I'm studying for an interview and. . .

[Wasatch CFI]

Trying to figure out the advantages and dissadvantages of AC vs. DC. How 'bout 28vdc vs. 14vdc? And finally, alternator vs. generator.

Any input would be appreciated.

 

[typhoonpilot]

First of all, good luck on your interview.

The advantage of A.C. over D.C. are:

1) A.C. power systems provide a maximum amount of power for the weight of electric equipment in the aircraft. The voltage of A.C. power may be changed at will by means of transformers. This makes it possible to transmit power at a high voltage with low current, thus reducing the size and weight of wire required.

2) Alternating current can be produced in a three-phase system, thus making it possible to use motors of less weight for the same amount of power developed.

3) A.C. machinery, such as alternators and motors, do not require the use of commutators; hence, service and upkeep are greatly reduced.



Hope that helps.

Typhoonpilot

 

[FlyboyPhil]

Alternator vs. Generator

A generator has permanent magnets to create the magnetic field that the wires spin through to make the juice, so you can start it spinning with no other electrical input and it will work.
An Alternator requires some juice to make the magnetic field.. so if the battery is really dead you could start the engine and the alternator would spin round and round and accomplish nothing.
Generators are much more rpm dependant than Alternators, so you need relatively high rpm to get power .. Alternators have a flatter power curve and produce good juice at a wider range of rpm.
Most (all?) turbine ac use generators for two reasons that I can think of.. first turbine engines idle at 60-70% of max rpm so low rpm generator problem isnt an issue.. and a generator is basically an electric motor being used bass ackwards, so they use the generator as the starter motor, then after the turbine gets going they use it to make juice.
Been a while since I instructed or looked at mr machados book on aircraft systems so you will have to excuse any errors and my abundance of technical terms.. Luck with the interview, Philip

p.s. just saw your 14v vs 28v question. The voltage lost/heat generated in a wire is proportional to the current through the wire. At a lower voltage you need more current for the same amount of power. Higher voltage means you can send your power with less current/less loss enroute and/or in smaller wires. Thats why big power lines are 50 kazillion volts.. so they dont have to be as big as gas pipelines and so we dont heat up the great outdoors..