Burnt primary on transformer

[Electro Noob]

I dissasembled a burnt transformer that was taken out of car battery charger recently to find out that the wire on the primary side burnt in half. Although wires are much thinner on primary i dont get it why would it burn out on primary instead on secondary where the load is. Isn't the wattage on primary always the same regardless of the load? Also the side i suppose is secondary(it is probably step down transformer) is center tapped for some reason. For what purpose such a tap could be useful? Thanks for your helpful suggestions in advance.

 

[Nigel Goodwin]

No, your understanding is completely wrong, primary wattage varies with secondary loading, as it has to.

Secondary tap is probably for a 6V charger?, or a switchable charging level (high/low).

 

[Electro Noob]

So i assume then magnetic flux flows similar to electricity (from higher density to lower) and load lowers secondary magnetism and primary draws more current to equalise it? Correct me if im wrong im just trying to understand how it all works.

You are probably right about the center tap on secondary.

 

[Nigel Goodwin]

So i assume then magnetic flux flows similar to electricity (from higher density to lower) and load lowers secondary magnetism and primary draws more current to equalise it? Correct me if im wrong im just trying to understand how it all works.

You're overthinking it, there's no need to even mention flux or anything else.

A transformer is the most nearly perfect machine man has ever invented (large transformers are exceptionally efficient), so basically power out equals power in, less the small amount of losses. That's all you really need to know about transformers

If you take no load from a transformer, then it will still take some primary power, but it's relatively small. The worst type of transformers are small ones, these are less efficient than bigger ones, but still pretty good in the scheme of things.

 

[Electro Noob]

Oh great i dont have to think about magnetic fields to understand it they always confused me I was thinking if i reconect burnt connection, apply new varnish on wires and rewind it maybe i can make it work again.Would it need new wire thats all from one piece to work?

 

[Nigel Goodwin]

If the primary has burnt out you must replace the entire winding, if it's got that hot it will have ruined the insulation elsewhere in the wire.

 

[KeepItSimpleStupid]

The ideal transformer equation is basically Vp*Ip=Vs*Is; V*I is power; So, power in the secondary = power in the primary. On a power transformer, there are losses in the form of heat, so primary power is a little higher.

 

[Electro Noob]

Is it possible to reuse the burnt wire if i rewind it and dip whole transformer in automotive varnish(it is disconnected at one place but i can solder it)?

 

[Nigel Goodwin]

Is it possible to reuse the burnt wire if i rewind it and dip whole transformer in automotive varnish(it is disconnected at one place but i can solder it)?

No, you would have re-varnish the wire individually before winding, and probably with something better than automotive varnish?.

If you wish to rewind the transformer, then buy the correct wire to do it with.

 

[KeepItSimpleStupid]

The wire used in a transformer may look like bare copper, but it is not. One insulation that is used is Formvar. You generally have to scrape the wire before soldering to expose the bare copper.

 

[Electro Noob]

So it seems theres no way to repair wire so i would have to buy new one.The only thing that confuses me is that secondary consists of three separate wires soldered together.Is there a way i can turn it in untapped transformer by rewinding primary with new wire with same number of turns and then wind new secondary coil(i guess i would need to do some calculation so it wont heat up).I guess number of turns in secondary also has impact on heat dissipation of transformer?

 

[KeepItSimpleStupid]

There is an insulation that can be applied: https://www.mgchemicals.com/products/insulating-coatings/red-insulating-varnish-4228

There is a technique called bifilar or tri-filar where multiple wires are wound at the same time. This makes it easy to locate the center tap.

 

[rjenkinsgb]

Both ready-made transformers and enamelled copper wire (sometimes called transformer wire) are readily available from suppliers.
The insulation is now generally a plastic film rather than enamel lacquer but the name has stuck..

A new mass-produced transformer is probably cheaper than the wire needed to rewind the old one - and it's definitely safer and more likely to be reliable, as you cannot do the vacuum impregnation etc. used to seal the windings on commercially made ones.
eg.
https://cpc.farnell.com/c/electroni...ansformers/isolation-transformers/prl/results

https://cpc.farnell.com/search?st=enamelled wire


For info, the turns ratio between primary and secondary sets the voltages; the "turns per volt".

eg. a moderate size power transformer could use ten turns per volt; a 230V primary would have 2300 turns and a 20V secondary 200 turns.
It has to be adjusted to allow for losses due to resistance causing voltage to drop under load, but that's the basic principle.

The wire thicknesses also have to be appropriate for the currents involved.

 

[Electro Noob]

I got new wire and rewound burnt primary with same number of turns it had, 605. I wound it counter clockwise should i rewind secondary in same direction or opposite(does it even matter which way i wind)?

 

[Nigel Goodwin]

As far as I'm aware it shouldn't matter, but I can't say I've ever thought about it?.

Personally I'd wind it the same way, simply because you wound the first that way as it was 'easier' or more 'natural'.