DC Electrics - Aircraft Electrical Power Systems — Page 126, Lesson 127

I want to walk you through the next part of the DC electrical system. We've already covered how a generator or alternator normally supplies power to the bus bar. Now, let's look at what happens when that generator's output voltage falls. When the output voltage of the generator falls below the battery voltage, the current flow reverses. Instead of the generator supplying the bus, current now flows backward from the battery toward the generator. The falling voltage of the generator causes the magnetic influence of the voltage coil inside the cut-out relay to reduce. At the same time, because the current flow through the current coil is reversed, it reverses the magnetic field produced by that current coil. This reversed field opposes the field still being produced by the voltage coil. The net effect is that the magnetic force holding the contacts closed weakens enough that the spring can pull the contacts open. That disconnects the generator from the bus bar and prevents reverse current from flowing into the generator, which could damage it. Now, let's move to rectifiers. In an alternator, the rectifiers are mounted in the alternator end frame. Their job is to convert the alternating current, or AC, that the alternator produces into direct current, or DC, which the aircraft's systems need. They also permit current to flow out from the alternator but not into it from the battery. They achieve this by having a low resistance in the direction of normal current flow and a high resistance in the opposite direction. So they act as one-way valves for electricity. Next, we have inverters. These devices do the opposite of rectifiers — they convert DC into AC. There are two main types. First, the static inverter. A static inverter is a solid-state device — meaning it uses semiconductor components rather than moving parts — that converts DC into constant-frequency AC. A typical input to a static inverter would be 18 to 30 volts DC, and the output would be 115 volts AC at a frequency of 400 hertz. Inside, the circuitry contains standard electrical and electronic components such as oscillators, diodes, transistors, capacitors, and transformers. Second, the rotary inverter. A rotary inverter converts DC to AC by using a constant-speed DC motor to physically drive an alternator. Because the motor runs at a constant speed, the alternator it drives produces constant-frequency AC. Now, let's talk about the generator differential cut-out. This component is fitted in a multi-engine aircraft. Its purpose is to prevent circulating currents between a generator that is already on line — meaning it's already connected to the bus and supplying power — and one that is coming on line. The on-coming generator cannot be switched onto the bus until its output voltage is 2% above the output voltage of the generator that is already on line. That 2% difference in potential is measured between the on-coming generator's output and the battery bus bar. Next, the generator or alternator warning light. This light on the flight deck indicates to the pilot that the generator or alternator voltage has fallen below battery voltage. Illumination of the light is usually associated with the generator cut-out position or a reverse current detector — so it tells you the generator has been disconnected or that reverse current is being detected. Finally, the generator or alternator master switch. This switch enables the pilot to electrically isolate the generator or alternator. When the pilot opens the master switch, it breaks the generator field circuit or the alternator exciter circuit. With that circuit broken, the electrical output falls to its residual level, which is virtually zero. So the master switch gives the pilot a positive way to take the generator or alternator off line.