# identifying a transformer

Discussion in 'The Projects Forum' started by daddy, Apr 5, 2007.

1. ### daddy Thread Starter New Member

Apr 5, 2007
3
0
I too would like to build a plasma cutter (/stick welder /TIG / everything-else-dream-welding-machine). I figure it's just a high current power supply with a clever torch. You can buy the torches separately (it seems they're the most expensive part of the whole deal).

I have ordered a couple of these 20T0175 transformers to play with:

(http://www.surplussales.com/Transformers/SwitchingXmers.html The

only things I know about them are...

operating temperature 180C, 75A @ 28.9v

Primary
VBUS = 400 (seems like a typical off-line input voltage from 220V AC)
VEFF = 130 (I guess that means that the switched DC into the devices has a low RMS because of the low duty cycle)
Amps=11.5
CM/A = 322 (whatever that means ?)
Winding=15 turns bifialar 20AWG 2034 cir mills 3.89mH

Secondary:
Volts=28.9
Amps=50-75
CM/A=309
Winding=6 turns trifilar 14AWG 9766 cir. mills

I guess it gets hot in operation, so I may not be able to operate it continuously for too long.

I have the transformer on the desk in front of me. It looks like it has a ferrite core.

I think it would be suitable for an off-line Switching power supply design using a Texas Instruments UCD8220 controller.

Presumably if I can get, say 30A at 30 volts then I could combine three together to get 30A at 90V. If the 110v outlet won't do it, then I can always install a 220V outlet.

The problems (there seem to be plenty !) include how to determine what frequency to run this transformer at. How can I identify the type of core material and pick a suitable frequency - can I infer anything from the winding inductances ?

I was thinking that I might build the circuit and run it off of a much lower input voltage, say 6v AC from a transformer and change the frequency of the controller until I hit the right spot, that way I might live long enough to test the circuit, but how will I know when I've hit the best frequency ?

Any tips would be much appreciated. Including whether I can up the output voltage simply by changing the duty cycle of the input. It seems to me that it would be excess current that would kill this transformer rather than excess voltage.

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
I have moved your post so that it constitutes a separate post. It is not good forum etiquette to high-jack another member's post. The one you posted to was an older dormant post anyway.

This will give you a dedicated post for the discussion of your project.

Good Luck,
hgmjr

3. ### daddy Thread Starter New Member

Apr 5, 2007
3
0
Thanks hgmr.

For some reason, looking at this on the forum has given me another thought. Must just be the magic of the forum...

I guess CM/A must mean circular mils per Amp. I gather from http://www.du.edu/~jcalvert/tech/transfor.htm that the CM/A specs for this transformer are quite aggressive / reckless ?