Generator Altitude Limitations Question

[PHX767]

Why indeed?

If I may chime in...

I believe the original question is why the APU is limited to FL 350.... Most APU's have a start/run limit at high altitudes. E.g, they won't start when: 1) Cold soaked 2) At high altitudes outside of their normal operating envelopes. This is unrelated to generator cooling limits which vary with altitude due to air density.

An operational reason for needing an APU at FL350 is for dispatch with a generator MEL'd. Not uncommon when you need to get a plane back from the wrong side of the pond. In this condition we would have to leave the APU running for the whole trip, even the high altitude portion across the ocean.

 

[bocefus]

There might be an operational need for a large, transport aircraft, but never for a corporate aircraft.

 

[Snoopy58]

Corporate aircraft never need to fly with an engine-driven generator inop???

Even if you never (!hah!) dispatch without everything working in "good as new" conditions, generators can fail enroute and Mr Big in the back might rather you fired up the APU and put it on the bus & continued on to his destination instead of either continuing with degraded equipment (which generator powers his laptop power port & entertainment center, btw?), or diverting to whatever Podunk is closest to wait until a replacement generator gets FedEx'd to you.

Who needs an operational APU above FL350 anyway?

It seems to be news to some here that this is a common procedure, but it is. Running APUs in-flight, even at high altitudes, as a common non-normal procedure (i.e. what you do if a generator is inop or fails) in many jets.

 

[bocefus]

The post was in reference to corporate aircraft. Yes, who does need an apu above 350 in a corporate aircraft? Nobody, this is why Dassault didn't bother going to the extra expense in certificating it to operate above 350. And, you might be surprised to know that there are plenty of transport aircraft that are NOT approved for in flight use of the APU, and some don't even have an apu.

 

[A Squared]

Getting back to the commutator question. It doesn't have to do with arcing betwen the commutator and the brush, it has to do with arcing between commutator segments. A Commutator is a segmented slip ring. Each segment must be insulated from the adjacent segment, as it voltage is always different than the adjacent segments. At higher altitudes the air has *less* resistance, not more. Actually, it has a lower dielectric coeficient, but "resistance" captures the general drift. The higher you go, and the thinner the air, the more likely you are to get electricity jumping from one segment to another. The fact that the brushes have coated everything with cinductive carbon doesn't help the situation.

Now, is this *the* reason the APU on the 2000EX has an altitude limitation? I don't know, and I'm not going to enter that pissing match. I do know that generators with commutator will get arcing at higher altitudes.

 

[pmcustom]

Thank you very much A Squared....that answers my original question completely.

 

[A Squared]

Also, non-segmented slip rings can arc too, the brushes should be in contact whith the slip ring surface, and normally that is where the current flows, through the brush contact area. At high altitudes, where the air is not a very good insulator, the electrons can jump from the slip ring to the brush at places other than the contact area (at the edges of the brush for instance)

The arcing tends to pit the surface of the commutator or slip ring, which increases brush wear.

Btw, that should have been "conductive carbon dust" in my previous post.

 

[NewBlue]

I can't supply you with a source but somewhere in the grey cell collection I remember something about cooling being a factor on some models of ac generators.

(yes it's colder aloft but the density is much less)