winding a transformer

[ljdavek]

Hi all, i was wondering if anyone could help i need a 300v 1a supply for my valve amp im building any idears! with out spending much money!!
im thinking could i rewind an existing transformer? i have one i could use its out of an av processor its quit a big transformer in there, rating of 350w so could it be possible to rewind the secondarys?

 

[MikeMl]

Have you ever tried to take the laminations of a big transformer apart? It is a lot of work. The laminations are usually dipped in a special heat-set varnish, and the finished transformer is baked in an oven to reflow the varnish to prevent the laminations from buzzing. The windings are sometimes wound on the laminated core; sometimes they are wound on a bobbin.

 

[QuietMan]

Instead of taking the lamination apart, how about neatly cutting it on one end to remove the bobbin? You would need to securely mount it back together without bolts in the lamentations, making a shell. This is pretty common practice anyhow. It will reduce the efficiency of the transformer somewhat though.

Dremels are your friend with projects like this, that and safety glasses.

 

[ljdavek]

ok what about rewinding a toroidal transformer? as i have one of them to and it looks to be 900va+ will have to try looking it up, if it is possibele id need some help calculating the number of windings

thanks
Dave.

 

[mramos1]

http://www.geofex.com/Article_Folders/xformer_des/xformer.htm

**broken link removed**

 

[mbarazeen]

if the power requirement by your valve is less than 350W, then you can go ahead.
povide the input and out put of the available transformer to help you further

 

[tcmtech]

If you need around 300 volts DC you could just use a standard issue 240/480:120/240 VAC isolation transformer in reverse. It will give you about 340 VDC with diode rectification or about 300 - 320 volts with tube rectification.

Its just a thought.

 

[BrownOut]

http://www.geofex.com/Article_Folders/xformer_des/xformer.htm

**broken link removed**

Interesting article. But after re-winding, do the transformer's laminations buzz since the varnish bonds were broken?

 

[ljdavek]

If you need around 300 volts DC you could just use a standard issue 240/480:120/240 VAC isolation transformer in reverse. It will give you about 340 VDC with diode rectification or about 300 - 320 volts with tube rectification.

Its just a thought.

ok i just checked up on the valves im going to use el84 and 300v is it maxium and i was going to run them a bit less than max, im going to have 4 valve in push pull per side, so was thinking now around 270v and aiming for 20ish watts output.

thanks
Dave.

 

[gary350]

I have rewound a lot of transformers. OK.....300v x 1a = 300 watts. Make sure the transformer you are taking apart is rated 300 watts. Use needle nose plyers to rip off the insulation material. The primary coil is usually wound on the coil form first so if your planning to use the same primary coil do not remove it from the coil form. Check the voltages on the transformer before you take it apart. Lets suppose it is 120 vac on the primary and 240 vac on the secondary this means turns ratio is 2 to 1. The factory has already done the math for losses so you don't need to worry with that. I am guessing the primary coil will have about 180 turns of wire for this watt rating so the secondary coil will have pretty close to 360 turns maybe 366 turns to allow for 2% core loss so don't worry with that either. Unwind the secondary coil and count the turns. Lets suppose your transformer has 365 turns of wire for 240 volts. 300 volts is 25% more than 240 so you need to multiply 365 turns by 1.25 = 456.25 turns of wire is what you will need for your 300 volt secondary coil. I hope I did not make a math error. Next you need to determine the wire size to get 1 amp. The 240 volt coil will have larger wire than a 300 volt coil at the same watt rating. The trick is more turns makes a physical larger size coil so the wire diameter needs to be smaller so it will fit in the available space on the transformer. You can use cerial box cardboard for insulation. Wrap 1 layer of wire on the secondary coil then paint it with polyurethane varnish and let it dry 24 hours. Next day wrap a layer of cerial box cardboard over the first layer of wire then wrap a second layer over the cardboard varnish it and let is dry 24 hours too. Do this day after day until you get all the wire wound on the transformer. Put on the last layer of cardboard wrap with fiberglass tape. Solder on the lead out wired fold them back and over the surface of the secondary and wrap with more fiberglass tape. You can buy a special fiber glass tape at any motor rewind shop that will allow you to varnish over the tape. The fiberglass tape comes in a roll about the same size of a roll of masking tape 1" wide white color. So this should give you the main idea of how it is done.

There are a few things you can do yourself that a factory will not do. For example I rewound a transformer last summer the secondary needed to be 12 volts 100 amps. I don't have any 100 amp wire but I do have some smaller wire so I wound the transformer with 5 secondary coils each being 20 amps each then I connected all the secondary coils in parallel to get 100 amps at 12 volts.

 

[mbarazeen]

do as gray350 told, this is what i also wanted to explain. since the transformer is small one, you can finish your secondary in one shot, put good varnish and bake it for some hours in an oven. it will be enough to give the insulation and strength to the winding.

 

[RODALCO]

Transformer formula

N1 : N2 = U1 : U2

where

N1 primary winding turns
N2 secondary winding turns
U1 primary voltage
U2 secondary voltage

The relation between voltage and amount of winding turns is the same, twice the amount of turns, twice the voltage

For the current applies the opposite, less windings, more current.

 

[ljdavek]

ok sounds fun so far!! so as my primary is 240v it will be 365 ish turns right?
and as i want the secondary to be 240v ish i need about the same?
with wire thickness of 18awg for the secondary.
or am i completely wong? so a toroidal transformer? or a EI type? wich would be the best type to do?

Thanks
Dave.

 

[mbarazeen]

you can modify any of the transofrmer which is capable to handle the power transfer you need.

you have to count the original number of turns on secondary to rewind it to 300V. ( new Ts= old Ts/old Vs x 300 )

select the wire that can give you optimum current required, it will help to accomodate the windings in the existing space.
doesnt matter which type of core you use.

 

[ljdavek]

Any one got any idears on how to remove the laminatons of a transformer so i can reuse them? iv read on the net about some solvent but dont know what!

Thanks
Dave.

 

[berty_a]

Use a sharp cutter positioned at one edge then push and remove the I then E lamination and so on.

 

[shortbus=]

Any one got any idears on how to remove the laminatons of a transformer so i can reuse them? iv read on the net about some solvent but dont know what!

Thanks
Dave.

Remove the wire first, then put core in a old(thrift store/yard sale) toaster oven. The heat will break down the varnish.

Use a stiff putty knife to knock apart lamination. Don't pry between them or they will bend. The laminations are hard to get flat enough again if they get bent.

 

[tcmtech]

ok i just checked up on the valves im going to use el84 and 300v is it maxium and i was going to run them a bit less than max, im going to have 4 valve in push pull per side, so was thinking now around 270v and aiming for 20ish watts output.

If the home built transformer rewind doesn't workout you could use a 120/240:208 step up/down/isolation transformer instead. Using a full wave bridge you would get around 294 volts peak and likely around 270 - 280 volts at load. That transformer voltage configuration is a very common one on commercial and industrial equipment so a second hand or new old stock unit is not hard to find.

I have tried the rebuilding transformers before and the only ones I have ever got to work right without developing considerable hum issues are the E & l core types and the U cores that dont have the interwoven laminations.

A good source of the split E & l core transformers is the arc lamp ballasts. Like what powers the common mercury vapor, metal halide, and high pressure sodium commercial and street lighting. They are the next best thing to the microwave oven transformer for rewind work!
Most are simple to disassemble, dirt cheap and most often made with the primary and secondary windings separate from each other. Plus they are very common in universal voltage primary winding configurations as well. 120/208/240/277 at 50/60 Hz all on one winding.

Its just a thought.

 

[ljdavek]

I have seen that you can soak the laminations in turps and meths!! has anyone done this successfully? as im trying is at the moment not much happing yet!

thanks
Dave.

 

[gary350]

I was never able to save any of that so I just ripped it away with plyers. Most os the time when you unwind the wire it brakes off the enamel coating and the wire can not be reused either.