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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm going to coil a 10" wide C-core for 70,000 volts using layers of 33AWG magnet wire on a 3" PVC. It will operate under oil but I'm wondering if oil needs to get into the coilings also to prevent corona. I was going to put several layers of wax paper between each layer (each layer having about 4kv potential between the next), but oil won't be able to easily get into all the coils. Will corona occur if there are large air pockets inside if the air bubbles are still surrounded by an oil environment?
If that will be a problem, how does one go about layering a transformer and getting oil all through it? Is there an insulation material that allows easy saturation of oil? The only way I can think of is building it under oil which I do not want to do.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
You'll probably want to heat the oil and get a vacuum pump and tank to get all those bubbles out. At the voltages you're describing, corona stands to be a major problem. Others have given their transformers long baths in hot wax. If you read the many home-built transformer threads on TCML, here, and elsewhere, you'll quickly realize that this is not a project to be underestimated!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi there Dude! Good to talk to you!
Don't on any account use waxed or impregnated paper, as this will actually block the self healing properties of dielectric oil, and form barriers where gas bubbles could be trapped, leading to corona breakdown.
The classic insulation of old high voltage high power transformers - many of which are still in service - is nothing more than dielectric oil and kraft paper, the kind of unglazed brown paper you wrap parcels in. The kraft paper absorbs the oil so if there should be a dielectric breakdown, oil will immediately pour in to fill the hole created by the spark or corona.
In the last few years, for environmental reasons, there has been a move away from using mineral oil to using vegetable oils, such as combinations of ordinary kitchen sunflower and rapeseed oil (Google: "vegetable dielectric oil") and some forum members here have used vegetable oils to good effect.
If you have the equipment, it is best to suck any trapped air out of the system with a vacuum pump, but if not the best thing to do is place the transformer in the oil, and subject it to repeated cycles of heating and cooling, and then leave it soaking in the oil for a few weeks before using it. Keep a tight lid on the pan at all times, as the fatty esters of vegetable oil are somewhat hygroscopic, and will absorb water from the air. Once this has happened, a constant boiling point mixture of oil and water has been formed, so no amount of furtherheating can drive the water out, and the dielectric oil is compromised.
A particular HV winding technique consists of winding each layer a little shorter than the one beneath it, so that the cross-section of the completed coil is traingular. If the insulation is maintained at full width, you'll see that as the potential rises with each succeeding winding, the distance around the insulating paper around which pre-discharge feelers would have to creep increases prortionality. Moreover, you'll see that the distance between the first layer, and the last layer at the apex of the triangle , is much greater thanin a coil with a rectangular cross-setion.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I actually do have a 50 micron vacuum pump that would work for that. I'll need to pick up an appropriate chamber, perhaps a pressure cooker. So vacuuming it will pull bubbles out of the transformer even deep under 10-15 layers of paper and solid density magnet wire coiling?
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Winding High Voltage Transformers
