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.

 

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.

 

Ok that makes sense. Thanks for explaining it!

 

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.

 

what about leading phase angle..wouldn't it get subtracted?