Help with question

[shnabz]

Calculate the phase and line current when three resistors each having a resistance of 15ohms are connected, to a 400V three-phase supply, in
A) Star
B) Delta

 

[Guest77]

Go on then :biggrin:

I can help but to be honest Richard and Archy are the main proffessors at this, archy will be back tommorrow, you may be lucky in catching Richard later, I can try to help, but am a designer rather than calculator now, that said I have a book I can reference :biggrin:

 

[shnabz]

Thanks!

I also have a book, but I still don't really understand what the question is asking. I think it's something to do with V/sqroot(3)

 

[Guest77]

Well first thing to remember is Ohms law and his formular, thats a start, you got your resistance and voltage.

 

[shnabz]

Okay, so resistance (series) = R1+R2+R3 = 15+15+15 = 45ohms
I=V/R = 400/45 = 8.9A

Parallel = 1/((1/15)*3) = 5A

 

[Guest77]

I make it around 15 amps for the star, you need to work out the line current by 230 volts, it may actually be just over, I cannot remember the exact formular, but it should be around 230 volts by 15 ohms which is 15 amps ish, the guys will confirm the actual figure later, delta will be different and probably 3 times the current, I will have to consult my book again, lol, I am learning again myself, I think you divide the 400 volts by 15 ohms, which is 25-26 amps then times that by the rootage of the design current which is around 15 amps which is 1.8 I think and it should give around 47 amps for delta, thats me done, I am knackered now, I am supposed to be retired from this lol

 

[shnabz]

Yeah, well that's what I thought, 400/sqroot(3) = 230v, then 230/15 = 15.333A

I have the City & Guilds Text Book, but there's not much in there about this.

 

[Guest77]

Yeah, well that's what I thought, 400/sqroot(3) = 230v, then 230/15 = 15.333A

I have the City & Guilds Text Book, but there's not much in there about this.

Looks right for star, then the delta needs to be calulated and I cannot remember the actual formular, it is around three times ish that figure though----------------Richard = help

 

[Guest77]

Yeah, well that's what I thought, 400/sqroot(3) = 230v, then 230/15 = 15.333A

I have the City & Guilds Text Book, but there's not much in there about this.

I think the 230 isn't right, it will be over that, around 230.5 ish, it is rooted from the 400 and 230 approximate.

 

[shnabz]

I find it highly annoying that the formulae for calculating Line & Phase current in Delta requires knowing either the Line OR Phase current.

 

[Simonslimline]

In star - IL = IP & VL = 1.732 X VP.



In Delta - IL = 1.732 X IP & VL = VP

 

[shnabz]

In Star - IL = Ip In Delta - IL = 1.732 x IP
VL = 1.732 x Vp VL = VP

Calculate the phase and line current when three resistors each having a resistance of 15ohms are connected, to a 400V three-phase supply, in
A) Star
B) Delta

 

[Simonslimline]

Star

VP = 400/1.732 = 230v.
IP = IL 230/15 = 15.33Amps.

Delta

IP = 400/15 = 26.66amps.
IL = 26.66 x 1.732 = 46.1 amps

Unit 309 by any chance? :computer::50:

 

[Guest77]

well I said 15 and 47 without calculating it lol, so I wasn't far away even though Simon has just a$$ raped me for getting it completely wrong :biggrin:

 

[Guest77]

Star

VP = 400/1.732 = 230v.
IP = IL 230/15 = 15.33Amps.

Delta

IP = 400/15 = 26.66amps.
IL = 26.66 x 1.732 = 46.1 amps

Unit 309 by any chance? :computer::50:

That doesn't work out hehe