AC Electrics - Alternators — Page 210, Lesson 226

I want to walk you through the AC electrical system for a multi-engine aircraft, focusing on the alternators and the control panel that manages them. Let's start with the Auxiliary Power Unit, or APU. The APU also drives a generator, but this one does not need a constant speed unit because the APU itself runs at a constant speed — so the generator output is already at the right frequency without any extra regulation. Now, look at the generator field switch lights on the panel. These control the field excitation circuit, which is also called the exciter control relay. The "flow bar" in the "close" switch light illuminates to tell you the generator field circuit is complete — meaning the magnetic field inside the generator is energized and ready to produce voltage. Once that's done, you can check the voltage and frequency by selecting the required generator on the rotary switch and reading off the voltmeter and frequency meter located in the upper right corner of the panel. The "trip" switch light, on the other hand, opens the field circuit, which reduces the generator voltage to zero — effectively de-energizing that generator. Next, we have the Generator Circuit Breaker, abbreviated GCB. The GCB is controlled by its own set of switch lights — close and trip. The GCB connects or disconnects the generator to its own AC bus bar. For the APU, the GCB connects it to the AC Tie Bus instead. The load on each generator can be monitored using the Real/Reactive load indicator, which displays either kilowatts (kW) or kilovolt-amperes reactive (kVAR). Normally, the meter shows kW, but you can see kVAR by pressing and holding the kVAR button located to the left of the gauges. The Bus Tie Breakers, or BTBs, are controlled in the same manner as the GCBs. Their job is to connect the generator busses to the AC tie bus so the generators can operate in parallel. On this aircraft, all three generators are normally connected in parallel to share the total aircraft electrical load — so each generator carries roughly one-third of the demand. Now, each AC bus feeds a Transformer Rectifier Unit, or TRU. The TRU converts 115 volts, 200 volts, 400 hertz, three-phase AC into 28 volts DC. That DC power then feeds the individual DC busses. These DC busses are also normally paralleled through tie breakers, and all of those tie breakers are controlled by a single switch called the DC Bus Isolation switch. You can also check the DC part of the system for voltage and current. There's another rotary selector and separate meters for DC volts and DC amps, so you can monitor the DC side just like you monitor the AC side. Finally, let's talk about the standby bus bars. These can be fed from the normal electrical supply — both AC and DC — or from the battery in the event of a total supply failure. That gives you a backup source for essential systems. The red fail lights on the panel indicate no voltage on the Essential or Standby bus bars — so if you see a red light, you know that bus has lost power.