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
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
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