AC Electrics -Alternators — Page 194, Lesson 206

I want to walk you through brushless alternators, which are a key development in aircraft AC electrical systems. Let's start with the basic construction. A brushless alternator has an exciter generator mounted on the same shaft as the main generator. So you have two generators on one spinning shaft — the exciter and the main generator. The purpose of the exciter generator is to provide a current for the main generator's rotating field. Think of it as a small generator that feeds the big generator. Now, here's where the name "brushless" comes from. The exciter produces AC in its armature, but the main rotor field needs DC. So a rotating rectifier is mounted on the shaft — it converts the AC produced in the exciter armature into the DC required for the main rotor field supply. Because the rectifier spins with the shaft, there's no need for brushes or slip rings to get DC into the rotating field. That's the whole point. Voltage regulation is done by controlling the exciter field strength. By varying the current in the exciter's stationary field coil, you control how much current the exciter generates, which in turn controls the current strength at the main rotor field coil, and therefore the main alternator's output voltage. So the regulator doesn't touch the high-power main output directly — it works through the small exciter field. Brushless alternators have three main advantages over brushed types. First, they are very reliable — fewer moving contact points means fewer failure modes. Second, there are no brush wear problems — brushes on a conventional alternator wear down and need inspection and replacement, and they also produce carbon dust that can cause problems. Third, they have a high power to weight ratio — you get more electrical power for the weight of the unit. Modern brushless alternators may have a third generator on the same shaft, called a Permanent Magnet Generator, or PMG. The PMG provides excitation current for the exciter generator. So the chain becomes: PMG feeds the exciter, the exciter feeds the rotating rectifier, and the rotating rectifier feeds the main rotor field. The PMG uses permanent magnets, so it doesn't need any external power to start producing — it's self-starting. The typical output from a brushless alternator on a large aircraft is 115 V / 200 V / 400 Hz / 3 phase. Let me unpack that. 115 V is the phase-to-neutral voltage, 200 V is the phase-to-phase voltage, 400 Hz is the frequency — much higher than the 50 or 60 Hz you see in household mains — and 3 phase means three separate AC outputs spaced 120 degrees apart. There are two basic types of brushless alternator. First, externally excited — this type has no residual magnetism in the exciter. That means if you lose the external excitation supply, the alternator stops producing. Second, self-excited — this type has some residual magnetism in the exciter. That residual magnetism allows the alternator to build up its own voltage when it starts spinning, without needing an external source to kick it off. Now let's talk about Frequency Wild Alternators. If an alternator is driven directly from the engine gearbox — meaning there's no constant-speed drive or electronic speed regulation in between — then its speed varies directly with engine speed. Since frequency is directly proportional to rotational speed, the output frequency varies with engine RPM. An output from such a generator is said to be Frequency Wild. There's an important operational limitation here. The note says: The connection of two frequency wild generators in parallel is not possible. Why? Because for two AC generators to run in parallel, their frequencies must be exactly matched and stay matched. With frequency wild generators, the frequencies are constantly changing with engine speed, and you can't keep two engines at exactly the same RPM at all times. So you cannot parallel them — they must operate independently, each feeding its own separate bus.