Generator Altitude Limitations Question

[pmcustom]

Maybe someone can clear up this question I have. Why exactly do generators have altitude limitations? I've been told that it has to do with the "normal" air gap between the brushes and commutator, which has a higher resistance because of the decreased air density, and therefore increases the excitation current needed (which increases temperature, which is the ultimate limiting factor). But why is there an air gap in the first place? I can't imagine that any gap between the commutator and brush would be acceptable, but they're saying that it's there AND normal. In case you're wondering, the particular aircraft we're talking about is a Falcon 2000EX, which has an APU altitude limitation of 35k.

 

[Tina Fey's Scar]

Are you sure it's the APU gen that is the limiting device? Perhaps it's just harder to start/operate the APU in the thin air.

 

[pmcustom]

He said that the altitude restriction was there because of the APU start/gen. I said I didn't understand what the generator had to do with altitude, and that's when he told me about the supposed air gap and it's consequences, and he is a Flight Safety instructor so he is reputable. The first thing I thought of when I heard about the altitude restriction was, like you said, the air density affecting the engine...maybe we just misunderstood each other (and no one else in the class spoke up).

 

[bocefus]

hmm, dontcha reckon the aircraft powerplant generators would have the same limitation? It's has nothing to do with air gap, brushes, or any of that nonsense.
Purely a certification issue related to extra testing required to verify the compressor portion of the APU powerplant can handle it. Who needs an operational APU above FL350 anyway?

 

[pmcustom]

Hmmm, well yes I thought of all that too. Just thought I'd check to see if I was missing something...doesn't look like I am...he must be mistaken.

 

[bocefus]

Hmm, those that can, do- those that can't, teach.
I'd be surprised if any of the generators on the 2000EX have brushes at all.

Ask the esteemed instructor to explain a scenario in which the APU would be needed above 350

 

[pmcustom]

I/he knows the APU won't be needed that high...that's not the point of my post. All I wanted to know was already answered, so thanks. Also, the main engines are air start.

 

[bocefus]

and the main generators are???

 

[pmcustom]

you said "the aircraft powerplant generators would have the same limitation" and i was pointing out that that isn't the case. nevermind anyway, i'll go talk to the p&w rep, this message board sure isn't helping.

 

[bocefus]

you didn't point out anything, you said the main engines are air start. What does that have to do with the main generators, or any limitation thereof?

 

[pmcustom]

you said the main generators would have the same limitations (if there were any) as the apu, but since they're obviously not the same generators, then they most likely would not...but either way it's besides the point, and this argument still does not help, so again, nevermind.

 

[bocefus]

Why are they "obviously not the same"? Because the engine has air starters? Bad assumption. You are better off asking the Dassault rep if you are trying to find out info on the engine driven generators, they are installed by the airframe manufacturer, not the engine manufacturer.

 

[pmcustom]

hey i'll be the first to admit that i don't know much about the plane...i work on them here and there, but not very often. i'm taking a flight safety class for the new 2000EX EASy. the instructor either came from dassault or still works for them...i'm not sure. you mentioned a brushless generator...could that be what he was talking about? maybe he just used the term "brushes" for convenience.

 

[bocefus]

Most likely, the engine driven generators are brushless, probably made by Thales. Can't speak for the Honeywell APU

 

[pmcustom]

Ok this is how it goes. The main engine generators are actually brushless alternators. They are brushless because at high altitudes the air has less resistance, and arcing became a problem between the brushes and slip ring (arcing is obviously not good, and greatly reduces life). The APU, though, has a standard starter/generator (not brushless, probably because it would cost more and no one cares if they can use the APU at higher altitudes) and therefore cannot be operated above a set level. In actuality, it can run just fine as high as the engine was designed to go, but it would need overhauled all the time I imagine.