microwave oven tansformer

[dheeru kiran]

I have a microwave oven transformer of 120v rating and my home mains is 220v I'm planning on making it a high current stepdown transformer for spot welding by replacing the secondary . I want it to work on 220v input , can I do it by replacing the primary focus with more no of turns , if possible how much turns of primary do i need and what gauge , pls help!!......

Sggest me the equations with which I can find these

Thank you

 

[jpanhalt]

Basically, you have a steel transformer core, and you want to wind both the primary and secondary, but have limited experience with transformers. I suggest you get a transformer designed for your mains supply and just focus on the secondary. MOT spot welders are not hard to make. I used 4 awg welding cable for my secondary, because it is much more flexible than standard household wire of the same gauge.

John

 

[spec]

Hi dk,

The basic theory of what you want to do is this:

(1) first establish the TPV (turns per volt) (magnetizing force) for the particular transformer that you have. A large transformer would be around 1 turn per volt and a small transformer as much as 20 turns per volt. So take an example, the primary on your transformer. It may have 180 turns and you know that the input voltage is 110V so the TPV= 180/110= 1.5 TPV.
(2) but you want a primary of 220V, so the number of turns on the primary must be V * TPV= 220 * 1.5 = 330 turns.
(3) for a spot welder you would probably want an open circuit voltage of around 5V ( I am guessing here). So the secondary would need to have: Voltage secondary * TPV = 5 + 1.5 = 7.5 turns (don't worry about the odd half turn here or there when you are winding the transformer)

So now the number of primary and secondary turns have been defined, but what about the wire thickness? That is simple: calculate the weight of the copper in the original primary and use wire that gives the same total weight. In this case the new wire would have to be around half the cross section area of the original primary. Put the same weight of wire on the secondary as the original secondary or the same weight as the primary.

The only critical thing is the TPV: you must never put on fewer turns per volt on the primary than the original transformer was designed for. This is because of the transformer core characteristics. But you can put as many extra turns on as you like. You can make the wire thinner or thicker it is up to you. But by doing this you will change how the voltage on the secondary sags under load. In general the thicker the wire the better, but there is a physical size limit to what weight of copper you can wind on to a particular transformer bobbin.

I think that answers your questions but please listen to John- you must not even think about building a spot welder. Apart from the fact that you appear to have little experience, a welding transformer is a special type and has to produce 50 Amps and more into a short circuit. An ordinary transformer will not do without special techniques.

YOU MUST NOT TRY TO MAKE A WELDING TRANSFORMER IT IS FOUGHT WITH DANGER.

 

[jpanhalt]

I think that answers your questions but please listen to John- you must not even think about building a spot welder. Apart from the fact that you appear to have little experience, a welding transformer is a special type and has to produce 50 Amps and more into a short circuit. An ordinary transformer will not do.

YOU MUST NOT TRY TO MAKE A WELDING TRANSFORMER IT IS FOUGHT WITH DANGER.

That is not quite what I said, or at least not what I intended to advise. Modified appropriate MOT's are regularly used for making spot welders. I have even made one. My recommendation was based on whether the OP should try to wind the primary too. That is usually connected to mains and presents the usual risks plus some,if it is not done right. If the OP were going from 220 V to 110 V without changing the primary, I would be far less worried. In fact, I ran my 110V MOT spot welder from a Variac. I also included a zero crossing timer to limit the weld time to just several cycles of the supply line frequency (60 Hz).

John

 

[spec]

That is not quite what I said, or at least not what I intended to advise. Modified appropriate MOT's are regularly used for making spot welders. I have even made one. My recommendation was based on whether the OP should try to wind the primary too. That is usually connected to mains and presents the usual risks plus some,if it is not done right. If the OP were going from 220 V to 110 V without changing the primary, I would be far less worried. In fact, I ran my 110V MOT spot welder from a Variac. I also included a zero crossing timer to limit the weld time to just several cycles of the supply line frequency (60 Hz).

John

The point here is not what can be done it is whether the OP should take this task on. You are obviously very experienced, the OP seems not so.

 

[dheeru kiran]

I will take the help of a welder near my home where he builds his own transformers for welding purpose , I had a dought on if it can be done so I enquired Thanks for the help and suggestions I'll be fine

 

[spec]

I will take the help of a welder near my home where he builds his own transformers for welding purpose , I had a dought on if it can be done so I enquired Thanks for the help and suggestions I'll be fine

That's good. I went a bit overboard with the caution bit because I was concerned for your safety. Good luck with your project

 

[Les Jones]

If you can get another IDENTICAL transformer you could connect the primaries in series and the secondaries in parallel. The secondaries must be identical wire, the same number of turns and the length of wire to the point where the parallel connection is made must be the same.

Les.

 

[spec]

If you can get another IDENTICAL transformer you could connect the primaries in series and the secondaries in parallel. The secondaries must be identical wire, the same number of turns and the length of wire to the point where the parallel connection is made must be the same.

Les.

Hi Les,
Good idea. I was trying to workout a similar approach. Do the transformers have to be identical? I was thinking that the secondaries would be better in series.

Another approach would be to get a second 110V transformer and wind a 240V secondary on it. Then connect the 240V secondary to the 240V mains and connect what was the 110V primary to the 110V input of a second transformer. If the transformers were not identical the more powerful transformer should be the one connected to the mains. I think this approach is probably the best and involves about the same amount of transformer winding work as the series approach.

spec

 

[Les Jones]

Hi spec,
As long as the turns ratio of the two transformers is the same then the idea should work. If the turns ratio was different then the voltage across the two primaries would not be the same. Putting the secondaries in series should also work. I have used the method I suggested to get 6 volts at 8 amps using two 12 volt lighting transformers. (Real transformers. It would not work with electronic transformers as the switching timing would not be in phase.) for a hot wire cutter. I think rewinding a microwave transformer with a large number of turns would not be worth trying is the laminations are normaly welded so the bobbin could not be removed. I find it strange that the OP wnts to use a microwave transformer of a different voltage rating to the local supply voltage. In our country you see lots of microwave ovens scrapped.

Les.

 

[spec]

Hi spec,
As long as the turns ratio of the two transformers is the same then the idea should work. If the turns ratio was different then the voltage across the two primaries would not be the same. Putting the secondaries in series should also work. I have used the method I suggested to get 6 volts at 8 amps using two 12 volt lighting transformers. (Real transformers. It would not work with electronic transformers as the switching timing would not be in phase.) for a hot wire cutter. I think rewinding a microwave transformer with a large number of turns would not be worth trying is the laminations are normaly welded so the bobbin could not be removed. I find it strange that the OP wnts to use a microwave transformer of a different voltage rating to the local supply voltage. In our country you see lots of microwave ovens scrapped.

Les.

Ah, I see. Thks for info.