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Series Connected MOT's

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Frosty_47

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Hello, I was wondering if I can safely connect 5 or 6 Microwave Oven Transformers in Series to get 10KV-12Kv output. I am thinking of submersing them in transformer oil. However, due to the dangers involved, I would like to hear some expert opinion on this issue.

Thanks
 
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dknguyen

Well-Known Member
Most Helpful Member
Nothing necessarily wrong with connecting many transformers in series to produce a transformer with a higher turns ratio...but can the insulation on the transformers handle 10kV-12kV? Because if they can't, you better not. Period.
 
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Leftyretro

New Member
Nothing necessarily wrong with connecting many transformers in series to produce a transformer with a higher turns ratio...but can the insulation on the transformers handle 10kV-12kV? Because if they can't, you better not. Period.
Does each transformer insulation value have to equal the total series voltage rating of all the series connected secondary windings? I didn't think so, but maybe you could explain your thinking. Maybe true if you ground one side of the output voltage and the transformers are mounted to a grounded frame. However if mounted in a isolated oil bath?

PS: What is the typical secondary voltage and current rating for a typical microwave transformer?

Thanks;

Lefty
 
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dknguyen

Well-Known Member
Most Helpful Member
Does each transformer insulation value have to equal the total series voltage rating of all the series connected secondary windings? I didn't think so, but maybe you could explain your thinking. Maybe true if you ground one side of the output voltage and the transformers are mounted to a grounded frame. However if mounted in a isolated oil bath?
I've been thinking about it a bit more and I realized I might have been wrong. After all, inductors in series produce an equivelant inductor with a higher voltage rating.

My reasoning was the the isolation between the two windings of the transformer stays the same no matter how many transformers are in series. If both sides of the compound-transformer were referenced to ground and the compound-transformer was wired to reverse polarities, then one of the transformers on the end would experience the full voltage difference between ground and the highest voltage in the windings. If the compound-transformer was wired to maintain the same polarity, then then one of the transformers on the end would experience the difference in voltage between input and output. So obviously the both input and output windings should be completely isolated from each other (ie. the output winding should be left floating so that it operates like an isolation transformer).

My concern about the oil submersion was that it would not increase the isolation voltage between windings (which still holds true I think). But...it does increase the isolation voltage all around the transformer. So if you left the output winding floating and submerged the whole thing in an oil bath then it might work. I think it might be a bit unpredictable to have such a high floating voltage, since it just might spark to something by accident under the right conditions, so the oil bath would insulate the floating output from the outside world to remove some of the unpredictability.
 
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Mikebits

Well-Known Member
I don't know why, but this whole idea sounds very hazardous to me.
 

Mikebits

Well-Known Member
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Frosty_47

New Member
PS: What is the typical secondary voltage and current rating for a typical microwave transformer?

Thanks;

Lefty
Well they all have different ratings. Lowest is 0.5Amp at 2000VAC I think. I plan to use this for my Tesla Coil Project. I will limit the input current to each transformer by the use of Triode Circuit installed on its primairy. I will not be exceeding 1000VA load at any point in time.

Thanks
 

Mikebits

Well-Known Member
Well they all have different ratings. Lowest is 0.5Amp at 2000VAC I think. I plan to use this for my Tesla Coil Project. I will limit the input current to each transformer by the use of Triode Circuit installed on its primairy. I will not be exceeding 1000VA load at any point in time.

Thanks
Now do a wattage calculation. :eek:


 
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Frosty_47

New Member

Mikebits

Well-Known Member
One mistake and...



Probably less comical however.
 
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Frosty_47

New Member
One mistake and...



Probably less comical however.
LOLZ. So the point in the example above is not to use cheap extension cord ?

I was wondering if there is a way to check if my MOT's have floating terminals or one terminal GNDed ? If they all have floating terminals than I should not have a problem. However, if at least one MOT has a GNDed terminal than I am realy screwed...Or should I say Melted ?
 
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Mikebits

Well-Known Member
No, my intent of above post is that your whole plan is dangerous. I will not supply any suggestions that may lead to your early demise. Then I would have to live with that.
 

Frosty_47

New Member
No, my intent of above post is that your whole plan is dangerous. I will not supply any suggestions that may lead to your early demise. Then I would have to live with that.
Well It's not like I will be connecting them when there is power aplied to the primairy ;)
 

Frosty_47

New Member

I was wondering if there is a way to check if my MOT's have floating terminals or one terminal GNDed ? If they all have floating terminals than I should not have a problem. However, if at least one MOT has a GNDed terminal than I am realy screwed...Or should I say Melted ?

Typicaly I think they should be floating...

Oh and some sites recomend earthing the core. I can attach a bolt to the bottom plate of each Transformer before I submerse the whole damn thing in oil. Though, the purpose of earthing the core is unclear to me...
 
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JimB

Super Moderator
Most Helpful Member
This sounds like a seriously bad idea.
The insulation between the primary and secondary windings of the transformers must be able to withstand the full combined secondary voltage (12kv).
I doubt that the insulation on the transformers is anywhere near good enough, having been designed and made at the lowest possible cost.

JimB
 

Mikebits

Well-Known Member
Well It's not like I will be connecting them when there is power aplied to the primairy ;)
Like I said, I will not be part of assisting you in becoming a chapter in the Darwin Awards book, and I hope others follow my lead.

Good luck, and no offense.
 

Frosty_47

New Member
This sounds like a seriously bad idea.
The insulation between the primary and secondary windings of the transformers must be able to withstand the full combined secondary voltage (12kv).
I doubt that the insulation on the transformers is anywhere near good enough, having been designed and made at the lowest possible cost.

JimB
Thanks for the NFO. I will see if I can find some datasheets on the MOTs. Perhaps overvoltage breakdown will be specified there. If not, than I will probably search for something else like a power pole transformer....

Thanks
 

MikeMl

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Most Helpful Member
If the primary (120V) windings are driven such that they all tie to a single common point, and the secondary windings (HV) are connected in series, you will break down the insulation between the secondary winding and the core laminations of the transformer.

When the transformer was built, it was subjected to what is called a HiPot test, were a high voltage is applied between secondary-primary, secondary-core, primary-core until the insulation breaks down and an arc-over occurs. The spec may be something like 5KV for the secondary to anything else, and 2KV from primary to core.

If you connect the secondaries in-series, the end transformers secondary to core breakdown voltage will be exceed by two or three times what they have ever been tested to. Very likely, you will have a arc-over and a huge hazard.

Even if you cheat and try to mount the cores so that they are electrically floating (instead of being bolted to a metallic chassis), the primaries are still tied to common point, so the arc-over will just move to the primary windings.
 

ronsimpson

Well-Known Member
Most Helpful Member
12kv @ 5kw??? How many times do you want to kill your self?
If you only want to kill your self 10 times over use a neon sign transformer.
I have seem them at:
6.5kv @ 30mA
7.5kv @ 30mA
9kv @ 30mA
15kv @ 30mA
10kv @ 60mA
12kv @ 30mA
 
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