Airline Pilot Central Forums - AC/DC difference

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Quote:

Originally Posted by Hacker15e (Post 2745123)

"AC lies, DC dies" is the phrase you're looking for.

This works for analog gauges, but not necessarily for other functions. AC powered gauges tend to stop in position or invert with power loss, while DC powered gauges tend to simply go back to zero (or in some cases indicate last position); the "lies" function of AC primarily points to AC anaolog gauges occasional propensity to invert or indicate out of phase or incorrectly.

Quote:

Originally Posted by Geronimo4497 (Post 2745195)

https://www.youtube.com/watch?v=ATFq...&frags=pl%2Cwn

Awesome thanks, nothing like visuals to understand sth.

Quote:

Originally Posted by rickair7777 (Post 2745246)

If you can understand and articulate this, that should suffice for an interview...

Direct Current (one way flow) is used in aircraft where batteries are involved, since they only do DC.

Alternating Current (Sloshes back and forth) is what is naturally produced by alternators/generators, and allows for more power density in motors and generators (weight savings), and lighter wires. Higher frequency AC is smoother, and better for precision electronics (hence 400Hz on aircraft vice 60 Hz at home).

You can convert back and forth if needed.
AC => DC: Transformer/Rectifier
DC => AC: Inverter
A motor-generator can also couple AC and DC systems with power flow possible in either direction, with instant, seamless reversal available.
Turbine airplanes typically have equipment to both supply some AC systems from battery DC, and all DC systems from the normal AC power system (to avoid depleting, and charge, the batteries).

Higher voltage is more efficient, especially over longer wire runs, but as voltage increases, insulation weight increases and much above 200V you need to start spacing the wires apart to avoid inductive interference (or arcing).

Motorcycles might use 6V (short wire runs). Cars use 12V, modern airplanes use 28VDC. AC will probably be somewhere between 100V and 400V.

Thank you so much. Always helpful!

Quote:

Originally Posted by Erj135dude (Post 2745261)

Thank you so much. Always helpful!

Sure.

Unrelated to interviewing, future "more electric" or even "all electric" airplanes are envisioned which will replace pneumatic and hydraulic systems with electrics. Will likely use much higher voltages to reduce weight in wires and equipment. The 787 was the first step in that direction.