For smaller aircraft models, automotive generators and alternators work in conjunction with lead acid batteries to provide electricity and power throughout the plane itself. In 1976, the standard for power rose from 14 VDC to 28 VDC – the manufacturers stayed the same. When it came to onboard equipment, certain aircraft required 400 Hz AC (navigation equipment). For this, DC to AC inverters are installed to adequately switch when necessary.
Powering Medium-Sized Aircrafts
Medium-sized aircrafts utilize turbine engines that have an electrical system onboard that is vital to the engine. A starter generator is typically installed and performs both tasks admirably. Usually, you’ll find the starter installed directly in the accessory gear train of the engine, so it can assist with the turbine starting process, and then switch over to serve as a generator. This versatile function is accomplished through the use of heavy duty relays that appropriately connect the windings.
Turbine engines in the medium class have always been configured to perform at 28 VDC. Now, since turbine engine instrumentation requires AC, small inverters are installed to adequately power the aircraft. Many avionics in the medium class AC as well, so you’ll often see two inverters installed to supply the appropriate AC power.
Powering Larger Aircrafts
Large aircraft engines are equipped with large 3 phase 115 VAC rotary generators. These specific generators power most of the electrical equipment and are driven by 3 phase electric motors – they are also rectified to create DC buses. Now, some of these larger aircraft place a battery on this line, while others do not – it depends on the gas turbine start up sequence as well as the model of the plane.
Furthermore, larger aircraft may also install an APU (Auxiliary Power Unit) to run systems while on the ground without having the engine idle when there is no ground power available – such as a gas electric hybrid GPU.