AC vs DC Power

[Macgyver]

Hey guys,

Can someone explain the difference between alternating current and direct current output? I just don't really understand why some systems require AC, while others require DC. What makes one system require one type of power input, but not the other?

Thanks in advance.

[vapilot2004]

115V 400hz Alternating current is generally more efficient than 28V Direct Current when powering motors. The wiring for 115V current is also lighter, due to the reduced currents involved compared to a similar 28VDC device.

Example:

A 100W lamp will draw .86amps @ 115VAC while the same 100W lamp would require 3.6amps @28VDC by comparison. Higher amperage loads require heavier wires to carry the current. Most lamps can operate on DC or AC as long as the voltage is correct.

The majority of the main aircraft systems using motor-driven pumps and fans are usually AC powered. The windings and cores on an AC type motor are lighter than a DC motor with comparable output.

All DC powered items are usually there as a backup or standby unit for redundancy. There are also convenience considerations for when APU/Engine Generators/Ground Power is not available.

Many electronic units, while they may be mainly AC powered, have internal TRUs to reduce the voltage and convert AC to DC. Practically all electronic devices from transistors to integrated circuits rely on DC power (whether converted AC or direct from DC) for operation. +/- 5V, +/- 12V and 28V are all common internal operating voltages. Some units will also internally convert 28V DC into AC for operation.

There are many systems that require both DC and AC power sources to fully function and the loss of one or the other require special procedures to overcome system shortcomings.

In the days before the switching power supply was in use, an inverter (of sorts) consisted of a DC motor directly connected to an AC generator and was quite common aboard aircraft pre-1960s. The vacumn tube era comes to mind here. :)

Direct Current:
Electron flow is steady from negative to positive
Is the most reliable power source aboard due to the aircraft batteries
Cannot be easily up-converted

Alternating Current:
Electrons are bouncing to and fro at the output frequency
Is subject to loss with engine outs and generator/inverter failures
Can be easily and efficiently up/down converted VIA a transformer

Cheers

[JamesA]

Some years ago I was a trade examiner in the R.A.F., and asked a young lad the same question.
He rapidly responded, 'That's easy - Direct current goes directly to the switch and on to the component by the wire. The alternating current is directed around the aircraft structure before it reaches the switch or component.'
I always recall this when I hear the question. Thanks for the reminder.

[Farmer 1]

Did he pass?

[hetfield]

Guess he became chief-pilot;)

[stilton]

While on the subject, anyone know a good reference book for beginners to aircraft electrical systems?

Sort of an idiots guide to electricity.

[vapilot2004]

While on the subject, anyone know a good reference book for beginners to aircraft electrical systems?

Sort of an idiots guide to electricity.

I highly recommend Aircraft Electrical Systems by EHJ Pallett, a former UK CAA official and aero engineer. It's not exactly a beginners book, but is clear and concise and contains numerous illustrations.

[West Coast]

Reminds me an oral question.

What is the non essential bus for?

It takes the First Officer to the airport.

[JamesA]

1. Was that you?
2. Are you the man to interview me for a job next week?

[cwatters]

http://en.wikipedia.org/wiki/Direct_current

http://en.wikipedia.org/wiki/Alternating_current