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guydewdney
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« on: November 02, 2011, 09:12:09 PM » |
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Cheap site 110v transformers always seem to have two outputs - does anyone have one, and a multimeter handy? Could it be re-wired to produce 240v output, isolated? I wonder if the 'live' on one side is 240v (220) off the oposite plug? isolating transformers are quite expensive, and 110v ones are very cheap, and I need loads of them....
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clivejo
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« Reply #1 on: November 02, 2011, 09:30:51 PM » |
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Just one question, WHY?!?
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EccentricAnomaly
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« Reply #2 on: November 02, 2011, 09:41:05 PM » |
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Searching for one on the Screwfix site finds the text: Used for stepping down UK mains voltage to 110V CTE (centre tap earthed) ...
which seems to support your supposition. What puzzles me is that the 3 kVA is £57.10 whereas the 1.5 kVA is £61.26. How does having the more powerful one be cheaper make much sense? |
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guydewdney
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« Reply #3 on: November 02, 2011, 10:04:14 PM » |
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yeah - it might be centre tapped - but is each one centre tapped from differnt points? Thus - to earth - you have:
(-55 0 +55) (+110 +165 +220) where the 165 is actually only 55v from each terminal.
why? 'cos a 3kw 110v transformer is 50 / 60 quid - a 240v isolating transformer is 150+ quid....
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clivejo
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« Reply #4 on: November 02, 2011, 10:09:44 PM » |
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What is your cunning plan?
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johnrae
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« Reply #5 on: November 02, 2011, 10:29:02 PM » |
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110volt portable tool transformers manufactured to european standards must be wound with their secondary centre tap connected to earth to limit the output voltage to a maximum of 55 volts. In other words 55-0-55. By definition this means the secondary 110 volt winding must be isolated from the primary winding since the primary neutral connection is nominally at earth potential (connected to earth at the substation). If the secondary windings were not isolated from the primary winding one of the 0-55 windings would effectively have a short circuit across it.
Where a transformer has multiple 110 outputs, this will be accomplished by either tapping multiple sockets from the same secondary windings (cheap) or have separate secondary windings (expensive). You will find it hard to identify which by using a resistance meter since all secondary windings (if more than one) are connected together via the common earth bond.
Note that no connection is made from any part of the primary winding to the earth terminal. However there MUST be an earth connected from the feeder cable to the transformer earth point.
If a transformer you have which is described as a portable tool transformer and its secondary winding is formed by tapping the primary (an autotransformer) then it is a death trap looking for a victim.
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« Last Edit: November 02, 2011, 10:31:28 PM by johnrae »
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guydewdney
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« Reply #6 on: November 02, 2011, 10:45:30 PM » |
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poooh. Oh well. Anyone got 2 x 3kva cont rated isolation transformers, and 1 x 4.2kva cont rated kicking about?
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Justme
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« Reply #7 on: November 02, 2011, 11:09:26 PM » |
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Check out boat forums. They use IT's as well as GI's to stop corrosion.
Edit to add.
Not as well as but instead off.
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
200w (250w peak) 12v turbine as a tester
6kva genny
6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h @12v
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guydewdney
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« Reply #8 on: November 02, 2011, 11:13:36 PM » |
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knighty
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« Reply #9 on: November 03, 2011, 12:17:43 AM » |
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poooh. Oh well. Anyone got 2 x 3kva cont rated isolation transformers, and 1 x 4.2kva cont rated kicking about?
how quickly do you need them ? i could probably sort some out in a month or so ? always have builders etc.. retiring/giving up and trying to sell me stuff... (work is right opposite a big builders yard) |
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guydewdney
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« Reply #11 on: November 03, 2011, 08:26:00 AM » |
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edit - anyone got any 240v transformers.....  |
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fred bloggs
Jr. Member
 Offline
Posts: 59
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« Reply #12 on: November 03, 2011, 09:13:51 AM » |
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Guy
You could connect up 110v tool transformers of a suitable rating back to back, to give 240v in to 240v out. Just make sure you remove the 110V centre tap earth connections, preferably put them both into a single box then no one will think they are site transformers!!!. Be awaren that cheap/most 110V tool transformers are not continuosly rated and are quite lossy. You would be better to look for normal 240V step down transformers and wire a pair back to back.
Best Regards
Fred
If you want a sketch / diagram just shout
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rogeriko
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« Reply #13 on: November 03, 2011, 01:46:19 PM » |
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Ebay has some large 30 kilo site transformers going for peanuts. Can you open them up? If they are open frame transformers it would be easy to remove the outer 110 volt winding (thicker wire) and rewind with thinner( same as 240v winding) wire. Probably not more than 80 turns ive done it before, dosnt take long. To make it easier you can leave the ends sticking out and connect them later so you dont have such a long piece of wire to pull around the winding. Every turn will increase the voltage by 2 or 3 volts so you can measure it as you go. Its the size of the metal core that denotes the Kva rating so buy the biggest & heaviest.
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guydewdney
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« Reply #14 on: November 03, 2011, 01:49:00 PM » |
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thanks - but I dont have time to do that. Ill just buy them new and high spec - the site ones, as mentioned above, are lossy.
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