Discuss So i am having some dramas as where to start for this practice theory exam question can someome please show me how to work it out :) in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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A load consisting of three idnentical coils with a resistance of 11 ohms and inductance of 50 mH are connected in (i) star, and then (ii) delta to a 400v 50Hz 3 phase suply. Determine the total power dissipated in each case
 
I’d probably start by drawing it out so I can visualise what I’m doing / what there asking for. I used to find it made things easier to deal with.

You must have worked examples of similar questions from college, and a big book of electrical formulas you use?
 
OK there are a number of shortcuts, all coming from the BALANCED aspect - never apply these if it isn't balanced!

Firstly if its BALANCED you can work out the voltage across each star connected load as being a simple factor of the line to line voltage - this factor will be available from your studies.

Secondly if its BALANCED you only need to work out the values for one and the total will be 3 x this.

So now you need to work out the total impedance of the load, this is the resistance plus the reactance of the inductor - you will have a formula from your studies which gives the XL from the L - you will need the inductance and frequency for it. You now have resistance and reactance, but you can't just add them, (not 3+4 =7), instead you will have a formula from your studies involving squaring and square roots, so this would need to be applied (and would give the correct value of 5 for the 3+4 example above).

Now you have two lots of work to do:
Firstly delta, you calculate the current using the line to line voltage (400v) and impedance of the load you have just calculated - now given this current you can calculate the power from i^2 r then multiply it by 3 to get the total.

Secondly star, you calculate the current using the star voltage you worked out earlier and impedance of the load you have just calculated - now given this current you can calculate the power from i^2 r then multiply it by 3 to get the total.

These would be the two answers - compare them!

You will see a fundamental difference between them - this is very useful to understand
 
As above. It wont help you if we just give you the answer. Better that you work through the task, to the best of your ability, post your effort up here and we can confirm you are correct, or give you guidance as to what may need some re-work.

Cheers
 
Can you draw phase angles using a protractor for this sort of calculation or is it an entirely different section of physics/maths? Man, I hated the maths bit in college!
 
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OK there are a number of shortcuts, all coming from the BALANCED aspect - never apply these if it isn't balanced!

Firstly if its BALANCED you can work out the voltage across each star connected load as being a simple factor of the line to line voltage - this factor will be available from your studies.

Secondly if its BALANCED you only need to work out the values for one and the total will be 3 x this.

So now you need to work out the total impedance of the load, this is the resistance plus the reactance of the inductor - you will have a formula from your studies which gives the XL from the L - you will need the inductance and frequency for it. You now have resistance and reactance, but you can't just add them, (not 3+4 =7), instead you will have a formula from your studies involving squaring and square roots, so this would need to be applied (and would give the correct value of 5 for the 3+4 example above).

Now you have two lots of work to do:
Firstly delta, you calculate the current using the line to line voltage (400v) and impedance of the load you have just calculated - now given this current you can calculate the power from i^2 r then multiply it by 3 to get the total.

Secondly star, you calculate the current using the star voltage you worked out earlier and impedance of the load you have just calculated - now given this current you can calculate the power from i^2 r then multiply it by 3 to get the total.

These would be the two answers - compare them!

You will see a fundamental difference between them - this is very useful to understand
So i am having some dramas as where to start for this practice theory exam question can someome please show me how to work it out :) 20201028_211206 - EletriciansForums.net
Thankyou so much for the help I think I have got it, the hard thing was I was not given nor taught some of these equations so I had to do a little research, I wouldn't ask you to do the maths but does it look like I'm onto a good thing here?
 
You're on the right lines. You would help yourself immensely if you drew out the problem; impedance triangle, power factor triangle, phasor diagram, you'll be surprised how much it helps.
 
Sorry wasn't around much this week, yes your latter calculations look fine, but just think about the last one - instead of 4.75/14 - what do you get if you do the calculation the other way around - does it look like a familiar number to do with 3 phase?


Also, although there is nothing wrong with your latter equations and work - it does involve a number of steps, working out power factor, remembering when to use root 3 etc.

What steps would you have if you just used I^2 R per phase - is that an easier calculation?
 

Reply to So i am having some dramas as where to start for this practice theory exam question can someome please show me how to work it out :) in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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