Opt laminations isolation

[M Gregg]

Originally Posted by RuiR

Yes it have, much smaller though, between two laminations.

When i think in all the care i had with this project wasted in one minute of pure distraction

Thank you all.

Leave the two laminations alone..you might make the situation worse..

If I was going to do anything I would polish with a very very fine abrasive only in the damaged area...untill I could see the lines of the laminations..then Coat with a varnish ordinary polyurathane, Waterbased is not a good Idea or better still is PCB lacquer..Don't make the damage any bigger...You could just leave it alone. It will be OK..but as the saying goes if it isnt broken don't fix it...

Regards
M. Gregg

[Rundmaus]

Originally Posted by nazaroo

Actually, the welding on transformers such as microwaves is designed to create small eddy currents for safety reasons and RF issues, not to save money on screws.
This practice would be disastrous for audio transformers.

Actually seen this on small power transformers as well as on cheap OPTs, I don't know if there are practical reasons for it in special applications. The ones I saw where for cost cutting reasons. 'Disastrous' sounds a bit harsh, as all it does is increasing the iron losses, i.e. the power dissipated in the core, heating it up.

Can you explain in more detail how eddy currents and increased core losses help in the RF range?

Trying to repair the small damage shown in the picture will most probably increase the number of shorted laminations, as M Gregg wrote above, better to leave it just like it is.

Greetings,
Andreas

[Rundmaus]

I have an 'h' for sale, almost new and very cheap. Buyer has to pull it from the post above. o.O

[nazaroo]

Originally Posted by Rundmaus

Actually seen this on small power transformers as well as on cheap OPTs, I don't know if there are practical reasons for it in special applications. The ones I saw where for cost cutting reasons. 'Disastrous' sounds a bit harsh, as all it does is increasing the iron losses, i.e. the power dissipated in the core, heating it up.

Can you explain in more detail how eddy currents and increased core losses help in the RF range?

Greetings,
Andreas

Again, the reason microwave transformers are welded is to prevent a disaster if the microwave is turned on with nothing in the oven.

The microwave transformer is a specially engineered CONSTANT CURRENT DEVICE, not a normal transformer:

"These transformers are designed with as little copper as possible. The primary for 115 VAC is typically only 120 turns of thick wire - thus about 1 turn per volt input and output (this is about 1/4th as many turns as in a "normal" power transformer. (It's usually possible to count the primary turns by examining how it is wound - no disassembly required!) So there would be about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for the transformer mentioned above.



The reason they can get away with so few turns is that it operates fully loaded about 90 percent of the time but is still on the hairy edge of core saturation. The HV components are actually matched to the HV transformer characteristics.



Performance will suffer if the uF value of a replacement HV capacitor is not close to that of the original.


There is also generally a "magnetic shunt" in the core of the transformer. This provides some current limiting, possibly to compensate for various magnetron load conditions. However, it's not enough to provide any reduction in the likelihood of electrocution should you come in contact with the HV winding!

High voltage transformer

(From: John De Armond.) "The transformer goes by several names, depending on where you are. Variable reluctance, leakage flux, stray flux, etc. It is exactly the same construction and operating principle as a neon transformer, some kinds of HID light ballasts and some series streetlight constant current transformers.
The core is an almost standard "E" core (or "H" core if you prefer) with one exception. The center leg has an air gap. The windings are on the end legs of the "E" instead of the center leg.
There are two magnetic paths around the core for the field set up by the primary to travel. Around the periphery and across the secondary and around the center leg and across the air gap. The field that travels along the center leg does not cross the secondary and induces no voltage.
With no load applied, the bulk of the field travels the peripheral, very much lower reluctance solid iron path, inducing full secondary voltage proportional to the turns ratio. As current flows in the secondary, counter-MMF raises the reluctance of the peripheral path so that some of the flux travels through the center leg. With less flux traveling around the periphery and cutting across the secondary, the secondary voltage drops as the current remains about the same. At the limit, if the secondary is shorted, the peripheral path has so much reluctance that most of the flux travels the center leg and across the air gap. The same current as before flows through the secondary but at zero volts.
When the dimensions of the core and gap are set up correctly, the transformer behaves as an almost perfect constant current device. That is, the secondary voltage varies as necessary to keep the same current flowing through a varying load. Just what the doctor ordered to keep the magnetron happy.
The secondary current can be increased by opening up the air gap. This raises the reluctance of that path and forces more field through the secondary leg. Closing the gap has the opposite effect.
The center leg is often called the magnetic shunt and frequently it is a separate piece of laminated iron stuck between the coils and TIG welded in place. It is a common trick for Tesla Coilers to open up a neon transformer and either knock out the shunt entirely or grind it down to open the air gap. This modification causes the transformer to output much more current than it is designed for - for a little while, at least :-) The same thing works with microwave oven transformers (MOT).
This design in a microwave oven is a vital part of keeping the magnetron anode current within spec. The magnetron is electrically a diode. A diode that isn't emission-limited would draw destructive current if not externally limited. With this design, the filament can be heated good and hot for long life and not have the tube run away. The design also is vital for protecting the magnetron from potentially damaging conditions such as operating the oven empty, arcing, etc.
It's popular to use several MOTs to build an arc welder. This works quite well specifically because these transformers are constant-current devices - exactly the characteristic stick welding needs. If they were conventional transformers, the first time the rod touched the work and shorted the secondary, fault current would flow and the breaker would trip or blue smoke would leak out.
Along similar lines, one can cut off the high voltage secondary and replace it with a suitable number of turns of heavy wire, connect a bridge rectifier and have a nice constant current battery charger. Select the turns carefully and it'll do the bulk/absorption stages of the smart 3 stage charging algorithm."

[RuiR]

Greetings

Managed to clean it and reduced the number of shorted laminations. I will leave as it is now because i cant see myself doing a better job


Thank you all
Rui

[Rundmaus]

Nazaroo,

thanks for the nice explanation, didn't know these details about 'constant-current transformers' before!

Greetings,
Andreas

[nazaroo]

Originally Posted by Rundmaus

Nazaroo,

thanks for the nice explanation, didn't know these details about 'constant-current transformers' before!

Greetings,
Andreas

My pleasure.
One of the most common dangerous mistakes people try,
is to try to use a Microwave transformer for a HV supply.
This a worthless exercise but can also be LETHAL.

Another common mistake is to try to read the voltage on HV transformer.
That also can be lethal, and will usually destroy a voltmeter.

Max ratings for most voltmeters is 700 volts.