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Transformer fiddling

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Dunc1

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#1
Many YouTube sites have plans for adapting a microwave transformer to supply a spot welder. European (including British) sites are working with 220 volt, 50(?) cycle transformers.

Can the transformers in North American microwaves (110-120 volts) be used with the 220 volt circuits for
clothes dryers & ranges? Using the higher primary voltage would give greater power (I think?).

Assuming this is doable, how would it be wired? Is a neutral required?
 

Ulma Doctor

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#3
Hi Dunc1,
you are correct, Europe is on 50 cycles frequency

no sir, you should not hook 220v up to a transformer rated for 115v operation
furthermore, you should not hook a 115v transformer to 220v.
both scenarios will damage the transformer and possibly anything hooked up to it

if you are looking for a transformer with higher amperage potential, you will want a higher VA rating for the proper voltage.
for example, your transformer is rated for 115v, 150VA but doesn't have enough power to operate a given circuit.
you would purchase/find a transformer with maybe a 115v, 200VA (or greater depending on necessity)

in the MOT transformers used for spot welders,
the primary windings are kept, but the secondary windings are removed and replaced with only a few turns with very heavy wire/cable.
the secondary voltage is low due to the small number of winds around the transformer core, but the amperage can reach impressive limits even with relatively small components.
the output will be AC unless rectified, but you'll want to use heavy diodes suitable for welding in the design
AC spot welders work ok, but DC spot welders offer better penetration in shorter time periods.
 

gr8legs

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#4
The conversion process uses only the primary winding of the microwave transformer. The primary winding (the one you keep) is the one with the fatter wire and fewer turns.

The secondary winding (more turns of finer wire) is cut away and completely removed as a first step and replaced with a couple of turns of heavy gauge cable - usually multi-strand welding cable. This turns what was a voltage step-up transformer into a current step-up transformer. Be extremely careful removing the secondary winding. If you nick or break a wire on the primary I recommend you scrap that transformer and find another instead of trying to fix it.

If you're starting with a 110 volt transformer then it will probably work on either 110 or 220 volts. (Insulation on the primary winding is probably good to at least 600 volts so 220 is no big deal). Wiring the primary: hot and neutral with 110, both hots and no neutral with 220. In either case, be sure to ground (green wire) the case and transformer laminations.

Yes, you will get more heating power with the 220 volts on the primary.

However, the nature of your questions suggests you find someone more knowledgeable to help with building this. It's not complicated but either the 110 or 220 can kill you.

Stu
 

markba633csi

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#5
This is a special case, a spot welder is intermittant duty so you could probably use a 110 unit on 220 long as you let it cool down periodically
Mark
 

cathead

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#6
You could use two 115 volt transformers and wire the primary in series and the secondary in series as well. That
way both the transformers would be operating at their rated voltage. The transformers would not overheat in
this configuration and you would have double the output power.
 

tq60

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#7
That would work in theroy but be sure to keep windings in sync so to speak.

It could be possible if using a single secondary wire through both to have one acting out of phase with the other netting nothing out.

Just something to consider if the final product does not work as expected.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 

markba633csi

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#8
Then you would just reverse the leads on one primary and you would be good to go
Mark
 
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