Induction Furnace Energy Loss in Copper Coil?

Yes, if you can afford the cost, use silver wires in which the resistive losses would be about 4-5% less. A cost analysis would give you the payback.

Of course, silver was $50 an ounce a year or so ago, now under $15.

Just replace them with silver coils.

INTERESTING BUT NOT ECONOMICALLY SOUND....ANY OTHER ...

If you have a 5000 gallon retort that uses 500 KW and is in use 14 hours per day = 7000 KW hours per day x 300 days x 12 cents per KWH = $252,000 used annually.

5% of that = $12,600 or $126,000 in 10 years. The savings will go on and on

Maybe heavier copper coils are available (more copper cross section, less loss). You would have to contact a manufacturer on what is available. Whatever you do, it's going to cost and you need to see how much energy you would save and how long it would take to cover the cost.

The purpose of an induction furnace is to heat things. So where else is the energy going?

Waste heat recovery system...

http://foundrymag.com/feature/energy-efficiency-system-enhancements-induction-melt-shop

...intelligent control system...

http://forgingmagazine.com/feature/intelligent-induction-boosts-productivity-energy-efficiency

...ref link...

http://ispatguru.com/induction-furnace-and-important-operational-aspects/

How do you define "wasted energy"?

Without any usable information about part configuration, coil design, power supply capacity or anything else, there is no way anybody can educate you.

Maybe you can revisit: Superconductors in Medium Frequency Induction Furnaces.

Silver plating is the way to go, but with a little caution.

First, a few skin depths is all you need, say 2 thou/50 microns at 400kHz.

Secondly, it is vital to get a continuous layer deposited. In the 1960's decorative platers went over to bright deposit formulations, saving polishing costs but with islands of silver. The gaps made it useless at RF conduction improvement.

The good textbooks at that time were translated from Russian, they were comprehensive.

A LOT more DETAIL would be helpful! What you consider to be medium frequency, another person may consider high, while yet another may consider it low. Provide a number, or a range of possible values.

Since you say the coils are cooled by circulating water, I presume they are coils of tubing, not wire, but I can't be sure unless you specify. Is that water de-ionized to avoid currents in the water circulation system?

I suspect you are calculating the 16% loss based on the flow and temperature rise of the cooling water. Do you know what fraction of that 16% loss is due to IR and other losses in the copper, and what fraction is due to heat transferred from the parts being heated and from the inside of the furnace via radiation and convection?

What about the furnace configuration? How big is the furnace? How big are the coils? How big are the parts being heated? How much spacing is there between the coils and the parts being heated? Are the parts centered in the coils, or just near them?

How long are the coils energized at one time? What is the duty cycle? What are you trying to accomplish via the heating (softening, annealing, heat-treating, brazing, melting, etc.?

I believe answers to all of these questions could be pertinent to your problem, or possibly could lead to the conclusion that 16% is reasonable...

Are we up to 20 questions yet?

I think that was 21.

If there is a reactive load consuming power but not contributing heat, where is there energy going?

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Seems like if the frequency is not at resonance that consumed power is not as high. The reactive load in an induction heater is what is heating the workpiece, IIRC.

The primary heating in an induction furnace results from the induced current and magnetic fields in the work piece which acts as a shorted secondary winding in the equivalent of an air cored transformer where the heating loop forms the primary winding. The massive currents flowing in the short circuit plus high frequency reversals of magnetic domains in the work piece create the intense and rapid heating effect. The energy put in is volts x amps (VA) but what is useful is VAcosØ where Ø is the phase angle between the current and the voltage, usually referred to as power factor in motor circuits. If the phase angle was off by 32° then only 84% of the power would be delivered in generating induced heat. The remainder goes to producing resistance heat, but that is a much less efficient process and is all generated in the copper so must be removed by the water cooling to prevent the coil from melting. It would contribute to work piece heating by transfer of radiated heat across the air core (the gap between the coil and the work piece) if the work piece was at a lower temperature than the coil. As this is not the case, radiated heat transfers out if the work piece into the coil. So all the resistance heat energy plus some induced heat energy is lost to the cooling water. Adjusting the circuit to bring the power factor back up to 1 will eliminate a large proportion of the losses.

I had no idea that the remainder went exclusively to resistance heating in the coil. Any ideas on the mechanism by which resistance is increased?

The amount of resistance heating is governed by two parameters, current and resistance, the circuit resistance does not change, it is the current that goes up (I²R where it is the I that changes). An analogy would be mechanically overloading an electric motor so that it runs slow. That results in lower induction so there will be a less back emf and a higher current goes through the winding coils. Eventually they heat up and the motor burns out. Or in this case where you are water cooling the coils, runs inefficiently. You see the same effect as a power surge when a motor speed increases on starting up. All that actually changes is the proportion of current used for inductive power and the proportion used for resistive power and which the OP is interpreting as losses. My original answer was to tweak the power factor nearer to 1 which has the same effect as it would on a motor, increases the efficiency. In a 50/60Hz motor circuit you would just add an extra capacitance to correct the power factor but in a high frequency circuit it is not that simple, and the techniques used fall beyond my level of competence. He needs an expert on induction furnaces.

"The massive currents flowing in the short circuit plus high frequency reversals of magnetic domains in the work piece create the intense and rapid heating effect."

I'm definitely NOT an expert in this area, but in induction cooktops, the eddy current heating is apparently minimal, since current units (that I've seen and/or used) will not heat copper or aluminum pans, and both of those materials should have significantly higher eddy currents than iron alloys. Thus it sounds like it is the magnetic domain reversals that do most of the heating.

Although the OP is heating vastly larger quantities, he is heating mostly magnetic materials, so it would seem like similar principles should apply. I have no idea what happens when the material reaches it Curie temperature. My induction cooktop uses frequencies near 45 kHz at low power, dropping to around 25kHz at full power (as measured by the digital frequency readout on my oscilloscope).

Again, I know nothing about the OP's size/kind of furnace, but 500Hz sure sounds like a very low frequency to me.

The Curie Temperature of iron is 1043°K at which point the domain reversal effect switches off. But the OP is melting iron at 1811°K so another mechanism must be raising the temperature from 1043°K to 1811°K. That second mechanism is eddy currents which add a substantial proportion of the heat below Curie Temperature and continue to heat the metal above Curie Temperature. Non magnetic metals like copper, aluminium, titanium, silicon and gold are regularly melted in induction furnaces, but the lack of domain reversal in these metals means they have to rely entirely on eddy currents, making the process less efficient and cost effective. Where contamination or oxidation could pose a problem non contact induction heating offers benefits like melting in a vacuum or inert gas that outweigh the higher costs.

One of the few things that induction cooking and induction heating have in common is the word induction. Induction cooking uses the vessel as the heat transfer agent, induction heating heats the melt directly. Induction cooking relies on eddy currents to heat a magnetic material, induction heating uses skin effect to heat an electrically conductive material. Induction cooking tends to much higher frequencies (>40kHZ) while induction heating tends to much lower frequencies to allow for deeper skin effect while the induced eddy currents contribute to the mixing of the melt.

One exception is induction heating used for case hardening of gears and bearing surfaces where higher frequencies are used to deliberately to minimize the depth of penetration of the hardening process.

Is the '16% wasted energy' a newly observed phenomena, or recently calculated without changes in operation?

If what you describe is a new phenomena, i.e. 16% more energy is being used and increase in delta T can account for most of that, then look for changes that have taken place. Has there been damage to the coils? How is the surface roughness? Has the coolant show corrosion/erosion products? Are there abnormal hot spots on the coils? Has there been a change in coolant flow rates?

Regarding energy losses in your medium frequency induction furnace. I sispect you energy loses are due in part to skin effect, where AC current tends to flow best only at the outer surfaces. See:

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

for a good discussion of skin-effect. particularly note the reference ot "litz wire". But flat ires also can increase the surface of a wire and help with better conduciton in ther condustor's skin. Formulas are included in the Wiki discussion.

It is possible that oxidation over the years has thickened the oxide coating= increased losses?

One would expect the makers of the furnace to anticipate this and advise the owners to remediate the coils from time to time.

Welcome to CR4!

Since the OP has mentioned water-cooled copper, I presume his furnace is using coils of copper tubing, which inherently take advantage of the skin effect.

For future posts, please re-read and edit your text and/or use a spell-checker before pressing the Submit button.

You may have losses through the water in your cooling system. Replace the water with antifreeze and a closed loop circulating pump and electrically insulate the couplings coming from the power system to the closed loop system. Ground the closed loop system to the case ground of the amplifier equipment.

If your amp has a tube, adjust your grid current down and raise your voltage as well.

How much power will this cooling system consume?

How much will this increase total system efficiency?

In other words, will it really save anything?

Since these furnaces use water coolled, heavy wall copper tubing coils for the inductive heating, you will lose effiency to the water. I am guessing that when you say medium frequency, you mean 3kHz.

You want to increase effiency? Simple, turn off the water! You should get about 3 minutes before the smoke starts. Another couple of minutes before the copper melts at the refractorys weakes point, and then the metal river starts! Been there, seen that!

What is the mechanism of power loss or efficiency loss to the water? The water path has the same number of turns as the copper path. The copper conducts better at a lower temperature. Although the conductivity of water is lower than copper, it is providing additional conductivity, so actually slightly lowering total resistivity.

here are the details for your reference and solution if any-:

We are having medium frequency induction melting furnaces for melting of iron , the following are the details-:

coil design -; electrolytic grade copper tube, duly taped and inter-turn class F insulated

coil cooling mode- water cooled.

rated output KW 3500 kw,

rated voltage- 3000v

rated input 2x900v

rated frequency - 500 Hz

medium for cooling -dm water.

the mains input power source voltage level shall be 33kv. The duty converter transformer is provided to step down the voltage at an appropriate level needed by static frequency converter.

inherent in the principle of melting in an induction furnace which include the inefficiency in electrical bus bar losses, eddy current losses, refractory losses, and cooling water losses etc.,

Efficiency of induction furnace is expressed as a total, deducting electrical and heat transfer losses. Electrical losses consist in transformer, frequency converter, condenser, wiring, cable, coil, etc. Loss in coil is essential factor, on which the furnace capacity depends. Heat losses in induction furnace consist of conduction loss of heat escaping from furnace wall to coil side, radiation loss of heat released from melt surface, absorption loss in ring hood, slag melting loss, etc. The coils of furnace are water cooled which also results in heat loss. Heat efficiency of high and medium frequency furnaces (60 % - 78 %) is slightly larger than that of low frequency furnace (58 % - 71 %).We are attaching a diagram for better understanding and request members to let us know if we can reduce power loss in any parameters which is depicted.

Usually the 16% of power loss takes place in the coil region. plz provide a solution.

Thank you! That helps considerably. I have no experience in furnaces of that size or frequency, so I'll leave the advice to others.

Based on the references provided by SolarEagle and others, it sounds like your 16% loss in the coils is pretty much expected. Find some other industrial process that can make use of that waste heat, or perhaps use a Sterling engine to power some of the movement of raw materials or finished product.

Your answer is right in front of you, "...conduction loss of heat escaping from furnace wall to coil side..." That conductive heat loss from the melt into the crucible, then into the coils, plus any radiative and convective heat gain as well, cannot be avoided until you can figure out how to shield the coils from them.

Unfortunately the 16% loss in the coil includes the heat gained from the melt along with any copper loss from the actual current passing through it. A simple experiment should convince you, simply run your coils at full current with no melt in the crucible and measure the real power consumed while recording the temperature rise of the cooling water, it should be much less due to the lack of heat gain from the non-existent melt.

If you can't run it empty, try measuring the difference in water temperature over time from a cold start to target temperature for a full melt, you should see the water temperature rise as the melt temperature rises, which indicates that the water temperature rise is a result of the melt, not the copper losses which are constant. This is not different than trying to separate the copper losses from the magnetizing losses in an unloaded transformer; after all, this is just a multi-turn transformer primary (the induction coil) feeding a short-circuited single turn secondary (the melt) that we're analyzing.

Increasing the thickness of the conductor wall will do very little since the skin effect governs the overall resistance of the circuit; i.e., for a given outer diameter increasing the wall thickness follows the law of diminishing returns. On the other hand, increasing the outer diameter for a given wall thickness will lower the resistance, as will decreasing resistivity of the conductor, perhaps by silver plating the outer surface, using a silver-bearing copper alloy, and/or (as others have suggested) using silver or (lmao) gold for the coil.

At 500 Hz, skin effect shouldn't present much of a problem for reducing resistance via increases in wall thickness....as long as wall is less than a few inches thick.

Actually skin effect is noticeable at 50/60Hz, that's why overhead transmission line cables can have a steel center core and high power bus bars are hollow. At 500Hz the skin depth is 1/10th that at 50Hz, or roughly 125mil (1/8"); basically any extra wall thickness is there to carry mechanical load not electrical current.

My mistake, I had things off by an order of magnitude. Skin effect depth at 500 Hz is around 3 mm.

Could make a difference.

There must be some stray inductance, which doesn't seem to be accounted for in your breakdown.

I also agree with Ramconsult, in that heat transfer from the workpiece to the coil is likely some portion of that 16% figure.

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There are a couple avenues that may improve efficiency, though I wouldn't bet on a quick return on investment.

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You could look into:

Nonconductive barrier to infrared between workpiece and coil

Refractory insulation between workpiece and coil

Flux concentrator.

Perhaps resistance of the copper is simply going up as more heat is coupled back to it as the work piece gets hotter. Electrical resistivity coefficient of hard-drawn copper is 1.771 microohms centimeters at 20 C, and the temperature coefficent of resistivity is 0.393-0.404 % per degree, depending on source, and that also increases with temperature, and I could not find the data on that.

Bottom line: you are basically stuck with most all that 16% loss of the coils until you can decouple heat transfer back to the coils from the work piece (that adds to temperature and increases the losses). This assumes your system is phase angle matched to the load, as pointed out by the other astute posters.

Things you might possibly try:

• mica between the coils and the work to impede heat transfer
• quartz tubes- although this material will melt at around 1300 K
• very careful silver plating - lowers resistivity of the conductor, and also improves radiation reflection.
• polish the coils before each work session???? If the coils do not appear nice and shiny, I suspect they will heat up more.

IThe like the idea of a smooth polished surface, but I wonder how long until material reduction from.polishing leads to a noticable increase in resistance?

I expect that would take a long time, unless they plan on using 00 grit sandpaper! I was thinking something more along the lines of Tarn-X, or perhaps something from Lyn-Dor Industries.

Chemical metal cleaners like Tarn-x and Brasso do remove a non-negligible amount of the of thr metal surface. These cleaners are acids that remove at least the passivation oxide layer, and probably some base metal under that. If memory serves me, I think Brasso feels like it has very fine pollishing grit in the formula.

Anyway, even if it were just the oxidelayer and it had to corm a new.oxide layer every day, I think the tubes would get thin to the point of noticeable increase in resistance in a couple months at the most, probably less.

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With a small precision postal scale, some Brasso and 20¢ in pennies, we could get an idea about removsl rates per area.

Or we could just polish the darned coil until it leaks.

'corm a new oxide layer'

I love that spell check let that through, but insists I must mean other words for a number of things I spell correctly.

It really can be an irritant at times. corm new oxide hide?

corm is a perfectly good word (corm, bulbo-tuber, or bulbotuber is a short, vertical, swollen underground plant stem that serves as a storage organ used by some plants to survive winter or other adverse conditions such as summer drought and heat).

So spell check has no problem with it. It just doesn't happen to be the word you intended (form).

Thanks.

It does 'correct' some of the words I use that are not incorrect. I assumed a bias towards words likely to be heard in typical conversation/informal communication. 'Corm' doesn'the seem to fit that criterium.

....or does everyone laugh and return to delightful conversations about corm as soon as I leave the room?

We don't always wait until you leave the room. Sometime we have clandestine conversations about corm before you even enter the room. Ever notice that quiet few seconds where everybody stops talking as you walk in?

Wait. You mean that silence isn't deep respect and hopeful anticipation of pithy observations?

That's funny, I was just having a similar thought as I was reading through the latest posts. Corm on, gentlemen, corm on!

Disclaimer: Bold used to separate key points.- Not shouting.

1. Does furnace(s) insulation, access door fit, and physical integrity meet OEM design criteria?

If not, the energy conversion efficiency issue may be due to simple furnace structural heat loss.

2. The impedance/resistivity of copper and other conductive materials is increased when exposed to high temperatures over a long period of time.

The existing coils may have reached their end of life and will require replacement to restore energy conversion efficiency.

3. Also: Depending on what materials are being processed through the furnace(s) there is a significant possibility that the coils are suffering from migration of heated material particles into the copper thereby increasing impedance which will negatively affect energy conversion efficiency.

A simple conductance test of all in-service coils and comparison of the gathered DATA to a new coil conductance reading would yield valuable information.

4. Remote possibility:

If all the above is ok:

Have you investigated the electrical system for the presence of destructive and/or disruptive harmonics?

If unwanted harmonics are present: Is the filter system intact and fully functional to OEM specifications? If not, it must be repaired or replaced.

Find and electrical engineer.

Something like do an adjustment on Voltage perhaps and sizes of coils.

Recall Power= VI =IR² , you may reduce the current (I) and go high on Voltage (V) to maintain the same power.

Reduced (I) current, you would have smaller coil cross section does Resistance is reduced. When resistance is reduced heat will also likewise be minimized. You may otherwise increase the frequency from medium to high.

This is how transmission lines are made (high voltage, small wires) or you opt to replace better conductive material other than copper like silver or "Graphene" perhaps? Nope, forget it.

Now, where is my pay?

In post #22 the OP says:

"We are having medium frequency induction melting furnaces for melting of iron , the following are the details-:

coil design -; electrolytic grade copper tube, duly taped and inter-turn class F insulated

coil cooling mode- water cooled.

rated output KW 3500 kw,

rated voltage- 3000v

rated input 2x900v

rated frequency - 500 Hz

medium for cooling -dm water.

the mains input power source voltage level shall be 33kv. The duty converter transformer is provided to step down the voltage at an appropriate level needed by static frequency converter."

He states: "rated output KW 3500 kw". That's 3.5 MEGA Watts OUTput! With rated input of 2x900V, and an efficiency of 69%,

I=P/(E*eff)= 3,500,000/(2*900*0.69)=2,820 Amps per unit, at 900 Volts.

This is (at least in my limited experience) a humongous amount of power. I have no idea what size or wall thickness of copper tubing is used, nor how many turns there are in a coil, but I really can't imagine applying 900 Volts to such a coil at only 500Hz, much less increasing that voltage.

The OP doesn't just need to find an engineer, he needs to find a Team of engineers, highly experienced in this narrow, high power induction field.

------

You state: "Reduced (I) current, you would have smaller coil cross section does Resistance is reduced. When resistance is reduced heat will also likewise be minimized"

I presume English is not your first language, as this paragraph does not make sense in my first language! It is true that if he could reduce the current, he could use tubing with a smaller cross section, but he would need to use more turns of the smaller tubing to accept the higher voltage. When the cross section is reduced, the resistance is increased, not reduced. Since he would need more turns of the smaller tubing, the resistance would be increased further, so I²R heating would be significantly increased.

"he would need to use more turns of the smaller tubing to accept the higher voltage"

Yes I see (I messed up I²R), but how much resistance difference the length influenced other the lateral cross section?

It seems like the only resort they would need is to literally, cool the coils approaching zero temperature absolute, like the ones they did in the LHC. But, cooling, it would mean additional energy inputs. Yet, it is interesting to find how much conductivity of copper is improved per dropped of temperature from normal.--It's worth a research.

BUT, lets say total efficiency = [Energy delivered Heating the Load/(Electrical Energy Input + Cooling Energy for the coils)] say increased from 69% to 80%. It's worth to implement.

The resistance of this coil is somewhat of less importance, but still relevant, than the actual inductance value. This determined how much power can be induced into the secondary side of this air-gap transformer (the work piece), whether it is mostly due to magnetization hysteresis heating, or through eddy currents, the current in the work piece has to be astronomical in value!

https://www.heat-processing.com/fileadmin/HPO/Dateien_Redaktion/Selected_Reports/2015_02_hp_FB_Doetsch.pdf
Maybe this link provides some insight. It appears that in the case of large pour induction furnace equipment, the liquid steel reduces gradually the thickness of the crucible, and after 30-40 pours they must break it out, and reline the vessel.

Interesting and it is a related subject, but I am having difficulty putting together how a discussion of crucible material loss might provide insight into the OP about coil resistance.

It may be coupling issues between the external coil and the crucible itself....I am certainly not the resident expert on this, never done it, but it seems to me that as the melt interacts with the spinel of the crucible, changes in transimpedance will result.

That link is amazing! 16 megaWatts of power at only 250Hz. What kind of electrical supply must they have?

Some humongous Main contactor units, probably operating off a large step-down transformer from medium voltage 3-phase delta to low(er) voltage 3-phase delta, with some really awesome large conductor bars carrying the enormous current. Current is obviously the kicker to this arrangement, and the higher the input current, the higher the induced magnetization hysteresis and/or eddy currents in the work. I am not entirely sure if 3-phase, but I consider that three separate and distinct copper coils could be used as primaries in the induction furnace. What I cannot envision is the interplay between the input coils magnetic fields and the single work piece, that seems a bit unruly, and no I have not even attempted to look at the differential equations for that.

More likely it is a 480v, 3ph, inverter. AC to DC, SCRs and then into a high frequency water cooled transformer, then into a capacitor bank to match the system to the load in the furnace. Yes, the conductors are large. Buss bars out near the furnace and then water cooled leads from the buss bars to the coil in the furnace.

The furnace is basically a transformer with a shorted secondary.

Jim B

From the username, I assume this installation is in India. I don't know what the standard industrial voltages are there...

Is there such a thing as a 16MW inverter (the one in Germany) or even a 3.5MW inverter (the OP's unit)? I find either of those hard to visualize!

I think you have at least one problem in your description: SCR's don't work on DC. I agree that the incoming power is probably three phase (the OP says 2X900V, which could mean 2-3Ø transformers, each with 900V, 3Ø output), rectified to DC and then inverted somehow to get the 250Hz.

I'm guessing, but I suspect they use such a low frequency in order to maximize the mixing of ingredients in the melt by turbulence...

Induction Melting equipment come in a variety of input voltages. Most of the ones that we had would operate from 400 to 500vac. We had one that they got a good deal on that ran on 575vac. Each "melter" had its own transformer from 4160 to what ever we needed for input voltage on the melter. Our equipment was considered low voltage compared to what is available.

That's odd, of the 15 induction power supplies that I have serviced for 30 years all used SCR's in the DC side of the power supply. The 480vac units had a dc buss voltage of 720vdc that chopped the dc into a square wave, then it went into the high frequency transformer, to the tank circuit, then to the furnace. Our units ranged from 125kw to 900kw, 500hz to 3khz.

And yes, there is turbulence in the furnace once the metal is liquid. And if you really want to see turbulence, be there when the refractory fails and the copper coil melts through to the water!

Jim B

Come to think of it, I have heard of/seen circuits where one SCR turning on essentially shorts across the other, thereby turning it off. Is that what they call a crowbar circuit?

All the circuits I've personally worked with used the zero crossing of an AC voltage to turn off the SCR and ready it for the next firing.

I stand corrected!

The SCR's each have a snubber circuit that brings the voltage back to 0V momentarily. There is a minimum TOT (Turn-off-time) that if it goes below that time, dead short! I saw one of the engineers from the manufacturer play with this one time, blu the hock puck right out of the clamps that they are mounted in.

THe incoming HT voltage IS 33000 volts+\- 2000 volts.

So I gather you have two transformers that drop the 33kV down to 900V. I assume these are three-phase transformers. Is that correct?

What kind of device/circuit is used to create the 500Hz?

we do have 4.5 mva transformers. yes. i guess to know more about working of the way induction furnace works you can take a look at http://www.megatherm.com/index.php?option=com_content&view=article&id=56&Itemid=579&lang=en.

It appears your system is more or less "state-of-the-art" level stuff. I fail to see anything anyone could do (other than possibly MEGATRON engineering staff) to improve delivery of heat into the work from the power input.

Good luck, just make sure you communicate with them frequently, read and take to heart all of the literature and technical advice you can get from them, and be a very religious person when it comes to (1) the safety aspects and (2) the maintenance aspects of this fine instrument of heating metal.

Agreed! (...except I'd replace the word "religious" with "diligent" or something similar.)

I have to confess I had no idea how far solid state devices had climbed up the power ladder! Thanks for the education!

^{_{OT: I had a short but exciting flight in a P-38 in 1948! It had been converted for cloud seeding in Colorado.}}

The OT part was most interesting. Do you remember much about the flight? Airspeed, altitude, etc. As I recall P-38 was turbocharged enough for make it able at altitudes way over 30,000 ft. Were you piloting this, or was it set up in 2-seat configuration?

There are certainly inverters and rectifiers of that magnitude and a whole lot larger (by several orders of magnitude). How do you suppose they get 1 million volts DC for the DC transmission lines going into Los Angeles? How do you suppose they convert that back to local std. 3Φ line voltage when it gets there? This is not a silly parlor trick, it is big-time electrical engineering at its finest.

Thank you much for the short clarification, as that removes a cloudy issue in my mind, obviously, I was not thinking clearly as 60 Hz (or 50 Hz) 3φ is not equal to 250 Hz single phase, which what makes the cat purr.

The best thing that can be done for efficiency, is make sure the melters (operators) are moderating the power level to match the load. We had some guys that would slam the power control to full power with only 1/2 the load in the furnace. That is a big waste of energy. Usually succeeds in expelling the metal out of the furnace once it becomes liquid.