Re-wire 4x110 V isolation transformer output to provide 2x 220 V? - Page 1
Forum section: electronics / projects

author: Lomax
author_rank: Frequent Contributor
timestamp: November 30, 2016, 12:37:37 am
content: I've got this hefty 110 V "safety" transformer and wonder if it might be possible to (safely) re-wire the transformer outputs to get 2x 220 V instead of the current 4x 110 V? Instinctively I'd say yes, but I'm always extra careful with high voltages, so I figured I'd try the idea on my fellow eevbloggers first... Any reason why this wouldn't work / be safe?

author: sbennett1298
author_rank: Contributor
timestamp: November 30, 2016, 02:25:29 am
content: How safe could it be if you accept an answer from a stranger on the internet?But safety aside.....It sounds like it would work. Are all the 110 secondaries isolated from each other?Do you understand polarity of a secondary? If you wire two secondaries in series they will add or subtract depending on polarity.Without seeing a schematic of the device I am not sure what you are dealing with.-Steve

author: Lomax
author_rank: Frequent Contributor
timestamp: November 30, 2016, 07:05:32 am
content: Quote from: sbennett1298 on November 30, 2016, 02:25:29 amFair point!I promise not to come and haunt you if I get electrocutedQuote from: sbennett1298 on November 30, 2016, 02:25:29 amIt looks to me like there are four completely separate secondaries with centre taps (green wires in the photo). IthinkI understand the "polarity" - if I reverse one of the secondaries in a pair (by connecting red to red) they would work against each other (180 deg out of phase), but not if I connect red to black. Or is it the other way around?

author: Siwastaja
author_rank: Super Contributor
timestamp: November 30, 2016, 09:59:20 am
content: 1) Pick the two wires, measure them unpowered with ohm meter to verify infinite resistance - so that they belong to different windings.2) Connect the wires together.3) Power on and measure the AC volts on the expected two other wires. If 220, you have the polarity right. If near to 0V, power off and swap the polarity of either one winding.Nothing bad should happen with this procedure, as long as you take care of all the precautions you need when handling mains AC voltages.

author: Lomax
author_rank: Frequent Contributor
timestamp: November 30, 2016, 02:02:36 pm
content: Quote from: Siwastaja on November 30, 2016, 09:59:20 amPerfect, that's basically what I was planning to do - I just worried what might happen if I got some winding in reverse, like lots of heat etc... Thanks for confirming this should not be a problem, for a short while at least. I'm pretty sure the transformer layout looks like this:I'm guessing I'd have to do away with the centre taps as well, right? I mean the junction between two secondaries effectively become new centre taps, and the old ones "1/4" & "3/4" taps?Edit: Something like this?

author: Lomax
author_rank: Frequent Contributor
timestamp: November 30, 2016, 02:36:20 pm
content: If that looks a bit weird (it does to me!), see here:http://carroll-meynell.com/cms/page/view/page_id/10/QuoteIn my situation (220 V) I would have thought this arrangement makes more sense:

author: sbennett1298
author_rank: Contributor
timestamp: November 30, 2016, 09:52:53 pm
content: Which secondary wire goes to ground depends on what you are planning to use this for which you have not told us.If you are making an isolation transformer nothing is grounded.-Steve

author: Lomax
author_rank: Frequent Contributor
timestamp: November 30, 2016, 10:05:57 pm
content: Quote from: sbennett1298 on November 30, 2016, 09:52:53 pmOk, full disclosure: I did not actually own this transformer when I initially posted my question - I wanted to find out if it may be suitable for my intended use (more on that below) before shelling out. Also, it was on sale for just £90 delivered, which considering it's a 25 kg 3000 VAbeastis a considerable bargain, and I didn't want to alert others to its availability...This transformer is going into a (steel) boat, to galvanically isolate the on-board electrical system from shore power. I needed something in the region of 2500 VA to cover peak demand, so wiring this up as two 110 V secondaries in series, then paralleled, should do the trick - now that I know this can be safely done...

author: Lomax
author_rank: Frequent Contributor
timestamp: December 01, 2016, 09:50:09 pm
content: Quote from: sbennett1298 on November 30, 2016, 09:52:53 pmActually, if I understand the principle correctly, for this particular use the transformer is there (at least partly) in order to substitute the land-power ground with a new "local" ground, which is normally taken from the neutral end of the secondary. This is then fed to an RCD, and an MCD, before going on to power on-board systems. Did I get something wrong?

author: langwadt
author_rank: Super Contributor
timestamp: December 01, 2016, 10:12:40 pm
content: Quote from: Lomax on November 30, 2016, 10:05:57 pmOn ships I've been on the centertap is "grounded" i.e. connected to the ship, that way a voltmeter between ground and a resistive divider can detectleakage from either conductor which probably means something is getting wet

author: Lomax
author_rank: Frequent Contributor
timestamp: December 01, 2016, 11:30:31 pm
content: Quote from: langwadt on December 01, 2016, 10:12:40 pmCool, that's good to know - I do understand the reason they do this on 110 V "site" transformers; you'd only ever see 55 V between any one conductor and earth, which is a relatively safe voltage, but the schematics I've seen for boat isolation transformers usually take the earth from one end of the secondary. I guess the 110 V you get in a centre tapped 220 V set-up (between one wire and ground) is perfectly capable of killing you, but your argument about detecting faults on either conductor is very good. I might ask on a boat-type forum before I start plugging things in as well...

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2016, 11:47:28 pm
content: Ideally you should use an RCD which will cut off the power if there's any earth leakage.I would think it would be safer to only earth one side, rather than the centre tap, if it's going to be used with equipment designed for a single phase 230V supply, earthed at one side.

author: Lomax
author_rank: Frequent Contributor
timestamp: December 02, 2016, 01:04:33 am
content: My head hurts. Doesn't grounding the centre tap mean there remains a 110 V potential in a device with a blown fuse? The internal fuse should be on the live wire, and neutral is bonded to earth - but then how does that work in European countries where (domestic) plugs are non-polarised?

author: Monkeh
author_rank: Super Contributor
timestamp: December 02, 2016, 02:23:32 am
content: Quote from: Lomax on December 02, 2016, 01:04:33 amFuses are not isolation. Therefore, which conductor is separated in a fault not involving a short to ground is irrelevant. Sufficient protection must be present on both poles upstream to ensure the fault is cleared if the downstream fuse is in the wrong pole.For use in a boat, keep things simple and 'ground' (CPC) one end. Leave the centre taps alone and parallel the two pairs of windings.

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2016, 11:46:56 am
content: Quote from: Lomax on December 02, 2016, 01:04:33 amIn Europe, a fast 16A breaker protects a small number of circuits so is more than adequate.Here in the UK, a 32A breaker is often used for a large number of sockets so a plug top fuse is required in each device.You're right, a potential of 110V is not be ideal after a fuse has blown. It would also mean there is no protection from short circuit on one side of the circuit, unless double pole breakers with current sensing on both poles are used, which is typically not the case on a system designed for use with conductor earthed. One could argue that an RCD should provide protection from either side shorting to earth, but it's not as reliable as having both a circuit breaker as well as an RCD.