winding a transformer

[ljdavek]

Hi all
i finaly got the laminations all 120 odd off!! have started to unwind the bobin there was a small winding to sart with then a much bigger one it had 36 turns then a tap then another 36 turns and now some plastic insulation with what seems to be more windings! one of them has a center tap agen and the other don't and theres seems to be one more with much thiner wire with some white sleeves on it but cant get a reading from my multimeter off them!

anyone got an idea as to how meny turns il need for 250v on secondary?

Dave.

 

[RCinFLA]

Maybe you can find an old 'valve' (vacuum tubes for yanks) color TV to scronge one from.

Been there done that, in 300-500 watt range usually about 1.5 turns per voltage, maybe a little less for 50 Hz as the core has to be larger then 60 Hz.

You will lose effective permeability on the core because it is unlikely you will get the laminates back together again as tightly as original new core.

Thought you would get a kick from the attached homemade 'Caribbean' arc welder transformer.

 

[Hero999]

Do you know the voltage of the windings you've just unwound?

If not, wind 10 turns, connect the primary, measure the voltage and divide by 10 for the volt/turn ratio.

 

[ljdavek]

Maybe you can find an old 'valve' (vacuum tubes for yanks) color TV to scronge one from.

Been there done that, in 300-500 watt range usually about 1.5 turns per voltage, maybe a little less for 50 Hz as the core has to be larger then 60 Hz.

You will lose effective permeability on the core because it is unlikely you will get the laminates back together again as tightly as original new core.

Thought you would get a kick from the attached homemade 'Caribbean' arc welder transformer.

love that transformer!!

 

[ljdavek]

Maybe you can find an old 'valve' (vacuum tubes for yanks) color TV to scronge one from.

Been there done that, in 300-500 watt range usually about 1.5 turns per voltage, maybe a little less for 50 Hz as the core has to be larger then 60 Hz.

You will lose effective permeability on the core because it is unlikely you will get the laminates back together again as tightly as original new core.

Thought you would get a kick from the attached homemade 'Caribbean' arc welder transformer.

so i did a bit of maths lol as my house get a nice 230v in id need 345 turns ish for the new prim
and 376 for the secondary for 270v ish.
dose that sound about right?

Dave.

 

[Hero999]

THere was a whole thread about it a few years ago.
https://www.electro-tech-online.com/threads/arc-welding-safety-first.31825/

 

[RCinFLA]

Did you not start with a 230vac primary transformer? Anyway, too late now. Usually you measure secondary voltage first so you can count the turns you remove and find out turns per volt. Also normally you try to save the primary so it doesn't have to be rewound.

The 1.5 turn per volt is a good enough guess assuming you are starting with transformer that was capable of the wattage you claim. The larger the core area the more VA handling capability and the lower the turns per volt.

Approximate design equations for 50 Hz power transformer using laminated iron transformer E-core:

primary turns = 45 * primary voltage / core area

secondary turns = 48 * secondary voltage / core area (48 instead of 45 to make up for some losses)

core area = 1.1 * sqrt ( P )

Where:
core area = cross square ares of the core going through the coil in square centimeters
primary voltage = AC voltage fed to prmary in volts
secondary voltage = AC voltage wanted on secondary in volts
P = transformer power

These equations are going to give you greater then 2 turns per volt but not super critical as long as you are redoing primary. You will likely need the slightly lower turns per volt to get the number of turns in available winding area.

The wire gauge to handle the current and the number of required turns must fit the available area.

I used to use paper grocery bags with shellac painted on each paper layer between winding layers.