AC Electrics - Introduction to AC — Page 174, Lesson 177

I want to walk you through the next topic in AC Electrics: Inductive Reactance and then Capacitance in AC Circuits. Let's start with inductive reactance. In an AC circuit containing inductance, the opposition to current flow isn't called resistance — it's called Inductive Reactance. The reason it's called reactance rather than resistance is important: the effects of inductance depend not only on the value of the inductance itself, but also on the frequency of the supply. So it's a frequency-dependent opposition. Inductive reactance is measured in ohms, just like resistance, and it's given the symbol XL — capital X with a subscript L. To calculate inductive reactance, we use this formula: XL = 2 π f L Let me break that down. π is a constant — roughly 3.14. f is the frequency of the AC supply, measured in hertz. L is the inductance, measured in henries. Now, what does this formula tell us? As frequency increases, the value of inductive reactance increases. That means the circuit current would decrease. Conversely — and this is the more important point for you as a pilot — as the circuit frequency decreases, the inductive reactance decreases, and the circuit current increases. So lower frequency means more current can flow through an inductive circuit. Now let's move to Capacitance in AC Circuits. Capacitance is the ability of a circuit to store an electrical charge. A device used to introduce capacitance into a circuit is called a Capacitor. A capacitor consists of two plates separated by a material called a dielectric. You can see this in Figure 11.10. Dielectrics can be, amongst other things, air, mica, or waxed paper. Three factors affect how much charge a capacitor can hold: 1. The area of the plates — larger area means more charge storage. 2. The distance between the plates — closer plates allow more charge. 3. The material used to separate the plates — the dielectric itself affects the capacity. A capacitor stores an electric charge, much like a hydraulic accumulator stores fluid under pressure. But first, it needs to be charged. When you connect a capacitor to a battery, as shown in Figure 11.10, electrons are removed from the plate connected to the positive terminal of the battery and added to the plate connected to the negative terminal. Conventional current flow is from positive to negative. This process continues until the plates become saturated — meaning they can't hold any more charge — and no more current will flow. At that point, the potential difference between the plates is at its maximum, and the capacitor is fully charged. Its voltage is equal to the battery voltage. If you now move the switch to a mid position, the charging circuit is disconnected, and the capacitor will hold its charge indefinitely — similar to an accumulator. In practice, there will be some leakage, which allows the capacitor to discharge over a period of time, but ideally it holds the charge. Now, if you use the switch to connect the capacitor to an external circuit, the capacitor will discharge. Current will flow around the circuit in the opposite direction until the potential difference across the plates has become equal. Notice two key points here: first, the capacitor discharges in the opposite direction to which it was charged. Second, electrons do not pass between the plates through the dielectric — the charge is stored on the plates, not flowing across the gap. That's the foundation for understanding how a capacitor behaves in a DC circuit. In the next part, we'll look at how it behaves in an AC circuit, which is where it becomes really useful for aircraft electrical systems.