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Registered Member #1159
Joined: Fri Dec 07 2007, 02:10AM
Location: Hudson Valley of NY State
Posts: 84
Hi all, Working on getting my 125 KV @300 Ma X-Ray transformer up and running for Halloween. It's back in the tank under fresh Shell Diala A oil. The federal plugs have been replaced with porcelain transformer bushings.
Right now, it's hooked up to hand held vacuum pump and it's holding 9 PSI of vacuum. Is that enough vacuum to remove air bubbles from the secondary windings? The can seems to be able to withstand 9 PSI of vacuum, but I don't want to overdo it and have a catastrophic failure.
This unit will be fed from a 240 VAC, 50 Amp circuit, through a 1256D Variac and current limited with a Miller Thunderbolt welder. Anything I'm missing here?
Registered Member #1159
Joined: Fri Dec 07 2007, 02:10AM
Location: Hudson Valley of NY State
Posts: 84
Another question: Does it make any difference where in the circuit the variac or welder is placed? 240 VAC source, variac, welder, transformer or 240 VAC source, welder, variac, transformer. If it does make a difference, I would like to know why.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
Hi sparky99, I guess that the welder is being used as a ballast. Is the output of a welder sinusoidal for the transformer? You may be able to use a bank of oil type, AC can type, capacitors in series as balast. I did a fiew calculations and a 125 KV @300mA output is 37500 watts. With 156A input you will only be able to run it at 1/3 power or 100mA out. Do you have any pictures of the beast and its plate?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
sparky99 wrote ... ... 9 PSI of vacuum. Is that enough vacuum to remove air bubbles from the secondary windings? The can seems to be able to withstand 9 PSI of vacuum, but I don't want to overdo it and have a catastrophic failure.
Hi Bob. I'm envious of your heavy duty stuff. You've clearly invested a lot & don't want to cut corners.
I've been preparing to vacuum pot my own XRT, and have done lots of reading on the subject. The factory tank is strong enough to hold a full vacuum safely. (as are pole pig cans, and big rectangular pad-mounted transformer boxes with oil cooling radiators). It's not uncommon for flat tank surfaces to deflect visibly but elastically under vacuum. Apparently, some oil-immersed applications have thin tanks, whose vacuum processing (if any) must be done inside a vacuum oven. I don't see the point, for tanks that hold heavy mains-frequency transformers.
In commercial practice you evacuate the tank to less than 1 torr, without oil, and hold it there for a while (preferably with transformer moderately hot) to extract any moisture absorbed during storage (e.g. in Kraft paper between winding sections). Then introduce the oil. Carl P. told me he has seen oil admitted as a spray into the vacuum, for last minute drying/degassing of the oil. Do you have access to a vacuum pump and micron gauge for refrigeration service?
If you pull a deep vacuum on a tank already full of oil, your bubble expansion ratio at the bottom is limited by the hydrostatic pressure of the oil.
Be sure to show us pictures! Where did you get your porcelain HV bushings? What are you going to do with the original Federal HV connectors?
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Really BIG Jacobs ladder coming together after more than two years......Some questions
