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Remove shunts from MOT: Drill out, or punch out?

Marco Polo

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So, the college kids are all moving away for the summer and/or for good, and they're leaving behind tons of stuff, which includes microwaves. I managed to score me an old one, which must be at least 30 years old by the looks of it. Didn't have time to actually take it home, so I stripped it right there on the curb. The MOT is huge, and both coils are copper.

For the time being, I won't be hooking this up to line power. I don't have the setup to do it safely and I won't risk it. I did power it up with a variac and isolating transformer and the step-up ratio is stupid. It's above 18 to 1. 493 volts out for 27 volts in.

This MOT is so old it's made in the USA by Basler Electric. It's no cheapie Galanz unit.

I want to remove the magnetic shunts from it so that secondary current isn't limited by the leakage inductance, but I don't want to damage the windings. I feel like punching them out will do damage. Has anyone ever tried to drill the shunts with a drill press? I'm thinking 1/16" pilot holes at either end of the shunts and then widening that to 5/16" or so, then filing any remaining metal to breach the edges. After which I would pry/lever out anything left. The shunts are surrounded by paper and glued in place, so it should be doable without hitting either coil in the process.

Thoughts? Thanks! :)
 

argon

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Hello Marco

I would not try to remove the shunts, because your MOT will overheat very very quickly without the shunts and most certainly it will blow your fuse. If you want to draw big arcs, you could compensate the leakage inductance by adding two paralleled microwave capacitors (2µf) in series to the MOTs secondary. This will give you big arcs, but you should do that only for short periods of time. If you have 2 MOTs and 4 capacitors, you could take the two MOTs secondaries in series (with grounded middlepoint and antiparalleled primaries) and then take 2 parallel-pairs of caps in series with the windings, see here, but without the third current-limiting MOT:



Best regards
argon
 

Seoul_lasers

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Hello Marco

I would not try to remove the shunts, because your MOT will overheat very very quickly without the shunts and most certainly it will blow your fuse. If you want to draw big arcs, you could compensate the leakage inductance by adding two paralleled microwave capacitors (2µf) in series to the MOTs secondary. This will give you big arcs, but you should do that only for short periods of time. If you have 2 MOTs and 4 capacitors, you could take the two MOTs secondaries in series (with grounded middlepoint and antiparalleled primaries) and then take 2 parallel-pairs of caps in series with the windings, see here, but without the third current-limiting MOT:



Best regards
argon

Actually I'd like to chime in here as I've had many experiences with them.

1# Definitely, MOTs are poorly shunted and will pop your circuit break if you
run them without the capacitive ballast.

2# A large 2.3-2.5Kv MOT can easily deliver 2000-3000mA without a ballast. No second chances here. ("radar range" 2KVA Sanyo XMFRs in particular )

3# knocking the shunts out of the XMFR makes these transformers work cooler without a load and more efficiently. The shunts actually are detrimental to the life of the transformer. however, if the capacitor fails on the secondary , pop goes the main breaker.

4# Can be used in 2 banks for 240Vac dual phase operation. Great care must be taken with ballasting and cooling. A setup using 4 transformers can easily deliver 9-10Kv at 400-650mA.
 
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argon

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The capacitors in the shown schematic are not a ballast, they are for achieving 50Hz/60Hz resonance for producing big arcs.

A setup using 4 transformers can easily deliver 9-10Kv at 400-650mA.
Sooner or later the insulation will break down if you put 4 transformers in series. normally they are earthed on the inner side of the high voltage windings at the core, and if you cut that connection to the core, the windings will float on high potential against earth. Then the insulation will break down and internal arcing against the core will occur. In such a case you can throw the MOT away.

Best regards
argon
 

Seoul_lasers

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The capacitors in the shown schematic are not a ballast, they are for achieving 50Hz/60Hz resonance for producing big arcs.


Sooner or later the insulation will break down if you put 4 transformers in series. normally they are earthed on the inner side of the high voltage windings at the core, and if you cut that connection to the core, the windings will float on high potential against earth. Then the insulation will break down and internal arcing against the core will occur. In such a case you can throw the MOT away.

Best regards
argon
MOT XMFRs use secondary side/ Reactive ballasting (capacitor) along with the shunts which are added as an extra safety. Mots will still draw up their max current with the shunts removed. As you can see by the links 4 pack mot and 6 pack mot supplies are possible and have been used to make Tesla Coils.

I have made many multi-mot supplies. Largest was 6 mots 3 | 3

We run those under shell dialex transformer oil with the least transformers on each leg modified with the high high voltage lead isolated from the XMFR case. We're using the large Sanyo XMFRs here not the puny compact ones. Tiny mots ( more typical nowadays) won't work for this kind of PSU.

http://kb3ewy.com:8000/old/motpsu.htm
3-phase high voltage supply with 6 MOTs
https://www.youtube.com/watch?v=2rhz52K9qkU
 
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DashApple

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In normal use the capacitor is used in a negative voltage douber inside the microwave and performs current limiting

the capacitors in that circuit are for resonance , not ballasting as I have made the same circuit ( I used 4 transformers 2P / 2S ) and then 4 1uF capacitors in 2P/ 2S and I had to use a external inductive ballast to limit the current to 20A

https://www.youtube.com/watch?v=yCSXVY89ra4
 
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argon

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ionlaser555, those arcs are really angry :yh: I like the sound...
 

Marco Polo

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I'm not planning on drawing any arcs, at least not beyond a couple for shits and giggles. I want to cut out the HV secondaries and rewind them for low voltage and high current. I want to remove the shunts to remove the limitation on maximum current that the shunts otherwise impose.

Regarding the MOT's pulling way too many primary amps and burning themselves up, to prevent that I figure I can wire two or three MOT's in series (primaries in series, I mean) which should increase the effective turns per volt since at that point the primaries would only be seeing half (or 1/3, with three in series) of the line voltage.

Unless I'm mistaken the main issue with these things is core saturation / excessive magnetic flux density through the core, which should be mitigable via either reducing the forward emf to the primaries, or else increasing the number of turns per volt. Wiring primaries in series sort-of accomplishes both, unless I'm understanding it wrong.

This is why I'd prefer euro-type MOT's since they're designed for 240v/50Hz, whereas I'm going to be applying 120v/60Hz. But when the college kids are moving away and throwing these things in the dum pster, you complain less and harvest more ;) I have two MOT's now, one old and one new. The difference between the two is striking. The old one is big and heavy, and all copper. The new is smaller, and the primary is aluminum, not copper.

But yeah, as cool as pulling arcs is, I'm not sure I want to risk death to do it. Even if I do pull some arcs, I won't be using a screwdriver to do it. I'll use a meter of PVC pipe with the wire taped to the far end of it. Low-voltage, high-current sounds more like my kind of fun. Although, with primaries wired in series, secondaries should be fine if also wired in series because the voltage produced by each secondary should be reduced in accordance with the effective reduction of voltage in the primary that feeds it, correct?
 
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Seoul_lasers

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I'm not planning on drawing any arcs, at least not beyond a couple for shits and giggles. I want to cut out the HV secondaries and rewind them for low voltage and high current. I want to remove the shunts to remove the limitation on maximum current that the shunts otherwise impose.

Regarding the MOT's pulling way too many primary amps and burning themselves up, to prevent that I figure I can wire two or three MOT's in series (primaries in series, I mean) which should increase the effective turns per volt since at that point the primaries would only be seeing half (or 1/3, with three in series) of the line voltage.

Unless I'm mistaken the main issue with these things is core saturation / excessive magnetic flux density through the core, which should be mitigable via either reducing the forward emf to the primaries, or else increasing the number of turns per volt. Wiring primaries in series sort-of accomplishes both, unless I'm understanding it wrong.

This is why I'd prefer euro-type MOT's since they're designed for 240v/50Hz, whereas I'm going to be applying 120v/60Hz. But when the college kids are moving away and throwing these things in the dum pster, you complain less and harvest more ;) I have two MOT's now, one old and one new. The difference between the two is striking. The old one is big and heavy, and all copper. The new is smaller, and the primary is aluminum, not copper.

But yeah, as cool as pulling arcs is, I'm not sure I want to risk death to do it. Even if I do pull some arcs, I won't be using a screwdriver to do it. I'll use a meter of PVC pipe with the wire taped to the far end of it. Low-voltage, high-current sounds more like my kind of fun. Although, with primaries wired in series, secondaries should be fine if also wired in series because the voltage produced by each secondary should be reduced in accordance with the effective reduction of voltage in the primary that feeds it, correct?
Going back to my original point about knocking out the shunts. This is why I mentioned that I had better performance when they had been removed. The also ran far cooler.
Also the larger RADAR range MOTs are significantly overbuilt and deliver 2.5Kv rather than the standard 1600-1800Vac.

Getting shunts out requires a punch and a hammer to knock them out.
 
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Down with Umbrella

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Hey folks, hate to necro post, I just don't want to start a new thread.
So, I've graduated from NSTs, they were fun.
I built a VTTC using one mot for a filament heater and the other as my main HV source. That coil works splendid(atm)
Question, if you don't know the power of the microwaves how do you ascertain the power of the mot? Measuring resistance of the secondary and the greater ohms the greater number of turns hence more power? The arbitrary value it seems is 2kV.
Is Advance transformer a good American made brand? I'm scanning flea bay not but not sure which ones to buy.

If you're running a duel mot system do they have to be identical?
Let's assume I do punch the shunts out would the transformer operate under oil?

Few questions as I'm planning of upping the power.
 
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Benm

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Measuring the resistance of the secondary probably is not a reliable method: It could use thicker wire for current handling resulting in a lower dc resistance, it could use more turns resulting in a higher dc resistance.

I guess the best way to estimate power is just by size, or size (as in total volume) of the secondary winding if you can determine it.
 

Down with Umbrella

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I agree, measuring secondary resistance was just a thought. I had two mots. One actually had thicker secondary gauge and greater resistance. That mot performed much more effectively then the first.
I can't decipher a difference on what's printed on the mot. Looks like it's a trial n error type thing.
I need to find a dinosaur!
 

Seoul_lasers

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Hey folks, hate to necro post, I just don't want to start a new thread.
So, I've graduated from NSTs, they were fun.
I built a VTTC using one mot for a filament here and the other as my main HV source. That coil works splendid(atm)
Question, if you don't know the power of the microwaves how do you ascertain the power of the mot? Measuring resistance of the secondary and the greater ohms the greater number of turns hence more power? The arbitrary value it seems is 2kV.
Is Advance transformer a good American made brand? I'm scanning flea bay not but not sure which ones to buy.

If you're running a duel mot system do they have to be identical?
Let's assume I do punch the shunts out would the transformer operate under oil?

Few questions as I'm planning of upping the power.
Yes, I would keep the transformers the same. Esp. if you intend on making a
Multi-MOT supply.
The large Sanyo (Japanese) XMFRs are the transformers you want. They're dated around late 1980s to early 1990s. They're generally given a 2KVA rating. (they can esp. the large transformers well exceed this rating)
They are extremely heavy and the windings are made of low .ga copper. They are built to take a direct lightning strike and keep working ( small exaggeration).
 
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Down with Umbrella

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Another way I tested the two transformers was using a DMM rated for 1000volts. Connect to a variac set to 0% I would slow increase power until the meter measured across the secondaries 500VAC. Power down. If The second transformer was more or less without adjusting the variac you could determine at least which had a greater output. Of course this is without being under a load.
I'll keep an eye out. Currently I'm in Panama so I cant rummage through my local junk yard.

This Sanyo is on flea bay.
https://www.ebay.com/itm/131911679290
 

Benm

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You'd really have to test these things under load though.

This is not crazy difficult, as a test load you could put a bunch of tungsten lighbulbs in series (about 5 230V ones or 10 US ones) to get a better idea of power output vs input.

You might want to ramp things up with a variac slowly using this test load as the resistance of -cold- tungsten filament lamps is very low compared to at operating voltage/temperature, which could perhaps trigger circuit breakers and such if powered up suddenly. I doubt the actual transformer would be damaged by this brief peak of current (the magnetron tube is not that friendly a load either).
 

Seoul_lasers

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Another way I tested the two transformers was using a DMM rated for 1000volts. Connect to a variac set to 0% I would slow increase power until the meter measured across the secondaries 500VAC. Power down. If The second transformer was more or less without adjusting the variac you could determine at least which had a greater output. Of course this is without being under a load.
I'll keep an eye out. Currently I'm in Panama so I cant rummage through my local junk yard.

This Sanyo is on flea bay.
https://www.ebay.com/itm/131911679290
Actually the best way to have the transformers matching is by simply having both from the same kind of oven. They MUST be the same size of transformer if you are going to use them together.
The old Sanyo radar range XMFRs are very heavy and massively overbuilt. this is good. It means they can be run above spec (~ +50%) without having much in the way of issues. The smaller XMFRs aren't very durable.

Benm is also quite correct here as well. So I'll second his opinion as well. :beer:
I'd also like to add that the large transformers will start smoking long after your house wiring starts burning.
This is why I explained about the shunts actually not being very helpful in controlling current draw,
but a single (radar range) mot without a secondary cap will draw about 35-40A at 120Vac.
This is also why the 2000VAC rated cap in the secondary is required.


I used 6 of these on a Multi-mot supply for a large jacobs ladder and a TC supply about 11/12 years ago. The output was around ~12 -13Kv with a maximum current of just under 800mA
(limited with a capacitive ballast and a primary inductive ballast each leg of the 2 sets of 3 transformers ). The last 2 transformers on each side of the MOT pack had their HV leads isolated with High density Derlin under shell dialex oil.

:can:




 
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