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Need more power ! (transformer questions)

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NicktheGeek

Thu Apr 17 2008, 10:46PM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Hi guys,

I'm craving more power for the largest of my coils and have several power supply questions..

Does anyone have or know where I could get hold of a suitable 3Kva transformer (to power the coil) ?
Is it possible to parallel an NST and several OBITS together, given they are all 10Kv ?
What other options do I have - given that the coil uses a vacuum gap and I live in the UK ?
Does anyone have any experience of and should I try 4 MOT's in series ?

Any advice or pointers (or kit for sale) would be greatly appreciated.

Thanks,

Nick

J. Aaron Holmes

Fri Apr 18 2008, 12:25AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Absolutely you can parallel NSTs with OBITs, provided they're the same voltage as you say. Generally speaking, I hate MOTs (for SGTCs, anyway). Coiling with higher voltages makes for nicer SGTC operation all around, and MOTs require precarious seriesing and/or voltage multiplication to get you there, among other issues (like maybe ballasting, which NSTs and OBITs don't require). You do have to be a bit more careful with NSTs and OBITs as they have fragile secondaries, but that's not a big deal. A "Terry filter" is easily built with cheap parts.

Richard Hull paralleled nearly 4kVA's worth of NSTs one time and made a fantastic coil.

The nicest thing, of course, would be a small PT. I've never had any trouble buying them straight from the electric company's salvage yard where I am, but I've got no clue where you'd pick one up in the UK. Sorry!

Cheers,
Aaron, N7OE

NicktheGeek

Sat Apr 19 2008, 06:19PM

Registered Member #1335 Joined: Tue Feb 19 2008, 09:14PM
Location: Woking, Surrey, UK
Posts: 6

Brilliant, thanks, i'll give it a go...

MOT_man

Sun Apr 20 2008, 02:17AM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

I have 3 NSTs in parallel.

I am now running twin NSTs off 208 V -- wired in series - so my secondary voltage is going to be significantly lower. However one of my NSTs is running 114V off a split feed--- the voltage difference between NSTs is 2000 V - so I am a little worried about how these guys are going to work when running. I'm wondering if the NST outputs will average out....
What do you think Aaron ? They don't seem to be fighting each other - but I am getting + 30 mA more current than usual.... guessing that the secondaries must be running at 10 -11 kV rather than 12 kV.

J. Aaron Holmes

Sun Apr 20 2008, 03:20PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

One question: Why???? Are your services too low-current to just stick all the NSTs on the same phase? Can you not pull a dedicated 30+A circuit for your hobby?

Let me see if I even understand your setup:
1) You have 208V Y three-phase service
2) You also have the standard U.S. 120/240V "split-phase" service
3) You have seriesed the *primaries* of two NSTs (paralleled the secondaries--seriesing secondaries of NSTs rarely works) and placed them across two phases of your 208V Y three-phase service.
4) You also have another NST connected to 120V from your split-phase service.
5) You are trying to parallel the NSTs from #3 with the one in #4.

If all this is correct, then I expect it probably won't do what you want. Your split-phase service is probably just one phase from your 208V Y three-phase service. So you should be able to find one phase in your three-phase service that is in phase with it. However, none of the three phases (or your split phase) will be in phase with the *phase-to-phase* (208V) voltage. At best, they'll be 30 degrees out of phase with it. Thus, at best, this effectively means you are paralleling transformers that don't put out the same voltage, and hence you won't get all the volts you expect. At worst, by paralleling NSTs that are subtly out of phase like this, you are "backfeeding" the phase difference and causing distortion on your AC line. That would seem to be "uncool".

EDIT: By the way, if you have three-phase service, you're a lucky bastard! You should be thinking big DC power supplies and resonant charging topologies

Cheers,
Aaron, N7OE

MOT_man

Sun Apr 20 2008, 09:53PM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

No my home has just a simple 240/120V service. The school I work at only has 208V Y- Three phase with some 114/120V outlets. Its but what gets me is the lack of a neutral wire --- I see lots of 120V outlets but they are rated a 15A -- so I thought why not try the 240V outlet - if I wire the 2 transformers in series and then spilt the feed on the single transformer I might just get away with running these transformer without tripping the breakers - I measured the output and it turns out to be running 208V at 15A .... so there I have it.
I just pulled some arcs off my transformer bank and all seems to be running well. I'm pulling close to 2200 VA when I run my transformers... I had to make do with what the shop had. Its all 3 phase stuff.
Wouldn't the secondary outputs be averaged due to the impedance of the transformers - they are all of the same kind/make - except for the one PFC 12kV/60mA unit Aaron?

I WONDER... I'll have to test this on Wednesday.

J. Aaron Holmes

Mon Apr 21 2008, 03:19AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

There will be some amount of "fighting", as you put it, even in NSTs that are a good match. If there is a lot, then you can probably spot it by doing something like:

1) Measure the open-circuit current draw of each NST individually, then add all these up to get current A.
2) Measure the open-circuit current draw of all the NSTs together when connected in parallel to get current B.
3) Subtract A from B to get current C.
4) Multiply C by the line voltage to approximate the VA wasted due to this "fighting".

Note that I've never tried this, but it seems reasonable. Maybe somebody else has a better idea.

EDIT: Additionally, seriesing NSTs, even just on the primary side, seems likely to be bad news. If the transformers aren't absolutely identical, then the 208V may not be evenly divided between them; one transformer may see more voltage, the other less. The one seeing more may see too much. NSTs are not the most robust transformers, so...

Cheers,
Aaron, N7OE

Tinfoil_Cat

Mon Apr 21 2008, 08:55PM

Registered Member #1450 Joined: Sun Apr 20 2008, 09:59PM
Location:
Posts: 10

American grounded-leg Y systems seem very odd, I'm extremely glad for our grounded star-point 400v service.

Also... I've recently acquired access to a 35KVA generator.

Suggestions for a huge coil?

J. Aaron Holmes

Mon Apr 21 2008, 10:37PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Tinfoil_Cat wrote ...

American grounded-leg Y systems seem very odd, I'm extremely glad for our grounded star-point 400v service.

Who said anything about a grounded leg? 208V Y means the same thing as 208V "star". I have heard of 240V delta service where the midpoint of one phase was grounded to derive the standard U.S. 240/120V service. With Y service, the 208V phase-to-phase voltage is sometimes just substituted for 240V, except when 240V motor use is anticipated.

What to do with a big generator would be another topic.

Cheers,
Aaron, N7OE

RogerInOhio

Tue Apr 22 2008, 01:00PM

Registered Member #1034 Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154

You could go to DC resonant charging and use two sets of two MOTs in series with voltage doublers. I have been doing this for quite some time using just two rather large MOTS.

I too have been craving more power and I am now working on a DC resonant charging Tesla coil that will use a 5kva , 7200 volt distribution transformer with a voltage doubler .

Buy the way if this thing is rated at 5kva continuous, dose any one know what the absolute maximum output would be. I have already had it loaded to 7 kva running the old coil and it doesn't even get warm.

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