Seeking to build a 400+kHz IH, ZVS platform, high performance components

[JimOB]

Hi, first post and thankful to be here. I am interested in building an IH operating above 400kHz. At this time I have the standard 2.5kW mail order ZVS PCB. I am currently burning up caps, MOSFETS, and diodes as I increase the frequency. I'm looking for configuration advice and parts selection advice.
Here are some facts:
1. The workcoil is 2" and 2 turns
2. The workpiece is a 2"L X 1/4" ID alloy steel tube connected to a source of fluid that is constantly streaming through the workpiece.
3. We are testing caps in series, parallel, series-parallel, and parallel-series. We have tested with as many as 12 caps and as few as 1 cap.
4. Caps are .33uF  600V/1200V standard issue. They are holding up, so long as we use 8 gauge or 6 gauge wire to connect them together.
5. MOSFETS are standard issue 260s and we've burned up several as we proceed.
6. Chokes are standard issue.
7. Power supply is 3 kW AC to DC; 240VAC In, 48W Output.
Here are some questions and observations:
~ I am looking at Silicone Carbide MOSFETS and Diodes from Wolfspeed as high temp, high speed replacements. Anyone have some experience with Silicone Carbide components?
~ We need more voltage so we need water cooled caps connected in series. I need to find a source that are cheaper than what Celem offers. Any help here would be greatly appreciated.
~ And finally, we need some help configuring this thing. For instance, do we need 6 MOSFETS or 4 or 8? And do we need larger chokes than the little ones that came with the board?
~ And we contemplate that once we dial this unit in with the right components, the wiring, cooling system, and PCB will have a different look than the typical mail order system, so we are seeking advice on how commercial systems are configured so that we may take advantage of some industry standards that we do not know about now.
Thank you very much.

[T3sl4co1l]

Hi,

I have some questions:

0. Does it have to be coreless induction?  Can it not be, for example, shaped into a horseshoe or eye-hook shape so that the work itself goes around one turn (and might be shorted across with a brazed or welded on strap), into which a core (e.g. laminated steel) can be placed, and so on to basically make a 50/60Hz transformer?

1. Does the frequency have to be so high?  What is the O.D. of the alloy steel tube?  Why is the work coil so much larger?  (Or is 2" the length?  You are missing several dimensions; a diagram would be helpful.)

2. Silicon (not -one) carbide is an attractive alternative to silicon, at high voltages -- they are available in ratings from 600 to 1700V and more, with 900 and 1200V being most common.  They are not useful at such low voltages.  They also have different gate drive requirements, and will not work in the standard ZVS push-pull self-excited oscillator circuit.

3. What's wrong with Celem prices?  Is this a one-off build or targeted for production?  I find it worrying, in that: you're apparently trying to do something quite fancy and high tech, but haven't budgeted anywhere near enough to actually achieve it.  Product development is easily five digits $USD and up.  If you don't have that kind of money, no, you shouldn't necessarily just give up, but understand that you may be committing a tremendous amount of time to something that probably won't pay off.

4. If you're looking at getting a new PCB made anyway, that's again an investment of time and/or money.  KiCAD is free, and quite good these days I understand (and always improving), but you may not want to just copy (and change incrementally) the standard circuit.  Depends on requirements.  IMHO, 3kW is really pushing it for a basic style design, and I strongly recommend a conventional commercial approach, using a PLL type control with monitoring voltages and currents for regulation and protection.  The ZVS circuit is basically uncontrollable by itself; you must vary supply voltage instead.  A PLL type control can be varied over a wide range, and can respond as quickly as the resonant tank allows.

Tim

[klugesmith]

Yeah, why does the frequency have to be so high? 

And for a given frequency, did you consider the voltage/current tradeoff?
(For a given work coil size and shape, you can adjust L over a wide range by changing conductor diameter and turns count.)

As Tim said, can you change the geometry to be more efficient?

If the ultimate goal is to heat the fluid flowing through hollow workpiece, what makes inductive heating better than resistive heating by current in the alloy steel tube itself, or a heating element around it?  Heat flux density at the fluid interface is enormous; is this for a rocket engine or something?

[johnf]

Maybe another approach
ie use microwaves to heat your fluid directly
it would be many many percent more efficient

if your fluid is not microwave absorbing maybe a SiC tube to be heated to heat the fluid

this seems to be taking a knife to a gun fight!!

[JimOB]

Thank you for your questions and insights.
The ideas and advice are greatly appreciated.
I've attached a pic of the workpiece in the coil.
The project is in its infancy now and the ZVS circuit will likely be replaced with a bridge or half-bridge soon.
We are in need of experiential knowledge, as you can surely surmise, and my small group has a limited budget at this time.
To answer a few questions, the 2 turn coil is closely coupled to the workpiece and we are getting our best results yet from this configuration. We have used multi-turn coils and larger workpieces at lower frequencies as well, but the combination in the attached photo has given us the best results to date.
We can't use electric resistance heating for this project because isolation between the working fluid and the heat source is critical.
Microwave heating is efficient but not as quick as IH for our purposes.

Many, many thanks!
Jim

[JimOB]

See attached pic

[petespaco]

We can't use electric resistance heating for this project because isolation between the working fluid and the heat source is critical.

Jim-  Isolation can easily be accomplished with an isolation transformer.  If more "protection" is needed, use a low voltage high current output such as you might find in an old heavy duty battery charger, such as the Solar 480.  I just bought two of them (non working) for $14 each.  You won't need the rectifier diodes, which often are bad anyway.  This one even has 4 or 5 output voltage choices, using multiple AC input taps.

OT, but I just bought them for the transformers, but I just ordered a $20 set of diodes and will see if I can get one running.  Why?  I don't know; just because it's there, I guess.

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

[Twospoons]

Is there any reason you can't coil your pipe through an iron core? Then you can add a copper primary, and a shorting bar on the pipe. You could then probably get efficient heating just with mains frequency, given the excellent coupling provided by the iron core. Or at least you can run your inverter at a more reasonable frequency, and without the resonant tank.
 Its the same method used to heat bearings.

And since a picture is worth a thousand words ...

[ritaismyconscience]

Consider using resistive wire and mica for insulation. Mica is cheap, a good electrical insulator, and thermally conductive. This method is much more cheap and reliable than the induction heater approach.

[klugesmith]

Yeah, what is gained by requiring the heater transformer to be air-cored?
If it is air-cored, as in Jim's picture, the flux coupling is very poor because workpiece occupies such a small fraction of the work coil area.  Why not use work coil with smaller diameter & more turns, to get same inductance if you're designing for inductance?

Nice rendering by Twospoons.  I will use only words to present a variant.

Original design shows helical copper tube primary, and helical steel tube secondary with a shorting bar.
Current in the workpiece is longitudinal instead of circumferential, which makes it easier to achieve a practical impedance.
It helps that the workpiece is a corrosion-resistant alloy with high electrical resistivity; stainless steel can be 40x that of copper, and some nichrome is 100x.
One way to eliminate the shorting bar and its cooling requirement: make the secondary a single turn.  Start with a long enough straight tube and make a 360 degree bend in the middle, with just enough offset that the input and output pipes scrape past each other while being electrically connected.



If the workpiece needs to be short and straight, as shown in Jim's drawing,
it can be part of a 1-turn secondary whose circuit is closed with a thick copper jumper.   

What good is high frequency except to reduce the required core size?
Look up the size of small variacs, say 300 to 600 VA.  Same unit could operate at 400 Hz with 7 or 8 times more power and volts-per-turn, needing only air cooling for core and primary winding.  Place workpiece tube at the axis, if there's enough radial clearance.  Secondary circuit can be copper bars or plates.  Mind the power density and thermal conduction at the interfaces.

[Twospoons]

Nice rendering by Twospoons. 

Thankyou! I was a bit bored so I decided to spend a couple of hours improving my Solidworks skills ...

Alternatives to the shorting bar: single turn with the pipes in electrical contact , as suggested above; multiple turns with the pipes bent so they can contact; multiple turns with the turns compressed into contact.
Any of those would work.

[Steve Ward]

I'm also building an IH (well, ive built a few now...) aimed at higher freq's in this range.  I'd be happy to share details, but its still in the works.  Also, as you already know, building these kinds of things are a steep learning curve with plenty of ways to go wrong. 

I'm using CREE SiC devices with a boot-strap type gate driver.  I have a post called "a three phase "tree phase" tesla coil" that has schematics attached for my half-bridge design, which im using the same design for the IH project.  I'm gonna try the C3M0065100K in full-bridge as im limited to about 12KW, which at 600VDC is only 20A or so (the switches might see 30A peak).   

The topology of the heater is a series resonant tank circuit fed by a step-down ferrite transformer with litz wire primary and water cooled secondary and a ratio of 35:1.  The bridge switching will happen strictly at or above the resonance frequency as this offers lower switching losses for the mosfets.  Power can also be controlled in this manner by operating off-resonance to control the boost that the resonance offers.

Alternatively, if the switching losses are too great, i will use a pulse-skipping method where the full-bridge can alternate between supply voltage at resonance, versus connecting both sides of the bridge to the same voltage, essentially putting zero voltage into the resonant circuit, letting the resonant current decay, before pumping it back up again.

I would say the SiC stuff is relatively easy to use if you are aware of a few things, like keeping gate drive parasitic inductances to an absolute minimum, and also watching the loop inductance of the half-bridge itself, because the higher switching rates available can more easily generate voltage transients.  The gate drive can oscillate and self-destruct if its sloppy (too much stray inductance).  I highly recommend the mosfets with a 4th "kelvin source" connection which eases the gate drive issues.

I've scored many CELEM capacitors on ebay recently, for pretty low prices.  Im using a pair of CSM150's of the .66uF type in parallel for this latest iteration. I do worry that some of them might be worn out, but the cost was small enough to take the risk.  The capacitance checks out OK on the 6 of them that i bought so far (i also got a pair of CP100, and a pair of CHF3 type 6). 

With all that being said, if you are in a hurry i would consider other options first as developing something like this is very challenging and likely to have a lot of hurdles.

I guess its worth saying why I want to build another IH, particularly with higher frequency capacity.  This heater is aimed at heating small copper pieces for purposes of selectively soldering them together as they are stacked with insulators and solder paste to form high-fill-factor motor windings.  The higher frequency will drive up the losses in my copper work pieces, getting it to soldering temperature even faster than my 210khz induction heater can now, which can take up to 60 seconds to achieve soldering temp, even with 500Apk circulating in my work coil.  Shorter heat time should limit the heat flow into neighboring coils and the stator core.