What exactly is the star point?

[Lectrician]

Done a quick phasor diagram to show neutral current.

I used the same figures above.

It is not exact, biro is not exactly an acurate thing to use!

L1 - 50amp
L2 - 20amp
L3 - 5amp

My neutral would be 39amps.

 

[ericmark]

If we consider three ladies (with big hats) at the sales find a set of three underpants connected with one price label.

Untitled

• ericmark
• 9 Mar 2009
• 1

As each lady pulls on the elastic waist band the label will be in a different place above the table.
All three equal pull then it will be central and if Lady 1 pulls more than others it will be displaced more to her side.
The pull on the waist band is like how much current is flowing centre of table when no current is flowing.
If lady 2 and 3 let go label will be with lady 1 so same current in neutral and live.
But as each pulls the label will never go past any of the ladies so will be always closer to table centre i.e. always less current than then the largest phase current.
I hope this lets you see how the neutral current (label) would vary according to the current in any phase (ladies pull).

 

[ericmark]

Thank you Lectrician great diagram can I try to do maths.
We need to split it into right angle triangles yes there are quicker ways but I want to show how it relates to Lectrician’s great diagram.
So I have split it into a load of simple sums using

and Pythagoras Theorem H² = a² + b² so here we go:-

Angle at L2 = 60º and Cos 60º = 0.5 (from tables)
Centre to L2 = 20
Sum 1
Cos 60º = x/20 = 0.5 x 20 = 10 so L2 is 10 below zero
Sum 2
Red L1 is therefore 50-10 = 40 above 0
Angle at L1 red = 60º
Cos 60º = x/5 = 0.5 x 5 = 2.5
Sum 3
40 – 2.5 = 37.5
Sin 60º = 0.866 (from tables)
Sum 4
Sin 60º = x/5 = 0.866 x 5 = 4.33
Sum 5
Sin 60º = x/20 = 0.866 x 5 = 17.32
Sum 6
17.32 – 4.33 = 13
Sum 7
Sq root of 13² + 37.5² = 39.686 Amp

Untitled

• ericmark
• 9 Mar 2009
• 1

 

[ricicle]

Or you could just get your pair of compasses out, swipe arcs, and measure the Neutral current

 

[ericmark]

I would agree using cosine rule would also be quicker but I was trying to kept it to school boy level.

 

[Lectrician]

It's all fun, which ever way you do it

A clamp meter is even easier

 

[DIYedboy]

Thank you lads for all your help/ diagrams etc it's much appreciated, I'm gonna go through them slowly and hope that that penny is gonna drop.