Electrical Question

bc2209

bc2209

Well-Known Member
What are the advantages or disadvantages for AC and DC power in regards to an airplane?

What is the difference between an alternator and a generator?
 
AC advantages: easy to step voltage up and down with minimal losses in efficiency, some components work really well with it (any motor that needs to run at a constant speed, lots of avionics components).
AC cons: I'm pretty sure it's witchcraft with all that current leading or lagging voltage and parallel RLC circuit nonsense, AC motors aren't optimal for all applications, you have to have monkey motion like CSD units or solid-state VSCFs to keep a constant frequency as engine rpm changes, did I mention it's like voodoo?, you can't really store AC in a battery.

DC advantages and disadvantages are pretty much opposite of AC.

Alternator vs generator is kind of semantics, in this context I'm guessing that the desired answer is that a generator pumps out AC while an alternator has a built in rectifier so it puts out DC. However in GA land a generator is a device that puts out DC via brushes and a commutator instead of slip rings and a rectifier.
 
Roger Roger said:
AC advantages: easy to step voltage up and down with minimal losses in efficiency, some components work really well with it (any motor that needs to run at a constant speed, lots of avionics components).
AC cons: I'm pretty sure it's witchcraft with all that current leading or lagging voltage and parallel RLC circuit nonsense, AC motors aren't optimal for all applications, you have to have monkey motion like CSD units or solid-state VSCFs to keep a constant frequency as engine rpm changes, did I mention it's like voodoo?, you can't really store AC in a battery.

DC advantages and disadvantages are pretty much opposite of AC.

Alternator vs generator is kind of semantics, in this context I'm guessing that the desired answer is that a generator pumps out AC while an alternator has a built in rectifier so it puts out DC. However in GA land a generator is a device that puts out DC via brushes and a commutator instead of slip rings and a rectifier.
Click to expand...
So I read in a book that AC is usually for high-draw items and DC is used for "general items"...
Is that just pertaining to the amount of draw the circuit breaker usually tells you... like higher draw items would be radios, aircraft lights, starter, landing gear, AC vs lower draw items like instruments, gauges, lights in cockpit sort of thing?

When you talk about turbine systems, you would consider them to be AC generators and DC generators vs piston where alternators produce AC and generators produce DC? As far as I remember from alternators and generators, alternators were better in the training aircraft because it was lighter and much more efficient but other than that I was not told why? Why is it more efficient that every trainer I've flown has an alternator...leading me to believe that alternators rule generators and then study turbine systems where we don't use the word "alternator" and just call them AC and DC generators?
 
At one point in history GA airplanes did have generators (DC with brushes) because that's what cars had.

Now they have alternators just like modern cars, many of them are are described by their auto name such as Ford style.


Sent from my StarTac using Etch A Sketch.
 
tcco94 said:
So I read in a book that AC is usually for high-draw items and DC is used for "general items"...
Click to expand...

AC can allow for thinner gauge wiring, which is lighter and costs less. This is mostly because AC is typically a much higher voltage as compared to DC. In GA aircraft, the currents involved don't justify the added weight and complexity of transformers and separate electrical buses. Larger aircraft - it can shave off enough weight to be substantial. While DC could technically be more efficient at higher voltages, there are lots of practical reasons that you don't want a 400vDC electrical system: It arcs, promotes corrosion, difficult to step down, no batteries available at those voltages...

It is also much easier to get "clean" AC power without too much noise in it. DC tends to have a lot of ripple, so needs to have voltage regulators which add to the losses.
 
gotWXdagain said:
Cue the discussion about alternators really technically put out AC and without a rectifier can only produce AC and now I'm running for the exit dodging tomatoes.
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You've got it all wrong, AC generators technically *pushes glasses up on nose* produce DC, they just change the voltage constantly and the polarity several times a second.

Also, I will cut you.
 
hook_dupin said:
Settle down. This is just a phase.
Click to expand...
Electricity and magnetism...
1285770302993.jpg
 
Roger Roger said:
You've got it all wrong, AC generators technically *pushes glasses up on nose* produce DC, they just change the voltage constantly and the polarity several times a second.

Also, I will cut you.
Click to expand...

You might even say, the current pattern alternates at a regular frequency?
 
The main thing to remember is that while generators can produce either AC or DC power, the only way to STORE DC power is in a battery. Without a battery, the only way to get AC power is to generate it by engine rotation. So, an engine failure in an AC only airplane also becomes an electrical failure. You could have a DC-only powered airplane, but then everything has to be much more heavy duty, because it has to be able to carry the entire voltage load at all times (red line), whereas an AC system only carries the full load at intermittent times (green line). The practical upshot of this is that AC systems are lighter, which means less aircraft weight, less fuel burn, and more payload/useful load....




...or at least, that's what I was taught.
 
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