AC Electrics - Alternators — Page 210, Lesson 225

I want to walk you through two important AC power sources that you'll encounter on modern aircraft: the Static Inverter and the Ground Power Unit. Let's start with the Static Inverter. A Static Inverter is a Solid State Device — that means it uses electronic components like transistors and diodes, not moving parts or rotating machinery. Its job is to supply the aircraft with 115 volts, 200 volts, 400 Hertz, three-phase AC power. But notice the phrase "for the limited operation of instrument and radio services." This isn't a primary power source; it's a backup or emergency supply for essential avionics when the main alternators aren't available. Where does the Static Inverter get its power? It's powered by the aircraft batteries or from an essential DC bus bar. So it takes DC power — either directly from the battery or from a critical DC bus that stays live even in an emergency — and converts it into the specific AC output I just described. Now let's move to the Ground Power Constant Frequency Supply System. When an aircraft is on the ground and its engines aren't running, we need an external source to power the electrical systems for maintenance, pre-flight checks, or engine start. The standard modern Ground Power Unit output is exactly the same as the Static Inverter's: 115 volts, 200 volts, 400 Hertz, three-phase. When you plug that ground power unit into the aircraft, it can be used to supply all the aircraft electrical services — not just limited services like the inverter, but the entire electrical distribution system. However, there are critical safety protections built into the ground power unit circuitry. These are automatic protection systems, and they ensure three things. First, ground power cannot be connected to the aircraft distribution system if the system is already being supplied by its own alternators. You don't want two sources trying to power the same bus at the same time unless they're properly synchronised. Second, ground power cannot be connected if the phase sequence of the supply is incorrect. The three phases have to arrive in the correct order — A, B, C — or the motors and generators on the aircraft could run backwards or be damaged. Third, if overvoltage occurs, the ground power will be rejected and switched off at source. That means the ground power unit itself shuts down to protect the aircraft's systems. Now let's look at typical controls and indications for a three-engine paralleled system. Figure 12.14 shows this arrangement. The panel uses what are called "switch lights" — these are a combination of a switch and an indicator in one unit. They can have either a momentary action or an alternate action. For example, a momentary action: you push once to activate a generator disconnect switch. An alternate action: you push once to switch something "on", then push a second time to switch it "off" — like a galley power on/off switch. The indicator in the switch light shows either the switch position or the system status. Each engine on this three-engine aircraft drives a constant frequency generator. That generator is called an Integrated Drive Generator, or IDG. The IDG maintains a constant output frequency regardless of engine speed. On the panel, you'll see oil temperature indications for each generator, along with overheat and low pressure warning lamps located right in the disconnect switch. And that disconnect switch is guarded — it has a physical cover or shield — to prevent inadvertent operation. You don't want to accidentally disconnect a generator in flight.