Welders and inductance

[FridgeFreezer]

After stumbling across this post on the MIG welding forum, and having just dissected my NBC I found that it has a fixed choke in the front disguised as a transformer and connected to the earth lead, it also has a bank of capacitors tapped into the choke about 2/3 of the way down the windings.

Choke is the one on the left:

Capacitors with resistors:

For those too lazy to click, here's the science bit:

I've been researching... on the Lincoln site and others. It's the inductance control, used in short circuit migging ( what most of us are doing). It controls the "wetness" of the toe, apparently!

The lincoln welding guide says...

Inductance plays a role in the frequency of droplet transfer per unit of time: as the inductance increases, the frequency of short-circuit metal transfer decreases. Each droplet contains more energy and toe wetting improves. As the inductance decreases, the shortcircuit events increase, and the size of the molten droplet decreases. The objective for the variable inductance control feature, on any given power source, is to transfer the smallest molten droplet possible with the least amount of spatter, and with sufficient energy to ensure good fusion.

This (I think, if I've understood this properly) explains why it's so much nicer to weld with the NBC than a hobby MIG

Out of interest can anyone point me to a bit more theory on this as the cap bank looks a bit past it's best and I was contemplating replacing the caps/resistors with new ones - would bigger ones or different resistors give any improvement or should I not tinker with ancient wisdom from the time of flared trousers?

I found a box of actual flux capacitors a while ago and I really want to have a reason to fit one to something

[disco tony]

After stumbling across this post on the MIG welding forum, and having just dissected my NBC I found that it has a fixed choke in the front disguised as a transformer and connected to the earth lead, it also has a bank of capacitors tapped into the choke about 2/3 of the way down the windings.

Choke is the one on the left:

Capacitors with resistors:

For those too lazy to click, here's the science bit:

This (I think, if I've understood this properly) explains why it's so much nicer to weld with the NBC than a hobby MIG

Out of interest can anyone point me to a bit more theory on this as the cap bank looks a bit past it's best and I was contemplating replacing the caps/resistors with new ones - would bigger ones or different resistors give any improvement or should I not tinker with ancient wisdom from the time of flared trousers?

I found a box of actual flux capacitors a while ago and I really want to have a reason to fit one to something

Have you thought about a Delorean??

[FridgeFreezer]

I bin learnding things so I thought I'd share:

From ESAB's website:

Power sources do not respond instantly to load changes. The current takes a finite time to attain a new level. Inductance in the circuit is responsible for this time lag.

The maximum amount of current attainable during a short is determined by the slope of the power source. Inductance controls the rate of rise of short circuit current. The rate can be slowed so that the short may clear with minimum spatter. The inductance also stores energy. It supplies this energy to the arc after the short has cleared and causes a longer arc. In ”short arc” welding, an increase in inductance increases the ”arc on” time. This, in turn, makes the puddle more fluid, resulting in a flatter, smoother weld bead. The opposite is true when the inductance is decreased. Figure 3-6 shows the influence of inductance on the appearance of ”short-arc” welds made both with an argon-oxygen gas mixture and with a helium-argon-carbon dioxide mixture. Weld No. 1, made with a mixture of 98% argon and 2% oxygen shielding gas and no added inductance, is rolled or peaked as seen in the top cross-section. Midway along the sample, inductance of about 500 micro-henries was added. Freeze lines are not as prominent, and the bead remains convex.

Weld No. 2, made with the He-Ar-Co2 mixture is also convex. Spatter on the plate is considerable. When inductance is introduced midway through the sample, the reduction in spatter is dramatic; the bead becomes flat and the cross-section on the bottom right shows penetration of the weld bead into the workpiece has increased. In spray arc welding, the addition of some inductance to the power source will produce a better arc start. Too much inductance will result in erratic starting. When conditions of both correct shorting current and correct rate of current rise exist, spatter is minimal. The power source adjustments required for minimum spatter conditions vary with the electrode material and size. As a general rule, both the amount of short circuit current and the amount of inductance needed for ideal operation are increased as the electrode diameter is increased.

I also love the fact that inductance is measured in Henries, the milli-Henry is possibly the best unit of measurement ever

[Jon White]

Hello Henry!

So..........how is the inductance changed? I've never seen a welder with a control for this I dont think?

Jon

[FridgeFreezer]

Seems to be only big/old sets that have a variable inductance:

Done with a sodding great big variable choke, V pricey these days I would think. Edit: Actually they'd probably do the same thing with electronics instead these days...

[simonb]

Out of interest can anyone point me to a bit more theory on this as the cap bank looks a bit past it's best and I was contemplating replacing the caps/resistors with new ones - would bigger ones or different resistors give any improvement or should I not tinker with ancient wisdom from the time of flared trousers?

I found a box of actual flux capacitors a while ago and I really want to have a reason to fit one to something

Caps look ok to me, unless they are leaking or bulged at the ends leave alone. The value of the choke and capacitors will all be matched to the transformer, so don't add/change capacitance. If they do fail replace like with like, new caps must have a voltage rating at least equal or higher than the originals.

[ThreeSheds]

I found a box of actual flux capacitors a while ago and I really want to have a reason to fit one to something

I think I've got some of those on an old Interossiter in the shed...

[simonr]

I think I've got some of those on an old Interossiter in the shed...

Cool film! Always wanted to build one! I'm sure Nokia will beat me to it with a death ray video phone though!

Back to the inductance thing -

This (I believe) is why all welders have a 'sweet spot' - one combination of settings where they lay down a nice weld really easily!

Si

[FridgeFreezer]

Well who doesn't have an interossiter these days?

[HoSS]

Caps do age, and if those are 25 years + it may be worth replacing.

You won't necessarily see that they have dried inside. If you do replace, dont change the capacitance, as it is probbaly designed that way & will affect the inductance. Use some with equal or higher voltage.

Old caps, when they do go, tend to go short (& sometimes explode)

I may have some around - tell me the capacitance, V & can dimensions.