Maker Pro
Maker Pro

How to limit current from spot welder?

E

Eric R Snow

Jan 1, 1970
0
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.
Thanks,
Eric
 
E

Eeyore

Jan 1, 1970
0
Eric said:
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.

It depends entirely on what's in the box that 'controls' it. It doesn't sound
like there's much there. Conceivably your best bet would be to run it on 120V.

Graham
 
R

Rich Grise

Jan 1, 1970
0
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.

Tolerance of what? Why do you want to lengthen the time period? If four
pulses weld .010" mild steel, then why change it? You put your parts in
the jaws, clamp them down, and weld. Do you want to do .005" mild steel?
Then try two pulses.

Or do you just want to slow it down so you can watch the weld taking
place in slo-mo? ;-)

Good Luck!
Rich
 
E

Eric R Snow

Jan 1, 1970
0
Tolerance of what? Why do you want to lengthen the time period? If four
pulses weld .010" mild steel, then why change it? You put your parts in
the jaws, clamp them down, and weld. Do you want to do .005" mild steel?
Then try two pulses.

Or do you just want to slow it down so you can watch the weld taking
place in slo-mo? ;-)

Good Luck!
Rich
Greetings Rich,
I want to slow it down because the timing relies on how fast I can
turn it on and off. I'm the timer. When doing .005" sheet I melt
through too often.
ERS
 
K

kell

Jan 1, 1970
0
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.
Thanks,
Eric

Make and model of the welder?
 
J

Jamie

Jan 1, 1970
0
Eric said:
Greetings Rich,
I want to slow it down because the timing relies on how fast I can
turn it on and off. I'm the timer. When doing .005" sheet I melt
through too often.
ERS
get your self a high current mercury relay to run the xformer and
get 2 adjustable delay on timers.
the first timer gets started with your clamp switch to insure you have
full compression. The first timer when it times out and comes on will
then start the second timer. the Second timer will use a NC (normally
Close) contact which will operate the main Mercury relay to turn on your
welder xformer. when this second timer times out, the contacts will
then open and wait for you to release the clamp to start the next cycle..
We did this once our self's for a shop, with the use of timers we removed
the arching that takes place when you're clamping and releasing because
you never know when the switch gets out of alignment with the electrodes
and energize under light contact.
Get your self short delay timers.
 
K

Ken Moffett

Jan 1, 1970
0
get your self a high current mercury relay to run the
xformer and
get 2 adjustable delay on timers.
the first timer gets started with your clamp switch to
insure you have
full compression. The first timer when it times out and
comes on will then start the second timer. the Second timer
will use a NC (normally Close) contact which will operate
the main Mercury relay to turn on your welder xformer.
when this second timer times out, the contacts will then
open and wait for you to release the clamp to start the
next cycle..
We did this once our self's for a shop, with the use of
timers we removed
the arching that takes place when you're clamping and
releasing because you never know when the switch gets out
of alignment with the electrodes and energize under light
contact.
Get your self short delay timers.

The problem with electromechanical relays are the delays on
make and break. In the case of the spot welder, "make" delay
is no problem. But these delays are on the order of hundreds
of milliseconds. Since one cycle is ~16 mSec, once the timer
shuts-off the relay current, it would still power the welder
primary for 16 or more cycles. What he needs is an
electronic timer that can have a start-stop response time of
1 cycle @60Hz, and a solid state relay that can also respond
in that short of time. The SSR should have a current rating
twice that of the spot welder.

I built one of these several years ago, after trying to spot
weld 0.010" stainless with a large Miller LMSW-52
(220vac/2.5KVA) spot welder, without a timer. And the timer
that Miller sold cost more than the spot welder, and stand
w/foot operated tongs, put together. And it was
electromechanical, and was never ment to do really short
cycle times.

The one I put together is archived in the Metalworking
Dropbox 2001, under "Miller Time"

If I were doing a lot of light weight sheetmetal I would get
a lower powered spot welder.

Ken
 
J

Jamie

Jan 1, 1970
0
Ken said:
Jamie <[email protected]>
wrote in



The problem with electromechanical relays are the delays on
make and break. In the case of the spot welder, "make" delay
is no problem. But these delays are on the order of hundreds
of milliseconds. Since one cycle is ~16 mSec, once the timer
shuts-off the relay current, it would still power the welder
primary for 16 or more cycles. What he needs is an
electronic timer that can have a start-stop response time of
1 cycle @60Hz, and a solid state relay that can also respond
in that short of time. The SSR should have a current rating
twice that of the spot welder.

I built one of these several years ago, after trying to spot
weld 0.010" stainless with a large Miller LMSW-52
(220vac/2.5KVA) spot welder, without a timer. And the timer
that Miller sold cost more than the spot welder, and stand
w/foot operated tongs, put together. And it was
electromechanical, and was never ment to do really short
cycle times.

The one I put together is archived in the Metalworking
Dropbox 2001, under "Miller Time"

If I were doing a lot of light weight sheetmetal I would get
a lower powered spot welder.

Ken
yeah, that's all well and fine how ever, you must pay close attention
to the type of SSR you use to insure zero crossing control and make
sure there is always a load with a HV suppresser on the electrodes.
arching loves SSR's
personally, unless micro spot welding is being done, i don't see why
the dwell time is so short.
I mean, it's a manual machine as it is now, how can he even be close
to that ? I think some one with that kind of speed should be in track :)
 
P

Phil Allison

Jan 1, 1970
0
"Jamie"

yeah, that's all well and fine how ever, you must pay close attention
to the type of SSR you use to insure zero crossing control ....


** Risky idea - ensures maximum possible " inrush surge " peak currents
every time.




......... Phil
 
P

Phil Allison

Jan 1, 1970
0
"John Popelish"
Yep. A shorted secondary, current limited transformer (I think that is a
pretty good description of a spot welder) is mostly inductor. If you
could add logic to fire the SSR, on the first half cycle, near peak
voltage, the transformer would get less hot and so would the SSR. But the
shorted secondary also reduces the tendency of the core to saturate
(except for the small magnetic shunt), so it may not be so important.



** Wrong to assume the secondary is always effectively shorted.

There will be very many occasions when the secondary is open.

Expect inrush surge peaks equal to the AC voltage peak / primary ohms.

My well exceed the SSR's repetitive I peak rating.

Splat.


......... Phil
 
J

John Popelish

Jan 1, 1970
0
Phil said:
"Jamie"




** Risky idea - ensures maximum possible " inrush surge " peak currents
every time.

Yep. A shorted secondary, current limited transformer (I
think that is a pretty good description of a spot welder) is
mostly inductor. If you could add logic to fire the SSR, on
the first half cycle, near peak voltage, the transformer
would get less hot and so would the SSR. But the shorted
secondary also reduces the tendency of the core to saturate
(except for the small magnetic shunt), so it may not be so
important.
 
J

Jamie

Jan 1, 1970
0
John said:
Yep. A shorted secondary, current limited transformer (I think that is
a pretty good description of a spot welder) is mostly inductor. If you
could add logic to fire the SSR, on the first half cycle, near peak
voltage, the transformer would get less hot and so would the SSR. But
the shorted secondary also reduces the tendency of the core to saturate
(except for the small magnetic shunt), so it may not be so important.
The problem i originally was stating is that Welders arc at various times.
We have copper tape welders at work that one of our electricians
tried to use a 3 phase SSR to drive the xformer. Not only was the idea
bad to start with, the xformer was a single phase type and his thinking
was that combining all three legs of this SSR would triple the duty of
the SSR was idiotic. I gave up trying to explain to this individual the
theory of diodes, SCR load sharing, arching, HV problems etc...

Needless to say, after shorting a few of these expensive SSR units,
the problem was past to me. To solve this problem i simply used a large
mercury relay. Problem solved.

We do have induction heaters at work that blow an SCR in the bridge
once in a while from arching. i must say, you should hear and feel the
HUM that comes out of that before it takes out the fuses!
 
K

Ken Moffett

Jan 1, 1970
0
"John Popelish"



** Wrong to assume the secondary is always effectively
shorted.

There will be very many occasions when the secondary is
open.

Expect inrush surge peaks equal to the AC voltage peak /
primary ohms.

My well exceed the SSR's repetitive I peak rating.

Splat.


........ Phil

Phil,

In mine, I use the switch that is built into the lever
mechanism for the jaws. In the original design it was just a
power switch in the primary. Jaws close=power on...jaws
open=power off. But the switch operates after the jaws make
contact and before contact is lost. In my design it just
triggers my timer. No timing or primary current unless the
jaws are closed...and I don't open the jaws until the timer
has cycled off. I suppose this where you could get powered
open secondary...if you let up on the jaws on a long time
cycle.

Ken
 
P

Phil Allison

Jan 1, 1970
0
"Ken Moffett"
In mine, I use the switch that is built into the lever
mechanism for the jaws. In the original design it was just a
power switch in the primary. Jaws close=power on...jaws
open=power off. But the switch operates after the jaws make
contact and before contact is lost. In my design it just
triggers my timer. No timing or primary current unless the
jaws are closed...and I don't open the jaws until the timer
has cycled off. I suppose this where you could get powered
open secondary...if you let up on the jaws on a long time
cycle.


** If the jaws close onto an insulator ??

Even lightly rusted, paint coated or grime/oil contaminated surfaces are
insulators, particularly at low voltages.




......... Phil
 
S

Si Ballenger

Jan 1, 1970
0
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.
Thanks,
Eric

You might try making a gizmo to shunt some of the output to the
weld contacts into another load. Maybe a carbon rod or some thing
that can handle the short burst of current and reduce the amount
being put into the weld itself.
 
H

Homer J Simpson

Jan 1, 1970
0
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.

You seem to imply you believe you can reduce the current and allow for
longer weld times. That is doubtful. You would need a tapped weld
transformer or at least a choke to do this.

A resistor in the primary is difficult. Possibly an electrical heater
element could be used but you may find the welding performance falls off
quickly.
 
J

Jamie

Jan 1, 1970
0
Homer said:
You seem to imply you believe you can reduce the current and allow for
longer weld times. That is doubtful. You would need a tapped weld
transformer or at least a choke to do this.

A resistor in the primary is difficult. Possibly an electrical heater
element could be used but you may find the welding performance falls off
quickly.
Phase controlled SCR's can adjust the voltage to the primary side.
 
A

amdx

Jan 1, 1970
0
Eric R Snow said:
Greetings All,
I have a handheld spot welder, the type used for autobody sheet metal
among other things. I am converting it to a stationary spot welder and
one thing that would be nice is to be able to limit the current for
thinner material. The input voltage is 220 volts @ 15 amps. Can I just
use an appropriately rated power resistor on the input side to reduce
the current? The welding current is 4000 amps and the suggested weld
times are specified in pulses at a pulse rate of 60 Hz. For example
the suggested weld time for .010" mild steel is 4 pulses. This is an
awfully short time period and I would like to make it a little longer
so that the timing periods allow more tolerance.
Thanks,
Eric

The following response assumes there are no control circuits that require
the full 220v input voltage.

Try to find a large Variac, a voltage variable transformer, usually
configured
as an auto-transformer. See http://www.elect-spec.com/variac_tutorial.htm
for more information. This would give you fine control of your spot welding
current.

Or,
you could wire in a series opposing (bucking) transformer on the primary
side. For example, If you had a transformer with a 40v 15a secondary, this
could be
wired in series (with opposite phase) with the primary of your spot welder.
This
would lower the spot welder input voltage to 180 volts and the output
proportionately
lower. But this only gives you one fixed voltage, and I don't know how do
you would
pick the right bucking transformer without testing.

Or,
Since large Variacs can be expensive, find a 220v to 110v stepdown
transformer with
a 15a secondary rating. Wire this series opposing. Then connect a smaller
Variac to the
primary of the stepdown transformer. I think the Variac could be much
smaller.
(This off the top of my head)
If you had the Variac at full voltage, your spot welder would be supplied
110 volts,
the spot welder primary current would be 1/2 or about 7.5 amps. That means
the
current on the primary of the step-down transformer would be 3.75 amps,
which is
the same current supplied by the Variac. So a much smaller Variac.
I have not crunched any other examples, so I'm not sure this is the worst
case scenerio,
I may have it backwards and this could be the best case.
Group, HELP verify a couple more scenerios)
Mike
I got a brand new 240volt 9amp Variac at a hamfest for $15. The seller
commented,
"oh, I should have looked in the box before I gave you the price."
 
J

John Fields

Jan 1, 1970
0
You seem to imply you believe you can reduce the current and allow for
longer weld times. That is doubtful.

---
Hardly.

Since the resistance of the materials is causing heat to be
generated when charge flows through them, the rate of change of
temperature is going to depend on the quantity of charge flowing and
the amount of time that charge is allowed to flow.

In other words, it will take the metal longer to melt with 2000
coulombs per second flowing through it than it would with 4000
coulombs per second flowing through it.

If changing the current doesn't change the time, what do you suppose
does?
 
Top