AC Electrics - Alternators — Page 186, Lesson 199

I want to walk you through the start of AC Electrics, specifically alternators. Let's begin with the big picture. As passenger aircraft grew larger, they needed more electrical power to run bigger equipment. That's why most large commercial aircraft today use alternating current, or AC, distribution systems instead of just DC. The industry standard for constant-frequency AC aircraft is this: 115 volts, 200 volts, 400 hertz, three-phase. Let me unpack that. 115 volts and 200 volts are the voltage levels used — 115 volts is the phase-to-neutral voltage, and 200 volts is the phase-to-phase voltage. 400 hertz is the frequency, much higher than the 50 or 60 hertz you'd see in a household mains supply. And three-phase means the AC is delivered on three separate conductors, each with its own alternating waveform offset from the others. Now, what about the DC equipment that still needs power? That's handled by transformer rectifier units, or TRUs. A TRU takes the AC from the distribution system and converts it to 28 volts DC. The battery is retained purely for emergency use — it's not the primary DC source during normal operation. The AC distribution system is laid out with a hierarchy of bus bars, very similar to what you'd see in a DC aircraft. The key design priority is that the system can cope with a failure while losing the minimum number of electrical services. So if one generator fails, the bus bar hierarchy ensures that essential loads stay powered. Also, just like in a DC system, AC generators can be operated in parallel if the designer chooses to do so. That means multiple generators can feed the same bus bar simultaneously. Now let's talk about the generators themselves. In a DC generator, the rotating part is always the armature — that's the part where the voltage is induced. In an AC generator, that's not generally true. Another name for an AC generator is an alternator. There are two types of alternator: rotating armature and rotating field. Let's start with the rotating armature alternator. Its construction is similar to a DC generator — the armature rotates inside a stationary magnetic field. As it rotates, an EMF — electromotive force, or voltage — is induced into the armature. But instead of being converted to DC by a commutator, as in a DC machine, this EMF is taken out as AC through slip rings. Slip rings are continuous circular contacts that allow the AC to be collected from the rotating armature without needing a commutator. However, the rotating armature design is only used in very small output alternators. It is not generally used for supplying AC systems on aircraft. The reason is that for high power, you'd have to pass large currents through the slip rings, which causes wear and limits reliability. That's the foundation. Next, we'll look at the rotating field alternator, which is the type you'll actually find on aircraft.