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SOOT: Autotransformer

I am aiming to use a centre-tapped trannie to power a Cobra MIG Wire feeder that I asked about before.

The feeder uses 115V and I have 240V. I bought it cheap on the off chance, untried. If it works I am a winner bigtime. Otherwise I will use a few parts and minimize my losses.

I have a 115V -> 240 Volt trannie, which I am going to use in reverse for testing it, because I don't particularly want to cough up for a trannie I have no other use for, if this beastie is a waste of time.

I can get 115V OK. But this thing is a centre-tapped trannie, what I call and auto-trannie, not fully isolated. So in the end I will have to get the real thing if all goes to plan.

To my question. The transformer is in a box. Into the box I feed 240V Active, neutral and earth. I have switched the Active 240V. Out comes

115V AC Active, neutral and earth. Now between 115V active and neutral, no troubles. 115V with switch on. 0V with switch off.

But with the switch either on OR off, I get 240V between either (active or neutral) and (earth). This 240V is not some sort of "float". It will power a 150W light globe to 240 Volt from either leg of the 115V to Earth.

I am puzzled.

It's a pity actually. This thing is capable of around 1000w (10A

115V). It weighs maybe 12-15 lb??

The cct as I see it

switch

240V A---\----][ ][ ][------------------- A ][ 115V | N--------][---------------N 240V? 240V? | Earth ----------------------------- |

Any help, as usual, appreciated. Hope I don't end up feeling silly.

Reply to
Old Nick
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Did you confirm with continuity checker that the two wires you label N above are tied together inside the box? I could see a situation like yours if the 115-volt-side wire labelled N really went to 240. Try reversing the input connections and see what happens.

Reply to
Don Foreman

On Fri, 31 Dec 2004 22:57:42 -0600, Don Foreman vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

OK. I will, but both Active 115V and Neutral 115V are at 240V to Earth, not each other. That's what bothers me.

Reply to
Old Nick

This is a common problem in Europe. The miltary sell a polarity tester in the Exchange to resolve this issue. German plugs are reversable, which creates this problem. When correctly plugged in, the output socket will have

115 on the phase connection, 0 volts on neutral and 0 volts on safety earth. However, when the input is reversed, the output is 240 volts on phase, 115 volts on neutral and 0 volts on earth. In Europe the problem is resolved by simply reversing the input leads, but this will not work in the US. This is because 240 volts references both ends of the distribution xformer, which has a center tap neutral. In Europe, the distribution xformers are end tapped neutral. In useage, if no reference is ever made between neutral and safety earth at the load, there will be no problem, but it is unsafe. You will need to use a conventional E plate, isolated primary and secondary winding xformer. Be advised that many of these autoxformers produce 100 volt or 105 volt output in order to reduce overheating of 60 Hz inductive loads on 50 Hz power. Steve
Reply to
Steve Lusardi

On Fri, 31 Dec 2004 22:57:42 -0600, Don Foreman vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

OK. You seem to be correct. Reversing the input fixed the problem. The wire colouring in the trannie fooled me. I simply followed colours.

I sat and thought, in hindsight, and I can see what is happening with the connections reversed.

Well caught, thanks.

You're here for being insane, not stupid, obviously!

Reply to
Old Nick

On Sat, 1 Jan 2005 11:44:36 +0100, "Steve Lusardi" vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

OK. Thanks. I live in Oz (nee Europe )

As Don Foreman said, and as you have confirmed, reversing it corrected the problem.

What made it more confusing when looking at it, but more sense when I see it in hindsight, is that turning the power switch Off amde the Earth and one of the outputs 240V, while having it one made the other one 240V with Earth.

As I said I am going to get a proper isolation trannie if I get the machiery working.

Reply to
Old Nick

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