# Induction Heaters



## TheDude_Abides

Anyone have any experience with any induction heaters? Namely Ajax-Tocco. We are a forge facility and have a 600kw Pacer 2 heater that will shutoff if you turn the power above 40%. I ran about 20 bars through our coils at 40% no problem. Turn it up to 50% and it trips out on overcurrent. I checked all the bolted connections with a Fluke 568 and everything was within tolerance (60-70 degrees F, the bus is watercooled). Anyone with any experience would be appreciated.


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## Navyguy

Check the SCRs and the caps. If the caps are banked, then they will cut in and out as the load requires. if the bank of caps is shorted it trips the breaker.

The other issue I would check to see if the coil is isolated and not shorting out on carbon dust.

Cheers
John


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## TheDude_Abides

Checked all SCR's and had to replace two. The Caps are banked with either 2 or 4 resistors. 2 caps per SCR. We have a coil bed with three coils on it. We are planning on changing the coils out in the morning because there is no way to inspect inside the coil without cutting it in half due to the cast refractory. Do you have alot of experience with these type heaters? If so I would like to pick your brain.


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## Navyguy

I worked maintenance in a forge for a few years. Our furnaces were pretty old and we were always "tweaking" them for different product runs.

We would send out control boards for repairs calibration as necessary. We built / rebuild out own coils or did repairs to the cooling when needed. The only time we sent coils out is when we had a backlog to do and needed inventory. We stocked about 100 or so different coils, some singles and but mostly doubles.

Please take the time to fill in your profile and I am sure others will assist.

Cheers
John


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## John Valdes

Welcome to Electrician Talk Dude.
Please take a few minutes and fill out your profile.


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## TheDude_Abides

Done and Done.

So, can anyone explain to me exactly how this process works. I mean I dont understand entirely why you need thyristors (SCR's) is parrallel with capacitors and resistors for each phase. I understand what a phase reactor does but these...I dont know. I guess what Im getting at is, I need to fully understand the functioning of certain parts so I can piece it all together.


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## paulengr

TheDude_Abides said:


> Done and Done.
> 
> So, can anyone explain to me exactly how this process works. I mean I dont understand entirely why you need thyristors (SCR's) is parrallel with capacitors and resistors for each phase. I understand what a phase reactor does but these...I dont know. I guess what Im getting at is, I need to fully understand the functioning of certain parts so I can piece it all together.


The SCRs are basically diodes with a switch to turn it on. They are used as variable output rectifiers. Lamp dimmers work the same way except they are AC output. The caps smooth out the voltage ripples. DC in coils is about 300% more efficient than AC so you rectify. Also its not just for temperature control. At a certain point you get negative resistance in induction furnaces so you have to cut the voltage back or Go into thermal runaway (burn up). The point where this happens is very fast so don't think an operator will catch it in time.

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## TheDude_Abides

Are the SCR's a rectifier or a diode? Both? If youre rectifying AC into DC what ripples are there in a DC sine wave? Why do you need resistors in series with a capacitor?


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## TheDude_Abides

So we got the heater to work for about 8 hours and it shutoff on overcurrent again. I attempted to break the bus bars going to the heating elements and now Im getting a start voltage fault. I imagine the power supply has to see a load to keep it running as maybe a prove circuit. What can I put in the circuit to simulate the load? A resistor of some kind? Can I jumper the logic board, if I do do I risk damaging the power supply?


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## TheDude_Abides

The operators said they noticed some arcing from the new coils. Thats why I am attempting to isolate.


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## paulengr

TheDude_Abides said:


> Are the SCR's a rectifier or a diode? Both? If youre rectifying AC into DC what ripples are there in a DC sine wave? Why do you need resistors in series with a capacitor?


WTF? A diode (ignoring the avalanche knee) blocks reverse bias voltage. An SCR blocks reverse bias current (not voltage! Subtle difference). In the forward direction it blocks voltage until you pulse the gate. Then it does not turn off until the current reverses. Not even getting into forced commutation or issues with self commutation.

Rectification means cutting off the reverse polarity. It's a function. Diodes and SCRs are devices. DC by the very name does not have ripples. That's AC or something else riding on the DC. But pure rectified AC is pulses. Capacitors store energy as they charge then release it as they discharge. If you feed it pulsed waveforms it smooths them out.

You don't need a resistor in series assuming your load has enough burden. Lots of induction furnace drivers don't use one because there is enough inherent resistance in the circuit. They already monitor current/voltage and control the SCR as needed to limit current. Not the same as a linear power supply. Induction furnaces are current control, not voltage. Most use fast fuses just to protect the SCRs or caps from exploding in the event of a dead short instead of the controller and a resistor. If you rely on that scheme some day when an SCR or a cap goes bad you're going to kill someone. Not that some idiots (Siemens) haven't done it.

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## paulengr

TheDude_Abides said:


> So we got the heater to work for about 8 hours and it shutoff on overcurrent again. I attempted to break the bus bars going to the heating elements and now Im getting a start voltage fault. I imagine the power supply has to see a load to keep it running as maybe a prove circuit. What can I put in the circuit to simulate the load? A resistor of some kind? Can I jumper the logic board, if I do do I risk damaging the power supply?


Yes. It needs some burden to control it but the resistor will be pretty bug wattage wise to simulate a load. Mostly I've seen 55 gallon drums filled with salt water for this. It sounds scary but it works. Shredders on wound rotor motors use the salt water trick to ramp up torque if the shredder starts to stall.

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## Navyguy

I have never found a load to work other than a coil; thanks for this tip Paul. If we had a suspect coil we would simply remove it and put in a new coil. Coils were "matched", not for any particular reason other then to keep them together for life cycling. Never run the furnace without a load in the coil or without a coil (unless Paul's idea works).

I found that the coil has to be pretty close to the billet size or it goes out on current overload. I found it was a balance between getting coils that were close fitting to not ramp out of control and large enough to allow some variation in the billet shape (often the shear end). Running too small material in the coil will wear out the refractory material on the bottom and will ramp out of control because there is not enough material.

If the coil is shorting out and if you are not leaking coolant, simply put a patch in. Heat the coil up with a larger billet then when patch is set, change the material back to the correct size. Again, it is important that the operator keeps the coils, rails and decking clean of carbon dust or it will track and blow a hole in your coil or rails. The cap cabinet should be cleaned out every shift if it is not well sealed, which most of them are not.

Cheers
John


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## paulengr

Agreed. The trick with salt water resistors is adjusting the bus bar electrode lengths, water depth, and salt concentration to hit the right resistance. Google liquid resistor or liquid rheostat. These are popular in Europe and when you need a rheostat. Never popular in the US because they’re downright scary in concept.

Coil matching is necessary if you don’t have one coil per driver. Otherwise you get heat = IxIxR so if R is off too much/little.


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## TheDude_Abides

So We broke the bus bars and I ohm'd everything out. Since the bus is watercooled there are hose clamps on the Neg that were touching the Pos. We've buttoned everything up and are going to try again after break. Previously I was getting a reading bus to bus and then a reading to ground. Now I have neither. Next question. When I megger these coils what kind of reading can I expect to get in respect to ground. I understand from positive to negative is a dead short. It should read completely open to infinity right? Im meggering at 1000v and it will only build to 70v with around 0.048M ohms. That to me doesnt seem right. If you have a completely open circuit then their should be no resistance. Yes the coils are isolated.


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## TheDude_Abides

Also, if the water conductivity hasnt been checked in a while, is it possible that there may be too much mineralization in the water to cause a fault?


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## Navyguy

I agree that there needs to be open between the coil to ground, you might be getting a reading if the coil is leaking / damp and is dirty.

I don't recall hard water causing a fault, except for cooling. Our water softener ran overtime to keep the water cool and in supply. We dumped about 40% of our makeup water to make our cooling water... not sure if there was a better way, but we went through a lot of water.

Cheers
John


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## TheDude_Abides

Also, if the water conductivity hasnt been checked in a while, is it possible that there may be too much mineralization in the water to cause a fault?


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## TheDude_Abides

So, still faulting on overcurrent. Checked DC mA between buses and I'm getting around 5 mA. Changed out one coil and im getting around 3.7mA. So I guess I'll be changing the coils. Any cause for the coils doing this? Theyre freshly rebuilt. The ones I am pulling off were freshly rebuilt as well. Albeit, they did run for about 8 hours before causing the fault. I know there is always the chance the builder could be causing some discrepencies but I dont want to point fingers unless im 125% certain.


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## Navyguy

Hard to tell without being there, even then it is best guesses at this point if you don't know your furnace.

I don't know if those numbers are good or not, but they should be similar to similar coils, that is why they need to be matched. My first couple of guesses might be that the coil is not properly centred in the refractory or perhaps the coils do not have the same turns on them; might be the same length, but one might have an extra turn...

Are you getting the same heat out of each or is one hotter then the other?

Cheers
John


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## paulengr

TheDude_Abides said:


> So We broke the bus bars and I ohm'd everything out. Since the bus is watercooled there are hose clamps on the Neg that were touching the Pos. We've buttoned everything up and are going to try again after break. Previously I was getting a reading bus to bus and then a reading to ground. Now I have neither. Next question. When I megger these coils what kind of reading can I expect to get in respect to ground. I understand from positive to negative is a dead short. It should read completely open to infinity right? Im meggering at 1000v and it will only build to 70v with around 0.048M ohms. That to me doesnt seem right. If you have a completely open circuit then their should be no resistance. Yes the coils are isolated.


Generally over 100 megaohms is acceptable for wire, medium voltage, etc. On under 1000 V motors it's 5 megaohms or more though because the coil insulation is thin for power density on AC. It's not unusual though for rotor coils and similar thin, high current coils to be 1 megaohm per 1000 V which is more likely in your case. The standards for under 1000 V is to use 500 V whether AC or DC according to IEEE and NFPA. Only NETA who don't usually have any rhyme or reason for what they do recommends going that high. This is an insulation test, NOT a conductivity test. Check conductivity with standard ohm meter. Better yet if you can test inductance which can be used like a turns ratio test to detect shorted turns.

You still have something shorting somewhere or the coil is burned up.

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## paulengr

Also megger tests are HIGHLY sensitive to contamination. It's almost really a contamination test. And they are temperature sensitive.

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## TheDude_Abides

So I was poking around our store room and found a 5kv hipot. Seems to be older but well taken care of. Assuming its calibrated, can I hipot the coils? I megged the capacitor this morning and all is well but I know I can hipot that as well. Anything else? Do I need to isolate anything is the power supply itself?


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## TheDude_Abides

So I hipotted the coils to ground. The ones that are on the heater right now will not build voltage past 700vDC. The DC mA spike at 5 and the breaker trips the tester. I checked some of the others in the coil room and everything on the front of racks are doing the samething. However the ones torward the back of the racks built to 1250vdc with minimal rise in current. I believe I may have found the red hering.


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## TheDude_Abides

I did isolate the coils from the power supply.


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## Navyguy

I have never Hipot tested a coil; or anything for that mater...

My question are the coils damp / full of cooling water? I would drain the coils and heat them up with an external source to dry them out and then see what you get.

Having said that, if the coils are shorting out, it could be for a number of reasons, the primary reason being there is a pinhole and the cooling water is migrating through the refractory. There also could be a gap in the refractory or there could be carbon building up somewhere on the inside if the refractory is not smooth. This happens when product is too small for the coil or the rails are not setup properly and the billets drag along the bottom of the coil. Carbon dust builds up in the crevices and eventually works it way to the coil and blows it out.

Cheers
John


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## John Valdes

Thanks for taking the time to fill out your profile.


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