AC Electrics - Alternators — Page 201, Lesson 211

I want to walk you through two important topics in AC Electrics: self-excited generators, and then load sharing or paralleling of constant frequency alternators. Let's start with self-excited generators. A self-excited generator is one that has some permanent magnetism built into its exciter generator. On initial rotation — when the shaft first starts turning — the flux from these Stationary Permanent Magnets causes an induced AC voltage, and therefore current, to flow in the rotor of the exciter. That rotor output is then fed directly to a rotating rectifier. The rotating rectifier converts that AC into DC, and that DC supply feeds the rotating field coils of the main generator. So the main generator's field gets its initial DC from the permanent magnets in the exciter, through the rotating rectifier. Now, the output of the main generator stator is tapped — meaning a portion of that AC output is taken off — and that tapped supply is fed back to control the exciter field. This provides a regulated supply to the exciter field, which enables the voltage of the whole system to be controlled. So once the generator is running, it regulates its own output voltage by adjusting the exciter field current from its own stator output. Let's move to load sharing or paralleling of constant frequency alternators. When you run two or more constant frequency alternators in parallel — meaning their outputs are connected to the same bus — they must be controlled so that each one takes a fair and equal share of the load. This process is called load sharing or paralleling, and it requires that two parameters are regulated: Real Load and Reactive Load. Real Load is the actual working load output available for supplying the various electrical services on the aircraft. It is measured in kilowatts — that's real power or true power. Real Load is directly related to the mechanical power or torque being supplied to the alternator drive by its prime mover. The prime mover is either the engine or the Constant Speed Drive Unit, the CSDU. So if you want to share Real Load equally between two alternators, you control the Constant Speed Drive Unit and adjust the torque at its output shaft. If the torque of two or more CSDUs is equal, then the real load taken by each generator is the same. Reactive Load is the so-called wattless load. It is the vector sum of inductive and capacitive currents and voltages, expressed in kVAR — that stands for Kilovolt-Amperes Reactive. Reactive Load Sharing is achieved by controlling the Voltage Output, specifically the Exciter Field Current, of each generator that is connected in parallel. If their voltages are identical, then the reactive load on each generator will be the same. So to summarise: Real Load sharing is about torque from the CSDU; Reactive Load sharing is about voltage from the exciter field current. Both must be balanced for proper paralleling.