Star and delta questions

[pitkin2020]

Evening all, could do with a little bit of help with regards to star and delta calculations. I have 3 questions and have attempted them but I have a feeling I have got the delta part incorrect (haven't covered all calcs for that yet)

I have posted the questions below and if any trained eyes could give me an opinion it would be much appreciated.

Cheers

Q1) A three phase star connected supply feeds a delta connected load. If the star connected phase voltage is 230v and the phase current is 30 A calculate,
A) The line voltages and line currents in the star connection
Phase voltage UP = 230V
Line voltage UL= Phase voltage UP x 1.73 230 x 1.73 = 397.9V
Phase current IP = Line current IL 30A = 30A


B) the line and phase voltages in the delta connection
UL=UP 397.9V = 397.9V (originally had 230V but read the question again)
IL = IP x 1.73 30 x 1.73 = 51.9A



Q2) Three identical loads of 60 Ω resistance with inductive reactance 80 Ω are connected to a 400v 3 phase supply. Calculate the phase and line currents if the loads were connected as follows
A) In star

Z= √R[SUP]2 [/SUP]+ XL[SUP]2[/SUP] Z= √60[SUP]2[/SUP] + 80[SUP]2 [/SUP]Z= 100 Ω
UP =UL /1.73 UP = 400 / 1.73 UP = 231.21V
IP = UP/ Z IP = 231.21 / 100 IP = 2.31A
IL =IP IL = 2.31A

B) In delta
UL = UP 400v = 400v
IP = UP/ Z IP = 400 / 100 IP = 4A
IL = IP x 1.73 IL = 4 x 1.73 IL= 6.92A



Q3) Three identical loads of 50 Ω resistance with capacitors of reactance 30 Ω are connected to 11Kv three phase supply. Calculate the phase and line currents if the loads were connected as follows.
A) In star

Z= √R[SUP]2 [/SUP]+ XL[SUP]2[/SUP] Z= √50[SUP]2[/SUP] + 30[SUP]2 [/SUP]Z= 58.31 Ω
UP =UL /1.73 UP = 11000 / 1.73 UP = 6358.38V
IP = UP/ Z IP = 6358.38 / 58.31 IP = 109.04A
IL =IP IL = 109.04A

B) In delta
UL = UP 400v = 400v
IP = UP/ Z IP = 11000 / 58.31 Ω IP = 188.65A
IL = IP x 1.73 IL = 188.65 x 1.73 IL= 326.35A

 

[happysteve]

All looks good to me!

Sounds like you've "got it", but the following is a useful reminder from the Trevor Linsley Level 3 book, page 121:

 

[pitkin2020]

Brill thank you puts my mind at rest

 

[ElectroChem]

Evening all, could do with a little bit of help with regards to star and delta calculations. I have 3 questions and have attempted them but I have a feeling I have got the delta part incorrect (haven't covered all calcs for that yet)

I have posted the questions below and if any trained eyes could give me an opinion it would be much appreciated.

Cheers

Q1) A three phase star connected supply feeds a delta connected load. If the star connected phase voltage is 230v and the phase current is 30 A calculate,
A) The line voltages and line currents in the star connection
Phase voltage UP = 230V
Line voltage UL= Phase voltage UP x 1.73 230 x 1.73 = 397.9V
Phase current IP = Line current IL 30A = 30A


B) the line and phase voltages in the delta connection
UL=UP 397.9V = 397.9V (originally had 230V but read the question again)
IL = IP x 1.73 30 x 1.73 = 51.9A

All the rest checks out but this bit is slightly off.
The line current has been stated to be 30A, since it's a star supply and delta load. Therefore to find the phase currents in the load you would divide by root(3) for a phase current of
IP = IL / 1.73 = 30 / 1.73 = 17.3A

 

[pitkin2020]

So I have the line current (30a) not the phase current as I have worked it above. I see what you mean and agree with that.

Question b is actually )the line and phase voltages and currents in the delta connection. (I missed a bit out not that I think it matters as I did cover them all)

Thank you

 

[happysteve]

All the rest checks out but this bit is slightly off.
The line current has been stated to be 30A, since it's a star supply and delta load. Therefore to find the phase currents in the load you would divide by root(3) for a phase current of
IP = IL / 1.73 = 30 / 1.73 = 17.3A

Yep, well spotted Oops.

I made the fatal error of not drawing it out.