AC Electrics -Introduction to AC — Page 158, Lesson 165

Alright — let's start the AC Electrics chapter. I want to walk you through why large modern transport aircraft use alternating current, or AC, instead of direct current, DC. There are several clear advantages, and I'll take them one by one. First, AC generators are simpler and more robust in construction than DC machines. That means fewer moving parts and less complexity, which is always good for reliability in an aircraft. Second, the power-to-weight ratio of AC machines is better than comparable DC machines. In aviation, weight is everything — a lighter system that delivers the same power is a big win. Third, the supply voltage can be converted to a higher or lower value with almost 100% efficiency using transformers. You can't do that easily with DC — transformers only work with alternating current. That near-perfect efficiency means very little energy is lost when you change voltage levels. Fourth, any required DC voltage can be obtained simply and efficiently using transformer rectifier units, or TRUs. So even though the primary system is AC, you can still produce the DC voltages that some equipment needs, without a heavy separate DC generator. Fifth, three-phase AC motors — which are simpler, more robust, and more efficient than DC motors — can be operated from a constant frequency source, meaning the AC generators themselves. Three-phase motors are the workhorses of aircraft systems like hydraulic pumps and air conditioning packs. Sixth, AC machines do not suffer from the commutation problems associated with DC machines. Commutation is the process of reversing current direction in a DC motor's rotor using brushes and a commutator — it causes wear, sparking, and reliability issues, especially at high altitude where the air is thinner and sparking can be more dangerous. AC machines avoid that entirely, so they are more reliable. Seventh, high-voltage AC systems require less cable weight than comparable power low-voltage DC systems. If you can transmit the same power at a higher voltage, the current is lower, and you can use thinner, lighter wires. That saves significant weight across the whole aircraft. Now, let's look at the nature of alternating current itself. If the electrons flowing in a circuit move backwards and forwards about a mean position — that is, they oscillate rather than flowing steadily in one direction — then the current produced is called alternating current, or AC. The simple AC generator shown in Figure 11.1 illustrates the basic principle. A loop of wire, called the armature, is rotated in a magnetic field. As it rotates, the loop experiences a continuously changing magnetic flux through it. Because the flux is always changing, a voltage is induced in the loop as long as rotation continues. That induced voltage is what drives alternating current in the circuit. So, to sum up: AC is preferred in large modern transport aircraft because generators are simpler and more robust, the power-to-weight ratio is better, voltage can be transformed with nearly 100% efficiency, DC can be obtained easily via TRUs, three-phase AC motors are more efficient and reliable, there are no commutation problems, and high-voltage AC saves cable weight. And the fundamental nature of AC is that the electrons oscillate back and forth, produced by a rotating loop of wire in a magnetic field.