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Induction Heater problem
2

Induction Heater problem

Induction Heater problem

(OP)
I've got to go look at an industrial induction heater tomorrow.

What I know:
It's an EASYHEAT 7590LI solid state induction heater.
It's less than 2 years old.

It worked great for heating a tube up for a subsequent insertion of some piece into it for shrink fitting.
I'd do ~150 pieces a day typically.
One day it started not heating as well, taking longer and longer to heat the workpiece. No material/process changes.
They sent it to the manufacturer to check. Manufacturer said there were some minor issues, certainly no show stoppers, and that they took care of them. Returned it with a clean bill of health.


Still not working.

That's all I know so far. Besides the usual suspects of bad power, damaged cables, etc., is there anything typical of induction machines that's problematic?

Skogs I know you've worked on one.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

They sent the control unit/power supply only, or the induction coil too? Is the coil water-cooled, and is it getting good flow. Can't remember if there are conductivity limits on the coolant...

RE: Induction Heater problem

It's presumably all about driving (AC/RF) current into the coil. Google says 9kw and 150 to 400 kHz (hmmm... be careful).

If the "transmitter" has been checked out, then concentrate on the feed line and coil. Perhaps just a bad connection.

Sad Google results: "Your search - easyheat 7590LI diagnostics - did not match any documents." You'd think it would provide some operator feedback and system self diagnostics about the output power and load.


RE: Induction Heater problem

(OP)
Thanks guys.

Today's visit was.. odd. One of those "we can't run it because they're still using the press part of it".

They've replaced it with a bank of toaster ovens. They gun the work pieces and when they're at temp they grab them out and stuff them into the press part and run it.

btrue; It has a chiller and is water cooled. It has flow sensors that seem happy.
Conductivity limits.. I'll check that.

VE1BLL: Yes it's 9kW.
The unit has the power unit - a big 19" rack unit - and the "Transformer" (shoe box) which has The Capacitor in it and a crazy football shaped RF transformer with a fist-full of taps.

As you run the machine it will suggest you move the taps UP or DOWN if the coupling is not so good. They reported 'something' to the factory and the factory suggested they "move the tap down". Now the unit itself is suggesting they raise the tap. This is still not supposed to be a show stopper just an optimization.

If they put the machine into manual mode and jumper everything on the control terminals it appears to run OK. Yet if the control is handed back to the PLC and the terminal block on the rear, it has issues.

That's pretty much it. I'll be going back tomorrow when -theoretically- they won't be using the machine and I can.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Make sure that the 9kw of MW RF isn't getting into the controller.

But it'll probably be something completely unexpected. Good luck.

RE: Induction Heater problem

I built one of these things a long time ago at university, although mine was at lower frequency. Some topologies rely upon a tuning capacitor to bring the work coil close to resonance. You might want check the condition of the tuning cap if this unit has one.

The second from last paragraph - do you mean that in manual mode the unit is performing normally? If so then that presumably indicates that power stage is working ok and the fault is in the control or possibly in the resonance detection if the unit has a self-trimming capability.

RE: Induction Heater problem

"Yet if the control is handed back to the PLC and the terminal block on the rear, it has issues. "

So, it sounds like there's something amiss in that block...

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Induction Heater problem

(OP)
What a day.

We sat there working as assembly workers all day waiting for the machine to have one of it's conniptions. This, under the theory of, "it will crap-out after we do a few". 350 later... Nothing. I put all the covers back on and fully installed control unit back into the rack as we continued to run another hour of product.

After an hour or so it started to act up. Instead of 400 Amps it dropped to about 385A, occasionally and the display put up an asterisk which signifies the output being more than 2% off target. So the controller knows about the problem.

The controller automatically alters the drive frequency to optimize the power transfer. After running at 266kHz all day it was now wandering around to lower values like 185kHz during these anomalies.

This whole thing hinges on a tank circuit that couples the RF power thru a transformer to the user's heating coil. There's a key capacitor involved. Can anyone think of a way for a cap to shift 'quality' or capacitance without failing outright?

Since it did this after an hour of having the covers on, it cast the possibility of temperature as a participant. So I got a temp gun and a heat gun and sequentially heated every major parts group or circuit board in the controller. I got them them up to about 130F. This did not cause the anomaly to ever appear. Note: Nothing in the controller seems to get over 82F in normal operation Everything major appears to be water cooled anyway.

Still puzzled.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

At 9kw it must be something almost the size of a softball or maybe a shoe-box. (?)

What type of capacitor? Fixed or variable? What type of capacitor-making technology?

RE: Induction Heater problem

Sorry to be late, Smoked.
Sorry that I cannot help much, I'm afraid.
I think that you may have a load problem. If you could check coil current an voltage on a scope, that would probably help. You can see if the compensation is OK, current and voltage are well aligned (phase angle near zero) and it is easy to see if there are any deviations. But, be careful with the current clamp. If they are not made for those frequencies, they may start a fire or even melt down.

Tip: make your own clamp for higher frequencies. I have a few of those that I needed for a special investigation. Shall see if I can find a description. BBL.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction Heater problem

Quote (Gunnar)

...check coil current an voltage on a scope....current and voltage are well aligned (phase angle near zero)...

The E and I should be in phase (resistive matched load) at the input (transmitter side) to the matching circuit (assuming that the matching circuit is working correctly); but the coil itself (its current and applied voltage) should be on the inductive side of the Smith chart and ELI the ICE man would apply. Unless I'm confused (?) smile.

RE: Induction Heater problem

Here it is! Fresh from the oven.

http://www.gke.org/presentationer/files/Green%20fe...

The pdf describes a current clamp that I needed lots of for a bearing current investigation. I made ten of them. This particular one has 50 turns secondary and a 100 milliohms burden resistor. It can easily measure 10 A, probably more. If you need to measure more current, adjust number of secondary turns accordingly.

The ferrite quality is a high permealibity type with around 1 MHz band-width. For phase comparison, I think you can use just about any decent quality.

Broken? Yes! I broke it. If you hit the toroid with a mallet, it will break nicely and the irregular cuts aids in keeping the halves in place. Use nylon strip to keep them together.

Of course, if you can break the current path, you do not need to break the toroid.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction Heater problem

Smoked,

Is it possible that the power supply and induction coil are just fine and doing what they are supposed to do, and the fault instead lies in the part? You say the parts are tubular - are they welded into tubes or drawn? If welded, I wonder if linear flaws or variations like that in the weld seam interrupt the eddy currents that the induction coil is trying to generate. Might be worth doing that day-long experiment again, but this time grab samples of parts that heat ok to compare to parts that give the controller fits. Maybe it's as simple as wall thickness variations in the parts, or more complicated like local areas that are more heavily strain hardened, or have more oxide inclusions...dunno but it might be worth looking at. Finally, if the 385 amp parts still got hot enough, is there really an issue?

RE: Induction Heater problem

(OP)

Quote (VE1BLL)

At 9kw it must be something almost the size of a softball or maybe a shoe-box. (?)

What type of capacitor? Fixed or variable? What type of capacitor-making technology?

Sorry VE1BLL. I'm not actually sure. It is a white labeled cylinder about the diameter of a coke can and about 3" long. It has a 1/4" stud sticking out the center of each epoxy potted end. Does that give us any clues?

Quote (Gunnar)

Sorry to be late, Smoked.
Sorry that I cannot help much, I'm afraid.
I think that you may have a load problem. If you could check coil current an voltage on a scope, that would probably help. You can see if the compensation is OK, current and voltage are well aligned (phase angle near zero) and it is easy to see if there are any deviations. But, be careful with the current clamp. If they are not made for those frequencies, they may start a fire or even melt down.

Hi! Better late than never! :)
I actually don't know what voltage to expect or what a DMM will think of 400A at 250kHz. At that current I'm expecting a really low voltage, but I'm not sure. Will the DMM even give me a correct voltage value? Since I don't yet know the voltage, I've hesitated hooking a scope probe across the coil.

Quote (VE1BLL)

The E and I should be in phase (resistive matched load) at the input (transmitter side) to the matching circuit (assuming that the matching circuit is working correctly); but the coil itself (its current and applied voltage) should be on the inductive side of the Smith chart and ELI the ICE man would apply. Unless I'm confused (?)

"ELI the ICE man" haven't heard that one in a while! Current on the heating side should lead the voltage. Makes sense. Nice to know there shouldn't be much difference on the supply side. That's something I can check. Is that why the controller messes with the frequency? To try to avoid any phase difference so maximum power is transferred?

Quote (Gunnar)

Here it is! Fresh from the oven.

http://www.gke.org/presentationer/files/Green%20fe...

The pdf describes a current clamp that I needed lots of for a bearing current investigation. I made ten of them. This particular one has 50 turns secondary and a 100 milliohms burden resistor. It can easily measure 10 A, probably more. If you need to measure more current, adjust number of secondary turns accordingly.

The ferrite quality is a high permealibity type with around 1 MHz band-width. For phase comparison, I think you can use just about any decent quality.

Broken? Yes! I broke it. If you hit the toroid with a mallet, it will break nicely and the irregular cuts aids in keeping the halves in place. Use nylon strip to keep them together.

Of course, if you can break the current path, you do not need to break the toroid.

Very cleaver Gunnar! Thanks MUCH for the Data Sheet. Where do I order these? What are the nylon tubes for?, to tie-wrap the halves together?
10A huh? Did you notice I'd have to measure 400A? Suggestions?

Quote (btrueblood)

Smoked,

Is it possible that the power supply and induction coil are just fine and doing what they are supposed to do, and the fault instead lies in the part? You say the parts are tubular - are they welded into tubes or drawn? If welded, I wonder if linear flaws or variations like that in the weld seam interrupt the eddy currents that the induction coil is trying to generate. Might be worth doing that day-long experiment again, but this time grab samples of parts that heat ok to compare to parts that give the controller fits. Maybe it's as simple as wall thickness variations in the parts, or more complicated like local areas that are more heavily strain hardened, or have more oxide inclusions...dunno but it might be worth looking at. Finally, if the 385 amp parts still got hot enough, is there really an issue?

Hail btrue; No chance. The work pieces are forged precision-machined aluminum block-things, not tubes as I thought. Tubes are rammed into the blocks once they're heated (expanded). Once the 'issue' occurs it hangs around,(so I'm told), which precludes the actual parts being the cause.

Added information discovery:

This thing runs on 208V 3ph. The factory tells us the only thing that would cause what we're describing would be if the line supply voltage is sagging. It does sag but from about 210 to 207.5V and we couldn't see any direct correlation between the sag and the output dribbling off the few minutes we saw the phenom.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Smoked
I think that you can order them at the Flaming Systems Company somewhere near S.F, CA. Look them up in the Yellow Pages bigsmile

"to tie-wrap the halves together?
10A huh? Did you notice I'd have to measure 400A? Suggestions?"

Yes, to tie-wrap the halves together.

400 A. I was wrong when I said 10 A. The actual measurement is 53 A and nothing says it can't take 100 A. Use 200 turns secondary and .1 ohm burden to get 400 A capability. Use AWG 20 or thereabouts.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction Heater problem

Have you looked at the temperature and flow of the cooling water?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Induction Heater problem

Quote (itsmoked)

If they put the machine into manual mode and jumper everything on the control terminals it appears to run OK. Yet if the control is handed back to the PLC and the terminal block on the rear, it has issues.

The above quote seems to be the most important clue. Maybe it should be very carefully rechecked. If it's true, then it excludes many possible faults.

RE: Induction Heater problem

(OP)
Skogs; Thanks! Will do.

Bill; Yes. This thing requires a chiller. The chiller is about 3 feet away with a large active temp display on the front. You can see the coolant temp at all times. You can watch as it climbs every cycle and the returns almost to pre-cycle temp. If production is continuous you can see it's steady state value rise a few degrees. It keeps the coolant around 76F. I'm guessing this prevents condensation issues.

VE1BLL; This was considered a priority. I essentially spent hours focused on the connector. I went so far as to replace it. They'd made a statement to me that, "When there's a problem the current drops to nothing", which seemed black and white. Later interrogation had them recant this, then I saw the 3~8% decline myself, confirming the reality that it doesn't, "drop to nothing", like an intermittent control signal might.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Keith,

Must admit, I have no clue how that auto-tuning circuit works. Units I have seen/used in the past were strictly of the turn it on and run it til the part gets hot variety*. What is the feedback that the PLC/controller is using to vary the frequency - the phase angle b/n current and voltage at the coil? Is that somehow correlated to the part's bulk temperature? Is there any direct measurement of part temperature? Sorry if this is of no help, but I'm just curious how you can get from coil RF measurements to the power dissipated in the part (I sorta see how that could work, but there are assumptions to that equation that I'd not take lightly) - but from power dissipated by the coil to part temperature rise would seem to require some more information, like part mass and specific heat, and cooling rate vs. temperature, and...

* along those lines - what prevents this device from being operated with manual/fixed settings for frequency and time, and just set it and forget it, once a few tests are run to find a "best" setting? You seemed to state that if the controller is left out of the loop, the unit functions just fine?

RE: Induction Heater problem

(OP)
Btrue; A typical open loop method is using straight time as the control. That's what you see on the front display. Time, frequency, current set, current delivered. When GO is enabled the unit starts counting down to zero.

In this application a PLC is reading the temperature of the workpiece. There are two non-contact temperature sensors that are looking at the workpiece. Once they both reach temp the PLC commands OFF to the EASYHEAT.

This is all a mute point because the "anomaly" is something the PLC cannot cause. That is, the delivered current dribbling off. The PLC cannot, in any way, command or adjust the power setting. Only ON/OFF.

I think the reason the unit worked with the PLC removed from the equation was because the unit was uncycled for the time it took them to change the connectors out. Same as my experience. It just started to happen then we ran out of workpieces. By the time we got more it refused to crapout again. Hence that reported 'condition' was only seen once and not repeatable.

The frequency is automatic. The unit just adjusts it each cycle continuously. Normal deviation is about +/- 1kHz during each cycle. The unit adjusts it to maximize power output. When the anomaly appeared the freq started deviating widely. By ~100kHz. This makes me suspect something in the "power head" or other output stuff. It's just physically hard to look at that stuff, plus I am not good a fixing things that are working correctly. I've asked them for some special workpieces that we can rapidly cycle thru the machine to stress it and hopefully make the anomaly show up on demand in minutes instead of hours and depending on workpiece availability.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Interesting update on the controller. I've had cases where the reported evidence 'proved' that the fault could only exist in an alternate dimension of space (excluding all possible causes in This Universe). I've learned to mentally append the word "maybe" to all such reported evidence.

Where does the operating frequency (150 to 400 kHz) originate? Being a communications guy, I had assumed that there would be a low power, adjustable (controlled) oscillator that is then amplified to 9kw. An alternative architecture is a big silly free running 9kw oscillator. The key difference between the two is that in the former (low power oscillator under some frequency control), the unexplained frequency shift is more likely to be an effect (red herring) rather than a cause. In the latter (free running power oscillator), the frequency shift might be a direct part of the failure mechanism.

Is there a schematic of the "RF" section?

Does it use a bank of transistors in parallel to make the 9kw? If you have power transistors that are heat-sinked and soldered into a PCB, then there might be stress failures of the solder joints at the PCB (thermal expansion of the Al heat sink yanking on the devices' solder joints).

In my experience, about half of all such failures are visible under close inspection (the other half being internal to a component). It can be worth a close inspection of the likely areas (hot, stressed) under a high power microscope.

RE: Induction Heater problem

Another idea: if you haven't already, monitor the internal power supply voltage(s). If they drop coincident with when the RF output drops, then the root cause must be in the PS. If the voltages ever so slightly tick up with the reduced output, that's normal and expected IR drop delta. If they're steady, that's good too.

RE: Induction Heater problem

VE1BLL,

If these are ferrous workpieces then the magnetic properties change as the workpiece reaches the Curie temperature and the load coil resonant frequency changes. Most commercial heaters have the ability to track the resonant point to maximise power transfer into the workpiece.

The actual choice of frequency is based on the desired penetration depth - for localised surface heating for (e.g.) to harden slideways the frequency is high to limit the skin depth, where for bulk heating the frequency is relatively low to uniformly heat the workpiece.

RE: Induction Heater problem

I saw it has serial interface... what can that tell you? perhaps you can watch some pesky internal variables real time and see which one(s) change to see where in the circuit the change occurs to help narrow down if it is in the osc, power amp, feedback, or load?

RE: Induction Heater problem

Scotty,

Smoked said they were aluminum parts. I'd agree that you'd want the freq. fixed once you've figured out the heating cycle you want for the particular part.

Smoked, you mentioned getting some more parts to test. I'd think you/they could come up with a) about 3 or 4 scrap or salvaged parts that can create reasonable facsimiles of the production parts, and b) several large ice chests filled with ice and maybe some water to drop the hot parts into to cool them down before running the heat cycle again, and c) some tasty beverages to help fill the excess volume in said ice chests and to keep the test operator(s) occupied whilst waiting for the phenomena to recurr. I hereby volunteer to assist you with the tasks of 1) specifying the beverages, 2) maintaing the ice chest with the beverages at the proper temperature (to be determined by repeated random sampling of said beverages), and 3) by pushing the little button that makes Skog's test-o-scope go "ping".

RE: Induction Heater problem

(OP)
mikekilroy; If you look further... You will see the serial comm is "optional". They didn't check that box. :) Otherwise that's a great idea.

btrueblood; Excellent idea. Also exactly what I recommenced. We made about 20 rejects so there are lots available. I described the cold water bath and was told, "I'm uncomfortable with water near the machine". It would work absolutely fine with no issues and not even potential issues. Heck, the actual jig is even plastic! But I didn't feel like pressing the issue at the time.

Last night I tore my LCL**, so one thing I know.. I won't be wondering around any factories in the immediate future.



** Lateral Collateral Ligament

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Aw Keith. The military never worries about collateral damage, why should you? grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Induction Heater problem

"I'm uncomfortable with water near the machine".

And you of course didn't remind him that the coil is filled with water (gasp), under pressure (shudder). But that (and the blown LCL) is why you have the trusty assistant - to fetch cold objects from the bath, and carry hot ones back to the bath. Put the bath around the corner, down the hall, out the door, around the parking lot, etc. Or even in the pub across the street.

RE: Induction Heater problem

We haven't heard from you in this thread for awhile Keith. I hope that you are healing well.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Induction Heater problem

Getting worried, too.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Induction Heater problem

(OP)
I'm here guys. Thanks for checking. Nothing has transpired on this project. Left them with the assignment to jig up so the problem can be brought on in a reasonable time.

No admission to this achievement yet. Just as well, I'm still pretty chair bound, haven't left the house since.. Ice, Ice, Ice, Ice, Ice, my 'walking' orders.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Let me guess; Playing sports with the kids. grin.
I had a little temporary pain last fall. I went riding with my small son to help a friend move some cattle. My horse stopped abruptly.
As my body continued forward, the saddle pushed my legs out sideways. Then inertia bent me over at the hips and left me with my arms wrapped around the horses neck. I have a little arthritis that usually doesn't cause any pain, but in that position.. OUCH. But the pain soon left and my son and I have some wonderful memories to share for years to come. (ps. I'll probably do it again.)
I did decline to go skiing with him this year and just sat and watched.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Induction Heater problem

((( I was preparing the car for the trip South in October, forgot that I had installed a hitch ball, and banged my knee on the ball support. I spent an extra month with the knee elevated and immobilized and painful while trying to get a decent image and a diagnosis, which an MRI eventually revealed was more or less equivalent to a spall on the convex surface of the tibia. By November I could drive, and walk, slowly, with a cane. Now I don't need the cane everywhere, but i have to go up and down stairs one leg at a time. Up with the good, down with the bad. Forgetting brings immediate pain from the bad knee. ... which is still healing, slowly. )))

Part of my first job, 45 years ago, involved taking care of axle scanners, which induction heat and then quench an axle shaft while it's rotating. The cycle took maybe 30 seconds to 'scan' the shaft with a hollow copper coil, two-ish turns, maybe 3/8" diameter, thick walled, brazed to large copper terminations. New coils were fairly rigid, but after a couple thousand parts, i.e. every few days, the copper would get sort of microcracked so they didn't conduct properly, and the locomotive size M-G sets on the mezzanine would complain and the case depth (which was checked hourly) would go out of spec, at which time we'd have to call the scanner tech to install new coils.

I.e., when you start banging out a lot of parts, induction coils become a consumable.

In between coil changes, they'd still have to be removed and cleaned and flushed of stuff like boiler scale that appeared on both internal and external surfaces.


Mike Halloran
Pembroke Pines, FL, USA

RE: Induction Heater problem

(OP)
AH man you guys give me pain just thinking out your injuries. Ouch.

I was on a 3 mile hike with the dog. It got dark on us. I stopped at an oft missed turn to study it. The path I was standing on collapsed down a steep hill under my left foot. Not desiring to fall a hundred or so feet I immediately pitched with everything I had forward. Everything went onto my right knee cap, even skinning it. It hurt but after 5 minutes dwindled off. I trekked the mile and a half back to truck without issue.

Couple of days later I got up to walk dogOwitz after dinner and my knee hurt enough I found myself limping a bit on the evening mile. Next day my LCL was a little sore. That night I was really limping right out the gate. I cut the jaunt short I subsequently went to my office.

Coming out late the big security gate was closed. I limped over to open it. It's a big ten foot wide chain-link vehicle gate with slats thru it making it really heavy. It's so heavy it has a set of outrigger wheels on the opening end. Sometimes while looking at the latch you run one of those wheels into your foot.

Tonight I looked at the inside wheel, where I was, and thought what if I push that wheel with my foot? I did it with my right limping foot. The gate blew open! 'Wow that works really well'. Must be the effective gear ratio or something. I walked thru then had to reverse the process. I immediately realized I couldn't use my right leg to reverse the process because 1) I had to make the gate come towards me now instead of a shove-away. The outside wheel was now next to my left foot so I stood on my right limping leg and tried to "foot" the gate towards me with my left foot. This put all my weight on my bad knee. And kicking back towards oneself is.. cumbersome. I promptly lost my balance with the twisting/footing motion and to recover it, my bad leg had to provide the restorative torque. There was a tearing sound in my head and a lighting bolt of pain out of (what I now know) was my LCL. It brought tears to my eyes. Crossing the road 30' to my car took me five minutes. I could weight bear on it at all the next day. Luckily my son was just off his crutches (Lyme) so I was able to commandeer them.

Mike: Thanks for that info. I am pretty sure the problem is in the "head" where the matching transformer and the coils connect or the coils. I'm not sure the coils are water cooled. The coil wire is really small and wrapped with some burnt orange colored tape. They sent the head back for testing but not the coil. Definitely something to check.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

Sorry for the late contribution to this thread. Just wanted to toss in my experience with a similar problem of a wander resonant frequency on an induction brazing system (for attaching carbide tips to saw blades). It turned out to be gradual failure of the film capacitors that are used to form the resonant tank with the work coil. The proximate cause was (painstakingly) traced to a failure of the joint between the terminals and the "schoopage", likely from thermal cycling. In short, whenever there is a problem with an induction heating system I suspect the tank capacitors first.

BTW - solved that particular problem by replacing the film capacitors with ones designed for GTO snubber use. AFAIK, that induction heater is still in service some 10 years later.

RE: Induction Heater problem

(OP)
Very good info! Tell me that cap would be in the work head instead of the controller?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

itsmoked: yep, the tank capacitors were inside the work head. Another important factor was that there were several capacitors in parallel and the one closest to the induction coil had the most internal damage - obviously because the circulating current between the L and C wasn't dividing equally into each capacitor. I don't remember the exact value of the capacitance, but it was somewhere in the 1-2uF range and was easily accommodated by a single GTO snubber capacitor. I also recall that the resonant frequency of the system changed enough to technically take it out of "spec", but the but the controller/driver had no problem with it.

RE: Induction Heater problem

(OP)
Thanks again Magic!

You mentioned "gradual failure". How exactly did the symptoms manifest?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Induction Heater problem

I wasn't really given an exact sequence of the progression of the symptoms from the client, just a complaint that the brazing process was taking longer. What I concluded based on the internal damage to the capacitors was that the ESR of the capacitors increased as the connection between the end spray metallization ("schoopage") deteriorated, leading to more heating and more schoopage deterioration. Not precisely the same symptoms as what you've reported here, but I figured it was worth mentioning.

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