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Join Date: May 2008
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Induction-Melting Furnaces

07/15/2017 2:29 PM

What is the difference in between low medium and high frequency induction coreless melting furnaces?

We in India are using induction furnaces mostly medium frequency for melting DRI,PIG IRON AND SCRAP in various ratio to manufacture billets. I would like to know which frequency type coreless furnace one should buy , if one wants to buy 10 mt or 15 mt coreless induction furnace and why?

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#1

Re: Induction melting furnaces

07/15/2017 3:54 PM

..."Operating frequencies range from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required. Generally, the smaller the volume of the melts, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. For the same conductivity, the higher frequencies have a shallow skin depth—that is less penetration into the melt. Lower frequencies can generate stirring or turbulence in the metal."...

https://en.wikipedia.org/wiki/Induction_furnace

You might consider energy saving strategies such as adding flux to reduce slag buildup....but have it professionally engineered and follow instructions carefully....link below gives more detail...

http://foundrymag.com/feature/saving-electrical-energy-coreless-induction-furnaces

"Induction melting efficiency has increased to more than 98% and, though it’s unlikely to increase much more than that, the current technological standard provides unprecedented reliability and productivity, as demonstrated in this “real-world” example."

..."Now, medium-frequency power supplies providing 700 kW per ton are common and units able to apply 1,000 kW per ton are in operation at many foundries. Inductotherm alone lists more than 2,000 induction batch melting installations worldwide boasting power units of 2,500 kW or larger, applying up to 1,000 kW per ton of furnace capacity.

In fact, medium-frequency power supplies of up to 18,500 kW at 200 Hz are in operation, and there has been a solid-state unit providing 42,000 kW of induction power for heating massive steel slabs. Also, the efficiency of these units has increased from 85% in the 1970s to more than 98% today.

While it is not likely that tomorrow’s induction equipment will show further significant increases in electrical efficiency, what is proving meaningful to foundry operators, however, are today’s new levels of equipment reliability and utilization."...

http://foundrymag.com/feature/more-powerful-induction-furnaces-and-inverters

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#2
In reply to #1

Re: Induction melting furnaces

07/17/2017 2:37 AM

I would use the size of the vessel to determine the frequency. The larger the the vessel the longer the wavelength you need. Also, the number of turns in the inductor Primary can be greater at a lower frequency to induce a higher current in the secondary (the work being heated). The number of turns in the secondary is always ONE when doing this calculation.

Utility frequencies 50-60 Hz can easily couple with a large load in the single turn secondary if the primary coil is multiple turns and high impedance relative to the supply. The coils have to be big in diameter but the full length for the number of turns should be equal to at least a 1/4 wave at 50-60Hz.

How big in diameter is the secondary (load) that you are heating?

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#3

Re: Induction Melting Furnaces

07/17/2017 11:13 AM

I vote with Solar Eagle and deefburger on this one. You need calculations that show that your frequency is an appropriate match for the scale of the furnace.

Since you are talking about 10 mt to 15 mt scale, probably the lowest frequency will be better, but it still depends on core geometry. Is that scale rating for batch melting of 10 mt? Or are we discussing a furnace that will produce 10 mt/hr?

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#4

Re: Induction Melting Furnaces

07/17/2017 11:37 AM

The frequency determines how the furnace is operated. I worked in a cast iron foundry using one 4KHz and one 60 Hz melting furnaces. The 60 Hz furnace required a melted charge maintained in the furnace at all times. 60 Hz could add heat to a melted charge, but could not generate enough induced current to start from cold metal. The 4KHz furnace could be started from a cold scrap charge, run down to empty and shut down on a repeated basis.

Once we relined the 60 Hz furnace and restarted it by melting a charge in the 4 KHz furnace and then poured the charge into the 60 Hz furnace.

If you will maintain a constant melted charge in the furnace, then the capital cost savings makes a 50/60 Hz furnace run direct from line frequency as a best choice. If you will be running batches and shutting down or running the furnace to empty, then the higher frequency melter is necessary. The higher the frequency, the faster the charge will melt.

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#5
In reply to #4

Re: Induction Melting Furnaces

07/17/2017 11:59 AM

This is true because the frequency limits the number of turns you can have in the primary, and so limits the voltage induced in the secondary and so limits the current in the secondary. The higher frequency signal can drive more turns and so induce a higher voltage, however, the lower frequency signal can drive a higher current in a larger coil.

Basically, you have a transformer. So impedance/voltage/current matching is important. This gets more difficult for larger loads at higher frequencies than it does for the same large loads at lower frequencies.

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#6
In reply to #5

Re: Induction Melting Furnaces

07/17/2017 12:22 PM

And as JPfalt just stated, the low frequency furnace does not couple well with cold metal. You still need a charging melt...

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