DC Welding

patrick_g · May 20, 2007

Sound Guy, Unfortunately the "standard" approach is often not standard. The gap between tech writers and technicians and engineers often interferes with things or language differences can cause translation problems.

Often times when "PEAK" power is listed it does not actually indicate the actual power in Watts when the product of the instantaneous voltage and current values are at the maximum. It is a statement of the highest RMS Wattage reached in conditions such as all burners on high, during starting surge of a motor, inrush current to a heating element or in some instances inrush to charge a capacitor. Rated this way you get an idea of how large a fuse has to be to not blow or a breaker to not trip under this "peak" load. It does NOT necessarily relate to the instantaneous power peak during a half cycle when the product of amps and volts is at its maximum value.

Big time philosophical difference or point of view between the electrician and the electronic tech or engineer.

Warning ANNECDOTE FOLLOWS!!!

In my spare time I was helping a friend who was placed in charge of electronically outfitting several diesel powered aluminum fishing boats for use in Alaska that were being made at the Campbell shipyard in San Diego. He was loaned manpower from the ranks of the electricians. They were having problems. Lots of electronics on these boats: RADAR, SONAR, VHF, SSB SATNAV and on and on. Short circuits, popped breakers, vaporized fuses in pigtails to radios, and on and on.

My friend was so busy putting out little fires he didn't GET the big picture which was that the electricians were using the black wires for "HOT" and the electronic technicians were using the black wires for ground (like the manufacturers intended.) Between them they were wiring the 12 volt busses directly to ground through breakers and fuses. Everyone followed THEIR standard but the electricians were not used to DC color codes.

My point is that sometimes we need to take a step back and pull our noses out of contact with the bark of a tree to better observe the forest.

Pat

Soundguy · May 21, 2007

I'd be interested to learn exactly what 'standard' is being used on that UL tag.

Is it 'peak' power in electronics terms.. or is it the highest expected RMS draw. Since it was an organization like the UL.. I had 'assumed' ( here we go! ) that if they used the word 'peak'.. that thet refered to the actual quantifiable and calculated peak waveform in keeping wtih electronic conventions.. IE.. pk 2 pk, pk, rms, average.. etc..

Soundguy

patrick_g · May 21, 2007

Soundguy, Have you ever dealt with UL? I have sent prototype products in for testing. What a bureaucratic outfit. One item, a moving message sign, came back with the required engineering change that was to wrap all the insulated wire nuts with UL approved electrical tape and they gave the specs for the tape. My consulting customer said he supposed they couldn't find anything substantive so they invented something trivial to justify their part in the process.

Pat

Soundguy · May 22, 2007

Not to mention that they probably tried to hit you up for another application fee after the rejection.. right?

Soundguy

patrick_g · May 22, 2007

Soundguy said:
Not to mention that they probably tried to hit you up for another application fee after the rejection.. right?

Soundguy

Not that time. They gave me conditional approval, with the condition being to wrap a minimum of three layers of vinyl electrical tape over each insulated with nut.

The highest voltage was 120VAC +/- normal fluctuations. What boggles the mind is that the insulated wire nuts were UL approved. Why in H E double toothpicks did they want us to wrap insulation around a UL approved insulator approved for a much higher voltage? Like my customer said, they were probably justifying their existence. If they couldn't find something to gripe about then of course they weren't needed.

You ain't seen nuttin' till you design something that plugs into the wall AND attaches to a patient! I built a prototype hot/cold therapy machine for use on race horses and human athletes. VERY STRINGENT leakage standards and such. Hospital equipment standards make household UL approval look like a kindergarten game.

I remember various experiences with different peoples electric guitars and amps. You could get electrocuted!!!! Hard to be much more surprised than to close talk a microphone and get sparks between the mike and your lips!!!

Pat

J_J · May 22, 2007

Casey1 said:
DC output adds better control on thin work and easier out-of-position welding, as well as fewer arc outages, less sticking and spatter.

I pulled that out of a Miller ad. However I have wondered why the same welder puts out 225 amps AC but only 150 amps DC. I am sure the answer is simple and obvious but for some reason I have never run across an explanation.

I believe the answer is that question is the AC is being rectifier by a half-wave rectifier. If it were a full wave rectifier, it would be 225. The full explanation can be found in a basic electronics, or electricians book.

Casey1 · May 22, 2007

J_J said:
I believe the answer is that question is the AC is being rectifier by a half-wave rectifier. If it were a full wave rectifier, it would be 225. The full explanation can be found in a basic electronics, or electricians book.

I thought that too but that begs the question why not use the full wave rectifier? Cost perhaps? Wouldn't it be nice to have 225 amps DC at your disposal for the price, that is if the cost of the full wave rectifier is not prohibitive.

I barely know enough about electronics to ask dumb questions but all I have read here, and in some welding forums, none have addressed the question of the difference in amps between ac and dc in the same welder. Perhaps the question is too basic.

patrick_g · May 22, 2007

Sorry JJ but I don't think so. Borrow an O'scope and look at the wave form or open the case and measure the RMS voltage (AC) on the secondary of the transformer (or autoformer depending on the design) and then with a true RMS meter measure the DC output. I think you will find you are in error.

Casey1, Most welding forum stuff revolves around what happens outside the box (including the setting knobs) rather than inside the box. The two are totally different knowledge bases.

Pat

dayve · May 22, 2007

I got curious, so I went and grabbed the schematic from Miller's site(see attachment). Unless I'm reading it wrong, they do use a full wave rectifier. I don't have a definitive answer on why the DC amp rating is so much lower than the AC amp rating, though.

Lazy · May 22, 2007

Hi
I don't believe UL approves anything anymore they only list the Item. I haven't seen a label in 50 years that said UL approved only UL listed. But I have been wrong before.

Charlie

patrick_g · May 23, 2007

dayve said:
Unless I'm reading it wrong, they do use a full wave rectifier.

Nothing wrong with your analysis, that is a full wave bridge circuit where the full voltage of the transformer secondary is expressed at the diodes rather than a two diode circuit with a center tapped secondary.

Pat

Soundguy · May 23, 2007

That appliction fee is why so many devices use a 'standard' ul listed wall wart xformer.. that is detachable.. and sold as a bundle.. vs building their own design.. too much payola to spread out over a sales life of an item..

Soundguy

patrick_g said:
Not that time. They gave me conditional approval, with the condition being to wrap a minimum of three layers of vinyl electrical tape over each insulated with nut.

The highest voltage was 120VAC +/- normal fluctuations. What boggles the mind is that the insulated wire nuts were UL approved. Why in H E double toothpicks did they want us to wrap insulation around a UL approved insulator approved for a much higher voltage? Like my customer said, they were probably justifying their existence. If they couldn't find something to gripe about then of course they weren't needed.

You ain't seen nuttin' till you design something that plugs into the wall AND attaches to a patient! I built a prototype hot/cold therapy machine for use on race horses and human athletes. VERY STRINGENT leakage standards and such. Hospital equipment standards make household UL approval look like a kindergarten game.

I remember various experiences with different peoples electric guitars and amps. You could get electrocuted!!!! Hard to be much more surprised than to close talk a microphone and get sparks between the mike and your lips!!!

Pat

Soundguy · May 23, 2007

I agree.. and it looks like the current is based on what tap they are using to feed the diode bridge.... etc..

Soundguy

patrick_g said:
Nothing wrong with your analysis, that is a full wave bridge circuit where the full voltage of the transformer secondary is expressed at the diodes rather than a two diode circuit with a center tapped secondary.

Pat

Casey1 · May 23, 2007

A question to some that have that type of welder. Can you weld as large of stock with the 150 amp dc setting as the 225 amp ac in a single or equal passes.

Brad_Blazer · May 23, 2007

According to the "typical operating Procedures" in this document:
http://content.lincolnelectric.com/pdfs/products/literature/c210.pdf

DC runs at about 10% lower amps for a given electrode. I am guessing that would also apply to size of stock welded in a single pass. That would mean about 205 DC amps to equal 225AC.

gunta · May 27, 2007

The advantage is it welds much better,forget about ac,,just use dc,,if you don't know this than its hard to tell you much more,,,get you some 6010 rods and use dc,,elecrode positive,,but all rods will burn better on dcep,,,,thingy

patrick_g · May 28, 2007

gunta said:
The advantage is it welds much better,forget about ac,,just use dc,,if you don't know this than its hard to tell you much more,,,get you some 6010 rods and use dc,,elecrode positive,,but all rods will burn better on dcep,,,,thingy

Agreed, mostly. A reason to go AC on my Lincoln Tombstone is that you can get more heat at full power AC than you can get with full power DC. This is a RARE requirement for me (your needs may be different.) Within the heat available with DC it is typically superior.

Trivia questions for all you welding guys out there:

1. When do you want to use electrode negative (and why?)

2. The real welder dudes say straight and reverse polarity when referring to the polarity of the setup. Which is which? Is the rod + or is it - when using straight polarity?

Pat

cp1969 · May 28, 2007

I am by no means a real welder but will take a shot at it.

Don't you use 'reverse' polarity (electrode positive) when you want the electrode to get most of the heat rather than the workpiece, such as when welding thin material?

patrick_g · May 28, 2007

cp1969 said:
I am by no means a real welder but will take a shot at it.

Don't you use 'reverse' polarity (electrode positive) when you want the electrode to get most of the heat rather than the workpiece, such as when welding thin material?

It is a plasma physics thing... Ions and such... You select the polarity to get the most heat at the work or the rod. One way gives more heat at the rod like if you want more fill material for the amount of heat/melting of the workpiece (thin workpiece is one reason) and the other way gives more heat/melting at the workpiece for the amount of rod used.

You have no choice with AC and that is yet another reason AC is not as good as DC.

DCSP (Direct Current Straight Polarity produces a narrow, deep weld. Since the heat is concentrated on the work, the welding process is more rapid and there is less distortion of the base metal. Usually, straight polarity is preferred over reverse polarity because you can achieve better welds. DCRP (reverse) makes a wider and more shallow weld and is not used as much. An exception is when it is aluminum. (something that nearly escapes me in practice)

Positive ions bombarding the workpiece tend to clean it and break through any oxide coating, an advantage for reverse. IT is sort of an advantage for AC since half of the tie the current is Reverse and so AC will weld rusty metal better than DC Straight Polarity (rod emitting electrons.)

The Weldor's Weld
by Sonia Balcer 8/21/82

Wherefore must I wear a mask when
I hold in my hand, the pen
wherewith I write poetry with fire?

How I long to discover the secrets that are hidden
in the theater before me;
to behold the movement of tiny, metal
particles, as they waltz and interlock
in a world within a world;
as they pirouette between the boundaries set
by the fire which frees them to move.

Oh, Lord in heaven! Why are not human eyes made
to see this wonder directly?
Must I always hold this dark glass before me?

Hark! He causes my heart, to see the mystery!
The metals are assaulted, by electric energy,
carried by heaving, heavy wires. In a molten flash,
I see the crystals breaking, and sighing;
the silent order of the solid surface, giving way
to rushing, hotly-radiant tides
that crash together like waves at a beach.

It swirls before me, an intricate dance
which I cannot see, but yet feel inside.

I delight to caress the molten piece
in my heart.
It is inside of me, and I am inside of it.
I slowly feel over and underneath
the hot liquid surfaces.
I move into its every contour, and through
the whole of it.

Bodies of metal, which once were separate;
the boundaries are fading.
It swirls and whirlpools within me-
It is all blurred now, caught
Into a quickly-freezing body of what once
was separate, but now is together,

a single piece.

Pat