Three phase wiring questions

Dear All,
With the recent, ongoing thread on inverters, could I chip in and ask a couple of questions concerning three phase wiring please? 1. If wiring up a 4-pin three phase plug/socket, is there an analogous mnemonic to a 240v plug (bLue, left, bRown, right, etc.)? The conductor wires out of my converter are black, blue and brown; the socket is marked L1, L2, L3. At the moment, I have it wired alphabetically (i.e. black-L1)
2. On said set-up, how (if at all) can I connect a transformer [440--12v] to power the machine light? As received, the machine had the live of the light connected to one of the phase wires, and a separate (black) neutral (?) wire. As I understand it, this would require three phases plus neutral (5-pin plug) and send 240v into the light. Can I just connect the transformer across any two of the phase wires?
A tip I recently read: when wiring a plug, make the earth wire longer than the others. If the cable pulls out, the earth is the last to 'let go'.
Thanks for any tips.

On Tue, 02 May 2006 19:49:12 +0100, John Montrose
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Doesn't matter as you often have to swop phases over to get the machine to run the right way.
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If it is a 440 - 12v transformer than yes any two phases to the transformer and 12v out.
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-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /

John Stevenson Wrote:
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Note that EACH of the 3 power leads is a hot lead. Each will have a voltage compared to ground. So use care when dealing with this stuff...
This aspect is similar to the way 110/220 systems are set up in the US. It can sort of be looked at as 220 1 phase center-tapped to ground for the 110. (Its relatively normal to see the ground and the "common" wires joined to a single buss-bar in the breaker box...)
Yes, its a good idea to have the ground wire be slightly longer if there's any chance of the wires being strained. It certainly can't do any harm and it just might prevent a fire or save a life. Be reasonable about the extra length.. 1 inch is probably plenty and adds negligible resistance. (all wire has SOME resistance)

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Just for clarification, the US system you refer to is NOT similar to the three phase system used to power motors. The most common US domestic supply is 110V-0-110V i.e. a single phase secondary with a centre tap, socket outlets are wired across 0-110V and heavy appliances across the full 220V. The two phases, if you wish to look at it that way, are 120 degrees apart and thus useless for generating a rotating magnetic field in an induction motor, in a three phase system they are 120 degrees apart so can generate a rotating field. Note some states are 115V or 120V instead of 110V.
For completeness, the alternative US domestic supply that is becoming common is to supply each apartment in a building with two phases and the neutral from a three phase 4 wire system at 120V phase to neutral, each apartment having a different pair of phases to balancing the load. The heavy appliances have to be 208V for this system.
Greg

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the US.
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for
the
supply
socket
220V.
apart
induction
generate a
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becoming common
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neutral
apartment
....mmmm..... I think you will find that the two US 'phases' are actually 180 degrees apart not 120
AWEM

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You are of course correct, it was a typo.
Greg

On Wed, 3 May 2006 23:03:30 +0100, "Greg"
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The original question was on UK sourced three phase supplies and bringing the crappy US electrical system into it not only clouds the issue it's not even relevant. However they managed to get a man on the moon when they can't get three wires across 48 states beats me. I'm sure Capricorn One was for real.
-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:- http://www.homeworkshop.org.uk /

On Wed, 03 May 2006 22:24:46 GMT, John Stevenson
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Quite easy really... The man on the moon thing was "Vorsprung Durch Technik" <BG>
Mark Rand RTFM

On Tue, 02 May 2006 19:49:12 +0100, John Montrose
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Under the old scheme we had:- L1=Red L2=Yellow L3=Blue Neutral=Black
Under the lethal new French scheme we have:- L1=Brown L2=Black L3=Grey Neutral=Blue
If you mix the old and the new colours anywhere it is recommended that the cores on both cables are marked L1, L2, L3, N with ferrules, if possible, or tape If you can't afford ferrules. You must also put a label on the cover of the junction pointing out that both colour schemes are in use.
Labels are available from electrical suppliers. e.g.:- http://www.tlc-direct.co.uk/Main_Index/Cable_Accessories_Index/Cable_Colour_Labels/index.html
Don't know if the likes of B&Q do them, they bloody well should.
As John indicates, you have an evens chance of having to swap things over the first time you wire up a motor, but If you change things round so that the phase rotation of the cable cores is correct after the first connection then you should have a more predictable set up in the future.
Mark Rand RTFM

John
I would like to point out one VERY important item in your original posting, you stated that the the machine light was connected to one of the live phases and a seperate Black (?) neutral cable so the machine must have originally had a 5 wire connection.
Unless I am mistaken, your machine light is a 230v single phase unit and as such MUST NOT be connected across two of you three 400v phases - it can only be connected to a 230v supply, i.e. a phase and neutral. You will need a 5 pin 400v plug or have a seperate fused single phase 230v feed to your machine light.
Hope this helps.
Best regards
Dave

On 3 May 2006 03:38:40 -0700, "Steam Train Dave"
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That's what I thought, hence the first thing I did was to disconnect the light completely and remove the extra wire.
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My original question was if it was possible to connect a 440-12 _transformer_ across the phase wires. As I understand it, mains voltage machine lights are not a good idea: my lathe has a 12v light; the surface grinder a 50v one. Presumably, it is absolutely imperative that the transformer is an isolating one.
Thanks to everyone who responded to this question. Yet again, ukrme does what google can't.

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My alarm bells are ringing.... If a machine has a neutral, it normally uses it to power a 240 V control system... I would check the voltage of the contactor coils... do not put 415 V ( now 400 ) onto 240 V coil contactors.
Regards Jonathan
remove AT to reply

On Wed, 3 May 2006 20:35:39 +0100, "Jonathan Barnes"
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Thanks for your concern. I appreciate your efforts to steer me on the correct path. Be assured, though, that the _only_ place the neutral was connected related to the light. The main control box (for want of a better expression) contains four connections mounted on, and insulated from, a rail; three brown and one yellow. As bought, all earth connections went via one of the rail mounting screws: the yellow connector which should be the earth was being (wrongly) used for the neutral to the worklight.
Thanks once again for your input.

Actually... The US power system works fine... thanks... and you can ge
a rotating magnetic field on a single phase... otherwise my 110 vol 2-wire fan with a squirrel cage rotor wouldn't work.
The aspect I pointed out as being similar really is similar... We ge 110 by going to ground, where you get your 220 or 240 off of your 44 3-phase by going to ground.
We have several differnt power systems over here... 110, 220, 408, 440 4160... and use each as is appropriate for the load.
As for Capricorn One being real... impossible. My Father was one o the Flight Surgeons who examined the astronauts after 2 of the moo missions. It is more likely for Queen Elizabeth to be a black man i drag than for that movie to have anything to do with reality
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fhhuber506771 wrote:
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Yeah. But that's a cludge using either a starting winding or a shaded pole.
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But a single system more the majority of domestic/light industrial situations has far less chance of error.

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Did I say it didn't ?
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110 volt
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Yes of course there are several types of single phase motor. The fact remains that the US 110-0-110 supply is fundamentally different to a 3 phase supply, the 180 degrees between the two phase will not, on it's own, generate a rotating field but the three phase 120 degree supply will, and it is exactly this fact that drives the three phase motors used on machine tools.
Greg

take any 3-phase AC motor with a squirrel cage rotor...
Clip one input power lead.
Turn it on (best to remove the workload... you have severely reduce the power output it can give without overheating)
It will run.
Now... if the single phase AC can't produce a rotating magnetic fiel without playing games... why will the 3-phase motor even turn when yo only apply power to one phase?
The sine wave AC input inherantly rotates the magnetic field. 1 phase 2 phase 3 phase .... 90 phases if you wanted to make such a system
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May I suggests that rather than boring people here you go and read a book on the subject, eventually you will realise your assertion that the US 110-0-110 system is similar to a three phase supply is fundamentally incorrect.
Greg

fhhuber506771 Wrote:
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The single phase AC field can be looked at as two counter-rotatin fields. The torque produced from both combined with the rotor at rest cancels The motor won't start. The starter windings produce a slightly larger field rotating in on direction, and the motor begins to run. ( Or you can manually spin the motor in one direction or the other) Due to the shape of the torque-slip curve, once it is running in on direction the torque produced by the field rotating in the opposit direction becomes very small, while the torque produced from the field rotating in the same directio increases.
Pat MacKenzi
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Exactly, but we're getting into some heavy theory for a model engineering group 8-). Greg