Transformer ?

[potter]

I found this welder at the dump today.I have been wanting a 120 to 240 step up transformer and was wondering if I could re-configure one of these to work ?  . Any help Most appreciated.

         Potter

[LowGear]

Hi Potter,

To be honest I don't have any idea but you need to bring your resolution 640x480 or so if want us to see what you found.  Or is there a simple way to do it from our end?

Casey

[TimSR2]

Well, if you have a big enough core to handle the wattage, you can rewind a transformer  to do anything you want. I suspect it was made with a 8 to 1 or 10 to 1 turns ratio, and the primary is almost certainly on the inside.  Buzzboxes are usually set up to give about 24volts (loaded) , and are driven by 230v mains.  So you are trying to turn an 8 or 10 to 1 multiple tap transformer into a 1 to 2 transformer. You will need to unwind the secondary, and rewind it with lighter magnet wire to get your new turns ratio.  Ham radio guys in the 1950's used to do this all the time to make their own plate transformers for tube  rf amps.

You just have to figure out the math and wind it carefully. Lacquer every pass, and tape it down really tight with fiberglass tape.  It's probably best to run it backwards and use the original primary as the output.  

 

[potter]

Thanks for the info TimSR2 I am Only looking for about 3kw these cores wound must weigh 150 lbs each . I was hoping I could reconfigure with the taps I Guss not so lucky. Will look to see how hard to remove the secondary thanks

    Potter

[TimSR2]

use a small 12 volt doorbell transformer for your power supply during experimentation. (  That way you can't kill yourself.)

[potter]

Thanks good advice  no seriously I am very careful when around high voltage or Lowe voltage high amps.

    Potter

[Ronmar]

If that dosn't work out for you, the 2:1 transformer is a pretty common item to allow people to run US appliances in europ and vis-versa...  They are not all that expensive.  I got one to allow me to spread my 1300W microwave load out across both 120V legs as it is a pretty significant hit on just one leg of a 3KW genset:)

http://www.nextag.com/Simran-AC-3000-Step-838459597/prices-html

[BruceM]

There are plenty of the 120-240 step up transformers on ebay, too, perhaps you can find a good deal on price+shipping to your location there.  Typically these are auto transformers; they do not provide isolation, there is only one winding.  You could also potentially do this with your welding transformer; if the primary is 240V with a center tap, you'd be in luck.

[potter]

BruceM are you saying If I have a 240v transformer with a center tap on the primary and put 120 leg to center tap that I will get 240 leg to leg???

   Thanks Potter

[rcavictim]

BruceM are you saying If I have a 240v transformer with a center tap on the primary and put 120 leg to center tap that I will get 240 leg to leg???

   Thanks Potter

Yes.  I'm saying it.  That is knwn as an autotransformer, the use of one tapped winding to give voltage step up or step down.  It doesn't give you galvanic isolation however, and sometimes you need that too.  Other windings if present do nothing if they are not shorted or connected to anything.

[BruceM]

Yes, as RCAvictim says.  You can keep the maximum voltage relative to ground lower by connecting your 120V neutral leg (I'm assuming one leg of your generator output is grounded) to the center tap.  A bit better shock safety that way, just as your typical US home power panel provides.

[mobile_bob]

+1 for finding a already made transformer to do the job

the problem with most welder transformers for you application are as follows

a. they are made without much concern or efficiency, built to be very rugged or sure at the expense of efficiency
b. they by design are often ferro-resonate or prone to early saturation, basically as a protection device
c. they often use welded construction techniques to strengthen the core, making it hard to take apart for rewind, and
making or more hysteresis losses (core losses due to shorted lamination's at the welds.
d. the core stacks are generally too small for a 60 hz stepup or step down transformer where one would need decent regulation
bear in mind that a welder has horrible self regulation, often times open circuit voltage is around 70volts and drops to around 30volts
once the arc is established.  i would expect to have to derate heavily a core to get acceptable regulation, perhaps a 10kva core used in a welder
might be ok derated to 2-3 kva used as a step up or stepdown and provide decent regulation.

for the effort involved not sure the end result would be worthwhile for something you would want to use on a continuous basis.

now for intermittent duty, such as providing for a pump or something, perhaps a good case could be made for rewinding a welder transformer.

bob g

[potter]

hoping I can just reuse the taps now to figure them out.

[TimSR2]

well, at worst you will get 2 bucks a pound for the windings material, without even cleaning it up .

[Chris]

how about these?
http://www.world-import.com/transformers.htm
Prices look almost too good to be true.
Chris