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3 Phase unbalanced load AC electronics question.

by rolls
Tags: electronics, load, phase, unbalanced
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Nov16-09, 12:49 AM
P: 51
1. The problem statement, all variables and given/known data

This is a past exam questions which I am doing as revision, now I am confused about how to calculate the answers.

3. The attempt at a solution
I have tried using millmans theorem which is
Vsn = (Van/Za + Vbn/Zb + Vcn/Zc) / (1/Za + 1/Zb + 1/Zc)

Assuming the following:
Van = 240v @ 0 degrees
Vbn = 240v @ 120 degrees
Vcn = 240v @ -120 degrees

As complex impedances:
Van = 240
Vbn = 195 + 139i
Vcn = 195 - 139i
Za = 10
Zb = 15
Zc = -20i

Vsn = 273 +32i
Vsn = 275v @ 6.6degrees

Now I feel like I am really guessing here and have no idea if what I have done is correct (I don't think it is) so any help would be greatly appreciated

For the currents it is just Ia = Van/Za = 240/10 etc which is easy and then you can easily construct the phasor diagram as well from these. Either way I don't think my values for Van, Vbn, Vcn or my method for calculating Vsn is correct.
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