[SOLVED]

Hi all,

I'm working on the following question.

It's under the heading of "Wye & Delta Transformations" in the textbook. I've attempted it a couple of times, but can't see where I'm going wrong.

I transform the middle Wye configuration into a Delta with resistors 140, 70 and 35 Ohms. I then reduce resulting circuit to get an effective circuit resistance of 33.793 Ohms. This produces a final answer of 0.710 A.

The textbook says 0.9974 A is the correct answer.

 

Your first reduction of the wye part is correct - at least with respect to the element values.

You've obviously gone astray after that. You would do better to show the subsequent working - particularly showing a network drawing so that your work can be checked.

The book answer is correct.

 

It is very easy to associate the wrong leg of the delta or wye in the transformation.

Label the nodes, rather than the resistances and refer to the resistance between two nodes.

You could try using the other wye in the diagram for practice and see if you get the same answer.

I confirm TNK findings.

 

The rest of my working is, as follows (Starting from top-right of transformed circuit and working around the delta network clockwise):

Combining branches of delta configuration with right-most branches of circuit
60 ohms // 140 ohms = 42 ohms
35 ohms // (20 + 50) ohms = 23.333 ohms

Adding 2 sets of newly combined resistors in series
23.333 ohms + 42 ohms = 65.3333 ohms

Adding in parallel with remaining (leftmost) branch of delta configuration
65.333 ohms // 70 ohms = 33.793 ohms

Io = 24V / 33.793 ohms = 0.710 Amps.

 

Does your circuit look like that attached?

 

... no, I have the 35 ohm and 70 ohm switched around. Hmmm. That is likely to be the problem.

Thanks for your help, t_n_k.