Microwave oven transformer - safe use and setup questions

[Guineafowl]

Before we start, this transformer will be used in a closed shop, on a clear bench and via a remote switch. I will be in full view of the setup. I hope to try some Lichtenberg patterns.

I see that the secondary, c. 2000V winding is referenced to the transformer core. What is the purpose of this? Would it not be safer to remove this connection and have a floating output?

Also, would removing the magnetic shunts improve or hinder the performance for this application? As far as I can find out, these provide a kind of constant current regulation.

 

[stephenkohnle53]

I don't know much about electrical engineering but I would recommend not messing with microwave transformers unless you have a lot of experience with high voltages and transformers in general. I have seen people use it and its not very safe, I suggest wearing very good protection and be very careful if you use it. I have seen people use lichtenberg patterns before and i would recommend doing lots of research before making it toe ensure you know every step you will do before beginning. I believe it works best if you put a certain film on the material but again I have very little engineering experience so take my advice with a grain of salt.

 

[anorlunda]

Thread temporarily closed for moderation.

 

[anorlunda]

Thread reopened. But it will be closed if anyone posts information here potentially dangerous to the public.

 

[Guineafowl]

Thanks anorlunda. I will say, for anyone on here or who might read this, that there will be no question of ‘messing with’ the device. Its output voltage and current capabilities are such that there will be no warning shock or ‘close call’ - it’s just plain lethal. The only time it will be energised will be via remote switch, activated by me, next to the closed door of the room. That way, no-one can burst in. Before approaching it, I will unplug the setup.

That said, the high voltage patterns created (which I believe are used as models for lightning propagation) are fascinating. I believe it’s possible, with care, to have a sensible discussion of the subject. Better here than in some Youtube comment section.

I think the best solution is sodium carbonate, painted on the surface of the wood. Table salt tends to give off chlorine gas.

 

[Guineafowl]

Anyone got ideas on this?

I’m mainly interested in why the secondary is referenced to the core, and whether it would be better to delete this.

From some research, I gather the magnetic shunts are there to shunt some of the magnetic field from the primary to reduce inrush current, and to provide some current limiting on the output. Is this correct?

 

[Guineafowl]

Is it this you're trying to do?
https://i.pinimg.com/736x/a4/00/cf/...4--lichtenberg-figures-fractals-in-nature.jpg

Is that acrylic? I think that’s done with very high voltage discharge.

My intention is to burn similar patterns into wood, by painting the surface with saturated sodium carbonate solution. The electrodes go on either side. Being AC, the patterns grow from both sides, giving a pleasing tree-branch or fractal-like effect.

It’s more of an interest in the engineering behind the process than a desire to actually create the patterns.

 

[jim hardy]

Anyone got ideas on this?

I’m mainly interested in why the secondary is referenced to the core, and whether it would be better to delete this.

From some research, I gather the magnetic shunts are there to shunt some of the magnetic field from the primary to reduce inrush current, and to provide some current limiting on the output. Is this correct?

Here's what i think but do not know for certain... that is I've been unable to find basis documents for MOT design.

The shunts are primarily to limit output current, both for when a fault occurs and to limit running current to the magnetron..
Since the MOT is capable of delivering a kilowatt under normal operation, a fault can pass enough current to make a sizeable explosion.
Shunts make such a fault far less spectacular giving the primary fuse time to blow without undue pyrotechnics.
Observe the hobby guys who use them for making big arcs knock out the shunts first thing.
Note that limiting HV winding current makes life easier for the rectifier diodes that charge the HV capacitor..
So, the shunts divert primary flux away from the secondary to lower the voltage there...

Grounding the HV winding assures that a fault in the HV circuit to oven frame will quickly blow the fuse , not elevate some other part of the circuit above ground.
That's the same thinking as behind "Separately Derived System" in IEEE grounding standard 142(Green Book) and NEC.
http://adammunich.com/microwave-transformers/

I know you understand the danger involved with MOT's. They are killing amateurs.
I always admonish experimenters "Never energize a MOT until you have physically sawed off its high voltage winding" .

I strongly urge you to hacksaw the HV winding off yours when finished with your experiment.
Tiny fingers are curious...

old jim

 

[Guineafowl]

Here's what i think but do not know for certain... that is I've been unable to find basis documents for MOT design.

The shunts are primarily to limit output current, both for when a fault occurs and to limit running current to the magnetron..
Since the MOT is capable of delivering a kilowatt under normal operation, a fault can pass enough current to make a sizeable explosion.
Shunts make such a fault far less spectacular giving the primary fuse time to blow without undue pyrotechnics.
Observe the hobby guys who use them for making big arcs knock out the shunts first thing.
Note that limiting HV winding current makes life easier for the rectifier diodes that charge the HV capacitor..
So, the shunts divert primary flux away from the secondary to lower the voltage there...

Grounding the HV winding assures that a fault in the HV circuit to oven frame will quickly blow the fuse , not elevate some other part of the circuit above ground.
That's the same thinking as behind "Separately Derived System" in IEEE grounding standard 142(Green Book) and NEC.
http://adammunich.com/microwave-transformers/
View attachment 216940
I know you understand the danger involved with MOT's. They are killing amateurs.
I always admonish experimenters "Never energize a MOT until you have physically sawed off its high voltage winding" .

I strongly urge you to hacksaw the HV winding off yours when finished with your experiment.
Tiny fingers are curious...

old jim

Thanks Jim. From what you say, I’m better off leaving the MOT as it is. The current limiting is likely to give a more gentle progression of the pattern. As for the grounded frame, it’ll sit on a wooden bench, and I shall be well away from this when it’s active.

The MOT is currently stored away on a high shelf, and separately from any means of plugging it into the wall, but no, it won’t be thrown away in a usable condition.

 

[anorlunda]

@Guineafowl , It is unlikely you'll get more practical advice on this thread, and the risk of danger remains. I'm going to close it again. I sent you a PM.