I'm confused when to use the apparent power S=VI and when to use S=VI* (where * denotes conjugation of current)? References that I have just say when dealing with complex power, use S=VI*. However, when running into practice exam questions, the questions look the same, but solve differently and so I'm not really sure when to use which equation.
apparent power question
- Thread starter Rei
- Start date
Help Support Professional Engineer & PE Exam Forum:
Flyer_PE
Jr. PE / Sr. Company Pilot
S is ALWAYS equal to sqrt3*VI*.
In the practice problems, what you are most likely seeing is that they give you a kVA value and a power factor value. They will also usually tell you if the pf is a leading or lagging value. The lead/lag information gives you the sign for the angle between voltage and current. They will show the equation as S=sqrt3*VI and use the lead/lag information to apply the correct sign to the angle.
In the practice problems, what you are most likely seeing is that they give you a kVA value and a power factor value. They will also usually tell you if the pf is a leading or lagging value. The lead/lag information gives you the sign for the angle between voltage and current. They will show the equation as S=sqrt3*VI and use the lead/lag information to apply the correct sign to the angle.
