A basic 3-phase question

Mar 01, 2020 Last reply: 6 years ago 23 Replies
A

So in the coffee bar this morning chat pondered on 3-phase and how to know which of the 3 wires was which phase. I've done some googling and would like the experts to confirm:


  1. It doesn't matter which wire is which phase providing the sequence of 1-2-3 2-3-1 or 3-1-2 is maintained when connecting, or any pair swapped if that is what is required.


  1. The voltage between any two phases is 440v and it doesn't matter which two



and


  1. The voltage between any phase and ground/neutral is 240v

Is that correct?



Are there any conditions when the phase needs to be known? If so how is it detected?


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C

when connecting up a 3 phase gennerating plant to the grid. Can't remember, I never became a poer engineer.

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R

Totally ignorant comment here:

Surely there is no absolute phase (in power distribution at least), just phase in relation to the other two phases. So no real life equipment could know the difference as long as you can swap any two if it is wrong.

Is it good practice in real life to keep the same colour wires throughout the whole installation for each incoming phase?

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A

Yes

As long as you know the 1-2-3, 2-3-1 or 3-1-2 rotation you should be OK.

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R

As long as they were in the right order wouldn't they just slip into sync?

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J

If you connect a 3-phase motor up wrong, it won't work. Does that count ?

This has the suggestion of leading to a bridges of Koenigsberg puzzle ... what's the minimum number of wires you'd need to swap to get it working :)

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R

From a position of no theoretical knowledge and little practical experience, I thought it would go backwards. Am I wrong?

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P

The motors at work did - switch on, look at the fan or feel the airflow, sod's law... Can't remember (>30 years ago) what happened with the 3-phase fridge motors. The condensr fan might have been the indication, but it might have been single phase. We had a 3-phase USP but that was just for providing 3-off 1-phase supplies.

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H

It depends. A 3ph motor can be connected in "star" or " delta/mesh" (Mesh=USA) Reversing any two phases reverse the motor.

However if you swop the connections on one of the windings, (ie reverse the Live and Neutral connections,) it won't run.

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N

400v +10% - 6%
230v +10% -6%.

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T

yes

The only time you need to know is when connecting to a true AC motor when reversing any two phases will reverse the motor direction

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T

yes.

yes.

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T

depends if its CAPABLE of going backwards

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C

It matters quite a bit if you close an interconnector without the two sections being synchronised.

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B

Would it not confuse a motor though? Brian

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A

What about when it's being connected to the grid?

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H

All you need is three lamps.

Eg:-

formatting link

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B

I remember reading of a case where the lamps were wired incorrectly. On the first attempt to sync, there was an expensive bang.

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