3 phase Question

[dbh6]

I want to verify something. Say you have a typical 3 phase 230VAC supply power. If you measure between any two legs you would read 230VAC. If you measure between any leg and neutral you would read 120VAC. Now say i wanted to power up a PLC for example with 230VAC this is ofcourse assuming the plc can handle that rating and to my surprise a lot of them are, i would then proceed to wire the L1 terminal of the PLC to one leg and the L2/N terminal of the plc to the second leg. Since the 'legs' are 120 degrees out of phase with each other or lagging in a sense the second leg in this case the L2/N terminal acts as the return path for the current, so my question is, technically speaking would that be the reason why a second leg can act as a return path?? Would the scenario be any different in a single phase as opposed to 3 phase?? feel free to elaborate.

 

[MasterBlaster]

I understand your scenario, but not the actual question. That aside, if you have 120vac available, and the neutral conductor is present, why would you not just use that? To my thinking, if you want 240vac single-phase, a better way is use a 120/240v transformer to get the 240v. The "missing" 60 degrees of phasing doing it your way may cause some heating issues with the PLC power supply. On the other hand, the P/S is converting all incoming AC to DC anyway, so the job of rectifying the AC may be made "easier" by the missing 60 degrees of phase difference. In either case, I'm still unsure your actual question about return paths. Sorry to ramble.

 

[dbh6]

i found this on another forum on google chekc link below, the seconds posting answered my question, @ masterblaster thanks for your info.

http://www.electriciantalk.com/f30/120v-240v-residential-180-out-phase-7596/

 

[balash]

Now say i wanted to power up a PLC for example with 230VAC this is ofcourse assuming the plc can handle that rating and to my surprise a lot of them are...

it is not a surprise, because, for instance, EU voltage is 230VAC and you can choose if you're comfortable with that as PLC power supply for some small remote sites or you want to buy power converter to 24VDC. tha's why you have series in 24VDC and 230VAC

as for the rest in every multiphase system can be used as single phase, where resulting voltage would be difference between them. as in your case we have:

V+=Vmax*sin(fi)
V-=Vmax*sin(fi+180)

DeltaV = (V+)-(V-) = Vmax(sin(fi)-sin(fi+180)) = Vmax(sin(fi)-(-sin(fi))= 2Vmax*sin(fi)

it ain't difference in compare to N, it's just for reference that we use N as 0V potential, especially if it is grounded.
There are cases where you have a floating neutral

EDIT:
i would just add that in this case (120+ 120-) you will have two "live" or "hot" ends. don't forget that, cause PLC won't see that but your hands could.

 

[shooter]

this is true for low power demand, however the Null will carry current, so you will need a Null connected, when you have a three phase transformer and you only use two legs the voltages will get corrupted a bit, that is why you have to keep the power down.
here in netherlands every house is connected to another phase, about 100 houses for each phase so 300 on one cable is normal.

 

[DickDV]

Assuming the 230VAC 3ph power is from a supply transformer with a wye secondary and the center of the wye is the neutral, the voltage is NOT 120VAC but 133VAC. In order to get 120VAC phase to neutral, you would need 208VAC phase to phase.

It is, of course, possible that the 230VAC the OP is referring to is actually high leg 230 where the transformer secondary is delta connected with one of the coils centered-tapped to get 115VAC. That would be 115VAC from either phase at the end of the center-tapped side to the centertap. If, by mistake, the connection is made from the high leg to neutral, there would be around 190VAC available.

The OP needs to tell us more about how his power supply is configured before this question can be properly answered.

 

[JesperMP]

Dont know about the US, but for the EU, AC control voltage MUST be derived from the main supply voltage via a transformer (exception is for very small control system consisting of a few relays).
On the same subject, supply to a machine or a plant doesnt normally include a Neutral. Only when there a large single-phase consumers (such as lighting or heating) is there a Neutral. And in that case you are not supposed to tap into this Neutral for the control voltage.

If you can switch to all-DC control voltage, you can bypass the problem completely. Nowadays i is common to use DC PSU's supplied by 2 phase or 3 phase AC.

 

[balash]

well in my country i always get phase(s), neutral and ground, with neutral and ground together. when higher power is needed transformer station is obligated. when you get supply you use it for whatever you like (i.e either power or control circuit or both).
don't know will that be changed now when we are in EU, but somehow i think not...

 

[TConnolly]

This is where you might be getting confused

Since the 'legs' are 120 degrees out of phase with each other or lagging in a sense the second leg in this case the L2/N terminal acts as the return path for the current,

so the question doesn't make any sense.

so my question is, technically speaking would that be the reason why a second leg can act as a return path??

There is no leg that is a dedicated return path. Both legs are supplying current and returning current, with current flowing from the leg with the most negative potential to the one with the most positive potential with respect to each other.

As for the notion that they are 120 degrees out of phase, that only applies when you have a third leg for which you can reference what being in phase is. Without the third leg you cannot resolve phase or which one is lagging. Yes, your original source is three phase so I understand how it is tempting to keep thinking the same way about a single phase source when its taken from a three phase source. Voltage is ALWAYS relative to some reference, you measure only the voltage drop, there is no such thing as absolute voltage (ever see that helicopter lineman video?). With only two legs you have nothing to fix a reference point. So your power supply sees a single 60HZ sine wave with peaks 266V apart and an RMS voltage of 230V.

 

[James Mcquade]

dbh6,

#1 why would you wan to do this?
you are increasing the arc flash requirements of the person getting into the panel.

#2 NFPA 79 - standard handbook for industrial machinery states that control voltages above 120 volts are not allowed. goto www.nfpa.org and look at the free online catalog.
sorry, I don't have my codebook with me or I would cite the code.
this has been in place for quite a while.
I know that this will be a power circuit, but I still consider it control voltage.

#3 95% or more of the plc's are powered with 120 volts, if this plc dies, the replacement might not handle 220 volts. if this happens, you have smoked your spare as well.

regards,
james

 

[TConnolly]

Good point. I can see that one might be tempted to do it if all control voltage was 24V. However, even though you can drive the DC power supply from line voltage, you still technically need to isolate the PLC power source with a transformer (NFPA 79 9.1.1), so you may as well step it down to comply with 9.1.2.1 (120V limit on control voltage)

There are exceptions for for elements that monitor line voltage or devices that would consume more than 20A at 120V.

 

[gbeaker]

Am I missing something here or would this power supply not read 208 line to line instead of 240? I have never seen 240 3phase in Canada.

 

[TConnolly]

gbeaker,

In some parts of the US there are 240V 3 phase delta systems. In a 240V 3 phase delta system one transformer secondary winding is center tapped.

The center tap is grounded and becomes a neutral, giving you 120V between the center tap and the two adjacent legs. The third leg is the high leg (also called wild leg or red leg) and is not connected. Just the two lines and the neutral can then be supplied to the residence, giving it 120V and 240V power. This supposedly had the advantage of lowering costs for single phase residential and light commercial utility service where three phase loads were small or nonexistent (though I don't quite follow how). Most of the systems in the western US are 208 WYE systems, but I believe its still very common on the East. Primarily found in older service areas, it is gradually waning and some jurisdictions now deprecate it where three phase service is provided, leaving the choice of 208/120 WYE or 480/277 WYE.

 

[strantor]

This is where you might be getting confused



so the question doesn't make any sense.

There is no leg that is a dedicated return path. Both legs are supplying current and returning current, with current flowing from the leg with the most negative potential to the one with the most positive potential with respect to each other.

As for the notion that they are 120 degrees out of phase, that only applies when you have a third leg for which you can reference what being in phase is. Without the third leg you cannot resolve phase or which one is lagging. Yes, your original source is three phase so I understand how it is tempting to keep thinking the same way about a single phase source when its taken from a three phase source. Voltage is ALWAYS relative to some reference, you measure only the voltage drop, there is no such thing as absolute voltage (ever see that helicopter lineman video?). With only two legs you have nothing to fix a reference point. So your power supply sees a single 60HZ sine wave with peaks 266V apart and an RMS voltage of 230V.

I was particularly baffled by this early on in my learning. The mind wants to hold on to that "120 degree phase separation" thing even while talking about single phase loads. It wasn't until I put my o-scope across 2 legs of a 3 phase system that I "got it" - 120 degrees out of phase of WHAT? There's only one wave on the screen! - And it looks just like a single phase... because it is! Epiphany moment. 230VAC from 2 legs of a 3 phase system is 230VAC single phase.

 

[gbeaker]

I was particularly baffled by this early on in my learning. The mind wants to hold on to that "120 degree phase separation" thing even while talking about single phase loads. It wasn't until I put my o-scope across 2 legs of a 3 phase system that I "got it" - 120 degrees out of phase of WHAT? There's only one wave on the screen! - And it looks just like a single phase... because it is! Epiphany moment. 230VAC from 2 legs of a 3 phase system is 230VAC single phase.

amazing explanation!