AC Electrics - Alternators — Page 206, Lesson 221

I want to walk you through the next part of AC Electrics — we're looking at the systems that let us safely connect alternators together and monitor their performance. First, let's talk about Synchronizing Units. Before an alternator can be connected to a bus bar that's already common to another alternator, three things must be within very strict limits and in the same order: its voltage, its frequency, and its phase sequence. The Synchronizing Unit is the device that checks these values and only allows connection to a common bus bar when they're all within limits. There are two methods in use: Automatic Control and the Manual (Dark Lamp) Method. With Automatic Control, the system will not allow the BTB — that's the Bus Tie Breaker — or the GCB, the Generator Control Breaker, to close and parallel the generators until the voltage, frequency, and phase sequence of the oncoming generator are within limits. This can be achieved by circuitry inside a bus bar protection control unit, or inside the Generator Control Units (GCUs) of a modern IDG system — Integrated Drive Generator. The Manual (Dark Lamp) Method is much older but still remains in use on a few aircraft. Here, Synchronizing Lights on the alternator control panel show when there are differences between the phases of two supplies. Synchronization is indicated when the lamps are "dark" — meaning no voltage difference across them — and at that point the BTB or GCB can be closed manually by the pilot using a switch. Next, the Generator Failure Warning Light. This light will illuminate when its associated GCB is tripped — that is, when the generator's breaker opens. The Centralized Warning System will operate simultaneously with the Generator Warning Light, and in some aircraft, Aural Warnings are also generated — so you get both a visual and an audible alert. On aircraft with electronic systems management display units, the failure will be shown on the display along with the associated schematic diagram. Now let's move to Load Meters. In paralleled alternator systems, we use kW / kVAR Meters to indicate the Real Power, measured in kilowatts (kW), or the Reactive Power, measured in kilovolt-amperes reactive (kVAR). Only one meter may be used to indicate both parameters — a selector switch determines which of the two is shown. Typically, the switch is selected so that the kW output is normally displayed. Let me define those two types of load carefully. Real Load is the part of the alternator output which is available to do work at the bus bar — that's the power that actually turns motors, lights lamps, and runs equipment. Reactive Load is the part of the alternator output which is used to create electromagnetic and electrostatic effects in the circuits. It's the so-called Wattless Load — it doesn't do useful work, but it's necessary for creating magnetic fields in transformers and motors. It's the vector sum of the inductive and capacitive currents and voltages. On modern electronic display units, load meters may only show a percentage of the maximum power being taken, rather than an absolute value. Finally, Voltage and Frequency Meters. Voltage and frequency indications are provided for each generator. Typically, only one voltmeter and one frequency meter are provided in systems with several alternators in circuit. You select which alternator you're reading by a Multi-position Switch. That switch can usually be positioned to show not only the supply frequency and voltage of the engine-driven alternators, but also that of the Auxiliary Power Unit (the APU), the Ground Power Unit (external power), or the Emergency Ram Air Turbine (the RAT), if that's fitted on the aircraft.