low-cost 1800-amp heating source

[Winfield Hill]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

 

[Fred Bloggs]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

The primary current is 1/225th of 400A or <2A, doesn't seem like much of
an issue there. That's easily off-the-shelf SSR territory. I wouldn't
think an evaporator would require all that much precision of fast response.

 

[Paul Probert]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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

 

[MooseFET]

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.

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.


[....]

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

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.

 

[Spehro Pefhany]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

These transformers have plenty of room inside their
center hole (1.5") to pass even several turns of big
wire, etc.

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

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.

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

 

[Ben Jackson]

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.

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.

 

[[email protected]]

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,http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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.

 

[John Popelish]

Winfield Hill wrote:
(snip)

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.

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.

 

[Paul Hovnanian P.E.]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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.

 

[John Larkin]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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

 

[colin]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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 =^.^=

 

[Spehro Pefhany]

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

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


Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

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.

Get him to turn the knob on a variac?

Colin =^.^=

Best regards,
Spehro Pefhany

 

[MooseFET]

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

The primary current is 1/225th of 400A or <2A, doesn't
seem like much of an issue there. That's easily off-
the-shelf SSR territory.

I'm concerned with transformer core saturation.
Perhaps if I use the 220V primary windings, with
an SSR-phase-controlled 120V ac input.

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.

 

[John Larkin]

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

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

 

[Rene Tschaggelar]

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, http://toroid-transformer.com/
which they sell directly or via eBay and PayPal.
http://stores.ebay.com/Antek-inc

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.

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

Rene

 

[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.

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.

 

[Spehro Pefhany]

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

Yup. A single SCR on a the DC side of bridge rectifier with the AC
side in series with the primary would assure symmetry.


Best regards,
Spehro Pefhany

 

[MooseFET]

Yup. A single SCR on a the DC side of bridge rectifier with the AC
side in series with the primary would assure symmetry.

This is assuming there is not mains ripple getting into the triggering
circuit. It only ensures that it is as symmetrical as the triggering
is. A triac running from that same trigger signal would be about as
good.

We could put a large capacitor in series to ensure no DC component.
It leads to some fun waveforms and interesting shopping trips at these
current levels, however.

 

[[email protected]]

Yeah, and that the diodes are pretty much the same.


Triacs, especially those of the sensitive-gate persuasion, don't like
to commutate when there is much dv/dt. Alternistors are better, SCRs
are the best thyristor.

how about two back to back FETs or one FET and a diode bridge, then
you can you can also chop anyway you like.

-Lasse