Author Topic: Fit a full bridge rectifier in order to use full 230 amp output with DC  (Read 549 times)

Alsabony

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I have a welding machine that has a rated output of 230 amps with the AC output. The DC output is 140 amps. I was a bit puzzled as to why this was, but from what I can tell they use a lower voltage tap on the output of the transformer. Since the output is rated at 140 amps, they likely selected an automotive rectifier from an alternator to keep production costs down.

I intend to fit a full bridge rectifier to allow me to use the full 230 amp output with DC welding current. The AC frequency is 60 Hz from the mains. So switching time is not likely to be an issue. Although the welder is rated at 230 amps, the peak output current can be much higher. Closer to 300 amps. The voltage listed on the front of the unit is 38 VAC with no load.

An additional design constraint is that the output has an inductor in series with it. This is done so that smoother current is delivered to the workpiece. It can cause a large voltage spike when welding stops.

Another constraint is that I want to add a high frequency high voltage current source in parallel with the welding leads. I intend to use a bypass capacitor to try and prevent the rectifiers from seeing this, but they could still see some of the voltage.

How should I go about selecting rectifiers for this unit? How much safety margin should I select in the rectifier's peak current rating? Is it generally cheaper to find an integrated unit (4 diodes in a package) or should I buy four independent diodes.




    V1 is mains current
    D1-D4 are the diodes I need to add
    L1 is a current limiting inductor on the primary side
    L2 is a current smoothing inductor on the output
    V2 is a HF high voltage power supply. This is known as "arc-start"
    R1 represents the welding leads and workpiece. The actual impedance is whatever the sum of the leads, clamps, and arc happens to be.

BTDretire

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Re: Fit a full bridge rectifier in order to use full 230 amp output with DC
« Reply #1 on: January 16, 2018, 12:44:23 PM »
Here's a url of a guy that installed 6 parallel 50 amp bridge rectifiers in his welder to increase the DC current rating.
 The only thing I notice is he mounted his rectifiers back to back on the heatsink. It really depends on the heat produced by the rectifiers, but that would not be best at eliminating the heat. Separate or at least moving the rectifiers apart would be better. Give them room to get the heat out.
http://hildstrom.com/projects/ac-225/index.html
 He did a bunch of other mods also.
  btw, no problem with a single bridge, just has to be very big. Or 4 big stud rectifiers.
Here's a 300amp 1600V bridge, https://www.ebay.com/itm/New-60A-100A-150A-200A-300A-400A-Amp-Single-Phase-Diode-Bridge-Rectifier-1600V/332344586065?hash=item4d6147ef51:m:mRtzPZhKNAngnD0FzSwLjWA
 You may not need 1600V, but it won't hurt and is a nice safety margin. Not bad for $30.

Silverado

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Re: Fit a full bridge rectifier in order to use full 230 amp output with DC
« Reply #2 on: January 21, 2018, 06:42:32 PM »
The people over at allaboutcircuits.com are tremendously helpful. I am constantly amazed at the intelligence there.

HamMan

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Re: Fit a full bridge rectifier in order to use full 230 amp output with DC
« Reply #3 on: January 22, 2018, 08:48:27 AM »
I think I have the same welder, made by Lincoln Electric? You are not the first person to want to add functionality to your machine, this guy's website is similar and pretty interesting: http://hildstrom.com/projects/ac-225/index.html

One comment: the AC/DC version of the Lincoln welder (which I'm guessing you own) is worth about double what the basic AC-only version is. Since you want to redo the DC components anyway it's a bit of a waste to modify the AC/DC welder. If you sold the AC/DC welder you'd probably have enough money to buy an AC welder and to fund your modifications. Just a thought!

Another constraint is that I want to add a high frequency high voltage current source in parallel with the welding leads. I intend to use a bypass capacitor to try and prevent the rectifiers from seeing this, but they could still see some of the voltage.

I'm curious what you are planning to do for the high-frequency source. I'm guessing to use it for striking a TIG arc? Interesting project!