AC Electrics -Alternators — Page 201, Lesson 213

I want to walk you through what happens when we connect AC generators—alternators—in parallel. This is a critical part of operating multiple generators on an aircraft, and it introduces two separate control systems you need to understand. First, let's talk about the two separate load sharing circuits. When two or more generators are paralleled—meaning connected together to share the electrical load—we need to control two different kinds of load: real load and reactive load. Real load is the actual power doing useful work, like driving motors or heating elements. Reactive load is the power that sustains magnetic fields in inductive equipment like transformers and motors. To manage these separately, there are two distinct circuits: one that detects and controls real load, and a completely separate one that detects and controls reactive load. Now, here's a very important point I want to stress: until a generator is actually connected in parallel with one or more other generators, it will not be connected into these load sharing circuits at all. Think about engine start—only one alternator may be on line at that time. While constant frequency alternators are operating as individual units, their real load and reactive load sharing circuits are simply not connected. They only come into play once the generator is paralleled with another. Before we even attempt to parallel two alternators, we have to understand what they are. AC generators—alternators—are synchronous machines. That means their output frequency is locked to their rotational speed. When you operate them in parallel, they will lock frequencies together. The system frequency then becomes that of the alternator with the highest load. So if one alternator is carrying more load, it effectively pulls the frequency of the whole system. But here's where we have to be very careful. If two alternators are at different frequencies before they are connected in parallel, damage can occur. One generator will try to slow down and the other will try to speed up as they fight to synchronise. That mechanical struggle can cause serious damage. So they must be at the same frequency before paralleling. That's not the only condition. They must also have the same phase sequence. Phase sequence means the order in which the three phases—A, B, and C—reach their peak voltages. At any point in time, phase A on the first generator must be identical to phase A on the second generator, and the same for phases B and C. If the phase sequences don't match, you'll get a short circuit between phases when you connect them. Finally, the voltage of each generator being paralleled must also be the same. So to summarise the conditions required before paralleling: same frequency, same phase sequence, and same voltage. These are the three essential conditions, and they're illustrated in Figure 12.10. That figure shows you the conditions required before paralleling. Make sure you understand that all three must be satisfied before you close the paralleling contactor.