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?