240-415 Volt Transformers

Hi
I'm thinking of making a 3 phase converter to using an auto transformer to step up the voltage. The Jim Cox book on Electric Motors in the Home Workshop describes
rejigging an old welding transformer - and then tells you that you can no longer take this approach 'cos transformers are welded now together and not bolted! Does anyone know of a source of smallish transformers to base a converter around - (or alternatively a source of elderly welding transformers!)
Thanks
Charles
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Charles,
When I did this a few years back I got a large 240 to 240 volt isolation transformer that had 120v taps on primary and secondary, wired the primary and secondary in series and fed the 240 in at the two tap points (an idea cribbed from seeing the circuit diagram of a Transwave converter). It worked fine and I hope it still is (Tim Leech please confirm <g>)
Andrew Mawson
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On Fri, 7 May 2004 09:49:04 +0000 (UTC), "Andrew Mawson"

Yes
Tim
Tim Leech Dutton Dry-Dock
Traditional & Modern canal craft repairs
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On Fri, 7 May 2004 09:49:04 +0000 (UTC), "Andrew Mawson"

Andrew
Do you know where you got the transformer or was it a lucky find!
Charles
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primary
That particular one came from Anchor Surplus, but it was HUGE - cannot exactly remember the rating but the convertor was running my Moog Hydropoint NC mill - just the hydraulic power pack was 10 HP iirc
Regards,
Andrew
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On Fri, 7 May 2004 09:49:04 +0000 (UTC), "Andrew Mawson"

That would work - but having got a source of 480V single phase how does one convert it to 3 phase?
I have seen 3 phase "converters" that are no more than applying single phase to one of a motor's windings and then starting the motor by applying the same voltage through a capacitor (to phase shift it a bit) to another of the motor windings. Is that it?
    Steve
---------------------------------------------------------- Steve Randall G8KHW snipped-for-privacy@btinternet.com UKRA #1072 Level 2 http://www.ukra.org.uk EARS http://www.spackington.com /
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On Fri, 7 May 2004 18:33:23 +0000 (UTC), steve randall

Basically yes. Or you could use the 440ish volts to power one leg of a 415v 3 phase in/out inverter. That should produce decent variable frequency 3 phase at about 2/3 of the nominal inverter rating. Inverters like that are cheap.
c
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wrote:

primary
idea
You may have to be a little careful here, with a normal three phase input, you use and have a 3 phase rectifier stack. When you try doing it with just two legs of 415 Volts it may be worthwhile beefing up the bridge rectifier and also upping the Smoothing caps a bit. Or down rate the converter by a third'ish!
You would have less current per diode in true 3 phase compared to single phase for the same load, and also you will get more ripple on the smoothing caps due to only getting a charge every 180 degrees instead of 120 degrees. More input volts may help to a degree.
Also be wary of the more sophisticated Inverters, they may well detect if a phase is missing, or if the input volts are out of the 415 V +/- a bit shut down.
Just a few thoughts there. I have certainly seen it done with mains and a 110V site transformer. The 110 volts is isolated from mains, so as such was connected to the live mains terminal and gave across the neutral and other end of the transformer 350 to 360 volts output. It worked and was enough for an old Jaguar inverter to run, but not a set-up I would like to try at home!
If you build your own converter with a transformer, motor and Caps, please make sure you use volts rated caps at least 480 Volts ac if you are after 415 output levels. Unbalances can lead to some pretty high volts on the output side.
When you have balanced the voltages under working load then it becomes quite good fun sticking caps across the mains input to improve the power factor!
What ever you choice, take great care.
-- Cheers Adrian.
http://www.aphtrading.co.uk/pages/apage1.htm Weekend Workshop http://www.aphtrading.co.uk/pages/apage2.htm Home made propane Foundry http://www.aphtrading.co.uk/pages/apage3.htm Learning CNC on a Vertical Mill
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You may well find that the invertor detects the presence of all three phases and shuts down if it detects either the loss of one or more phases or an imbalance in the phase input voltages. The 415V 3-phase input invertor that I have does this.
Charles Ping wrote:

snip
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steve randall wrote in message news:

building site stransformers... you want one in a metal case. you can get at it and pull some windings out to lower the voltage if you need to. well i got a pair of largish yellow things and they were both potted and i didn't want 480 volts. but the ones i had had remarkably low rating so weren't any good to me anyway..

you use the 415 volts as two of your phases, run a motor from these phases and the motor will generate from the third set of windings. The capacitor to start it is just to give some direction to it[like you said]. the winding will be generating less than 400 volt's on the generated phase but if you were to excite it a bit you could get some more, hence the capacitors from the other two phases to the generated one. withthe caps you can balance it so you have equal voltages under load..

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isolation
primary
idea
Just to clarify a few points:
a/ The transformer was originally a 240 - 240 volt isolation transformer with 120 volt tappings on both primary and secondary. I wired the primary and secondary in series and fed the 240 volts in at the 120 volt tapping points.
b/ I reduced the voltage from 480 to about 415 by winding a few turns 'antiphase' onto the transformer legs as it was easier than removing turns that were varninshed in.
c/ The three phases are generated thus: Phase one is the 'low leg' of the transformer, phase two is the 'high leg' of the transformer, and phase three is connected to phase two via the start / run capacitor network. (This is why mains input has to be at the tapping points, so mains neutral is nearer to the 'star point' voltage of the motor)
d/ With no motor connected the capacitor bank gives approx 90 degrees phase shift producing a second phase that will get the motor to start turning when connected, and as the motor spins up its internal generator effect with its coils spaced at 120 degrees drag the phase shift to nearer to the correct 120 degree relationship. Most commercial units monitor the voltage on the third phase with a voltage sensitive relay which drops the start capacitor out when at a reasonable level.
e/ Anyone reading this who is not experienced using high voltages and doesn't understand to forgoing and is not confident in their own abilites to stay safe SHOULD NOT experiment as i did.
Andrew Mawson
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On Fri, 07 May 2004 09:57:36 +0100, Charles Ping

With care and an angle grinder it IS possible to disassemble modern transformers by grinding away the weld metal. With laminated E+I or T+U cores the weld metal is located in a low flux density region of the core so the metal removed has little effect on the electrical characteristics. Jim
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wrote:

I think I have some 440v to 240 v transformers out of a scrapped CNC. I know I have one three phase one, three massive separate transformers but can't remember what the output was, could be about 160 to 180 volt knowing what they were for. Just got back from Harrogate, let me check in the morning
-- Regards,
John Stevenson Nottingham, England.
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