Question

I have been working to build a power transformer to develop use a 240v primary and a 800v secondary. The turns ratio appears easy enough, but I am not able to find information that would help me determine the right amount of primary turns. AS an example, I calculate that the primary needs 414 turns and the secondary needs needs (approximately, as I am not in front of my figures)1420 turns to make 800 volts. Not exactly the 1380. My primary is #16 awg and the secondary is #26 awg. Both are 2000c magnet wire. I have a primary current of 3 amps w/o secondary load. What needs to change?

Answer 1

Hi. I've had a look at the posting, and have decided to go with the following "guess".

This seems to be a step-up transformer. That implies to me that the current rating in the primary should be higher than that in the secondary. As the voltage goes up, so the current goes down argument.
3A primary sounds as though that might be a fairly warm transformer. Especially if it’s going into a narrow gauge wire. I don’t know the details of the wire specifications quoted, but from the looks of the figures quoted, 26awg would be better suited to the low voltage primary current side of the transformer. Presumably that would mean that the secondary, which won’t draw 3A because it’s a step up in voltage and so down in current transformer.
Also, the more turns you have in the primary at 3A current, the “even more” resistance you’ve got. So, you’ve got the narrow gauge and the long length.
I’d therefore guess that, accepting the 3A primary current on no secondary load, the gauge and length of the primary are adjusted so that the primary passes 3A but doesn’t get warm.
I’m referring to a UK A level Physics text book (Advanced Level Physics, Nelkon and Parker) as I write this, and I see that there’s this “... perfectly coupled and in which a negligible primary current flows when there is no load in the secondary ...”
http://perendis.webs.com

Answer 2

Thank you for your reply. After reviewing it I realized that my information was partly in error. The 3 amp rating is for the secondary. 240vac input and 800vac output. With this I can rectify with a voltage doubler circuit, and have a DC output of approximately 1 amp @2000vdc.
My concern was if there was another formula to calculate the number of turns on the primary. With the core area, my calculations shows 414 turns for the primary. I have sized the wire to 16awg magnet wire with a 200c rating. You are correct, more turns would only result in higher resistance. Thank you for your help.
Harry