AC Electrics - Introduction to AC — Page 180, Lesson 186

I want to walk you through the different kinds of power we deal with in AC circuits, starting from the purely reactive cases and moving into a practical circuit that has both resistance and inductance. First, let's define a key term. In any AC circuit, the product of the RMS voltage and the RMS current is called the apparent power. It's measured in VA (volt-amperes) or kVA (kilovolt-amperes). Apparent power is what you get if you simply multiply volts and amps without considering the phase relationship between them. Now, let's look at a purely inductive circuit. In this case, the current lags the voltage by exactly 90 degrees — that's the phase angle. Because of this 90-degree shift, the positive power and the negative power over one complete cycle are equal. That means the true power — the real power actually consumed — is zero. No real work is done. All the power in this circuit is reactive power, measured in VAR (volt-ampere reactive) or kVAR. The caption on Figure 11.16 puts it clearly: "No real power generated when current 90° out of phase with voltage." A purely capacitive circuit behaves very similarly, but with one important difference. Here, the current leads the voltage by 90 degrees instead of lagging. Once again, the positive power equals the negative power, so no real power is absorbed. The power required is only overcoming the capacitive reactance. So again, all the power is reactive — VAR or kVAR. And just like before, the RMS volts times RMS amps gives you the apparent power in VA or kVA. Now, let's move to a practical AC circuit. In the real world, an AC circuit will always have some resistance and some inductance — the amounts depend on how the circuit is constructed. It may also have capacitance if capacitors are fitted. Calculating power in such a circuit depends on the ratio of resistance to inductance or capacitance. And remember: inductance and capacitance have opposite effects. If both are present, the effects of one will partially cancel the other, leaving the circuit either more inductive or more capacitive depending on which is dominant. But the resistance is always there. Figure 11.18 shows a circuit with equal amounts of resistance and inductance. Notice the phase angle is 45 degrees. And critically, the amounts of positive power and negative power are not equal anymore — there is more positive power than negative power. The true power axis is now above the zero axis. If you draw a line dividing the power curve into two equal areas, that line shows the average power consumed in that circuit. That average power is the true power, measured in kW (kilowatts). So in a circuit with both resistance and inductance, we have three distinct quantities: - Apparent power (kVA) — RMS volts times RMS amps. - Reactive power (kVAR) — the power required to overcome the inductive reactance. - True power (kW) — the actual power consumed. And the relationship between them is given by the power factor. The power factor equals true power divided by apparent power — that is, kW divided by kVA. In this circuit with a 45-degree phase angle and current lagging voltage, the power factor tells you what fraction of the apparent power is actually doing real work.