Microwave transformer welder

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Microwave transformer welder

Postby Mjolinor » August 11th, 2017, 6:09 am

Off on another project, it's been a while.

So I got my microwave HV transformer and want to make a spot welder for batteries. All very easy or so the Internet would have you believe.

It all works fine when running through my variac but when I plugged it directly into the mains it whiffed all the fuses and let the smoke out of my 50 amp SSR (input side of transformer). (UK so 230 volts and up to 100 amps available) I suspect the inrush current is massive.

The DC resistance of the primary is 4 ohms, this seems low but seems inline with what the Internet is telling me for these transformers.

When I pulled the microwave apart there was a winding on the transformer of maybe 5 turns or so. I suspect this was a DC winding to limit inrush current by pre-magnetising the core though I don't really know.

I have two turns on the secondary but these are open circuit when I power on the SSR.

Any suggestions about how to not blow fuses at power on?

I am using an ESP12 with an OLED to switch the SSR, it gives me variable pulse length and delay between the two pulses. I could use an IO line to detect zero crossover but should I switch at Vmin or Vmax. I can also switch off at the same phase so the hysteresis remains in the direction to reduce inrush on next power on.

No one else seems to have this problem and I am just wondering if I should find another transformer, it seems some of them have a magnetic shunt, this one does not.
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Re: Microwave transformer welder

Postby k-ww » August 11th, 2017, 3:40 pm

I suspect that your SSR doesn't do zero-crossing turn on.

Turning it on at any other point will cause the type of surge you observed, and release the magic smoke from your SSR.

When you replace it, get one that does zero-crossing turn on.

Use the small secondary winding [was the filament winding for the magnatron, I suspect] and after full wave rectifying it, use it to count the half cycle's of AC you have turned the SSR on for.
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Re: Microwave transformer welder

Postby Mjolinor » August 12th, 2017, 6:39 am

It was a Chinese, 25 amp zero crossing SSR. I have replaced it with a non-zero cross 50 amp Crydom and decided that for now I will use it off the Variac. Use what is in the bits box if you can. :)

I am not convinced that V zero cross is the best time to turn it on. There will be significant residual magnetism in the core so I reckon that the best time to turn on is when the inrush current requested by the un-magnetised core would equal the residual magnetism. << If that is not the most convoluted way of saying it then I will eat my hat.

Or maybe the best thing is to switch off and on at zero cross.

I just checked and the Crydom is also zero switching.

It is interesting however that neither the Chinese one nor the Crydom actually zero cross switch.

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