Hello. I have dissasenble a microwave oven transformer, removed the secondary and put 1 turn of thick wire to make a spotwelder. Now i´m going to weld it back. The question is: Is it very important that the reassembly is perfect? The alingment of all the sheets? I know cores are laminated to improve overheating, but since I´ll use as spotwelder is not a problem. I´m worried about lossing efficency

What do you think?

Thank you

 

I think its better to not disassemble the Core in the first place.
But, the alignment is not critical.
I'm wondering how You got it apart without damaging the Primary-Winding.

Welding is not a requirement,
You can keep it from buzzing too loudly by reassembling the whole thing with Epoxy-Glue,
but don't make the Epoxy too thick as that could possibly make it run hotter than normal.
MOTs already run hotter than a "standard" design Transformer because
the manufacturers will all "cheap-out" as much as possible
on the amount of Copper used in the Primary-Winding, and the amount of Iron used in the Core.

MOTs are generally quite inefficient because of this type of cheap construction.

If You want to make a Spot-Welder that You can depend on for doing actual work,
it's much better to use 2 identical MOTs with the Primaries wired in series.
The secondary can be a single Wire, or strip of very heavy Copper,
that runs through both Cores.
This scheme will make the Transformers run much cooler.
.
.
.

 

Also ensure the magnetic shunt is removed (punched out) if it has one, as this will defeat it being efficient for welding

 

I am very interested on how you got the welded sections apart, because I have a similar plan for an MWO transformer. If I had access to a mill it would be simple to do an excellent job, but I do not have such access any more.

 

A common Angle-Grinder will do a quick and excellent job of removing the welds,
but it won't help You with the sticky-Varnish that some have gluing them together.
You'll need a ~1/8" thick Grinding-Wheel and less than ~10-minutes..
.
.
.

 

I have not seen a varnished microwave oven transformer in over 30 YEARS! The stacks of laminations may be stuck together, and that is fine because there is no need to have them come apart. And certainly the windings are done separately and slide onto the center post. An angle grinder with even the thinnest disc will remove more metal than I chose to have to replace. The big challenge is to not damage the magnetic flux path, because that reduces the efficiency. My target goal is to produce a 20 to 25 amp battery charger in a small package. Probably it will have a small mains powered fan inside.

 

Thanks for your answers.

With a angle grinder it is easy to cut along the welds. The secondary it is very hard to remove witout damging it. But the primary it is easy, using a hammer and a piece of wood to five taps in each side until gets out.

Despite not being the most important thing, I tried to keep everithing in place with C clamps



The new secondary is a thick wire made with other wires and wrapped in tape

 

Obviously the secondary consists of varnished wire(s) , how do you intend to ensure complete area contact to the electrode?

 

Hello. I have dissasenble a microwave oven transformer, removed the secondary and put 1 turn of thick wire to make a spotwelder. Now i´m going to weld it back. The question is: Is it very important that the reassembly is perfect? The alingment of all the sheets? I know cores are laminated to improve overheating, but since I´ll use as spotwelder is not a problem. I´m worried about lossing efficency

What do you think?

Thank you

Why? microwave oven transformers are amongst the most deadly things around the home, just buy something designed to do what you need.

 

Why 2 turns on the secondary?
How do you limit the secondary current?
I think I would have used a single strip or bar of copper or aluminum in a "U" shape.
The configuration of the laminations is not your major problem here!
I would have a fuse on the primary and expect it to blow. And rubber boots and gloves!!

 

Obviously the secondary consists of varnished wire(s) , how do you intend to ensure complete area contact to the electrode?

No no, is regular copper wire, extracted from other wires and wrapped in tape

Why? microwave oven transformers are amongst the most deadly things around the home, just buy something designed to do what you need.

The deadly part is the secondary, thousand of turns and 2 thousand volts. Mine have 1,5 turns, 1 or 2 volts, but around 1.000 amps, totaly safe.

Why 2 turns on the secondary?
How do you limit the secondary current?
I think I would have used a single strip or bar of copper or aluminum in a "U" shape.
The configuration of the laminations is not your major problem here!
I would have a fuse on the primary and expect it to blow. And rubber boots and gloves!!

I don´t want to limit the current. I use a control circuit that allow me to send very short pulses, from 0,01 seconds to 1 second.

It is not the first spot welder I make this way. But this one is improved. I´ll post pictures here when is finished

 

The spot welders I used to work with had originally a Thyratron & Ignitron tunbes to switch the primary!!
Seam welders had top and bottom wheel type rotary electrodes on them. Used for sealing automotive gas tank halves.

 

Spot welders mostly do not limit the current except by design. The weld is regulated by limiting the watt-seconds of power-on time.
And the serious danger associated with a MWO transformer that is disconnected and removed is the damage associated with dropping it on ones foot. Of course if some fool chooses to power the transformer and goof around with the high voltage there may be a hazard. But when I was in grade school I was experimenting with the effects of a 12000 volt neon sign transformer and also a 10,000 volt oil burner ignition transformer and never had any problems, nor ever got shocked. Knowing what one is doing is the method of providing safety.

"Natural Selection" is the alternative. Often called by the euphemism "Evolution", which sounds much nicer.

 

Why? microwave oven transformers are amongst the most deadly things around the home, just buy something designed to do what you need.

This use inverts the transformer feeding the secondary as the primary and replacing the primary with only a few turns. The result is a low voltage, high current output. It is not dangerous unless you come in contact with the mains voltage which is not an unmanageable hazard.

 

This use inverts the transformer feeding the secondary as the primary and replacing the primary with only a few turns. The result is a low voltage, high current output. It is not dangerous unless you come in contact with the mains voltage which is not an unmanageable hazard.

OK, that's a huge difference I admit, but for an application like this I think it calls for purchasing proper equipment.

I've seen no mention of the gauge of the wire for the new "secondary" or number of turns either.

 

OK, that's a huge difference I admit, but for an application like this I think it calls for purchasing proper equipment.

I've seen no mention of the gauge of the wire for the new "secondary" or number of turns either.

The one proposed by the OP in #7 has the typical single turn secondary.
For an design application such as this, I would recommend Earth grounding the secondary.
The one other design must, is pressure clamping the secondary electrodes at point of weld.

 

The one proposed by the OP in #7 has the typical single turn secondary.
For an design application such as this, I would recommend Earth grounding the secondary.
The one other design must, is pressure clamping the secondary electrodes at point of weld.

So isn't a spot weld more or less a short circuit?

 

Yes, a momentary, very high current through a small copper electrode welding tip area.

 

Yep, it's serious:

 

Thanks for your answers.

With a angle grinder it is easy to cut along the welds. The secondary it is very hard to remove witout damging it. But the primary it is easy, using a hammer and a piece of wood to five taps in each side until gets out.

Despite not being the most important thing, I tried to keep everithing in place with C clamps



The new secondary is a thick wire made with other wires and wrapped in tape

That polythene wrapping seems awfully flimsy, if it were to soften and the cable touch itself you'd be in for a problem.

This looks better: