Discussion in 'Homework Help' started by matt_t, Sep 13, 2012.

1. ### matt_t Thread Starter Member

Aug 18, 2012
30
0
I am stuck on a question similar to the delta question I asked:

Three devices are connected in wye to a three-phase four-wire 240 Volt supply.

• A-N Z = 30 ohms pf = .9 leading
• B-N Z = 20 ohms pf = .8 lagging
• C-N Z = 10 ohms pf = 1
Calculate the neutral current.

I know that the neutral current will be the vector difference of currents ABC.

First I need to find the currents:

Ia= 240/_0 deg / 30 /_25.84 = 8_/-25.84A

Ib=240/_120 / 20/_36.87 = 12_/83.13A

Ic=240/_240 / 10/_0 = 24/_0A

Next I find the difference between the currents:

In= 7.2-j3.49 - 1.435+j11.91 - -12-j20.78 = 17.765+j12.36 = 21.64/_34.82A

The answer given for the question is 14.6A????

2. ### matt_t Thread Starter Member

Aug 18, 2012
30
0
Any help would be appreciated. I realised the Phase to N voltage would be 138.56V instead of 240V but tried the same calculations and still did not get the correct answer. Any ideas anyone?

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You angles are out of kilter. For instance your A phase current has a lagging phase angle when it is explicitly stated that the A phase has leading power factor.

Referring to the attached solution - While I use different phase rotation than yours, you may be able to see how the solution would evolve using your preferred notation.

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Last edited: Sep 17, 2012
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4. ### matt_t Thread Starter Member

Aug 18, 2012
30
0
Thanks t_n_k I was so close but had my +/- on my angles all messed up. I appreciate your time and help.