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.