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some3

Sat Feb 21 2009, 10:00PM

Registered Member #1887 Joined: Sun Dec 28 2008, 02:19PM
Location: most na soÄi,slo
Posts: 8

Hi this si my first post.

I'm building a HV transformer (230V/10kV 50Hz 4kVA)
how can i limit the output current to about 350 mA (3.5kVA)
this transformer will be used for a tesla coil.

tnx

Proud Mary

Sat Feb 21 2009, 10:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

some3 wrote ...

Hi this si my first post.

I'm building a HV transformer (230V/10kV 50Hz 4kVA)
how can i limit the output current to about 350 mA (3.5kVA)

At 10kV, 350mA, special fuses are required so that an arc does not form in the metal vapour created by the catastrophic failure of the fuse wire. A re-settable overload current trip is another option.

With costly transformers, such as the one you describe, a temperature sensor port might be added to the case during construction, to enable constant monitoring of the oil via an auto cut-out device.

Rather than losing yourself for weeks in great piles of laminations, why don't you just use an inexpensive neon sign transformer to start with, like most of the other Tesla lads?

The old-fashioned (non-electronic) neon sign transformer ("NST" here) has magnetic shunts, which limit the output current to safe values in the event of short circuit.

Dr. Dark Current

Sun Feb 22 2009, 06:43AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Use an inductor on the primary side to set your current. The inductor must be rated at your voltage (230V) and current (15A), and should have variable air-gap to control your current.

some3

Sun Feb 22 2009, 09:53AM

Registered Member #1887 Joined: Sun Dec 28 2008, 02:19PM
Location: most na soÄi,slo
Posts: 8

1.

I'm from Slovenia and here i simply can't finde any NST that are cheap i don't want to give 100â‚¬ for a transformer of 900VA or maiby only 300VA

a fuse is not an option because it brakes the current.

2.
variable air-gap -- you mean like a welding transformer?
If yes can you help me calculate this coil

Now i'm using an measurement transformer it's power is 100VA now it's overloaded 20 times (2kVA)

Dr. Dark Current

Sun Feb 22 2009, 02:22PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

some3 wrote ...

2.
variable air-gap -- you mean like a welding transformer?
If yes can you help me calculate this coil

Now i'm using an measurement transformer it's power is 100VA now it's overloaded 20 times (2kVA)

If you have a variable current welder transformer, it should be perfect for the job.

By variable air gap I meant if you made your own inductor, you could change the gap in the core to get the desired current.

Proud Mary

Sun Feb 22 2009, 06:08PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

By my estimation, a 4KVA transformer, 240V primary/50Hz, flux 1T (silicon steel) will have a cross-sectional core area of about 72 sq. cm - a very major undertaking

Steve Conner

Sun Feb 22 2009, 06:19PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, but only major as in big. It might be harder to move around your garage, but everything else is easier.

The large core has lots of room for wire, and induces more volts per turn than a small one. So, the HV winding can be made with relatively few turns of thick wire: maybe 5,000 turns or whatever. I'd feel happier attempting this than a NST-sized thing with 50,000 turns of hair-thin wire.

Proud Mary

Sun Feb 22 2009, 06:50PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Yeah, but only major as in big. It might be harder to move around your garage, but everything else is easier.

Yes, of course, whether it is 'major' or not - the order of difficulty - depends upon your financial and workshop resources, and metal work skills.

In any case, I'm sorry if I sounded discouraging. There is no reason in principlel why anyone here shouldn't be able to construct such a transformer if they put their mind to it for long enough.

some3

Sun Feb 22 2009, 06:56PM

Registered Member #1887 Joined: Sun Dec 28 2008, 02:19PM
Location: most na soÄi,slo
Posts: 8

ok
my calculation for the HV. transformer:

core area: 64 cm2
primary current: 17,4A
diameter of the primary wire: 2,94mm (3mm)
diameter of the secondary wire: 0,44mm (0,5mm)
turns per volt: 0,58 turns/volt ( 1.2T )
numbers of turns fot the primary: 134,7 (136) turns
numbers of turns for the seconrary: 5800 turns

i think this calculation is correct if not pleas correct it.

Now for the current limitation:
Do i have to make a nother transformer on the welding core or do i heve to meke just a coil on the core.

Proud Mary

Sun Feb 22 2009, 07:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

1.2T sounds like some kind of silicon steel, is that right?

Edit: Using your figure of 1.2T, your 64 sq.com is good only for 3.086KVA, while your specification at the top of the page calls for 4KVA.

By my reckoning you need a core area of 73 sq.cm for 4KVA/230V/50Hz/1.2T

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