# Complex Power = VI* or VI?

Discussion in 'Homework Help' started by HBenzing, Feb 6, 2011.

1. ### HBenzing Thread Starter New Member

Feb 6, 2011
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This is more of an understanding problem then a homework problem.

Can anyone tell me why complex power is calculated using the conjugate of the current? Everywhere ive looked it just states S=VI* without really giving an explanation, why isn't it just S=VI

Anything that would help explain this would be much appreciated.

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
Suppose Vrms=|Vrms|/_θ1 and Irms=|Irms|/_θ2

[The /_ 'thingy' is meant to signify the phase angle]

To construct the complex power triangle one might assume (as you did) that this could be found from the product Vrms x Irms

However, this would give the incorrect relationship

S=|Vrms|x|Irms|/_(θ1+θ2)

whereas the apparent power should be given by

S=|Vrms|x|Irms|/_(θ1-θ2)

The complex conjugate form Vrms x Irms* in fact gives the correct relationship.

3. ### HBenzing Thread Starter New Member

Feb 6, 2011
2
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Ok that makes sense. Thanks for explaining it!

Oct 29, 2015
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5. ### Ankush Sharma New Member

Oct 29, 2015
2
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what about leading phase angle..wouldn't it get subtracted?