An AC circuit has three types of loads: Pure resistance, inductive and capacitive. True power (P) is the power consumed by the resistance elements only, and as someone pointed out P=IR. That would be exactly the same a calculating the power of a DC circuit. However, the apparent power (U) takes into account all resistances plus the inductvive and/or capacitive reactances. (reactance is kinda the same as resistance) A circuit can be predominantly inductive or predominantly capacitive. When you sum all the inductive and capacitive reactances, you end up with either a capacitive reactive power or an inductive reactive power.
To arrive at the apparent power (U), you must vectorially add the true power (P) and the reactive power (Q) (inductive or capacitive). Apparent power is always greater than or equal to reactive power. If you can imagine a graph, reactive power increases along the +ve x-axis, and inductive power goes up along the +ve y-axis and capacitive power goes down along the -ve y-axis. Apparent power points either up and to the right (inductive) or down and to the right (capacitive).
True power (P) is measured in watts (W). Apparent power (U) is measured in volt-amps (VA) and reactive power (Q) is measured in volt-amps-reactive (VAR).
The power factor (PF) is the ratio of true power to apparent power. PF=P/U. It is basically the circuits efficiency. It is not a costant value (like 0.8) but depends on the phase angle. PF=cosine (phase angle). PF=1 for purely resistive circuits, and PF=0 for purely reactive circuits. You might be thinking of the RMS voltage/current which is 0.707 times the maximum voltage/current.
Last edited by GearDoor; 30th November 2004 at 17:54.