3phase volt drop.

[Simonslimline]

This is one of a weekly question by sparks magazine posted on Facebook to win free tools each week. Everyone has answered B. 8Volts. I am not sure this is correct. Please could anyone help me with this. I have posted it up in here rather than just the trainee section so more people will see it. Not bothered about the prize. I just have a feeling that 8volts is wrong, but would happily be corrected on this. Thanks.

To comply with IEE Wiring Regulations the voltage drop on a three phase 400v Circuit should not exceed:
a) 4v
b) 8v
c) 20v
d) 32v

 

[Chr!s]

You have lost me now sorry.

You have three currents, ti find the neutral current you add them together, In = il1 + il2 + il3

Cheers

 

[Silly Sausage]

Without information as to the nature of those loads, no answer can be gien. If they were all single phase resistive loads the neutral current would be 40A. If there was a balanced three phase load pulling 10A and the rest single phase loads, the neutral current would be 30A. If it was all one inbalanced three phase load then you're quite right, the neutral current would be 17.3A

Do what?!?!
Workings please!

 

[Simonslimline]

You have three currents, ti find the neutral current you add them together, In = il1 + il2 + il3

Cheers

So adding 40 + 30 + 20 how do you get 15?

 

[ElectroChem]

Without information as to the nature of those loads, no answer can be gien. If they were all single phase resistive loads the neutral current would be 40A. If there was a balanced three phase load pulling 10A and the rest single phase loads, the neutral current would be 30A. If it was all one inbalanced three phase load then you're quite right, the neutral current would be 17.3A

I'm having a little trouble wrapping my head around your answer here Skelton. Are you using 'neutral current' to mean 'greatest current in the neutral conductor at any point in the system'? My reading of the question was that it wanted the overall neutral current i.e. in the supply neutral.

In this case even assuming a single unbalanced load there would be a short length of conductor between say the end of the load's winding and the star point that would carry the full phase current but I wouldn't think that would normally be considered a neutral current.

 

[Silly Sausage]

Re the voltage drop:
There's an approximation of root 3 lurking in that 8V answer somewhere.

 

[Chr!s]

So adding 40 + 30 + 20 how do you get 15?

Okay, forget equations, draw out a phasor diagram. It will help you see what your trying to achieve.

Cheers

 

[Silly Sausage]

Okay, forget equations, draw out a phasor diagram. It will help you see what your trying to achieve.

Cheers

You still get 17.32.
(I went complex for simplicity! )

 

[Simonslimline]

Re the voltage drop:
There's an approximation of root 3 lurking in that 8V answer somewhere.

Can you make sense of where the 8Volts may come from Archy?

I have tried to come up with 8 but cant see how it can be an answer!

 

[Chr!s]

You still get 17.32.
(I went complex for simplicity! )

you have three currents, assuming resistive loads.

take the 40 amp as your reference phase, 0 degrees.

what are the angles of the two other currents?

Turn them all into complex and add together.

Cheers

 

[Chr!s]

Archy, if you take(I 40) as your reference phase, 17.32 is the angle between the (I 20) and your reference phase.

Cheers

 

[Chr!s]

Can you make sense of where the 8Volts may come from Archy?

I have tried to come up with 8 but cant see how it can be an answer!

Why would you want to, balanced its a % of 400, unbalanced its a % of 230.

Cheers

 

[Silly Sausage]

I end up with 15+sqrt(3)*5j for the N phasor, so its magnitude is the root of its squares => 17.32

 

[Chr!s]

you have three currents, assuming resistive loads.

take the 40 amp as your reference phase, 0 degrees.

what are the angles of the two other currents?

Turn them all into complex and add together.

Cheers

Ps, i have drawn out the phasor and complex and the answer is the same. Try using kirchoffs current law, you will get 15 than

 

[Silly Sausage]

Can you make sense of where the 8Volts may come from Archy?

No!
I've tried reverse engineering (hahaha!) it but it don't make any sense.

 

[Simonslimline]

Why would you want to, balanced its a % of 400, unbalanced its a % of 230.

Cheers

It is a question for sparks magazine competition and everyone has said 8Volts is the maximum allowed volt drop on a 400volt circuit. I think they are wrong personally. I am just trying to make sure i have not overlooked anything stupidly.