R, L, C and Power Question

[jaydub]

Question: Three coils of resistance 40 Ω and inductive reactance 30 Ω are connected in delta to a 400 V 50 Hz three phase supply. The total power will be:

a) 13.31 kW

b) 7.68 kW

c) 9.6 kW

d) 16.63 kW

I now know what the answer is, however, I do not understand how to get there! I would be very interested to see what others think to this.

I will hold back the correct answer for the time being.

 

[Wilko]

Having now started the evening's decline, I will partake in this fine quest ... C ... (?)

 

[jaydub]

Now that is what I believe to be the answer. I also managed to get there using two different formulas.

However, according to the answers, that is not correct! Guess again

 

[The Ghost]

Mhmm dont think that pdf is any good too small,sorry

 

[jaydub]

The answer is apparently B, 7.68kW.

Would anybody be able to explain how to carry out the calculation? I have looked at the PDF Vortigern, but I still don't understand it...

 

[static zap]

Did you add 40 + 30 , they are not the same ?

 

[jaydub]

I thought you had to do this:

Z = square root of (Rsquared + XLsquared)

Z = 50 ohms

400/50 = 8amps

400 x 8 = 3200W

3200 x 3 = 9600W OR 9.6kW

Clearly though, this is incorrect, static zap, can you shed any light?

 

[The Ghost]

I found this part of the course I did the most exacting and understood it at the time, however I cannot get my head around it now without serious study and thinking about it. It is a different thing with the tutor there helping you to get through it and as I virtually never used it in general work it just goes away. This is for the industrial electricians I think. I am sure there must be some on here.

 

[static zap]

If you have a perfect coil and a reistor in series with current flowing which one gets hot ( you have current --nearly there)

 

[jaydub]

Sussed it now, cheers everyone.

P = Isquared x R

That's the solution!

 

[The Ghost]

As a matter of interest are you studying or using this in your daily work?

 

[static zap]

...... however I cannot get my head around it now without serious study and thinking ...........

Got a new name for it IsquaredR --memory losses !

 

[static zap]

As a matter of interest are you studying or using this in your daily work?

Power dissipation is more about safety ? (I'm nearly a maths nerd)
So well paid design work .... can always dream

 

[telectrix]

the last time i did any calculations like this, calculators had not been invented. a computer was the size of a house, and we still had steam trains. so i sit this one out.

 

[static zap]

the last time i did any calculations like this, calculators had not been invented. a computer was the size of a house, and we still had steam trains. so i sit this one out.

Used my 6x house brick sized computer (and calc app !)
..hate using phone

 

[jaydub]

Thanks for everybody's help on this, passed my 2365 302 exam today!

 

[static zap]

Well done !

 

[Simonslimline]

Can you possibly show the working out to get the answer below please?

The answer is apparently B, 7.68kW.

Would anybody be able to explain how to carry out the calculation? I have looked at the PDF Vortigern, but I still don't understand it...

 

[Simonslimline]

Can you possibly show the working out to get the answer below please?

The answer is apparently B, 7.68kW.

Would anybody be able to explain how to carry out the calculation? I have looked at the PDF Vortigern, but I still don't understand it...

 

[static zap]

Can you possibly show the working out to get the answer below please?

Read thru post( 7) ... the last good line reads =8Amps
(ignore everthything from then on)

The final answer is 3x the answer for 1 phase , so divide B= 7.68 /3 for clues !
Post (10) has the other formula needed .
" Copper Losses " is probably a good google phrase

(compared with other examples , impedance bits are handed on a plate )
(No Pi , frequency, current etc to get confused/distracted by hunger with )

 

[Simonslimline]

Thanks,

Also would it be corrrect to say that the apparent power of this is 9600VA

With a power factor of 0.8 making true power 7680watts?

Read thru post( 7) ... the last good line reads =8Amps
(ignore everthything from then on)

The final answer is 3x the answer for 1 phase , so divide B= 7.68 /3 for clues !
Post (10) has the other formula needed .
" Copper Losses " is probably a good google phrase

(compared with other examples , impedance bits are handed on a plate )
(No Pi , frequency, current etc to get confused/distracted by hunger with )