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Discuss phase to phase wiring question in the Commercial Electrical Advice area at ElectriciansForums.net

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raider94

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I am servicing a machine that uses a 3 phase power supply. Some of the components (motor, compressor, fan) use 3 phase power. Other components (solenoids) use single phase (phase to phase) tapped from L1 and L3. I understand regular single phase-120v and a nuetral. The nuetral creates the potential for the current to flow. How does the two hots, in this case (L1 and L3) create a potential without using a nuetral wire? If a soleniod is using two hots out of phase, how is that single phase and how does the current flow? How is there a potential created between the two phases?

Also,

Sometimes I test L1 to grd L2 to grd and L3 to grd. I always get 0 volts when I measure from L2 to grd. How is this if all 3 phases are coming out of the powerstation generation plant? How can one phase be grounded and we still get 3 phases?

Thanks for any help.
 
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R

randyrat

  • Thread Starter Thread Starter
  • #2
In a three phase supply, each of the phases is at a different point on the sinusoidal waveform at any given point in time. I.E L1 may be at a rising point on the Positive half of the wave, where L2 may be descending the negative, and L3 may be at zero. All at a given time. Its the average peak value at any point in time that denotes the overall voltage. As each of the wavelengths is the same, the relationship between the individual phase voltages is always the same. I.E the voltage total and between is always constant.
As far as your question ref neutral return path goes, when the supply is via two hots, then the return is via the opposite hot...i.e one wave form is always descending as the other rises, allowing each phase to return back down its opposite number. Think of it like a back and forward pulse domn a pipe....the current simply surges one way, then the other way as the relative potentials change between the phases.
Hope this helps......it's not the easiest concept to grasp!!!
randyrat
 

benji

-
Arms
where was the machine manufactured?you say three phase but what is the voltage is it 3ph 415v or 3ph 220v is there a 415/220 transformer somewhere in the machine for single phase components
yours benji
 
W

wattsup

  • Thread Starter Thread Starter
  • #4
You cannot get 0v from L2 to ground, no way no how, you have not got a 3-phase supply. You can still get 415v however between L1 and L3 (assuming UK supply)

It is normally a welding set that uses two phases only, though not always. I suspect if through a transformer, you have a winding down. If a direct supply from the utility suppliers, then they have a winding down. You cannot get 0v to earth from any phase at any time.

Sometimes contactors are wired wrong, sometimes mistakes are made with the line (hot) or neutral, you could be checking L2 when in fact it is neutral (if wired incorrectly) you need to check on the main incommer you have a real 3-phase supply
 
R

raider94

  • Thread Starter Thread Starter
  • #5
randyrat- thanks, thats the best answer I got yet anywhere.

To the other two- why did you ask me about a transformer when I specifically said in my post that L1 and L2 were being tapped for other single phase components in the circuit?

Also, in some areas, particulary some large cities, one of the phases of a three phase system is grounded. So your also wrong. I know 3phase when I see it and L2 to ground is reading 0.

Randy- you the man! that clears it up.

I still dont know why or how the L2 can read 0 to ground, even if one corner of the delta transformer is grounded.

"one wave form is always descending as the other rises, allowing each phase to return back down its opposite number"

That right there is what clears it all up! Returning down its opposite number creates the potential. You cant find this in books.
 
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Sorry my friend,but I think you are talking out of your arse.(ass)In your origional statement,you said you were getting 120v. This would lead me to assume that you are not based in uk and so probably were working with
a 120/240v three phase 4 wire system,though to fully diagnose your issue
phase to phase voltage would help(all phases) as would all phases to ground.You did not mention if your supply to machine was 3 or 4 wire.
Just for your information your statement that in big cities one phase is grounded, tsk tsk,you really havn't got the hang of this lark at all have you. The uk voltage L1 to L2,L1 to L3,L2 toL3 should all be in the region of 400volts. Measurements between L1 to ground,L2 to ground,and L3 to ground should be around 230volts (the same would apply to measurements between hots and neutral) It could be possable though unlikely that a neutral potential was obtained via a local ground connection. But all in all I would have to agree that it sounds like a phase down and in order to get uk voltage of 120volts a transformer has to be used.
 
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raider94

  • Thread Starter Thread Starter
  • #7
I happen to be in Asia but I have read that this situation is also encountered in the U.S. It is possible and its obvious its something you must of never encountered.
 
I happen to be in Asia but I have read that this situation is also encountered in the U.S. It is possible and its obvious its something you must of never encountered.
i would still be interested to hear what voltage measurements you got.(By the way phase to phase is not single phase.phase to neutral is.)
I think you are in you statement about delta transformers,you are refering to a 4 wire delta grounded system with a centre tapping to ground giving you a neutral and then you should have 120v ,hotA toN and hotA to hotB should give you 240v.,and if you have 3 phase hotC also.240v.
When you are taking measurements what you see is like the average peak value voltage.You need to understand frequency also ,in uk 50heartz per second,in us and possibly asia 60hz per second,the no of times your sine wave will flow every second.But your meter will only show the peak. Hopefully this has been a little helpfull.
 
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raider94

  • Thread Starter Thread Starter
  • #9
Its called a corner grounded delta system. Im still trying to figure out how it works.
 
P

PhaseShift

  • Thread Starter Thread Starter
  • #10
Its called a corner grounded delta system. Im still trying to figure out how it works.
Raider, a corner grounded delta is just that. It has one of the phases connected to ground. This is a simple way to provide a ground to a system that would otherwise be ungrounded. In this setup, you will read the same voltage phase to phase as you do phase to ground. We don't use them much anymore where I'm from, but I have run into them from time to time. In general, the grounded phase will be run and handled like a neutral, but it is not a real neutral. It is a grounded phase conductor. Hope this helps clarify things.;)
 
K

KathrynLeng

  • Thread Starter Thread Starter
  • #11
In a three phase supply, each of the phases is at a different point on the sinusoidal waveform at any given point in time. I.E L1 may be at a rising point on the Positive half of the wave, where L2 may be descending the negative, and L3 may be at zero. All at a given time. Its the average peak value at any point in time that denotes the overall voltage. As each of the wavelengths is the same, the relationship between the individual phase voltages is always the same. I.E the voltage total and between is always constant.
As far as your question ref neutral return path goes, when the supply is via two hots, then the return is via the opposite hot...i.e one wave form is always descending as the other rises, allowing each phase to return back down its opposite number. Think of it like a back and forward pulse domn a pipe....the current simply surges one way, then the other way as the relative potentials change between the phases.
Hope this helps......it's not the easiest concept to grasp!!!
randyrat

Great answer dude!!!
This is very good and helpful to us.
Thanks for sharing this type of information with us......
Thanks once again!!!!!
 
W

wattsup

  • Thread Starter Thread Starter
  • #12
If you don't get 230v to earth on all phases then you have a problem. That's simple my friend. (assuming no unconventional wiring in place).

Where you could be mistaken is if you are testing a neutral load, where depending which phase is energised for a given circuit.

If you do not have 230 ish volts line to earth on each phase then you have a problem to solve.

Please tell me which large cities have 0v L2 to earth?

It would seem you have a 415v coil not a 230v.

Or 120v or whatever, the same applies

>>In a three phase supply, each of the phases is at a different point on the sinusoidal waveform at any given point in time. I.E L1 may be at a rising point on the Positive half of the wave, where L2 may be descending the negative, and L3 may be at zero. All at a given time. >>

No matter, line to earth, line to neutral should be the same voltage or very similar on all phases.
 
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