Looking for advice on homemade spot welder design
plis...@gmail.com
---
Hi. Lately i've become interested in building a small spot welder for
DIY use. The idea is to end up with a (relatively) portable unit, with
controlable power, suitable for spot-welding anything from battery
contacts to 1.5mm sheet metal. I've ruled out buying a commercial
welder - mainly because they're rather expensive where i live, and
also because i happen to like the challenge :)
So, right now i'm contemplating two different designs, and having a
hard time deciding which one to go for:
* Capacitor discharge welder: The idea is to have a rather large
(100,000 to 300,000uF) capacitor bank which is discharged by a large
triac or similar electronic switch into the weld contacts. A cheap
microcontroller with an integrated ADC would control the charging
voltage, allowing for variable power, plus triggering the output
device.
* Mains transformer welder: The idea is to modify (or pay to have
built) a large (1 kVA to 3 kVA) transformer with only a few turs in
the secondary to get a low voltage, high current output. The power
controlling would also be handled by a microcontroller timing a relay
in the mains winding - i've had good sucess in switching off relays
very quickly (arround 5ms) by using a zenner in series with the back-
EMF catching diode.
I've considered also running a high voltage cap discharge through a
transformer to get a strong amperage pulse, as done in commercial
welders, but ruled it out - if anything, because it's overly complex
and overkill for a simple device as intended. Anyway, in both cases
i'm planning to build a simple clamp arm with copper contacts as weld
tips.
Right now i'm favoring the transformer option, if only because it ends
up being way cheaper than buying large electrolyitc caps + SCR. I have
concerns about it being used to weld small parts, as i feel that a
capacitor bank allows for much finer power control - yet, caps won't
perform as well in thicker parts... or will they?
So, my questions are... does anyone with experience with this type of
welders have any advice to share? If i go with the transformer option,
should i look into limiting the output surge current with an inductor
of some kind? How long can one expect electrolytic caps to last when
stressed with charge-discharge cycles like these? Any comments that
can help me make up my mind will be appreciated, thanks!
Ignoramus16379
electrical circuits. See how they make their spot welders.
i
On Thu, 26 Jul 2007 15:13:56 -0700, plis...@gmail.com <plis...@gmail.com> wrote:
> (this is a copy of a post made on sci.electronics.design)
>
>
RoyJ
.3 to .5 volt output to the tongs under full load. The simple hand held
units
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=45689
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=45690
http://www.millerwelds.com/products/spotwelders/msw_and_lmsw__air-cooled_/
have nothing more than a toggle switch for control. Clamp down, pull
trigger for whatever you think works, release trigger, release clamp.
Basic info on spot weldeing here:
http://www.millerwelds.com/pdf/Resistance.pdf
On 240 volt lines you need approximately 500:1 turns ratio. Core size
and input need to be suitable for the kva rating you are trying for.
Output cables and tongs need to be suitable for the amperage. Note that
at 6000 amps at .5 volt, connection resistances need to be on the order
of 1/100,000 ohm for things to work well.
For industrial applications in the 15kva to 125kva range, the weld
current and duration is controlled for each application. It's common to
have a specific series of pulses with varying timing and current.
http://www.on-b.com/weld/wd-lc-nu.htm
For a home brew I think I'd look for a suitable base transformer core
and primary windings in the 1.5 to 3 kva range. Pull out the secondary
windings, install new secondary windings made from copper braid.
Probably only need a couple of wraps. Hook them to some sort of clamped
tong assembly. A simple timer/trigger circuit with user adjustable timer
from perhaps .5 seconds to 3 seconds would be nice. I think you will
find that this is much more of a mechanical project with fabrication
issues than an electrical one.
plis...@gmail.com
16379.invalid> wrote:
> Miller welding machine manuals usually include diagrams of their
> electrical circuits. See how they make their spot welders.
I have a lot of tech documents for spot welders, both from Miller
(transformer based) and Image Industries (capacitor discharge based).
The electronics on these devices are rather simple - what i'm looking
for is for wisdom from people who built similar devices for
themselves, or have experience with both types to help me decide on
the welding system.
dca...@krl.org
>what i'm looking
> for is for wisdom from people who built similar devices for
> themselves, or have experience with both types to help me decide on
> the welding system.
I have played with building a couple of spot welders. The first one I
built with four big variac cores. The cores were from about a
kilowatt variac. I used the existing windings which were for 120
volts and wired two cores in series in parallel with the other two
cores in series. And then used 240 volts.
The secondary was made aluminum. It filled the hole in the variac
cores. So I had four cores with a laminated bar going thru them. I
never got to finishing it, but did use it to fuse two 3/8 diameter
bolts together. Just crossed the two bolts and clamped an electrode
on each side with a C clamp. As the bolts melted into each other I
tightened the C clamp until the bolts were pretty much in the same
plane. Took a minute or so.
Dan
plis...@gmail.com
> Note that at 6000 amps at .5 volt, connection resistances
> need to be on the order
> of 1/100,000 ohm for things to work well.
Interesting, this is a figure i didn't find elsewhere. Guess it
excludes any metal but copper for the clamp.
> For a home brew I think I'd look for a suitable base transformer core
> and primary windings in the 1.5 to 3 kva range. Pull out the secondary
> windings, install new secondary windings made from copper braid.
> Probably only need a couple of wraps. Hook them to some sort of clamped
> tong assembly.
Yes, that was pretty much the idea all along. Perhaps even isolated,
bent solid copper bars to reduce resistance in the winding.
> A simple timer/trigger circuit with user adjustable timer
> from perhaps .5 seconds to 3 seconds would be nice. I think you will
> find that this is much more of a mechanical project with fabrication
> issues than an electrical one.
I know - i want to make a fancy uC controlled welder, and even then is
a rather simple project (electrically :). I have some experience
building mechanical assemblies, and i have a pretty good idea on how
i'd build the clamp - basically two thick L shaped copper bars with
fittings for replaceable tips and a hand operated spring assembly to
close it with some force. The whole assembly kept as close
electrically to the secondary as possible.
Do you know if this kind of welder is well suited for small works,
where localized heat might be a problem? I'm asking because all the
literature i found suggested a cap discharge assembly for "finer"
work. I can control the welding time with precisions in order of 25mS
(realistically, with 10ms for both latch and release),and i stil don't
know if that would be enough. The nice thing about capacitor discharge
gives an exponential decrease, which means that the part is stressed
(a lot) for only a brief moment, with a current slope following, which
i think is gentler on the parts to weld.
plis...@gmail.com
probes or the transformer secondary?
Carl Ijames
> where localized heat might be a problem? I'm asking because all the
> literature i found suggested a cap discharge assembly for "finer"
> work. I can control the welding time with precisions in order of 25mS
> (realistically, with 10ms for both latch and release),and i stil don't
> know if that would be enough. The nice thing about capacitor discharge
> gives an exponential decrease, which means that the part is stressed
> (a lot) for only a brief moment, with a current slope following, which
> i think is gentler on the parts to weld.
I've used both a capacitive discharge and a small transformer unit (the
one sold at www.sisweb.com) to spotweld small filaments to posts, stuff
like 0.001" thick by 0.030" wide rhenium ribbon to 0.03" thick stainless
steel, and the capacitive discharge units were much easier to control.
Both got the job done, though, with some practice. At work we have a
15kva spotwelder that looks like it was made in the 60's, with an
electronic control box from the 90's added on. It does zero crossing
switching so the time is in increments of 16.7 msec - you program by
telling it how many cycles to apply. That would probably make your
current switch last longer if you go the transformer route.
--
Regards,
Carl Ijames carl dott ijames aat verizon dott net
(remove nospm or make the obvious changes before replying)
RoyJ
with it.
I'm into auto body work, small electronic boxes, robot base parts, etc.
Combine a 3 in 1 (press/shear/roll) with a 2.5 kva spot welder and a
whole lot of projects become quite easy. I haven't gotten those items
for my home shop but I did get the prototype lab at work to pop for a
shear, brake, and a borrowed 120 volt/1.5 kva spot welder.
If you are doing anything with smaller wire (thermocouples, etc) then
you want the capacitive discharge units. I looked at a surplus unit a
few months back: 1 to 100 joules, 30 pounds, $30.
plis...@gmail.com
> If you are doing anything with smaller wire (thermocouples, etc) then
> you want the capacitive discharge units. I looked at a surplus unit a
> few months back: 1 to 100 joules, 30 pounds, $30.
Nah, the smaller i would go would be welding small part for boxes or
tabs to batteries, all the way up to welding sheet metal to build
enclosures. I'm pretty sure i'll go the transformer way - my only
concern was that i wouldn't be able to control output energy as finely
as with a capacitor device.
dca...@krl.org
> Nice! Did you recall getting too much overheat on either the output
> probes or the transformer secondary?
The secondary was the laminated aluminum bar about 1.5 inches in
diameter. I don't recall it changing temperature. The primarys were
the original winding on the variacs. They must have warmed up a
little, but not enough for me to remember.
Dan
dca...@krl.org
i have a pretty good idea on how
> i'd build the clamp - basically two thick L shaped copper bars with
> fittings for replaceable tips and a hand operated spring assembly to
> close it with some force. The whole assembly kept as close
> electrically to the secondary as possible.
>
> Do you know if this kind of welder is well suited for small works,
> where localized heat might be a problem? I'm asking because all the
> literature i found suggested a cap discharge assembly for "finer"
> work. I can control the welding time with precisions in order of 25mS
> (realistically, with 10ms for both latch and release),and i stil don't
> know if that would be enough. The nice thing about capacitor discharge
> gives an exponential decrease, which means that the part is stressed
> (a lot) for only a brief moment, with a current slope following, which
> i think is gentler on the parts to weld.
You can buy the replacement tips at WW Grainger. And I would
recommend doing that. The commercial tips are a copper alloy , not
pure copper.
I think the major difference between the transfomer and cap discharge
spot welders is that the cap discharge can adjust the energy in
smaller steps. With a transfomer unit, you are limited in how fast
you can turn on and off. Pretty hard to control to less than 16 ms.
The current slope probably does nothing. You have a big copper heat
sink clamped on the weld area.
Microwave oven transformers are a cheap source for the cores. The
secondaries are would separate from the primary so can be removed
fairly easily. Need bigger core, use two or more transformers.
Dan
RoyJ
not have a timer at all. You just pull the trigger, wait for the spot to
get a suitable color of red, release. After about 3 welds on a
particular combination of metal and you will have a good sense of what
is "not enough", "just right', and "too much" I did some pull tests of
student welds done with a portable unit, the timing was not particularly
critical.
In the sheet metal range it takes something like 50,000 amps per inch of
total metal in the weld area. It doesn't take long to find that more
power is better. Also, it doesn't work on aluminum, carbon and stainless
only. It works on galvanized stock but you need extra power and the tips
crud up quickly.
Projects are fun but is there any way you can just buy one of the import
knockoffs? Sale prices around here are US$150. Not an issue to get some
sort of suitable base transformer but a set of copper tongs will set you
back US$40
awright
build a capacitor-discharge spot welder with transformer-coupled
output for welding weldable strain gauges to railroad rails several
decades ago. About 50 Watt-seconds energy level, as I recall. All
analog control circuitry using a stud-mounted SCR and a PUT
(Programmable Unijunction Transistor) for triggering via a microswitch
on an aluminum channel handle actuated by an aluminum electrode-
holding beam mounted at the rear of the channel handle.
As I recall, there is a "resistance welding" industry group that
offers technical literature and sets standards of welding tip alloys
and shapes. I don't remember the exact name of the group.
Worked great for its purpose. Weldable strain gauges are mounted on
thin sheet SS (I think) - or maybe nickle. They are mounted by making
a string of spot welds to the test object all around the active strain
gauge element.
The whole motive in building my welder was to get portability for
welding remote from line power (and to have fun), so it was operated
off two 67-volt "B" batteries in series. They charged the caps to an
adjustable voltage (we always used the maximum available voltage, as I
recall). This was before the days of readily available, inexpensive,
DC-DC converters, as I would use today with a 12-volt gel cell power
source.
The transformer was a steel-core inductor that had (as many inductors
do) extra space available in the E-I core window for a copper-braid
secondary of very few turns. I think a microwave oven transformer
with secondary removed would work for a capacitor discharge output
transformer. It was chosen somewhat arbitrarily on the basis of
having reasonable looking core and magnet wire size - perhaps by
looking at literature on commercial units. From memory, I'd guess the
core was about 3.5" x 4.5" x 2" stack thickness. I used about 3 feet
of relatively small welding cable between the transformer secondary
and the ground clamp and welding probe.
At the time, very high capacitance, low voltage caps were rare and
expensive, which was the motive for using the output coupling
transformer. I would probably investigate using capacitors directly
today, but worry about sparking and erosion from the relatively
uncontrolled discharge current.
Which reminds me, why are you thinking of using a triac instead of an
SCR in a capacitive discharge welder? I haven't priced them, but
would expect to pay much more for a high current triac than an
equivalent SCR, and very high current SCRs are readily available and
cheap on the surplus market. Check out candhsales.com in Pasadena
(but do it soon, as they are going out of business).
As one raised on tube electronics, I also wonder what the advantage is
of using microprocessor control for a capacitor discharge welder.
Control of both charge voltage and triggering is pretty
straightforward with analog electronics.
Many years later (but still many years ago) I tried using the same
homemade welder for welding nickle tabs on AA, C and D cell Nicads.
It worked, but only marginally. I think about 100 Watt-seconds would
be desirable for welding battery tabs reliably. However, my problem
may have been contact resistance, not welder capacity. I used a two-
pronged copper fork to make ground contact with the battery terminal
to avoid having the welding current pass through the active part of
the battery. I don't know if the welding current would damage the
battery, but I didn't want to risk it. Seems like a bad idea. The
tab to be welded and the welder probe fit inside the fork tines with
just enough clearance to avoid direct contact.
I think it might be a mistake trying to create a welder capable of
both sheet metal work and battery tab attachment, as that covers a
wide range of capacities and the handpiece/clamp and control
requirements are radically different.
Have fun!
awright
plis...@gmail.com
> do) extra space available in the E-I core window for a copper-braid
> secondary of very few turns. I think a microwave oven transformer
> with secondary removed would work for a capacitor discharge output
> transformer. It was chosen somewhat arbitrarily on the basis of
> having reasonable looking core and magnet wire size - perhaps by
> looking at literature on commercial units. From memory, I'd guess the
> core was about 3.5" x 4.5" x 2" stack thickness. I used about 3 feet
> of relatively small welding cable between the transformer secondary
> and the ground clamp and welding probe.
Nice work! I was consdiering looking for a scrap microwave transformer
with an intact primary - those have rather big cores and can handle
powers in the order of kVAs.
> At the time, very high capacitance, low voltage caps were rare and
> expensive, which was the motive for using the output coupling
> transformer. I would probably investigate using capacitors directly
> today, but worry about sparking and erosion from the relatively
> uncontrolled discharge current.
I'm kinda leaning away from the idea of caps or caps/transformer
combination - i like the way you can control the output power of such
configurations with ease, but caps are still expensive and, worst,
even low-ESD caps rated for such discharge work get worn out very
quickly, i'm told.
> Which reminds me, why are you thinking of using a triac instead of an
> SCR in a capacitive discharge welder? I haven't priced them, but
> would expect to pay much more for a high current triac than an
> equivalent SCR, and very high current SCRs are readily available and
> cheap on the surplus market.
I just happen to have a few triacs laying arround. SCRs would do fine
though - you can get ones rated for 1,2kA surge for very cheap.
> As one raised on tube electronics, I also wonder what the advantage is
> of using microprocessor control for a capacitor discharge welder.
> Control of both charge voltage and triggering is pretty
> straightforward with analog electronics.
Ahh another purist :) I usualy prefer analog electronics myself, but
this process is very well suited to be controled with, say, a 16F628
microcontroller. You get very accurate timing/charge monitoring and
can program a chain of operation easily, which helps making it safe.
The PIC is cheap and would only need a minimum of parts to interface
with the rest - i was planning to add a nice LED display though.
> However, my problem
> may have been contact resistance, not welder capacity. I used a two-
> pronged copper fork to make ground contact with the battery terminal
> to avoid having the welding current pass through the active part of
> the battery. I don't know if the welding current would damage the
> battery, but I didn't want to risk it. Seems like a bad idea. The
> tab to be welded and the welder probe fit inside the fork tines with just enough clearance to avoid direct contact.
Actually, as long as you have both contacts on the same terminal
there's little risk of current flowing into the battery. What *is* as
problem would be localized heat - batteries can be damaged rather
quicky in this way. Pulses must be strong but very brief (i.e.,
perfect job for caps) in order to avoid excessive heating.
> I think it might be a mistake trying to create a welder capable of
> both sheet metal work and battery tab attachment, as that covers a
> wide range of capacities and the handpiece/clamp and control
> requirements are radically different.
Sorry. I meant about the device itself being able to handle such jobs,
i.e, powerful enough to solder thick metal yet precise enough to be
useful for smaller work. Of course, the clamp assembly for soldering
sheet metal won't be of much use in finer work, but that could be
replaceable.
Batteries are not my main concern right now - but the ability to
control solder timing to work in different sheet metals would be very
nice to have.
Ignoramus21381
> I'm kinda leaning away from the idea of caps or caps/transformer
> combination - i like the way you can control the output power of such
> configurations with ease, but caps are still expensive and, worst,
> even low-ESD caps rated for such discharge work get worn out very
> quickly, i'm told.
>
>> Which reminds me, why are you thinking of using a triac instead of an
>> SCR in a capacitive discharge welder? I haven't priced them, but
>> would expect to pay much more for a high current triac than an
>> equivalent SCR, and very high current SCRs are readily available and
>> cheap on the surplus market.
>
> I just happen to have a few triacs laying arround. SCRs would do fine
> though - you can get ones rated for 1,2kA surge for very cheap.
How would you use an SCR if you are leaning away from capacitors?
I thought that the use of an SCR is to trigger discharge of the caps.
Without caps, you can use [two] SCRs to switch primary side, kind of
like those solid state relays, but you do not need a big one?
Am I missing a anything?
i
plis...@gmail.com
>
> I thought that the use of an SCR is to trigger discharge of the caps.
>
> Without caps, you can use [two] SCRs to switch primary side, kind of
> like those solid state relays, but you do not need a big one?
>
> Am I missing a anything?
SCRs were mentioned as a trigger if i used a capacitor-based design :)
I'll use a normal SPST relay in the primary for the transformer - been
doing some tests, and i can switch it on and off with a precision of
arround 20mS. I expect this to be a *tad* more under load (relay
contacts might arc a bit) so i'm settling on 25mS intervals just to be
safe.
plis...@gmail.com
> spot welders is that the cap discharge can adjust the energy in
> smaller steps.
I just had an epiphany... what if i placed a (swictheable) half-wave
rectifier in the primary winding? I'd have to test if the 50/60hz
pulses stresses the core too much, but it would cut output power in
half. I don't recall ever seeing this in a resistive welder, would be
interesting how it performs in real usage.
awright
> I just had an epiphany... what if i placed a (swictheable) half-wave
> rectifier in the primary winding? I'd have to test if the 50/60hz
> pulses stresses the core too much, but it would cut output power in
> half. I don't recall ever seeing this in a resistive welder, would be
> interesting how it performs in real usage.
Not sure that I understand exactly what you are referring to here, but
putting a rectifier on the INPUT of a transformer is a no-no. The
transformer core will saturate from the DC component of the rectified
waveform and when that happens primary current is essentially
unlimited except for DC resistance of the primary and input wiring.
That's why phase-control thyristors are used to vary the primary power
in a symmetrical manner.
awright
Bob Engelhardt
Shouldn't be too hard to find.
Here's a different approach:
http://www.green-trust.org/junkyardprojects/FreeWelderPlans/SpotWelder%232.pdf
It's basically a holder for a stick rod. In operation, one presses the
rod to the work and releases after a moment. There are a couple of
small springs pushing back, but I can't see how the rod is not going to
stick. Is there a special rod that would not stick in use like this?
Bob
Bob Engelhardt
> I'm pretty sure there's a homemade spot welder in the DropBox archives.
> Shouldn't be too hard to find.
This is the one that I was thinking about. Not in the DropBox after all:
http://www.5bears.com/welder.htm
Bob
plis...@gmail.com
> putting a rectifier on the INPUT of a transformer is a no-no. The
> transformer core will saturate from the DC component of the rectified
> waveform and when that happens primary current is essentially
> unlimited except for DC resistance of the primary and input wiring.
I was thinking of a simple half-wave rectifier - namely, a diode in
series with the primary winding. This way you get a half-sine pulse in
every cycle, then zero volts. Like i said, i never seen this before
and would have to check on how it stresses the transformer.
Another option would be using a reactance in series (most probably a
capacitor, to compensate a bit for the inductive load) to limit the
primary current. The capacitor needed would probably need to be quite
large (and mains rated) though.
plis...@gmail.com
The tips won't stick as long as their combined resistance is lower
than the one of the parts being welded, IIRC. Even then, they get
deformed and worn out, and need eventual replacement. Usually these
tips are copper alloy.
BTW, that very site started my curiosity on these devices :)
dca...@krl.org
> I was thinking of a simple half-wave rectifier - namely, a diode in
> series with the primary winding. This way you get a half-sine pulse in
> every cycle, then zero volts. Like i said, i never seen this before
> and would have to check on how it stresses the transformer.
There is a reason why you don't see this done. And Awright explained
what it is. But I will also explain and maybe you will understand one
of the explanations.
With a diode in series with the primary, the current will only flow
one way thru the transformer. So all the magnetizing force will be in
one direction. And the core will magnetize in one direction until
the transformer is fully magnetized in that direction. After that
there will be no flux change, and without any flux change there will
be no secondary current. Even worse there will be no back emf to
counter the voltage applied on the primary. So the only thing that
will limit the current will be the dc resistance of the primary. So
the current will be quite high and your fuse will blow. Your fuse as
in the diode you have in series.
Dan
Martin H. Eastburn
A cap in series of the transformer would likely - depending on size
would reduce the impedance of the input and cause a short when power is applied.
A cap - who has a reactance equal to the reactance of the transformer would drop
it, but a Transformer with a tap switch providing the mains power to the welder
might help - but another large core transformer.
The best way is to put a high wattage resistor in the output and adjust it.
Transformers that electronics can control are typically RF or higher frequency
than 50/60 Hz. In older machines they controlled the amount of metal
in the core - the coupling between primary and secondary was movable
and this provided the 'loss' of lines or power and the output would drop.
Just not an easy thing to do without a lot of bench testing and experiments.
Martin
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/
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plis...@gmail.com
> what it is. But I will also explain and maybe you will understand one
> of the explanations.
Yeah, i see it now. Bad idea, thanks.
plis...@gmail.com
> would reduce the impedance of the input and cause a short when power is applied.
My bad - i was thinking about a parallel cap (as in power factor
correction :), which of course will do nothing to reduce current
through the transformer. I need to stop posting late at night...
An inductor as ballast could do, but it'd need to be near the primary
in value in order to produce any noticeable effects - which means a
big iron core or an even bigger air one. Still, an option.
> A cap - who has a reactance equal to the reactance of the transformer would drop
> it, but a Transformer with a tap switch providing the mains power to the welder
> might help - but another large core transformer.
A switcheable tap would be ideal, but i don't think i'll be able to
find such transformers in those power ranges...
> The best way is to put a high wattage resistor in the output and adjust it.
Not feasible in this design - the resistance would need to be
comparable to the weld resistance, which is small fractions of an ohm
and means a lot of power to dissipate. Switching it in and out of the
system would be a problem too in order not to increase secondary
resistance by much. The only practical way to control power is in the
primary.
Anyway, it was just a thought. I also considered using a solid state
relay, which has much smaller switching times, but SSRs able to handle
3kVA loads are expensive and require a lot of heatsinking.
Martin H. Eastburn
Martin
Martin H. Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot's Medal.
NRA Second Amendment Task Force Charter Founder
IHMSA and NRA Metallic Silhouette maker & member.
http://lufkinced.com/
plis...@gmail.com wrote:
>> A cap in series of the transformer would likely - depending on size
>> would reduce the impedance of the input and cause a short when power is applied.
>
> My bad - i was thinking about a parallel cap (as in power factor
> correction :), which of course will do nothing to reduce current
> through the transformer. I need to stop posting late at night...
>
Need to watch the tank frequency - less the wrath of the power grid overlords.
> An inductor as ballast could do, but it'd need to be near the primary
> in value in order to produce any noticeable effects - which means a
> big iron core or an even bigger air one. Still, an option.
>
>> A cap - who has a reactance equal to the reactance of the transformer would drop
>> it, but a Transformer with a tap switch providing the mains power to the welder
>> might help - but another large core transformer.
>
> A switcheable tap would be ideal, but i don't think i'll be able to
> find such transformers in those power ranges...
>
>> The best way is to put a high wattage resistor in the output and adjust it.
>
Sure it is - you just don't use the resistors I used. Carbon piles that
are the size of a tower computer. The resistance is variable by turning
a screw that squeezes the stack. We dumped > 1000 amps through ours -
thought we were in trouble as we exceed the maximum suggested current
of 800 amps steady. Our sense wires were on the pile so the wrist size
of thousands of wires (copper) wouldn't drop voltage. The stack was smoking!
Not feasible in most shops due to carbon catching on fire :-)
I think the best way is to generate (like a switcher)
it in HF and rectify it to be the DC you want.
AC would have to be generated from the powerful
DC or perhaps the internal AC.
Hope you saw the 5bears design - that seems reasonable and knowing
him for some years and I bet it was in full working order before
a web page was developed.
> Not feasible in this design - the resistance would need to be
> comparable to the weld resistance, which is small fractions of an ohm
> and means a lot of power to dissipate. Switching it in and out of the
> system would be a problem too in order not to increase secondary
> resistance by much. The only practical way to control power is in the
> primary.
>
> Anyway, it was just a thought. I also considered using a solid state
> relay, which has much smaller switching times, but SSRs able to handle
> 3kVA loads are expensive and require a lot of heatsinking.
>
----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
awright
> A switcheable tap would be ideal, but i don't think i'll be able to
> find such transformers in those power ranges...
A method of reducing the size of a transformer used to control the
input voltage of your welder transformer is to use a transformer with
a high-current, low-voltage secondary and put the secondary in series
with the welding transformer primary in bucking mode. In this
configuration, the input control transformer power is only the bucking
voltage times the input current of the welding transformer - way less
than the full input power of the welding transformer (depending upon
the magnitude of the bucking voltage, of course). You could enhance
this setup by using a small Variac to control the primary voltage of
the input control transformer - at least smaller than would be
necessary to directly control the welding transformer input voltage.
> Anyway, it was just a thought. I also considered using a solid state
> relay, which has much smaller switching times, but SSRs able to handle
> 3kVA loads are expensive and require a lot of heatsinking.
An alternative to an expensive very high current SSR is a surplus dual
SCR module. You place the two SCRs in inverse parallel and provide a
low current switch or relay with series resistance between the two SCR
gates to create a bilateral static switch. I don't remember details
at this moment but could look it up in my 1960s era Westinghouse SCR
manual if you are interested. Don't build anything based only on my
vague description from memory above.
High current, dual SCR modules are a dime a dozen these days. One
that I just happened to buy a few of on a family visit to the Los
Angeles area last week is:
"DUAL SCR POWER BLOCK SW Semiconductor (ltaly) #STT 091/08. Current
rating is 91 amps. Peak reverse voltage is 800 VDC. Dimensions 3 5/8"
wide X 13/16" deep X 1 3/16" high. It contains two diodes which have a
common anode/cathode conneciton and separate connections for the
opposite anode and cathode. This allows the unit to be used in bridge
type applications. All units have heavy duty screw type terminals and
an electrically isolated thermally conductive baseplate. The mounting
bases of all the units have two mounting holes. The connections to the
gates of the device is via 1/8" quick connect tabs. RFE Stock #PD2202A
$5.00"
Search for the stock # in the online store at www.candhsales.com.
With a little more sophistication you could turn this into an input
power phase control, rather than merely a static switch. Note also
that you have to be very careful about current surge in applying SSRs
to control of inductive loads like motors. I don't know much about
this myself, but was advised once long ago by an SSR manufacturer's
tech rep. that an SSR used to control a motor had to be way over-rated
to tolerate starting surge.
awright
awright
> A switcheable tap would be ideal, but i don't think i'll be able to
> find such transformers in those power ranges...
A method of reducing the size of a transformer used to control the
input voltage of your welder transformer is to use a transformer with
a high-current, low-voltage secondary and put the secondary in series
with the welding transformer primary in bucking mode. In this
configuration, the input control transformer power is only the bucking
voltage times the input current of the welding transformer - way less
than the full input power of the welding transformer (depending upon
the magnitude of the bucking voltage, of course). You could enhance
this setup by using a small Variac to control the primary voltage of
the input control transformer - at least smaller than would be
necessary to directly control the welding transformer input voltage.
> Anyway, it was just a thought. I also considered using a solid state
> relay, which has much smaller switching times, but SSRs able to handle
> 3kVA loads are expensive and require a lot of heatsinking.
An alternative to an expensive very high current SSR is a surplus dual
plis...@gmail.com
> are the size of a tower computer. The resistance is variable by turning
> a screw that squeezes the stack.
>
> Not feasible in most shops due to carbon catching on fire :-)
Heheh, no, not really. An easier way would be to use a patch of heavy
gauge cable and soaking it in a water container to avoid needing to
cool it after a few welds. Still, if it ain't portable it's not
very... mm... handy :)
dca...@krl.org
>
> A switcheable tap would be ideal, but i don't think i'll be able to
> find such transformers in those power ranges...
>
No sweat. Use two or more microwave oven transformers in a row so the
secondary can be straight and run thru all the transformers cores,
make a bend and run back thru all the transformers cores again. Then
to vary the power just decide on how many transformer primarys to use.
Dan
Bob Engelhardt
>... Use two or more microwave oven transformers in a row so the
> secondary can be straight and run thru all the transformers cores,...
What a cool idea! So simple and easy & so powerful. You're adding-up
the kva of the transformers.
If I understand correctly, the magnetic field creating the voltage in
the secondary is the sum of the fields in the separate transformers.
You would use 1/2 the number of secondary windings to get the same
secondary voltage, but twice the secondary current.
You could get a 5 kva spot welder by using 4 1250 "watt" microwave
transformers, 2 pair of 2 in series, in parallel, with 240v input (240v
at 20a = 5 kva+-).
Bob
plis...@gmail.com
It is neat, but physically arranging the transformers so the cores
line up might be an issue, specially when short, low resistance
winding is needed. Still something i'll definitely keep in mind
though!
> If I understand correctly, the magnetic field creating the voltage in the secondary is the sum of the fields in the separate transformers.
It is - you're harnessing the flux of both transformers in the
secondary. The primaries appear (electrically) as a single one and you
basically get an output with the sum of the power capabilities of both.
plis...@gmail.com
> SCR module.
You know, for some reason i never considered this... a triac (dual-
scr) rated at 600V 60A can be found for like $5 and will do the job
nicely... in fact, more reliably than a relay, which i kinda worried
wouldn't last long under the stress of quick switching.
> With a little more sophistication you could turn this into an input
> power phase control, rather than merely a static switch. Note also
> that you have to be very careful about current surge in applying SSRs
> to control of inductive loads like motors. I don't know much about
> this myself, but was advised once long ago by an SSR manufacturer's
> tech rep. that an SSR used to control a motor had to be way over-rated
> to tolerate starting surge.
Indeed - since the welder will be controled with a uC, adding a zero-
cross detector in the mains and firing it in mid cycle (phase control)
is rather trivial and allows for very precise and fine timing.
And yes, triacs are *very* sensitive of inductive loads, because back-
emf when the current is swicthed on can (and will :) fry the device
rather quickly. I don't quite remember the details, but this can be
solved adding a capacitor & zenner in parallel to the primary winding
so the voltage spike is clipped. Of course, everything connected to
mains (z-c detector & triac) must be optoisolated.
I'll definitely go this route, thanks for the heads up!
dca...@krl.org
> You could get a 5 kva spot welder by using 4 1250 "watt" microwave
> transformers, 2 pair of 2 in series, in parallel, with 240v input (240v
> at 20a = 5 kva+-).
>
> Bob
You got it.
Dan
Bob Engelhardt
>... but physically arranging the transformers so the cores
> line up might be an issue, specially when short, low resistance
I don't think that they need to be aligned. It's all a matter of the
total flux enclosed by the secondary winding. The cores could be in
different rooms and the flux still enclosed by the winding, to take an
extreme and totally impractical example.
Bob
plis...@gmail.com
> total flux enclosed by the secondary winding. The cores could be in
> different rooms and the flux still enclosed by the winding, to take an
> extreme and totally impractical example.
Yes, but you'd have 20mts of secondary winding between them :) What i
mean is that the configuration most likely to result in shorter wiring
for the secondary would be something like:
+-------------------+
| +----+ +----+ |
| | t1 | | t2 | |
| +----+ +----+ |
+-------------------+---
|
| Secondary loop(s)
Like i said, still a good idea. Just a trickier implementation, but
two 1,5kva transformers are cheaper than one 3kva...