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Canyonman
11-21-2009, 12:26 PM
Hi All; With a special ping to Ernie Henne,

I read with interest your Article in vol.22 on the Induction Heater. I have the Induction Heating book by Frank W Curtis. But it appears I'm Electrically Impaired.

I would like to build one of these, a simple one. I have read the book, and I have a very basic understanding of the design. I understand Induction, but lack the knowledge to "slap something together."

I have a very healthy respect for High Voltage Electricity!

I have a whole crate full of Computer power supplies, a transformer from a Microwave, Capacitors of all kinds. It would seem to me, especially after reading your article, that I have enough "Stuff" to build one of these.

Are you going to do a build article? If not, is there any way I could get a schematic of your build?

Thanks.

Take Care and Be Well,

Ken

millhand
11-21-2009, 05:20 PM
I sent a message to a friend about this article, asking if the appliance might have been a microwave oven. I knew that the output of the oven was microwave frequency, but did not know what drove the magnetron. Thought maybe the power supply might have been lower frequency. He replied with:

Nope---not a microwave---they operate with 2440 Megacycle pulses from a magnetron which is operated from a rectifier which is fed by a transformer directly off the incoming line power.

30 kHz says switching power conversion to me and household appliance and 900 watts leads me on to thinking the writer is talking about one of the flat glass stove tops that have an induction heating coil underneath. I'll bet that he has a induction stove power conversion unit to which he has fitted a coil of his design to.

Carl

Canyonman
11-21-2009, 07:09 PM
Hi Millhand, Carl,

I don't know. I was hoping to "Crack the Egg" here. But nothing so far.

I don't have the Magnetron from a Microwave but I do have the Transformer. Lemme tell ya, it's a Honker! With an almost giant copper wire as final winding.

I get lost at the "Frequency" stuff. Except for 400Hz which is run in Aircraft. But our Power units just made it, we never really gave it a thought.

One of those control units from a flat top stove??!!!! That is a heck of an idea, I wonder if that could be made to work???

Well, Here's Hoping We get an Answer!

Take Care and Be Well,

Ken

J Tiers
11-21-2009, 08:57 PM
The article says 30 kHz, which is fairly high for high power, once you get to 10x that frequency most motor drives etc use lower frequencies to avoid losses. Still it is quite possible at 3x that power, i have designed 3kW SMPS using higher frequencies. It would almost definitely be some form of SMPS driving the coils. the 900W is well within the capability of a very reasonable full-bridge or even half-bridge converter. The basic technology is the same as is used in the millions of SMPS (line to low voltage converters) and "inverters" (battery to 120V/240V converters.

However, 30 kHz is only one choice. There are many others. The induction heating depth differs by material and frequency. The so-called 'skin" effect determines the effective depth to which the current flows. The current is what heats the material, so the frequency effectively determines the depth of heating.

Induction heating frequency would also be affected by size. Physically large items are best heated by lower frequency, and an electric induction furnace could run at or close to 50/60 hz.

Earlier induction heaters at higher frequencies were driven by "frequency changers" of various sorts. Motor-generator sets, with the generator (alternator) wound to produce higher frequency, magnetic frequency doublers and triplers, which used various effects including magnetic saturation to provide energy at harmonics of the input frequency, lots of methods are possible.

Using SMPS technology could allow some flexibility in the choice of frequency, depending on the capabilities of any internal magnetic components.

I'm not at all sure that the technology is easy to describe in an article. There would be quite a bit of specialized knowledge used, one would almost have to have a source for PC boards and for the magnetic components, which are tricky to wind at high frequencies.

dp
11-21-2009, 09:30 PM
Here's a guy who's built a small one:

http://webpages.charter.net/dawill/tmoranwms/Elec_IndHeat1.html

macona
11-22-2009, 09:16 PM
Check this out:

http://www.repairfaq.org/sam/lasercon.htm#consrp

http://www.hvguy.4hv.org/ih/indheat.htm

I need to build a small one for activation neon sign electrodes. I have some nice 710/6011 triodes and I used to have a schematic for a power supply design using them but cant find them. Ugh...

MarshSt
11-22-2009, 09:58 PM
I just got my magazine and saw the article description for the induction heater in the index. I was very interested and immediately started reading the magazine from that page. I must say I was quite disappointed. I expected to find a description of how the project was built with schematics and explanations etc. With the usual "safety disclaimers" of course. The article was only one page and didn't really provide much more than an introduction,a very vague description of his induction heater and a referrence to a book on the subject. The article obviously got a discussion started here but, no offense intended to the author, what was the point of this article for the general readership?

Steve

macona
11-22-2009, 10:49 PM
Steve, thats exactly what I thought reading through the article, Whats the point?

Canyonman
11-22-2009, 10:49 PM
Hi JTiers,

With all due respect, I only understood about 1/2 of your post. SMPS??? What I did get, and feel free to correct me, is that different frequencies will result in different depths of heating. Now from a mechanical standpoint I can see the usefulness of that in brazing, silver soldering, case hardening, and tempering/annealing. But Thank You for the reply.

Hi MarshSt,

Bullseye Steve! I had those exact same thoughts! I was even checking to see if some of the pages were stuck together!

And I have the referenced book. But some help is still required. And I hope it is in the way of a "Hard Wire Schematic" to follow. Because I have no way to make printed circuit boards and would need help with that too!

I hope the editors of MW are catching this.

Macona & dp,

I have not yet had a chance to scan your links, so I'm NOT snubbing you in any way.

Take Care and Be Well,

Ken

dp
11-22-2009, 11:10 PM
[SIZE=3]Macona & dp,

I have not yet had a chance to scan your links, so I'm NOT snubbing you in any way.

Take Care and Be Well,

Ken

SMPS means Switched Mode Power Supply. The switches being electronic devices now days.

I get the feeling these machines are not healthy for people wearing pacemakers. :)

Canyonman
11-22-2009, 11:44 PM
macona & dp,

I downloaded some info. Thanks.

Oh, and Thanks for the definition.

But I still don't feel "Build Confident."

Can't someone make a simple one??? Induction for Dummies! :rolleyes:

Take Care and Be Well,

Ken

dp
11-23-2009, 12:00 AM
It may actually be possible using over the counter components. Class D amplifiers for boom box cars are now over 5kw. I don't know what the frequency response is, but if they can go high enough you have a furnace. Class D amps are a type of amp that uses switched mode and PWM to create buttloads of power without overheating the box.

Edit: And wouldn't you know, somebody has done it: http://www.youtube.com/watch?v=W47rx5jSt7g

Canyonman
11-23-2009, 12:31 AM
Hi dp,

I sure would like to have one that works like this one! {One of these 2 links should work} Maybe a bit bigger around and not quite as long.

http://www.youtube.com/watch?v=1SZCRJEr6Lc

http://www.youtube.com/watch?v=1SZCRJEr6Lc


Take Care,

Ken

darryl
11-23-2009, 12:53 AM
I'll throw in an idea- you have computer power supplies, these are SMPS. Don't know the frequency they work at, but probably in the 20khz to 60 khz range. Maybe up to 250khz. They are rated at what- 150 watts to 350 watts-

Tap into the secondary winding that delivers the high current levels, the 5v winding. That's going to be one ground wire and one wire from the point that leads to the largest rectifier in there. No need to actually go into the transformer itself, just bring out wires from those two points on the circuit board. The rest of the circuit can stay as is, except you might need to put a small load on the 5v output so it will work.

For the work coil, you'd have to find a diameter and number of turns that would put some load on the power supply without a workpiece in the coil. Inserting a workpiece would then raise the power level consumed by the power supply, with the extra power drawn being absorbed by the workpiece.

I've never built or experimented with this, so it's just an idea. You wouldn't have any control over the output frequency since the power supply would be changing it to suit the loading, but you'd still get induction heating.

One of the first things you'd want to know is how much power it's actually going to take to heat your part. Obviously it's a function of part size amongst other things, but I think you could have something quite useful.

Maybe not enough power though.

dp
11-23-2009, 01:40 AM
Here you go - cheapest I've seen yet. All that's needed to to change the heating element: http://www.webstaurantstore.com/countertop-induction-range-1800-watt-120v/922IC1800%20%20%20120.html

Suitable for experimentation.

Hmmm - found a 220AC unit that is rated 3kw. That's getting serious. http://www.foodservicewarehouse.com/eurodib/ihe3097-p1/p371975.aspx

macona
11-23-2009, 03:59 AM
darryl, check out the first link I posted.

Evan
11-23-2009, 06:29 AM
I expected to find a description of how the project was built with schematics and explanations etc. With the usual "safety disclaimers" of course.

Safety disclaimers are no protection against liability and only serve to prove that the hazards were known in advance with corresponding increase in liability damages.

It is unlikely that Village Press would be willing to publish details/U build it for anything that involves high voltages at high power levels. That is not something for the untrained amateur to be playing with and could easily produce a business ending outcome.

A warning about microwave ovens: Operating the oven without proper containment of the microwave radiation is an easy way to lose your vision permanently.

andy_b
11-23-2009, 07:53 AM
A warning about microwave ovens: Operating the oven without proper containment of the microwave radiation is an easy way to lose your vision permanently.

Or interfere with the GPS guidance control of missiles, oops, I mean your TomTom.

andy b.

wierdscience
11-23-2009, 09:01 AM
Nice one showing the various operations HF heating can be used for,from China where else?

http://www.youtube.com/watch?v=N9mCETmHutw&feature=fvw

lazlo
11-23-2009, 09:15 AM
Can't someone make a simple one??? Induction for Dummies! :rolleyes:


There's no such thing as a simple induction heater.

Induction heating works by dumping a high frequency AC power source into an electromagnet. Old school induction heaters used a parallel resonant tank circuit as a high frequency oscillator -- that's what you'll find in an inductive stove.

Modern induction heaters use a high-power mosfet H-bridge to generate a high voltage alternating current.

Both induction power supplies (analog or digital) have a "work coil" (the burner coil on an induction stovetop) -- the electromagnet, with another tank circuit to match the power supply to the induction coil.

You can dump high frequency AC into an electromagnet, play around with the frequency, and eventually get the metal to heat up, but the really complicated part is that you're creating a inductive resonator, and the fundamental frequency increases as the metal gets hotter. So on the very few amateur induction heaters I've seen, they spend a lot of time experimenting with the frequency necessary to heat up that specific part.

Commercial units have a closed-loop PID controller between the AC power supply and the work coil that senses and constantly adapts to the state of the inductive tank circuit.

For a really good description of the electronics (and emag theory!) involved:

http://www.richieburnett.co.uk/indheat.html

George Bulliss
11-23-2009, 09:42 AM
I have had a good amount of mail on this article, mostly complaining of the lack of information in the article.

I received this article in my email sometime last year and thought it was an interesting idea. Last spring, I met Ernie at the NAMES show and had a few words with him about the article. I tried to push for a complete build project but I think Ernie was worried about liability issues and did not want to be responsible for any injuries that may have resulted from the project. Also, Ernie is from Australia and he was unsure if all the components for the build would be available to those in other countries. He simply wanted to get the photo in the magazine to show people what he had done.

We get these types of submittals all the time and they are typically run in the letters to the editor section of the magazines. If we have enough to stretch things out to a page, I often choose to run the submittal by itself. This way, the author gets a few bucks for his trouble. We have done this frequently over the years with various models, shop tools, etc.

Some of these submittals will include enough information to build the item, though they aren’t really full blown projects. Until this article, the typical response to a page of the magazine showing an authors completed project has been something along the lines of “Wow, that guy does nice work.” The word filters won’t let me quote the typical response I have received on this one.

I consider these articles to be similar to what you would find at a hobby show. You can view some great examples of a craftsman’s hard work, but you would certainly not expect complete plans and instructions for everything you saw at the show.

I’m sure that if I had run this in the “End Notes” section, there would have been no nasty letters but I am not going to stop placing some of the larger and more interesting letters in the paying section of the magazine. A few bucks for the shop fund never hurts. I suppose I could have included a statement at the beginning of the article but I felt it was clear as to what the article was and I thought the book reference would be helpful for readers wishing to find out more.

So, for those wishing to find more information, I’m sorry to say I don’t have any. I’m also pretty sure that Ernie will not be doing a full build article on this, but I won’t rule it out.

George

AlleyCat
11-23-2009, 10:08 AM
Has anyone tried a high power ultrasonic power supply for induction heating? I have two Branson Ultrasonic power units that I could use or modify for induction heating if that's feasible. The small one is 1200 watts and the larger one is about 5000 watts. Both switch around 40 kHz. Any ideas??

ckelloug
11-23-2009, 10:22 AM
AlleyCat,

I have a complete 750 W ultrasonic disperser from Sonics and Materials that I use for sonochemistry research. I think it's likely the ultrasonic power supplies won't work for induction heating because they are tuned to their frequencies pretty much down to the Hz. Mine I believe is something like 20,000Hz plus or minus 5 Hz. Lazlo pointed out that there is a high degree of frequency variability required for an induction heater so I think the ultrasonic units will not provide the necessary degree of variability.

--Cameron

dp
11-23-2009, 11:54 AM
George - what this has demonstrated is the simbiosis between the printed and on line pages. The article was a great idea starter.

Ries
11-23-2009, 12:38 PM
A friend of mine, Grant Sarver, imports induction forges from China-
here is a great video of his machines.

http://www.youtube.com/watch?v=k4xsqw463Hs

the Kaynes, at http://www.blacksmithsdepot.com/
were selling them- I dont know if they still are or not, you would have to contact them.

Basically, Grants machines are sold with very little markup, and cost between $4000 and $6000.
Large american companies that sell industrial models have traditionally started at over $10,000 and headed towards $20k or $30k very quickly.

This pricing is not because they are ripping you off- to do this right is complicated.

Grants basic machine, shown in the vid above, is about as complex as a good 250 amp tig welder. It costs more than a tig welder because the volume is so low- if he could sell 50,000 a year, he could probably sell em for around $2,000 or so- but the market is a tiny fraction of that.

He uses a tig welder water cooler system to cool the coils, and bends his own custom coils quite quickly and easily.
His machines are very reliable, flexible, and easy to program to do different things- he sells them primarily to blacksmiths and knifemakers, and people are forging a wide variety of things with them, as well as using them for very controlled heat treating.

Just like you COULD build your own AC/DC inverter tig welder, you COULD make your own induction forge- but most people with that degree of electronics experience can probably make more doing another job, and just buy one.

They are very cool to see in operation, though. I want one.

dp
11-23-2009, 01:18 PM
Just like you COULD build your own AC/DC inverter tig welder, you COULD make your own induction forge- but most people with that degree of electronics experience can probably make more doing another job, and just buy one.


The same could be said for airplanes - but there's a very robust home built community. Sometimes we do things for the challenge and entertainment value. And sometimes the result is a very useful thing.

dp
11-23-2009, 01:32 PM
Both induction power supplies (analog or digital) have a "work coil" (the burner coil on an induction stovetop) -- the electromagnet, with another tank circuit to match the power supply to the induction coil.

You can dump high frequency AC into an electromagnet, play around with the frequency, and eventually get the metal to heat up, but the really complicated part is that you're creating a inductive resonator, and the fundamental frequency increases as the metal gets hotter. So on the very few amateur induction heaters I've seen, they spend a lot of time experimenting with the frequency necessary to heat up that specific part.

This is actually quite basic stuff. Any amateur radio operator who has worked with an auto-tune antenna tuner can relate. The phase angle of voltage and current in the tank circuit is easily measured. In the case of induction heating devices the two variables are frequency applied to the tank circuit, and the resonant frequency of the tank circuit. In the case of antenna tuners the tank circuit is changed to get the desired phase angle while in the induction heater the frequency is adjusted to get the phase angle. In both cases this is done dynamically.

A common analog phase detector is a ferrite core to sense current (just like you'd use in your power panel), and a small capacitor coupled voltage sensor.

lazlo
11-23-2009, 02:13 PM
The phase angle of voltage and current in the tank circuit is easily measured. In the case of induction heating devices the two variables are frequency applied to the tank circuit, and the resonant frequency of the tank circuit.

If it's easy, no amateur has successfully done it -- every amateur induction heater I've seen, like the one you posted and the one I posted, are simply high voltage power supplies with fixed frequency dumping into a work coil.

You really have to d!ck around with the induction of the work coil and the rapidly changing resonance of the workpiece (which is why the commercial units have a closed-loop PID control).
And like Ries says, the work coil gets really hot, so you need to have forced liquid cooling.

But if you can build one Dennis, by all means -- you'll be an Internet hero! :)

lazlo
11-23-2009, 02:24 PM
A friend of mine, Grant Sarver, imports induction forges from China-
here is a great video of his machines.

http://www.youtube.com/watch?v=k4xsqw463Hs.

That's impressive that he can touch the coil immediately after a heat -- he must be pushing a lot of water through those coils.

15 KW -- no margin for error if you're a hobbyist trying to build one in your home shop :)

2ManyHobbies
11-23-2009, 04:49 PM
That's impressive that he can touch the coil immediately after a heat -- he must be pushing a lot of water through those coils.

15 KW -- no margin for error if you're a hobbyist trying to build one in your home shop :)

I doesn't take much to keep the coils cool. In that demo, the coils probably pick up more radiated heat from the metal being heated than they generate via ohmic heating. A TIG cooler wouldn't really sweat over it. Some of the homebrew setups use gravity feeds or a fountain pump for circulation.

15kW is outside of the range of a 60A breaker on 6ga wire. For the hobbyist, that would mean more along the lines of two power supplies drawing in the 8-10kW range with coupled outputs. Not impossible, but more expensive than building a single power supply on 3-phase.

A multi-use 0-10kW SMPS is on my want to build list some day. I don't see a reason why one couldn't recycle most of the power stage for welding, plasma cutting, induction heating, or even battery charging. With 5 taps on the output transformer, everything else is just a matter of the correct combination of push buttons, brains, and sensors.

Ries
11-23-2009, 04:50 PM
The same could be said for airplanes - but there's a very robust home built community. Sometimes we do things for the challenge and entertainment value. And sometimes the result is a very useful thing.

Absolutely.

But nobody is homebuilding fly by wire jets.

They are putting VW engines in glorified hang gliders.

Which, when you think about it, is just like making a forge from an old propane tank.

Which a lot of people do, as well.

The level of sophistication of homemade airplanes is, by and large, about on par with the level of sophistication of homemade forges. Neither of which, usually, features a lot of scratch built engineered electronic circuits at high voltages and frequencies.

Sure, people like Burt Rutan build extremely sophisticated stuff- but its still CONCEPTUALLY pretty simple. No matter how you cut it though, autoclaved carbon fiber is never cheap.

I would love to see a homemade induction forge, that works as well as a commercial one- but I still say the electrical engineering involved, and a build quantity of one, means that it would cost MORE, not less, than a commercial unit.

The best way to get into induction heating cheap is to buy a used one- Jack Slack, a buddy of Grant's, and Grant himself, have found used machines from Boeing Surplus and on ebay for pennies on the dollar.

2ManyHobbies
11-23-2009, 05:33 PM
Absolutely.

But nobody is homebuilding fly by wire jets.

They are putting VW engines in glorified hang gliders.

Which, when you think about it, is just like making a forge from an old propane tank.

Which a lot of people do, as well.

The level of sophistication of homemade airplanes is, by and large, about on par with the level of sophistication of homemade forges. Neither of which, usually, features a lot of scratch built engineered electronic circuits at high voltages and frequencies.

Sure, people like Burt Rutan build extremely sophisticated stuff- but its still CONCEPTUALLY pretty simple. No matter how you cut it though, autoclaved carbon fiber is never cheap.

I would love to see a homemade induction forge, that works as well as a commercial one- but I still say the electrical engineering involved, and a build quantity of one, means that it would cost MORE, not less, than a commercial unit.

The best way to get into induction heating cheap is to buy a used one- Jack Slack, a buddy of Grant's, and Grant himself, have found used machines from Boeing Surplus and on ebay for pennies on the dollar.

It will happen, just a matter of convergence. SMPS is still bursting onto the scene everywhere as are switching components rated over 1MW. At some point it will be a mix and match like building a PC. Right now the hobbyist has to fab their own circuit boards and acquire buckets of little components, much like building a computer in the early 1980s. We are entering a stage where you can buy a rectifier, H-bridge, and driver stud and spade terminals. Not quite plug and play on the brains yet, but fast approaching the old ISA card era were everything has jumpers or DIP switches to set. Electric cars are going to really drive down the prices for the electronic hobbyist and we'll see all sorts of fun with robotics, transportation, induction heaters, and off-grid power.

dp
11-23-2009, 05:49 PM
If it's easy, no amateur has successfully done it -- every amateur induction heater I've seen, like the one you posted and the one I posted, are simply high voltage power supplies with fixed frequency dumping into a work coil.

http://www.youtube.com/watch?v=dxjD9jz9ImM

Good example of PLL control, bad example of induction heating - not enough power. With class D amplifiers you no longer need power switching - you just need a signal source (a simple voltage controlled oscillator), a PLL chip, a class D amplifier, and a tank circuit. This is probably more expensive than imagined as you need very high current, high Q capacitors. The coil can be copper wire or tubing.

Notice the O'scope as the frequency changes with movement of the work in the coil.


You really have to d!ck around with the induction of the work coil and the rapidly changing resonance of the workpiece (which is why the commercial units have a closed-loop PID control).

I was doing this kind of thing in the 1970's when PLL became all the rage for all manner of things.

boslab
11-23-2009, 06:02 PM
i'm not up to the challenge of DIY induction furnaces, i repair the industrial ones made by LECO from the US and to be honest they can be a nightmare, particularly when the oscillator starts its inevitable downward spiral [a dirty great glass valve about 5" tall and damned expensive]
i would love to build a more accesable unit but even though i do electrical engineering i dont have the expertise to produce anything chucking out 1200 amps consistently, maybee once or twice but i dont think it would stand up to 'industrial' use, i'm going to stick with propane for now, slow and low tec
if i found a good design i might be tempted to have a go depending on cost.
regards
mark

lazlo
11-23-2009, 06:03 PM
http://www.youtube.com/watch?v=dxjD9jz9ImM

Good example of PLL control, bad example of induction heating - not enough power.

Notice the O'scope as the frequency changes with movement of the work in the coil.

That's the typical feed-forward induction heater you see on the web. It's actually a copy of Richie Burnett's circuit that I posted earlier in the thread:

http://www.richieburnett.co.uk/indheat.html

It's hand-tuned for that specific setup, which is why it has really sucky performance. In other words, it's completely impractical to use in a shop.

By the way, he's using IRPP450's, which are 500V, 14A Power Mosfets, and he complains that they're getting hot at 350mA!. He's got a serious bug in his circuit! :D

The commercial units like Ries posted have an inductive LCLR matching network, and self-tune with a PID loop as the resonant frequency changes (as the part heats up).

lazlo
11-23-2009, 06:16 PM
Here's a much more sophisticated amateur induction heater. Uses an old-school parallel tank circuit for the oscillator. Note how fast the bolt heats up, and the heat is consistent as he holds it:

http://www.youtube.com/user/tonskulus#p/u/23/6IUTKUwtUuY

Here's the resonant circuit. Notice the 22 Kva hockey-puck capacitors. You can't use electrolytics because it's AC:

http://www.elisanet.fi/tonskulus/kuvei/ih_osc.jpg

dp
11-23-2009, 07:41 PM
The commercial units like Ries posted have an inductive LCLR matching network, and self-tune with a PID loop as the resonant frequency changes (as the part heats up).

I'm not sure I understand your point. A PID and a PLL are functionally equivalent in this situation. A matching network is used in all circuits I've seen. The PID and PLL both control the frequency that is fed to the heating coil (the LCR tank).

In very elegant systems where wide frequency ranges are used it is certainly possible to adjust the L and C elements (that is how antenna tuners work) as well as the frequency. The objective is to achieve zero phase angle (resonance) at the tank and to maintain it as an object is moved by the operator or it's characteristics are modified by the process. An open question is what do you use for feedback to recognize resonance and that is where phase detection comes in to play as one solution.

lazlo
11-23-2009, 07:46 PM
I'm not sure I understand your point. A PID and a PLL are functionally equivalent in this situation. A matching network is used in all circuits I've seen.

Agree that PLL is a form of PID, but despite the title of the video, and judging from the dismal performance of his induction heater, he's not doing any feedback -- he's just dumping the output of an H-Bridge into an LC circuit, and that's enough to eventually heat-up the bolt by brute force. That's what the vast majority of amateur induction heaters do, and why they're not useful.

dp
11-23-2009, 07:55 PM
Agree that PLL is a form of PID, but despite the title of the video, and judging from the dismal performance of his induction heater, he's not doing any feedback -- he's just dumping the output of an H-Bridge into an LC circuit, and that's enough to eventually heat-up the bolt by brute force. That's what the vast majority of amateur induction heaters do, and why they're not useful.

To get to that analysis you have to ignore the o'scope wave form and the claim of using a pll to control frequency over the range of tuning required to maintain resonance in his sample. I don't see a problem with using an h-bridge if it is a saturated switched amp or oscillator, and the LC circuit is what is used by the system Ries discussed.

There is no way I know of in a modern system to get a 15kw system without switched saturated mode devices in the final stage.

BobWarfield
11-23-2009, 08:43 PM
Heaters like the one shown in Ries' video are for sale on eBay, $2700.

See eBay item #310182521493 for example.

While they're cool, watcha gonna use them for? Heat treat? Blacksmithing?

Much cheaper than $10K, but still not exactly cheap to play with.

Cheers,

BW

Canyonman
11-23-2009, 09:07 PM
Hi All,

While what I was looking for was "Induction For Dummies," it has grown! At this point all I can say is "Huh?"

But PLEASE Carry On!

Thanks for some of the suggestions, but I've got a box full of Odds and Ends, $2700 is out of the question on a Retired Fixed Income.

Which reminds me, my subscriptions are coming due.

Take Care and Be Well, and Safe,

Ken

Don Young
11-23-2009, 09:42 PM
I know very little about this subject but I do have an old tube type 5KW heater which is basically a simple high power push-pull oscillator with the work coil and the work as part of the tank circuit. Any work coil or work reactance is coupled back into the tank circuit. It automatically oscillates at whatever frequency the whole business is resonant at. There is a variable tuning capacitor to shift the frequency but it can never become non-resonant regardless of the properties of the thing being heated.

dp
11-23-2009, 10:11 PM
I know very little about this subject but I do have an old tube type 5KW heater which is basically a simple high power push-pull oscillator with the work coil and the work as part of the tank circuit. Any work coil or work reactance is coupled back into the tank circuit. It automatically oscillates at whatever frequency the whole business is resonant at. There is a variable tuning capacitor to shift the frequency but it can never become non-resonant regardless of the properties of the thing being heated.

Some of those systems used broadcast radio tubes - got a picture? And that reminds me of something long forgotten. At High School in Berkeley I built a parallel line tuned UHF oscillator:

http://www.vias.org/basicradio/basic_radio_38_04.html

Don't recall the size of the tube, but it put out enough energy to light up a fluorescent tube. The frequency was changed by sliding the shorting bar on the transmission line. I wonder if it was still around when Evan went to school there.

Don Young
11-23-2009, 10:36 PM
I don't have a picture and it's pretty hard to get at right now. I know whereof you speak, I am 78 years old and have been fooling with electronics for over 70 of them. Got my amateur license in about 1952.

dp
11-23-2009, 10:39 PM
I don't have a picture and it's pretty hard to get at right now. I know whereof you speak, I am 78 years old and have been fooling with electronics for over 70 of them. Got my amateur license in about 1952.

Beat me by 10 years!