AC Electrics - Alternators — Page 206, Lesson 218

I want to walk you through how reactive load sharing works in a paralleled AC generator system, and then we'll look at alternator cooling. Let's start with the key component: the mutual reactor. This is a phase-shifting transformer. Its job is to make sure the error detector only sees the part of the generator current that is 90 degrees out of phase with the voltage. That 90-degree-out-of-phase component is what we call the reactive load — the part of the current that does the magnetising work in the system, not the real power that turns the propellers. So the mutual reactor isolates that reactive current component. Once the error detector has that signal, the system amplifies it and sends correcting signals to the generator field circuit. The correcting signals do two things: they increase the voltage on the generator that is running low, and they reduce the voltage on the generator that is running high. That action balances the reactive load between the paralleled generators. Now, who actually controls this balancing? The Voltage Regulators. Reactive load sharing is controlled by the Voltage Regulators matching the voltage outputs — and that matching is done by adjusting field excitation. So when I say 'field excitation', I mean the current flowing through the generator's field windings, which directly controls the output voltage. The Voltage Regulators raise or lower that excitation to equalise the reactive loads. Let me show you the circuit. You can see three generators labelled GEN1, GEN2, and GEN3. Each generator has current values shown — for example, one leg shows 5 amps, another shows 4 amps, and so on. The numbers illustrate how the load sharing circuit distributes the reactive current among the three generators to keep them balanced. Now, let's talk about Load Sharing General — the bigger picture. It is typical to run three-generator and four-generator systems in parallel. However, most modern twin-engine aircraft with two generators do not run them in parallel. Instead, they run the generators in isolation, using what is called a Split Bus System. That means each generator powers its own busbar independently, and they are not connected together to share load. In those three- and four-generator systems, the load sharing circuits operate exactly as I just described, but they are extended to cater for the required number of generators. So if you have four generators, the same principle applies — the mutual reactors and Voltage Regulators work together for all four. There is an important rule to remember: if any generator in a parallel system is not connected in parallel — meaning it is isolated or off the bus — then it will not be connected to the load sharing circuits either. The load sharing circuits only act on generators that are actually paralleled. Let me give you a memory aid that the book provides. It separates the two types of load sharing: - Real load sharing is about speed, frequency, and torque — and that is controlled by the CSDU, the Constant Speed Drive Unit. - Reactive load sharing is about excitation current and field current — and that is controlled by the Voltage Regulator. So real load sharing balances the mechanical power; reactive load sharing balances the magnetic or electrical excitation. Finally, let's look at Alternator Cooling. The alternator stator windings generate heat because of the current flowing through them. That heat has to be removed, so some form of cooling system is required. The type of cooling depends on the generator system: - Systems with frequency wild generators — generators whose output frequency varies with engine speed — or systems with constant frequency generators that have a separate CSDU typically use ram air cooling in flight. Ram air is the outside air forced into the cooling ducts by the aircraft's forward motion. On the ground, when there is no ram air, they need some other means to induce an airflow — usually an electric fan or a blower. - Integrated Drive Generators — IDGs — or Integrated Drive Units — IDUs — use a different approach. They use their own oil to cool the stator windings. That oil picks up the heat from the stators and is then cooled in its own oil cooler, just like engine oil is cooled in an oil cooler. So to summarise: the mutual reactor isolates the reactive current, the Voltage Regulators adjust field excitation to balance reactive load, three- and four-generator systems parallel while most twins use a split bus, and cooling is either ram air or oil depending on the generator type.