Controlling AC voltage for an inductive load

Thread Starter

chris_22

Joined Nov 5, 2021
3
Hello I need your help to a project that I have to do.
There is an old 8kw 3 phase spot welder transformer. I have to design a controller to control the output current/voltage of it.

This can be done by controlling the AC voltage that flows into the primary windings, right?
I read that snubberless triacs are suitable for this kind of job.
I need your advices for this project.

(This is my first thread and English isn't my mother tongue. So please bear with me)
 

MaxHeadRoom

Joined Jul 18, 2013
24,980
Years ago it was done with a timer and a large Contactor.
Later Ignitron mercury tubes were used,
8Kw is a little large (at what voltage though ??) but you should be able to find Triacs large enough,
 

Thread Starter

chris_22

Joined Nov 5, 2021
3
Years ago it was done with a timer and a large Contactor.
Later Ignitron mercury tubes were used,
8Kw is a little large (at what voltage though ??) but you should be able to find Triacs large enough,
Input voltage from the main supply is 400v... The controller should be capable of adjusting it between 250v-400v..
 

MaxHeadRoom

Joined Jul 18, 2013
24,980
Generally the voltage is not controlled, just the conduction time.
The older devices previously mentioned, used this method, i.e. you cannot vary the current by means of a contactor or Mercury-Thyratron!
For varying heat control, a method of a number of cycles on and cyles off were sometimes used as in continuous seam welders.
 

LowQCab

Joined Nov 6, 2012
1,914
"Why" do You need to control the Current ?
What is the Current now, and what is the desired Current ?

The easiest way to reduce the Current is to disconnect 1 or 2 of the Phases going into the Transformer.
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shortbus

Joined Sep 30, 2009
9,322
It puzzled me too,
a 3-Phase Spot-Welder ?????
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You've never worked in an industrial shop ,huh? what do you think all of those robot spotwelders run on in a car plant? Even the small sheet metal shop where I worked the spot welders and all of the welders, Tig, Mig and Stick were 3phase. Even the portable motor/generator welders used 3 phase to power the motor.

A quick google gets - https://www.google.com/search?client=firefox-b-1-d&q=3+phase+spot+welder
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Last edited:

LowQCab

Joined Nov 6, 2012
1,914
"" You've never worked in an industrial shop ,huh? ""

The Thread-Starter only mentioned a Transformer, and provided no further information.
I have quite a bit of 3-Phase Industrial/Commercial experience,
but not much experience with permanently installed Welders/Spot-Welders.

You comment is not very helpful.
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LowQCab

Joined Nov 6, 2012
1,914
"" Most industrial spot welders are AC on the electrodes. ""

I'm aware of that,
and that means that they are not applying 3-Phase AC.
So what's wrong with this picture ?

Why don't we find out more about what the Thread-Starter is referring to ?
There must be more than a Transformer involved.
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shortbus

Joined Sep 30, 2009
9,322
"" Most industrial spot welders are AC on the electrodes. ""

I'm aware of that,
and that means that they are not applying 3-Phase AC.
So what's wrong with this picture ?

Why don't we find out more about what the Thread-Starter is referring to ?
There must be more than a Transformer involved.
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Why would you even think what? Just because you have a 3PH primary has no bearing on what the secondary is.

Oh, excuse me, my head is exploding a little early

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LesJones

Joined Jan 8, 2017
3,701
I did think of the possibility of having a three a three phase bridge on the output but decided that there would be too much power loss. If we assume 2000 amps total output current there would be two diodes in series with the output (2 of the 6 conducting at any one time.) so that would drop about 1.4 volts. this would mean that 2.8 Kw would be dissipated in the diodes.

Les
 

MaxHeadRoom

Joined Jul 18, 2013
24,980
Early on in my career, for a decade I worked in a UK Automotive factory maintaining the machinery electronics, much consisting of welding M/C's, Spot, seam, and projection versions,
Many made by the well known Sciaky Welding Co.
The largest were the projection welders where one machine had to be interlocked to the other to prevent both operators from activating both machines at the same time due to the large power consumption.
All the welding machines, with the exception of the small spot welders were controlled by single phase transformers using thyratron and ignitron control, because the ignitron is similar to a SCR, it only conducts in one direction, hence, two ignitrons had to be connected back to back .
And the secondary of course produces 1phase.

Below is a sample of one of the smaller spot welders,

100Kva 220-440v 1phase.

1637813007013.png
 
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