AC2 Fundamentals Unit 4 – Power in AC Circuits
In circuits containing resistance and reactance (XL or XC), the generator must supply both the
real power (P) and the reactive power (Q).
To determine this generator power, called the apparent power (S), multiply the generator
voltage (VGEN) by the circuit current (I). S = VGEN x I
Only a percentage of the generator's apparent power (S) is converted to real power (P) and is
dissipated in the circuit resistance as heat.
The generator also supplies reactive power (Q) to the reactive components, which alternately
consume and supply power (Q).
The ratio of real power (P) to apparent power (S) is the power factor (PF).
PF = P/SThe power factor (PF) is a measure of the real power (P) actually delivered to the circuit from the
generator.
NEW TERMS AND WORDS
real power (P) - power that is converted from one form of energy to another; the power
dissipated in a resistor as heat; the product of the applied voltage and circuit current in a
resistance, expressed in watts; also called true power.
reactive power (Q) - power not converted into another form of energy; the product of the voltage
across and the current through a reactive component such as an inductor or a capacitor. The unit
of measure for reactive power is the voltampere reactive (var).
apparent power (S) - the product of an rms voltage across and an rms current through an
impedance (Z). The unit of measure for apparent power is the voltampere (VA).
power factor (PF) - the ratio of the true power to the apparent power in a circuit; the cosine of
the phase angle between circuit current and applied voltage.