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Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
I thought I'd found the PT to end all PTs when I picked up a 200:1 double-bushing unit about a year ago. Last week, however, friend and fellow Tesla coiler Matt Stiger (of "Makers" fame) tipped me off to the existence of a half dozen 220:1 PTs at the local utility's salvage yard. 26kV! "Holy crap!" I thought, "I've gotta get one!" Grossly underestimating the size, I'll called up the salvage yard and had them set two aside for me. Today, Matt (who also plays the pivotal role of "Friend with a Truck"), fetched them for me and brought them by. My already cluttered garage feels half the size now
As you can see, they're monsters! I guess I'll wire 'em in series for 52kV Unlike most PTs, they're oil-filled. They even have little oil gauges on the tops of the bushings just like big substation transformers! Whee!! Actually, the salvage folks drained 'em, so I have to refill them. Not a big deal.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Electric Genie wrote ...
How much does one of thoes weigh?
Lots! The nameplates say 450lbs, which make 'em wayyy heavier than my 15kVA pig! The garage rafters were groaning as we worked the hoist. I'm glad the roof didn't come down! BTW, these are only 3kVA! From what little I know of PTs, the cores are usually super-sized to improve the voltage transformation ratio linearity. These would seem to be proof of that! Still, most folks seem to run PTs at 3 and 4x their rated power, and if the looks of these mean anything, I suspect they'll take more abuse than the average PTs!
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I'm not convinced you can run those in series without having arcing problems on the low side bushing. If you plan to put 26kv into that bushing you're probably going to have to put a large plastic corona shield around the threaded part and silicone it up good. Otherwise once you start a tracking arc, the current is going to make it grow horrendously and you'll have a huge flaming MOT like arc at 26KV. That would not be very fun.
My little 12kv 1.5kva PT can put out a 14" flaming arc, and yours...who knows, that could be 3 feet of disaster.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Hazmatt_(The Underdog) wrote ...
I'm not convinced you can run those in series without having arcing problems on the low side bushing. If you plan to put 26kv into that bushing you're probably going to have to put a large plastic corona shield around the threaded part and silicone it up good.
Shouldn't be a problem; that small bushing is actually meant to be grounded to the can. These are, for all practical purposes, single-bushing PTs. The hardware necessary to ground the bushing to the can was removed for transport (see pile of hardware in second picture). I just haven't put the grounding bracket back on there.
So for seriesing these, I'd just connect the cans and small bushings and ground them all to the same point, then drive them each 180 degress out-of-phase with respect to the other. A trivial way to do this while retaining use of my 120V variac-based ballast would just be to use the LV side of my pole pig as a 1:2 step-up autotransformer, which would "synthesize" the missing hot lead. That way nothing is left floating either. Seems like it should work great!
Registered Member #593
Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50
Aaron,
Awww, you aren't going to throw up a quick ladder with a single PT at least? I kept scrolling down hoping to see that you'd tried that already. I guess it's smart not to get too eager with those monsters. Those look really great.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Hi, Brett!
Hheheh, well, here's the stupid/boring truth: I ran out of transformer oil! I need to go buy some more Diala. I had two big six-gallon containers on the shelf that I thought were full of the stuff, but when I went and picked them up, I found that one was totally empty and the other not quite full Sigh... I'd emptied one of my pigs during my recent move, then recently sold that one to a local coiling buddy. When I sold it, I refilled it, and forgot to account for the refill in my oil ledger Whoops!
Unfortunately, all the places I know that sell transformer oil are only open during the week, which means taking time off work to buy it. I'll see what can be managed this week. I too can't wait to see one of these lit up!! If another week goes by and I still haven't gotten any oil, I may siphon some out of my other pig, although I haven't yet managed to do that without creating at least a small mess.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Well, it's been most of a month, but I finally got down to the local oil place today and picked up two five-gallon pails of Shell Diala AX. Ouch! $10/gallon!! Oh well, it's the only place I can find that sells it
Notice the happy little oil gauge on the bushing says "Full" now (yes, up and slightly to the left is where the "Full" indicator is)
Arc is ~26kV, ~115mA (3kVA). You don't get a proper sense of scale in this picture, but it still ain't all that spectacular. BUT...it works. I'd push harder, but as I haven't finished unpacking my garage, all I could find was some feeble 14AWG lamp cord, and it was getting pretty darn hot around 30A! Time to start unpacking, I guess! I just couldn't wait to test one of these! The other will have to wait until I can set aside enough geek-budget to buy another pail of Diala.
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New toys: 26kV PTs
Unlike most PTs, they're oil-filled. They even have little oil gauges on the tops of the bushings just like big substation transformers! Whee!! 
Actually, the salvage folks drained 'em, so I have to refill them. Not a big deal.
Whoops!
If another week goes by and I still haven't gotten any oil, I may siphon some out of my other pig, although I haven't yet managed to do that without creating at least a small mess.
