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low-cost 1800-amp heating source

Some evaporation sources need lots of current, but have modest voltage requirements, for example needing 400 amps, but dropping less than a volt.

I imagine one good way to get a high ac current would be to pass a big fat wire through the center hole of a toroidal 50 / 60Hz power transformer.

For example, AnTek makes nice low-cost toroidal transformers,

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which they sell directly or via eBay and PayPal.
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These transformers have plenty of room inside their center hole (1.5") to pass even several turns of big wire, etc.

I purchased a 1kVA part, p/n AN-10265, for $100. Using a one-turn secondary to measure the turns ratio, I found 225 primary turns. This means we'll get up to 530mV with a one secondary turn, 1.06V with two turns, or 1.6V with three turns, and we can draw up to 1875, 938 or 625A respectively.

The question then becomes, what's the best way to control the total output current, to obtain a precise oven temperature.

Reply to
Winfield Hill
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The primary current is 1/225th of 400A or

Reply to
Fred Bloggs

Win, One precaution is that your (leakage) inductance of your secondary circuit doesn't have to be much to limit your current. 1volt/400 amps =2.5 mohm, so for 2.5 mohm of inductive reactance at 60 Hz you only need about 6 microhenries to cut your current by .7. A little care with lengths and conductor separation might be necessary.

Paul Probert University of Wisconsin

Reply to
Paul Probert

Use a bunch of wires not just one. This way you can spread them around the core.

You may be able to do it on an E core using many wires too.

[....]

Do you need proportional control or would a band-bang control be good enough?

The primary side current will be low enough that SSRs and triacs etc can be used. A triac would need a good snubber circuit. You could add a little leakage inductance to the system on purpose to make the current waveform not have the sharp edge. A sharp 400A current change will almost certainly couple into however you are measuring the temperature.

Reply to
MooseFET

Some nice soft copper tubing might be a good way to go at those currents.

If there's enough thermal mass maybe you can time proportion with a cycle of a few seconds (of course you can do better than simple time proportioning especially around the likely operating point of ~30-70%), but more likely you're looking at SCR phase control. The time proportioning has the control advantage of power linearity. With trigger angle you have two nonlinearities to contend with.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

If you look around the net there are lots of people cutting the secondaries off of microwave oven transformers ("MOTs") and rewinding the secondary for low voltage, high current operation. People have used everything from welding ground wire (fat, but flexible) to flattened copper pipe.

For welding people use solid state relays (on the primary, obviously) with builtin settable one-shot timers.

Most of those designs also put the transformer VERY close to the load because the IR loss is terrible.

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Reply to
Ben Jackson

I had numerous 2kw toroids available and for a test set, a single,

30cm piece of 10mm^2 cable, looped through a toroid centre hole happily gave 150Amps~ from a 0.8V~ source. Didn't need control but this would most definitely have been by triac on the primary and by continuous phase angle control. Practical drawback with the big toroids is the tendency to huge inrush currents, dependant on core magnetism from the point in the mains cycle where they were last switched off. Even in my uncontrolled on/off setup I added a 'softstart' resistor and relay, to avoid blowing fuses etc.
Reply to
john

Winfield Hill wrote: (snip)

I once paper designed, but never built, a power controller based on alternating half cycle control. Every half cycle of voltage is followed by the opposite half cycle of voltage, with a variable, but even number, of missing half cycles in between, with a solid state relay as the final control element. I think this is the smoothest variable power output you can produce without chopping off chunks of individual half cycles. that produces a very low DC voltage component across the transformer.

The design I made originally, used discrete 4000 series logic, but now that I use PICs, it could be made much simpler.

Reply to
John Popelish

PWM the primary?

I did a little goofing around with high current, low voltage sources many years ago*. I tried the single turn through a transformer core, but it didn't work very well. I suspect the reason was that the single turn suffered from high leakage inductance due to its not filling the opening left behind by the removed secondary. The final, successful solution was to modify a heavy duty soldering gun, which is essentially an N:1 turn transformer.

*The project was to build a portable, easy to use current source that power company linemen could use to verify the proper operation of some clamp-on fault current sensors in the field. I had to generate 400A for a fraction of a second and a soldering gun with a copper loop in place of the tip did so quite easily. 
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Reply to
Paul Hovnanian P.E.

One of those Omega 1/4 DIN (or whatever) temp controllers should work. Some of them do full-cycle zero-crossing burst control, sort of the AC power equivalent of delta-sigma. SCR phase control is very nasty into transformer loads.

John

Reply to
John Larkin

maybe you could control it with magnetics either mechanically like with the old type welding transformers wich moved a magnetic shunt or saturate the shunt with a dc current, maybe even a welder might do the job anyway.

Colin =^.^=

Reply to
colin

Shouldn't be with a shorted secondary.. it should look pretty resistive.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Get him to turn the knob on a variac?

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Well I assume he'd get one of his students to do it.

a variac wouldnt give enough step down so youd need a transformer as well, unless you conected the input to the tap and had 1 turn as a secondary, thay are basically a toroidal transformer so wouldnt be to hard todo,

ofc you would need say 110v into a 240v variac and make sure you didnt turn it to less than 50%. that would give you a range of 50-100% maybe thats enough to control it ?

puting the thick wire for the secondary through would probably limit the movement to 50% anyway.

I was thinking more along the lines of those magnetic cores with a large amount of hystersys that used to be used for controlling switch mode secondary outputs, but realised they probably only practical with dc.

Colin =^.^=

Reply to
colin

I'm concerned with transformer core saturation. Perhaps if I use the 220V primary windings, with an SSR-phase-controlled 120V ac input. DigiKey offers one for $64, but doesn't have any in stock.

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But phase-control isn't too hard to design and make.

So, this would mean a 450:1 turns ratio, and 215mV with one secondary turn, 530mV for two turns, 745mV for three turns, and 1.06V for four turns. We can draw up to 3750, 1875, 1250, or 938A, respectively, on the secondary.

Hmm, the 400 or 700-VA xfmrs should work fine.

Surprisingly, from what I'm told, it does.

Reply to
Winfield

The real question would be whether you can even pass a "wire" that can handle 1800 Amps through the toroid.

How big are the transformers on arc furnaces?

from wiki:

A mid-sized modern steelmaking furnace would have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage between

400 and 900 volts and a secondary current in excess of 44,000 amperes. In a modern shop such a furnace would be expected to produce a quantity of 80 metric tonnes of liquid steel in approximately 60 minutes from charging with cold scrap to tapping the furnace. In comparison, basic oxygen furnaces can have a capacity of 150-300 tonnes per batch, or 'heat', and can produce a heat in 30-40 minutes. Enormous variations exist in furnace design details and operations, depending on the end product and local conditions, as well as ongoing research to improve furnace efficiency - the largest furnace (in terms of tapping weight and transformer rating) is in Turkey, with a tap weight of 350 metric tonnes and a transformer of 350 MVA.
Reply to
Spurious Response

I don't see why you are worried about core saturation. With the transformer loaded with a heavy load, the core will be a bit less likely to saturate because of the IR losses. If you are switching the current on, you should be able to control when it switches and do so at the peak of the waveform. With the phase control, you can always start at the late timing and move towards more on time slowly enough to ensure no saturation.

The run from the transformer to the load will be a bit of an issue. You need to keep its impedance near zero. I assume this run will be kept as short as you can.

As I suggested elsewhere, I worry a bit about the sharp edged current step causing trouble in other parts of the system.

Reply to
MooseFET

If the scr/triac conduction isn't perfectly symmetric, you can wind up with net DC into the primary, which can get weird. And sometimes an scr won't stay triggered when driving an inductive load, ie the transformer leakage inductance. There are hazards, that's all.

John

Reply to
John Larkin

For one-of : A variac between line and primary.

Rene

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Reply to
Rene Tschaggelar

He does have a hefty load on this transformer. The SCR isn't likely to have the problem of there not being enough current to hold it on just after it is triggered. He will have to put an RC snubber on the SCR to keep the dv/dt within reason. The resistance of the snubber will also be passing current just after the SCR fires.

Reply to
MooseFET

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