One man's insanity---the story of an induction furnace

The Photo Album is a place for "Shop Shots" as well as pictures and descriptions of projects that we are working on. Show off your Shops, Machines, and your Projects!

Moderator: Harold_V

rrnut-2
Posts: 691
Joined: Mon Jul 18, 2011 7:40 pm
Location: Bennington, NH

Re: One man's insanity---the story of an induction furnace

Post by rrnut-2 »

"I've been aware of the idea that a "heel" is needed when firing the furnace. This heel provides a large enough mass for rapid heating, and that makes sense, but by starting with a load of small pieces, you get heating, it's just a lot slower. Is that correct?"

The "heel" is made after the first melt. Tilt the furnace and allow the metal to cool to form the "heel". This will help with the next heat. We used a lot scrap from the auto industry, ie frame stampings, sheet metal pieces.

I suspect, especially after witnessing a high amperage but low voltage, that the wattage doesn't rise until the metal is molten, or that there is a large, one piece charge. I used the largest piece I had that I was willing to heat (didn't want to heat the larger piece of 17-4 PH, for obvious reasons).

If you started with a small amount of metal, yes. But, with your unit, you may have to adjust the lead/lag more often to get the wattage up and maintain it. Also once the charge hits the curie point, your power, lead/lag and voltage will change. Bear in mind that the last motor-generator set that I looked at while running was in 1978. Once the inverter type units came in, a lot of this was automatic. The melters would load the furnace and spin the power control clockwise and walk away for 10 minutes or so. The power would only be turned off while deslagging.

I intend to use a length of pipe as my forms. They will be melted and become a part of the first heat. For the furnace you provided, schedule 40, 6", and for the furnace I had, schedule 40, 5". I have read that the first charge should be slightly overfilled (with more added when the metal is molten) to ensure sintering of the lining above the full melt line.

Yes, sinter the refractory up high on the furnace. I've seen the melters run to the top of the lining where if they dropped another ounce in, it would overflow. Also, first heat on the lining slow and easy does it.

Would you please address this idea, and provide further guidance?

As my primary interest is in melting steel to produce gray or ductile iron, is there a particular lining you'd advise? Also, as the top and spout are formed from a different refractory, what would you advise for that function? I hesitate to adhere to the information provided by Ajax, as it relates to technology of the early 60's, and much has changed since then.

I will have to get back to you on the refractory material. I am not the expert there. But I know who is!

Jim B
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: One man's insanity---the story of an induction furnace

Post by Harold_V »

tornitore45 wrote: Wed Jan 29, 2020 7:47 am I know nothing about induction ovens but do know about electromagnetism.
The transformer cores are laminated to avoid eddy current losses. The idea is that as the current path is broken up the resistance increase reducing eddy currents and the power lost.

Clearly the opposite is desirable in a melting situation. In an induction oven, serious losses and heat coupling does not rise until a bird nest of small pieces coalesce into an interconnected molten mass.
As my knowledge in electricity is limited to personal experiences, as well as what I learned in high school, which was many years ago, I may say something stupid here, so please bear with me, and feel free to correct anything I say that isn't correct. I hope to not mislead anyone.

An induction furnace is looked upon as a transformer with a shunted secondary. It relies on eddy currents for not only melting, but stirring action, which is rather abrupt, and a good thing, as that ensures a homogeneous heat.

Melting is reliant on frequency. As the size of the heat diminishes, the frequency must increase accordingly. The power supply I have was built to operate with a 3,000 Hz output, 400 volts, single phase. The identical unit was available with a 10,000 Hz output, which would allow for melting starting with smaller items, although the overall capacity would be the same.

It is advised that chips (swarf to some folks, but that isn't proper terminology in the US. Ever hear of a swarf conveyer for a CNC machine?) be placed at the bottom of a charge, as they don't heat well. By placing them at the bottom, they will readily combine with the molten metal that reaches them. The stirring action of the furnace assures that everything that melts will be combined.

Jim's mention of charging a new furnace with small sized pieces allows for a slow heat, which is desirable to expel any moisture. The heating rate can be seriously high, resulting in damage due to steam explosions from residual moisture in the lining. A slow initial heat allows for its escape without doing damage.

For those with an interest, here's a graph that displays the ideal frequency versus heat size. (right click to open a large display)
page 3 cropped.jpg

H

Swarf: A name commonly applied to the fine particles resulting from grinding.
Wise people talk because they have something to say. Fools talk because they have to say something.
User avatar
neanderman
Posts: 896
Joined: Mon Jan 09, 2012 7:15 pm
Location: Cincinnati, Ohio, USA

Re: One man's insanity---the story of an induction furnace

Post by neanderman »

Harold_V wrote: Thu Jan 30, 2020 1:57 am I may say something stupid here...
I intensely dislike the use of "stupid" in a forum like this.

You can be unknowing, perhaps uneducated or un/misinformed -- possibly even ignorant, though I consider that a willful state, which, again, is unlikely in a forum like this-- but not stupid!

I'm all for self-deprecation, but let's not undersell ourselves.

Stupid: Lacking in intelligence or exhibiting the quality of having been done by someone lacking in intelligence.
Ed

LeBlond Dual Drive, 15x30
US-Burke Millrite MVI
Atlas 618
Files, snips and cold chisels

Proud denizen of the former "Machine Tool Capitol of the World"
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: One man's insanity---the story of an induction furnace

Post by Harold_V »

Chuckle!
Just trying to cover my bases, as I am not the person I once was, and tend to forget the obvious. Some things I say do sound stupid, although I do my best to not display that side. :wink:

There are times when I think I remember something, only to be surprised that it isn't correct. Thus my request to be corrected. I have no qualms with being told when I'm wrong. To do otherwise would be a mistake, as I wouldn't continue to expand my small world of knowledge.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
User avatar
neanderman
Posts: 896
Joined: Mon Jan 09, 2012 7:15 pm
Location: Cincinnati, Ohio, USA

Re: One man's insanity---the story of an induction furnace

Post by neanderman »

For better, or worse, none of us are the "person we once were!"

Here's to never believing we are infallible, while always admitting that even an old bear can be taught new tricks!
Ed

LeBlond Dual Drive, 15x30
US-Burke Millrite MVI
Atlas 618
Files, snips and cold chisels

Proud denizen of the former "Machine Tool Capitol of the World"
rrnut-2
Posts: 691
Joined: Mon Jul 18, 2011 7:40 pm
Location: Bennington, NH

Re: One man's insanity---the story of an induction furnace

Post by rrnut-2 »

An induction furnace is looked upon as a transformer with a shunted secondary. It relies on eddy currents for not only melting, but stirring action, which is rather abrupt, and a good thing, as that ensures a homogeneous heat.

I was always told by Inductotherm guys that it is a shorted secondary, not shunted. Shunting in a furnace is something else and your furnaces are not big enough for that.

Melting is reliant on frequency. As the size of the heat diminishes, the frequency must increase accordingly. The power supply I have was built to operate with a 3,000 Hz output, 400 volts, single phase. The identical unit was available with a 10,000 Hz output, which would allow for melting starting with smaller items, although the overall capacity would be the same.

Yes. My experience is that the smallest furnaces will run up to 15kHz and the largest furnaces that we had ran at 500Hz. We didn't have any, but they go all the way down to 60Hz, but you are talking many tons of capacity at this point. The small LECO analyzer furnace that Liz runs, works at 15kHz. But it will only do 2 grams at a time. Typical load is 1 gram.

It is advised that chips (swarf to some folks, but that isn't proper terminology in the US. Ever hear of a swarf conveyor for a CNC machine?) be placed at the bottom of a charge, as they don't heat well. By placing them at the bottom, they will readily combine with the molten metal that reaches them. The stirring action of the furnace assures that everything that melts will be combined.

If you do use chips, make sure that they are dry. You might want some liquid before adding chips. I can't really comment too much on chips as the foundry always used bigger stuff. All metal stock had to be checked by the lab (Liz) first. Chips are hard to certify. Adding anything wet or rusty will give you spectacular fireworks!

Jim B
User avatar
tornitore45
Posts: 2077
Joined: Tue Apr 18, 2006 12:24 am
Location: USA Texas, Austin

Re: One man's insanity---the story of an induction furnace

Post by tornitore45 »

How about "temporary stupidity"? I consider myself an intelligent chap but I have experienced some real goof in the shop that can only be explain by stupidity. Anyway from doing something wrong comes the valuable lessons we call experience.
Mauro Gaetano
in Austin TX
John Hasler
Posts: 1852
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: One man's insanity---the story of an induction furnace

Post by John Hasler »

Jim B writes:
> I was always told by Inductotherm guys that it is a shorted secondary, not shunted.

A better conceptual model is a transformer with a single turn secondary with a very low resistance load connected to it. When melting iron or steel it is initially an iron core transformer with a large air gap. When the temperature passes the Curie point it becomes essentially an air core transformer. Your goal is to couple as much power as possible into that load resistance while dissipating as little as possible in the resistance of the coil and keeping the primary voltage and current from becoming excessive and letting the magic smoke out.

An accurate model is probably rather complicated especially after the charge is melted because then it becomes sort of an induction motor.
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: One man's insanity---the story of an induction furnace

Post by Harold_V »

tornitore45 wrote: Thu Jan 30, 2020 7:59 am How about "temporary stupidity"? I consider myself an intelligent chap but I have experienced some real goof in the shop that can only be explain by stupidity.
Yeah, that's the idea to which I was alluding. I learn something that is not in my normal experience---and I think I remember it well, but time manages to change my thoughts, so I say something absurd, not realizing it isn't correct. That's a particularly sore spot for me, as I abhor a liar, and don't want to be seen as one. A dreamer? Sure, I can live with that, so long as the dream is based on reality.

I expect that I have credibility, that if I speak on a subject that readers will have confidence that I am properly informed. When I have doubts, such as in this case, I like to make note, so readers don't assume what I say is true. Once one's credibility is suspect, there's really no reason to listen to them unless you enjoy being taken for a ride. I hope to never be placed in that category.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: One man's insanity---the story of an induction furnace

Post by Harold_V »

John Hasler wrote: Thu Jan 30, 2020 10:27 am When the temperature passes the Curie point it becomes essentially an air core transformer. Your goal is to couple as much power as possible into that load resistance while dissipating as little as possible in the resistance of the coil and keeping the primary voltage and current from becoming excessive and letting the magic smoke out.
That helps me understand what we experienced. Exciting the generator is accomplished by a Variac. I took note that I could raise the amperage (output of the generator) easily, but the voltage remained low (yet well beyond 100 amps). A little more voltage from the Variac resulted in an instant disconnect of the field of the generator (a contactor drops out). It's clear that that function is to protect the generator.

I concluded that, because the load was small, I couldn't apply the full load (total wattage), and that was reflected by the meters of the power supply (low voltage). The best results were achieved when the highest voltage of the high frequency transformer was selected (which I recall to be the 650 volt tap).

If I understand the Curie point, that's when the normally magnetic material becomes non-magnetic. If I am not correct, would you please address that issue?

H
Wise people talk because they have something to say. Fools talk because they have to say something.
John Hasler
Posts: 1852
Joined: Tue Dec 06, 2016 4:05 pm
Location: Elmwood, Wisconsin

Re: One man's insanity---the story of an induction furnace

Post by John Hasler »

Harold writes:
> If I understand the Curie point, that's when the normally magnetic material becomes non-magnetic.

Correct. That's at 770 °C (1418 °F), though. At that temperature the resistivity of the steel will have more than tripled.
User avatar
Harold_V
Posts: 20231
Joined: Fri Dec 20, 2002 11:02 pm
Location: Onalaska, WA USA

Re: One man's insanity---the story of an induction furnace

Post by Harold_V »

Ahhh! Now it makes even more sense to me why the voltage was so low. Thanks for that tidbit.
Given enough time, I'm going to understand this magic geewizz gizmo quite well.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
Post Reply