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High Wattage Current Limiting Resistor for a Pole Distribution Transformer

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RacerX

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I bought a pole distribution transformer for my tesla coil project. It's a 15KVA 14.4kv 300 pound beast! The transformer runs off 240Vac. But it does not have current limiting. It essentially has no resistance across the primary coil. If I tried to plug it in I would essentially short out my house.

Any ideas on a VERY high wattage resistor? I was thinking about a heating element from an electric stove. But it's resistance would increase and the element heated up. I'm not really sure how many ohms it should be but I'm guessing 15-20ohms. Any ideas? Any links on some low cost 5KW resistors?



BTY, I know of the dangers of high current electricity. This is not my first project with high voltage. I've built a few tesla coils in the past.

-Tony
 

RODALCO

Well-Known Member
I use a 4.6 kW heating element from a range in series to limit the current to 20 Ampères.

Not for a tesla transformer but for melting sand and other experiments.
 

RODALCO

Well-Known Member
4.6 kW element

I use this element(s) often on 240 Volts experiments in series with the equipment under test.
Made up from 3 stove elements picked up from the inorganic collection on the side of the road.

Cost 0.00

Look on my YouTube channel under iron sand melting where you can see it in use with a experimental 20 Amp arc furnace.
 

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marcbarker

New Member
I use an electric kettle for these kind of experiments (last one was a blue laser).

You can make tea at the same time :D
 

tcmtech

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If you worked with high voltage before you would know that the inductance of the transformer windings will cause the transformer to draw a very limited amount of power until you shorted out the high voltage secondary.

Circuit breakers will keep it from taking down your house system as well.

Good chance your going to get killed if you dont know this sort of stuff!
See you on the news and possibly in the 2010 Darwin awards!
 

tcmtech

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For a high power resistor just get the heating elements from an electric furnace. They are easy to work with and not hard to find. Plus the can work well in the ten plus KW ranges. Or use the heating elements from an electric hot water heater. Just set them up in a barrel or bucket as the load dump device.

For high powered Tesla coil driver circuits a variable voltage and variable inductance circuit is best. the in line resistance method is fair but not great.

An alternate method is capacitive reactance current regulation. That is you just use a AC power factor capacitor bank in series with the power side of the transformer.
 

Hero999

Banned
I did think about suggesting two aluminium 330µF capacitors connected back-to-back but the current rating won't be anywhere near high enough.

Another problem with the capacitive method is you need to take the inductance of the transformer into account.

Is the centre tap of the transformer connected to the metal case?

You should earth the case anyway but it will probably pop the RCD (GFCI) if the case is connected to the centre tap.

I wouldn't recommend using this huge transformer, like tcmtech says, if you need to ask the question, then you probably don't have enough experience to play with HV.

Get a neon sign transformer which is current limited and far safer.
 

Hero999

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Mr RB

Well-Known Member
Personally i would stick some good triacs on a large heatsink in parallel then drive it with a typical phase angle driver. That would be efficient adjustable and reliable, you would probably be driving it at only a few kW so it's really not that big a load.
 

Hero999

Banned
I don't think the TRIAC are going to last long, when an arc occurs across the secondary of that fat transformer.:D
 
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RacerX

New Member
Thanks for your concerns. I've worked with NSTs before. And therefor have never had to deal with high wattage current limiting. I know that the inductance of the primary would limit it's own current but it would still be too high and flip the breakers.

I'm just trying to power up the coil with a jacobs ladder set up. There are no primary capacitors (not yet).

I'm gonna try the heating element route. I'd like to keep the cost low.

BTY I bought this refurbished coil for 100 bone.

-Tony
 
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tcmtech

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Actually I am going to get a big pole or pad mount transformer just for the Grid Tie Inverter development and testing. I Know someone that has a few I just Have not found something he thinks he really wants so we can trade.;)

SO when you Jacobs ladder this thing post some pictures! :)

The phase angle control concept does actually work quite well even with big transformers though. Just use a pair of industrial SCR's in a reverse parallel connection with a small random turn on AC solid state relay as the gate control device. With the right setup you can get either current or voltage control with simple phase angle switching. Most welders use that for the output power control.

I have scrapped some massive voltage correction units that did in fact use a simple phase angle control system to keep the output voltage stable and they were around 250 KVA if I recall properly so 15 KVA is no problem.

If you need some big SCR's that can handle 240 VAC (or more) at a few hundred amps let me know. I have a whole bucket of them! $20 each. I even have the heat sinks that they mount to as well.
 

tcmtech

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You just need phase angle referenced source to tell the SCR's when to turn on in relation to what the line voltage is at that instant in regards to the sine wave.
Basically all you need is a zero crossing reset of a simple timer that will delay the turn on of the SCR's from front to back time wise.
Turning them on latter in the sine wave gives you a lower voltage turning them on at the front of the sine wave gives you a higher voltage.

A simple UJT oscillator thats set up with an adjustable delay from 0 to 1/120 of a second will work as a timing reference source. Each time the sine wave crosses the zero point it resets the UJT. Then so many milli seconds later the UJT fires and sends power to the Opto coupler that fires the SCR gates.

This is the basic way to be able to use phase angle voltage control to get a variable voltage with a inductive load like a transformer. By varying the one time of each half of the sine wave you get a basic PWM type effect. the inductance of the transformer smoothers out the chopped wave form and then gives a reasonable smooth sine wave on the secondary as a result.

By adding a feed back loop to either vary the output voltage or to limit the output current you can get a stable and constant voltage or a constant current. Or with the right circuits a combination of both. Voltage limiting up to a preset current and then decreasing voltage as the current is limited.
 

MikeMl

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Most Helpful Member
I bought a pole distribution transformer for my tesla coil project. It's a 15KVA 14.4kv 300 pound beast! The transformer runs off 240Vac. But it does not have current limiting. It essentially has no resistance across the primary coil. If I tried to plug it in I would essentially short out my house.
Tony, you obviously do not have a clue what you are doing. What is the goal when putting a resistance in series with the 240V winding? If 240V ac is connected to the 240V winding, then with nothing connected to the HV winding, only the core-loss excitation current ~1/2A) will be drawn. It will not "short out my house"!. If you do not understand how transformers work, you have no business screwing with 15KV.

Is the goal of inserting resistance in the primary circuit it to vary the secondary voltage? The best way is to get a 240V Variac. Putting a series resistance will generate 100s of W of heat and result in very poor regulation of the HV output.

An easy way to get 1/2 of the rated voltage is to feed the 240V side with only 120V.
 
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Mr RB

Well-Known Member
And if you power it up to 240v and it's making a funny noise, you need to listen to those big white things on top... Stick your head right in between them so you can listen to one with each ear.

Sorry, sorry, couldn't resist. :D
 

tcmtech

Banned
Most Helpful Member
Be nice now. Every body knows you lick your fingers and then place each hand on the top terminals and feel for the electron flow! :D

When you see the bright light walk towards it! The answer is in that bright lit room at the end of the tunnel. ;)

It its not a white light but feels hot and humid well... :eek: Maybe you should have petted the neighbors dog and not his wife. :p
 

RacerX

New Member
You just need phase angle referenced source to tell the SCR's when to turn on in relation to what the line voltage is at that instant in regards to the sine wave.
Basically all you need is a zero crossing reset of a simple timer that will delay the turn on of the SCR's from front to back time wise.
Turning them on latter in the sine wave gives you a lower voltage turning them on at the front of the sine wave gives you a higher voltage.

A simple UJT oscillator thats set up with an adjustable delay from 0 to 1/120 of a second will work as a timing reference source. Each time the sine wave crosses the zero point it resets the UJT. Then so many milli seconds later the UJT fires and sends power to the Opto coupler that fires the SCR gates.

This is the basic way to be able to use phase angle voltage control to get a variable voltage with a inductive load like a transformer. By varying the one time of each half of the sine wave you get a basic PWM type effect. the inductance of the transformer smoothers out the chopped wave form and then gives a reasonable smooth sine wave on the secondary as a result.

By adding a feed back loop to either vary the output voltage or to limit the output current you can get a stable and constant voltage or a constant current. Or with the right circuits a combination of both. Voltage limiting up to a preset current and then decreasing voltage as the current is limited.
Ok I'm gonna look into this. Thanks alot!!!

To all you that think this might be over my head... you may be right. But I'm not an idiot. That's why I'm here asking questions. With my previous TCs I've used NSTs with a 120v variac. It was much simpler. However, with this transformer I don't want to build a 12 foot TC so I need a way to adjust the wattage, mmmkay!

Now I'm off to plug it into 120vac and measure the secondary with my Radio Shack voltmeter right next to some tanks of gas, while in my backyard, while it's raining. Be back in a bit.

-Tony
 

tcmtech

Banned
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Now I'm off to plug it into 120vac and measure the secondary with my Radio Shack voltmeter right next to some tanks of gas, while in my backyard, while it's raining. Be back in a bit.
big-explosion-04.jpg

:eek: :D
 
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