3ph and neutral

[brizospark]

If there is a 3 phase 6 way d.b. lets say all circuits are single phase.

How come it is ok to have the phase and neutral feeds to d.b. the same size when the neutral can be carrying load of all 3 phases?

 

[DurhamSparky]

it just is "white mans magic"!!

 

[Sintra]

It's because the loads on all phases if equal will cancel each other and leave the neutral with no load. So even if the loads over the three phases are unbalanced the neutral will only have to carry the load of the unbalance and never the total load of three phases added together.

 

[brizospark]

even i every circuit is single phase?

 

[brizospark]

anyone able to elaborate?

 

[topquark]

Sintra has pretty much nailed it. Remember in 3 phase all of the voltages are out of phase by 120 deg (and so is the current, but 90deg out of phase with the voltage). If you draw out what is happening to the voltage and current over time you'll see why it cancels out.

 

[somersetsparks]

The current doesn't cancel each other out, it goes down the neutral just 120 deg apart so the neutral only ever takes the load of 1 phase at a time not all three.

 

[topquark]

The current doesn't cancel each other out, it goes down the neutral just 120 deg apart so the neutral only ever takes the load of 1 phase at a time not all three.

If all 3 phases are exactly equally balanced there is NO neutral current.

 

[topquark]

Here's a little spreadsheet that helps to show this. I have calculated the current for 3 (balanced) phases at each of the 360 (degrees ... arbitary choice but it demonstrates it). If you have open office/excel have a peak and you'll see why there is no net current flow on the neutral at any point in time.

Edit: Attachement removed as it's wrong (see later post)

 

[somersetsparks]

sorry what I wrote didn't sound quite right, In a perfectly balanced system there is no neutral current. But in a normal 3 pahse sytem with single phase loads, the nueutral carries the inbalance, quite often see places with lighting wired with shared neutral, even with the same light fittings you always end up with quite an inbalance if you clamp it?!

 

[topquark]

I think maybe a better way of saying it would be that there is no net electron movement. There are electrons heading in both directions but the net movement cancels out. It's not that there is no electron movement at all, which may be what it appears to the casual observer. Imagine it like several (3 in this case) different paths inside the neutral conductor.

 

[topquark]

Here's a little spreadsheet that helps to show this. I have calculated the current for 3 (balanced) phases at each of the 360 (degrees ... arbitary choice but it demonstrates it). If you have open office/excel have a peak and you'll see why there is no net current flow on the neutral at any point in time.

Nobody seems to have spotted the (not deliberate) mistake

I didn't think the data looked right in the light of day. The SIN() function takes radians not degrees, so I've redone it (and plotted it). Sorry for the mistake guys!

View attachment 5644

 

[BlueToBits]

Another way of looking at it:
AC current flows forward and backwards through the conductor +ve and then -ve. The current in each conductor of a three phase current does the same but is 120 degrees out of sync with each other. So, at any given moment in time the neutral current in a three phase balanced circuit for one phase may be in the +ve direction with the other 2 phases in the -ve direction. Because the magnitude of the current follows a sine wave pattern, the magnitude of (say) the +ve current will always be equal to the total of the other 2 -ve currents. As they are flowing in opposite directions the net current is therefore zero.
If the phases are un-balanced, the total current in one direction will not be the same as the other, some of the current will be cancelled out leaving just the unbalace.

 

[johnboy6083]

the max neutral current in a 3ph system will be maximum the value of one phase. this would only happen if only one phase was loaded and the otherphases not pulling anything

 

[RogertheBodger]

Yet another (simpler) way of looking at it is, 3 people stuffing their face with beer but there's only one Loo.

Probably wouldn't have posted this If I hadn't been testing out this very theory today.

Good original question though, anybody ever seen a perfectly balanced three phase system? apart from motors etc.

 

[davelerave]

the max neutral current in a 3ph system will be maximum the value of one phase. this would only happen if only one phase was loaded and the otherphases not pulling anything

could be higher with harmonic currents-in theory

 

[Electro-tech]

Where the neutral current is expected to be higher than the line current, then the cable size should be selected on the basis of the neutral current. Appendix 11 BS7671. Also 523.6.3.

 

[topquark]

Where the neutral current is expected to be higher than the line current, then the cable size should be selected on the basis of the neutral current. Appendix 11 BS7671. Also 523.6.3.

Curious (and useful to keep in mind). Not sure I'd "expect" it to be though. I guess it depends on where your 3 phase supply is coming from and whether any harmonics have been sufficiently filtered. Looks like someone has decided to add VAT to be on the safe side (well at the old rate anyway).

Is this something you have come across frequently (pun intended )? Do you take this into account with fault scenarios, for example?

 

[Electro-tech]

Maybe over the last decade, occassionally.Fault scenarios where you have over-heating of neutral conductors.Non-linear loads generate high levels of harmonic currents. Typical non-linear loads include desktop computers, ac variable speed drives, inverters, and welders. Of these non-linear loads the main cause for concern is the switch mode power supply found in desktop computers and other office equipment.

 

[topquark]

Ah, so more load side than supply side related (must admit my thought track was on the supply side). I could see 100's of PC's being a consideration (the current's involved would be so small for the harmonics for an individual PC) ... but they should normally be divided amoungst several circuits anyway.

Welders, garage lifts etc I can see being a problem and potentially difficult to balance the load on 3 phase in that type of environment.

Not something I get involved with ... everyday's a school day, thanks!

 

[davelerave]

yes load side

 

[telectrix]

no i'm confused. my van has 2 side loading doors, so does that mean i've got 2 load sides? or is it 3 with the back door, or is the back classed as an end and not a side?

 

[topquark]

It's 3 and they're all out of phase (but in this case by 90deg and 180deg) with each other.

 

[telectrix]

things are becoming clear now, or not, as i don't have side windows.

 

[jeffducati]

Wow its frightening question??
dont go there if you do not understand it !!
these are course work questions.
its ok though had student put his test leads on PFC across 2 sad to say it never worked again!!!

 

[topquark]

Wow its frightening question??
dont go there if you do not understand it !!
these are course work questions.
its ok though had student put his test leads on PFC across 2 sad to say it never worked again!!!

Don't take the last few bits in here toooo seriously jeffducati We do like to have a little fun (once the original posters question has been answered of course).