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Limiting AMP Draw of a Transformer? Answered

Hello guys. So i hacked a microwave transformer open and changed the primary coil out. It now has 10 windings and so i get around 15-16kV on the secondary coil. (which i need for a tesla coil)
Now the problem is the transformer draws more than 16A and so the fuses in my home "go out" ? (I hope you know what i mean; European household -> 230v 16A)
How can i limit how mutch Amps are getting "used" by the transformer ?

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Jack A Lopez
Jack A Lopez

4 years ago

I predict your hacked MOT (microwave oven transformer), will not work as a source of high voltage, as you have imagined it.

Although if you want to energize it, just to observe and make measurements on it, without the inconvenience of tripping your breaker, I suggest connecting the primary winding in series with some large resistive load, like an incandescent light, or maybe a toaster, or hair dryer. Then you can at least put your measuring tools, voltmeter, ammeter, etc, on it.

Although you should still be very careful not to touch it, while it is powered up, because MOTs are dangerous animals, even when wounded. Probably better to connect the instruments while it is turned off. Then switch on the power. Then read the display with your hands behind your back.

The reason why I expect your hacked MOT will not work as high voltage transformer is somewhat complicated to explain.

I mean, you increased the turns ratio, but the real magic inside a transformer comes from changing magnetic flux.

Vout/Nout = (dPhi/dt) = Vin/Nin

where Phi is magnetic flux, and dPhi/dt is the time derivative of Phi. That is, dPhi/dt is the rate of change of magnetic flux inside the core. You might have seen this formula before as Faraday's Law

V = N*(dPhi/dt)

https://en.wikipedia.org/wiki/Electromagnetic_indu...

https://en.wikipedia.org/wiki/Transformer

Anyway, in your MOT's original design, the magnitude of Phi was already about big as they could make it. It turns out there is a limit to the magnitude of Phi inside a transformer, the kind with ferromagnet core, and this is due to an effect called "saturation"

https://en.wikipedia.org/wiki/Saturation_%28magnet...


Saturation does not affect "air-core" transformers, but that's a whole different story.

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steveastrouk
steveastrouk

4 years ago

You broke it: changing the primary require maths, and is usually "right" for a given design anyway.

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rickharris
rickharris

Answer 4 years ago

I don't see how he can have changed the Primary and put in 10 turns, this would give him a massive (well short cct) step down transformer.

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steveastrouk
steveastrouk

Answer 4 years ago

The primary will be completely saturated, I doubt the juice gets much chance at the secondary, if the OP MEANT the primary.

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Downunder35m
Downunder35m

4 years ago

You can't as you already violated everything required by changing the primary.
If you need high voltage use 4 identcial mots and capacitors, plenty of examples out in the web for different configurations.
A normal mot is already close to the limit of a standard outlet, so there really is no chance to make your cripple work.

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rickharris
rickharris

4 years ago

Ah, you can't easily, Unfortunately electrical products draw as much current as they need to do their task, i.e. that for which they are designed.

You could design a current limited supply but that would be more trouble than it was worth at this current.

You could add resistance to the primary, that will limit the current and drop some of the the voltage as well and reduce your output.

16 amps at 230v is 3680 watts ! a fair sized electric fire (or resistor) . I suggest you re-examine the design of your transformer. Are you sure you altered the primary? and should you have done so, the original primary winding was sized to work with your 230v mains electricity.