Wiring a Step Down Transformer for Isolation

[ak_47_boy]

I would like to use a Europe to US transformer to make isolated 120vac out of 2 phase (240vac).

Does this seam right?
The two ac legs go to the primary on the trans.
The ground to one side of the secondary.
Leave neutral disconnected.

I am confused about what to do with the neutral and ground.

[KBK]

Considering the lethal amount of power that is being handled, it would be good, first, if you supplied a model number, brand name, etc, of the isolation transformer you are attempting to re-wire.

I would expect that this is the minimum amount of information required here, before any reasonable people at this forum would offer any advice. Even then, it may be more than a bit dangerous for you. Either there is the appearance of a slight language barrier, or we are not quite sure that this is a safe thing for you to be doing at your particular level of knowledge of electronics and electricity.

This is why the minimum of the transformer's wiring schematic, model number, and brand of transformer would be a nessessary minimum in order to respond. We wouldn't like to be associated with any potential (jk!) issue you may currently (jk!) run into.......

Messing with unfused 120/240 AC is serious business, and once again, it can be lethal. Or burn your house down. Or both.

~~~~~~~~~~
ok. I looked at your profile, it seems you are not that innocent in the ways of electrocution.

The problem for most responders is that the 240AC is a Canadian phenomenon, and I don't believe the numbers are the same in the US, per the rating. I think they have 220 2 phase in the us, which is the same as our 240. Or the 2 phase 208 (in the US) is the equivalent wiring as our 240 rated 2 phase. I can't quite remember that aspect. I think the 220 is the equivalent.... Regardless...

Look up Ontario spec on 2 phase, ensure the voltages are exactly 180 degrees, measure with a meter, one probe on ground, one on one leg. Then measure the other leg . You should get 120VAC RMS on each, when tested against ground. Then measure the two AC legs together, you should get the 240VAC RMS. Try each leg of the 240VAC against the neutral as well, should test the same as against ground. (or very close. the neutral to ground should also test to ground with virtually no voltage or none)

At that point, I'd give the tranny a shot, in terms of hook up, but FUSE IT! TWO FUSES... One on each active AC leg. ie, about a 3 amp fuse, max. and use the ground, as usual. The neutral gets tied off. Not used. Or get brave and ignore my advice, your call.

I just checked my clothes dryer socket, which is 240AC RMS two phase. came up as two legs of 120, a neutral and a ground. I measured the 120AC legs against one another and found 240VAC RMS.

Should work fine but the usual warning, especially with 30 amp 240AC wiring....:

This message board bears no responsibility for my advice to you.

As a matter of fact, I am using that exact socket to get a single phase 240 from it, via the two 120's which have 180 degree phase difference. This is for an electric welder wiring AC socket. In this case, an extention or 'jack' point.

[ak_47_boy]

The transformer is a cheap no name Europe to America step down. It weights 60lbs, and has a primary and secondary, it is not a auto.

I have 120vac on both legs. 180 degree phase shift.

Unfortunately safety ground is always connected to neutral in the breaker box. That means if i use the ground as usual isolation will be useless. I suppose i could run another ground line to a spike in the earth.

[KBK]

Originally posted by ak_47_boy
The transformer is a cheap no name Europe to America step down. It weights 60lbs, and has a primary and secondary, it is not a auto.

I have 120vac on both legs. 180 degree phase shift.

Unfortunately safety ground is always connected to neutral in the breaker box. That means if i use the ground as usual isolation will be useless. I suppose i could run another ground line to a spike in the earth.

Ultimately dangerous. The ground (resistance) levels will never be the same, leading to potential across the 'faked' ground.

I am using mine in the exact way stated, with zero issues, with a 50 foot extension to keep the welder in the garage. I'm using 10g 3 wire 600V outdoor flexible cabling.

noise will be inherently reduced via the 180 phased legs and the fact that the 240 is inherently quieter, due to general application of that 240V-this, compared to the noise on 120V.

I'm not using it for audio. For audio, I'm using a 50KVA 450lb isolation transformer wired as a balanced AC unit.

[pmkap]

KBK,

I believe both the States and Canada (2 great countries separated by a common language) have the same SPLIT-PHASE 240vac consumer electricty in addition to the typical 120vac single phase power. 2 phase power would have each leg 90 degrees out of phase. The 3rd wire in that 240vac is safety ground, not neutral (grounded conducter) and power never flows to this conductor unless there is a fault condition. 240vac in North America is balanced power, but unless the component can be wired for it, with both legs fused and switched, it isn't practical or safe to use it for audio.
As you know, unless our ballistic friend's transformer has a dual or center tapped secondary on that step-down transformer, he isn't going to be able to derive balanced/technical power.
Besides, if it is a cheap, no name transformer, it is unlikely that that there is either an shield between primary and secondary, or separate bobbin for primary and secondary, which makes it a poor choice for isolation/noise attenuation.

FWIW,
Paul

[ak_47_boy]

The neutral is the center tap on a pole pig. Neutral is connected to ground in the fuse box. Therefore if the ground from your fusebox is used in your audio setup you no longer have isolation. The only purpose that a ground serves is to provide a 0v reference so your amp chassis does not float. The chassis can never be above or below reference correct?

It does not matter if the 240v primary floats because there will always be 240 volt drop over the primary. So it does not need a center tap.

Also the iron core will soak up hf and spikes. The only thing a split bobbin would do is stop flashover from primary to secondary correct?

[pmkap]

AK,

While the major coupling mechanism of a transformer is electromagnetic, the other is capacitive.....
When primary and secondary is overwound, they are capacitively coupled. (the typical toroidal trans makes a rather good audio transformer and can have a bandwidth extending to 100 kHz)
Two techniques are typically used to limit the capacitive coupling -
1. a grounded shield between primary and secondary which functions as a capacitve shunt to ground.
2.Physical separation between primary and secondary which minimizes that capacitive coupling, split/multiple bobbins.

While transformers lacking these techniques will offer a modicum of noise attenuation, they are simply not, all things being equal, as effective as transformers that use either of the above techniques.

FWIW,
Paul