How do inverters in aircraft work? And why are they necessary?
Inverter is a switching-circuit that takes DC as input and converts it into AC of the desired frequency and waveform.
(Note that an inverter will not change the voltage. This can be remedied by fitting a transformer downstream of the inverter.)
An inverter can be supplied with direct current from:
TRU stands for Transformer Rectifier Unit. It takes AC input and converts it into DC of the desired voltage. It can be supplied by AC of any waveform/frequency. The supply first passes through the transformer which changes its voltage, then through the rectifier which converts it into DC output.
On most aircraft, the TRU usually provides a 28VDC output; this is because the 24 volt batteries on board require this as their charging voltage. Also, this allows the TRU output to act as battery output during normal operations.
However, modern aircraft AC systems operate at 115VAC @400Hz. This means that a transformer downstream of the inverter will be necessary to boost the voltage. The inverter converts 28VDC into 28VAC @400Hz, and the transformer then converts that into 115VAC @400Hz, which can then be supplied to the AC bus. The transformer is usually an integral part of the inverter unit.
A combination of TRU and the inverter unit can therefore convert any random AC source into AC of the desired voltage, frequency and waveform. When the engines are unavailable, the TRU is replaced by the batteries.
Some applications of the inverter
In those aircraft whose generators are not equipped with a Constant Speed Drive Unit, generator output (wild-frequency AC) is not usable as such. It must first be converted into DC supply by the TRU, then finally into a usable AC supply by the inverter.
In those aircraft whose generators are equipped with CSDU, the generator output is 115VAC @400Hz as it is, and can be directly supplied to the AC bus. However, there may be applications where a different frequency/voltage is desirable - for example, the standard 115/230 VAC @50/60Hz outlets. Here, a combination of TRU and inverter can provide this supply.
Inverter is also an essential part of circuitry required to drive synchronous motors, since they require a constantly varying frequency input during acceleration/deceleration. Here, a device called ESC is used, which combines a variable-inverter and a variable-transformer. The combination is capable of providing alternating current varying in frequency and voltage. Synchronous motors are often used as engine starters.
On a relatively simple turboprop like a Twin Otter, the output from the generators is 28V DC and this powers most of the electrical equipment on board eg fuel pumps and the hydraulic pump.
Some flight instruments require 115V 400 Hz AC power and this is supplied from inverters. After reading the above good answer, I understand that a transformer must also a part of this process to increase the voltage.
On larger aircraft the generator power is AC and most electrical equipment is powered by this. Equipment that needs DC power are supplied from Transformer-Rectifier Units which on the B767 eg are supplied by the Main AC Busses. The B767 also has a Standby AC Bus for supply to some essential items. In some situations, the ship’s batteries (DC) can power this bus through the Standby Inverter.