Gallery, Projects and General > The Design Shop
Microwave owen transformer spot welder?
PekkaNF:
Oh, I never have used those small spot welders, they look a little special and probably something worth imitating.
If you do the math, there must be very little of restance on every joint, therefore my goal is to minimize number of components and connections on secondary. Plan is to terminate other end of the copper rope (secondary winding) on lower (fixed) arm and the other end to pivoting upper arm.
I was toying with the idea of using steel profile/pipe as arms and put the conductor iside of them, but big current here might interfere here and my intuition tells that this might be of complication.
Thanks to Andrew, I got a bag of M6 MIG wire tips, they feel like a good candidate for this small spot welder.
I have one grounding electrode, OD is about 13 mm and it looks like copper.
Which is better, linear movement of the upper arm or pivoting arm? On my doodles looks like a over arm is the most simple solution. Long arm should have very small radial movement. I.E. lateral displacement of the tip should small enough not cause too much trouble.
Pekka
awemawson:
From a theoretical point of view, a straight line squash is best, but an arc should be fine, as the displacement is microscopic (or should be). I set mine up (with the power off !) to firmly nip the pieces together. Then adjust performance by varying the welding time. It often pays to do a few trials to check what time is best. In my test piece the one on the right is when I was happy with it functionally and cosmetically.
Clean electrodes help enormously :clap:
The electrodes in mine are fixed using a tapered cotter in like the old bicycle pedals used but without the retaining nut, so they are just tapped in - rather crude but it seems to work.
PekkaNF:
Thanks. More doodles with arm then...
Another avenue: Which type of solid state relay to choose? I have used triacs with zero crossing switching...apparently not that great here: Highly inductive load.
Pekka
awemawson:
Assuming that you intend to switch the primary of your transformer (which I'm sure that you do), so long as you use a solid state relay that is very well over specified for the load you should be fine. Some useful gen here:
https://www.omron.com/ecb/products/pdf/precautions_ssr.pdf
PekkaNF:
Yesh...primary. And that one is chalenging eneough. Some swears on zero (voltage) crossing switching some swears at it. I tried to google answers and I got even more confused that I were before.
Like this, halfway down and not geting anywhere....
http://electronics.stackexchange.com/questions/181874/when-where-to-open-and-close-a-transformers-output
Finally I setled here:
http://www.automation.com/library/articles-white-papers/industrial-io/understanding-solid-state-relays
Transformers
In controlling transformers, the characteristics of the secondary load should be considered, because they reflect the effective load on the SSR. Voltage transients from secondary load circuits, similarly, are frequent in transformers and can be imposed on the SSR. Transformers present a problem in that, depending on the state of the transformer flux at the time of turn-off, the transformer may saturate during the first half cycle of subsequently applied voltage. This saturation can impose a very large current (10 to 100 times rated typical) on the SSR that far exceeds its half-cycle surge rating. SSRs having random turn-on may have a better chance of survival than a zero cross turn-on device, for they commonly require the transformer to support only a portion of the first half cycle of the voltage. On the other hand, a random turn-on device will frequently close at the zero cross point, and then the SSR must sustain the worst-case saturation current. A zero cross turn-on device has the advantage that it turns on in a known mode and will immediately demonstrate the worst-case condition. The use of a current shunt and an oscilloscope is recommended to verify that the half-cycle surge capability is not exceeded.
A typical approach in applying a SSR to a transformer load is to select a SSR having a half-cycle current surge rating greater than the maximum applied line voltage divided by the transformer primary resistance. The primary resistance is usually easily measured and can be relied on as minimum impedance, limiting the first half cycle of inrush current. The presence of some residual flux, plus the saturated reactance of the primary, will then further limit, in the worst case, the half-cycle surge safely within the surge rating of the SSR.
Although. I was about to give it up ans use a contactor and RC-snubber (that's hard eneoug..)
Oh, and I found this, but those rocks looks diminutive.
http://www.avdweb.nl/Article_files/Tech-tips/Spot-welder/Spot-welder-controller-circuit.jpg
http://www.avdweb.nl/arduino/hardware-interfacing/spot-welder-controller.html
Second link confirms of switching SCR:s on near maximum voltage.
Pekka
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