DC Electrics - Aircraft Electrical Power Systems — Page 126, Lesson 122

I want to walk you through the start of DC Electrics — Aircraft Electrical Power Systems. This is where we lay the foundation for how electrical power is generated, distributed, and controlled on an aircraft. Let’s begin with the power system for a single-engine aircraft. It consists of a generator or an alternator, along with the control and indication equipment needed to supply all the electrical power once the system is on line. The term on line means that the generator or alternator has been switched into the electrical system and is actually supplying power to the system — not just spinning, but actively feeding the bus. For multi-engine aircraft, two or more generators or alternators are installed in parallel. That means they work together to share the load. The ampere capacity — the current-carrying ability — of an aircraft electrical system is determined by the number of power-consuming devices fitted. So the more electrical equipment you install, the higher the required capacity. Now, let’s look at how the circuit is completed. There are two main types of wiring systems. First is the Dipole or Two Wire System. A dipole system is required when an aircraft is made of a non-conductive material — for example, a composite or wooden airframe. Current needs a complete circuit to flow. So you need a negative wire to connect the load to the negative side of the generator, and a positive or ‘live wire’ to connect from the bus bar — that’s the distribution point — to the load. Two wires for every circuit. Second is the Single Pole (Unipole or Earth Return) System. This is the most common type of system on an aircraft with metal construction. Here, the metal airframe itself is used as the negative conductor — it completes the circuit for current flow. The negative side of the generator is connected to an ‘airframe earth’, and the negative side of each load is also connected to the airframe earth. So you only need one wire — the positive live wire — and the return path is through the metal structure. Now let’s move to the sources of electrical power: Generators and Alternators. Both are used to convert mechanical energy — from the engine — into electrical energy. But they do it differently. A generator produces direct current, DC, by using a rotating armature, a stationary field, and a commutator. That’s the classic DC generator design described in the previous chapter. An alternator, on the other hand, produces alternating current, AC, by using a rotating field and a stationary armature. If you need to convert the AC output of an alternator to DC — which is common in aircraft DC systems — a diode rectifier is used, fitted in the end frame of the alternator. Most modern light aircraft have a direct current system that is powered by an alternator. Why? Because of the RPM relationship. The full power output of a generator is closely related to the RPM of the engine and is usually attained with the engine running at half speed. In contrast, the full power output of an alternator can be attained at slow running — that’s one obvious advantage an alternator has over a generator. Also, the generator is driven at a speed which is approximately three times that of the engine — so it needs to be geared up. Finally, we have Voltage Regulators. The voltage regulator maintains the output voltage of the generator or alternator at a constant value, regardless of engine RPM or electrical loads. It does this by controlling either the current flow in the field coils of a generator, or the current flow in the exciter field of an alternator. The excerpt introduces this as the basic voltage regulator set — we’ll build on that as we go. So to summarise: we have two wiring systems — dipole for non-conductive airframes, unipole for metal airframes. We have generators producing DC with a commutator, and alternators producing AC with a rectifier. Alternators have the advantage of full output at low RPM. And voltage regulators keep the output steady no matter what the engine or loads are doing.