Sciencemadness Discussion Board

Massive transformers

Ramiel - 31-1-2003 at 20:46

This idea here is really more like MadPhysics...

My gR4nD 1dEa!!!!1
primarily the grand idea was to have one multi-transformer, if you will. it works like this:



Here it is adapted for rapid electrolosys.

I am the first to say that this is a pretty dangerous device, that sort of power is enough to zap someone dead many times over i believe. Therefore would also be making a grounding thingie.

Do you think this device will work? will it induce correctly and work essentially like three transformers in series?

On a slightly different topic, does anyone know how to limit current into a circuit? I think i would need to do that so the device didn't coverheat eh?

ps. If it does work and no-one has thought of it before, then it's copy-left. If someone patents it, I will hunt them down, kill them, (eat them?) revoke the patent and make it copy-left.

???? ! ??

Organikum - 3-2-2003 at 09:17

This makes no sense to me sorry. I plain don´t understand what function the middle two windings have. I suppose a kind of stepping up. I also suppose that this cannot work on a common iron-core.

After the negative part has to follow a constructive one:

What you want can easily be achieved by rebuilding the transformer of a microwave. You rip away the output windings and rewind the coil with something usable for the more current coming, say 10 to 15 windings of 1-2cm diameter wire. Wire? Not wire anymore thats more cable. I have a link somewhere as it was made this way for a homemade electro welder, I will dig it up.

Limiting current? Your design doesn´t need this. It will blow every fuse at once. The law of Mr Ohm might be helpful, something like: more resistance less current? Some lightbulbs in line before the transformator might give a easy regulation......

ORG

here it is, the transformer....

Organikum - 3-2-2003 at 09:28

thats the way it is done:
Quote:
Microwave oven transformers are step up transformers. That means that the voltage at the secondary winding is higher than the primary. In microwave ovens, the primary accepts standard house current, 120 volts. The secondary voltage is typically 4000 volts. The secondary winding must be removed and a low voltage winding put in its place. The new secondary winding has a typical open circuit voltage of 10 volts. Under an arc welding load, this voltage will drop to between 2 - 4 volts, and up to 250 amps. You will use #6 single conductor wire for the new secondary winding. Many people ask exactly how many turns I put on this new secondary, and I always say as many as you can fit! If you must know, I got 12 to 15 turns on each transformer.


from this page:
http://www.dansworkshop.com/Homebuilt%20arc%20welder.shtml

Hey this is worth a look - the guy buildt a electro furnace for melting aluminium too!
This should be hot enough to fulfill the most perverted dreams ;-)

mine at least
ORG

lucifer - 3-2-2003 at 15:47

Sorry to say ramiel, but this transformer will not work, the red and blue windings are shorted and there four will also short the black winding, result a blown fuse.
What kind of transformer are we talking of, it looks like a three phase transformer.


Also the turn ratio’s are wrong.
You will get 68V on the output (if red and blue are open)
With only 35 turns on the primary it should really be a very big transformer.
Or it will still blow the fuse

Ramiel - 3-2-2003 at 21:14

It looks like it will not work because of those many annoying laws of physics. But just imagine that there was a way for the wires to induce only those coils parralell to them, and the top two coils were ~3500 and ~1000 coils. That might work, no?

I had a look on hv4 for MO transformers, and that looks like the right way.

I looked at that dan's workshop site while I was searching about for furnace designs, the only problem is that the plans for the furnace cost money. I was thinking of purchasing it anyway (then i could give it to the community i guess).

microwave transformers

Organikum - 10-2-2003 at 01:49

They are for free and rewinding is easy. You can make an connection at winding 10/12/14/16 for example and have various voltages output available this way.

What advantages has the hv4 MO transformer? A link to this or short description would be appreciated - I always love to see better concepts for something.

thanks
ORG

Ramiel - 12-2-2003 at 18:02


"MOTs and MOTs"

"MOT coilers unite"

Oh no! what have i done? i've linked to another forum and stolen posters. noooo

nbk2000 spare my soul!

axehandle - 11-2-2004 at 23:33

I can't think of any reasons for having more than 2 different transformers:

1) An ATX power supply. An old one can give +12V at 9A and +5V at 30A, new ones typically more. And they are dirt cheap, or free.

2) A Neon Sign Transformer of respectable power. I've got one capable of 9kV at 60mA, which comes to 540W of power. This one wasn't cheap though, but not that expensive either. Approx. 200 Euro including VAT, with a 3 year warranty =)

(3?) A bridge rectifier connected directly to the mains in case you REALLY want to electrolyze the hell out of something.

Edit: Actually you'd probably want to reconsider point 3. When electrolyzing using a salt bridge made out of toilet paper impregnated with KNO3(!), I lost patience with the slow rection at 5V, and connected the electrodes to a full-wave rectifier driven directly from the mains (220V AC). The paper boiled and carbonized. Perhaps a little too much power

[Edited on 2004-2-12 by axehandle]:o

[Edited on 2004-2-12 by axehandle]

Organikum - 12-2-2004 at 09:34

to 3.
If you dont use an saltwaterheostat or other ballast between your mains and the bridge rectifier you will probably only blow your fuses.

axehandle - 12-2-2004 at 09:35

Organikum: Nope, only got some toilet paper to almost catch fire. But I won't do it again, I'm a one mistake learner.

rikkitikkitavi - 15-2-2004 at 12:12

In most EU countries cheap (chinese made) welding transformer can be bought for around
50-90 €, they have a low voltage winding of around 20-30V, capaple of sustaining a good 6-800 W continously.

Most of them also have a current limit function by adjusting the iron core, but it will not work as a fuse.

/rickard

jimwig - 15-2-2004 at 16:29

Number 6 copper wire will not hold 250 amps for any lenght of usable time.

The highest rating for that wire is around 80 amps and that's free air single conductor.

250 amps would require around 4/0 Cu or several smaller wire sizes in parallel.

Take a look at the buzz box welder/transformers sometime and tell me you see anything short of large copper secondaries on the transformer. These are rated at 225 amps and that's only for intermittent use. Duty cycle around 15%.

So if you want to do welding or more like use it as a supply for electro stuff you had best check the wire sizing.

Oh somelthing I saw recently in an old Popular Mechanics magazine. When you are winding using silicon iron laminations and they are closed - meaning you cannot wrap the wire around and around - use a shuttle cock like is used on a loom. Wrap your wire onto the shuttle and pass it through over and over. Great idea in those old mags.

JimWig

rikkitikkitavi - 16-2-2004 at 10:04

duty cycle 15 % *225 A => 33 A *25 V = 800VA aprox.

But unfortunately it is a little bit lower in reality , since the actuall output reduces a bit since the duty cycle /power is not linear.

if you can measure the wire size, aprox 3 A/mm2 cross section is ok with no cooling.

Good idea about the shuttle cook. It is a hell to rewind an E-I core.

/rickard

rewinding cores

Organikum - 16-2-2004 at 10:56

If you are to rewind the low voltage side of a transformer like from a MOT it is much easier to use Cu or Al SHEET (with an isolating layer or painting - is understood) to wind the low voltage - high amperage side. The sheet is much easier and closer to fold and you will have almost no noise generated from loose windings (whats nervy like hell). As almost no space is wasted like with round wire you can use more cmxcm effective and go for a higher load this way.

"laminar transformer"?
Not sure on the name.

jimwig - 3-3-2004 at 13:29

rikkitikkitavi

a bit more on duty cycle. what is meant here is that a total of 15% of the time used welding can actually be the rated max current through the unit. TRX.

this is not rms or pulsed - the unit still delivers the selected output current just not a 100 per cent of the time. otherwise it burns up. overheats.

Out of welding 100 minutes only 15 of those minutes can be at full power with a down cycle for cooling else you loose your insulation on the windings. it melts..


So its a time factor not a power factor.

disipating the heat via fan etc would probably improve the duty cycle somewhat.

But now that I think about it if 800va output could be set (that's about 20 amps) or so then maybe a 100% duty cycle could be achieved.

But that would only be enough current to weld tin cans maybe.

Marvin - 3-3-2004 at 18:49

jim, rickard is correct, the time factor derives from the power factor, not the other way around. His math applies to a system where its being used to supply power continuously, and still be within operating tolerences.

or..

Hermes_Trismegistus - 3-3-2004 at 22:15

Quote:

disipating the heat via fan etc would probably improve the duty cycle somewhat.


Submerging the whole shebang in a tub of oil would work even better.

Silicone transformer oil is best, but peanut oit works in a pinch.

The larger the mass of oil the longer the duty cycle. The oil will usually mix itself via convection

as a temporary measure to really increase the heat capacity, pre- freezing the oil or adding frozen stones to the tub will also help.

Oil is cheap, so use lots and a suitabley large and heat resistant container. It the container is metal it will also dissipate heat to the surrounding atmosphere faster, but yer gonna want to ground it, just in case (get it......in case).:D

axehandle - 4-3-2004 at 07:37

<b>or...</b>

Submerge the entire thing in Fluorinert(r), cooled by liquid nitrogen running through pipes in the coolant intermediary. :)

Someone once did that to successfully overclock a Pentium II by a factor of 10...

Edit: <b>Yes</b>, I'm post-whoring right now.... the 250 limit is sooooo close....

[Edited on 2004-3-4 by axehandle]

rikkitikkitavi - 4-3-2004 at 08:50

but you have to cool the oil in some way, otherwise you end up with a hot transformer in warm oil. compare to the cooling flanges/tubes on large power transformers. And actually, a large pot filled with oil doesnt provide much more cooling surface compared to the transformer, and the oils adds another heat transfer resistance, especially when it is not forced circulated.

Ciruclating the oil in a radiator might do it, but then it becomes much more complicated.

Also, it is the inner (primary) winding that will be hottest , regardless of cooling method. Oil will not improve this since it will not contact the primary winding.

/rickard

axehandle - 4-3-2004 at 10:40

That's why I suggested Fluorinert. (Even though I did so mainly in jest.) Its viscosity is much better for penetrating between wires. It's horribly expensive though, but completely electrically inert. Combined with, say, water cooling and some mechanical stirring, it would work like a charm. The world's most efficient and most expensive welding transformer. :)

rikkitikkitavi - 4-3-2004 at 11:48

but the problem is the liquid (regardless of type of liquid) penetrating between the wires. There will be very little circulation regardless of the circulation- or liquid for that matter. Smaller transformers have the winding so closely wound that liquid wont flow through easily, or might even not, if the winding is impregnated.

large transformer has the windings shaped to cooling liquid can easily flow through.
Sometimes the core itself has cooling channels.

iof course fluorinert is a jest, I guess it is over the top for most of us...
whats the current price, 500US$/liter or so?

Big sigh!

Hermes_Trismegistus - 4-3-2004 at 12:16

Quote:
Originally posted by rikkitikkitavi
but you have to cool the oil in some way, otherwise you end up with a hot transformer in warm oil. compare to the cooling flanges/tubes on large power transformers. And actually, a large pot filled with oil doesnt provide much more cooling surface compared to the transformer, and the oils adds another heat transfer resistance, especially when it is not forced circulated.

Also, it is the inner (primary) winding that will be hottest , regardless of cooling method. Oil will not improve this since it will not contact the primary winding.
/rickard


Nooo.....but then again, it's unlikely that anything you do (on the cheap) will turn this ugly little surplus transformer setup into a High-Perfomance, internally cooled and jacketed wonder of modern science.

Will it?

I also didn't say it was perfect, just better than air.

Any liquid is better than air at cooling a substance regardless of the final transfer involved.
(because the molecules are closer together)

Having a hot transformer in warm oil is better than having a glowing transformer in air any day.

If you are driving so much power through a transformer that the heat doesn't even have time to dissipate from the internal windings to outer portions of the transformer. Then you are just putting WAY too much power through it and you should look into modifying your setup.

Circulation through convection is significant.

At five bucks a gallon, a five gallon bucket filled with cold corn oil, will absorb one HELL of a lot of heat energy.

That will lengthen your duty cylce tremendously.

You don't have to cyle it through a radiator to make it worthwhile. Having a ten minute duty cycle and then having to wait half an hour is hugely better than having a 3 minute duty cycle and having to wait forty five minutes.

and you'll be far less likely to push it too far and if you really wanted, you could drop in a copper coil around the edges and run cold water through it for cooling. It would be unlikely to increase your duty cycle significantly, but would decrease the down time greatly.

What I would find easiest though, would be to simply buy some more oil, attatch a strap, and yank the transformer out and plunk it down in the other bucket of oil.
:P

rikkitikkitavi - 5-3-2004 at 10:33

yes , the oil will increase duty cycles if it is allowed to cool in between. I m not killing your idea, I have played with it myself , my point is rather that it is difficult to say that it is going to increase capacity this and that...


I would think though that by using a fan, the simplest cooling is achieved...

/rickard

Experience.

Hermes_Trismegistus - 5-3-2004 at 11:39

Having run large HID transformers in an extremely hot humid enviornment :P for 12-18 hours a day I often ran into overheating problems.

There are situations in which having your power interrupted for even short periods of time while the transformers cooled enough to restart caused serious problems.

This was my solution. It worked. I was able to run them for longer and they cooled faster.

Fans were my initial idea, and they worked up to a point, then the heat they produced and the amount of electricity they consumed became a problem.

using 8 1KW transformers in each room during a heatwave was difficult. I arranged five boxfans in a circle around the square rooms so that each blew air in the same direction.

We jokingly called it (the first one the Cyclone room. As a teenager I liked to chatup the oldtimers at the local spot.

For those of you who've been around the scene and know what I'm talking about, I was in the headshop across the street from marc emery's old bookstore where they were challenging the laws by openly selling clones.

anyway, an oldtimer was nice enough to sit me down and explain the difference between the heat capacity of wet and dry air and the way heat energy is transmitted through gasses and solids verses through fluids.

and the difference between kinetic and radiant heat transfer.

I didn't retain alot of it, but kept enough to find that a clean five gallon steel tar bucket filled with oil was a magic trick for transformer cooling. It also kept the thing from rusting.

Another sweet trick/but more time consuming and temporary, was to lay the transformer on a brick or a cinderblock that had soaked in water for a couple days and then lay another on top. making sure the longest steel surface touched the largest flat cement/clay surface.
(also making sure the brick was wet but not dripping)

and replacing every four hours, that coupled with air cooling was pretty damn effective. (if I was using cinder blocks I found arranging them so the air blew through the holes was better but dried them way faster.

good luck:cool:

[Edited on 5-3-2004 by Hermes_Trismegistus]

rikkitikkitavi - 5-3-2004 at 11:54

ok, i guess it worked fine for you :)

actually wet and dry air has almost the same heat capacity, so using a fan shows no difference...

wet cinderblocks+fan, hmm sounds a lot like evaporative cooling, very efficient..

/rickard

Hermes_Trismegistus - 5-3-2004 at 12:31

Quote:
Originally posted by rikkitikkitavi
actually wet and dry air has almost the same heat capacity, so using a fan shows no difference...
/rickard


You're joking right!:o

Organikum - 5-3-2004 at 13:17

The thing used with HID lamps is no transformer but a so called "ballast".

The best solution si to use an bigger transformer (or for HID lamps an ballast of higher quality) as this in the end is cheaper and less hassle than all this cooling with oil and timberbricks and fans. For the HID ballasts I would say some deeper investigation of this "electricity" phenomen would bring up astonishing solutions, for example that there is no need for a separate ballast for every lamp, as a ballast only limits the amperage and does this for one as for ten lamps, doesnt matter at all......
(you need actually two ballasts, one for starting one lamp after the other and one for running ALL lamps)
hehe.....

rikkitikkitavi - 5-3-2004 at 13:53

no, i am serious , wet and dry air has almost similar heat capacity.

so you wont see any difference when using a fan in dry or humid air for convective cooling.

that is what I meant, it could be interpreted otherwise i realise.

Technically, the convective heat transfer coefficient will be a bit different depending on humiditiy but not much...

/rickard

axehandle - 5-3-2004 at 14:05

Quote:

no, i am serious , wet and dry air has almost similar heat capacity.

Interesting. Do you have the numbers? I'm a bit confused, since sitting in a sauna brings very different experiences based on the humidity of the air.

I'm not taking sides here, I just want the numbers. It's quite interesting.

Sigh!

Hermes_Trismegistus - 5-3-2004 at 15:23

Quote:
Originally posted by Organikum
The thing used with HID lamps is no transformer but a so called "ballast".
(you need actually two ballasts, one for starting one lamp after the other and one for running ALL lamps)
hehe.....


Y'know Orgi, I'd hate to condradict you (cuz yer such a clever chemist)

but...

I called the ballast a transformer because I wanted people unfamiliar with HID's to know what I was talking about, and also because it is a transformer, transforming the high amperage/ low voltage house hold AC current into the lower amperage / higher voltage DC current required by the HID bulb.

The other portion of the HID light is commonly called the "starter".

What it really is, is a giant capacitor whose function is to initiate the arc through in the metal vapour inside bulb(simultaneously heating the metal to the vapour point)

The other important parts that are so small (and usually attatched to the step up transformer) are usually overlooked but are just as important to the function of the system...

they are..:
a full-wave rectifier connected to an AC power line to produce an unregulated pulsating DC output, A power factor correction (PFC) circuit. The PFC circuit includes a first semiconductor electronic switch whose activation is controlled to bring the input current and voltage in time-phase with one another, thereby imparting a high power factor raring to the ballast. The pulsating DC output of the PFC circuit is applied to the storage capacitor circuit (whose function we discussed earlier),a second electronic switch whose activation is controlled to discharge the capacitor so as to cause the capacitor circuit to yield in its output, a regulated DC which is fed to a power control (PC) circuit.

The PC circuit includes a third electronic switch whose activation is controlled to maintain the wattage of power supplied to the HID lamp at the rated wattage of the lamp.

This is a necessary part of the sytem because as the bulb heats up the wattage requirements change, and in the long haul they also change as the bulb ages.
----------------------------------------------------
I am not so well versed in electronics and electrical systems as I might seem in a general sense, but at one point the food on my table depended on the proper functioning of those damn lights and I REALLY didn't feel comfortable letting repairmen into my house!:(

So I learned how to troubleshoot.:D

Jeeez...what are they teaching in school these days!

Hermes_Trismegistus - 5-3-2004 at 15:41

Quote:
Originally posted by rikkitikkitavi
no, i am serious , wet and dry air has almost similar heat capacity.
/rickard


Our air is composed of mostly Oxygen and Nitrogen and water vapour.

The specific heat capacity of Nitrogen is 1042 J/KgK
the specific heat capacity of Oxygen is less, approx 800-900 J/KgK


whereas the specific heat capacity of water vapour is extremely high ~2000 J/KgK

so as you can plainly see, varying the ratio of water to other gases varies the heat capacity quite significantly.:cool:

[Edited on 5-3-2004 by Hermes_Trismegistus]

rikkitikkitavi - 5-3-2004 at 23:20

no , i didnt learn it at school , but at university...

simulation with chemcad shows that air , saturated with water vapour at 20 C has a cp of 1,02kJ/kgK, increasing temperature to 40 C, but still saturated air, cp increases to 1,047 kJ/kgK. a 2 % change...

it will different at higher temperatures , where the air can take higher water content before it saturates, but I was assuming you used a fan to cool it with normal air that surrounds us , and you dont put your transformer in a sauna...

speaking of saunas, the reason for the high temperature is that water is condensing on
all surfaces ,especially a 35 C human body and then gives away its condensation heat
, about 2250 KJ/kg.



/rickard

[Edited on 6-3-2004 by rikkitikkitavi]

Yes...but air saturation levels are NOT constant!

Hermes_Trismegistus - 6-3-2004 at 10:32

Quote:
Originally posted by rikkitikkitavi
simulation with chemcad shows that air , saturated with water vapour at 20 C has a cp of 1,02kJ/kgK, increasing temperature to 40 C, but still saturated air, cp increases to 1,047 kJ/kgK. a 2 % change...
/rickard
[Edited on 6-3-2004 by rikkitikkitavi]


You're figures aren't showing us the effect of Humidity change, Because your figures don't use constant Temperature.

You aren't even effectively showing us the effect of temperature change because you are varying the level of humidity(saturation level varies dramatically with T, warm air absorbs far more water than cold air.)

Your math may be correct, but the basis of your calculations is fundamentally flawed.

So please, input those numbers again with absolutely dry air at 20 deg C and saturated air at 20 deg C (or dry air at 40 deg C and sat air at 40 deg C)

We await your figures.

edit: (courtesy)

[Edited on 6-3-2004 by Hermes_Trismegistus]

Organikum - 6-3-2004 at 10:43

Might you please cool down a little bit?

It might help to clearly define the QUESTION again as it seems to me that here is the main problem.

Actually it is by no way clear to me WHAT exactly you are arguing about and I have the impression it isnt clear to the gladiators in this arena also. Or at least it seems clear to every single gladiator but the definitions dont match.

regards
ORG

rikkitikkitavi - 6-3-2004 at 11:01

I dont like to be called fool, whatever way you put it. So I suggest you tag it down and read what I write carefully. I dont try to owerrun anyone with data, as I believe you would have the capacity to grasp my posts.

The values for cp that I have calculated are for air saturated with water at respective temperature, so the amount of water , measured in kg/kg dry air IS different.

If you dont trust the numbers and believe I am bullshitting you, please look up a air-humidity chart and se the numbers yourself.

Dry air 20 C ,0 kg H2O/kg dry air, cp 1,00 kJ/kg K
saturated air 20 C, 0.015 kg H2O/kg dry air , cp 1,02 kJ/kg K

saturated air 40 C, 0,05 kgH2O/kg dry air, cp = 1,04 kJ/kg K

dry air 40 C , 0 kg H2O/kg dry air , cp = 1,00 kJ/kg K

Now I would like to see you show us why cp should variate so much with a megre 5 % change in composition (0,05 kg H2O/kg dry air) you can assume cp for each component to be constant over such small temperature range.
I use chemcad and similar programs almost daily as tools to design equipment like cooling towers, heat exchangers e t c,To my experience a 80000 Us$ program is quite correct, but if you prefer I can calculatet it by hand to.

Engineering is applied science, actually.

/rickard

[Edited on 6-3-2004 by rikkitikkitavi]

Hermes_Trismegistus - 6-3-2004 at 11:37

Quote:
Originally posted by rikkitikkitavi

Dry air 20 C ,0 kg H2O/kg dry air, cp 1,00 kJ/kg K
saturated air 20 C, 0.015 kg H2O/kg dry air , cp 1,02 kJ/kg K

saturated air 40 C, 0,05 kgH2O/kg dry air, cp = 1,04 kJ/kg K

dry air 40 C , 0 kg H2O/kg dry air , cp = 1,00 kJ/kg K
[Edited on 6-3-2004 by rikkitikkitavi]


I didn't call anyone a fool, but obliterated any reference to the word, to ensure not misunderstandings occur.

(this is, after all, just an academic disagreement)

for clarification and confirmation, are your figures indicating that a Kg of air, at 20 C is saturated with water when it contains 15 grams of water and a kg of air at 40 degrees C is saturated when it contains 50 grams of water?

rikkitikkitavi - 6-3-2004 at 12:06

yes.

/rickard

IgnorantlyIntelligent - 10-3-2004 at 17:03

Does anyone know how one might aquire one of those huge transformers used by the city(the ones on the telephone poles) Does anyone know the ratio or w/e for those? Like 1:30? input:output
I'm not against illegal activities but it would seem very difficult to steal a massive transformer...

[Edited on 11-3-2004 by IgnorantlyIntelligent]

Hermes_Trismegistus - 18-3-2004 at 14:21

OK rikki.....I checked out your figures in my spare time.

You're right, I was wrong. Humidity levels have suprisingly little effect on the heat capacity of air.

rikkitikkitavi - 19-3-2004 at 08:17

never mind, errare humane est
or maybe alea jacta est ?

/rickard

chemoleo - 20-3-2004 at 10:25

I happen to have an extremely big transformer with a 6 kW output, input 220V, output 42 V, 120 A.
I have used this transformer on occasion, the foremost success was to make Calcium carbide.

Anyway, the reason why I couldnt use the transformer much was because it tripped the fuse of the house electricity system, but *not* because of drawing too much power out of it, but rather, once the damn thing is switched off! Even if the transformer is not hooked to something, i.e. it is running but not drawing power!
I guess this is some inductive effect, where, when the magnetic fields collapse, backinduce a current into the system.
It is NOT because the transformer is earthed, and leaking current (simply because it's not earthed).

Now, what's the solution to avoiding this?
I guess one needs a capacitor or two in the right places, but unfortunately I don't know enough about electrical engineering to know what uF /V specifications the capacitor should have, and also as I don't know how to connect the capacitors.

Anyone got some ideas?

Mr. Wizard - 20-3-2004 at 13:09

I think you are right about the field collapsing and causing a spike of voltage into your primary winding. perhaps you could hook up a high wattage load across the primary just before you disconnect and have it absorb the spike? A capacitor might work too, but I don't know how to calculate its size. Instead of cutting off the power to the transformer suddenly, you could add more resistance to the input until you had the current at a safe level, and then disconnect it. You could use old heating elements from electric stoves. How many amps does it draw without a load on the secondary? How many square centimeters of iron in the cross section of the core?