Quick three phase question

[La Poste]

Greetings.

I apologise if this is a bit of a daft question but I don't know much about three phase.

In a three phase system there is only current in the neutral if there is an imbalance of the phases right?

So looking at the transformer that supplies our street it is a three phase transformer that supplies various houses on different phases.

Each house has a line and neutral so does the above principle mean that the neutral cable in my own house will be carrying less than the phase?

If the current in the neutral cable is a result of an imbalance between the phases then does this principle apply to houses that are supplied from the same three phase transformer?

Any opinions appreciated.

Thanks.

 

[Knobhead]

Sorry not you Jud

 

[Moses]

This is not what I said.

I said that I interpreted it the way it should have been expressed (which is as you posted) because I understood the connection between the calculations on either side of the subtraction operator i.e. one cancels the other out (in a balanced load).

When entered into a calculator it would look like...

(20² + 10² + 10²) - ((20 x 10) + (20 x 10) + (10 x 10)) and then you would press the √ button which would give you the Square Root of the result of the calculation.

The way its written, then the SQRT only applies to (20² + 10² + 10²) and not the rest of the calculation.

 

[Knobhead]

Go and get your eyes tested, you've missed a set of brackets.

 

[Moses]

Neutral curent is given by:

In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

Tony my friend, The above is your post.

That formula will not give you the correct results. That is the wrong formula.



But the formula you posted in your last post is correct and will work.

i.e In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

You just missed a set of brackets. Thats all.

 

[Darkwood]

Think we all need reminding of basic maths ..... if its bracketed it needs working out first... if multi brackets then sectionally the inner most brackets need working first then head outwards, if a sq-rt sign is used it can be related to the whole equation in 2 ways (a) by adding an extra set of brackets or extending a line across the whole formular where the line stops is where you apply up to, putting a shortform sq-rt sign without extra brackets within the formular may commit it to only part of the sum dependent on the sum itself.

 

[Moses]

Think we all need reminding of basic maths ..... if its bracketed it needs working out first... if multi brackets then sectionally the inner most brackets need working first then head outwards, if a sq-rt sign is used it can be related to the whole equation in 2 ways (a) by adding an extra set of brackets or extending a line across the whole formular where the line stops is where you apply up to, putting a shortform sq-rt sign without extra brackets within the formular may commit it to only part of the sum dependent on how its written.

exactly my point


thankyou for putting it so eloquently.

 

[Knobhead]

Now you’ve pointed me back to the post then yes I was wrong.
What is puzzling me now is how the hell did it happen? I copied and pasted it direct out of a list of formulae I have saved. Had I done it from memory I could understand the mistake.

Apologies to all,
In=√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))) copied perfectly.

Or if you want the Excel version
=SQRT(((B35*B35)+(B36*B36)+(B37*B37))-((B35*B36)+(B35*B37)+(B36*B37)))
Where:
B35 = Ia
B36 = Ib
B37 = Ic

 

[Silly Sausage]

Tony, does this mean you're now going to have to email several 100 amended spreadsheets?

 

[Knobhead]

No.
Because I copied and pasted out of the Excel sheet in my last post. You’ve got a copy of it, check it for me.

 

[Silly Sausage]

Deleted!

 

[Silly Sausage]

No.
Because I copied and pasted out of the Excel sheet in my last post. You’ve got a copy of it, check it for me.

Tomorrow.

 

[Knobhead]

Try these figures

R 8.9
Y 4.2
B 5.3

You should get 4.25793

You’ve had a copy from before this argument started so I haven’t changed it.