AC Electrics - Introduction to AC — Page 174, Lesson 180

Let’s talk about what happens when we put a capacitor into an AC circuit — because it behaves very differently from what you saw in a DC circuit. When a capacitor is fitted in an AC circuit, as shown in Figure 11.11, the capacitor will be constantly charging and discharging. Why? Because the applied voltage and the current flow are constantly reversing polarity and direction — that’s the nature of AC. As the applied voltage rises, the capacitor charges. As the applied voltage falls, the capacitor discharges current back into the circuit, but in the opposite direction, and its own voltage falls along with it. This charging and discharging cycle has a very important effect: it shifts the voltage out of phase with the current. In a purely capacitive circuit — meaning there is no resistance or inductance to speak of — the current will lead the voltage by 90 degrees. You can see this phase relationship illustrated in Figure 11.12. Now, let’s define the unit of capacitance. The unit is the farad, and a capacitor is given the symbol C. Here’s the formal definition: if a current of 1 ampere flowing for 1 second creates a potential difference of 1 volt between the plates of a capacitor, then that capacitor has a capacitance of 1 farad. But because of the values we actually deal with in aviation electrical systems, a 1 farad capacitor is not a practical size — it’s enormous. The more common units you’ll see are the microfarad, which is one millionth of a farad, and the picofarad, which is one millionth of a microfarad. Now, just as an inductor opposes changes in current, a capacitor opposes the flow of current in its own way. That opposition is called capacitive reactance. It is measured in ohms, just like resistance, and it is given the symbol X sub C — X with a subscript C. The amount of capacitive reactance depends on two things: the frequency of the AC supply, and the value of the capacitor in farads. You can calculate capacitive reactance using this formula: X sub C equals 1 divided by 2 pi f C. Let me read that clearly: X sub C = 1 over (2 π f C). In that formula, f is the frequency in hertz, and C is the capacitance in farads. From this formula, you can see an important relationship: as frequency increases, the value of capacitive reactance decreases. That means the circuit current will increase. Conversely, if frequency decreases, capacitive reactance increases, and the circuit current will decrease. So in a capacitive AC circuit, higher frequency means less opposition and more current; lower frequency means more opposition and less current. That’s the direct opposite of what you saw with inductive reactance, where higher frequency meant more opposition.