home made isolation transformer

[aneng]

Something's just occurred to me....  those transformers are rectangular, not square.  Looking from the top, it looks as if there could be two separate transformers in the core.  I've never seen one like this before.  What's going on inside then ? - Is it two, or one big bugger that has rectangular windings ?

[Ian.M]

I'd have to see one in-situ to be certain but I suspect its width is *either* simply constrained by the UPS chassis form factor, *or* they sub-optimally  change the VA rating by implementing the required change of core area by adding/removing fixed size laminations rather than having different sizes of lamination that  increase/decrease the 'face' area to keep the center limb squarish.    Of course that increases your copper costs and copper losses as the windings are longer per turn . . . . .

[aneng]

Bit of a necro-bump but.... I finally got around to wiring it up and testing it.  I have connected the mains earth to the transformer laminations and one side of the low-voltage link between the trannies.

I only lost 2 Volts across the whole thing, though I have yet to test it with a load.




I have a couple of questions though....   I am reading significant voltage between the output of the second transformer (TX2) live and mains earth - so I disconnected the earth from both transformer chassis and the low voltage link, leaving just two transformers, back to back with only live and neutral.

With it like this, I'm reading 148V from the live output of TX2 to the chassis of the transformer and 7.8V from the neutral to the chassis.  If I measure the TX2 transformer chassis to mains earth, I get 77V.
Obviously, I get the full 238V across the TX2 output.

The chassis of TX1 is at 106V relative to mains earth.

When I say 'chassis' - I mean the laminations.  I've noticed that the bolts holding the laminations together have insulating washers at either end and I'm curious why this would be so.  If I measure from the insulated bolts to any of the input or output lines, or mains earth, on either transformer, I get roughly 20V.

At first I thought perhaps the transformers were junk and there were internal shorts, but measuring with my meter that does 100M ohms, I'm getting infinite DC resistance between any lead and either the laminations or the bolts.

What's going on here ?

[bdunham7]

You're measuring what is probably perfectly normal capacitive leakage, but without a well defined impedance (your voltmeter).  You need to measure leakage current by connecting resistor across each thing you want to measure and then measure the voltage across that.  A 1K resistor will give you 1mv/uA, but I'd recommend you try a 100K first, then if the voltage is low, go to 1K.  Obviously not across the mains....

[ArthurDent]

"I've noticed that the bolts holding the laminations together have insulating washers at either end and I'm curious why this would be so. "

There only needs to be an insulating washer at one end of each screw and generally a paper tube like a drinking straw the length of the the screws through the laminations. If the screws aren't insulated they will in effect act like a shorted turn, waste power, and the transformer will heat up.

[Alti]

The problem with wiring two back2back is that output voltage == input voltage at 0A load.

Typical 230V/24V 40VA transformer is going to have 28V under no load and 24V under nominal load. Now, if you feed 24V into secondary of the other one, you will get under 200V under nominal load.  You need a third 24V transformer to compensate for that.

I think a more practical idea would have been a set of 10 small 230V/24V transformers wired with secondaries in series.

If you want identical pieces, either 3*40VA transformers or 10*4VA transformers.

Alternatively, you could use 2*400VA transformers which would be idling at 40VA with minimal voltage drop, to get near 230V.

[aneng]

You're measuring what is probably perfectly normal capacitive leakage, but without a well defined impedance (your voltmeter).  You need to measure leakage current by connecting resistor across each thing you want to measure and then measure the voltage across that.  A 1K resistor will give you 1mv/uA, but I'd recommend you try a 100K first, then if the voltage is low, go to 1K.  Obviously not across the mains....

Right....  measured with a 980 ohm resistor connected to mains earth at one end, and the other end connected to a) - gives b) mV across the resistor.

a) Laminations of T1     b) 27.3mV AC
a) Laminations of T2     b) 19.1mV AC
a) Hot output of T2       b) 32.5mV AC
a) Neutral output of T2  b) 17mV AC
a) one side of LV link between T1 & T2    b) 30.8mV AC

So, that means the largest leakage current in the whole thing is 32 uA - which I'm guessing is absolutely fine ?


As an aside, I'm a member of another forum (not electronics related) where I've mentioned that I've built this isolation transformer and one of the guys on there, who I've previously regarded as knowledgeable about electronics, has gone crazy saying I'm going to kill myself if I use this, that it defeats the RCD (I know that !) and that if I were to touch either of the isolated output leads, I would get a potentially fatal shock due to the "capacitance of the human body".  I can't see the logic in his claim, as unless I complete the circuit in some way (impossible if the output isn't earth-referenced and I'm not stupid enough to grab both output leads!), I can't see how any current could flow through me, let alone give me a potentially fatal shock.

He's stated that "the circuit is actually completed by your capacitance to earth (all your surroundings, in fact) and the capacitance of the transformer secondary to earth in series - which you have also made far worse (approximately doubled, I think) by earthing the low-voltage link between the two transformers".

Is he talking out of his *** ?

[bdunham7]

a) Laminations of T1     b) 27.3mV AC
a) Laminations of T2     b) 19.1mV AC
a) Hot output of T2       b) 32.5mV AC
a) Neutral output of T2  b) 17mV AC
a) one side of LV link between T1 & T2    b) 30.8mV AC

So, that means the largest leakage current in the whole thing is 32 uA - which I'm guessing is absolutely fine ?

Yes, those numbers look OK.

He's stated that "the circuit is actually completed by your capacitance to earth (all your surroundings, in fact) and the capacitance of the transformer secondary to earth in series - which you have also made far worse (approximately doubled, I think) by earthing the low-voltage link between the two transformers".

Is he talking out of his *** ?

Well, the numbers matter.  Yes, the circuit is completed by your impedance (of whatever type) to earth and the transformer's capacitance to earth (which you just measured) in series.  Assuming you are standing barefoot in a puddle, if you touch the various areas that you just measured, the corresponding current will to through you.  Below 500uA you'll be OK, although you might feel it.  As for earthing the center section (and T2, for that matter) I don't think it is helpful.  I don't know why there is all the worry about the transformer breaking down and so on.  I suspect you will have the lowest leakage with either both the center section and the case of T2 isolated completely from ground, or the center section grounded and the case of T2 isolated.  It's worth an experiment.

[The Electrician]

With a proper shoulder washer at each end of the bolts, no paper tube is needed:

https://www.tecni.uk/Nylon-Shoulder-Washer-7.9mm-x-6.5mm-172.150.068

[tkamiya]

What are you going to use this for?

As proposed, 6.3V 3A transformer won't give you much of capacity at 120V.  I doubt it would actually be useful.  The purpose of isolation transformer is for safety.  If you are unsure of what and why certain things are important, I would strongly suggest purchasing a small one already made for the purpose.

[Ground_Loop]

I have a pair of 500VA transformers wired back to back that I have been using for years.  Works just fine.