[SOLVED] Arc Welding: AC Current Limiter

[denujith]

When the arc welding transformer is switched on, it takes ~3 A. Sometimes transformer is on for a while though welding doesn't take place. During this time it wastes energy. Can you please let me know whether is there a way to switch on the transformer when the welder touches the work piece with the electrode?

 

[lakshmi narayanan]

you could probably put another push down switch in the handle and ask him press it while welding

 

[denujith]

They do not want to have such a solution. I proposed it. But they need an automatic solution rather than manual intervention.

 

[FvM]

Did you measure the real power consumption of the transformer in idle state?

 

[denujith]

Yes current drawn was close to 3A.

 

[FvM]

Yes current drawn was close to 3A.

3 A hasn't directly to do with power consumption, because the transformer idle current is mostly reactive.

 

[dick_freebird]

I imagine you could make a resistor divider w/ the workpiece and sense the
low-going edge, but you might just end up starting into a stuck electrode that
way.

Didn't you just post this question recently? What the heck is wrong with your
workplace discipline? If your operators can't follow instructions of the simplest
sort, why do you let them do welding when that requires much greater
attention to detail?

 

[denujith]

workplace discipline

I have no influence in the way they work. They want me to come up with an automatic solution. I am thinking of different solutions and nothing came to mind as yet. That is why I am posting this.

3 A hasn't directly to do with power consumption, because the transformer idle current is mostly reactive.

Can you please elaborate this a little bit more? (I didn't measure the power consumption)

 

[FvM]

To know the real power consumption of the transformer at no-load, you need a power meter or have to count the meter revolutions. I guess that 10 % or maximum 15 % of the said 3 A are active current.

 

[denujith]

Thanks FvM, I'll do that ASAP. Thank again for your input.

FvM, I just wanted to know how this reactive power works and why 3A doesn't mean a lot in this case. Can you please direct me to a resource so that I can learn a little bit?

----


I think below one helps:
Reactive Power

 

[FvM]

See also:
Lessons In Electric Circuits -- Volume II (AC) - Chapter 11
AC power - Wikipedia, the free encyclopedia

 

[denujith]

Thank you a lot! (Y)

 

[BradtheRad]

First, try seeing if the idle current really can blow a 3A fuse. Or whether it can heat up short lengths of different gauge wire. This should prove whether genuine power is really being wasted.

Also, is the transformer in decent shape? Too much current, or too long current, can overheat it. Varnish insulation might break down. Metal plates might separate, creating a bulging transformer. Windings might make electrical contact with one another. Greater idle current (wasted power). The transformer suffers a permanent decline in performance.

A visual inspection may reveal transformer damage. Check coil isolation, winding isolation, etc.

Can you please let me know whether is there a way to switch on the transformer when the welder touches the work piece with the electrode?

My suggestion:

A welder wants to get immediate arc simply by touching the rod to the work. (I have experience with a 230 amp AC welder.)

Your circuit shall be like a watchdog, sensing immediately when he touches rod to work.

This can be done by sensing a change in voltage or current, in the secondary coil. (Theoretically voltage should fall across the secondary when the rod touches work and completes the circuit.)

Your sense circuit will then close a relay on the mains. (You might get by with a beefy triac.)

Primary coil gets power. Secondary coil gets power. Welder gets immediate arc (or almost immediate).

While he welds, your sense circuit holds the relay closed all that time.

When he stops welding, the secondary goes quiescent again.

Your sense circuit opens the relay after a while. It returns to watchdog mode.

So now everyone says "But if the relay is open then there's no voltage on the secondary for the sense circuit to sense."

My answer is that it should be possible to supply auxiliary power to the secondary.

Perhaps by attaching a second transformer to the main one inside the welder. It may even come to winding a hundred turns on the main core to create an auxiliary primary.

This is just brainstorming.

There will be a lot of testing required to see what electrical havoc takes place in the secondary during welding. Testing to see how robust the sense circuit needs to be. Etc.

 

[FvM]

First, try seeing if the idle current really can blow a 3A fuse. Or whether it can heat up short lengths of different gauge wire. This should prove whether genuine power is really being wasted.

3A is 3A as measured by the origanl poster, but a fuse or a current meter doesn't tell if it's representing active or reactive power. The only way to determine the difference must access both current and voltage (because the phase angle matters), as a regular electric meter does.

Your idea of using an auxilary supply for an automatic arc welder power switch should basically work. It's not a realistic idea to modify the transformer, I think. But it can be made as an external device connected between the mains and the arc welder, without modifying the arc welder. It supplies the transformer with a low voltage of 5 or 10% of the regular mains voltage through a small (e.g. 50 VA size) auxilary transformer, which should be sufficient to detect the electrode "short" at the transformer output and reduces transformer no-load losses to about "nothing". It triggers an off-delay timer, that controls the mains contactor. A current transformer has to retrigger the timer during normal operation. An auto-off time of e.g. 5 min seems reasonable.

If it's economic to install this kind of power switch in a workshop depends on how much active power is actually consumed by the transformer in no-load.

 

[denujith]

Thank you all for your input. I am waiting for the measured watts before proceeding with anything. As FvM pointed out, I want to see is it worth implementing such automatic control. If the active power is low under no-load, no point of having external circuitry. I'll post with the data [power consumption] I am about to receive soon.