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Science Forum Index » Electronics - Basics Forum » How to limit current from spot welder?
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| Homer J Simpson |
 Posted: Sat Feb 10, 2007 10:20 pm |
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Guest
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"Eric R Snow" <etpm@whidbey.com> wrote in message
news:3sfms258rh41shrbi7ijo1unahmkkrmanq@4ax.com...
Quote: 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. |
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| Jamie |
| Posted: Sat Feb 10, 2007 10:59 pm |
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Homer J Simpson wrote:
Quote: "Eric R Snow" <etpm@whidbey.com> wrote in message
news:3sfms258rh41shrbi7ijo1unahmkkrmanq@4ax.com...
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.
Phase controlled SCR's can adjust the voltage to the primary side.
--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5 |
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| Homer J Simpson |
| Posted: Sun Feb 11, 2007 1:05 am |
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"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:Hzvzh.2401$xa2.146@newsfe04.lga...
Quote: 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.
How well will that work with such a device? |
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| amdx |
| Posted: Sun Feb 11, 2007 8:36 am |
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"Eric R Snow" <etpm@whidbey.com> wrote in message
news:3sfms258rh41shrbi7ijo1unahmkkrmanq@4ax.com...
Quote: 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." |
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| John Fields |
| Posted: Sun Feb 11, 2007 8:50 am |
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Guest
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On Sun, 11 Feb 2007 02:20:54 GMT, "Homer J Simpson"
<nobody@nowhere.com> wrote:
Quote:
"Eric R Snow" <etpm@whidbey.com> wrote in message
news:3sfms258rh41shrbi7ijo1unahmkkrmanq@4ax.com...
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.
---
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?
--
JF |
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