inductance of work coil. I calculated it to be 3.33µH, which is quite a big improvement from the old coil (only 1.14µH). This work coil has 6 turns with 10cm in diameter, using 7mm-diameter copper tube (previous is 5 turns, 5cm diameter, same tube). I ran out of copper tubes else I had made more turns.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F21FZx6W.jpg&hash=2aa70984213956f676972b09167834ad)

Measurement time for no workpiece. I used 23V input and the input voltage only drops to 21.92V. The input current is 2-2.13 A (approx 43-45W). A great reduction from the previous 16A (in the OP). It seems that direct soldering on components and using bigger wire as ground greatly improve efficiency. Tank voltage peak is 70V (or 148V on the oscilloscope) which is roughly pi*Vin. At this time I felt like a fool for expanding the filter cap so much. But well, the spike at start should be quite powerful so being careful isn't so bad.

When I put the door hinge into the coil, the consumption increased to 6.93A. The input voltage dropped to 21.06V for approx 146W consumption. Every measurement on the oscilloscope is still fine.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FYDmUQUg.jpg&hash=c459c37ccd68c27925fa1e9416eefd91)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FtapIqY7.jpg&hash=d3df391593a668e58898f29cf2f5c811)

I also took time to measure the heating of the work coil when without any workpiece. The graph below shows the evolution of temperature based on time. I haven't flowed water in the coil tube yet, but after using the induction heater for more than 10 minutes to heat 5 bolts to more than 300°C (the maximum that my multimeter can measure) the temperature of the work coil is still under 70°C.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FlKd0XVj.png&hash=7b53055666681640a4430cabe5c336e3)

I'm still looking for method to increase the output. It can reach 10A but it is still low. I'll test the machine at higher voltage later to see if it can fare well.
Title: Re: Royer induction heater
Post by: Mads Barnkob on July 24, 2017, 11:56:42 AM
Great update on the project!

I love the absolutely over-engineered parts like 25mm^2 ground connection, its better to make things sturdy and never have failures :)

While you trippled the inductance of the new work coil, compared to the old, you would also limit the current to 1/3 of before. I also think that the diameter is possibly too big compared to the work piece, so there is simply not enough coupling to transfer energy. You need to have work coils that fit pretty close to the work piece.

Higher input voltage will of course help on the performance.

Regarding your LC circuit current transformer that does not work, did you add a load resistor across it? A current transformer merely transforms current in a ratio, f.ex. 1:50, but you need a burden resistor to have a measurable voltage across it, figured out by Ohms law to the voltage that you need for your control system/meter.
Title: Re: Royer induction heater
Post by: afk on July 25, 2017, 08:17:58 AM
I already tried to put a 0.5 Ohm resistor (actually 2x 1Ohm in parallel) as shunt but there is barely a signal outside. Actually, I don't know where I should latch the ground of the oscilloscope on. The current transformer is pretty much on air.

As for increasing the size of the coil, I think that it will be fine. My intended workpiece will be big enough. I leave more space in order to avoid direct heat loss from the workpiece to be transferred to the work coil. Tripling the inductance only reduces the oscillation current by sqrt(1/3) according to my calculation, as impedance of inductor is Lω where L is increased by 3 and ω is reduced by sqrt(1/3) from inductance increase. I will tweak around its size later when I get my crucible.

I am thinking of wrapping alum foil to reflect heat from the workpiece. It should help reducing loss and overheating the work coil. Since the surface of the alum foil will be in parallel with the magnetic field inside the coil there shouldn't be a problem of the foil heating up via eddy current.

A bit off-topic, I'm also buying a thermocouple to measure temperature. I found a K-type for less than $10, but there is a risk of green rot damaging the thermocouple.
Title: Re: Royer induction heater
Post by: afk on July 26, 2017, 04:26:31 PM
So I tested the circuit at 60V and... darn the heat was immense.

Without putting a workpiece the work coil heated up quite fast. I measured it to be 8-9.3A consumption (supply at 62.15V) so the loss is already at 500-600W, which is huge. This is rather troubling as I expected the loss to be increased nine-fold (from around 43-45W 22V). I hope that when I add water cooling it can help mitigating the loss from increased resistance due to heat.

Other than that, other components heated up quite fast as well, especially the MOSFETs. The choke also got a bit warm. I will need something to dissipate the heat from the choke as the enamel of the wire could get degraded.

This is what I got from the oscilloscope, though. As usual, yellow and blue are the two sides of the coil and the red one is the tank voltage:

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FdVy24Y0.jpg&hash=9053bb863e425ac042d94810e4ed4c02)

Heating up 1 bolt, the current raised to 11.07A. Heating up the door hinge again, it was 21.73A (supply dropped to 59.22V).

The video below shows the heating of 8 steel bolts. Supply current read at 33.33A and supply voltage dropped to 57.33V. It took around 30s to get red hot. 1900W is already enough for me to use, but at least 600W got wasted (actually more since there are more current flowing through parasitic resistance), which is not really desirable.


By the way, I have a question concerning water cooling. Do I have to distill water for cooling? It is a bit troublesome to distill water. Somebody said that tap water contains some sodium or calcium minerals that can cause electrolysis within the copper tube and corrode it. Since it is an alternative current at a few dozens kHz I doubt electrolysis might happen, but I just want to ask just in case.
Title: Re: Royer induction heater
Post by: kamelryttarn on July 27, 2017, 09:01:37 AM
Very impressive induction heater! When I see your cap bank I am a bit concerned if my bank will not have enough capacitance. I couldn't afford and fit more than 1900uF so I am expecting a bit of ripple but I can't see how that would be a big problem. Just like you I plan to use beefier zener diodes, but isn't 630 Ohm gate resistor a bit much? I had planned to use 100 Ohm instead. Have you looked at the gate signal with your oscilloscope?
Title: Re: Royer induction heater
Post by: afk 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.
Title: Re: Royer induction heater
Post by: kamelryttarn 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
Title: Re: Royer induction heater
Post by: Mads Barnkob 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.
Title: Re: Royer induction heater
Post by: afk 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.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FZEF4Doy.jpg&hash=27b38c87dba90d39b70dfa0b6548fcbe)

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:

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.
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FXPciwAp.png&hash=decd2cde5945aa2fb279fa6e5923bb6a)

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).
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FE4VbV0m.jpg&hash=053c423328f9a04c963b1b7ffcbf71cd)
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FdiavhLk.jpg&hash=0e3a586ea427a8d2f60a40876a5f2ed6)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2F5l1Yoyl.jpg&hash=f87f918af99f56d8b344f940bea7eda6)

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:
(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FvB14Qw2.jpg&hash=1297045d0fdd4a1860fbcb340a237e28)(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FMnK7eAO.jpg&hash=0c77a5402a157e9cef56765950a6613c)

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.
Title: Re: Royer induction heater
Post by: petespaco 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
---------------
Title: Re: Royer induction heater
Post by: Mads Barnkob 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.
Title: Re: Royer induction heater
Post by: afk 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.
Title: Re: Royer induction heater
Post by: petespaco 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.
Title: Re: Royer induction heater
Post by: afk 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).
Title: Re: Royer induction heater
Post by: petespaco 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
---------------
Title: Re: Royer induction heater
Post by: afk 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.
Title: Re: Royer induction heater
Post by: kamelryttarn 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?
Title: Re: Royer induction heater
Post by: afk 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.
Title: Re: Royer induction heater
Post by: Mads Barnkob 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.
Title: Re: Royer induction heater
Post by: ldm314 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.
Title: Re: Royer induction heater
Post by: afk 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.

(https://highvoltageforum.net/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FoGUQtMU.png&hash=6dbbfd2bd0506b7be78287fbbb9efd27)

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.
Title: Re: Royer induction heater
Post by: petespaco 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
---------------
Title: Re: Royer induction heater
Post by: afk 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).