Aircraft Electronics and Instrument Systems Questions

[tribute]

Hi, I am researching on aircraft electronics and i would like to ask a few questions. Thanks for your help and responds.

Questions:
1.Do we need constant frequency and constant voltage for AC motor for running it at constant speed?
2.Assuming that a DC source is available in the aircraft, explain the working of a single-phase inverter using IGBTs.
3.How to maintain the frequency output of an inverter constant?
4.How to maintain the voltage output of an inverter constant?

 

[Boncuk]

Hi,

as far as I know constant frequency is a must since multiple generators (one generator driven by one (jet) engine) have to be connected to supply the entire avionics of an aircraft from one energy bus.

I used to fly RF4-E (Phantom recce plane with the triple-U pilots (Unarmed - Unescourted and Unafraid)) which had a 60KVA generator attached to each engine. As soon as both engines were running the "bus tie" had to close automatically (meaning both generators deliver the same voltage and frequency in phase to connect buses).

For weight reduction of DC powered devices the "mains" frequency is 400Hz to save on iron for transformers. Standard aicraft generators deliver 115V/400Hz.

A "bus tie open" generally meant abort of any mission since safety circuitry (to avoid overload of a generator) prohibited use of certain electronic equipment.

Batteries on board of aircraft supply the most vital functions as engine ignition and the VHF/UHF radio (for mayday calls). They are normally rated 48V/110Ah on combat planes (not larger than a truck battery).

Any aircraft loosing 100% generator power depends on radar guidance for a safe landing.

Boncuk

 

[MikeMl]

Boncuk outlined practise in Military Aircraft. Passenger Jets might be similar.

Civil Aircraft from puddle jumpers up to light jets are very different. There is no AC distribution system at all; 115V 400Hz is not used. Instead, all instrumentation, navigation, and communication is powered with DC; 28VDC in larger and newer aircraft, and 14VDC in light aircraft built before ~1980 and almost all homebuilt aircraft.
The 14VDC aircraft use an alternator system very similar to automotive systems. The 28V aircraft use a similar alternator that is modified for 28V. Typical battery capacity is about 35AH for smaller aircraft and helicopters with up to 200AH for Jets and light transport.

Be advised that any equipment designed for aircraft must meet very strict EMI standards (DO161, etc). Switching power-supplies are a total pain in the ass when it comes to getting them certified...

 

[Reloadron]

Hi, I am researching on aircraft electronics and i would like to ask a few questions. Thanks for your help and responds.

Questions:
1.Do we need constant frequency and constant voltage for AC motor for running it at constant speed?
2.Assuming that a DC source is available in the aircraft, explain the working of a single-phase inverter using IGBTs.
3.How to maintain the frequency output of an inverter constant?
4.How to maintain the voltage output of an inverter constant?

Problem I see here is the questions are all pretty open ended. Military aircraft or civilian aircraft and then large commercial or small aircraft?

Most aircraft that have AC systems use a CSD (Constant Speed Drive) off the engine(s) to maintain the AC frequency. Regardless of engine speed the generator/alternator rotates at a constant speed.

Many aircraft that have DC systems frequently use a 28 VDC starter generator which can start for example a turbo prop aircraft and after start function as the aircraft DC Generator with associated GCU (Generator Control Unit).

There are just so many variations of aircraft power systems it is hard to answer the questions posted other than in generic terms.

Just My Take
Ron