AC Electrics -Alternators — Page 201, Lesson 209

I want to walk you through the drive disconnect system for alternators, and then introduce a completely different type of AC power system — the Variable Speed Constant Frequency system. Let's start with the Drive Disconnect Unit, which is also called a Dog Clutch Disconnect. This is a safety mechanism. In the unlikely event of a malfunction in the CSDU — that's the Constant Speed Drive Unit — or in the alternator itself, the engine's input drive to the CSDU can be physically disconnected. When that happens, both the drive unit and the alternator become stationary. The purpose is to eliminate any chance that the malfunction inside the generator system could affect engine performance. You're protecting the engine from a failed generator. The disconnection can be carried out at any time while the engine is running. However — and this is an important operational limitation — reconnecting the drive may only be done manually on the ground, after the engine has been shut down. You cannot reset it in flight. Let me describe how the disconnect is operated. The pilot selects a momentary action switch called the 'Drive Disconnect' switch. That switch operates a solenoid, and the solenoid causes a mechanical separation of the input drive from the engine to the constant speed unit. It's a one-shot mechanical action in flight. Exceptionally, some aircraft may allow automatic disconnection of the generator drive. In those cases, a Generator Control Unit — a GCU — can trigger the disconnect under certain fault conditions, without the pilot having to press the switch. Now, there's a special type of Integrated Drive Generator — IDG — that you should know about. Some IDGs are known as Permanent Magnet Generators, or PMGs. In this design, the generator actually has three separate generators all mounted on the same shaft. Let me break that down. First, there is a permanent magnet generator, which provides the initial excitation. That initial excitation goes to the exciter generator, and the exciter generator controls the main generator field. So the chain is: permanent magnet generator → exciter generator → main generator field. This type of generator is invariably controlled by a Generator Control Unit — again, a GCU. Now let's shift to a completely different approach to AC power: the Variable Speed Constant Frequency system, abbreviated VSCF. A VSCF system uses what is called a frequency wild generator. That means the generator is driven directly by the engine, and its output frequency varies with engine speed — it's not constant. The key innovation is that this variable frequency output is then electronically converted into a constant frequency supply of 400 Hz. This conversion is achieved by a unit called the Generator Converter Control Unit, or GCCU. Here is how the GCCU works, step by step. First, it passes the variable frequency supply through a full wave rectifier. At the rectifier, the AC is rectified — turned into DC — and then filtered. After that, the DC goes to an inverter, where it is formed into a 115 volt, 200 volt, 400 Hz, three-phase supply. So the output is a clean, constant-frequency three-phase AC supply. The big advantage here is that this system eliminates the need for a hydromechanical Constant Speed Drive Unit — the CSDU — and all its associated controlling mechanisms. That improves reliability and flexibility in the installation, because the electronic circuit does not necessarily have to be located in the engine compartment with the generator. You can place the GCCU somewhere cooler and more accessible. VSCF systems are currently fitted to Boeing 737 aircraft and to several military aircraft. So this is a real-world system you will encounter.